Article

Aminonaphthalic Anhydrides as Red-Emitting Materials: Electroluminescence, Crystal Structure, and Photophysical Properties

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  • AFRL/UES Inc
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Abstract

The red and orange emitters (ANA-1-3) consisting of a 4-amino-1,8-naphthalic anhydride group were synthesized. The lowest absorption band of these ANA molecules centered at approximately 450 nm is assigned to be a charge-transfer transition with emission at 514-536 nm in nonpolar solvents such as n-hexane and at approximately 590-640 nm in polar solvents such as THF and CH(2)Cl(2) and in the solid states. Emission lifetimes are measured with time-correlated single photon counting. Shorter lifetimes are observed for the ANA molecules when dissolved in polar solvents compared with those in nonpolar solvents. Strong dipole-dipole interaction of ANA molecules with solvents is indicated. At high concentrations the measured emission lifetimes, generally shortened from self-quenching, are found to remain about the same order of magnitude in ANAs. This implies that the exciton states of aggregates are formed and they exhibit a relatively long lifetime. Crystallographic data of 4-(phenyl antracen-9-yl) (ANA-2) and 4-(phenyl-2-naphthyl) amino-1,8-naphthalic anhydrides (ANA-3) show that the molecules exist as dimeric structures with antiparallel head-to-tail stacking of naphthalic anhydride planes in addition to other pi-pi stacking. The strong dipole-dipole interactions and the pi-pi stacking account for the observed red-shifted emissions of ANAs in the powders. For films prepared from vacuum sublimation, a structure similar to that in the crystal but with less crystalline order is expected based on the emission wavelength. Several electroluminescent devices based on these ANAs are reported here; they emit orange-red light at 602-628 nm with high brightness and steady external quantum efficiency.

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... In general, the ICT absorption maxima depended on the conjugation lengths and the electronic effects of the functional group attached to the oxadiazole moiety. Table S1), suggesting the absence of positive solvatochromism due to their relatively low polarity in the ground state [35][36][37][38][39]. Figure 3) which was due to the dipolar stabilization of the highly polar excited state [35][36][37][38][39]. ...
... In general, the ICT absorption maxima depended on the conjugation lengths and the electronic effects of the functional group attached to the oxadiazole moiety. Table S1), suggesting the absence of positive solvatochromism due to their relatively low polarity in the ground state [35][36][37][38][39]. Figure 3) which was due to the dipolar stabilization of the highly polar excited state [35][36][37][38][39]. ...
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Crystal structures of isomeric N-(X-picolyl)-1,8-naphthalimides (XD2,3,4) are compared with that of N-(benzyl)-1,8-naphthalimide. The comparison on the disposition of aromatic rings from torsion angles determined theoretically and experimentally suggests that the packing patterns in all these four derivatives except in the case of N-(2-picolyl)-1,8-naphthalimide are controlled by �–� and C–H,� interactions. In the case of N-(2-picolyl)-1,8-naphthalimide electronic factor decides the packing pattern. Crystal structures of N-(X-picolinium)- 1,8-naphthalimide perchlorates (XD2, 3, and 4) show that these compounds self-assemble through intermolecular hydrogen bonding involving the pyridinium N+–H, CBO and the perchlorate anions. Aromatic �-stacking interactions between the 1,8-naphthalimide units are also observed in the structures of the perchlorate salts. The compounds N-(2-picolinium)-1,8-naphthalimide perchlorate and N-(3-picolinium)-1,8-naphthalimide perchlorate are characterized by hydrogen bonded dimeric motifs that are held by intermolecular N+–H,OBC and C–H,� interactions; however, intermolecular interactions in N-(4-picolinium)-1,8-naphthalimide perchlorate result in the formation of hydrogen bonded polymeric structure
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The photophysical, electrochemical, surface morphology and thermal properties of two novel blue lightemitting materials were studied. Results indicate that the molecules offer potential as non-doping light-emitting materials with good electron injection capabilities for fabrication of blue organic light-emitting diodes.
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Novel blue light-emitting materials were designed by the substitution at the 4-position of 1,8-naphthalimide with electron-donating phenoxy group. The effect of molecular structure on the photophysical, electronic structure properties of the derivatives was explored by UV-visible absorption spectroscopy, photoluminescence spectroscopy, cyclic voltammetry and quantum chemical calculations. Both UV-visible absorption and emission spectra of derivatives indicate that the emission is in blue region. Electrochemical studies of the molecules revealed that they have low-lying energy levels of the lowest unoccupied molecular orbital (LUMO) and energy levels of the highest occupied molecular orbital (HOMO) indicating that the derivatives possess good electron-transporting or hole-blocking properties. To further reveal the electronic structure and the optical properties, the structural and electronic properties of the synthesized derivatives were calculated. These results indicate that molecules may offer potential as dopants as well as non-doping light-emitting materials with good electron injection capabilities for fabrication of blue organic light-emitting diodes.
Article
A series of 1,8-naphthalimide derivatives has been designed to explore their optical, electronic, and charge transport properties as charge transport and/or luminescent materials for organic light-emitting diodes (OLEDs). The frontier molecular orbitals (FMOs) analysis have shown that the vertical electronic transitions of absorption and emission are characterized as intramolecular charge transfer (ICT) for electron-donating and aromatic groups substituted derivatives. However, the ICT character of the electron-withdrawing substituted derivatives is not significant. The calculated results show that their optical and electronic properties are affected by the substituent groups in 4-position of 1,8-naphthalimide. Our results suggest that 1,8-naphthalimide derivatives with electron-donating −OCH3 and −N(CH3)2 (1 and 2), electron-withdrawing −CN and−COCH3 (3 and 4), 2-(thiophen-2-yl)thiophene (5), 2,3-dihydrothieno[3,4-b][1, 4]dioxine (6), 2-phenyl-1,3,4-oxadiazole (7), and benzo[c][1,2,5]thiadiazole (8) fragments are expected to be promising candidates for luminescent materials for OLEDs, particularly for 5 and 7. In addition, 3 and 7 can be used as promising hole transport materials for OLEDs. This study should be helpful in further theoretical investigations on such kind of systems and also to the experimental study for charge transport and/or luminescent materials for OLEDs. The optical, electronic, and charge transport properties of 1,8-naphthalimide derivatives have been theoretically investigated. Eight derivatives have been designed by introducing different groups with the aim to design good candidates for luminescent and/or charge transport materials for organic light-emitting diodes (OLEDs).
