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The absorption and emission spectra of fluorescent thiophene dye, namely, 2,5-di-(5-tert-butyl-2-benzoxazolyl) thiophene, have been recorded at room temperature in solvents of different polarities. The excited state dipole moments (μe) were estimated from Lippert’s, Bakhshiev’s, and Kawski–Chamma–Viallet’s equations using the variation of Stoke’s shift with the solvent dielectric constant and refractive index. The optimized geometry of the molecule and μg were calculated theoretically by Gaussian 03 software using B3LYP/6-31g* level of theory. The μg and μe were calculated by means of solvatochromic shift method and μe was determined in combination with μg. It was observed that μe were higher than those of the μg, indicating a substantial redistribution of the π-electron densities in a more polar excited state for the selected thiophene dye. Further, the change in the dipole moment (Δμ) was calculated both from solvatochromic shift method and on the basis of microscopic empirical solvent polarity parameter [Formula: see text] and values are compared.

We estimated the relative florescence quantum yield (Φ) of 8-methoxy-3-[1-(4,5-dicarbomethoxy-1,2,3-triazoloacetyl)]coumarin [8MDTC] using a single-point method with quinine sulfate in 0.1 M of sulfuric acid used as a standard reference. The fluorescence lifetimes, radiative and non-radiative decay rate constants are calculated. Relative quantum yields were found to be less in the non-polar solvents, indicating that the solute exhibits less fluorescence in a non-polar environment. The fluorescence quenching of [8MDTC] by aniline was studied at room temperature by examining the steady state in five different solvents in order to explore various possible quenching mechanisms. The experimental results show a positive deviation in Stern-Volmer plots in all solvents. Ground state complex and sphere of action static quenching models were used to interpret the results. Many quenching rate parameters were calculated using these models. The values of these parameters suggest that the sphere of action static quenching model agrees well with the experimental results. Further, a finite sink approximation model was used to check whether these bimolecular reactions were diffusion limited or not. The values of the distance parameter R' and the diffusion coefficient D were determined and are compared with the values of the encounter distance R and diffusion coefficient D calculated using the Stokes-Einstein equation.

The Photophysical properties like ground state and excited state dipole moments, change in the dipole moment and fluorescence quantum yield of a boronic acid derivative 5-chloro-2-methoxy phenyl boronic acid (5CMPBA) are characterized. The study is carried out in various solvents at room temperature using absorption and steady-state fluorescence technique. The emission wavelength of 5CMPBA is quite sensitive to the polarity of solvents. With the increase in solvent polarity red shift or bathochromic shift of about 9 nm has been observed. The excited state and ground state dipole moments are estimated using solvatochromic shift method and effect of solvents on spectral properties of the molecule are investigated using Kamlet-Taft multiple linear regression approach. The changes in dipole moment (Δμ) are calculated both from solvatochromic shift method and microscopic solvent polarity parameter (E T (N) ), and the values are compared. The ground state dipole moment is also evaluated using quantum chemical calculations. The bathochromic shift of the emission spectra and the increase in the excited state dipole moment indicates π → π* transitions as well as the possibility of intramolecular charge transfer (ICT) character in the emitting singlet state of 5CMPBA. The relative quantum yield (Φ), radiative and non-radiative decay constants are calculated using single point method. It is found that the quantum yield of the molecule varies from 11 to 64% with the change in the solvent polarity indicating the dependency of fluorescence nature on the solvent environment.