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Highly Efficient Solid-State Near-Infrared Emitting Material Based on Triphenylamine and Diphenylfumaronitrile with an EQE of 2.58% in Nondoped Organic Light-Emitting Diode
... Phosphorescent materials include transition complex materials. According to the literature, organic NIR electrofluorescent materials consist of organic small molecules and polymers in the D-A configuration [5,[41][42][43][44][45][46][47][48][49][50][51][52][53]. They share many common characteristics and continuous π-conjugated structures, which can make the emission spectra of the materials redshift to the NIR region. ...
... In 2015, Han's team reported D-A-type NIR fluorescent compound 2 based on diphenylfumaronitrile and triphenylamine [42], as shown in had an NIR (EL) peak at 675 nm. The maximum brightness was 7025 cd m −2 , and the maximum EQE was 2.58%. ...
Near-infrared (NIR) refers to the section of the spectrum from 650 to 2500 nm. NIR luminescent materials are widely employed in organic light-emitting diodes (OLEDs), fiber optic communication, sensing, biological detection, and medical imaging. This paper reviews organic NIR electroluminescent materials, including organic NIR electrofluorescent materials and organic NIR electrophosphorescent materials that have been investigated in the past 6 years. Small-molecule, polymer NIR fluorescent materials and platinum(II) and iridium(III) complex NIR phosphorescent materials are described, and the limitations of the development of NIR luminescent materials and future prospects are discussed.
... Meanwhile, its near-planar structure would allow more π conjugations compared with a single benzene ring and lead to light absorption and emission redshift [31]. Therefore, triphenylamine derivatives have been widely used in hole transport materials and donor-acceptor systems of diodes [32][33][34][35], sensors [36,37], solar cells [38][39][40], and photochromic materials [41][42][43]. ...
Density functional theory calculations at PBE38/6-311+G** level by involving the polarizable continuum model in solvent dichloromethane were employed to explore the geometries, electronic excitations and associated properties of the donor–acceptor-donor (D–A–D) di-triphenylaniline-modified thiophenes of 4,4′-(thiophene-2,5-diyl)bis(N,N-diphenylaniline) (TPA–Th–TPA), 4,4′-([2,2′-bithiophene]-5,5′-diyl)bis(N,N-diphenylaniline) (TPA–ThTh–TPA) and 4,4′-(thieno[3,2-b]thiophene-2,5-diyl)bis(N,N-diphenylaniline) (TPA–TT–TPA). The spectral properties were investigated with the time-dependent density functional theory at the same theoretical level, involving 37.5% of the Hartree–Fock exchange energies and 50% of the local and non-local contributions, respectively, for the rest of the energies. It was found that the most stable TPA–Th–TPA has no symmetry (C1 point group) in the ¹A electronic state, while the most stable structures for both TPA–ThTh–TPA and TPA–TT–TPA have C2 symmetry in the ¹A state. Their vertical absorption spectra were examined with the twenty lowest excitations, while the emission spectra were equivalently simulated by the vertical transition (from S1 to S0) of the structure of the S1 state. Both the theoretical absorption and emission spectra agree very well with the experiments in terms of absolute wavelengths and their sequence for different compounds. For the absorption with the maximum wavelength and strength, the theoretical wavelengths reproduced the experiments with deviations of only 4.4, 0.6 and 7.3 nm for TPA–Th–TPA, TPA–ThTh–TPA and TPA–TT–TPA, respectively. While the emission peaks have slightly larger deviations by 44.5, 90.5 and 53.3 nm. Detailed features for the next intense peak, as well as their peak shoulders, were explored. For the electronic properties associated with the S0 → S1 transition, the hole-electron, frontier orbital and natural transition orbital analyses supported charge transfer characteristics. The inter-segment charge transfer analyses provided the magnitude of inter-segment charge transfer of TPA–Th–TPA, TPA–ThTh–TPA and TPA–TT–TPA by 67.1, 60.6 and 66.4%, respectively, within which the transfer from donors to acceptor(s) is dominant. In addition to the largest π conjugation of the ThTh group that leads to the largest redshift of the spectra and charge redistribution, TPA–ThTh–TPA has the largest vertical electron affinity energy, electronegativity and global electrophilicity with 2.01, 3.68 and 4.05 eV, respectively. All the molecules have electrostatic potentials in their S0 and S1 states by approximately 54% in the negative potential region, supplied mainly by the lone pair electrons of the S and N atoms as well as the π electrons of the C atoms. This leads to the compounds being more susceptible to electrophilic reactions. Similar atomic natural charge distributions for the different compounds in their S0 and S1 states were found, with the S atom(s) having the most positive charge (~ 0.42 e) and the N atoms having the most negative charge (~ − 0.51 e). Small changes in the atomic charge were found in the excitations, indicating that the charge transfer does not significantly change the atomic charge distributions.
... Meanwhile, its near planar structure would allow more π conjugations compared with a single benzene ring and lead to light absorption and emission red shift [31]. Therefore, triphenylamine derivatives have been widely used in hole transport materials and donor acceptor systems of diodes [32][33][34][35], sensors [36,37], solar cells [38][39][40], and photochromic materials [41][42][43]. ...
Density functional theory calculations at PBE38/6-311 + G** level by involving the polarizable continuum model in solvent dichloromethane were employed to explore the geometries, electronic excitations and the associated properties of the donor-acceptor-donor (D-A-D) di-triphenylaniline modified thiophenes of 4,4'-(thiophene-2,5-diyl)bis(N,N-diphenylaniline) (TPA-Th-TPA), 4,4'-([2,2'-bithiophene]-5,5'-diyl)bis(N,N-diphenylaniline) (TPA-ThTh-TPA) and 4,4'-(thieno[3,2-b]thiophene-2,5-diyl)bis(N,N-diphenylaniline) (TPA-TT-TPA). The spectral properties were investigated with the time dependent density functional theory at the same theoretical level by involving 37.5% of the Hartree-Fock exchange energies and with 50% of the local and non-local contributions, respectively for the rest of the energies. It was found that TPA-Th-TPA has one stable structure in ¹ A electronic state without symmetry, while both TPA-ThTh-TPA and TPA-TT-TPA have most stable structures with C 2 symmetry in ¹ A state. Their vertical absorption spectra were examined with twenty lowest excitations while the emission spectra were equivalently simulated by the vertical transition (from S 1 to S 0 ) of the structure of the S 1 state. Both the theoretical absorption and emission spectra agree very well with the experiments by absolute wavelengths and their sequence for different compounds. For the absorption with the maximum wavelength and strength, the theoretical wavelengths reproduced the experiments by deviations of only 4.4, 0.6 and 7.3 nm for TPA-Th-TPA, TPA-ThTh-TPA and TPA-TT-TPA, respectively. While the emission peaks have slightly larger deviations by 44.5, 90.5 and 53.3 nm. Detailed features for the next intense peak as well as their peak shoulders were explored. For the electronic properties associated with the S 0 →S 1 transition, the hole-electron, frontier orbital and natural transition orbital analyses supported charge transfer characteristics. The inter-segment charge transfer analyses provided the magnitude of inter-segment charge transfer of TPA-Th-TPA, TPA-ThTh-TPA and TPA-TT-TPA by 67.1, 60.6 and 66.4%, respectively. within which the transfer from donors to acceptor is dominant. In addition to the largest π conjugation of the ThTh group that leads to the largest red shift of the spectra and charge redistribution, TPA-ThTh-TPA has the largest vertical electron affinity energy, electronegativity and global electrophilicity with 2.01, 3.68 and 4.05 eV, respectively. All the molecules have the electrostatic potentials on their S 0 and S 1 states by approximately 54% of the negative potential region supplied mainly by the lone pair electrons of the S, N atoms and the π electrons of the C atoms. This leads to the compounds being more susceptible to electrophilic reactions. Similar atomic natural charge distributions for the different compounds in their S 0 and S 1 states were found with the S atom(s) having the most positive (~ 0.42 e) and the N atoms having the most negative charges (~-0.51 e). Small changes of the atomic charge were found in the excitations indicating that the charge transfer does not significantly change the atomic charge distributions.
... This allows to separate HOMO and LUMO and thus to obtain a negligible energy gap between singlet and triplet excited states (ΔEST). [1][2][3][4][5] Nevertheless, these emitters are bound by a trade-off between the singlet radiative rate and HOMO-LUMO overlap: the latter must be suitably small to achieve ΔEST ≈ 0 but large enough to retain high photoluminescence quantum yield (PLQY). For this reason, various strategies have been proposed to achieve the best compromise between these two factors. ...
In this work we present three donor-acceptor thermally activated delayed fluorescence (TADF) molecules comprising a 2,3,5,6-tetrafluorobenzonitrile acceptor with various electron donor units: phenoxazine (Phx-BzN), phenothiazine (Pht-BzN), and carbazole (Cz-BzN). These molecules have been studied using steady-state and time-resolved photophysical techniques in solution, film and in crystal state. While Cz-BzN displays TADF in solution and PMMA films, Phx-BzN and Pht-BzN are non-emissive in solution and somewhat emissive in polymer films. More interestingly, while Pht-BzN remains virtually non-emissive in all studied solvents, it exhibits strong photoluminescence and TADF in crystal state, attributed to Crystallization Induced Emission (CIE). We demonstrate through computational studies that the CIE properties arise due to intermolecular interactions in the crystal structure that result in locking the ground state molecular geometry and blocking relaxation in the excited state. As a result, the oscillator strength in the crystal form is enhanced leading to a highly luminescent behaviour, while in solution it equals nearly zero due to the molecule adopting a perfectly orthogonal D-A orientation in the excited state.
... 6,7 The TADF phenomena can occur when a small singlet-triplet energy splitting (ΔEST) is obtained through, e.g. a charge transfer (CT) transition using donor-acceptor (D-A) compounds. [8][9][10] Indeed, in D-A compounds the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are separated on different moieties of the molecule minimizing exchange energy and thus also ΔEST. This leads to efficient reverse intersystem crossing (rISC) from T1 to S1. ...
Control of photophysical properties is crucial for the continued development of electroluminescent devices and luminescent materials. Preparation and study of original molecules uncovers design rules towards efficient materials and devices. Here we have prepared 7 new compounds based on the popular donor-acceptor design used in thermally activated delayed fluorescence emitters. We introduce for the first time benzofuro[3,2-e]-1,2,4-triazine and benzothieno[3,2-e]-1,2,4-triazine acceptors which were connected to several common donors: phenoxazine, phenothiazine, carbazole and 3,6-di-tert-butylcarbazole. DFT calculations, and steady-state and time-resolved photophysical studies were conducted in solution and in solid states. While derivatives with azine moieties are non-emissive in any form, the compounds comprising 3,6-di-tert-butylcarbazole display TADF in all cases. More interestingly, the two derivatives substituted with a carbazole donor are TADF active when dispersed in a polymer matrix and phosphorescent at room temperature in neat films (microcrystalline form).
