Effect of substituents in benzoxazole derivatives of biphenyl on the optical properties and photostability of their thin films

Abstract

established that fluorine atoms in the biphenyl moiety of the benzoxasole derivatives significantly improve the stability of the TVD films relative to oxidative aging compared with hexyloxy substituents. Irradiation with UV light stimulates quenching of luminescence and structuring of the TVD film of the difluoro-substituted derivative.Introduction. The spectral properties of poly(phenylene vinylenes) (PPVs), used as thin-film electroluminescent materials, depend considerably on the nature of the substituents on the PPV backbone and side chains [1-3]. Thus the electroluminescence turn-on voltage for diodes containing thin films of poly[2-(2-ethylhexyloxyl)-5-(4-trifluoromethylphenyl)-1,4-phenylene vinylene] and poly[2-(2-ethylhexyloxy)-5-(3,5-bis(trifluoromethyl)phenyl)-1,4-phenylene vinylene] as emitting layers is 4.0 V and 3.3 V respectively [4], and 18 V for poly[2-(2-ethylhexyloxy)-5-phenyl)-1,4-phenylene vinylene] not containing electron-acceptor trifluoromethylphenyl substituents [5]. Very high photoluminescence efficiency (>60%) is observed for PPV films containing alkylsilyl substituents [6].The position of the substituents also affects the photoluminescent properties of PPV, and this effect is especially important in sold samples in which the major role is played by the steric factor and intermolecular interaction. For a poly(2,3-dibutoxy)-1,4-phenylene vinylene film, not only a hypsochromic shift has been observed but also a significant increase in the photoluminescence efficiency (~40%) [7] relative to a poly(2-methoxy-5-ethylhexyloxy)-1,4-phenylene vinylene film (~15%-20%) [8,9].We should point out that introducing bulky oxyalkyl side substituents is a widely used method for improving the stability of PPV films relative to crystallization. However, it has been established [10] that in TVD films of model low-molecular weight octyloxy-substituted phenylene vinylenes, the crystal phase content is higher than in octyl-substituted analogs.According to the data in [11], substituted biphenyls have a low tendency to crystallize and are convenient for studying low-molecular weight analogs of PPV. Owing to high emissivity in the solid state and good electron transport properties, TVD films of benzoxazole derivatives of biphenyl are also promising for use in electroluminescent devices [12].We showed earlier in [13] that 2,2′-oxyhexyl substituents in the biphenyl moiety of 4,4′-bis[(E)-1-(1,3-benzoxazol-2-yl)-2-ethenyl-1,1′-biphenyl (BP) reduce the photostability and promote aggregation of hexyloxy-substituted TVD films of biphenyl on storage, while a TVD film of unsubstituted biphenyl is stable relative to irradiation and oxidative aging.