Influence of solvents on the photoinduced birefringence in chitosan films incorporating azo dyes

The photoinduced birefringence arising from trans – cis – trans isomerization has been studied in chitosan cast films doped with the azo dye Ponceau‐S. The chitosan/Ponceau‐S films are highly resistant to several solvents, which allowed the study of their optical storage features after immersion in various solvents. We demonstrate that the induced birefringence and its dynamics are influenced by the solvent. From experiments using such different solvents as water, dimethylsulfoxide, ethylene glycol, methanol and chloroform, it was found that the induced birefringence and the writing rate increase linearly with the dielectric constant of the solvent. At high dielectric constant, photoinduced molecular reorientation is facilitated, leading both to a larger birefringence and faster dynamics.