... Moreover, introduction π unit between D and A unit can realize the co-existence of light emission with charge-transfer (CT) and local excited (LE) characteristic. It shows two advantages: firstly, the LE characteristic provides a high fluorescence quantum yield (φ PL ) in consequence of high radiative transition rate; secondly, the CT characteristic can acquire high exciton utilization due to a tiny singlet-triplet energy splitting, which can both improve efficiency of blue fluorescence materials [22][23][24][25][26][27][28][29][30]. More important, the introduction of π bridge can also decrease the triplet excitons concentration, weaken triplet excitons annealing and efficiency roll-off at high current density eventually, so that the stability of blue emitting materials is improved. ...
In this work, we designed and synthesized two novel blue-emitting compounds (m-TAC and p-TAC) with a twisted D-π-A configuration by incorporation of 2,4,6-triphenyl-1,3,5-triazine as an acceptor and carbazole as a donor through an anthracene π-spacer. Therein, p-TAC exhibits a hybridized local and charge-transfer (HLCT) characteristic with a sky-blue emission at 462 nm in toluene while m-TAC shows emission maximum at 431 nm. The two compounds exhibit superb thermal stability in the company of a thermal decomposition temperature (Td) as high as 507 and 516 °C. It is noticed that the non-doped devices with m-TAC and p-TAC as blue emitters display a turn-on voltage (Von) of 3.2 V and achieve sky-blue emission at 460 and 476 nm. In particular, the optimized device based on p-TAC realizes a maximum luminance above 10 000 cd m⁻² and the maximum external quantum efficiency (EQEmax) reaching 5.03%. As a sky-blue HLCT emitter, p-TAC reveals high-efficiency electroluminescence (EL) performance, providing a new insight for further industrial applications.
... In the following years, many purely organic, aromatic and heteroaromatic RIM undergoing AIEgens were described. Specifically, AIEgens bearing cores such as tetraphenylethene, thiophene-triphenylamine, tetraphenylpyrazine, quinoline, 9,10-distrylanthracene, dithiole, and derivatives have been extensively reviewed in recent articles [21,25,63,[68][69][70][71][72][73][74]. ...
Aggregation-induced emission (AIE) compounds display a photophysical phenomenon in which the aggregate state exhibits stronger emission than the isolated units. The common term of “AIEgens” was coined to describe compounds undergoing the AIE effect. Due to the recent interest in AIEgens, the search for novel hybrid organic–inorganic compounds with unique luminescence properties in the aggregate phase is a relevant goal. In this perspective, the abundant, inexpensive, and nontoxic d10 zinc cation offers unique opportunities for building AIE active fluorophores, sensing probes, and bioimaging tools. Considering the novelty of the topic, relevant examples collected in the last 5 years (2016–2021) through scientific production can be considered fully representative of the state-of-the-art. Starting from the simple phenomenological approach and considering different typological and chemical units and structures, we focused on zinc-based AIEgens offering synthetic novelty, research completeness, and relevant applications. A special section was devoted to Zn(II)-based AIEgens for living cell imaging as the novel technological frontier in biology and medicine.
The spin-allowed doublet emission of luminescent radicals has recently attracted significant attention. However, the spectral range of most reported luminescent radical emitters and their corresponding organic light-emitting diodes (OLEDs) is confined to the red and deep red regions, with only a few extending to the near-infrared region, specifically in the context of an emission peak exceeding 800 nm. Herein, a luminescent radical, 2-(4-(bis(2,4,6-trichlorophenyl)methyl radical)-3,5-dichlorophenyl)-4-phenyl-4H-thieno[3,2-b]indole (TTM-2PTI), with NIR emission peaking at 830 nm in toluene, was obtained through attaching a 4-phenyl-4H-thieno[3,2-b]indole group to the TTM radical core. An organic light-emitting diode (OLED) utilizing TTM-2PTI as the emitter exhibits electroluminescence (EL) emission peaking at 870 nm, which is the longest EL wavelength among the doublet-emissive near-infrared (NIR) OLEDs. This work provides a simple molecular design strategy to achieve NIR emission of radicals by leveraging the lower steric hindrance and electron-donating ability of thiophene.
High power efficiency and low efficiency roll‐off at practical luminance are two requirements for new‐generation energy‐saving lighting technologies, which are still bottlenecks of thermally activated delayed fluorescence (TADF) white organic light‐emitting diodes (WOLED), despite the advantages of TADF materials and devices in low cost and high sustainability. Herein, we developed a spiro phosphine oxide host named SSOXSPO , which can form multiple and multidirectional intermolecular hydrogen bonds (IHB). The resulted multilevel IHB network integrates long‐range ordered and short‐range disordered alignments for suppressing triplet‐polaron quenching (TPQ) and triplet‐triplet annihilation (TTA). Electronic characteristics of SSOXSPO matrix are further regulated, leading to the optimal exciton allocation through balancing energy and charge transfer. As consequence, using SSOXSPO as host, the single‐emissive‐layer TADF WOLEDs realized the record performance, including ultralow operation voltage as ∼4.0 V, power efficiency beyond fluorescent tube (70.1 lm W ⁻¹ ) and negligible external quantum efficiency roll‐off (3%) at 1000 nits for indoor lighting. This work demonstrates that multiple interplays supported by host matrixes in TADF WOLEDs can facilitate the synergistic effects of TADF emitters on 100% exciton utilization.
The development of novel organic fluorescent materials with strong luminescence in the aggregate state as well as stimuli-response under external forces is of great importance for practical applications in the...
Amphiphilic di-cationic fumaronitrile derivatives with peripheral n-alkyl chains formed organic nanoparticles (NPs) through self-assembly in aqueous media. The NPs exhibited enhanced luminescence in both steady-state and delayed mode in comparison to their molecularly dissolved state. Due to the presence of the multivalent array of positive charges on their surface, they were found to bind heparin, a bio-polyanion which is routinely employed during surgery as an anticoagulant. The electrostatically driven co-assemblies resulted in a significant enhancement in the steady-state and delayed luminescence of the NPs. This provided a highly sensitive detection of the polyanion in aqueous buffer as well as in highly competitive serum and plasma media. Furthermore, and most notably, the heparin based co-assemblies were found to act as efficient donors exhibiting fluorescence resonance energy transfer (FRET) to acceptor dyes with the energy transfer efficiency reaching up to 88%.
The stimulus-responsive smart switching of aggregation-induced emission (AIE) features has attracted considerable attention in 4D information encryption, optical sensors and biological imaging. Nevertheless, for some AIE-inactive triphenylamine (TPA) derivatives, activating the fluorescence channel of TPA remains a challenge based on their intrinsic molecular configuration. Here, we took a new design strategy for opening a new fluorescence channel and enhancing AIE efficiency for (E)-1-(((4-(diphenylamino)phenyl)imino)methyl)naphthalen-2-ol. The turn-on methodology employed is based on pressure induction. Combining ultrafast and Raman spectra with high-pressure in situ showed that activating the new fluorescence channel stemmed from restraining intramolecular twist rotation. Twisted intramolecular charge transfer (TICT) and intramolecular vibration were restricted, which induced an increase in AIE efficiency. This approach provides a new strategy for the development of stimulus-responsive smart-switch materials.
Molecular structural tailoring is one of the versatile approaches for developing functional organic fluorescence molecules with desired properties for practical applications. In this manuscript, we report the synthesis of triphenylamine...
Considerable efforts have been made to design efficient thermally activated delayed fluorescence (TADF) emitters for organic light-emitting diodes (OLEDs). However, the development of efficient red/near-infrared (NIR) TADF materials with emission...
Conjugated polymers (CPs) have attracted great attention due to their excellent optical properties (such as large absorption cross section, signal amplification, high photostability etc.). As representative electron acceptors and organic small molecules which are easy to be synthesized and modified, cyano-substituted stilbene (CSS) derivatives are widely used to construct photoelectrical materials. Despite donor-acceptor (D-A) conjugated polymers based on CSS have been applied in sensing and super-resolution imaging, systematic studies about the effects of different CSS structures on the photophysical properties of CPs have rarely been reported. Therefore, we have synthesized a series of D-A conjugated polymer nanoparticles (CP NPs) based on different CSS units, and found that the photophysical properties of CP NPs including the bandgap and ΔES-T were closely associated with the structure of CSS derivatives. Moreover, the introduction of tetraphenylethylene (TPE) can relieve the aggregation-caused quenching (ACQ) effects of CSS conjugated polymers to varying degrees. The theoretical calculation further corroborated that by regulating the number and distribution of cyanide groups in the repeating units, the stronger D-A strength resulted in a redshift in the emission spectrum and the more efficient capacity of total ROS (¹O2, O2•− and •OH) generation. We then selected CP6-TAT NPs, with the near infrared (NIR) emission and best ФPS, to characterize its performance in photodynamic therapy (PDT). It was revealed that CP6-TAT NPs can be regarded as an ideal candidate for PDT. The results provided a new reference for regulating the structure-effect relationship of CPs and a comprehensive method for constructing photosensitizers based on CPs.
Thermally activated delayed fluorescence (TADF) white organic light‐emitting diodes (WOLED) hold great potential for daily lightings, but should overcome the current limitation in power conservation. Despite amount of efforts on developing efficient TADF emitters, it is demonstrated here that the host matrix actually controls exciton formation and allocation, therefore strongly determines power efficiency. Through multiplying P≐O orientations, 248DBTTPO matrix establishes “diffusion predominance” mode to support multidirectional carrier/exciton migration, which combines the advantages of “recombination predominance” and “transport predominance” modes in alleviating dopant–dopant interactions and rationalizing carrier and energy transfer. 248DBTTPO matrix effectively balances carrier flux, singlet radiation, and triplet quenching suppression of TADF emitters, and exciton allocation in its complementary white TADF systems. As consequence, 248DBTTPO endows its trilayer single‐emissive‐layer WOLEDs with the state‐of‐the‐art external quantum efficiency up to 27%, ultralow driving voltages, and consequently the record‐high power efficiency of 108.6 lm W⁻¹, which breaks through key bottleneck of TADF lighting.
To date, the most commonly applied method for detecting constituents in exhaled breath is gas chromatography (GC) coupled with mass spectrometry (GC-MS). Despite the ability to accurately detect a wide range of volatile organic compounds (VOCs), GC-MS-based instruments are frequently large and expensive, and the analysis is time consuming. Modern exhaled breath analysis requires on-line sampling and real-time selective biomarker detection with high sensitivity, precision, and accuracy. Therefore, optical sensing strategies are increasingly adopted and have emerged as a promising alternative method to meet these requirements. In particular, the mid-infrared wavelength regime has substantial potential, as almost any biomedically relevant gaseous molecule has species-characteristic vibrational, ro-vibrational, and rotational transitions in this frequency range. This chapter provides an overview on the fundamental principles and recent developments in this emerging field with a focus on technological and application examples in the infrared regime, as these sensing devices have already demonstrated their potential serving as a smart diagnostic tool in exhaled breath analysis.KeywordsExhaled breath analysisMid-infrared sensing techniquesOptical sensing techniques
Near-infrared emission has gained increasing attention owning to their application of optical communication, night-vision readable displays, remote sensing and medical diagnostics. However, near-infrared molecules suffer from intrinsic limitation of low...
