THEORETICAL-STUDY OF THE SOLVATOCHROMISM OF A MEROCYANINE DYE

Molecular dynamics (MD) simulations of a strongly polar merocyanine dy e in solutions with acetone and methanol were performed using a quantu m 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 W-visible absorption band observed when the dye is t ransferred from acetone to methanol was analysed. Semiempirical electr onic structure calculations on configurations obtained randomly from t he data collection period of the MD run were used to study the effect of the solvent on the W-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 mod el. The semiempirical method AM1 was employed to calculate absolute en ergies, 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 pol arities and therefore their solvatochromic effects are due mainly to s pecific interactions. Accordingly, we found that only the supermolecul e 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 zwitte rion resonant form, that predominates over the quinone in both solvent s. A detailed analysis of zwitterion-solvent specific interactions sug gests that a nucleophile-carbonyl interaction in acetone and a hydroge n bond in methanol would be the main causes of the solvatochromism. (C ) 1997 Elsevier Science B.V.