Refractive index dependence of solvatochromism

Refractive index (n) function phi(n(2)) (phi)(n(2))= (n(2) + 2)/(n(2) + 2)) stands for average electronic polarizability density of the matter. Sets of nonpolar and highly polar (epsilon >= 25) liquids were used in parallel for recording the pi(n(2)) dependence of absorption and fluorescence spectra of several push-pull chromophores. Fluorescence maxima of Nile Red (NR) and laser dye 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) shift much less in polar solvents (coefficient corresponding to the sensitivity to phi(n(2)), p-4000 cm(-1)). Fluorescence in nonpolar solvents, as well as absorption in both solvent sets has almost constant and much larger p of similar to-8000 cm(-1.) Similarly, zwitter-ionic Betaine 30 has also different p values in nonpolar and highly polar media for absorption (Renge, J. Phys. Chem. A 114 (2010) 6250). The phi(n(2)) dependent induction shift is a function of solute dipole moments squared (mu(2)(g) - mu(2)(e)). This shift is suppressed in polar environment, if the dipole of the initial state is large (epsilon for fluorescence, mu g for absorption), in qualitative agreement with Li theory of non-equilibrium solvation (Huang et al., J. Theor. Comput. Chem. 5 (2006) 355). As compared to absorption, conspicuous fluorescence bandwidth narrowing by a factor of similar to 2 takes place in highly polar liquids for NR and DCM. The unusual narrowing indicates a dramatic molecular structure change in the excited state affecting the Franck-Condon factors for vibronic coupling. The reaction field created by the large dipole of solute in polar liquid exceeding 103V/m can modulate the bond alternation in a conjugated polymethine system. Dipole moments mu(g), mu(e) and the polarizability difference Afx were estimated for NR as 14 +/- 3 D,18 +/- 3 D, and 38 +/- 10 angstrom(3), respectively. Finally, a set of guidelines for solvatochromic data assessment is proposed. (C) 2017 Elsevier B.V. All rights reserved.