In order to improve the optoelectronic performance of π-conjugated molecules with electronic donor and electronic acceptor, people often chose multi-branch molecule structure, which can enhance the charge transfer ability....
Tetraphenylpyrazine (TPP) is a conspicuous heterocyclic fluorophore with aggregation-enhanced emission (AEE) character. In general, the TPP building block is considered as either the molecular rotor or electron acceptor in construction...
Three deep‐blue emissive donor‐acceptor (D−A) fluorophores, 2‐(4′‐(1‐(4‐(tert‐butyl)phenyl)‐1H‐phenanthro[9,10‐d]imidazol‐2‐yl)‐[1,1′‐biphenyl]‐4‐yl)benzo[d]thiazole (PTPISN), 2‐(4′‐(1‐(3‐(trifluoromethyl)phenyl)‐1H‐phenanthro[9,10‐d]imidazol‐2‐yl)‐[1,1′‐biphenyl]‐4‐yl)benzo[d]thiazole (m‐CFPISN) and 3‐(2‐(4′‐(benzo[d]thiazol‐2‐yl)‐[1,1′‐biphenyl]‐4‐yl)‐1H‐phenanthro[9,10‐d]imidazol‐1‐yl)benzonitrile (m‐CNPISN) based on benzothiazole with varying substituents at N1‐position to tune the properties of the materials were designed and synthesized. The synthesized fluorophores were structurally confirmed by spectroscopic techniques. All the fluorophores exhibited good thermal properties with high thermal degradation temperatures. An optical study reveals that the fluorophores are capable of emitting in the blue spectral region in solution and solid. The quantum chemical investigations were also performed with the aim to understand the electronic structures of the fluorophores. Further, solution‐processed OLED device was fabricated by using PTPISN as an emissive dopant and found that bright deep‐blue electroluminescence with CIEy of (∼0.08). These results suggest that the introduction of benzothiazole moiety is a promising design strategy for obtaining new deep‐blue emitters. A series of deep blue fluorophores based on benzothiazole as acceptor core was designed and synthesized. All the fluorophores shows deep blue emission, high PLQY, and positive solvatochromism effect. The doped device based on PTPISN shows deep blue EL emission with CIE coordinates of (0.15, 0.08) with good EL performance of PEmax, CEmax, and EQEmax of 0.2 lm/W, 1.1 cd/A, and 1.3 % at the brightness of 1000 cd/m2.
Organic near‐infrared (NIR) emissive materials integrating the intrinsic NIR light advantages and organic semiconductor characters have generated immense scientific interest in fundamental science and practical application, including optical communications, night vision, surveillance, and biomedical applications. Nevertheless, the intrinsic energy gap law engenders a huge challenge to simultaneously meet longer emission wavelength and higher luminescence efficiency for organic NIR emitters. Herein, it is suggested that the purposefully molecular design and crystal engineering for novel organic NIR materials are urgently studied and explored. Herein, recent advances in novel organic NIR materials, mainly focusing on the hybridized local and charge transfer (HLCT) compounds, thermally activated delayed fluorescence (TADF) emitters, CT cocrystals, neutral π‐radicals, and excited‐state intramolecular proton transfer (ESIPT)‐active materials, are summarized. It is hoped that this perspective can afford a new insight into the fine design and synthesis of the desired organic NIR emitters. Organic near‐infrared (NIR) materials with high penetration ability and low phototoxicity have attracted considerable research attention for their fundamental science and practical applications. This perspective summarizes the recent advances in rational design and synthesis of the desired organic NIR emitter and gives an outlook, providing inspiration for their rational design and promising applications.
Organic light-emitting diode (OLED) materials based on a reverse intersystem crossing (RISC) triplet harvesting mechanism have attracted tremendous attention. Currently, many research efforts have been devoted to the construction of D-A or D-A-D RISC-OLED materials whose charge-transfer-featured singlet excited states (¹CTD-A) are very close to the local triplet excited states of their D or/and A moieties (³LED or/and ³LEA), so that a small singlet–triplet energy splitting and a large spin–orbit coupling matrix element can be acquired concurrently. However, the chief technical difficulty lies in the accurate prediction and delicate tuning of the ¹CTD–A energy levels of these compounds. Herein, by using PCz-TXO2 as an example, we demonstrated that high-performance RISC-OLED materials can be readily achieved by integrating an appropriate D’ subunit into a traditional D-A fluorophore that lacks RISC property, i.e., constructing a D–A–D’ triad whose ¹CTD-A is close to its ³LED’. In comparison with its D-A or D-A-D counterpart of P-TXO2 or DP-TXO2 showing satisfactory photoluminescence efficiency, pure blue gamut but poor RISC, the presence of a carbazole-based D’ subunit has negligible influence on the transition feature of its lowest singlet excited state (S1), but triggers significantly enhanced RISC property. Consequently, PCz-TXO2 shows pure blue electroluminescence (EL) inherited from DP-TXO2 [CIE1931: (0.160, 0.089) vs. (0.154, 0.102)], but significantly enhanced external quantum efficiency (EQEmax: 9.5% vs. 6.1%) and exciton utilization efficiency (EUEmax: 93% vs. 42%). Our results present a new method to facilely access RISC materials and can greatly extend the design rationales for high-performance OLED materials.
In the mid-infrared (MIR) spectral range, a series of applications have successfully been shown in the fields of sensing, security and defense, energy conservation, and communications. In particular, rapid and recent developments in MIR light sources have significantly increased the interest in developing MIR optical systems, sensors, and diagnostics especially for chem/bio detection schemes and molecular analytical application scenarios. In addition to the advancements in optoelectronic light sources, and especially quantum and interband cascade lasers (QCLs, ICLs) largely driving the increasing interest in the MIR regime, also thermal emitters and light emitting diodes (LEDs) offer opportunities to alternatively fill current gaps in spectral coverage specifically with analytical applications and chem/bio sensing/diagnostics in the focus. As MIR laser technology has been broadly covered in a variety of articles, the present review aims at summarizing recent developments in MIR non-laser light sources highlighting their analytical utility in the MIR wavelength range.
Graphical abstract
Bioimaging is increasingly becoming an indispensable tool in disease diagnosis, clinical trials and medical practice. Fluorescence bioimaging is minimally invasive, affordable and portable, with the potential to become a widespread medical imaging technique. Currently, a serious challenge obstructing the large-scale clinical applications of fluorescence technique is the shallow penetration depth. Three-photon fluorescence offers several advantages over near-infrared and two-photon fluorescence, such as deeper penetration, more confined excitation areas and higher resolution. On the other hand, fluorophores displaying solid-state fluorescence are intriguing because they can emit bright fluorescence in the condensed phase, which is beneficial to imaging applications demanding intense emission signals. This review highlights the recent advances in small organic AIEgens for three-photon fluorescence bioimaging in vivo. The progress suggests that three-photon fluorescence imaging offers deep penetration, good photostablity and high signal-to-background contrast, which is valuable in fluorescence imaging in vivo.
This work describes the first hot exciton fluorescent material based on benzo[c][1,2,5]thiadiazole and chiral binaphthol enabling circularly polarized luminescence (CPL) through a chiral perturbation strategy. The new molecular architecture displays CPL, hybridized local and charge transfer (HLCT) properties concurrently. Utilizing it as the emitter, circularly polarized organic light‐emitting diodes (CP‐OLEDs) achieve an external quantum efficiency (EQE) of 7.2% with a good exciton utilization (36%) and a moderate circularly polarized electroluminescence (CPEL) dissymmetry factor (gEL, 2.1 × 10⁻³). In addition, the CP‐HLCT molecule is sensitized by a thermally activated delayed fluorescence material, significantly ameliorating the efficiency of HLCT fluorescent CP‐OLEDs. Excellent performances of twofold maximum EQE (EQEmax) of 15.3% and 82% exciton utilization are obtained in the sensitized device, regarding an extremely low‐efficiency roll‐off of 2.6% at 1000 cd m⁻² as well as CPEL with a gEL value of 2.0 × 10⁻³.
Near-infrared(NIR) fluorescent materials with high photoluminescent quantum yields(PLQYs) have wide application prospects. Therefore, we design and synthesize a D-A type NIR organic molecule, TPATHCNE, in which triphenylamine and thiophene are utilized as the donors and fumaronitrile is applied as the acceptor. We systematically investigate its molecular structure and photophysical property. TPATHCNE shows high Tg of 110 °C and Td of 385 °C and displays an aggregation-induced emission(AIE) property. A narrow optical bandgap of 1.65 eV is obtained. The non-doped film of TPATHCNE exhibits a high PLQY of 40.3% with an emission peak at 732 nm, which is among the best values of NIR emitters. When TPATHCNE is applied in organic light-emitting diode(OLED), the electroluminescent peak is located at 716 nm with a maximum external quantum efficiency of 0.83%. With the potential in cell imaging, the polystyrene maleic anhydride(PMSA) modified TPATHCNE nanoparticles(NPs) emit strong fluorescence when labeling HeLa cancer cells, suggesting that TPATHCNE can be used as a fluorescent carrier for specific staining or drug delivery for cellular imaging. TPATHCNE NPs fabricated by bovine serum protein(BSA) are cultivated with mononuclear yeast cells, and the intense intracellular red fluorescence indicates that it can be adopted as a specific stain for imaging.
Organic near‐infrared (NIR) luminogens have attracted intensive attention considering their vast potential applications in areas like bioimaging, organic light‐emitting diodes (OLEDs) and night‐vision telecommunication. However, organic NIR luminogens with high solid quantum efficiency are scarce, limiting the ir applications. Here, we reported an electron‐deficient acceptor, BSM, based on dithiafulvalene and benzothiadiazole, could work as a strong acceptor to produce highly efficient NIR emitters with aggregation‐induced emission (AIE) property. One of the AIEgens, TBSMCN emitted at 820 nm with a solid quantum yield of 10.7%. When applied to solution‐processed OLEDs, an outstanding external quantum efficiency (EQE) of 9.4% was achieved with a peak wavelength at 728 nm. Moreover, its non‐doped device could achieve an extraordinary EQE of 2.2% peaking at 804 nm. In the further optimized configuration, when an extra sensitizer was added to harvest triplet excitons, the EQE unprecedentedly soared up to 14.3% with a peak wavelength of 750 nm.
