Theoretical Studies Of Excited State Hydrogen Bond For Coumarin Molecular Systems

The weak interaction between solute and solvent is needed to be considered for studying the photochemical behavior of organic molecules system in the solution. As a site-specific interaction between hydrogen donor and acceptor molecules, hydrogen bonding is one of the most important types of solute-solvent interactions and plays an important role on physical and biochemical processes of hydrogen bond complex upon photoexcitation. For example, the internal conversion from the excited to ground state is enhanced by hydrogen bond strengthening of excited state.In present work, excited state hydrogen-bonding effects on the photochemistry of coumarin102in ethanol solvent has been studied using TDDFT method and CPCM model. By monitoring the spectral shifts of the characterized vibrational modes involved in the formation of hydrogen bonds in different states, it has been indicated that intermolecular hydrogen bond between C102and ethanol is strengthened in the excited state. The C=O and O-H stretching bands are strongly red-shifted due to the formation of the intermolecular hydrogen bond. The excitation energy and frontier molecular orbital indicate the S1of the C102-EtOH is the locally excited state. Furthermore, we have demonstrated that the internal conversion from the excited to ground state is enhanced by hydrogen bond strengthening.In our work, the TDDFT method is carried out to investigate the locally excited (LE) and twisted intramolecular charge-transfer (TICT) state of the Coumarin152in the protic methanol solvent. From the geometric structures and stretching vibrational frequency in different states, we have demonstrated that the intermolecular hydrogen bond are significantly strengthened in the LE and TICT states and more strengthened in TICT state. The ICT electronic excitation can significantly induce the C=O and O-H stretching vibrational frequency redshift. The calculated strong absorption peak and norm fluorescence peak are in agreement with the experimental results. From the frontier MOs and excitation energy analysis, the LE and TICT states correspond to the orbital transition from HOMO to LUMO. LE state is with ππ*transition mode, and TICT state is with nπ*character.