A Theoretical Study On The Photochemistry Properties Of Hydrogenbonded Complex

The compounds with both electron-donor and electron-acceptor units have become a hot-spot in recently years. Throughout theπ-conjugated-bridge, the both units connect together and form the"D-π-A"compounds. Upon photo-excitation, these compounds may undergo intramolecular charge transfer (ICT) process. The ICT states are very sensitive to the local environment, attracting many photo-chemistry and photo-biological researchers′attention. Therefore, these compounds have been used widely as fluorescence probe in the study of the microenvironment. Moreover, the ICT process has become important tool to study the photochemistry and electronic properties of D-π-A compounds.The compounds naamine A, naamidine A and naamidine G studied in this work are also D-π-A compounds, which are from marine sponges. As we know, most researchers have paid their main attention to the synthesis and bioactivity study of these compounds. However, there are little study on their photochemistry and electronic structure properties in excited states. In this work, the time-dependent density functional theory (TD-DFT) method has been used to study their properties in excited states. Much information about structure in ground and excited states and the change of different units before and after photo-excitation has been provided. By the analysis of electronic structures, distribution of electron density and Mulliken charge, we demonstrate that all the S1 states of them also are intramolecular charge transfer states, which provide prospects for the study of their photochemistry and photophysics properties.As a ubiquitous phenomenon in chemistry, biology and other branches of science, hydrogen bonding plays a crucial role in solute-solvent interactions, recognition in biological macromolecules, and so forth. As the development of the laboratory equipment and theoretical computing methods, the concept of hydrogen bonding has been expended. Many new concepts have been added, such as dihydrogen bonding, metal-hydrogen bonding. The excited-state hydrogen and dihydrogen bonding of a dihydrogen-bonded phenol-borane-dimethylamine (BDMA) complex is investigated by use of time-dependent density functional theory (TDDFT) method. The coexistence of an intermolecular dihydrogen bond (B-H…H-O) and hydrogen bond (N-H…O) is confirmed by the optimized geometric structure of the dihydrogen-bonded phenol-BDMA complex. Upon photoexcitation, the hydrogen bond (N-H…O) is weakened while the intermolecular dihydrogen bond (B-H…H-O) is strengthened in excited states respectively. There will be a dehydrogenation process if they close to each other too much. The study provides prospects for the study of material with hydrogen storing potential. Then, the keto and enol conformers of 2′-deoxyguanosine and its monohydrated complex have been studied by TDDFT method. By the analysis of electronic structure, infrared spectra, and distribution of charge, it is demonstrated that the intramolecular hydrogen bond of keto conformer is weakened in excited state S1, while it is strengthened for enol conformer. For monohydrated complex, the intramolecular hydrogen bond is strengthened in excited S₁ state, while both the intermolecular hydrogen bonds are weakened. In contrast to isolated 2′-deoxyguanosine, the fluorescence intensity of 2′-deoxyguanosine-H₂O complex is increased. Therefore, a conclusion can be made that the weakening of intermolecular hydrogen bonds induces the strengthening of 2′-deoxyguanosine'fluorescence. Together with the theory that the strengthening of intermolecular hydrogen bond induces the quenching of fluorescence, both of them can be used to explain the fluorescence behavior of solute in different solvents.