Refractive index dependence of solvatochromism

Abstract

Abstract Refractive index (n) function ϕ(n2) (ϕ(n2) = (n2 − 1)/(n2 + 2)) stands for average electronic polarizability density of the matter. Sets of nonpolar and highly polar (e ≥ 25) liquids were used in parallel for recording the ϕ(n2) 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 ϕ(n2), p ∼ −4000 cm−1). Fluorescence in nonpolar solvents, as well as absorption in both solvent sets has almost constant and much larger p of ∼−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 ϕ(n2) dependent induction shift is a function of solute dipole moments squared (μg2 − μe2). This shift is suppressed in polar environment, if the dipole of the initial state is large (μe for fluorescence, μg 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 ∼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 109 V/m can modulate the bond alternation in a conjugated polymethine system. Dipole moments μg, μe and the polarizability difference Δα were estimated for NR as 14 ± 3 D, 18 ± 3 D, and 38 ± 10 A3, respectively. Finally, a set of guidelines for solvatochromic data assessment is proposed.