Degradation of organic materials is responsible for the short operation lifetimes of organic light-emitting devices, but the mechanism by which such degradation is initiated has yet to be fully established. Here we report a new mechanism for degradation of emitting layers in blue-phosphorescent devices. We investigate binary mixtures of a wide bandgap host and a series of novel Ir(III) complex dopants having N-heterocyclocarbenic ligands. Our mechanistic study reveals the charge-neutral generation of polaron pairs (radical ion pairs) by electron transfer from the dopant to host excitons. Annihilation of the radical ion pair occurs by charge recombination, with such annihilation competing with bond scission. Device lifetime correlates linearly with the rate constant for the annihilation of the radical ion pair. Our findings demonstrate the importance of controlling exciton-induced electron transfer, and provide novel strategies to design materials for long-lifetime blue electrophosphorescence devices.

Phosphorescent Pt(II) complexes having tetradentate ligands have emerged as promising materials for use in organic light emitting devices (OLEDs). One drawback that retards the full exploitation of their electroluminescence is a strong propensity for bathochromically shifted emissions. The chromic shift results from intermolecular association. Molecular strategies that avoid the intermolecular interactions are needed, particularly for producing blue electrophosphorescence. We have designed and synthesized a series of phosphorescent cycloplatinate complexes having bis(1-pyrazolylphenyl)methane tetradentate ligands (PtSN1−3). The ligands have been systematically modified to incorporate methyl substituents at different positions of the pyrazole moieties (PtSN1, no methyl substituent; PtSN2, 4-methyl substituents; PtSN3, 3-methyl substituents) with the aim of preserving intrinsic sky-blue phosphorescence while suppressing intermolecular interactions. The synthetic modifications control the extent of out-of-plane distortions in the cycloplatinate scaffold. Excimer emission is obtained from PtSN1−2 of planar platinacycles in diluted solutions (10 μM) or in thin mCBP:TSPO1 (8 wt %) films (1:1, wt/wt; mCBP = 3,3-di(9H-carbazol-9-yl)biphenyl; TSPO1 = diphenyl(4-(triphenylsilyl)phenyl)phosphine oxide) due to strong spontaneous excimer formation with free energy changes of −4.4 to −3.1 kJ mol −1 . By contrast, helically distorted PtSN3 is capable of preserving its inherent sky-blue phosphorescence in concentrated states as it effectively suppresses excimer formation. An additional benefit of our synthetic control is improved stability against degradation for PtSN3. Finally, electroluminescence performances were evaluated by constructing and analyzing multilayer OLEDs employing the PtSN complexes as dopants. As expected, the PtSN3 devices produced sky-blue electrophosphorescence with a Commission Internationale de l'E ́clairage coordinate of (0.16, 0.24) and a peak external quantum efficiency of 8.5%. The electrophosphorescence spectra of the PtSN1−2 devices were contaminated with excimer luminescence. These results collectively demonstrated the effectiveness of the helical distortion approach for obtaining blue electrophosphorescence.

Although electrofluorochromism enables unique optoelectronic applications, its utility has been limited by poor reversibility. It is demonstrated that high reversibility in electrofluorochromism is obtainable from molecular dyads having a redox‐stable acceptor and an aromatic or antiaromatic donor. The structural control aims to generate excited‐state conjugation that produces twisted intramolecular charge‐transfer fluorescence, while suppressing the ground‐state conjugation in order to confine electrochemical processes exclusively within the acceptor unit. Overpotential‐free electrofluorochromism can be achieved with a high fatigue resistance against repeated electrochemical cycles. The electrofluorochromism is investigated using structural, spectroscopic, electrochemical, spectroelectrochemical, and quantum chemical techniques. The studies reveal that electrochemical gating of intramolecular charge transfer is the key mechanism underlying the improved electrofluorochromism performance. The study will provide novel insights into the future development and applications of electrofluorochromic devices.

The combined use of a singlet oxygen photosensitizer and 1,3-diarylisobenzothiophene enables efficient generation of hydrogen sulfide under visible light illumination.