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A detailed study on the photophysical properties of a series of alkoxy substituted diphenylbutadienes in solution and in the solid state providing a molecular level understanding of the factors controlling their solid-state luminescence behavior is reported. Our studies provide clear evidence for exciton splitting in the solid state resulting in red-shifted emission for this class of materials. The role of the number of alkoxy substituents and the alkyl chain length in controlling the nature of the molecular packing and consequently their fluorescence properties has been elucidated. Whereas in the di- and tri-alkoxy substituted derivatives, the solid-state fluorescence was independent of the length of the alkyl chains, in the monoalkoxy substituted derivatives, increasing the length of the alkyl chain resulted in a visual change in fluorescence from green to blue. On the basis of the analysis of the molecular packing in the single crystals, this difference could be attributed to fluorescence arising from aggregates with an edge-to-face alignment in the molecules possessing short alkyl chains (methyl and butyl) to monomer fluorescence in the long alkyl chain containing derivatives.
Article
Aggregation-induced emission compounds containing triphenylethene and tetraphenylethene moieties with high thermal stability and good device properties have been synthesized. Their maximum fluorescence emission wavelengths are 469–493 nm in solid states. The glass transition temperatures range from 138 to 180 °C and the decomposition temperatures are 495–557 °C. The unoptimized device fabricated with the triphenylethene compound combined with three tetraphenylethene groups as emitters turns on at 6 V and the maximum luminance is observed at 1908 cd/m2 and 15.5 V. The electroluminescence peak of the device is at 474 nm and the Commission Internationale de l’Eclairage (CIE) chromaticity coordinate values are (0.18, 0.31) at 10 V.
Article
A series of aminobenzanthrone derivatives, possessing a keto and an amino group on the aromatic ring, are synthesized and their photoluminescence (PL) and electroluminescence (EL) properties are studied in detail. These compounds emit strongly in solution and in the solid state, with the emission maxima in the range of 528–668 nm resulting from charge-transfer transitions from the amino group to the keto moiety. The emission wavelength depends greatly on the polarity of the solvent. A red shift of nearly 100 nm is observed from n-hexane to dichloromethane for each of these compounds. The PL quantum yields of these molecules also depend tremendously on the solvent. The values are between 88 and 70 % in n-hexane and decrease as the polarity of the solvent increases. The single-crystal X-ray diffraction data reveal that the aminobenzanthrone planes of these molecules stack in the crystals in an antiparallel head-to-tail fashion. This strong dipole–dipole interaction accounts for the observed red-shifted emissions of the aminobenzanthrone molecules in powders and in films relative to those in nonpolar solvents. Electroluminescent devices using aminobenzanthrone derivatives as the host emitters or dopants emit orange to red light in the range 590–645 nm. High brightness, current efficiency, and power efficiency are observed for some of these devices. For example, the device using N-(4-t-butylphenyl)-N-biphenyl-3-benzanthronylamine as the emitter gives saturated red light with a current efficiency of 1.82 cd A–1, brightness of 11 253 cd m–2, and Commission Internationale de l'Éclairage (CIE) coordinates of (0.64,0.36); the device using N-(2-naphthyl)-N-phenyl-3-benzanthronylamine as the emitter gives orange–red light with a current efficiency of 3.52 cd A–1, brightness of 25 000 cd m–2, and CIE coordinates of (0.61,0.38).
Article
The synthesis and characterisation of a series of novel 4-acylamino and 4-alkylamino-N-1,8-naphthalimides is described. The UV-visible absorption and emission properties of the compounds are reported. Significant solvent effects are noted for 4-n-butyl-9-n-butyl-1,8-naphthylimide. The incorporation of acetyl and chloroacetyl groups into the 4-substituent markedly increases the fluorescence quantum yield compared with 4-alkylamino substituemnts.
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A novel intramolecular donor–acceptor system of four isomers consisting of 7H-benzimidazo(2,1-a)benz(d,e)isoquinolin-7-ones and diarylamine units was synthesized and characterized; the absorption and fluorescence spectra of the system in a variety of solvents were investigated. Intramolecular charge transfer was confirmed within the system by virtue of shifts in emission maximum with increasing solvent polarity; a high dipole moment for the intramolecular excited state was calculated using the Lippert equation. Shorter lifetimes were observed in polar solvents compared with those in non-polar solvents, indicating strong dipole–dipole interactions occurred. The ground-state geometry, lowest energy transition and the UV–vis spectrum of the system were studied using density functional theory and time-dependent density functional theory at B3LYP/6-31G∗ level, which showed that the calculated outcomes were in good agreement with experimental data.
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Substitution at the 4-position of 1,8-naphthalimide with electron-donating phenoxy or tert-butyl modified phenoxy groups, novel naphthalimide derivatives were obtained which emitted blue fluorescence with emission peaks of 425–444 nm in chloroform solution under UV irradiation, with highest relative photoluminescence quantum efficiency of 0.82. When in solid film, only compounds that contained ortho-tert-butylphenoxy substituents displayed blue photoluminescence of 438–451 nm, with highest absolute fluorescence quantum yield of 0.29; whereas other compounds showed greenish blue fluorescence at 471–478 nm, with highest absolute fluorescence quantum yield of 0.42. Cyclic voltammetry studies revealed that the molecules have low-lying energy levels of the lowest unoccupied molecular orbital (LUMO) ranging from −3.29 eV to −3.24 eV, and energy levels of the highest occupied molecular orbital (HOMO) ranging from −6.26 eV to −6.16 eV, suggesting they may possess good electron-transporting or hole-blocking properties. The findings indicate that the molecules offer potential as dopants as well as non-doping light-emitting materials with good electron injection capabilities for fabrication of blue or greenish blue organic light-emitting diodes.
