Thermally Activated Delayed Fluorescence in a Y3N@C80 Endohedral Fullerene: Time‐Resolved Luminescence and EPR Studies

The endohedral fullerene Y₃N@C₈₀ exhibits luminescence with reasonable quantum yield and extraordinary long lifetime. By variable‐temperature steady‐state and time‐resolved luminescence spectroscopy, it is demonstrated that above 60 K the Y₃N@C₈₀ exhibits thermally activated delayed fluorescence with maximum emission at 120 K and a negligible prompt fluorescence. Below 60 K, a phosphorescence with a lifetime of 192±1 ms is observed. Spin distribution and dynamics in the triplet excited state is investigated with X‐ and W‐band EPR and ENDOR spectroscopies and DFT computations. Finally, electroluminescence of the Y₃N@C₈₀/PFO film is demonstrated opening the possibility for red‐emitting fullerene‐based organic light‐emitting diodes (OLEDs).     

Flexible Alkylene Bridges as a Tool To Engineer Crystal Distyrylbenzene Structures Enabling Highly Fluorescent Monomeric Emission

To design ultrabright fluorescent solid dyes, a crystal engineering strategy that enables monomeric emission by blocking intermolecular electronic interactions is required. We introduced propylene moieties to distyrylbenzene (DSB) as bridges between the phenyl rings either side of its C=C bonds. The bridged DSB derivatives formed compact crystals that emit colors similar to those of the same molecules in dilute solution, with high quantum yields. The introduction of flexible seven-membered rings to the DSB core produced moderate distortion and steric hindrance in the DSB π-plane. However, owing to this strategy, it was possible to control the molecular arrangement with almost no decrease in the crystal density, and intermolecular electronic interactions were suppressed. The bridged DSB crystal structure differs from other DSB derivative structures; thus, bridging affords access to novel crystalline systems. This design strategy has important implications in many fields and is more effective than the conventional photofunctional molecular crystal design strategies.     

Synthesis and property of diazocine derivatives substituted with imidazole in various positions

ABSTRACT

TIACA-I, TIACA-II were synthesized by changing the substitution position of the imidazole group in the diazocine core. TIACA-I, TIACA-II in the film state showed absorption in the range of 354 to 392 nm and exhibited blue photoluminescence (PL) emissions at 448 and 462 nm, respectively. The PL wavelength of TIACA-II is red-shifted by 14 nm than that of TIACA-I due to the electron-donating intensity depending on the position of the imidazole group. The use of TIACA-II in a non-doped OLED device resulted in blue emission with current efficiency of 2.84 cd/A and CIE of (0.15, 0.18).     

Substituent Effects on Crosslike Packing of 2′,7′‐Diaryl‐ spiro(cyclopropane‐1,9′‐fluorene) Derivatives: Synthesis and Crystallographic,Optical, and Thermal Properties

A series of 2′,7′‐diarylspiro(cyclopropane‐1,9′‐fluorene) derivatives are efficiently synthesized and characterized to determine the reason for the “green‐light” emission of these compounds. These compounds exhibit bright‐violet to blue photoluminescence (PL) (λ^PL_max = 353–419 nm) with excellent PL quantum efficiencies (Φ_PL = 83–100 %) in solution and show high thermal stabilities (T_d = 267–474 °C). The variation of the optical properties of these molecules in the solid state depends on the different stacking modes of these compounds containing different substituents, which are revealed by crystallographic analysis. CH…π hydrogen bonds instead of intermolecular π–π interactions act as the driving force between adjacent fluorenes, even though a very small dialkyl group (cyclopropane) is introduced at the C‐9 position of fluorene. The crosslike molecular stacking efficiently reduces the energy transfer between the herring‐like aggregates and therefore results in the absence of a “green‐light” emission tail. In order to determine the cause of the “green‐light” emission tails, the fluorescence spectra of the films annealed in N₂ or in air are recorded. Broad green‐light emission tails were observed for the films annealed in air, which might be caused by fluorenone defects generated during processing or during the course of the photophysical analysis by reaction with residual oxygen.     

Solution‐Processible Multi‐component Cyclometalated Iridium Phosphors for High‐Efficiency Orange‐Emitting OLEDs and Their Potential Use as White Light Sources

The synthesis and photophysical studies of several multifunctional phosphorescent iridium(III) cyclometalated complexes consisting of the hole‐transporting carbazole and fluorene‐based 2‐phenylpyridine moieties are reported. All of them are isolated as thermally and morphological stable amorphous solids. Extension of the π‐conjugation through incorporation of electron‐pushing carbazole units to the fluorene fragment leads to bathochromic shifts in the emission profile, increases the highest occupied molecular orbital levels and improves the charge balance in the resulting complexes because of the propensity of the carbazole unit to facilitate hole transport. These iridium‐based triplet emitters give a strong orange phosphorescence light at room temperature with relatively short lifetimes in the solution phase. The photo‐ and electroluminescence properties of these phosphorescent carbazolylfluorene‐functionalized metalated complexes have been studied in terms of the coordinating position of carbazole to the fluorene unit. Organic light‐emitting diodes (OLEDs) using these complexes as the solution‐processed emissive layers have been fabricated which show very high efficiencies even without the need for the typical hole‐transporting layer. These orange‐emitting devices can produce a maximum current efficiency of ～ 30 cd A^–1 corresponding to an external quantum efficiency of ～ 10 % ph/el (photons per electron) and a power efficiency of ～ 14 lm W^–1. The homoleptic iridium phosphors generally outperform the heteroleptic counterparts in device performance. The potential of exploiting these orange phosphor dyes in the realization of white OLEDs is also discussed.     

