Theoretical investigation of twisted charge-transfer-promoted intramolecular proton transfer in the excited state of 4 '-dimethylaminoflavonol in a highly polar solvent

Theoretical insight is provided into excited-state intramolecular proton transfer based on a time-dependent density-functional theory method for 4'-dimethylaminoflavonol in the highly polar solvent acetonitrile. The calculated absorption and fluorescence spectra are in good agreement with the experimental results. Calculated hydrogen-bond energies and infrared vibrational spectra point to a strengthening of hydrogen bonding in the excited state. A frontier molecular orbital analysis illustrates that the nature of the hydrogen-bond enhancement is charge redistribution upon photo-excitation, which has been quantitatively confirmed by Mulliken, Hirshfeld, and natural bond orbital charge analyses. A reduced density gradient function provides a visual confirmation of the observed phenomenon of hydrogen-bond strengthening. Thus, the 4'-dimethylaminoflavonol molecule in highly polar acetonitrile can adopt a twisted intramolecular charge-transfer state. According to our calculations, intramolecular hydrogen bonds can facilitate intramolecular proton transfer in the twisted intramolecular charge-transfer state. Potential energy curves show that excited-state intramolecular proton transfer can occur because of the relatively low potential energy barrier