Solvatochromism quantitative calculations

This general, multi-layer model will be our reference in the following paragraphs devoted to the numerical methods based on QM and classical MM theories, suitable for the quantitative calculation of solvent effects on the electronic distribution of the chromophore, which determines, in turn, the solvatochromic effects. 

At about the same time as this work was published, Znamenskiy and Kobrak simulated the absorption spectrum of betaine-30, a commonly used solvatochromic probe molecule, in [C4inim][PF6]. They investigated the interactions responsible for the solvatochromic shift. Because this shift is used experimentally to assess solvent polarity, the calculations can thus provide a direct window into the nature of polarity in ionic Hquid systems. To conduct the study, a single molecule of betaine-30 was immersed in a Hquid containing 200 ion pairs. Twelve independent 1-ns runs were then carried out, and from that the absorption spectrum was computed. They observe two distinct time scales one on subpicosecond time scales and one that is on the order of 100 ps. This result is consistent with previous simulation studies as well as time-resolved fluorescence spectroscopy experiments. Although the actual absorption spectra computed do not agree quantitatively with experimental results, the qualitative features do. 

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