Theoretical Studies Of Excited State Proton Transfer For Small Molecules

In general, the process of excited state proton transfer (ESPT) involves transfer of a hydroxy (or amino) proton to an acceptor, and meanwhile a proton transfer tautomer is formed. ESPT has an important role in a variety of biological and chemical phenomena. It is also a hot topic at the experimental and theoretical research.In this dissertation, we investigated the excited state intramolecular and intermolecular proton transfer mechanism of small molecules using density functional methods (DFT) and time-dependent density functional methods (TDDFT). By calculating the molecular structure, dipole moment, frontier molecular orbitals, absorption spectrum, fluorescence spectrum, potential energy surface, and so on, we can explain the mechanism of ESPT.In this work, the excited state charge transfer/excited state double proton transfer (ESCT/ESDPT) reaction of 3-CNAI,5-CNAI, and 3,5-CNAI in methanol solvent, and the photoinduced solute-solvent proton transfer mechanism of 6-HQc (6-Hydroxyquinolinium) and 6-MQc (6-hydroxy-l-methylquinolinium) in water and 6-HQc in acetonitrile with residual water are investigated. The calculated results indicate that:1) The sites where cyano group substituted on 7-azaindole induce a large difference of electron density distribution in excited state. When cyano group substituted on meta-pyrrole moiety, the electron which is transferred to pyridine moiety for 7-azaindole at excited state will partly transfer to cyano group. This may make electron density distribution change a little. Cyano group substituted on meta-pyridine moiety will lead to the increasing of electron acceptability of pyridine moiety, which causes the electron density distribution varied largely. However, the formation of intermolecular hydrogen bond does not largely influence the orbital transition. In addition, for the PT-3-CNAI-MeOH, PT-5-CNAI-MeOH, and PT-3,5-CNAI-MeOH complexes, the electron density of the HOMO is mostly localized on the pyrrole moiety, while the electron density of the LUMO is mostly delocalized on pyridine moiety. Therefore, electron density distribution in tautomer excited state will be strongly influenced by the double proton transfer for PT-3-CNAI-MeOH, PT-5-CNAI-MeOH, and PT-3,5-CNAI-MeOH complexes.2) When photoexcited to S1 state for 6-MQc-H2O,6-HQc-H2O and 6-HQc-2H2O complexes, only the 6-MQc and 6-HQc moieties should be electronically excited. The absorption, fluorescence, and proton dissociation fluorescence spectra of the 6-MQc in water are redshifted relative to those of the 6-HQc in water and blueshifted relative to those of the 6-HQc in acetonitrile containing residual water. In addition, because of the solute-solvent proton transfer reaction, the dissociation fluorescence spectra are largely redshifted compared with corresponding fluorescence spectra for all complexes.We also investigate the excited state intramolecular proton transfer (ESIPT) mechanism of HBQ, DIHBQ, HBT (2-(2-hydroxyphenyl)benzothiazole) derivatives, and the 3-hydroxy-2-(pyridin-2-yl)-4H-chromen-4-one (1a). The calculated results indicate that:1) For the HBQ and DIHBQ, the enol phosphorescence peak of HBQ which has not been detected in experiments is at 669 nm, and the experimental phosphorescence spectrum of DIHBQ (λmax=735 nm) is the enol phosphorescence, which was assigned as keto-tautomer phosphorescence in the previous study. In addition, the calculated keto phosphorescence peaks of HBQ and DIHBQ are at 1331 nm and 1201 nm; 2) Because of the influence of electron withdrawing property of Br and donating property of OCH3; the intramolecular hydrogen bonds of Br-HBT are shorter than those of OCH3-HBT, and the absorption and emission spectra of Br-HBT are redshifted relative to corresponding spectra of OCH3-HBT. In addition, due to the larger rigid structure, the absorption and emission spectra of BBTHQ are redshifted relative to those of Br-HBT and OCH3-HBT.3) 1a had two hydrogen bonding configurations:one is hydrogen bond located at O-H…O=C, forming five-member ring (1a(O)), the other is hydrogen bond located at O-H…site, forming six-member ring (1a(N)). The energy of 1a(O) and 1a(N) in ground state, excited state, and tautomer excited state indicats that the 1a(N) is more stable than 1a(O).