Mechanistic study of trans reversible arrow cis isomerization of the substituted azobenzene moiety bound on a liquid-crystalline polymer

A mechanistic study of the trans reversible arrow cis isomerization of the azobenzene moiety in a side-chain liquid-crystal polymer system was carried out with six liquid-crystalline polymethacrylates in which different elect ron-withdrawing substituents were attached to the para-positions of the azo benzene chromophores. Compared to the non-nitro-substituted azo polymers, t he nitro-substituted azo polymers exhibited two quite different behaviors: an extraordinarily high reaction rate of the thermal cis-trans isomerizatio n and an unexpected composition of cis-trans isomers obtained from the phot ochemical trans-cis isomerization process. A potential energy profile for t he isomerization process was established on basis of the structures of the proposed transition states and was employed to elucidate the reaction mecha nism. The results confirmed that the nitro-substituted azo polymer system p roceeded via a rotation mechanism in either direction of the trans reversib le arrow cis isomerization reaction, whereas the non-nitro-substituted spec ies were more likely to follow an inversion mechanism. (C) 2001 John Wiley & Sons, Inc.