Photophysics of 4-dimethylamino cinnamic acid in different environments

Abstract

4-dimethylamino cinnamic acid (DMACA) has been studied by steady state and time-resolved fluorescence spectroscopy in different environments in order to get the information about its photophysics and photochemistry. Dual fluorescence in polar medium has been assigned to be arising out of delocalized excited state (DE) and twisted intramolecular charge transfer (TICT) state from different experimental and theoretical considerations. The computed excited state dipole moments in different twisted geometries evince that a twist of 90° of N(CH 3) 2 produces minimum energy state and maximum dipole moment change. The experimentally measured value (9.23 D) for dipole moment change from ground to excited state tallies with the corresponding value computed theoretically (7.5 D) for twisted geometry. Polarization studies in polar, nonpolar solvents and in β-CD cavity suggest that the emission transition moment is perpendicular to the molecular axis confirming the presence of twisted conformer. Weak acid protonates the carbonyl group while strong acid favors protonation of amino nitrogen. The solvent dependent absorption and emission spectra were found to be due to intramolecular structure change rather than ‘solvent coordinate’ effect. TICT band intensity is enhanced in β-CD inclusion complex due to modulation of photophysics by it while the normal (DE) band failed to appear. The fluorescence dependence of DMACA inclusion complex on polarity and viscosity has been discussed in detail. Quantum chemical calculations with AM1 Hamiltonian have been performed to find the actual geometry producing twisted intramolecular charge transfer state by twisting N(CH 3) 2 group to different angles and to confirm experimental findings qualitatively.