Hydrogen Bonding Dynamics And Effects On Luminescent Behavior Of Complex Molecular Systems

Hydrogen bonding as a very important solute-solvent interaction has been recognized widely for its importance in physics, chemistry, and biology. Intermolecular hydrogen bonding in solution which is a site-specific solute-solvent interaction between hydrogen donor and acceptor molecules not only plays a significant role in molecular non-equilibrium processes in liquids but also is the key to understand microscopic structure and function in many organic and biological systems. Under some circumstances, the solute molecules with the solvent molecules can form hydrogen-bonded complexes in the ground state. Upon photoexcitation, due to the charge redistribution in different electronic states, the solute and solvent molecules which are used for the formation of hydrogen bonds will be readjusted and then the dynamic response of the intermolecular hydrogen bonds to photoinduced changes should be generated. This process is known as the hydrogen-bonding dynamics.As we know, hydrogen-bonding dynamics determined by low-frequency motions of the hydrogen-bonded groups takes place on the ultrafast timescales. Thus, the femtosecond time-resolved vibrational spectroscopy has shown the potential to monitor hydrogen-bonding dynamics. At the same time, time-dependent density functional theory (TDDFT) method has been demonstrated as an effective way to theoretically study the hydrogen-bonding dynamics by monitoring the spectral shifts in different electronic states.In our work, we study the hydrogen bonding dynamics and photochemical properties of two different hydrogen bonding systems by time-dependent density functional theory (TDDFT). For the C120-(H2O)A,B,C system, we calculated the ground-state geometrical structures and electronic transition energies as well as corresponding oscillation strengths of the low-lying electronically excited states of three types HB complexes by the density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods, respectively. We can summarize the change of hydrogen bonding and the shift of electronic spectrum. For the hydrogen-bonded Cl20-(H2O)B complex, we calculated the lengths of hydrogen bonds and hydrogen-bonded groups and the infrared spectra of the C120-(H2O)B complex in ground state and the S1state. Therefore, a redshift of the O-H stretching mode can theoretically demonstrate that the intermolecular hydrogen bond should be strengthen in S1state. For the [CuCN-EIN] system, we study the effect of furcated hydrogen bond and coordination bond on luminescent behavior by means of theoretical calculations. The geometric structures, binding energies and IR spectra in both ground state and electronically excited state S1of the complex were computed using DFT and TDDFT methods. Furthermore, we also discuss the frontier molecular orbitals and the electron configuration of the fragment. We can come to a conclusion that luminescence of the fragment can be caused by the ligand-to-metal charge transfer (LMCT) form. The furcated hydrogen bonds HB-1and HB-2are both strengthened in the S1state slightly. And it is distinctly demonstrated for the first time that the strengthening of the hydrogen bonds in the S1state goes against the charge transfer from the ligand to metal and then should be in favor of the luminescence. In particular, the changes of coordination bonds in the excited state are also researched. We can also find that the coordination bond Cu7-N8is strengthened. And the strengthening of the coordination bond Cu7-N8should also be in favor of the luminescence.