The Effects of Substituents and Solvents on the Ground-State π-Electronic Structure and Electronic Absorption Spectra of a Series of Model Merocyanine Dyes and Their Theoretical Interpretation

Two series of new merocyanine dyes have been synthesised and the dependence of their electronic structure on substituents and solvents has been studied by NMR spectroscopy, by using both the NMR 13C chemical shifts between adjacent C atoms in the polymethine chain and the 3J(H,H) coupling constants for trans-vicinal protons. The widely used valence bond (VB) model based on two contributing structures cannot account theoretically for the observed alternating p-electron density in the polymethine chain. In addition, the prediction of zero-p-bond order alternation (or zero-bond length alternation) by this model is also incorrect. However, the results are consistent with the predictions of a qualitative VB model which considers the resonance of a positive charge throughout the whole polymethine chain. Based on this model and the Franck-Condon principle the effect of substituents and solvents on the fine structure of the electronic spectra of these dyes can be explained as vibronic transitions from the vibrational state v=0 to v', where v is the vibrational quantum number of the totally symmetric C?C valence vibration of the polymethine chain in the electronic ground state and v' is that in the electronic excited state. In contrast, neither the effects of substituents or solvents on the electronic structure of merocyanines and their electronic spectra can be accounted for by the simple two state VB model.