Synthesis and photochromic behaviour of a series of benzopyrans bearing an N-phenyl-carbazole moiety: photochromism control by the steric effect.

Five new N-phenyl-carbazole benzopyrans bearing different substitutions on one of the phenyl rings at the sp3 carbon have been synthesized. Their molecular structures were investigated by X-ray and NMR analyses and through quantum chemical calculations. The photochromic mechanism under UV irradiation in toluene, consisting of the consecutive formation of transoid-cis (TC) and transoid-trans (TT) isomers, was studied by UV-vis spectral and kinetic analyses. These molecules have been specifically designed to ascertain the possibility of favouring the formation of the less thermodynamically stable TT at the photostationary state, upon exploiting steric hindrance effects on the diene part of the molecule. The spectrokinetic study allowed the estimation of most of the spectrokinetic parameters, such as molar extinction coefficients, quantum yields of UV colouration and visible photobleaching, and the rate constants of the fast and slow thermal bleaching processes. Peculiar effects of substituents with different donor strengths on one phenyl ring located at the 3-position were observed on the spectrokinetic properties.