Solvatochromism of a typical merocyanine dye. A theoretical investigation through the CNDO/SCI method including solvation

Abstract

Solvatochromism of a typical merocyanine dye is tentatively explained by a semi-empirical method including σ and π electrons (the CNDO/SCI method) for the first time. The procedure used makes allowance for a variation in geometry induced by the solvent, and stabilisation of the energy levels is introduced through the solvaton model proposed by Constanciel and Tapia. The calculations show that the solvatation results in a modification of the geometry from a ‘quinonoid’ to a ‘benzenoid’ structure with a concomitant general blue shift of the n→π^* and π→π^* transitions which fits the experimental results. Moreover, the results indicate a weak red shift of the π→π^* transition for the lowest polarities. A detailed analysis of the composition of the π→π^* transition made by considering the dipole moments of the ground and excited states substantiates the above-mentioned results. An attempt to explain the importance of the oscillator strength in the solvatochromism background and to correlate the net charges with the ¹³C shifts as the solvatation changes is proposed.