Dual fluorescence emission of p-N,N-(dialkylamino) benzyl-idenemalononitrile and related systems: Evidence for direct excitation of ground state twisted intramolecular charge transfer (TICT) conformer

Abstract

The mechanism of dual fluorescence emission of a fluorophore is controversial ever since its first observation in p-N,N-dimethylaminobenzonitrile (DMABN). Excited state twisted intramolecular charge transfer (TICT) and planarized intramolecular charge transfer (PICT) models have been the two prominent theories used to explain the dual fluorescence mechanism in several systems. These mechanisms are based on excited state adiabatic structural changes of the fluorophore. Nevertheless, pieces of evidences based on excitation spectral measurements at different emission windows suggest the possibility of additional and/or alternative mechanisms based on ground state structural changes of the fluorophore. In this paper, we have presented a systematic steady-state absorption and fluorescence spectroscopy of a donor-πacceptor system, p-N,N-(dialkylamino)benzylidenemalononitrile, and related compounds in solvents of different polarities. The excitation spectrum of p-N,N-(dialkylamino) benzylidenemalononitrile is found to be dependent on the emission window. Furthermore, the emission spectra of the molecule are dependent on the excitation wavelength which suggest that the molecule consists of two stable ground state conformational isomers. The spectroscopic pieces of evidence together with results from DFT calculations are in favor of the solvent-induced ground state structural change of the fluorophore. Hence, two ground state conformers of p-N,N-(dialkylamino)benzylidenemalononitrile are attributed to the dual emission of the molecule. KEY WORDS: Dual fluorescence, Charge transfer, TICT, PICT, Excitation, Benzylidenemalononitriles Bull. Chem. Soc. Ethiop. 2024, 38(5), 1413-1427. DOI: https://dx.doi.org/10.4314/bcse.v38i5.17