Dynamics of photoinduced orientation of nonpolar azobenzene groups in polymer films. Characterization of the cis isomers by visible and FTIR spectroscopies

To better characterize the cis state and the reorientation processes in pol ymers containing azobenzene groups, the in situ dynamics of photoinduced or ientation of a polymer with azobenzene groups bearing no donor and acceptor substituents such as poly [4- [2-(methacryloyloxy)ethyl]azobenzene] polyme r (pMEA) was carried out using real-time UV-vis and infrared spectroscopies . Thin films of pMEA were irradiated at 488 nm with either linearly or circ ularly polarized light (I-pump = 10 mW/cm(2)). The different mechanisms occ urring during the photoisomerization cycles were revealed by following the time dependence of the normalized average absorbance (T-0) and the normaliz ed linear dichroism (T-2) during the orientation (laser on) and relaxation (laser off) periods. Therefore, the normalized average absorbance has allow ed an estimation of the cis isomer fraction (f(cis)) at the photostationary state. The photoisomerization is more efficient when the film is irradiate d using a circularly polarized light (f(cis) = 0.15) than using linearly po larized light (f(cis) = 0.11). The cis state has been characterized for the first time in azopolymers by infrared spectroscopy using a band at 1515 cm (-1) characteristic of the cis isomers. The time dependence of the intensit y of this band during the relaxation period has shown that the cis state is very stable and has a lifetime of about 12 h. Finally, a significant dichr oism of the trans and cis isomers has been observed when the films were irr adiated with a linearly polarized light.