Article
New thermally stable aggregation-induced emission enhancement compounds were synthesized. A non-doped red device showed a maximum brightness of 13,535 cd m(-2), a maximum current efficiency of 6.81 cd A(-1), a maximum power efficiency of 4.96 lm W(-1) and a low turn-on voltage of 4.3 V.
Article
A series of p-nitro-p'-alkoxy(OR)-substituted (E,E,E)-1,6-diphenyl-1,3,5-hexatrienes (1a, R = Me; 1b, R = Et; 1c, R = n-Pr; 1d, R = n-Bu) were prepared. The absorption and fluorescence spectra in solution were almost independent of the alkoxy chain length. The absorption maximum showed only a small dependence on the solvent polarity, whereas the fluorescence maximum red-shifted largely as the polarity increased. The solid-state absorption and fluorescence spectra were red-shifted relative to those in low polar solvents and were clearly dependent on the alkoxy chain length. The fluorescence maxima for the crystals of 1b and 1d were observed at 635-650 nm, which were red-shifted by 40-50 nm relative to those for 1a and 1c. The Stokes shifts were all relatively small (3000-3500 cm-1). For all four compounds, the fluorescence decay curves in the solid state were able to be analyzed by single-exponential fitting to give the lifetimes of 1.1-1.3 ns. This indicates that the emission of 1a-d is not originated from an excimer or molecular aggregates, but from only one emitting monomeric species. The fluorescence quantum yields of 1a-d were considerably high compared with the values for organic solids, which is consistent with their monomeric origin of emission. Single-crystal X-ray structure analyses of 1a, 1c, and 1d showed that the crystal packing was dependent on the alkoxy chain length. The crystals of 1a and 1c had herringbone structure, whereas that of 1d had pi-stacked structure. Strong pi-pi interaction in the crystal of 1d would be the cause of the spectral red shifts relative to those for 1a and 1c. No observation of excimer fluorescence from crystal 1d can be attributed to the limited overlap between the pi-planes of the molecules due to its "slipped-parallel" structure.
Article
The photophysical properties of two newly synthesized photoactive compounds with asymmetrical D-π-A structure and symmetrical D-π-A-π-D structure are investigated in different aprotic solvents by steady-state and femtosecond fluorescence depletion measurements. It is found that the asymmetrical DA compound has larger dipole moment change than that of the symmetrical DAD compound upon excitation, where the dipole moments of the two compounds have been estimated using the Lippert-Mataga equation. Furthermore, the steady-state spectral results show that increasing solvent polarity results in small solvatochromic shift in the absorption maxima but a large red shift in the fluorescence maxima for them, indicating that the dipole moment changes mainly reflect the changes of dipole moment in excited-state rather than in ground state. The red-shifted fluorescence band is attributed to an intramolecular charge transfer (ICT) state upon photoexcitation, which could result in a strong interaction with the surrounding solvents to cause the fast solvent reorganization. The resulting ICT states of symmetrical compounds are less polar than the asymmetrical compounds, indicating the different extents of stabilization of solute-solvent interaction in the excited state. Femtosecond fluorescence depletion measurements are further employed to investigate the fast solvation effects and dynamics of the ICT state of these two novel compounds. The femtosecond fluorescence depletion results show that the DA compound has faster solvation time than that of DAD compound, which corresponds to the formation of relaxed ICT state (i.e., a final ICT state with rearranged solvent molecules after solvation) in polar solvents. It is therefore reasonably understood that the ICT compounds with asymmetrical (D-π-A) structure have better performance for those photovoltaic devices, which strongly rely on the nature of the electron push-pull ability, compared to those symmetrical compounds (D-π-A-π-D).
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The potential of 4-aminophthalimide (AP) and its derivatives as fluorescence probes for organized media is highlighted. The fluorescence response of AP, as measured from the position of the fluorescence maximum, fluorescence intensity and lifetime, is highly sensitive to the polarity of the medium. The sensitivity of the fluorescence parameters is further enhanced due to the involvement of the emitting intramolecular charge transfer state in hydrogen bonding interaction with the solvent molecules containing hydroxyl groups. It is shown that the microheterogeneous environments of organized media such as cyclodextrins and micelles can be very conveniently monitored using this probe. The results of the investigations carried out employing AP and its derivatives as fluorescence probe molecules in these media clearly suggest that a combination of the hydrophobic interaction with the host media and hydrogen bonding interaction with the solvent molecules determine the location of the fluorophore, which in all cases is found to be the interfacial region separating the nonpolar core of the micelle or the cyclodextrin cavity and the polar aqueous environment. Guidelines for the design of probes of this class of systems for the nonpolar core region of the micelles are provided and possible ways to increase the sensitivity of the fluorescence response of the systems are suggested.
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Using the Langmuir-Blodgett technique, we fabricated a double-heterostructure electroluminescent (EL) device with a J-aggregated cyanine-dye bimolecular layer as an emitting layer. The EL cells were composed of an indium tin oxide electrode, a diamine hole-transport layer, a J-aggregated cyanine-dye emitting layer, an oxadiazole electron-transport layer and an MgAg electrode. Efficient EL was observed in the EL device owing to the confinement of charge carriers and excitons within the cyanine-dye bimolecular layer. The EL spectrum corresponded well to resonance fluorescence of the J-aggregate.
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Es wird eine spektroskopische Methode angegeben, mit der das Dipolmoment des ersten angeregten Elektronenzustandes von fluoreszenzfähigen Molekülen in polaren Lösungsmitteln bestimmt werden kann. Einige Beispiele enthält Tab. 2. Der Anteil der polaren Resonanzstruktur am angeregten Zustand von 4-Dimethylamino-4′-Nitrostilben und 4′-Cyanostilben wird zu ≳ 50% abgeschätzt.
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Several devices using a europium complex Eu(TTA)3(DPPz)(TTA = 2-thenoyltrifluoroacetonate, DPPz = dipyrido[3,2-a:2′,3′-c]phenazine) as dopant emitter were fabricated. The performances of these devices are among the best reported for devices incorporating a europium complex as a red emitter. One such device with structure TPD (50 nm)/Eu:CBP (4.5%, 30 nm)/BCP (30 nm)/Alq (25 nm) exhibits an external quantum efficiency 2.1%, current efficiency 4.4 cd/A, power efficiency 2.1 lm/W, and brightness 1670 cd/m2. © 2002 American Institute of Physics.
