Solvent Effects on the Spectral Properties of Rhodamine 6G: Estimation of Ground and Excited State Dipole Moments

Abstract

Spectroscopic and photophysical behavior of Rhodamine 6G (R6G) in seven solvents are reported in this work. R6G monomers exhibit strong electronic absorptions (due to π*←π transitions) in the 529–539-nm region, and the band positions were shown to depend on the refractive indices of the respective solvents. The Stokes’ shifts displayed a linear dependence on solvent polarity, with values ranging between 19 nm (in benzene) and 28 nm (in water). R6G’s fluorescence quantum yields (ΦF) are generally high (> 0.90), and this was ascribed to structural features (rigidity, planarity and presence of condensed rings) in the R6G molecule. These ΦF values are shown to vary with solvent viscosity in accordance with the Forster–Hoffmann model. Fluorescence quenching of R6G by 1,4-benzoquinone was diffusion controlled and occurred via an electron transfer mechanism. Theoretical treatment of the fluorescence quenching data yielded fluorescence lifetime values, which were also shown to be solvent viscosity dependent. Ground-state and excited-state dipole moments of R6G were obtained semi-empirically via the analysis of solvatochromic data, and the values are found to be 0.69 D and 1.97 D, respectively.