Solvation and thermal effects on the optical properties of naturaldyes: a case study on the flavylium cyanin

We present a first-principles study of the effects of both hydration and thermal dynamics on the optical properties of a natural anthocyanin dye, namely, \textit{cyanin} (Cya), in aqueous solution. We combine Car-Parrinello molecular dynamics and time-dependent density functional theory (TDDFT) [1] approaches to simulate the time evolution of UV-vis spectrum of the hydrated Cya molecule at room temperature [2,3]. The spectrum of the dye calculated in the gas phase [4] is characterized by two peaks in the red and in the blue, which would bring about a greenish hue incompatible with the dark purple coloration observed in nature. Describing the effect of the water solvent through a polarizable continuum model does not modify qualitatively the resulting picture. An explicit simulation of both solvent and thermal effects using ab-initio molecular dynamics results instead in a spectrum that is compatible with the observed coloration. This result is analyzed in terms of the spectroscopic effects of molecular distortions, induced by thermal fluctuations. [1] \textit{turbo}-TDDFT, B. Walker, A. Saitta, R. Gebauer, S. Baroni, \textit{Phys. Rev. Lett. }\textbf{2006}, $96$, 113001. [2] A. Calzolari, et. al , \textit{J. Chem. Phys.} \textbf{132}, 114304 (2010). [3] O.B. Malcioglu, A. Calzolari, R. Ghebauer, D. Varsano, and S. Baroni, preprint (2010). [4] A. Calzolari, et al, \textit{J. Phys. Chem. A} \textbf{113} 8801 (2009).