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Electroluminescent devices based on organic materials are of considerable interest owing to their attractive characteristics and potential applications to flat panel displays. After a brief overview of the device construction and operating principles, a review is presented on recent progress in organic electroluminescent materials and devices. Small molecular materials are described with emphasis on their material issues pertaining to charge transport, color, and luminance efficiencies. The chemical nature of electrode/organic interfaces and its impact on device performance are then discussed. Particular attention is paid to recent advances in interface engineering that is of paramount importance to modify the chemical and electronic structure of the interface. The topics in this report also include recent development on the enhancement of electron transport capability in organic materials by doping and the increase in luminance efficiency by utilizing electrophosphorescent materials. Of particular interest for the subject of this review are device reliability and its relationship with material characteristics and interface structures. Important issues relating to display fabrication and the status of display development are briefly addressed as well. # 2002 Elsevier Science B.V. All rights reserved.
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We demonstrate high-efficiency red electrophosphorescent organic light-emitting devices employing bis(2-(2<sup>′</sup>- benzo[4,5-a]thienyl)pyridinato-N,C <sup> 3 <sup> ′ </sup></sup>) iridium(acetylacetonate) [ Btp <sub> 2 </sub> Ir(acac) ] as a red phosphor. A maximum external quantum efficiency of η<sub> ext </sub>=(7.0±0.5)% and power efficiency of η<sub>p</sub>=(4.6±0.5)  lm/W are achieved at a current density of J=0.01  mA/cm <sup> 2 </sup>. At a higher current density of J=100  mA/cm <sup> 2 </sup>, η<sub> ext </sub>=(2.5±0.3)% and η<sub>p</sub>=(0.56±0.05)  lm/W are obtained. The electroluminescent spectrum has a maximum at a wavelength of λ<sub> max </sub>=616  nm with additional intensity peaks at λ<sub> sub </sub>=670 and 745 nm. The Commission Internationale de L’Eclairage coordinates of (x=0.68, y=0.32) are close to meeting video display standards. The short phosphorescence lifetime (∼4 μs) of Btp <sub> 2 </sub> Ir(acac) leads to a significant improvement in η<sub> ext </sub> at high currents as compared to the previously reported red phosphor, 2,3,7,8,12,13,17,18-octaethyl-12H, 23H-prophine platinum (II) PtOEP with a lifetime of ∼50 μs. © 2001 American Institute of Physics.
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Organic electroluminescent devices with a double‐heterostructure indium‐tin‐oxide substrate/hole transport layer/emitter layer/electron transport layer/MgAg have been fabricated by vacuum vapor deposition. The organic carrier transport and emitter layers were composed of amorphous films. In the double‐heterostructure devices, the luminance continued to lie in high level, even when the emitter thickness was 50 Å. The confinement of charge carriers and molecular excitons within a narrow emitter layer was achieved.
Article
A novel red-emitting polymer, poly{1,4-phenylenevinylene-(4-dicyanomethylene-4H-pyran)-2,6-vinylene-1,4-phenylenevinylene-2,5-bis(dodecyloxy)-1,4-phenylenevinylene} (PM-PPV), was prepared by the Heck coupling reaction between 2-{2,6-bis[2-(4-bromophenyl)vinyl]pyran-4-ylidene}−malononitrile and 1,4-bis(dodecyloxy)-2,5-divinylbenzene. The EL device based on a single-layer structure (ITO/PM-PPV/Al) showed EL emission with a maximum at 652 nm and an external quantum efficiency of 0.004% at 0.12 mA/mm2, which is higher than that of MEH−PPV (0.0005%, at 0.15 mA/mm2) measured under the same conditions. From the electrochemistry and UV−vis spectroscopy, the HOMO and LUMO energy levels of PM−PPV were figured out to be −5.44 and −3.48 eV, respectively, which are lower than those of MEH−PPV at −4.98 and −2.89 eV, respectively. It is concluded that, by lowering the HOMO and LUMO levels in PM−PPV, the injection rates of the holes and electrons are more balanced than in MEH−PPV. To improve the efficiency, the PPV layer was inserted between anode and PM−PPV layer. The device based on bilayer structure (ITO/PPV/PM−PPV/Al) showed EL emission with a maximum at 646 nm and an external quantum efficiency of 0.05% at 0.18 mA/mm2. The efficiency of the bilayer device was higher than that of the single-layer device by 13 times. The CIE chromaticity coordinates of the bilayer device were x = 0.69 and y = 0.31, which are very close to the CIE chromaticity coordinates (x = 0.67, y = 0.33) of NTSC for the red color.
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We report that organic electroluminescence devices with a hole transporting emitting layer composed of distyrylarylene derivatives realized highly efficient and bright emission in the blue-green region. Luminous efficiency was obtained to be 2.1 lm/W in the low-luminance region (135 cd/m2) using an indium tin oxide/emitting layer/electron transporting layer/Mg:Ag structure. The external quantum efficiency was estimated to be about 1.5&percnt;. The highest luminance was obtained to be 4000 cd/m2 for the device.
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We report cyclic voltammetry measurements for the blue electroluminescent conjugated polymer poly(9,9-dioctylfluorene). Both oxidation and reduction potentials are determined and thus estimates of both the ionization potential Ip and electron affinity Ea of the polymer are obtained for the same sample under the same experimental conditions. We estimate Ip=5.80 eV and Ea=2.12 eV. These results disagree with the common assumption that Ea is, to good approximation, given by the difference between Ip and the optical gap. Measurements on indium tin oxide/polyfluorene/calcium light emitting diode structures are consistent with the deductions from the electrochemical data.
