Bridge effect in charge transfer absorption bands ofpara-substitutedβ-nitrostyrenes

Raúl G E Morales,Carlos Hernández,Victor Vargas

doi:10.1155/1997/701640

Abstract

Charge transfer electronic transitions in a series ofpara-substitutedβ-nitro-styrenes are analyzed in order to characterize theπ-conduction channel between the electron-donor substituents and the NO2electron-acceptor group, particularly the bridge effect of the olefinic molecular structure.Absorption bands assignment of theπ → 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 theπ → π*(1La) excited state.Our present study based on ZINDO/S-CIS M.O. calculations permits us to analyse the role of the NO2electron-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 theseβ-nitrostyrenes range from 2.1 to 2.5 (Å/quantum).

Highlights

We have started a program of research on resistivities of molecular wires of D–Ph– [CH=CH]n–A type, where D = electron-donor, A = electron-acceptor and Ph = aryl group [1]
Our main interest is based on the behavior of the π-conduction channel when a photo induced charge transfer (PICT) process occurs in excited state, i.e., the bridge effect [1]
We have developed a spectral assignment of the charge transfer electronic absorption bands in the molecular series under study

Summary

Introduction

By means of UV-spectral absorption data and semiempirical M.O. calculations, we have characterized the πconduction channel in excited state and the polyene bridge role after the PICT process as function of the electron-donor group strength in the para-substituted β-nitrostyrene molecular series (see Fig. 1). Electronic transitions energies and molecular orbital analysis of the ground and excited states were obtained by means of standardized ZINDO/S-CIS calculations [9].1 The computational work was done on a PC network station in our Laboratory and on the RICS 6000 IBM-station at the DIC Center of the University of Chile.

Results

Conclusion