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Dynamic broadening alters triplet extinction coefficients in fluorene oligomers and polymers
Abstract
We report Tn ← T1 spectra and extinction coefficients, ε, and other properties as functions of chain length for a series of fluorene oligomers, oFn, and polymers, pFn, with n = 2–84 repeat units. We find that ε increases with length, peaking at 159 400 M⁻¹ cm⁻¹ for oF3 and then decreases for longer chains. ε does not scale with 1/n or e⁻ⁿ to reach a constant value at long length, as predicted by the commonly applied oligomer extrapolation approximation, although spectral shifts, oscillator strengths, and transition dipole moments do reach limiting values for chains near 10 units long. While computations describe the triplet in oF2 and oF3 as having similar geometries with a single flattened dihedral angle between units, computations and simulations suggest that in longer oligomers motion along the chains of the short 2–3 unit, the long T1 state is probably the source of the unusual changes in ε. These occur because hopping along the chain is sufficiently fast that the dihedrals between fluorene units cannot fully relax. At a length near 10 units, hopping and dihedral angle changes produce a steady state distribution of geometries with only small changes from the ground state, which persist for longer chains. Additional decreases in ε from pF28 to pF84 are plausibly due to a small number of chain defects which result in loss of triplets.
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In this work, the complex excited state dynamics of a sky-blue emitting co-polymer {poly-[9,9′-dioctylfluorene-co-bis-N,N′-(4-butylphenyl)-bis-N,N′-phenyl-1,4-phenylenediamine]} is determined, using a time-resolved gated spectroscopy method, which helps to investigate the origin of delayed fluorescence in thin film. It is clearly shown that delayed fluorescence arises from triplet–triplet annihilation mechanism, which opens an indirect pathway to generate singlet excitons. Experimentally, the delayed fluorescence yield varies from 20% at room temperature to 46 ± 2% at 20 K, suggesting that an alternative triplet harvesting mechanism is operative to convert the triplet into emissive singlet states.
Light‐emitting field‐effect transistors (LEFETs) are an emerging type of devices that combine light‐emitting properties with logical switching function. One of the factors limiting their efficiency stems from the spin statistics of electrically generated excitons. Only 25% of them, short lived singlet states, are capable of light emission, with the other 75% being long lived triplet states that are wasted as heat due to spin‐forbidden processes. Traditionally, the way to overcome this limitation is to use phosphorescent materials as additional emission channel harnessing the triplet excitons. Here, an alternative strategy for triplet usage in LEFETs in the form of thermally activated delayed fluorescence (TADF) is presented. Devices employing a TADF capable material, 4CzIPN (2,4,5,6‐tetra[9H‐carbazol‐9‐yl]isophthalonitrile), in both n‐type and p‐type configurations are shown. They manifest excellent electrical characteristics, consistent brightness in the range of 100–1,000 cd m‐2 and external quantum efficiency (EQE) of up to 0.1%, which is comparable to the equivalent organic light‐emitting diode (OLED) based on the same materials. Simulation identifies the poor light out‐coupling as the main reason for lower than expected EQEs. Transmission measurements show it can be partially alleviated using a more transparent top contact, however more structural optimization is needed to tap the full potential of the device.
Organic solar cells are a promising renewable energy technology, offering the advantages of mechanical flexibility and solution processability. An understanding of the electronic excited states and charge separation pathways in these systems is crucial if efficiencies are to be further improved. Here we use light induced electron paramagnetic resonance (LEPR) spectroscopy and density functional theory calculations (DFT) to study the electronic excited states, charge transfer (CT) dynamics and triplet exciton formation pathways in blends of the small molecule donors (DTS(FBTTh2)2, DTS(F2BTTh2)2, DTS(PTTh2)2, DTG(FBTTh2)2 and DTG(F2BTTh2)2) with the fullerene derivative PC61BM. Using high frequency EPR the g-tensor of the positive polaron on the donor molecules was determined. The experimental results are compared with DFT calculations which reveal that the spin density of the polaron is distributed over a dimer or trimer. Time-resolved EPR (TR-EPR) spectra attributed to singlet CT states were identified and the polarization patterns revealed similar charge separation dynamics in the four fluorobenzothiadiazole donors, while charge separation in the DTS(PTTh2)2 blend is slower. Using TR-EPR we also investigated the triplet exciton formation pathways in the blend. The polarization patterns reveal that the excitons originate from both intersystem crossing (ISC) and back electron transfer (BET) processes. The DTS(PTTh2)2 blend was found to contain substantially more triplet excitons formed by BET than the fluorobenzothiadiazole blends. The higher BET triplet exciton population in the DTS(PTTh2)2 blend is in accordance with the slower charge separation dynamics observed in this blend.
Solution-processed polymer organic light-emitting diodes (OLEDs) doped with triplet-triplet annihilation (TTA)-upconversion molecules, including 9,10-diphenylanthracene, perylene, rubrene and TIPS-pentacene, are reported. The fraction of triplet-generated electroluminescence approaches the theoretical limit. Record-high efficiencies in solution-processed OLEDs based on these materials are achieved. Unprecedented solid-state TTA-upconversion quantum yield of 23% (TTA-upconversion reaction efficiency of 70%) at electrical excitation well below one-sun equivalent is observed.
We present studies of steady-state photoinduced absorption (PIA) spectroscopy on photoexcitations in a series of well-defined α-oligothiophene (Tn, n=6, 7, 9, and 11) films and solutions. The PIA spectra and the excited state lifetimes are consistent with the signatures of a photoexcited triplet state. The PIA spectra consist of a strong vibronically resolved subgap absorption, which is readily observed in solid-state films and in solutions at ambient and cryogenic temperatures. The transition energy is linearly dependent on the reciprocal chain length and shifts to lower energy for longer oligomers. Variation of the modulation frequency and the pump intensity under matrix-isolated conditions reveals that the photoexcitation is created via an intrachain mechanism and decays nonradiatively with monomolecular kinetics. In solid films we find a significant contribution of a bimolecular decay process to the relaxation rate.