Article
A novel red luminescent material N,N-bis{4-[2-(4-dicyanomethylene-6-methyl-4H-pyran-2-yl)ethylene]phenyl}aniline (BDCM) with two (4-dicyanomethylene)-4H-pyran electron-acceptor moieties and a triphenylamine electron-donor moiety for application in organic light-emitting diodes (OLEDs) was synthesized. The resultant compound has a sterically well-hindered structure and a high fluorescence yield. The photoluminescence (PL) of this compound in solution and solid film and the electroluminescence (EL) have been studied. Based on its intense sterically hindered structure, the pure BDCM film prepared shows a bright red PL emission. The three-layered EL device with the structure ITO/CuPc/DPPhP/BDCM/Mg:Ag has a turn-on voltage of less than 4 V, which suggests that BDCM has an excellent electron injection property. A bright luminance of 582 cd m−2 is obtained for the device at 19 V.
Article
An isopropyl substituted red fluorescent dye, 4-(dicyanomethylene)-2-i-propyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTI), has been found to be a good dopant in a host MQ3 matrix which produces organic EL devices with essentially identical luminance efficiency and chromaticity to that of the t-butyl derivative, DCJTB. The key intermediate, 4-(dicyanomethylene)-2-(I-propyl)6-methyl 4H-pyran, involved in the synthesis of DCJTI is much easier to manufacture than that of DCJTB. In the interest of driving down the cost of OLED materials, DCJTI may prove to be a cheaper alternative for the red dopant DCJTB.
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A stable non-polymeric organic glass with a high glass transition temperature has been prepared and its properties are reported here. The purest blue emission so far achieved can be obtained with a non-doped, low molecular weight bis(spirobifluorenyl) anthracene derivative. High-quality amorphous films of this compound with high morphological stability could be prepared by vapor deposition.
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With two novel second ligands, 2-(2-pyridyl)benzimidazole (HPBM) and 1-ethyl-2-(2-pyridyl)benzimidazole (EPBM), two europium complexes, Eu(DBM)3HPBM and Eu(DBM)3EPBM (DBM = dibenzoylmethanato), were prepared and used as emitting materials in organic electroluminescent (EL) devices. The devices with the structures ITO/TPD/Eu(DBM)3HPBM (or Eu(DBM)3EPBM)/Al and ITO/TPD/Eu(DBM)3EPBM/AlQ/Al emit red light originating from the europium complexes. The EL luminance of Eu(DBM)3EPBM is much higher than that of Eu(DBM)3HPBM. A maximum luminance of 180 cd m−2 in the triple layered device of Eu(DBM)3EPBM was achieved at 18 V.
Article
Several twisted intramolecular charge transfer (TICT) fluorescence dyad compounds containing 1,8-naphthalic anhydride derivatives have been synthesized in this paper, and their electroluminescence properties were observed. It is more important that stronger solid state fluorescence and enhancement fluorescence as well as large Stokes shift due to the TICT process have been observed in these dyad twisted compounds.
Article
9-Anthryl and 1-pyrenyl terpyridines (1 and 2, respectively), key precursors for the design of novel fluorescent sensors have been synthesized and characterized by 1H NMR, mass spectroscopy and X-ray crystallography. Twisted molecular conformations for each 1 and 2 were observed in their single crystal structures. Energy minimization calculations for the 1 and 2 using the semi-empirical AM1 method show that the ‘twisted’ conformation is intrinsic to these systems. We observe interconnected networks of edge-to-face CH⋯π interactions, which appear to be cooperative in nature, in each of the crystal structures. The two twisted molecules, although having differently shaped polyaromatic hydrocarbon substituents, show similar patterns of edge-to-face CH⋯π interactions.The presently described systems comprise of two aromatic surfaces that are almost orthogonal to each other. This twisted or orthogonal nature of the molecules leads to the formation of interesting multi-directional ladder like supramolecular organizations. A combination of edge-to-face and face-to-face packing modes helps to stabilize these motifs. The ladder like architecture in 1 is helical in nature.
Article
A class of anthracene derivative which is suitable used as emitting materials for producing efficient and stable blue emission for full color organic electroluminescence (EL) devices has been developed. Multilayer organic EL devices using these fluorescent materials as an emitting layer produced blue emissions with good chromaticity and luminous efficiency as high as 3.5 cd/A. The half life of 4000 h of blue emission EL device with initial light output 700 cd/m2 has been achieved. (C) 2002 American Institute of Physics.
Article
We have discovered two stericly hindered green coumarin derivatives which showed significantly better thermal stability and overall electroluminescence (EL) performance than those of the corresponding 3-(2-benzothiazolyl)-7-(diethylamino)-2H-1-benzopyran-2-one (C-6) and 10-(2-benzothiazolyl)-1,1,7,7-tetramethyl- 2,3,6,7-tetrahydro-1H,5H,11H-benzo[l]pyrano[6,7, 8-ij] quinolizin-11-one (C-545T). The strategic incorporation of bulky spacer t-butyl groups in these dopants considerably delays the onset of concentration quenching. In particular, 10-[2(5,7-di-t-butyl)benzothiazolyl]-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-benzo-[l]pyrano[6,7,8-ij]quinolizin-11-one (C-545TB) with its superb thermal stability (Tg = 142 °C; Td = 327 °C), high luminance efficiency (∼13 cd/A), good Commission Interationale d’Eclairage (CIE) coordinates (x = 0.30; y = 0.64) and a nearly flat EL efficiency dependence on drive current stands out as one of the best choice of green dopants for use in the passive-matrix organic light-emitting diode displays. © 2001 American Institute of Physics.
Article
Efficient electroluminescence was revealed in single-layer light-emitting diodes based on electron-hole conducting polymers containing the nanocrystalline phase of cyanine dyes (J-aggregates). These species exhibit a very narrow emission band with a maximum in the red to infrared spectrum range. The J-aggregates play an active role in the charge carrier transport of the composites. © 1999 American Institute of Physics.
Article
We demonstrate an integrated, two‐color organic light‐emitting device for flat‐panel display applications. The device utilizes a unique stacked pixel architecture which allows for independent tuning of color and intensity. Each of the two addressable colors is sufficiently bright for video display applications, so that the compact stacked pixel, which maximizes display resolution, may form the basis of a new type of full‐color display. © 1996 American Institute of Physics.