Organic near‐infrared (NIR) luminogens have attracted intensive attention considering their vast potential applications in areas like bioimaging, organic light‐emitting diodes (OLEDs) and night‐vision telecommunication. However, organic NIR luminogens with high solid quantum efficiency are scarce, limiting the ir applications. Here, we reported an electron‐deficient acceptor, BSM, based on dithiafulvalene and benzothiadiazole, could work as a strong acceptor to produce highly efficient NIR emitters with aggregation‐induced emission (AIE) property. One of the AIEgens, TBSMCN emitted at 820 nm with a solid quantum yield of 10.7%. When applied to solution‐processed OLEDs, an outstanding external quantum efficiency (EQE) of 9.4% was achieved with a peak wavelength at 728 nm. Moreover, its non‐doped device could achieve an extraordinary EQE of 2.2% peaking at 804 nm. In the further optimized configuration, when an extra sensitizer was added to harvest triplet excitons, the EQE unprecedentedly soared up to 14.3% with a peak wavelength of 750 nm.
One of the most important issues of the organic light‐emitting diode (OLED) is the highly efficient blue‐emissive material, which demands both excellent photoluminescent quantum yield (PLQY) and balanced carrier mobilities. Herein, a series of blue‐emissive donor–π–acceptor (D–π–A) materials with fluorene π‐bridge and their D–A analogues are synthesized and discovered with a theoretical combined experimental method. Based on the excellent electron mobility of the oxadiazole (OXZ) acceptor, it is further proven that the insertion of the fluorene π‐bridge can not only contribute to the formation of hybrid local and charge‐transfer excited state with high PLQY, but also give rise to the hole mobilities by enhanced intermolecular face‐to‐face stacking. As a result, the non‐doped OLED of TPACFOXZ exhibits a high maximum external quantum efficiency approaching 10% with boosted and balanced hole and electron mobilities of 5.60 × 10−5 and 6.60 × 10−5 cm2 V−1 s−1, respectively, which are among the best results of the non‐doped blue fluorescent OLEDs. Incorporation of fluorene‐bridge in donor–acceptor (D–A) molecules is proven to demonstrates double functions: high photoluminescent quantum yield and balanced carrier mobilities due to decent stacking patterns. Remarkably, the non‐doped OLED of the fluorene‐bridged D–A materials demonstrate excellent electroluminescent performances with high efficiency, low driving voltage, low rolling‐off with boosted and balanced carrier mobility.
As an important class of stimuli‐responsive material, piezochromic organic materials have been widely investigated in recent years. To promote their further applications as pressure sensors, realizing a high sensitivity and big color contrast, especially under high external pressure, is of great importance. Herein, two pure cis/trans isomers, E‐ANTCN and Z‐ANTCN, based on anthracene and cyano substituted double bond are achieved. The molecular structures are determined by nuclear Overhauser effect spectroscopy, correlation spectroscopy, and X‐ray diffraction. At ambient conditions, both of them show typical mechanochromic characteristics under grinding−heating or grinding−fuming treatments. E‐ANTCN displays a sensitivity of 0.83 nm GPa−1 and a color contrast of 5 nm in a high‐pressure experiment, while Z‐ANTCN shows a much enhanced sensitivity of 22.64 nm GPa−1 and bigger color contrast of 110 nm, indicating the potential as a good colorimetric sensor for external pressure. The two cyano groups in the same side of double bond in Z‐ANTCN induce more intense intermolecular interactions with a resultant spatial packing mode being mainly responsive for its good sensing performance. This study fills the gap for a piezochromic property and high‐pressure experiment of E/Z isomers and provides useful information for the investigation of molecular structural changes under isotropic and anisotropic treatments. Pure cyano substituted E/Z isomers are obtained with distinct responses to mechanical grinding and hydrostatic pressure. Isomer Z‐ANTCN shows a better sensitivity of 22.64 nm GPa−1 and bigger color contrast of 110 nm compared to E‐ANTCN because of more intense intermolecular interactions induced by the two cyano groups in the same side of double bond.
An efficient formal alkenyl C-H cyanation reaction has been developed for the general synthesis of unsymmetrical diarylfumaronitriles in good to excellent yields. The reaction was achieved through tandem Michael addition and an oxidative process. The merits of this transformation include the use of K3Fe(CN)6 as a safe and nontoxic cyanide source, without an external noble metal catalyst, oxygen-involved reactions, easily available raw materials, good functional group tolerance, high stereoselectivity, and potential further application of the products.
Nonlinear optical organic molecules have advanced a wide range of fields spanning from integrated photonics to biological imaging. With advances in molecular design, an emerging application is multifunctional nonlinear organic...
“Hot excitons (HE)” channels highlight the reversed intersystem crossing (RISC) is happened from high-lying triplet to singlet states, plays important role in developing highly efficient organic light-emitting diode (OLED) emitters....
A novel dinuclear iridium (Ⅲ) complex containing a non- conjugated bridging picolinic acid ligand of (C8TPAPhOC2FIrpic)2BT with a D-A-D core was synthesized and characterized, in which the D-A-D core consists of benzo[c][1,2,5]thiadiazole (BT) and triphenylamino (TPA) units. Its optophysical, electrochemical and electroluminescent characteristics were primarily studied. It is found this dinuclear iridium (Ⅲ) complex exhibited similar photoluminescent and electroluminescent profiles, but higher brightness and efficiency in compared with the picolinic acid ligand in the single-emissive-layer organic light-emitting devices. The maximum luminance of 7404 cd/m² and current efficiency of 0.81 cd/A was obtained in the (C8TPAPhOC2FIrpic)2BT-based devices. The efficient energy transfer is observed from the terminal iridium complex unit to the picolinic acid ligand. This work demonstrates that the picolinic acid ligand with a D-A-D core can facilitate dinuclear iridium (III) complex to exhibit the ancillary ligand-based red emission.
Two single-crystal structures of cucurbit[n]uril mediated supramolecular complexes were obtained in which [1+3] and [2+3] self-assembly modes are adopted due to the different sizes of cucurbit[7]uril and cucurbit[8]uril. An obvious...
The fluorescent molecules utilizing hybridized local and charge-transfer (HLCT) state as potential organic light-emitting diodes materials attract extensive attention due to their high exciton utilization. In this work, we have performed the density functional theory method on three HLCT-state molecules to investigate their excited-state potential energy surface (PES). The calculated results indicate the T1 and T2 energy gap is quite large, and the T2 is very close to S1 in the energy level. The large gap is beneficial for inhibiting the internal conversion between T1 and T2, and quite closed S1 and T2 energies are favor for activating the T2 → S1 reverse intersystem crossing path. However, considering the singlet excited-state PES by twisting the triphenylamine (TPA) or diphenylamine (PA) group, it can be found that the TPA or PA group almost has no influence on T1 and T2 energy levels. However, the plots of S1 PES display two kinds of results that the S1 emissive state is dominated by charge-transfer (CT) or HLCT state. The CT emission state formation would decrease the S1 energy level, enlarge the S1 and T2 gap, and impair the triplet exciton utilization. Therefore, understanding the relationship between the S1 PES and molecular structures is important for designing high-performance luminescent materials utilizing HLCT state.
Four new phenanthroimidazole/N,N-diphenylnaphthalen-2-amine hybrid compounds, 6-(4-(1-(4-(tert-butyl)phenyl)-1H-phenanthro[9,10-d]imidazole-2-yl)phenyl)-N,N-bis(4-methoxyphenyl)naphthalen-2-amine (PPI-NPA-OMe), N,N-bis(4-(tert-butyl)phenyl)-6-(4-(1-(4-(tert-butyl) phenyl)-1H-phenanthro[9,10-d]imidazole-2-yl)phenyl)naphthalen-2-amine (PPI-NPA-t-Bu), 6-(4-(1-(4-(tert-butyl)phenyl)-1H-phenanthro[9,10-d]imidazole-2-yl)phenyl)-N,N-di-p-tolylnaphthalen-2-amine (PPI-NPA-Me) and 4,4'-((6-(4-(1-(4-(tert-butyl)phenyl)-1H-phenanthro[9,10-d] imidazole-2-yl)phenyl)naphthalen-2-yl)azanediyl)dibenzonitrile (PPI-NPA-CN), were successfully synthesized for application in blue light emitting. The thermal stabilities, photophysical and electrochemical properties of the compounds were investigated. Furthermore, the vacuum-processed devices based on these compounds were fabricated, in which the device based on the compound PPI-NPA-OMe exhibited the best electroluminescence performance with a maximum brightness of 12,593 cd/m², a maximum current efficiency of 9.23 cd/A and a maximum external quantum efficiency (EQE) of 4.92%.
The Hf(OTf) 4 -catalyzed three-component (3CR) was employed as a powerful tool for facile access to a library of 23 pyrimido[2,1- b ][1,3]benzothiazole (PBT)-based AIEgens with full-color tunability, solid-state fluorescence quantum yields up to...
Solution-processed organic light-emitting diode (s-OLED) is one of the strongest candidates for the next-generation display technology owing to its cost-effectiveness and broad applicability. However, its commercialization is being delayed due to the relatively low operational stability and abstruse mechanism of solvents’ influence on the device characteristics. Herein, we investigate the impact of the solvents on the characteristics of s-OLEDs by segmenting the solvents from tetrahydrofuran (THF) to chloroform (CF) and comparing them with thermally evaporated OLED as a reference device. Whereas the evaporated device offered current efficiency (CE) of 42.9 cd A⁻¹, the THF- and CF-based ones showed 38.5 cd A⁻¹ and 7.4 cd A⁻¹. Besides, the CF-based device showed significantly reduced operational stability with less than 10% of that of evaporated and THF-based ones. The main reason for the deviation between the devices is interpreted to be related to the residual chemicals and degradation of the organic materials of the CF-based emissive layer (EML), which is supplemented by Fourier-transform infrared spectroscopy (FTIR) analyses. We believe that the results will provide helpful insight into developing highly efficient and stable s-OLEDs.
Pure organic materials with piezochromic property have been intensively studied in recent years. Many examples describe response to both mechanical grinding and hydrostatic pressure, but the examples in realizing high color contrast as well as high sensitivity, especially under high external pressure, are very rare which hampers their practical applications as pressure sensors. Herein, two anthracene‐based structural isomers, NDHA‐p and NDHA‐m, are developed applying 9,10‐bis(diphenylmethylene)‐9,10‐dihydroanthracene (DHA) as the basic skeleton and substituted by four cyano groups on para‐ or meta‐positions of the periphery benzene rings, respectively. In high‐pressure experiment, as compared to the NDHA‐m which shows a color contrast of 60 nm and a sensitivity of 7.87 nm·GPa‐1, NDHA‐p exhibits a dramatically larger color contrast of 138 nm and higher sensitivity of 13.20 nm·GPa‐1, whose performance is among the best pure organic piezochromic materials so far. The systematical experimental and theoretical analyses indicate that NDHA‐p possesses a lower crystal density, relatively looser molecular packing mode, and higher degree of electronic communications than NDHA‐m, which are responsible for the excellent piezochromic performance. This design method will open a new avenue for controlling the molecular packing configuration of luminescent materials via structural isomerization, and further extending their applications. Two anthracene‐based structural isomers show distinct responses to mechanical grinding and hydrostatic pressure. Isomer NDHA‐p sustains higher external pressure of 14.7 GPa and exhibits a bigger color contrast of 138 nm, better sensitivity of 13.20 nm·GPa−1 comparing with NDHA‐m owing to the contribution of lower density crystal arrangement mode induced by CN groups at para‐position.