Time-resolved luminescence spectroscopy has been used to investigate exciton diffusion in thin films of poly(p-phenylene vinylene) (PPV)–based derivatives. Due to chemical modifications the PPV derivatives differ by three orders of magnitude in charge carrier mobility as a result of a reduced energetic disorder. From the photoluminescence decay curves of PPV/fullerene heterostructures, the exciton diffusion coefficient was found to increase by one order of magnitude with decreasing disorder. This increase in the diffusion coefficient is compensated by a decrease of the exciton lifetime, leading to an exciton diffusion length of 5–6 nm for the various PPV derivatives.
The nature of the charge carriers in 'conducting' polymers is of considerable interest at present1,2, largely on the basis of the technological potential of these materials for use as the semiconducting layer in field-effect transistors (FETs) and the emissive layer in light-emitting diodes3 (LEDs). One of the main outstanding questions concerns the relative importance of intra- versus inter-chain charge transfer in determining the overall rate of charge transport. Here we apply the pulse-radiolysis time-resolved microwave conductivity technique4 to dilute solutions of a soluble dialkoxy derivative of the semiconducting polymer poly(phenylene vinylene), PPV, by which means we determine the one-dimensional intra-chain mobilities of electrons and holes on isolated polymer chains free from inter-chain interactions. The values so obtained—0.5 and 0.2 cm2 V-1 s-1 respectively—are considerably larger than the mobilities measured previously for bulk PPV-based materials5, 6, 7, 8, 9. This suggests that considerable improvement in the performance characteristics (in particular switching time and maximum current) of organic FET and LED devices should be possible if material purity and structural order can be better controlled.
We developed a new method to accurately extract the singlet exciton diffusion length in organic semiconductors by blending them with a low concentration of methanofullerene[6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM). The dependence of photoluminescence (PL) decay time on the fullerene concentration provides information on both exciton diffusion and the nanocomposition of the blend. Experimentally measured PL decays of blends based on two narrow band gap dithiophene– benzothiadiazole polymers, C–PCPDTBT and Si–PCPDTBT, were modeled using a Monte Carlo simulation of 3D exciton diffusion in the blend. The simulation software is available for download. The extracted exciton diffusion length is 10.5 AE 1 nm in both narrow band gap polymers, being considerably longer than the 5.4 AE 0.7 nm that was measured with the same technique in the model compound poly(3-hexylthiophene) as a reference. Our approach is simple, fast and allows us to systematically measure and compare exciton diffusion length in a large number of compounds.
Donor-acceptor dyads consisting of octathiophene (T8) paired with three (di)imide acceptors (naphthalene diimide (NDI), benzene diimide (BDI) and naphthalimide (NI)) were synthesized and probed for their photoinduced forward electron transfer (ET) and charge recombination kinetics by using ultrafast transient absorption spectroscopy. The three acceptors have different electron affinity, leading to variation in the energy of the charge-separated state and the driving force (ΔG) for forward ET and charge recombination. Analysis of the TA spectra and kinetics allows assignment of rates for forward ET and charge recombination for each of the oligomers. Electrochemistry and photoluminescence spectroscopy are used to determine the ΔG values for the ET processes. For two of the oligomers (T8NDI and T8BDI) the rates for forward ET and charge recombination are very rapid (k > 3 x 1010 s-1). By contrast for the third oligomer (T8NI) the rates for both processes are considerably slower (k < 5 x 109 s-1). Analysis of the rate/free energy correlation for the series of oligomers reveals generally good agreement with Marcus semi-classical theory. The rapid dynamics for T8NDI and T8BDI are explained as arising due to the processes occurring near the barrierless region (-ΔG ~ λ), or slightly into the Marcus inverted region (-ΔG > λ). The slower dynamics for T8NI are explained as arising because the forward ET is weakly exothermic, whereas charge recombination is deep into the inverted region. This study is the first to produce experimental results that match a full Marcus bell-shaped curve with ET rates in the normal, barrrierless and inverted regions in dyads based on -conjugated oligomer donor.
A short exciton diffusion length (LD) is one of the main factors limiting the final photoconversion efficiency (PCE) of organic photovoltaics (OPV). Aiming at overcoming this barrier, there have been studies on the use of longer-lived triplet excitons, which can be introduced either through triplet sensitization by doping or through the use of donor materials containing heavy atoms that allow for intersystem crossing (ISC). This review focuses on how these systems have been shown to increase LD, leading to increased PCEs.
Excited state energy transfer in disordered systems has attracted significant attention owing to the importance of this phenomenon in both artificial and natural systems that operate in electron- ically excited states. Of particular interest, especially in the context of organic electronics, is the dynamics of triplet excited states, which due to their weak coupling to the singlet manifold often act as low energy trapping sites and are therefore detrimental to device performance. Herein, we explore the triplet energy transfer mechanism from dichlorobenzene to thioxanthone in methanol solution. We rationalise previous experimental observations as arising from preferential popula- tion transfer into the lowest triplet state rather than the higher lying triplet state that is closer in energy. The reason for this is a delicate balance between the electronic coupling, reorganisation energy and the energy gap involved. The present results provide the understanding to potentially develop a hot exciton mechanism in materials for organic light emitting diodes (OLED) to achieve higher device efficiencies.
