Photoreaction and molecular reorientation studies of ultra-violet light irradiated azobenzene containing polymer films using two-dimensional correlation infrared spectroscopy

A photosensitive polymer with azobenzene side groups was synthesized from free radical polymerization and characterized. Photoreaction and photoinduced molecular reorientation was investigated in films of poly(4-methacryloyloxyazobenzene) (PMAAz) through ultraviolet (UV) absorption spectra and two-dimensional (2D) correlation analyses of their infrared (IR) spectra. The PMAAz polymer molecules were found to undergo trans–cis photoisomerization upon UV irradiation. The fraction of the cis-isomer of azobenzene increased with an increasing UV light exposure dose. The analysis of the 2D IR correlation spectra revealed that the isomerization of azobenzene chromophores induced molecular movement of azobenzene moiety as well as alkyl main chains in PMAAz films. The mono-substituted benzene ring changed sooner than the para-substituted benzene ring for trans–cis isomerization. The azobenzene chromophores in the PMAAz film induced the molecular reorientation by photoreaction in the following sequence: mono-substituted benzene ring → NΦ bond → cis-NN → para-substituted benzene ring.