Secondary amino group as charge donor for the excited state intramolecular charge transfer reaction in trans-3-(4-monomethylamino-phenyl)-acrylic acid: Spectroscopic measurement and theoretical calculations

The emission properties of a synthesized donor–acceptor substituted aromatic system trans-3-(4-monomethylamino-phenyl)-acrylic acid (t-MMPAA) have been investigated both experimentally and theoretically. The molecule t-MMPAA shows dual fluorescence in polar solvents. The normal emission band is assigned to be arising from the locally excited state whereas the anomalous, solvent dependent, red shifted emission band arises from a twisted transient emissive state. Solvent dependency of the large Stokes shifted emission band and high dipole moment of the excited state support charge transfer character of the twisted excited state. Potential energy curves along the two possible twisting coordinates have been calculated for the ground and excited states both in vacuo and in polar aprotic acetonitrile solvent using time dependent density functional theory (TDDFT) and time dependent density functional theory including polarized continuum model (TDDFT-PCM), respectively. Both the twisted paths, i.e., along the donor (–NHMe) and acceptor (–CHCHCOOH) groups, favor the formation of stable twisted excited state, but the twisting through acceptor path is energetically favorable (low barrier height) than the donor path. However, localized nitrogen lone pair supports TICT process by twisting along the donor path. A more stabilized calculated twisted state in acetonitrile solvent correlates well with the solvent dependent red shifted emission.