Restricted by the energy-gap law and π-π stacking, developing highly efficient red emitting materials and corresponding organic light-emitting diodes (OLEDs) having the emission over 600 nm is a formidable challenge. Three red emitters, namely DPABz-TPA, 2DPABz-TPA, and 3DPABz-TPA, are developed bearing mono-, bis-, and tri-[2,1,3]benzothiadiazole (Bz) substituted triphenylamine (TPA) as the emissive core and bulky diphenylamine (DPA) as the steric protection units. All compounds display strong red emission with peaks at 645 nm and high efficiency up to 38%. Compared with reference molecule DPABz-TPA, both 2DPABz-TPA and 3DPABz-TPA exhibit higher radiative transition rate and thermal stability. The OLEDs based on 2DPABz-TPA and 3DPABz-TPA thus show superior EL efficiencies of the maximum current efficiency of 2.5 and 2.4 cd A–1, and external quantum efficiency of 2.9% and 3.2% compared with those of DPABz-TPA (1.9 cd A–1 and 2.1%). More importantly, the efficiency roll-off of OLED based on sterically protected 3DPABz-TPA obviously decreases owing to limited intermolecular packing. These results indicate that the strategy combined multiple donor-acceptor units and steric hindrance effect is favorable for constructing robust red emitters for efficient OLEDs.
Multimodal therapy is attracting increasing attention to improve tumor treatment efficacy, but generally requires various complicated ingredients combined within one theranostic system to achieve multiple functions. Herein, a multifunctional theranostic nanoplatform based on a single aggregation‐induced‐emission luminogen (AIEgen), DDTB, is designed to integrate near‐infrared (NIR) fluorescence, photothermal, photodynamic, and immunological effects. Intravenously injected AIEgen‐based nanoparticles can efficiently accumulate in tumors with NIR fluorescence to provide preoperative diagnosis. Most of the tumors are excised under intraoperative fluorescence navigation, whereafter, some microscopic residual tumors are completely ablated by photodynamic and photothermal therapies for maximally killing the tumor cells and tissues. Up to 90% of the survival rate can be achieved by this synergistic image‐guided surgery and photodynamic and photothermal therapies. Importantly, the nanoparticles‐mediated photothermal/photodynamic therapy plus programmed death‐ligand 1 antibody significantly induce tumor elimination by enhancing the effect of immunotherapy. This theranostic strategy on the basis of a single AIEgen significantly improves the survival of cancer mice with maximized therapeutic outcomes, and holds great promise for clinical cancer treatment. Integrated surgery and phototheranostics employing nanoparticles based on a near‐infrared aggregation‐induced emission luminogen (AIEgen) for image‐guided synergistic tumor therapy are explored to overcome respective limitations and realize maximized therapeutic outcomes and minimized recurrences. Moreover, the combined treatment of AIEgen nanoparticle‐mediated phototheranostics and programmed death‐ligand 1 antibody significantly induces tumor elimination by enhancing the effect of immunotherapy.
Organic light-emitting diodes (OLEDs) have been explored and utilized in many fields of flat-panel displays nowadays because of their plentiful advantages such as lightweight, high flexibility, high picture quality, etc, and are hopeful to be the next-generation displays. However, improvements to the efficiency of blue OLEDs are still vital to OLEDs as replacements for liquid crystal display technology. A blue light-emitting material is one of the key components in the preparation of OLED displays. Designing molecule based on donor-acceptor (D-A) structure with a hybridized local and charge transfer (HLCT) excited state is a appealing strategy for providing an efficient OLED with high external quantum efficiency through efficacious exciton utilization. Herein, a highly efficient blue emitter, PyI-Py, is constructed combining pyrene[4,5-d]imidazole (weak donor) and pyrene (weak acceptor). The non-doped blue OLED exhibits excellent performance with a maximum current efficiency of 15.74 cd A⁻¹, a maximum external quantum efficiency (ηext) of 9.13% and a maximum brightness of as high as 91097 cd m⁻² which is rarely obtained for a non-doped blue OLED. Moreover, the ηext can reach 9.05% at very high brightness of 10000 cd m⁻², displaying extremely low efficiency roll-off of 0.9% which displays great superiorities in contrast with most thermally activated delayed fluorescent materials. The device characteristics lie among the highest values in non-doped blue OLEDs based on HLCT emitter and correspond to the best performance achieved for non-doped pyrene-based blue OLEDs to date. A maximum exciton utilization efficiency of 65% is harvested. The superior performance is attributed to the proper donor-acceptor design strategy which results in a HLCT excited state with the generation of a high proportion of singlet excitons.
A comparative investigation on the photophysical properties of a quinoxaline derivative 4,4'-((1E,1'E)-quinoxaline-2,3-diylbis(ethene-2,1-diyl))bis(N,N-dimethylaniline) (QDMA2) was performed by employing many spectroscopies. Based on the pump-dump/push-probe measurement, it is found that a solvent-stabilized charge-transfer state can participate in the relaxation of excited QDMA2 with increasing solvent polarity. Meanwhile, the aggregated QDMA2 molecules were engineered into the organic light-emitting diode test, which showed a correlated color temperature value of 1875 K. With the help of a diamond anvil cell, the pressure-dependent photoluminescence of aggregated QDMA2 shows that the intermolecular interaction can affect the color and intensity of photoluminescence through adjusting the band gap and irradiative channel of the aggregated molecules. These results are important for understanding the structure-property relationships and the rational design of functional materials for optoelectronic applications.
The design and characterization of metal-organic complexes for optoelectronic applications is an active area of research. The metal-organic complex offers unique optical and electronic properties arising from the interplay between the inorganic metal and the organic ligand. The ability to modify chemical structure through control over metal-ligand interaction on a molecular level could directly impact the properties of the complex. When deposited in thin film form, this class of materials enable the fabrication of a wide variety of low-cost electronic and optoelectronic devices. These include light emitting diodes, solar cells, photodetectors, field-effect transistors as well as chemical and biological sensors. Here we present an overview of recent development in metal-organic complexes with controlled molecular structures and tunable properties. Advances in extending the control of molecular structures to solid materials for energy conversion and information technology applications will be highlighted.
The inherent flexibility afforded by molecular design has accelerated the development of a wide variety of organic semiconductors over the past two decades. In particular, great advances have been made in the development of materials for organic light-emitting diodes (OLEDs), from early devices based on fluorescent molecules to those using phosphorescent molecules. In OLEDs, electrically injected charge carriers recombine to form singlet and triplet excitons in a 1:3 ratio; the use of phosphorescent metal-organic complexes exploits the normally non-radiative triplet excitons and so enhances the overall electroluminescence efficiency. Here we report a class of metal-free organic electroluminescent molecules in which the energy gap between the singlet and triplet excited states is minimized by design, thereby promoting highly efficient spin up-conversion from non-radiative triplet states to radiative singlet states while maintaining high radiative decay rates, of more than 10(6) decays per second. In other words, these molecules harness both singlet and triplet excitons for light emission through fluorescence decay channels, leading to an intrinsic fluorescence efficiency in excess of 90 per cent and a very high external electroluminescence efficiency, of more than 19 per cent, which is comparable to that achieved in high-efficiency phosphorescence-based OLEDs.
Bright (maximum 10034 cd m(-2), 455 cd m(-2) at 20 mA cm(-2)) and efficient (maximum 2.4% at 4 mA cm(-2)) red (lambda(max)el 634-636 nm) organic light-emitting diodes employ arylamino-substituted fumaronitrile as the novel host emitter, which is readily prepared and easily purified.
Dual emitting cores for thermally activated delayed fluorescent (TADF) emitters were developed. Relative to the corresponding TADF emitter with a single emitting core the TADF emitter with a dual emitting core, 3,3',5,5'-tetra(carbazol-9-yl)-[1,1'-biphenyl]-2,2',6,6'-tetracarbonitrile, showed enhanced light absorption accompanied by a high photoluminescence quantum yield. The quantum and power efficiencies of the TADF devices were enhanced by the dual emitting cores.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Excited state characters and components play a decisive role in photoluminescence (PL) and electroluminescence (EL) properties of organic light-emitting materials (OLEDS). Charge-transfer (CT) state is beneficial to enhance the singlet exciton utilizations in fluorescent OLEDs by an activated reverse intersystem crossing process, due to the minimized singlet and triplet energy splitting in CT excitons. However, the dominant CT component in the emissive state significantly reduces the PL efficiency in such materials. Here, the strategy is to carry out a fine excited state modulation, aiming to reach a golden combination of the high PL efficiency locally emissive (LE) component and the high exciton utilizing CT component in one excited state. As a result, a quasi-equivalent hybridization of LE and CT components is obtained in the emissive state upon the addition of only an extra phenyl ring in the newly synthesized material 4-[2-(4′-diphenylamino-biphenyl-4-yl)-phenanthro[9,10-d]imidazol-1-yl]-benzonitrile (TBPMCN), and the nondoped OLED of TBPMCN exhibited a record-setting performance: a pure blue emission with a Commission Internationale de L'Eclairage coordinate of (0.16, 0.16), a high external quantum efficiency of 7.8%, and a high yield of singlet exciton of 97% without delayed fluorescence phenomenon. The excited state modulation could be a practical way to design low-cost, high-efficiency fluorescent OLED materials.
A series of 1,8-naphthyridine derivatives is synthesized and their electron-transporting/injecting (ET/EI) properties are investigated via a multilayered electrophosphorescent organic light-emitting device (OLED) using fac-tris(2-phenylpyridine)iridium [Ir(ppy)3] as a green phosphorescent emitter doped into a 4,4′-N,N′-dicarbazolebiphenyl (CBP) host with 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (a-NPD) as the hole-transporting layer, and poly(arylene ether sulfone) containing tetraphenylbenzidine (TPDPES) doped with tris(4-bromophenyl)ammonium hexachloroantimonate (TBPAH) as the hole-injecting layer. The turn-on voltage of the device is 2.5 V using 2,7-bis[3-(2-phenyl)-1,8-naphthyridinyl]-9,9-dimethylfluorene (DNPF), lower than that of 3.0 V for the device using a conventional ET material. The maximum current efficiency (CE) and power efficiency (PE) of the DNPF device are much higher than those of a conventional device. With the aid of a hole-blocking (HB) and exciton-blocking layer of bathocuproine (BCP), 13.2–13.7% of the maximum external quantum efficiency (EQE) and a maximum PE of 50.2–54.5 lm W−1 are obtained using the naphthyridine derivatives; these values are comparable with or even higher than the 13.6% for conventional ET material. The naphthyridine derivatives show high thermal stabilities, glass-transition temperatures much higher than that of aluminum(III) bis(2-methyl-8-quinolinato)-4-phenylphenolate (BAlq), and decomposition temperatures of 510–518 °C, comparable to or even higher than those of Alq3.
