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[<i>d</i>]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[(<i>N</i>-carbazole)styryl]biphenyl (BSBCz). In addition, BZ derivatives demonstrate high photostability and efficient electron transport, highlighting their great potential as organic semiconductor laser materials.