Chiral methacrylic polymers containing permanent dipole azobenzene chromophores. C-13 NMR spectra and photochromic properties

Dynamics in solution and photochromic properties of radical copolymers of (S)-4-[N-(2-methacryloyloxyethyl)-N- (2-methylbutyl)]amino-4'-cyanoazobenzene with butyl methacrylate were investigated. C-13 NMR and 2D heteronuclear correlated (HETCOR) spectroscopy allowed assignment of C-13 NMR signals and evaluation of main chain tacticity. C-13 relaxation times T-1 evidenced a rather limited mobility of azobenzene chromophores. Mobility increases with increasing content of butyl methacrylate units, that acts as an internal plasticizer. The limited mobility of the spacer between azobenzene and methacrylate groups is attributed to an interaction between carbonyl oxygen and aminic nitrogen stabilized by the internal charge transfer from the amino to the cyano group. Trans to cis photoisomerization rates turned out to be independent of molecular weight and chemical composition of the investigated photochromic polymers, whereas cis to trans thermal isomerization rates increase with the content of aromatic units. These data suggest that photo- and thermal isomerization processes imply an in-plane inversion and an out-of-plane rotation of one of the phenyl rings of the azobenzene chromophores, respectively. Preliminary second harmonic generation measurements indicate that the investigated polymers are endowed with significant non-linear optical properties.