Benzene-cored luminogens with multiple triarylvinyl units are designed and synthesized. These propeller-shaped molecules are nonemissive when dissolved in good solvents, but become highly emissive when aggregated in poor solvents or in the solid state, showing the novel phenomenon of aggregation-induced emission. Restriction of intramolecular motion is identified as the main cause for this effect. Thanks to their high solid-state fluorescence quantum yields (up to unity) and high thermal and morphological stabilities, light-emitting diodes with the luminogens as emitters give sky-blue to greenish-blue light in high luminance and efficiencies of 10800 cd m−2, 5.8 cd A−1, and 2.7%, respectively. The emissions of the nanoaggregates of the luminogens can be quenched exponentially by picric acid, or selectively by Ru3+, with quenching constants up to 105 and ∼2.0 × 105 L mol−1, respectively, making them highly sensitive (and selective) chemosensors for explosives and metal ions.
In an organic electroluminescent (EL) device, the recombination of injected holes and electrons produces what appears to be an ion-pair or charge-transfer (CT) exciton, and this CT exciton decays to produce one photon directly, or relaxes to a low-lying local exciton (LE). Thus the full utilization of both the energy of the CT exciton and the LE should be a pathway for obtaining high-efficiency EL. Here, a twisting donor-acceptor (D-A) triphenylamine-imidazol molecule, TPA-PPI, is reported: its synthesis, photophysics, and EL performance. Prepared by a manageable, one-pot cyclizing reaction, TPA-PPI exhibits deep-blue emission with high quantum yields (90%) both in solution and in the solid state. Fluorescent solvatochromic experiments for TPA-PPI solutions show a red-shift of 57 nm (3032 cm−1) from low-polarity hexane (406 nm) to high-polarity acetonitrile (463 nm), accompanied by the gradual disappearance of the vibrational band in the spectra with increased solvent polarity. The photophysical investigation and DFT analysis suggest an intercrossed CT and LE excited state of the TPA-PPI, originating from its twisting D-A configuration. This is a rare instance that a CT-state material shows highly efficient deep-blue emission. EL characterization demonstrates that, as a deep-blue emitter with CIE coordinates of (0.15, 0.11), the performance of a TPA-PPI-based device is rather excellent, displaying a maximum current efficiency of >5.0 cd A−1, and a maximum external quantum efficiency of >5.0%, corresponding to a maximum internal quantum efficiency of >25%. The effective utilization of the excitation energy arising from materials with intercrossed-excited-state (LE and CT) characters is thought to be beneficial for the improved efficiency of EL devices.
Light emission of 2-(2,6-bis((E)-4-(diphenylamino)styryl)-4H-pyran-4-ylidene)malononitrile (TPA-DCM) is weakened by aggregate formation. Attaching tetraphenylethene (TPE) units as terminals to TPA-DCM dramatically changes its emission behavior: the resulting fluorogen, 2-(2,6-bis((E)-4-(phenyl(4′-(1,2,2-triphenylvinyl)-[1,1′-biphenyl]-4-yl)amino)styryl)-4H-pyran-4-ylidene)malononitrile (TPE-TPA-DCM), is more emissive in the aggregate state, showing the novel phenomenon of aggregation-induced emission (AIE). Formulation of TPE-TPA-DCM using bovine serum albumin (BSA) as the polymer matrix yields uniformly sized protein nanoparticles (NPs) with high brightness and low cytotoxicity. Applications of the fluorogen-loaded BSA NPs for in vitro and in vivo far-red/near-infrared (FR/NIR) bioimaging are successfully demonstrated using MCF-7 breast-cancer cells and a murine hepatoma-22 (H22)-tumor-bearing mouse model, respectively. The AIE-active fluorogen-loaded BSA NPs show an excellent cancer cell uptake and a prominent tumor-targeting ability in vivo due to the enhanced permeability and retention effect.
A family of donor–acceptor–donor (D–A–D) type near-infrared (NIR) fluorophores containing rigid nonplanar conjugated tetraphenylethene (TPE) moieties was designed and synthesized through Stille coupling reactions with electron-deficient [1,2,5]thiadiazolo[3,4-g]quinoxaline (QTD) or benzo[1,2-c;4,5-c′]bis[1,2,5]thiadiazole (BBTD) as acceptors. The absorption, fluorescence, and electrochemical properties were studied. These compounds exhibited good aggregation-induced emission enhancement (AIEE) property, as a result of the twisted TPE units, which restrict the intramolecular rotation and reduce the π–π stacking. Photoluminescence of these chromophores ranges from 600 to 1100 nm, and their HOMO–LUMO gaps are between 1.85 and 1.50 eV. Non-doped organic light-emitting diodes (OLEDs) based on these fluorophores were made and exhibited EL emission spectra peaking from 706 to 864 nm. The external quantum efficiency (EQE) of these devices ranged from 0.89% to 0.20% and remained fairly constant over a range of current density of 100–300 mA cm–2. The device with the highest solid fluorescence efficiency emitter 1a shows the best performance with a maximum radiance of 2917 mW Sr–1 m–2 and EQE of 0.89%. A contrast between nondoped and doped OLEDs with these materials confirms that AIEE compounds are suitable for fabricate efficient nondoped NIR OLEDs.
A family of π-extended platinum(II) porphyrins has been synthesized and incorporated into solution processed polymer light emitting diodes (PLEDs) and vapor deposited multilayer organic light emitting diodes (OLEDs), giving rise to devices with peak emission ranging from 771 to 1005 nm. The longest wavelength emitter, platinum(II)-5,10,15,20-(3,5-di-tert-butylphenyl)tetraanthroporphyrin (Pt-Ar4TAP), shows an emission maximum at 1005 nm, an external quantum efficiency (EQE) of 0.12%, and a maximum radiant emittance (Rmax) of 0.23 mW/cm2 in single layer PLED architectures, which is enhanced to an EQE of 0.20% with an Rmax of 0.57 mW/cm2 upon vapor deposition of an electron transport layer. In an effort to understand substituent effects and enhance the performance of π-extended Pt-porphyrins in PLEDs and OLEDs, a family of Pt-tetrabenzoporphyrins (Pt-TBPs) with varying functionality was investigated. The luminescent lifetimes of the Pt-TBPs in solution and in films were measured, and a strong correlation was demonstrated between the film lifetimes and the PLED and OLED efficiencies. An improvement in external quantum efficiency (EQE) from 2.07 to 2.49% for PLEDs and from 8.0 to 9.2% for OLEDs was observed between the less substituted Pt-tetraphenyltetrabenzoporphyrin and the more substituted Pt-5,10,15,20-(3,5-di-tert-butylphenyl)tetrabenzoporphyrin. The PLED EQEs were further enhanced to 3.02% with the disubstituted Pt-5,15-(3,5-di-tert-butylphenyl)tetrabenzoporphyrin; however, this increase was not observed for the OLEDs where an EQE of 7.8% was measured.
Luminescent materials with aggregation-induced emission (AIE) property have attracted considerable interests for their promising applications in light-emitting and display devices and fluorescent probes for chemo- and biosensors. Tetraphenylethene (TPE) derivatives are the most attractive species for their notable AIE performance, facile synthesis, and flexible structure modification. To study the effects of donor and acceptor substitutions and extend the applications of TPE-based materials, three TPE kindred, TTPE, BTPEFN, and BATPEFN, are employed. TTPE film displays efficient green fluorescence (λem = 494 nm, ΦF = 100%), evident AIE characteristic (αAIE = 154), and reversible mechanochromism by grinding-fuming: from blue (λem = 472 nm) to green emission (λem = 505 nm). Replacing two phenyls by two cyano (A) groups on the central TPE moiety derives BTPEFN, whose film shows efficient orange fluorescence (λem = 575 nm, ΦF = 100%) and evident AIE (αAIE = 13). The mechanochromic behavior of BTPEFN (from yellow to orange emission, λem from 541 to 563 nm) is reversible by repeating both the grinding-fuming and grinding-annealing processes. The cyano groups bestow BTPEFN with evident intramolecular charge transfer (ICT) property, the emission color can be tuned from green to red-orange by changing solvent from hexane to THF, while the emission of TTPE shows much less response to solvent polarity. Cyanos also endow BTPEFN with better self-assemble ability in proper conditions, and the obtained regular microribbons emit bright green fluorescence. Further decoration of BTPEFN with N,N-diethyamino (D) groups results in BATPEFN. Due to the cooperative effects of D and A groups, BATPEFN shows dramatic red-shifted fluorescence (λem = 713 nm), evident ICT process, and enhanced solvatochromism (from red to infrared).
In principle, the ratio (Φ) of the maximum quantum efficiencies for electroluminescence (EL) to photoluminescence (PL) can be expected to approach unity, if the exciton (bound electron–hole pair) generated from the recombination of injected electrons and holes in OLEDs has a sufficiently weak binding energy. However, seldom are examples of Φ > 25% reported in OLEDs because of the strongly bound excitons for most organic semiconductors in nature. Here, a twisting donor–acceptor triphenylamine-thiadiazol molecule (TPA-NZP) exhibits fluorescent emission through a hybridized local and charge-transfer excited state (HLCT), which is demonstrated from both fluorescent solvatochromic experiment and quantum chemical calculations. The HLCT state possesses two combined and compatible characteristics: a large transition moment from a local excited (LE) state and a weakly bound exciton from a charge transfer (CT) state. The former contributes to a high-efficiency radiation of fluorescence, while the latter is responsible for the generation of a high fraction of singlet excitons. Using TPA-NZP as the light-emitting layer in an OLED, high Φ values of 93% (at low brightness) and 50% (at high brightness) are achieved, reflecting sufficient employment of the excitons in the OLED. Characterization of the EL device shows a saturated deep-red emission with CIE coordinates of (0.67, 0.32), accompanied by a rather excellent performance with a maximum luminance of 4574 cd m−2 and a maximum external quantum efficiency (ηext) of ∼2.8%. The HLCT state is a new way to realize high-efficiency of EL devices.
The development of near-infrared (NIR) organic light-emitting diodes (OLEDs) is of growing interest. Donor-acceptor (D-A) chromophores have served as an important class of NIR materials for NIR OLED applications. However, the external quantum efficiencies (EQEs) of NIR OLEDs based on conventional D-A chromophores are typically below 1 %. Reported herein is a butterfly-shaped D-A compound, PTZ-BZP. A PTZ-BZP film displayed strong NIR fluorescence with an emission peak at 700 nm, and the corresponding quantum efficiency reached 16 %. Remarkably, the EQE of the NIR OLED based on PTZ-BZP was 1.54 %, and a low efficiency roll-off was observed, as well as a high radiative exciton ratio of 48 %, which breaks through the limit of 25 % in conventional fluorescent OLEDs. Experimental and theoretical investigations were carried out to understand the excited-state properties of PTZ-BZP.
