Hydrogen Bonding Dynamics Of Aminophthalimides In Electronic Excited States

The solute-solvent interactions play a fundamental role on the photochemistry of organic and biological chromophores in solution. Intermolecular hydrogen bonding, as a site-specific interaction between hydrogen donor and acceptor molecules, is an important type of solute-solvent interactions and central to the understanding of microscopic structure and function in many complex molecular systems. Furthermore, dynamical behaviours of hydrogen bonds in electronic excited states plays an important role on determining the rates of many chemical, physical and biochemical processes that occur in hydrogen bonding surroundings. 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.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.In our work, 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). 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 4-aminophthalimide chromophore and hydrogen bonding solvents are significantly strengthened upon photoexcitation. The calculated results are consistent with the mechanism of the hydrogen bond strengthening in the electronically excited state, while contrast with mechanism of hydrogen bond cleavage.We have also investigated the Intra-and inter-molecular hydrogen bonding of 3-aminophthalimide dimer complex in the electronically excited state. The electronic excited-state conformations and vibrational absorption spectra of hydrogen-bonded systems are calculated by the TDDFT method. It has been demonstrated that the hydrogen-bonded 3-aminophthalimide dimer complex is of good symmetry in ground state. However, the symmetrical structure disappeared in the electronic excited state upon photoexcitation. The intramolecular hydrogen bond C3=O5…H8-N6 and intermolecular hydrogen bond C1=O4…H7'-N2' are considerablely strengthened in electronic excited state while the other intermolecular hydrogen bond C1'=O4'…H7-N2 and intramolecular hydrogen bond C3'=O5'…H8'-N6' of the hydrogen-bonded 3AP dimer are almost unchanged in the electronically excited state. The intra-and inter-molecular hydrogen bonding in the electronic excited state plays a very important role on the excited-state single proton transfer. The excited-state single proton transfer should be facilitated by the hydrogen bond strengthening in excited states.