Synthesis and photo-induced birefringence of pyrazoline substituted azo-dyes in PMMA films

Three push-pull type azo dyes, 4 ^′-{[(2-hydroxylethyl)methyl]amino}-4-nitroazobenzene (compound I), 1-[4-(4^′-nitrophenyl)-azophenyl]-3,5-biphenyl-2-pyrazoline (compound II) and 1-[4-(4^′-nitrophenyl)-azophenyl]-3-(3 ^′-trifluoromethyl)-phenyl-5-phenyl-2-pyrazoline (compound III) were synthesized. The cis-trans isomerization process and polarized photoinduced birefringence of those azo dyes doped PMMA films were investigated. It was found that the rise and relaxation process of birefringence signals were fit with biexponential functions and the molecular structures have effects on their photo-induced birefringence. This suggests that the three films have similar storage speed when the laser intensity is high enough. Compound III, which has the biggest substituent, has the largest long-term storage ability and can even keep 90% of its saturated birefringence.