Biphenyl-Modified Benzobisoxazole Derivatives with Reduced Concentration Quenching and Low Amplified Spontaneous Emission Thresholds for Organic Semiconductor Lasers

In this study, we investigate 6,6′-bibenzo[ d ]oxazole (BZ) derivatives, in which a biphenyl unit is incorporated into the benzobisoxazole (BOX) core, to achieve lower amplified spontaneous emission (ASE) thresholds. BOX derivatives exhibit low ASE thresholds of approximately 1 μJ cm –2 and demonstrate robust photostability. However, their photoluminescence quantum yields (PLQYs) decreased in solid films because of concentration quenching, even at 6 wt % doped into 4,4′-bis(carbazole-9-yl)biphenyl (CBP) films. Mitigating concentration quenching in solid films is expected to further reduce ASE thresholds. Since concentration quenching is primarily caused by dipole–dipole interactions, which can be suppressed by increasing the Stokes shift, we focus on BZ derivatives to relax the rigidity of BOX derivatives. As a result, BZ derivatives exhibit larger Stokes shifts than BOX derivatives, effectively mitigating concentration quenching in solid films. When doped at 6 wt % into CBP films, BZ derivatives show low ASE thresholds below 1 μJ cm –2 . Notably, BZCz2 exhibits an exceptionally low ASE threshold of 0.38 ± 0.06 μJ cm –2, even lower than that of 4,4′-bis[( N -carbazole)styryl]biphenyl (BSBCz). In addition, BZ derivatives demonstrate high photostability and efficient electron transport, highlighting their great potential as organic semiconductor laser materials.