Bond length alternation in unsymmetrical cyanine dyes and its influence on the vibrational structure of their electronic absorption spectra

Abstract

Based on the assumption that the unsymmetrical cyanine dyes can be regarded as a hybrid of the two corresponding symmetrical dyes it was expected that λmax of the unsymmetrical dye would be the arithmetic mean of λmax of the two symmetrical dyes. There is, however, often a remarkable difference between the arithmetic mean and the observed λmax of the unsymmetrical dye – the Brooker deviation. To date, this effect is explained in terms of increasing electronic energy differences. With increasing difference of the electronic structure between the ground and excited state, the difference between equilibrium geometry of ground and excited state increases and the relative intensity of the higher vibronic transitions 0–v increases and that of the 0–0 transition decreases. λmax represents the intensity maximum of an absorption band and could be related to the 0–0, 0–1 or another vibronic transition. We discuss the origin of the Brooker deviation on the vibrational structure of the spectra.