Solvent effect on the spectral properties of dipolar laser dyes: Evaluation of ground and excited state dipole moments

Abstract

The effect of solvents on absorption and fluorescence spectra and dipole moments of two medium sized dipolar laser dyes 2-(2,7-dichloro-6-hydroxy-3-oxo-3H-xanthen-9-yl) benzoic acid (Fluorescein 27) (F27) and N-[6-diethylamino)-9-(2,4-disulfophenyl)-3H-xanthen-3-ylidene]-N-ethylhydroxid (Sulfarhodamine B) (SRB) have been studied comprehensively in polar protic and polar aprotic solvents at room temperature (298 K). The bathochromic shift observed in absorption and fluorescence spectra of F27 and SRB with increasing solvent polarity signifies that the transition involved are π→π*. Solvatochromic correlations were used to obtain the ground and excited state dipole moments. The observed excited state dipole moments are found to be larger than their ground state counterparts in all the solvents studied. The ground and excited state dipole moments of these probes have also been computed from ab initio calculations and compared with those determined experimentally. Further, the experimentally obtained changes in dipole moment (Δμ) were compared with those using normalized polarity terms from Reichardt equation.