Bridge effect in charge transfer absorption bands of para-substituted β-nitrostyrenes

Abstract

Charge transfer electronic transitions in a series of para-substituted β-nitro-styrenes are analyzed in order to characterize the π-conduction channel between the electron-donor substituents and the NO₂ electron-acceptor group, particularly the bridge effect of the olefinic molecular structure.Absorption bands assignment of the π → π* electronic transitions in the near ultraviolet spectral region is carried out from an experimental and theoretical point of view. The photo-induced charge transfer spectral bands of these aromatic compounds follow the same pattern as nitrobenzene, benzaldehyde and acetophenone and the electronic transition takes place in the π → π*(¹L_a) excited state.Our present study based on ZINDO/S-CIS M.O. calculations permits us to analyse the role of the NO₂ electron-acceptor group, and the ethylenic group, where this last structure merges as a second electron-acceptor group. The calculated olefinic bridge molecular resistivities (ρo) of these β-nitrostyrenes range from 2.1 to 2.5 (Å/quantum).