Harvesting of both triplets and singlets yields electroluminescence quantum efficiencies of nearly 100% in organic light‐emitting diodes (OLEDs), but the production efficiency of excitons that can undergo radiative decay is theoretically limited to 100% of the electron–hole pairs. Here, breaking of this limit by exploiting singlet fission in an OLED is reported. Based on the dependence of electroluminescence intensity on an applied magnetic field, it is confirmed that triplets produced by singlet fission in a rubrene host matrix are emitted as near‐infrared (NIR) electroluminescence by erbium(III) tris(8‐hydroxyquinoline) (ErQ3) after excitonic energy transfer from the “dark” triplet state of rubrene to an “emissive” state of ErQ3, leading to NIR electroluminescence with an overall exciton production efficiency of 100.8%. This demonstration clearly indicates that the harvesting of triplets produced by singlet fission as electroluminescence is possible even under electrical excitation, leading to an enhancement of the quantum efficiency of the OLEDs. Electroluminescence employing singlet fission provides a route toward developing high‐intensity NIR light sources, which are of particular interest for sensing, optical communications, and medical applications.
A set of monodisperse macromolecular starbursts which are composed of triazine center substituted by heterogeneous carbazole and diphenylamine units with multibranched oligofluorenes (TF1 and TF2) have been designed, synthesized, and explored as gain media for organic lasers. The thermal, morphological, photophysical, and electrochemical properties and optical gain characteristics of the resulting starbursts have been investigated in comparison with those of their linear counterparts (2FCz and 4FCz) to shed light on better understanding the structure–property relationships. The results manifest that the resulting starburst architectures based on a triazine center with multibranched oligofluorenes are beneficial for depressing the crystallization tendency, leading to enhanced amorphous morphologies, excellent thermal stabilities, and favorable facile solution processability. The novel heterogeneous donor–acceptor core structure based on triazine center plays a key role to dominate the electronic properties of the resulting multibranched starbursts and endows the molecules with low-lying LUMO energy levels. Promising amplified spontaneous emission (ASE) characteristics are recorded for the multibranched starbursts, in which TF1 and TF2 exhibit rather low ASE threshold (EthASE) of 4.3 and 10.3 μJ/cm², respectively. Remarkably, TF1 and TF2 manifest enhanced ASE stability with no obvious spectral variations (within 2 nm for TF1 and 1 nm for TF2) and almost unchanged EthASE upon increasing the annealing temperature even up to 200 °C in air. In contrast, their linear counterparts 2FCz and 4FCz showed distinct EthASE variations with increasing the annealing temperature above 100 °C. The results suggest that the novel molecular design is beneficial for enhancing the thermal and optical stabilities as well as fine modulating the electrical properties, rendering the resulting multibranched triazine-centered starbursts advantageous as efficient gain media for electrically pumped organic lasers.
Bimolecular equilibria measured the one-electron reduction potentials and triplet free energies (G°T) of oligo(9,9-dihexyl)fluorenes and a polymer with lengths of n=1-10 and 57 repeat units. Accurate one-electron potentials can be measured electrochemically only for the shorter oligomers. Starting at n=1 the free energies change rapidly with increasing length and become constant for lengths longer than the delocalization length. Both the reduction potentials and triplet energies can be understood as the sum of a free energy for a fixed polaron and a positional entropy. The positional entropy increases gradually with length beyond the delocalization length due the possible occupation sites of the charge or the triplet exciton. The results reinforce the view that charges and triplet excitons in conjugated chains exist as polarons and find that positional entropy can replace a popular empirical model of the energetics.
In this work, the triplet state delocalization in a series of monodisperse oligo(p-phenyleneethynylene)s (OPEs) is studied by pulsed electron paramagnetic resonance (EPR) and pulsed electron nuclear double resonance (ENDOR) determining zero-field splitting, optical spin polarization, and proton hyperfine couplings. Neither the zero-field splitting parameters nor the optical spin polarization change significantly with OPE chain length, in contrast to the hyperfine coupling constants, which showed a systematic decrease with chain length n according to a 2/(1+n) decay law. The results provide striking evidence for the Frenkel type nature of the triplet excitons exhibiting a full coherent delocalization in the OPEs under investigation with up to five OPE repeat units, and with a spin density distribution described by a nodeless particle in the box wavefunction. The same model is successfully applied to recently published data on pi-conjugated porphyrin oligomers.
The field of organic photovoltaics has developed rapidly over the last 2 decades, and small solar cells with power conversion efficiencies of 13% have been demonstrated. Light absorbed in the organic layers forms tightly bound excitons that are split into free electrons and holes using heterojunctions of electron donor and acceptor materials, which are then extracted at electrodes to give useful electrical power. This review gives a concise description of the fundamental processes in photovoltaic devices, with the main emphasis on the characterization of energy transfer and its role in dictating device architecture, including multilayer planar heterojunctions, and on the factors that impact free carrier generation from dissociated excitons. We briefly discuss harvesting of triplet excitons, which now attracts substantial interest when used in conjunction with singlet fission. Finally, we introduce the techniques used by researchers for characterization and engineering of bulk heterojunctions to realize large photocurrents, and examine the formed morphology in three prototypical blends.
A family of trigonal starburst conjugated molecules (TrFPy, TrFPy, and TrF2Py) composed of a truxene core and pyrene cappers with various bridge lengths is synthesized and characterized. The incorporation of pyrene cappers successfully depress the crystallization tendency, resulting in enhanced glassy temperature and improved morphological stability of the thin films. The high photoluminescence yield in neat films and excellent thermal stability render these pyrene-capped starbursts promising lasing optical gain media. Low amplified spontaneous emission (ASE) thresholds (EthASE) of 180 nJ pulse-1 and 101 nJ pulse–1 were recorded for TrFPy and TrF2Py, respectively. One dimensional distributed feedback (1D DFB) lasers demonstrated lasing threshold of 9.3 kW/cm2 and 7.3 kW/cm2 for TrFPy (at 457 nm) and TrF2Py lasers (at 451 nm), respectively. The ASE performance of TrFPy and TrF2Py in an ambient condition was recorded with various annealing temperature (from 80 to 250 °C, 10 min). Surprisingly, TrFPy exhibited excellent ASE stability in an ambient condition, which is still detectable even after annealing at 250 °C for 10 min. The results suggest the pyrene-capped molecular design strategy is positive on improving the optical gain stability and meanwhile maintaining excellent lasing properties.