Article
The concept of a blue light emitting diode containing a hole blocking layer (HBL) is generalized to efficiently excite dye molecules introduced in the form of an emission layer on the hole transporting side. The concept allows the excitation of an emitting molecule even if the molecule does not act primarily as a recombination center. On the basis of this concept pure red emitting organic light emitting diodes were produced utilizing tris(dibenzoylmethane)(monophenanthroline)europium(III) [Eu(dbm)3phen] as the emitting layer in an Al/LiF/Aluminum tris(8-hydroxyquinoline) (Alq3)/HBL/Eu(dbm)3phen/N,N′-di(1-naphthyl)-N,N′-diphenylbenzidine (α-NPD)/copper-phthalo- cyanine (CuPc)/indium tin oxide structure. Additionally, the luminance output of such a device was improved by partially adding a hole transport material to the emitting layer in order to increase the penetration of holes into the emission region. A maximum luminance output of 200 cd/m2 at 15 V was achieved without optimizing the layer thickness. Furthermore, the effect of saturation of the Eu3+ emission is studied and discussed. The basic device architecture presented here is expected to work with other transport and emitting materials also. © 2001 American Institute of Physics.
Article
We constructed red organic electroluminescence (EL) devices with a reduced porphyrin compound, tetraphenylchlorin, that was doped at various concentrations (0.5, 1.7, and 3.7 wt %) within a tris(8-hydroxyquinoline) aluminum (Alq3) host layer. We measured their EL properties and found that all three devices emitted a red EL band at 660 nm with a width of 20 nm. Emission color of the 1.7 and 3.7 wt % devices was red (chromaticity coordinates x = 0.67, y = 0.29 and x = 0.78, y = 0.21) and the luminance maximum was 100 and 24 cdm−2, respectively. The 0.5 wt % device emitted a green Alq3 EL band as well, and showed an increase in relative intensity of the Alq3 emission with increasing applied voltage. © 1999 American Institute of Physics.
Article
The efficiency of electroluminescent organic light-emitting devices can be improved by the introduction of a fluorescent dye. Energy transfer from the host to the dye occurs via excitons, but only the singlet spin states induce fluorescent emission; these represent a small fraction (about 25%) of the total excited-state population (the remainder are triplet states). Phosphorescent dyes, however, offer a means of achieving improved light- emisssion efficiencies, as emission may result from both singlet and triplet states. Here we report high-efficiency (≤90%) energy transfer from both singlet ariel triplet states, in a host material dope d with the phosphorescent dye 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine platinum(II) (PtOEP). Our doped electroluminescent devices generate saturated red emission with peak external and internal quantum efficiencies of 4% and 23%, respectively. The luminescent efficiencies attainable with phosphorescent dyes may lead to new applications for organic materials. Moreover, our work establishes the utility of PtOEP as a probe of triplet behaviour and energy transfer in organic solid-state systems.
Article
The fluorescence of six secondary arenedicarboxamides has been investigated in single crystals, powders, melts, and ethanol solution. Single crystals of four rodlike arenedicarboxamides which pack in one-dimensional tapes display dual emission attributed to excited-state monomers and dimers. These tapes have a constant 5-Å separation between the long axes of neighboring arenes but differ in the arene−arene dihedral angle. The frequency shift between monomer and dimer fluorescence and the intensity and vibronic structure of the dimer emission are dependent upon the ground-state geometry. Single crystals of two naphthalenedicarboxamides which pack in two-dimensional sheets and have no close contacts between neighboring arenes within the same sheet display only monomer emission. Melted and resolidified samples display broad structureless emission attributed to fluorescence from a mixture of excited monomers and dimers with different ground-state geometries.
Article
Fluorescent emission and photodimerization of crystalline (1E,3E)-1,4-diphenylbutadiene (2, DPB), (1Z,3E)-1-cyano-1,4-diphenylbutadiene (3, CDPB) and (1Z, 3E)-1-cyano-3-methyl-1,4-diphenylbutadiene (4, CMDPB) have been studied. Although crystals of all the three dienes exhibit red-shifted fluorescence, only CDPB (3) crystals give [2 + 2] photoproduct on photolysis, despite having a rather long center-to-center distance of 5.04 Å between cyano-substituted double bonds. Large differences in the absorption and fluorescence excitation spectra are observed for crystalline dienes. For example, absorption by a solution of CDPB (3) in n-hexane is at 339 nm, while its crystals show excitation maximum at 414 nm. However, in solution, the excitation spectra are quite similar. Further, the excitation spectra of crystalline DPB (2) and CMDPB (4) are much broader and considerably blue shifted relative to that of CDPB (3). The difference in the photophysical behavior is related to the crystal structure differences and reactivity differences. The results are discussed in terms of the possible involvement of either excimers or ground state molecular aggregates in the observed photoprocesses.
Article
The aggregation properties of 4-(1-pyrene)butanoate in aqueous media have been quantitatively investigated. For the pyrene chromophore ground state, the monomer−dimer equilibrium was monitored using NMR methods, yielding a dimerization constant of 150 M-1 for pyrenebutanoate in alkaline water. In addition, the excited-state aggregation was studied by time-resolved emission spectroscopy. The excimer formation constant in water, which represents the equilibrium established by interaction of excited- and ground-state species, was determined to be 1.6 × 104 M-1 (about 100-fold larger than the ground-state dimerization constant). The role of solvent polarity and the influence of an aromatic cosolvent were examined in order to determine the types of interactions that contribute to the driving force for aggregation. The ground-state aggregation propensity of pyrenebutanoate was significantly diminished upon addition of methanol or pyridine to aqueous solutions, indicating that hydrophobic and/or π-stacking interactions play a role in the aggregation processes. Similar trends were observed also in the case of excited-state aggregation when the organic cosolvents were included. The investigation provides a quantitative assessment of the thermodynamics of interaction of pyrene fluorescence probes that are widely used in aqueous media in biophysical studies.
