Effect of solvent on fluorescence and absorption spectra of fluorescent coumarin derivative: Estimation of ground and excited state dipole moments

Abstract

Solvent effect on fluorescence and absorption spectra of fluorescent coumarin derivative viz, 4-[(5-amino-1,3,4-thiadiazol-2-thio)methyl]–2H-benzo[h]chromen2-one(4ATMBC) has been studied in different solvents at room temperature. The shifts in the position, intensities and shapes of the absorption and fluorescence bands have been observed. The ground and excited state dipole moment of the fluorescent molecule have been calculated from Solvatochromic shift method. The excited-state dipole moments were estimated from Lippert, Bakhshiev and Kawski-Chamma–Viallet equations by using the variation of the Stokes' shift with the solvent dielectric constant and refractive index. The Reichardt‘s microscopic solvent polarity parameter has been used to calculate change in dipole moment. It was found that the excited-state dipole moments were higher than those of the ground-state dipole moment. The large increase in dipole moment in the excited state is due to more polar excited state than in the ground state. The ratio of ground and excited state dipole moments is used to know the solubility of the fluorophore in different solvents. Further, we have calculated angle between the ground and excited state dipole moments which is found to be zero. This shows that ground and excited state dipole moments are parallel. Energy band gap values of different solvents were calculated using Tauc relation. Kamlet-Taft and Catalan methods were used to have the information of non-specific solute–solvent interactions and hydrogen bonding interactions.