Photoexcitation of conjugated poly-2,7-(9,9-dihexylfluorene) polyfluorenes with naphthylimide (NI) and anthraquinone (AQ) electron-acceptor end traps produces excitons that form charge transfer states at the end traps. Intramolecular singlet exciton transport to end traps was examined by steady state fluorescence for polyfluorenes of 17-127 repeat units in chloroform, dimethylformamide (DMF), tetrahydrofuran (THF), and p-xylene. End traps capture excitons and form charge transfer (CT) states at all polymer lengths and in all solvents. The CT nature of the end-trapped states is confirmed by their fluorescence spectra, solvent and trap group dependence, and DFT descriptions. Quantum yields of CT fluorescence are as large as 46%. This strong CT emission is understood in terms of intensity borrowing. Energies of the CT states from onsets of the fluorescence spectra give the depths of the traps which vary with solvent polarity. For NI end traps, the trap depths are 0.06 (p-xylene), 0.13 (THF), and 0.19 eV (CHCl3). For AQ, CT fluorescence could be observed only in p-xylene where the trap depth is 0.27 eV. Quantum yields, emission energies, charge transfer energies, solvent reorganization, and vibrational energies were calculated. Fluorescence measurements on chains >100 repeat units indicate that end traps capture ∼50% of the excitons, and that the exciton diffusion length is LD = 34 nm, which is much larger than diffusion lengths reported in polymer films or than previously known for diffusion along isolated chains. The efficiency of exciton capture depends on chain length but not on trap depth, solvent polarity, or which trap group is present.
Triplet excitons created in poly-2,7-(9,9-dihexyl)fluorene (pF) chains with end trap groups in solution are efficiently transported to and captured by the end groups. The triplets explore the entire lengths of the chains, even for ∼100 nm long chains, enabling determination of the completeness of end-capping. The results show that the chains are continuous: they may contain transient barriers or traps, such as those from fluctuations of dihedral angles, but they are free of major defects that stop motion of the triplets. Quantitative determinations are aided by the addition of a strong electron donor, TMPD, which removes absorption bands of the end-trapped triplets. For chains having at least one end trap, triplet capture is quantitative on the 1 μs time scale imposed by the use of the donor. Fractions of chains having no end traps were 0.15 for pF samples with anthraquinone (AQ) end traps and 0.063 with naphthylimide (NI) end traps. These determinations agreed with measurements by NMR for short (<40 polymer repeat units (PRU)) chains, where NMR determinations are accurate. The results find no evidence for traps or barriers to the transport of triplets, and places limits on the possible presence of defects as impenetrable barriers to less than one per 300 PRU. The present results present a paradigm different from the current consensus, derived from observations of singlet excitons, that conjugated chains are divided into "segments," perhaps by some kind of defects. For the present pF chains, the segmentation either does not apply to triplet excitons or is transient so that the defects are healed or surmounted in times much shorter than 1 μs. Triplets on chains without end trap groups transfer to chains with end traps on a slower time scale. Rate constants for these bimolecular triplet transfer reactions were found to increase with the length of the accepting chain, as did rate constants for triplet transfer to the chains from small molecules like biphenyl. A second set of polyfluorenes with 2-butyloctyl side chains was found to have a much lower completeness of end-capping.
The nature of low energy optical excitations, or excitons, in organic solids is of central relevance to many optoelectronic applications, including solar energy conversion. Excitons in solid pentacene, a prototypical organic semiconductor, have been the subject of many experimental and theoretical studies, with differing conclusions as to the degree of their charge-transfer character. Using first-principles calculations based on density functional theory and many-body perturbation theory, we compute the average electron–hole distance and quantify the degree of charge-transfer character within optical excitations in solid-state pentacene. We show that several low-energy singlet excitations are characterized by a weak overlap between electron and hole and an average electron–hole distance greater than 6 Å. Additionally, we show that the character of the lowest-lying singlet and triplet excitons is well-described with a simple analytic envelope function of the electron–hole distance.
Emission quenching by fullerenes covalently attached to both ends of a series of size-selected regioregular poly(3-hexylthiophene) samples was quantified and used to determine the intrachain exciton diffusion length. The diffusion length was found to be LD = 7.0 ± 0.8 nm. When the distance dependence of the quenching mechanism is considered, this is the same value that has been reported for emissive excitons in thin films. This result indicates that intrachain exciton transport is more facile for excitons localized to single chains than that for excitons that are delocalized between chains. In the context of solar cells, the result indicates additional complexity and the potential for competing issues when considering morphological design of the film to enhance both exciton and charge transport.
Liquid-filled photonic crystal fibers and optofluidic devices require infiltration with a variety of liquids whose linear optical properties are still not well known over a broad spectral range, particularly in the near infrared. Hence, dispersion and absorption properties in the visible and near-infrared wavelength region have been determined for distilled water, heavy water, chloroform, carbon tetrachloride, toluene, ethanol, carbon disulfide, and nitrobenzene at a temperature of 20 °C. For the refractive index measurement a standard Abbe refractometer in combination with a white light laser and a technique to calculate correction terms to compensate for the dispersion of the glass prism has been used. New refractive index data and derived dispersion formulas between a wavelength of 500 nm and 1600 nm are presented in good agreement with sparsely existing reference data in this wavelength range. The absorption coefficient has been deduced from the difference of the losses of several identically prepared liquid filled glass cells or tubes of different lengths. We present absorption data in the wavelength region between 500 nm and 1750 nm.