Article
Since 1987, high-luminance low-voltage driven devices based on tris(8-hydroxyquinoline)aluminum(III) (Alq3) opened the route to design low-cost large area displays and illuminators. Despite the large number of studies devoted to this material, very little is known about its basic structural and optical properties in the solid state. Therefore, we have investigated the structure(s) and the correlation between intermolecular interactions and optical properties in various Alq3 systems, including solution, amorphous thin films, and different crystalline forms. Two novel unsolvated polymorphs of Alq3, namely, α-Alq3 and β-Alq3, have been synthesized and their crystalline structures determined from X-ray diffraction data on powders (α) and single crystals (β). Crystals of α-Alq3 are triclinic, space group P-1, a = 6.2586(8) Å, b = 12.914(2) Å; c = 14.743(2) Å, α = 109.66(1)°; β = 89.66(1)°, and γ = 97.68(1)°; crystals of β-Alq3 are triclinic, space group P-1, a = 8.4433(6) Å, b = 10.2522(8) Å; c = 13.1711(10) Å, α = 108.578(1)°, β = 97.064(1)°, and γ = 89.743(1)°. Both these crystal structures consist of a racemic mixture of the mer isomer, but are characterized by different molecular packings involving well-defined short contacts between quinoxaline ligands belonging to symmetry-related Alq3 molecules with interligand spacings in the 3.5−3.9 Å range. A third “high-temperature” phase, γ-Alq3, was found to contain orientationally disordered mer-Alq3 molecules, lying about a 32 position of the trigonal P-31c space group, with a = 14.41(1) Å and c = 6.22(1) Å. In addition, a hemichlorobenzene adduct of Alq3 was also prepared and structurally characterized (monoclinic, space group P21/n, a = 10.786(1) Å, b = 13.808(2) Å, c = 16.928(2) Å, β = 97.90(2)°). Investigations of the different crystal phases, as well as of amorphous thin films and solutions by absorption, fluorescence excitation, fluorescence, and Raman spectroscopy, allowed the effect of the molecular packing on the emission properties to be elucidated, the nature of the photoexcitations to be clarified, and the vibrational fingerprints of the α and β crystalline forms to be highlighted. The spectral position of fluorescence is found to be correlated with both the molecular density of the packing and the length of interligand contacts between neigboring Alq3 molecules as a consequence of different dispersive and dipolar interactions as well as different π−π orbital overlaps (the shorter the contacts, i.e., the denser the crystal, the more the fluorescence is red-shifted). The low-temperature (4.2 K) vibronic structure of the fluorescence spectrum of Alq3 is resolved for the first time. It is assigned to the Franck−Condon activity of an in-plane bending mode at ca. 525 cm-1, and it is symptomatic of the ligand-centered nature of the optical transitions. From the analysis of the vibronic progression the existence of a strong electron−phonon coupling involving the 525 cm-1 mode with a Huang−Rhys factor of ca. 2.6 ± 0.4 is inferred. The origin of the amorphous nature of the vacuum-sublimed thin films is here explained on the basis of the accessibility of many different π−π links between homo- and heterochiral Alq3 molecules.
Article
Anthracenophanes and 1,2-dianthrylethanes were studied as models of excimers. It was found that (1) there are natively various structures of excimer, (2) the so-called "n = 3 rule" is not valid in the case of dianthrylalkanes, (3) the excimers of 1,2-dianthrylethanes show two different kinds of structures, and (4) excimer fluorescence lifetimes are reduced due to the photodimerization in the case of syn-[2.2](1,4)anthracenophane, [2.2](1,4)(9,10)anthracenophane, and 1,2-dianthrylethane (type 2).
Article
The synthesis and photophysical study of a family of cyclometalated iridium(III) complexes are reported. The iridium complexes have two cyclometalated (CN) ligands and a single monoanionic, bidentate ancillary ligand (LX), i.e., CN2Ir(LX). The CN ligands can be any of a wide variety of organometallic ligands. The LX ligands used for this study were all β-diketonates, with the major emphasis placed on acetylacetonate (acac) complexes. The majority of the CN2Ir(acac) complexes phosphoresce with high quantum efficiencies (solution quantum yields, 0.1−0.6), and microsecond lifetimes (e.g., 1−14 μs). The strongly allowed phosphorescence in these complexes is the result of significant spin−orbit coupling of the Ir center. The lowest energy (emissive) excited state in these CN2Ir(acac) complexes is a mixture of 3MLCT and 3(π−π*) states. By choosing the appropriate CN ligand, CN2Ir(acac) complexes can be prepared which emit in any color from green to red. Simple, systematic changes in the CN ligands, which lead to bathochromic shifts of the free ligands, lead to similar bathochromic shifts in the Ir complexes of the same ligands, consistent with “CN2Ir”-centered emission. Three of the CN2Ir(acac) complexes were used as dopants for organic light emitting diodes (OLEDs). The three Ir complexes, i.e., bis(2-phenylpyridinato-N,C2‘)iridium(acetylacetonate) [ppy2Ir(acac)], bis(2-phenyl benzothiozolato-N,C2‘)iridium(acetylacetonate) [bt2Ir(acac)], and bis(2-(2‘-benzothienyl)pyridinato-N,C3‘)iridium(acetylacetonate) [btp2Ir(acac)], were doped into the emissive region of multilayer, vapor-deposited OLEDs. The ppy2Ir(acac)-, bt2Ir(acac)-, and btp2Ir(acac)-based OLEDs give green, yellow, and red electroluminescence, respectively, with very similar current−voltage characteristics. The OLEDs give high external quantum efficiencies, ranging from 6 to 12.3%, with the ppy2Ir(acac) giving the highest efficiency (12.3%, 38 lm/W, >50 Cd/A). The btp2Ir(acac)-based device gives saturated red emission with a quantum efficiency of 6.5% and a luminance efficiency of 2.2 lm/W. These CN2Ir(acac)-doped OLEDs show some of the highest efficiencies reported for organic light emitting diodes. The high efficiencies result from efficient trapping and radiative relaxation of the singlet and triplet excitons formed in the electroluminescent process.
Article
4-Dicyanomethylene-chromene moiety has been introduced as a π-electron acceptor in red fluorescent dye molecules for organic light-emitting devices (OLEDs). On the basis of this moiety, a new family of red dopants with saturated emission, higher fluorescent quantum yield, and convenient synthetic procedures have been designed and synthesized. Their photoluminescent and electroluminescent properties have been examined and compared. On the basis of the results, useful guidelines for the molecular design of saturated red-emission fluorescent dopants for OLED applications are presented.
