Theoretical study of the solvatochromism of a merocyanine dye Original Research Article

Molecular dynamics (MD) simulations of a strongly polar merocyanine dye in solutions with acetone and methanol were performed using a quantum mechanical/molecular mechanical (QM/MM) mixed model. The solute was represented by its zwitterion and quinone resonance forms. The 102 nm blue shift of the UV-visible absorption band observed when the dye is transferred from acetone to methanol was analysed. Semiempirical electronic structure calculations on configurations obtained randomly from the data collection period of the MD run were used to study the effect of the solvent on the UV-visible spectrum. Different levels of solvent representation were considered: a self-consistent reaction field (SCRF) approach, a QM/MM mixed model and a solute-solvent supermolecule model. The semiempirical method AM1 was employed to calculate absolute energies, dipole moments and solute partial charges during the dynamics, whereas another semiempirical method, ZINDO/S, was employed to obtain the electronic spectra. The solvents considered have very similar polarities and therefore their solvatochromic effects are due mainly to specific interactions. Accordingly, we found that only the supermolecule model is able to predict the observed blue shift, that turns out to be produced by the effect of the first solvation shell over the zwitterion resonant form, that predominates over the quinone in both solvents. A detailed analysis of zwitterion-solvent specific interactions suggests that a nucleophile-carbonyl interaction in acetone and a hydrogen bond in methanol would be the main causes of the solvatochromism.