On the Electronic Transitions of the Methylene Violet Dye in Protic and Aprotic Solvents. A Theoretical and Experimental Study

G. JARA; C. GLUSKO; C. M. PREVITALI; C. CHESTA; H. A. MONTEJANO; A. B. PIERINI; D. M. A. VERA

Brasilia

Simposio; III Simpósio de Estrutura Eletrônica e Dinâmica Molecular; 2010

SEEDMOL

Methylene Violet (Bernthsen) is a neutral phenothiazine with potential as photosensitizer in different processes as well as a fluorescent probe. Absorption and emission spectra are strongly dependent on the polarity and nature (protic or aprotic) of the solvent. Since the Stokes´ shifts can not be rationalized by using standard solvatochromism models, we here present a combined Quantum Mechanics (QM), classical molecular dynamics (MD) and hybrid QM/MM study with the aim of understanding the photophysical properties of the dye in different media. The simulated media were benzene, acetonitrile and methanol. Three protocols were compared: 2) TD-DFT (B3LYP and PBE0 functionals with 6-311+G bases) on the optimized minima, using the IEFPCM continuum solvation model. 3) same as 1), including some discrete protic solvent molecules. 4) ONIOM/TD-DFT on a set of 150 selected MD simulation snapshots (using GAFF force field), including the protic solvent molecules involved in hydrogen bonds with the solute within high-level layer. The snapshots selection criteria were based on autocorrelation function of the potential energy. The relative number of configurations with two, one and zero hydrogen bonds between solute and solvent molecules in the selected set was the same as in the whole equilibrated trajectory. The comparison between protocols 1) against 2) and 3) reveals the relevance of the dynamic effects and the incorporation of discrete effects which improved the frequencies and oscillator strengths. Selected configurations were subject of natural bonding analysis in order to determine the degree of coupling between the solvent and substrate wave functions and its consequences in the orbitals involved in the transitions.