Study of novel fluorescent coumarin-3,4-dihydropyrimidin-2(1H)-ones dyads. Estimation of ground- and excited-state dipole moments from a solvatochromic shift

摘要

The photophysical properties like ground- and excited-state dipole moments, change in the dipole moment and fluorescence quantum yield of three coumarin-3,4-dihydropyrimidin-2(1H)-ones dyads 4a-c are characterized. The study is carried out in various solvents at room temperature using absorption and steady-state fluorescence technique. The emission wavelengths of the 4a-c are quite sensitive to the polarity of solvents. Increasing solvent polarity red or bathochromic shifts of about 10 nm have been observed. The excited-and ground-state dipole moments are estimated using solvatochromic shift method. It was observed that dipole moments of the excited state were higher than those of the ground state, indicating a substantial redistribution of the pi-electron densities in a more polar excited state for 4a-c. The changes in dipole moment (Delta mu) are calculated both from solvatochromic shift method and microscopic solvent polarity parameter. The bathochromic shift of the emission spectra and the increase in the excited-state dipole moment indicates pi -> pi* transitions as well as the possibility of intramolecular charge transfer (ICT) character in the emitting singlet state of 4a-c. The relative quantum yields (Phi) are calculated using single point method. It is found that the quantum yields of the molecules vary (9 to 22%) with the change in the solvent polarity indicating the dependency of fluorescence nature on the solvent environment. (C) 2016 Elsevier B.V. All rights reserved.