Excited states of organic molecules (excitons) are the heart of any organic electroluminescent device. They mediate the conversion of injected charges – electrons and holes – into photons. Phosphorescent emission originating from triplet excitons is especially important, as it is to date the only general route to enable unity charge-to-photon conversion efficiencies. In this paper, we discuss the key aspects of excitons, following the excited state lifecycle. First, we review fundamentals of singlet and triplet exciton formation in organic semiconductors, followed by a discussion of concepts that aim to alter the singlet-to-triplet formation rates to enable higher electroluminescence yields in the fluorescence manifold. Subsequently, we focus on the exciton distribution within the organic semiconductor material during its lifetime. The processes involved ultimately determine organic light-emitting diode (OLED) performance and are especially key in the development of concepts for white emission, where precise balance of the exciton between different emitter species control the emitted color. We close this paper with discussion of non-linear effects at high excitation levels that, to date, limit the high brightness efficiency of phosphorescent OLEDs.
Three new asymmetric light emitting organic compounds were synthesized with diphenylamine or triphenylamine side groups; 10-(3,5-diphenylphenyl)-N,N-diphenylanthracen-9-amine (MADa), 4-(10-(3,5-diphenylphenyl)anthracen-9-yl)-N,N-diphenylaniline (MATa), and 4-(10-(3′,5′-diphenylbiphenyl-4-yl)anthracen-9-yl)-N,N-diphenylaniline (TATa). MATa and TATa had a PLmax at 463 nm in the blue region, and MADa had a PLmax at 498 nm. MADa and MATa had Tg values greater than 120 °C, and TATa had a Tg of 139 °C. EL devices containing the synthesized compounds were fabricated in the configuration: ITO/4,4′,4′′-tris(N-(2-naphthyl)-N-phenyl-amino)-triphenylamine (2-TNATA) (60 nm)/N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (NPB) (15 nm)/MADa or MATa or TATa or 9,10-di(2′-naphthyl)anthracene (MADN) (30 nm)/8-hydroxyquinoline aluminum (Alq3) (30 nm)/LiF (1 nm)/Al (200 nm). The efficiency and color coordinate values (respectively) were 10.3 cd/A and (0.199, 0.152; bluish-green) for the MADa device, 4.67 cd/A and (0.151, 0.177) for the MATa device, and 6.07 cd/A and (0.149, 0.177) for the TATa device. The TATa device had a high external quantum efficiency (EQE) of 6.19%, and its luminance and power efficiencies and life-time were more than twice those of the MADN device.
It was suggested that in a series of osmium (II) polypyridyl compounds the properties of their metal-to-ligand charge-transfer (MLCT; Os/sup II/ ..-->.. ..pi..* (bpy) or (phen)) excited states including luminescence lifetimes, emission maxima, and redox potentials were systematically variable by making chemical changes. The authors have prepared a more extensive series of compounds and with the extended series are able to identify systematic variations in both nonradiative (k/sub nr/) and radiative rate constants (k/sup r/), and the systematic variations have important implications for transition-metal photochemistry. They give some relevant excited-state parameters for a series of mono- and bis-2,2'-bipyridine (bpy) or 1,10-phenanthroline (phen) complexes of Os(II).
A novel all-organic host-guest system for emission in the NIR is introduced and investigated with respect to its opto-electronic processes. The good agreement between theoretical and experimental results highlights the model character of this system and its potential for electroluminescent application. Comparative measurements provide access to the recombination mechanisms on molecular length scale and show that the emission behavior of the device under operation is controlled by charge carrier dynamics.
A series of low-band-gap copolymers of thiophene, benzothiadiazole, and benzobis(thiadiazole) were synthesized. The polymers were synthesized by Stille cross-coupling polymerization of distannylalkylthiophenes and dithiophenes with dibromo derivatives of benzothiadiazoles and benzobis(thiadiazole)s. The polymers were characterized using NMR, UV−vis, and size exclusion chromatography (SEC). The molecular weight, solubility, and film-forming ability were highly dependent on the choice of side chains. 3,7,11-Trimethyldodecyl side chains were found to give polymer products with high molecular weight, good film-forming ability, and good solubility. Band gaps were estimated from UV−vis to be 2.1−1.7 eV for polymers based on benzothiadiazole and 0.7 eV for polymers based on benzobis(thiadiazole). The band gap and electronic structure of the polymers were determined by a combination of UV−vis spectroscopy and ultraviolet photoelectron spectroscopy (UPS).
A series of a donor−acceptor−donor type of near-infrared (NIR) fluorescent chromophores based on [1,2,5]thiadiazolo[3,4-g]quinoxaline (TQ) as an electron acceptor and triphenylamine as an electron donor are synthesized and characterized. By introducing pendent phenyl groups or changing the π-conjugation length in the TQ core, we tuned the energy levels of these chromophores, resulting in the NIR emission in a range from 784 to 868 nm. High thermal stability and glass transition temperatures allow these chromophores to be used as dopant emitters, which can be processed by vapor deposition for the fabrication of organic light-emitting diodes (OLEDs) having the multilayered structure of ITO/MoO3/NPB/Alq3:dopant emitter/BCP/Alq3/LiF/Al. The electroluminescence spectra of the devices based on these new chromophores cover a range from 748 to 870 nm. With 2 wt % of dopant 1, the LED device shows an exclusive NIR emission at 752 nm with the external quantum efficiency (EQE) as high as 1.12% over a wide range of current density (e.g., around 200 mA cm−2). The radiance reaches the largest value of 2880 mW Sr−1 m−2 at an operating voltage of 15 V.
Two series of Pt(diimine)(dithiolate) complexes have been prepared in order to investigate the effects of molecular design on the excited-state properties of this chromophore. The first series comprises Pt(dbbpy)(dithiolate) complexes where dbbpy = 4,4‘-di-tert-butyl-2,2‘-bipyridine and the dithiolates are 1-(tert-butylcarboxy)-1-cyanoethylene-2,2-dithiolate (tbcda), 1-diethylphosphonate-1-cyanoethylene-2,2-dithiolate (cpdt), 6,7-dimethyl-quinoxaline-2,3-dithiolate (dmqdt), maleonitriledithiolate (mnt), and toluene-3,4-dithiolate (tdt). The second series comprises Pt(diimine)(tdt) complexes where the diimines are 3,4,7,8-tetramethyl-1,10-phenanthroline (tmphen), 4,4‘-di-tert-butyl-2,2‘-bipyridine (dbbpy), 4,4‘-dimethyl-2,2‘-bipyridine (dmbpy), 2,2‘-bipyridine (bpy), 1,10-phenanthroline (phen), 5-chloro-1,10-phenanthroline (Cl-phen), 4,4‘-dichloro-2,2‘-bipyridine (Cl2bpy), and 4,4‘-bis(ethoxycarbonyl)-2,2‘-bipyridine (EC-bpy). All of the compounds display solvatochromic absorption bands and solution luminescence, which are attributed to a common charge-transfer-to-diimine excited state. The excited-state energies can be tuned by approximately 1 eV through ligand variation. Solution lifetimes range from 1 ns to over 1000 ns and Φem range from 6.4 × 10-3 to less than 10-5 in CH2Cl2. Based on these data, the nonradiative and radiative decay rate constants have been calculated. For the Pt(diimine)(tdt) series, the nonradiative decay rate constants increase exponentially with decreasing energy, in agreement with the Energy Gap Law, while those for the Pt(dbbpy)(dithiolate) complexes do not exhibit a similar correlation. Excited-state redox potentials have been estimated for all of the complexes from spectroscopic and electrochemical data. The ability to tune the driving force for bimolecular excited-state electron-transfer reactions has been demonstrated for eight of the complexes using reductive and oxidative quenching experiments.
Contrary to the general rules, the radiative rates in intramolecular full CT excited states are temperature-dependent: fluorescence emission is thermally activated. This is demonstrated on the TICT states and on highly polar intramolecular exciplexes. The radiative (back) electron transfer from the equilibrium structure of these excited states is highly forbidden often due to zero or minimal orbital overlap (nodal plane effects); thus, the equilibrium structures in both classes of states correspond to practically pure CT configurations. Vibrational activation (100-600 cm{sup {minus}1}) is predominantly responsible for the observed fluorescence and presents a new key to the structure of these excited species. Thermally activated emission is predicted for other flexible molecules with a strongly forbidden transition.
A series of D-π-A-π-D type of near-infrared (NIR) fluorescent compounds based on benzobis(thiadiazole) and its selenium analogues were synthesized and fully characterized by 1H and 13C NMR, high-resolution mass spectrometry, and elemental analysis. The absorption, fluorescence, and electrochemical properties were also studied. Photoluminescence of these chromophores ranges from 900 to 1600 nm and their band gaps are between 1.19 and 0.56 eV. Replacing the sulfur by selenium can lead to a red shift for emission and reduce the band gaps further. Interestingly, compound 1 exhibits aggregation-induced emission enhancement effect in the solid state. All-organic light-emitting diodes based on M1 and M2 were made and exclusive NIR emissions above 1 μm with external quantum efficiency of 0.05% and maximum radiance of 60 mW Sr−1 m−2 were observed. The longest electroluminescence wavelength reaches 1115 nm.
A family of multi-heterocycle donor–acceptor–donor (DAD) telechelic conjugated oligomers designed for two-photon absorption (2PA) and emission in the near-infrared (near-IR) were prepared, and the relationship between their spectral, structural, and electrochemical properties were investigated. These oligomers, based on electron-rich thiophene, phenylene, and 3,4-ethylenedioxythiophene (EDOT) units as donors along with electron-deficient benzothiadiazole or its derivative units as acceptors, have been characterized through linear absorbance and fluorescence measurements, nonlinear absorbance, cyclic voltammetry, and differential pulse voltammetry to demonstrate the evolution of narrow HOMO–LUMO gaps ranging from 1.05 to 1.95 eV, with the oligomers composed of EDOT and benzo[1,2-c,3,4-c′]bis[1,2,5]thiadiazole (BBT) exhibiting the narrowest gap. The absorption maxima ranges from 517 to 846 nm and the fluorescence maxima ranges from 651 to 1088 nm for the different oligomers. Z-scan and two-photon fluorescence were used to measure the frequency degenerate 2PA of the different oligomers. The oligomer’s 2PA cross sections ranged from 900–3500 GM, with the oligomer containing EDOT donor units and a BBT acceptor unit exhibiting the largest 2PA cross section. The use of these oligomers in red to near-IR emitting polymer light-emitting diodes (PLEDs) was demonstrated by blending the soluble emitting oligomers into a suitable host matrix. Energy transfer from the matrix to the emitting oligomer can be achieved, resulting in PLEDs with pure oligomer emission.Keywords: near-infrared emission; PLED; two-photon absorption; donor−acceptor oligomers; controlled HOMO−LUMO gap
This short review surveys the development of red fluorescent materials for the application of organic light-emitting diodes (OLEDs) that generate red electroluminescence (EL). The merit and problems of current dopant-based, either fluorescent or phosphorescent, red OLEDs will be addressed first. Materials that offer unique EL characteristics, such as narrow and saturated red EL as well as current density or voltage-independent EL efficiency, are discussed. In addition to dopant-based and assist dopant-based red OLEDs for comparison purposes, the survey emphasizes nondoped red OLEDs that are fabricated with the newly emerging red fluorophores as the host-emitter. The advantage of host-emitting nondoped OLEDs compared with traditional dopant-based red OLEDs is described in view of the chemical and device structures of these materials.
