Methacrylic polymers containing permanent dipole azobenzene chromophores spaced from the main chain. C-13 NMR spectra and photochromic properties

Dynamics in solution and photochromic properties of radical copolymers of 4 -(4-oxy-4'-cyanoazobenzene)but- l-yl methacrylate, 6-(4-oxy-4'-cyanoazobenz ene)hex-l-yl methacrylate, and 8-(4-oxy-4'-cyanoazobenzene)oct-l-yl methacr ylate with (-)-menthyl methacrylate were investigated. C-13 NMR and 2D HETC OR spectroscopy allowed assignment of C-13 NMR signals and evaluation of ma in chain tacticity. C-13 T-1 relaxation times evidenced a rather limited mo bility of the azobenzene chromophores when inserted in polymer macromolecul es. Mobility increased with increasing the length of the polymethylene spac er that acted as a flexible joint between the aromatic chromophore and poly mer backbone. Both the trans to cis photoisomerization and the cis to trans thermal isomerization processes showed a small dependence on monomer struc ture and chemical composition of the investigated photochromic polymers. Th ese data seem to suggest an appreciable contribution of the in-plane invers ion mechanism to both isomerization processes of the azobenzene chromophore s. The absence of appreciable dichroic bands in the copolymer CD spectra an d the negative results obtained in preliminary second harmonic generation m easurements are discussed in terms of the polymer structural features.