Experimental And Theoretical Studies Of The Excited-state Hydrogen Bonding Dynamics Of Several Organic Molecules

As a weak interaction, hydrogen bond, formed by hydrogen donor and hydrogen acceptor, plays an important role in the photo-physics, photo-chemistry and photo-biology processes, such as, intermolecular charge transfer, molecular fluorescence quenching, excited state proton transfer and phase transition, etc. It is a tough task to measure experimentally the effect of site-specific hydrogen bonding on organic molecules fluorescence characteristics due to their short excited state lifetimes. Moreover, most previous theoretical studies focused on excited hydrogen bonding dynamics studies of some prototype molecules, so that some key site-specific hydrogen bonds tends to be neglected. Therefore, it is necessary to conduct excited state hydrogen bonding dynamics studies on real function molecules.In this paper, theoretical and experimental researches on the influence of hydrogen bonding dynamics on the excited state properties of organic molecules are carried out. Firstly, the rigid scan of potential energy curves of pyrrole monomer is performed with time dependent density functional theory (TDDFT) method along the rigid NH stretching coordinate. The results show that TDDFT is suitable to be applied to calculate the excited state potential energy surface (PES) of these biological molecules. Then, the excited state hydrogen bonding dynamics of Pyrrole-MeOH and Indole-MeOH complexes are studied at TDDFT level. A proton transfer process from the solute to the solvent molecule is found, which leads to a disappearance of a dissociation state on the first electronically excited singlet state PES and an appearance of a local energy minimum instead. This conclusion is further verified through high level accuracy calculations of complete active space self-consistent field (CASSCF) method and algebraic-diagrammatic construction (ADC) class methods based on Green function.Tryptophan (TRP) is chosen to conduct the further study of the excited state hydrogen bonding dynamics because it is one of the most important derivatives of indole, and its fluorescence is sensitive to the surrounding environment. The steady-state absorption spectra, fluorescence spectra and time-resolved fluorescence lifetime of TRP are measured in MeOH, EtOH and THF solvents, respectively. The experimental results suggest that hydrogen bonding dynamics is the main influence factor on the spectral characteristics of tryptophan in three kinds of solvents. Molecular dynamics simulation, infrared spectrum calculation and the potential energy surface scan indicate that the hydrogen bond located on the NH bond of indole ring of TRP plays a key role in the effect of hydrogen bonding dynamics on the TRP spectrum characteristics. We find that the solvation models (CPCM, COSMO) are not suitable for the calculation of the potential energy surface involving proton transfer process. The ONION model is adopted to simulate the solvent environment by increasing the amount of the solvent molecules. The results show that a local energy minimum exists on the first excited state PES due to the formation of the hydrogen bonds between TRP and solvent molecules, which is disappeared for free TRP. The barrier of the global energy minimum along NH stretching coordinate is higher for the TRP-MeOH complex than for the TRP-EtOH complex, and the barrier of the local energy minimum is lower for the TRP-MeOH complex than for the TRP-EtOH complex, which is the reason that it is more easy for H proton on indole ring in the TRP-MeOH complex crosses over the barrier between the global energy minimum and transition state and transfers to and stays more stably at the local energy minimum. Moreover, the energy gap between the local energy minimum on the first electronically excited singlet state PES and ground state with the same structure for the TRP-MeOH complex is lower than that for the TRP-EtOH complex. Therefore, it is easier for the excited TRP-MeOH complex to return to the ground state through internal conversion process than for the TRP-EtOH complex, which leads to a shorter fluorescence lifetime and lower fluorescence quantum yield in the TRP-MeOH complex.