Four ultrahigh energy gap organosilicon compounds [diphenyldi(o-tolyl)silane (UGH1), p-bis(triphenylsilyl)benzene (UGH2), m-bis(triphenylsilyl)benzene (UGH3), and 9,9‘-spirobisilaanthracene (UGH4)] were employed as host materials in the emissive layer of electrophosphorescent organic light-emitting diodes (OLEDs). The high singlet (4.5 eV) and triplet (3.5 eV) energies associated with these materials effectively suppress both the electron and energy transfer quenching pathways between the emissive dopant and the host material, leading to deep blue phosphorescent devices with high (10%) external quantum efficiencies. Furthermore, by direct charge injection from the adjacent hole and electron transport layers onto the phosphor doped into the UGH matrix, exciton formation occurs directly on the dopant, thereby eliminating exchange energy losses characteristic of guest−host energy transfer. We discuss the material design, and present device data for OLEDs employing UGHs. Among the four host materials, UGH2 and UGH3 have higher quantum efficiencies than UGH1 when used in OLEDs. Rapid device degradation was observed for the UGH4-based device due to electro- and/or photooxidation of the diphenylmethane moiety in UGH4. In addition to showing that UGH materials can be used to fabricate efficient blue OLEDs, we demonstrate that very high device efficiencies can be achieved in structures where the dopant transports both charge and excitons.
Efficient thermally activated delayed fluorescence (TADF) has been characterized for a carbazole/sulfone derivative in both solutions and doped films. A pure blue organic light emitting diode (OLED) based on this compound demonstrates a very high external quantum efficiency (EQE) of nearly 10% at low current density. Because TADF only occurs in a bipolar system where donor and acceptor centered (3)ππ* states are close to or higher than the triplet intramolecular charge transfer ((3)CT) state, control of the π-conjugation length of both donor and acceptor is considered to be as important as breaking the π-conjugation between them in blue TADF material design.
Rare red-fluorescent fluorene derivatives have been designed and synthesized. The long-wavelength red fluorescence is achieved by incorporating a di(4-tolyl)amino or diphenylamino electron donor and a dicyanovinyl electron acceptor. The single-crystal X-ray structures of the di(4-tolyl)amino (pTSPDCV) and diphenylamino (PhSPDCV) compounds indicate only weak non-π van der Waals contacts in addition to long-distance dipole–dipole interactions of the red-emitting fluorene molecules in the solid state. The aggregation of the dipolar fluorene is largely suppressed by introducing bulky 9,9-substituents (spiro-fused bifluorene) as well as a non-planar di(4-tolyl)amino or diphenylamino group. In the solid state, these fluorene derivatives all show red fluorescence that is much brighter than with the red dopants Nile Red and DCM (4-(dicyanomethylene)-2-methyl-6-[4-(dimethylaminostyryl)-4H-pyran]). The unique photophysical properties of red-emitting fluorene derivatives differ from other known red dopants and facilitate the fabrication of non-doped red organic light-emitting diodes (OLEDs). Authentic red (CIE, x = 0.65, y = 0.35) electroluminescence with a brightness of more than 12 000 cd m–2 (greater than 600 cd m–2 at 20 mA cm–2) and a remarkable external quantum efficiency as high as 3.6 % have been observed for the red-emitting OLEDs with pTSPDCV or PhSPDCV as the sole emitting host.
Cyanostilbene derivatives with the aggregation-induced emission enhancement (AIEE) activity are prepared by Knoevenagel and Suzuki reactions. Among them, the dye (Z)-2,3-bis(4'-(diphenylamino)-[1,1'-biphenyl]-4-yl)acrylonitrile (CNS-4) nanoparticle suspension shows the polarity-dependent characteristics of the fluorescence properties. By the fluorescence spectroscopy and transmission electron microscopy (TEM) analysis, the restriction of transfer from the local excited (LE) state to the intramolecular charge-transfer (ICT) state and crystal formation results in a blue-shift in emission and enhances the intensity in the aggregate state. Additionally, the luminophors CNS-3 and CNS-4 possess the AIEE effect as well as mechanochromic fluorescent properties. This mechanofluorochromic behavior originates from the change between the crystalline and the amorphous state.
We report efficient near-infrared (NIR) organic light-emitting devices (OLEDs) based on fluorescent donor-acceptor-donor conjugated oligomers. The energies of the highest occupied and lowest unoccupied molecular orbitals of these oligomers are controlled by the donor and acceptor components, respectively; hence the energy gap and therefore the emission wavelength can be tuned by changing the strengths of the donor and acceptor components. External quantum efficiencies (EQEs) up to 1.6% and power efficiencies up to 7.0 mW/W are achieved in NIR OLEDs based on 4,9-bis(2,3-dihydrothieno[3,4- b ][1,4]dioxin-5-yl)-6,7-dimethyl-[1,2,5]thiadiazolo[3,4- g ]-quinoxaline (BEDOT- TQMe <sub>2</sub> ), in which the electroluminescence peaks at a wavelength of 692 nm but extends to well above 800 nm. With a stronger acceptor in the oligomer, 4,8-bis(2,3-dihydrothieno-[3,4- b ][1,4]dioxin-5-yl)benzo[1,2- c ;4,5- c<sup>′</sup> ]bis [1,2,5]thiadiazole (BEDOT-BBT) based devices show longer wavelength emission peaked at 815 nm, although the maximum EQE is reduced to 0.51% due to the lower fluorescent quantum yield of the NIR emitter. The efficiencies of these NIR OLEDs are further increased by two to three times by using the sensitized fluorescent device structure, leading to a maximum EQE of 3.1% for BEDOT- TQMe <sub>2</sub> and 1.6% for BEDOT-BBT based devices.
Soluble molecular red emitters 1a/1b are synthesized by Stille coupling from
2-(3,5-di(1-naphthyl)phenyl)thiophene precursors. The compounds show
emission maxima at ca. 610nm in CH2Cl2 solution and 620 nm in solid films.
Replacing the n-hexyl substituent by 4-sec-butoxyphenyl produces a marked
increase of glass transition temperature (Tg) from 82 8C to 137 8C and
increases the solubility in toluene and p-xylene, thus improving the filmforming
properties. Cyclic voltammetry shows that the compounds can be
reversibly oxidized and reduced around þ1.10 and �1.20 V, respectively. A
two-layered electroluminescent device based on 1b produces a pure red light
emission with CIE coordinates (0.646, 0.350) and a maximal luminous
efficiency of 2.1 cd A�1. Furthermore, when used as a solution-processed red
emitter in optically pumped laser devices, compound 1b successfully
produces a lasing emission at ca. 650nm.
Molecular fluorophores of type A-π-D-π-A (D=donor, A=acceptor) demonstrate solid-state emission in the red to near-infrared region with high efficiency. The emission color can be tuned through the substituents on the diarylamino and cyanophenyl moieties. The electroluminescence performance of the designed fluorophore confirms its potential as an emitter for use in organic light-emitting devices.
To obtain highly efficient and stable blue electroluminescence, two novel star-shaped compounds are purposefully designed and synthesized, which are composed of two functional groups, central triphenylamine (TPA) and peripheral anthracene (AN) or phenanthrene (PA) with obviously segregative electronic density distribution characteristics between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO and LUMO). As a light-emitting layer, they exhibit not only highly efficient deep-blue electroluminescence, but also good stability in organic light-emitting diodes (OLEDs) due to the combination of several advantageous properties (electrochemical, thermal and morphological stability). In particular, for triphenylamine-cored phenanthrene (TPA-PA), the external quantum efficiency of 7.23% is the best reported result for non-doped deep-blue OLEDs, and the lifetime of the device is also improved, being twice that of 2-methyl-9,10-di(2-naphthyl) anthracene (MADN) under the same device conditions.
Full color luminogens are constructed from tetraphenylethene, benzo-2,1,3-thiadiazole and thiophene building blocks. OLED fabricated using one of the luminogens exhibits orange-red electroluminescence with high luminance and efficiencies of 8330 cd m(-2), 6.1 cd A(-1) and 3.1%, respectively.
Highly efficient deep blue phosphorescent organic light-emitting diodes (PHOLEDs) with external quantum efficiency above 20% are developed using a bipolar-type high-triplet-energy host material and a high-triplet-energy exciton blocking material. Maximum quantum efficiency of 25.1% and low roll-off (still 23.1% at 1000 cd m-2) are achieved in these deep blue PHOLEDs.
Organic electron-transporting materials are essential for the fabrication of organic p-n junctions, photovoltaic cells, n-channel field-effect transistors, and complementary logic circuits. Rylene diimides are a robust, versatile class of polycyclic aromatic electron-transport materials with excellent thermal and oxidative stability, high electron affinities, and, in many cases, high electron mobilities; they are, therefore, promising candidates for a variety of organic electronics applications. In this review, recent developments in the area of high-electron-mobility diimides based on rylenes and related aromatic cores, particularly perylene- and naphthalene-diimide-based small molecules and polymers, for application in high-performance organic field-effect transistors and photovoltaic cells are summarized and analyzed.
This Review summarizes the latest advances in the field of rylene dyes and rylene nanoemitters for applications in photonics, and describes the influence of the dye design on the optical properties, the self-assembly, the molecular interactions, as well as the labeling specificity of the compounds. The interplay between tailored (macro)molecular design and bulk/single-molecule spectroscopy enables complex processes to be explained, for example, the kinetics of energy-transfer processes or (bio)catalysis. Such investigations are essential for the ultimate design of optimized nanoemitters, and require a close cooperation between spectroscopists and preparative organic chemists.
The photovoltaic performance of polymer bulk heterojunction solar cells is studied systematically. Using a new benzodithiophene polymer (PTB7) and PC 71BM (see figure) a power conversion efficiency of 7.4% has been achieved in PTB7/PC71BMblend film, indicating a great potential and bright future for polymer solar cells (FF = fill factor, PCE = power-conversion efficiency). (Figure Presented).
A new class of aromatic fumaronitrile core-based compounds with different donors and linkers has been synthesized and well characterized. Comp