DUAL FLUORESCENCE EMISSION OF DONOR- Π- ACCEPTOR SYSTEMS: BENZYL MALONONITRILE AND MELDRUM’S ACID DERIVATIVE FROM GROUND STATE PERSPECTIVE

Abstract

The absorption, steady state fluorescence emission and excitation spectra of p-N, N- (Dialkaylamino) benzylidenemalononitrile (DMN), benzylidenemalononitrle (BMN), p-N, N- (Dialkaylamino) benzylideneMeldrum’s acid (DMA) and benzylideneMeldrum’ acid (BMA) were measured in various solvent with different polarities and the source of anomalous emission was determined with the help of both experimental and theoretical studies by reconsidering ground state conformational dynamics. The effect of solvents with different polarity were investigated and showed that in nonpolar solvents single emission band however, in polar solvents two emission bands, one from local excitation (LE) and the large red shift one from twisted intermolecular charge transfer state (TICT) were observed for structure DMN and DMA where as structure BMN and BMA show only single emission in both polar and nonpolar solvents. Analyses of the excitation wavelength and solvent polarity dependence of the emission spectra showed that the compound consisted of two conformational isomers which are also supported by theoretical computation. The slope analyses give a ground state dipole moment (μg1) of 9.4 D and excited state (μe1) dipole moments of 20.2 D and the change in dipole moment upon excitation of the first conformer becomes 10.8 D and similarly, the values of the ground state dipole moment (μg2) and excited state dipole moment (μe2) were found to be 7.3 and 19.1 D, respectively, for the second conformer, which gives a change of 12.0 D upon excitation, that was well reproducible with 11.1 D of theoretically calculated value for structure II. The results obtained from both experimental and theoretical calculations confirmed the existence of twisted ground state fluorophore, which is responsible for the second emission band in the dual fluorescence of structure DMN and DMA and in favor of a recently proposed mechanism, the solvent induced ground state structural change of the fluorophore .The main focus of this study was to provide additional evidence for the alternative mechanism which supports the ground state conformational dynamics responsible for dual emission.