Dynamic Effects Of Hydrogen Bonding On Ultrafast Process In Electronic Excited States

The solute-solvent interactions play a fundamental role on the excited states dynamic of organic and biological chromophores in solution. For example, it can always be found that the fluorescence emission behaviour of many organic and biological chromophores can be strongly quenched by the intermolecular hydrogen bonding between chromophores and protic solvents. Hydrogen bonding is a very common intermolecular and intromolecular interaction which is very different from the chemical bond. It plays a important role in chemistry, physics and biology. The formation of the hydrogen bond can bring great effect on the configuration and physical property of the system. However, how the intermolecular hydrogen bonding affect the fluorescence quenching? This need us to investigate the excited state hydrogen bonding dynamics.The time-resolved ultrafast spectroscopy, quantum chemical calculations for excited states, and excited-state dynamics simulations have been versatile tools for the study on the electronic excited-state ultrafast dynamics of complex molecular systems. It is very valuable to combine time-resolved spectroscopic experiments with excited-state quantum chemistry calculations and dynamics simulations. However, one can note that it is difficult to do quantum chemistry calculations and dynamics simulations on electronic excited states of complex molecular systems.At present, people still have different opinions on the excited state hydrogen bonding dynamics. In this papre, the electronic excited-state hydrogen bonding dynamics and its dynamic effects on the photochemistry of organic and biological chromophores in solution have been investigated by time-dependent density functional theory (TDDFT). The electronic excited-state conformations and vibrational absorption spectra of Coumarin151 and Coumarin 102 hydrogen-bonded systems are calculated by the TDDFT method. By monitoring the spectral shifts of some characterized vibrational modes involved in the formation of hydrogen bonds in different electronic states, it has been demonstrated that the intermolecular hydrogen bonds between coumarin 102 and coumarin 151 chromophore and hydrogen bonding solvents are significantly strengthened upon photoexcitation. Inaddition, we also find that Fluorenone in alochols can not only exist one hydrogen bond, bue also exist doubly hydrogen-bonded complex, moreover, both the two intermolecular hydrogen bonds are significantly strengthened in the S1 state of the doubly hydrogen-bonded complex. We have demonstrated that the radiationless deactivation via internal conversion (IC) from the fluorescent state to ground state for Fluorenone system can be enhanced due to the hydrogen bond strengthening in the fluorescent state, which is a good explanation of the fluorescence quenching.