The present manuscript reports the design, synthesis and characterization of three star-shaped polymers consisting of three different arylimides such as perylene (PR)-, naphthalene (NT)- and benzene (BZ) tetracarboxylicdiimide as core and polyfluorene (PF) as arms. Chemical structure of star-shaped polymers was aimed at broadening as much as possible their absorption profile. Arylimide cored star polymers (PF-BZ, PF-NT and PF-PR) were prepared through palladium catalyzed Suzuki polycondensation to tune the band gap of the polymers. The prepared polymers were characterized by elemental analysis, NMR, GPC, UV-Vis, photoluminescence and cyclic voltammetry studies. Electrochemical and optical responses of three polymers revealed the lowering of band gap from linear PF to star shaped polymers. TCSPC study confirmed the partial energy transfer from PF arms to arylimide cores. The unexpected keto defect in linear PF was also reduced by preparation of star polymer with large arylimide cores. TGA exhibited the enhancement of thermal stability of star polymer than linear PF. By using star polyfluorenes as the donor and [6,6]-phenyl C61- butyric acid methyl ester (PCBM) as the acceptor, bulk heterojunction (BHJ) solar cells of the structure ITO/PEDOT:PSS/star polymer : PCBM/Al were fabricated and studied with a solar simulator under AM1.5G (100 mW/cm2) irradiation intensity. Those cells showed the open circuit voltage (Voc) ~0.52-0.55 V, the short circuit current density (Jsc) ~0.84 –1.13 mA cm-2, the fill factor ~0.39 -0.44 and the power conversion efficiency (PCE) ~0.18 – 0.26%.
The optical properties of two differently substituted types of polyfluorenes, 9-monoalkylated PF (mono-PF) and 9,9-dialkylated PF (bi-PF) where studied by means of photo-induced absorption (PIA) and ultrafast pump and probe measurements. The photo-induced absorption was complemented by measurements on a fluorene–fluorenone copolymer, which can be seen as a model substance for the polyfluorenes containing keto-defect sites. By differential transmission measurements we show that for the 9-monoalkylated PF measurements the singlet and triplet signal is strongly reduced compared to the 9,9-dialkylated PF. Instead, the polaron signal becomes the dominant feature.
Polyfluorenes have emerged as versatile semiconducting materials with applications in various polymer optoelectronic devices, such as light-emitting devices, lasers, solar cells, memories, field-effect transistors and sensors. Organic syntheses and polymerizations allow for the powerful introduction of various periodic table elements and their building blocks into π-conjugated polymers to meet the requirements of organic devices. In this review, a soccer-team-like framework with 11 nodes is initially proposed to illustrate the structure–property relationships at three levels: chain structures, thin films and devices. Second, the modelling of hydrocarbon polyfluorenes (CPFs) is summarized within the framework of a four-element design principle, in which we have highlighted polymorphic poly(9,9-dialkylfluorene)s with unique supramolecular interactions, various hydrocarbon-based monomers with different electronic structures, functional bulky groups with steric hindrance effects and ladder-type, kinked, hyperbranched and dendritic conformations. Finally, the detailed electronic structure designs of main-chain-type heteroatomic copolyfluorenes (HPFs) and metallopolyfluorenes (MPFs) are described in the third and fourth sections. Supramolecular, nano and soft semiconductors are the future of polyfluorenes in the fields of optoelectronics, spintronics and electromechanics.
The triplet-triplet extinction coefficients of seventeen compounds have been determined in cyclohexane and benzene at room temperature by a method involving energy transfer. Large changes in extinction coefficient at the wavelength maximum ε(λmax) were observed upon changing solvent from cyclohexane to benzene. Values of ε(λmax) were determined relative to a value of ε(λmax) for the ketyl radical (2ĊOH) of 3220 M–1 cm–1 in water, assuming no change of oscillator strength with solvent. A number of triplet transfer rates, and other reactivities of benzophenone triplet and ketyl radicals, were also estimated.
The excited state absorption spectra of biphenyl and 2,5-dimethyl biphenyl have been investigated by femtosecond broad band pump–probe spectroscopy in organic solvents. The results show vastly different intensities that were analyzed by calculating transition density surfaces for characteristic S1→Sn absorptions. The analysis revealed strong symmetry-lowering substituent effects in 2,5-dimethyl biphenyl which directly affect the transition dipole moments for excited state absorption. By measuring the temporal evolution of the transient spectra of the biphenyl derivatives we observed a biphasic relaxation mechanism in the excited state characterized by two distinct time scales of 400 fs and 12 ps.
We present a compilation of spectral parameters associated with triplet–triplet absorption of organic molecules in condensed media. The wavelengths of maximum absorbance and the corresponding extinction coefficients, where known, have been critically evaluated. Other data, for example, lifetimes, energies and energy transfer rates, relevant to the triplet states of these molecules are included by way of comments but have not been subjected to a similar scrutiny. Work in the gas phase has been omitted, as have theoretical studies. We provide an introduction to triplet state processes in solution and solids, developing the conceptual background and offering an historical perspective on the detection and measurement of triplet state absorption. Techniques employed to populate the triplet state are reviewed and the various approaches to the estimation of the extinction coefficient of triplet–triplet absorption are critically discussed. A statistical analysis of the available data is presented and recommendations for a hierarchical choice of extinction coefficients are made. Data collection is expected to be complete through the end of 1984. Compound name, molecular formula and author indexes are appended.
