Excitation Wavelength Dependence of Dual Fluorescent Molecules

The effect of various polar solvents on the absorption and dual fluorescence of 4- (dimethylamino)benzaldehyde (DMABA) was investigated. It was found that in non- polar solvents DMABA shows only one emission band, however in polar solvents second emission band at longer wavelength was observed. The two emission bands show excitation wavelength dependence. The long wavelength emission band was found to increase in intensity at the expense of the short wavelength emission band. Area normalized intensity versus emission wavelength plot in dichloromethane and ethyl acetate shows isoemissive point. Also excitation spectrum depends on emission wavelength. Therefore in polar solvents two different conformers of the same molecule at equilibrium in ground state are responsible for the dual fluorescence observed. The experimental studies were supported by theoretical calculations. Potential energy surface calculation by Molecular Mechanics results in presence two low energy conformers in polar solvents at ground state and excitation energy calculation for the two conformers by time dependent density functional theory at B3LYP/6-31d level of theory shows agreement with experimental result. Based on solvatochromic shift, excited state dipole moments were calculated and found to be 6.25 D and 10.5 D; this suggests presence of charge transfer for the second conformer.