Article
Excimers are dimers with associated excited electronic states, dissociative ground states, and structureless emission spectra. Noble and other monatomic gases form atomic excimers. Aromatic molecules form excimers in fluid solutions, liquids, crystals and polymers, at crystal defects, and intramolecularly. Excimer interaction is attributed to configurational mixing of exciton and charge resonance states. The helium excimer and pyrene crystal dimer potential curves are compared. Aromatic excimers are discussed. Related photophysical studies on atomic excimers are considered. Saturated amines, which exhibit vapour and solution excimer fluorescence, provide a link between atomic and aromatic excimers. Aromatic molecules form complexes, exciplexes or mixed excimers with different molecules, and noble gas atoms form complexes or exciplexes with different atoms. Review completed in 1975.
Article
2,2′-bistriphenylenyl (BTP) was used as both a blue emitter and hole-transport material. It was demonstrated that BTP-based electroluminescent devices emit blue light with emission maximum at 440-460 nm with extremely high brightness, current efficiency, and excellent CIE coordinates.
Article
A convenient synthesis of new star-shaped, thieno[3,4-b]pyrazine-based molecules was developed. The molecules were found to be red-emitting as well as carrier-transporting. EL devices with saturated red emission were demonstrated.
Article
A novel molecular glass red fluorophore NPAMLI were synthesized and characterized. A red OLED, containing a non-doping NPAMLI red emitter which was simple and easy to fabricate was shown. As such, the performance of the device was comparable with or better than known OLEDs with red dopant. Since the synthetic flexibility, work was currently in progress to prepare other maleimide derivative with better properties, such as a narrower FWHM and higher fluorescence quantum yield.
Article
Various triarylamines can be readily prepared in excellent yields by palladium-catalyzed cross-coupling reaction of aryl halides and diarylamines. The amination reaction takes place rapidly by using the catalyst combination of Pd(OAc)2 and a bulky and electron-rich ligand. P(t-Bu)3.
Article
Electroluminescent (EL) devices are constructed using multilayer organic thin films. The basic structure consists of a hole‐transport layer and a luminescent layer. The hole‐transport layer is an amorphous diamine film in which the only mobile carrier is the hole. The luminescent layer consists of a host material, 8‐hydroxyquinoline aluminum (Alq), which predominantly transports electrons. High radiance has been achieved at an operating voltage of less than 10 V. By doping the Alq layer with highly fluorescent molecules, the EL efficiency has been improved by about a factor of 2 in comparison with the undoped cell. Representative dopants are coumarins and DCMs. The EL quantum efficiency of the doped system is about 2.5%, photon/electron. The EL colors can be readily tuned from the blue‐green to orange‐red by a suitable choice of dopants as well as by changing the concentration of the dopant. In the doped system the electron‐hole recombination and emission zones can be confined to about 50 Å near the hole‐transport interface. In the undoped Alq, the EL emission zone is considerably larger due to exciton diffusion. The multilayer doped EL structure offers a simple means for the direct determination of exciton diffusion length.
Article
Three derivatives of poly(paraphenylene) (PPP) have been synthesized, all with excellent solubility in common organic solvents. Efficient blue polymer light‐emitting diodes (LEDs) are demonstrated using these PPPs as the semiconducting and luminescent polymers. Double‐layer polymer LEDs (consisting of a hole transport layer in addition to the electroluminescent layer) emit blue light with external quantum efficiencies between 1% and 3% photons per electron, when using indium tin oxide as the anode and calcium as the cathode. Using internal field emission (Fowler–Nordheim tunneling) of single carrier devices for both electrons and holes, the energies of the top of the π band and the bottom of the π* band have been determined as, respectively, 5.7 and 2.3 eV below the vacuum. The operating voltages of these LEDs have been lowered by using a porous polyaniline anode, or by blending PPP with a hole transport material. LEDs using air stable cathodes, silver, indium, aluminum, and copper, were also demonstrated. Device quantum efficiencies, between 0.3% and 0.8%, were obtained. © 1996 American Institute of Physics.
Article
We describe the performance of an organic light-emitting device employing the green electrophosphorescent material, fac tris(2-phenylpyridine) iridium [ Ir(ppy) <sub> 3 </sub>] doped into a 4,4<sup>′</sup>-N,N<sup>′</sup> -dicarbazole-biphenyl host. These devices exhibit peak external quantum and power efficiencies of 8.0% (28 cd/A) and 31 lm/W, respectively. At 100 cd/m<sup>2</sup>, the external quantum and power efficiencies are 7.5% (26 cd/A) and 19 lm/W at an operating voltage of 4.3 V. This performance can be explained by efficient transfer of both singlet and triplet excited states in the host to Ir(ppy) <sub> 3 </sub>, leading to a high internal efficiency. In addition, the short phosphorescent decay time of Ir(ppy) <sub> 3 </sub> (≪1 μs) reduces saturation of the phosphor at high drive currents, yielding a peak luminance of 100 000 cd/m<sup>2</sup>. © 1999 American Institute of Physics.
Article
A novel electroluminescent device is constructed using organic materials as the emitting elements. The diode has a double‐layer structure of organic thin films, prepared by vapor deposition. Efficient injection of holes and electrons is provided from an indium‐tin‐oxide anode and an alloyed Mg:Ag cathode. Electron‐hole recombination and green electroluminescent emission are confined near the organic interface region. High external quantum efficiency (1% photon/electron), luminous efficiency (1.5 lm/W), and brightness (≫1000 cd/m<sup>2</sup>) are achievable at a driving voltage below 10 V.
Article
We demonstrate a general method for tuning the luminescent emission spectrum of dipolar molecules by adjusting the strength of intermolecular dipole–dipole interactions using a doped guest–host molecular organic thin film system. Spectral shifting is achieved by introducing dopants that change the local electric field in the film. This `solid state solvation effect' is used to continuously tune, by up to 75 nm, the peak emission wavelength of organic light emitting devices consisting of triarylamine host materials doped with polar luminescent dyes DCM2, aluminum tris(8-hydroxyquinoline), or both. Red, orange, yellow, green, blue, and white emission is demonstrated with the same set of organic molecules.
  • Kim Y.-H