The rates of formation of solute triplet and excited singlet states as a function of time have been measured over the first 70 ns in the pulse radiolysis of isooctane, cyclohexane, and n-hexane solutions. At 0.1 M solute (naphthalene or biphenyl), the fraction of the solute excited states formed by ion-recombination that are triplet is determined to be 0.5±0.1 in the time regime 2 to 70 ns. A similar value is obtained from analysis of the time profile of the triplet state of naphthalene for concentrations from 0.001 to 0.1 M in the same solvents. The implications of these results with respect to the non-homogeneous ion recombination processes occurring are discussed. The decay rate of the solute radical anion has also been determined, and the relationship between it and the rate of solute excited state formation has been analyzed.
A new method for determining these coefficients has been developed. The method employs pulse radiolysis for triplet excitation and, by means of triplet-triplet (T-T) energy transfer, leads to a comparison between the extinction coefficients of the benzophenone ketyl radical (phi_2COH) and the triplet states of several aromatic compounds whose triplet levels are lower than that of benzophenone. Since the ketyl extinction coefficient can be determined independently, the actual T-T extinction coefficients can be obtained. Values of the extinction coefficient at the absorption maximum were obtained for anthracene (57 200 M-1 cm-1), naphthalene (22 600 M-1 cm-1), phenanthrene (21 000 M-1 cm-1) and 1,2-benzanthracene (25 100 M-1 cm-1), all relative to an extinction coefficient for the benzophenone ketyl maximum around 540 nm determined to be 3220 M-1 cm-1. For several other compounds the above method for various reasons could not be applied. In such cases estimates were obtained by energy transfer experiments, leading to comparisons between their triplet absorptions and those of the compounds already determined. Estimates were thus obtained for the maximum T-T extinction coefficients of benzophenone (10 300 M-1 cm-1), biacetyl (6400 M-1 cm-1), diphenyl (35 400 M-1 cm-1), and acridine (28 800 M-1 cm-1). During the course of these experiments estimates were also obtained for the rates of the following reactions: e^-aq. + benzophenone (kII = 3.0 x 1010 M-1 s-1), OH^. + benzophenone (kII = 8.7 x 10^9 M-1 s-1) and hydrogen abstraction by triplet benzophenone from cyclohexane, to give phi_2COH (kII~ 8 x 10^5 M-1 s-1).
Using time dependent density functional theory, we investigate the chain length dependence of the energies of excited states of a series of conjugated 9,9-dihexylfluorene-2,7-diyl oligomers. Excited state optimization reveals that upon excitation the dihedral angle between two adjacent monomer units moves towards zero, forming a planar structure within the oligomer. The calculated energies of the optical transitions in absorption, fluorescence, phosphorescence and triplet–triplet absorption are compared with recently reported experimental data. The calculated as well as experimentally reported energies involved seem to saturate very fast as the chain length increases. The energy dispersion and saturation indicates that the triplet ground state is somewhat more confined than the first singlet excited state. Our calculated energies agree well with the experimental findings where available, showing small but systematic deviations.
The optical absorption of ordered and disordered π-conjugated molecules is described on the same microscopic footing. This gives a concise picture of the length dependence of the optical absorption of π-conjugated systems.
The BNL Laser-Electron Accelerator Facility (LEAF) uses a laser-pulsed photocathode, radio-frequency electron gun to generate greater than or equal to7 ps pulses of 8.7 MeV electrons for pulse radiolysis experiments. The compact and operationally simple accelerator system includes synchronized laser pulses that can be used to probe or excite the electron-pulsed samples to examine the dynamics and reactivity of chemical species on the picosecond time scale. (C) 2004 American Institute of Physics.
Density-functional methods are used to analyze the scaling of discrete oligomeric pi-electron conducting molecules towards idealized isolated polymer chains, treated in periodic boundary conditions. The band gaps of a series of conjugated oligomers of incrementally increasing lengths exactly fit a nearly-free-electron molecular-orbital picture and exhibit a smooth deviation from the classical empirical "1/N" trend for long oligomers and infinite polymers. The calculations also show a smooth convergence of bond lengths. The full band structures and densities of states of a polyacetylene, polypyrrole, polyfuran, and polythiophene show that band crossing, localized bands, and other effects cannot be accurately determined from simple extrapolation of oligomer electronic structures. Systematic comparisons of the electronic structure variations of the polymers investigated indicate that the electron affinity, rather than the electronegativity of the heteroatom or the bond-length alternation of the conjugated backbone, significantly affects the band gap of the resulting polymer as indicated by the presence of heteroatom states in the partial density of states of the conduction band, requiring revision of previous semiempirical analyses. Consequences for doping processes are also studied, along with a comparison of valence bandwidths, conduction bandwidths, and carrier effective masses as a function of heteroatom.
Optical absorption spectra of anions and cations in poly(3-decylthiophene), (P3DT), in solution were identified as single polarons. Pulse-radiolysis of P3DT in THF determined a spatial extent of one negative polaron to be ~11.5 thiophene units by observing transient absorption of P3DT-•, which are prinicpally free ions, at 850nm with ε = (7.25 +/- 0.47) x 104 M-1cm-1 and bleaching of neutral absorption band at 450 nm. P3DT-• was formed in a combination of diffusive reactions and fast "step" processes. Similarly, a positive polaron of P3DT was estimated to delocalize over ~8.7 thiophene units by pulse-radiolysis in chloroform. Chemical reduction by sodium and oxidation by FeCl3 injected multiple charges to single P3DT chain while showing absorption spectra in early stages of reaction resembling to those observed by pulse-radiolysis. The results indicated multiple polarons exist in single chain of P3DT before coalescing into bipolarons or transforming to other forms of polaron.