Soluble porphyrin-bisindolylmaleimides dyad and pentamer as saturated red luminescent materials

Novel soluble porphyrin-bisindolylmaleimides dyad and pentamer have been prepared conveniently by coupling of amino-porphyrins and bisindolylmaleic anhydride. The bisindolylmaleimide groups function as the antenna to efficiently enhance the intramolecular energy transfer to the porphyrin core. These compounds may serve as good candidates of red-light emitting materials for organic light-emitting diodes.     

有机层界面对有机电致发光器件性能的影响

采用真空蒸镀的方法,制备了以ITO/2T-NATA(15 nm)/NPB(25 nm)/Alq3(30nm)/LiF(1 nm)/Al(100 nm)为基本结构的绿光器件,实验中在NPB(25 nm)与Alq3(30 nm)有机层界面处加入周期性不同的NPB(10 nm)/Alq3(10 nm)结构的有机层.通过实验测得的数据,研究了周期性的空穴传输层与发光层结合这种特殊结构对绿光器件发光性能的影响.根据实验结果,发现在有机层界面处,加入周期不同的NPB(10 nm)/Alq3(10 nm)层虽然会提高器件的起亮电压,但会改善器件的发光效率,而对器件的发光波长与发光区域以及发光亮度影响不大.     

Effects of transparent bottom electrode thickness on characteristics of transparent organic light-emitting devices

We conduct both simulation and experiment studies of impacts of simultaneously varying the thicknesses of transparent bottom electrodes and semi-transparent top thin metal electrodes on optical characteristics (e.g., transmission, reflection) and efficiencies of transparent organic light-emitting devices (OLEDs). For the thickness range of both electrodes studied, the total electroluminescent (EL) efficiencies (including both bottom and top emission), EL spectra, and emission patterns remain similar; yet the ratio of top to bottom emission would be modulated by the semi-transparent top metal electrode thickness. The thickness of the transparent bottom electrode has weak effects on the efficiencies of top/bottom/total emission, but it does have definite effects on optical transmission/reflection spectra (e.g. peak/valley wavelengths). Meanwhile, the thickness of semi-transparent top metal electrodes mainly affect magnitudes of the optical transmission/reflection. Transmissive/reflective hues and appearances of the transparent OLEDs can thus be tuned by the thicknesses of bottom/top electrodes. Overall, we demonstrated efficient transparent green phosphorescent OLEDs exhibiting a high peak transmittance of up to 81% and rather high total external quantum efficiencies of up to 21–21.5% (corresponding to a total current efficiency of 80–82 cd/A and total power efficiency of 95–99 l m/W), among the highest (if not the highest) for planar transparent OLEDs using no other optical out-coupling structures. By varying the thicknesses of transparent bottom electrodes and semi-transparent top metal electrodes, the ratio of top to bottom emission, and the transmissive or reflective hues/appearances of transparent OLEDs in the off state can be tuned, yet without sacrificing total EL efficiencies or changing their EL colors/patterns. Such tunable optical characteristics of transparent OLEDs may find some interesting applications.     

Theoretical study of the electronic ground states and low-lying singlet excited states of thiophene-based spirofluorenes

The electronic structures and optical properties of oligothiophene substituted spirofluorenes are investigated theoretically with semi-empirical quantum chemical calculations. A theoretical investigation of the interaction between two perpendicular π-systems of various oligothiophene substituted spirofluorenes is conducted. The results demonstrate that the interaction between two perpendicular branches is reduced by oligothiophene substitutions. Photoexcitation induced relaxation is mainly located on one of...     

Atomic layer deposited TiOx/AlOx nanolaminates as moisture barriers for organic devices

Atomic layer deposited nanolaminates of alternating AlO_x and TiO_x thin-films are investigated as moisture barriers for organic electronic devices. Direct encapsulation on organic light emitting diodes (OLEDs) is tested in aging experiments and compared to calcium corrosion tests of equivalent barrier films. This allows for a direct assessment of moisture barrier performance in simple as well as more complex systems. Thickness variations are performed for the nanolaminate single and total layer thickness, with an optimum single layer thickness of 1–2 nm observed. This correlates to the maximum number of dyads once completely closed single layers are produced. For large single layer thickness and low dyad count, strong lateral diffusion from the edges occurs in the OLEDs, which likely correlates to poor mechanical stability. At optimum single layer thickness, barriers remain mechanically and chemically stable up to 100 nm total thickness. OLEDs encapsulated with such nanolaminate barriers show no significant degradation after 2500 h of continuous aging.