Dual fluorescence of (E)-N-(4-(dimethylamino)benzylidene)-2H-1,2,4-triazol-3-amine (DMABA-Amtr): A ground state perspective.

We present the absorption and fluorescence spectra of (E)-N-(4-(dimethylamino)benzylidene)-2H-1,2,4-triazol-3-amine (DMABA-Amtr), an electron donor-π-acceptor system. The molecule shows a single fluorescence emission band in non-polar solvents while dual emissions were observed in polar aprotic solvents. Although several researchers over the years provide different explanations for the mechanism of the phenomena, based on solvent assisted excited state geometry changes of such systems, it is still a matter of controversy since such systems are unique as they contradict Kasha's rule. The emission spectrum of the molecule shows strong dependence on solvent polarity and excitation wavelength. This observation together with a single iso-emissive point found in the area normalize emission spectra indicates the presence of two ground state equilibrium structures of the compound which are both fluorescent. Density functional theory (DFT) and time-dependent (TD-DFT) calculations also support the experimental findings.