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.
