Solvatochromism of a D205 indoline dye at the interface of a small TiO₂-anatase nanoparticle in acetonitrile: a combined molecular dynamics simulation and DFT calculation study

A combined molecular dynamics (MD) simulation and density functional theory (DFT) calculations were used to investigate the effect of the acetonitrile (ACN) solution on the absorption and the charge transfer in a D205 indoline dye at the interface of a TiO2 nanoparticle. DFT calculations were carried out to estimate the equilibrium geometry of a small Ti30O62H4-anatase nanoparticle and to derive interaction parameters for bidentate binding of a D205 dye to TiO2. A series of the D205 dye configurations anchored to Ti30O62H4 were generated from the MD simulations and used as input for the time-dependent DFT (TD-DFT) calculations. We found that the immersion of the D205-Ti30O62H4 composite system into the polar ACN environment results in conformational changes of the dye moiety, which are accompanied by the solvatochromic redshift of its long-wavelength absorption band up to 1900 cm-1. Our results show that the HOMO and LUMO energy level alignment of the dye and the nanoparticle suggests a favourable driving force for charge transfer from the dye to TiO2. The MD/TD-DFT-estimated solvatochromic shift for the D205 dye at the TiO2/ACN interface is in good agreement with the experiment, showing that such computational approach enables reliable predictions of optical properties of other dye-TiO2 composites in solution.