A large basis set of α-oligothiophenes with two to seven rings (α2−α7), also including thiophene, α1, have been investigated in five solvents regarding absorption, fluorescence and phosphorescence, quantum yields of fluorescence (φF) and triplet formation (φT), lifetimes of fluorescence and the triplet state, quantum yields of singlet oxygen production (φΔ), all rate constants kF, kIC, kISC, and several of the foregoing as a function of temperature. Ten different theoretical calculations across several levels including three levels of ab initio have been carried out regarding which conformer is lowest in energy and the ΔH's among all conformers of α2, α3 and α5, as well as calculations of transitions energies of the α-oligothiophenes. We have shown that the (l) 1Bu state is the lowest singlet state for all α2−α7 in any solvent, in contradiction to previous predictions for the higher members. Based on absorption and fluorescence data and calculations of atomic charges in S0 and S1, the ground state is twisted while the excited state is planar (quinoidal-like). Significant charge transfer occurs between S0 and S1 but not S0 and T1. For all α2−α7, φIC is small, k0F is approximately constant while kISC decreases significantly from α2 to α7. The decrease is kISC is believed to arise from a decrease in matrix elements of the type 1ΨCT|H‘|3Ψ1. The essential lack of phosphorescence is assigned as originating from inter-ring twisting mode coupling between T1 and S0. Triplet energy transfer to 3O2 to produce 1O2 is highly efficient for α2−α5. Based on all data, the first αn representative of α-polythiophene is α5.
Triplet excited states created in polyfluorene (pF) molecules having average lengths up to 170 repeat units were transported to and captured by trap groups at the ends in less 40 ns. Almost all of the triplets attached to the chains reached the trap groups, ruling out the presence of substantial numbers of defects that prevent transport. The transport yields a diffusion coefficient D of at least 3 × 10–4 cm2 s–1, which is 30 times typical molecular diffusion and close to a value for triplet transport reported by Keller (J. Am. Chem. Soc.2011, 133, 11289–11298). The triplet states were created in solution by pulse radiolysis; time resolution was limited by the rate of attachment of triplets to the pF chains. Naphthylimide (NI) or anthraquinone (AQ) groups attached to the ends of the chains acted as traps for the triplets, although AQ would not have been expected to serve as a trap on the basis of triplet energies of the separate molecules. The depths of the NI and AQ triplet traps were determined by intermolecular triplet transfer equilibria and temperature dependence. The trap depths are shallow, just a few times thermal energy for both, so a small fraction of the triplets reside in the pF chains in equilibrium with the end-trapped triplets. Trapping by AQ appears to arise from charge transfer interactions between the pF chains and the electron-accepting AQ groups. Absorption bands of the end-trapped triplet states are similar in peak wavelength (760 nm) and shape to the 760 nm bands of triplets in the pF chains but have reduced intensities. When an electron donor, N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD), is added to the solution, it reacts with the end-trapped triplets to remove the 760 nm bands and to make the trapping irreversible. New bands created upon reaction with TMPD may be due to charge transfer states.
On the basis of configuration interaction calculations, we first describe the nature of the lowest singlet and triplet excited states in oligothiophenes ranging in size from two to six rings. We calculate the vertical excitation energies from the singlet ground state S0 to the first one-photon allowed singlet excited state S1 as well as the energy difference between the ground state and the lowest triplet state T1. The computed transition energies are in very good agreement with the measured values and indicate a strong confinement of the lowest triplet. We also uncover the nature of the higher-lying triplet excited state Tn that is coupled via a large oscillator strength to T1. The evolution with chain length of the T1−Tn excitation energies compares well with the experimental evolution based on photoinduced absorption data. Next, we investigate the geometry relaxation phenomena occurring in the S1 and T1 states; more pronounced and localized bond-length deformations are calculated in the triplet state than in the singlet, confirming the more localized character of T1. We also analyze the influence on the lowest excited states of grafting electroactive end-groups on the conjugated path of terthiophene. Finally, the various mechanisms involved in the nonradiative decay of the singlet excitations are discussed, and results are presented as a guide toward the optimization of light emission efficiency in conjugated systems.
The valence effective Hamiltonian (VEH) technique is used to compute ionization potentials, optical transition energies, and electron affinities of oligomers and polymers in four conjugated systems: polyacetylene, poly(p-phenylene), polythiophene, and polypyrrole. The theoretical results compare very favorably with experimental data on gas-phase ionization potentials, optical absorption, and electrochemical redox potentials. The latter case is especially important, and the calculated oxidation and reduction potentials are in remarkably good agreement with experiment. For polyacetylene the predicted oxidation potential is 0. 4 v vs. SCE, and the predicted reduction potential is minus 1. 1 v, both of which are in good agreement with experimentally observed oxidation and reduction onsets. In these systems, the electronic and electrochemical properties predicted by VEH theory for the oligomers extrapolate to those of the polymer with an inverse chain-length dependence.
Thiophene oligomers nT with n = 2-6 repeat units were studied in solution upon excitation with UV pulses (349 nm, 100 mu J, 25 ps). Each nT showed transient absorption spectra S-n <-- S-I, induced fluorescence, and delayed appearing triplet absorption bands. The transient bands shift bathochrom with increasing repeat units n. Band shapes and extinction coefficients of the transient singlet and triplet bands were determined. The transient S-n <-- S-I absorption bands decay with a single-exponential function. The decay times agree with the corresponding fluorescence lifetimes of the nT. They increase from 51 ps for 2T to 1100 ps for 6T with the oligomer size. The triplet bands appear during the decay of the S-n <-- S-I bands. The spectral shift of transitions from the ground and excited singlet and triplet states can be excellently described by the extended FEMO model. Even average bond lengths of the conjugated chain of oligothiophenes at ground and excited states could be estimated using the FEMO model.
