Systematic photophysical investigation on pyrromethene 650 laser dye: Experimental and DFT approach

In the current study, we have chosen a highly fluorescent laser dye called pyrromethene 650 (PM-650) to investigate its photophysical properties using theoretical and experimental methods. We measured the absorption and emission spectra of PM-650 in polar protic and polar aprotic solvents at ambient temperature. Bathochromic and hypsochromic shifts were observed in the absorption and fluorescence spectra of the laser dye, confirming the presence of transitions between pi -> pi* and n -> pi* states. We further estimated the ground and excited state dipole moments of PM-650 through experimental (solvatochromic correlations) and theoretical (ab initio) techniques, and compared the results. Interestingly, the PM-650 displayed an increased excited state dipole moment when contrasted with its ground state. Furthermore, using the B3LYP/6-311G++(d, p) approach, we estimated the natural bond orbital and molecular electrostatic potential map of PM-650 and discussed the results. Additionally, we theoretically determined the HOMO-LUMO energy band gap of PM-650 in different solvents with varying polarities, and found good agreement with the experimentally obtained values. To assess the reactivity and softness of PM-650, we calculated global chemical reactive descriptor values based on the theoretically estimated HOMO-LUMO. Our results suggest that PM-650 is highly reactive and possesses a soft nature.