The electrooxidation of alpha,alpha-coupled thiophene oligomers with terminal alpha-(CH3)3Si groups (alpha-TMS) and beta-CH3 groups was studied in methylene chloride at room temperature. The oligomers with four, five, six, seven, and eight thiophene rings undergo two stepwise oxidations to produce the radical cation and dication, respectively, as confirmed by the ESR spectra. The redox waves are chemically reversible and separated by 180 mV for the hexamer, heptamer, and octamer, suggesting that the same electrochemical behavior should be observed with the next higher oligomers or with the pi-conjugated segments of polythiophene. Thus it is proposed that the broad, featureless voltammogram observed with films of polythiophene is not an inherent property of the polymer segments but may reflect complications from the solid-state nature of the film. Radical cations and dications of some of the oligomers were succesively generated with stoichiometric amounts of FeCl3 in CH2Cl2 and characterized by vis-near-IR. The energies of the sharp absorption bands in the visible and near-infrared of the neutral and oxidized oligomers are found to scale linearly with the inverse of the oligomer size. Extrapolation of the absorption energy to infinite chain length leads to excellent agreement with the values for neutral poly(methylthiophene), but significant differences are observed with those for the oxidized forms. The implications of this difference on the delocalization length of the polymer radical cation and dication are discussed. Some of the electronic transition assigments for the oxidized thiophene oligomers which appear in previous reports are reinterpreted.
On the basis of an extended series of monodisperse oligomers of the dialkoxy-substituted phenyleneethenylenes 1a–i the Eqs. (3) and (4) were conceived in order to determine the limiting values of the energies Ei and the wavelengths λi of the UV/vis absorption. The convergence of the Ei, and λi values with a growing number n of repeating units permits a precise prediction of the Ei,∞ and λi,∞ values of the corresponding polymer 1j as well as a statement about the overall effect of conjugation ΔEi and the effective conjugation length ECL. A great variety of different conjugated oligomers 2–14 can be evaluated by the same algorithm.
Keto defect sites play a key role as the source of low-energy emission bands in polyfluorene type materials. The formation of fluorenone defect sites can be regarded as a dominant degradation mechanism in light-emitting devices based on polyfluorenes. The superiority of difunctionalization (see Figure) at the methylene group in -CR2- bridged polyphenylene and polyarylene derivatives is illustrated.
Abstract— The extinction coefficient εT, of triplet benzophenone in benzene has been directly determined by absolute measurements of absorbed energy and triplet absorbance, ΔD0T, under demonstrably linear conditions where incident excitation energy, E0, and ground state absorbance, A0, are both extrapolated to zero. The result, 7220 ± 320 M-1 cm-1 at 530 nm, validates and slightly corrects many measurements relative to benzophenone of triplet extinction coefficients made by the energy transfer technique, and of triplet yields obtained by the comparative method.
As E0 and A0 both decrease, ΔD0T becomes proportional to their product. In this situation, the ratio R= (1/A0)(dΔD0T/dE0) = (εT - εG)φT. Measurements of R, referred to benzophenone, give (εT - εG)φT for any substance, without necessity for absolute energy calibration.
Both absolute and relative laser flash measurements on zinc tetraphenyl porphyrin (εT - εG at 470 nm = 7.3 × 104M-1 cm-1) give φT= 0.83 ± 0.04.
Electrochemical properties of monodisperse oligofluorenes (OFn, n = 2 to 7) and corresponding polyfluorene were studied by cyclic and differential pulse voltammetry. In combination with data of UV-vis absorption spectra, a series of linear relations such as the band gap, the oxidation potential, the ionization potential, and the electron affinity with the reciprocal number of the fluorene units (1/n) were deduced. When a chain length of ca. 14 repeat units is reached, a stable structure of about one positive charge per 3.5 repeat units is obtained.
Time-resolved exciton diffusion measurements of fluorescence coupled with a TiO2 quencher, in poly(3-hexylthiophene) (P3HT), was described. Ellipsometry was performed on P3HT films over a range of thicknesses and the data used to construct a uniaxial model from which the absorption as a function of film thickness was calculated. The PL decays show that, over all four thicknesses there is a clear trend with the P3HT on TiO2 decaying faster than its fused silica counterparts. Ultrafast electron transfer to TiO2 is demonstrated from MEH-PPV and a thiophene oligomer, supporting the use of an infinite quencher in the proposed model. Internal conversion to the lowest state is found to be the dominant relaxation pathway of the higher energy exciton in a polythiophene derivative.
A comprehensive review of the literature on electron transport materials (ETMs) used to enhance the performance of organic light-emitting diodes (OLEDs) is presented. The structure-property-performance relationships of many classes of ETMs, both small-molecule-and polymer-based, that have been widely used to improve OLED performance through control of charge injection, transport, and recombination are highlighted. The molecular architecture, electronic structure (electron affinity and ionization potential), thin film processing, thermal stability, morphology, and electron mobility of diverse organic ETMs are discussed and related to their effectiveness in improving OLED performance (efficiency, brightness, and drive voltage). Some issues relating to the experimental procedures for the estimation of relevant material properties such as electron affinity and electron mobility are discussed. The design of multifunctional electroluminescent polymers whereby light emission and electron-and hole-transport properties are combined in one material to achieve efficient single-layer OLEDs is also discussed. The review concludes with a brief perspective on the challenges that future research should address.
Electron–electron interactions in organic semiconductors split the lowest singlet and triplet states by the exchange energy, ΔEST. Measurement of singlet and triplet emission spectra in a large number of conjugated polymers yield an almost constant ΔEST value close to 0.7 eV. This is in contrast to the situation in molecules, where the exchange energy is found to depend on molecular size and to vary over a wide range. Quantum-chemical calculations are performed to address the origin of the constant exchange energy in phenylene-based conjugated polymers. The electron–hole separation in the lowest singlet and triplet excited states is found to be independent of the π-conjugated backbone, and saturates for chains longer than a few repeating units, resulting in a constant exchange energy. In shorter conjugated oligomers, confinement of the excitations destabilizes the singlet with respect to the triplet through exchange interactions and leads to a larger and size-dependent singlet–triplet energy separation.