Theoretical Study On Photoinduced Proton Transfer And Spectral Changes Of Several Molecules

In this thesis,the excited state hydrogen bonding dynamics and proton transfer process of several molecular systems are studied.The path of proton transfer process and fluorescence detection mechanism are explained by analyzing their spectral properties and fluorescence decay process.In the process of research,the density functional theory,non-adiabatic dynamics simulation and wave function analysis have been used.The dynamic behaviors and photophysical mechanism of hydrogen bonded systems are discussed comprehensively.Firstly,the excited-state molecular dynamics of 7-Hydroxyquinoline-8-carboxylic acid(HCA)were studied to observe its fluorescent properties and proton transfer process.The two intramolecular hydrogen bonds(O-H···O and O-H···N)of the Enol form were both strengthened after photoexcitation to the first excited(S₁)state.The experimentally observed fluorescence emission(465 nm)was attributed to the theoretical Keto form(470 nm),which implies the occurrence of an excited-state intramolecular double proton transfer(ESIDPT)process.The nonadiabatic dynamics results demonstrate that a single proton transfer from the carboxylic to the nitrogen atom(process-A)occurs in 55 fs,which excludes the ESIDPT concert pathway.The potential energy surface results indicate that process-A(0.08 kcal/mol)induces the occurrence of a second proton transfer from the phenol to the oxygen atom(process-B).Compared to the stepwise ESIDPT reaction that begins with process-B(1.53 kcal/mol),process-A is energy favorable in the S₁state.Therefore,we propose a reaction path of the following:Enol in the ground state(S₀)?Enol in the S₁ state?proton transferred Keto in the S₁state(stepwise pathway begins with process-A)?Keto in the S₀ state(fluorescence emission at 470 nm)?Enol in the S₀state(reversed proton transfer).Secondly,the fluorescent behaviors and reaction mechanism of 10-hydroxybenzo[h]quinolone(HBQ)on the detection of phenylboronic acid(PBA)have been theoretical studied.The potential curves scan of HBQ in the S₁ state indicates that the enol form transfers to keto form without any energy barrier.This enol?keto transfer is induced by excited state intramolecular proton transfer(ESIPT)in the S₁ state,which occurs in 50 fs with fluorescence emission located at 615 nm.The fluorescence peak of hydrogen bonded HBQ-PBA blue-shifts about 25 nm compared to monomer HBQ,which dues to the formation of intermolecular hydrogen bond.Thereafter the intermolecular hydrogen bond interrupts the ESIPT and forms reaction products of benzo[h]quinolin-10-yl hydrogen phenylboronate(BQHP)and water.The result of nonadiabatic dynamics indicates that the internal conversion of BQHP complex is forbidden and the fluorescence emission is the main decay pathway.The BQHP exists a fluorescence emission located at 564nm(S₁),which blue-shifts about 50 nm with the HBQ molecule.This work provides a reasonable mechanism of HBQ on the detection of phenylboronic acid.Finally,The detection mechanism of 2-(4-nitro-1,3-dioxoisoindolin-2-yl)acetic acid(CORM3-green)on CO-Releasing Molecule-3(CORM-3)and their corresponding fluorescent behaviors have been theoretical studied.The Keto form of reaction product 2-(4-amino-1,3-dioxo-2,3-dihydro-1H-inden-2-yl)acetic acid(PTI)exists the fluorescence peak located at 423 nm,which transfer to PTI-Enol with the fluorescence peak at 689 nm induced by the ESIPT.This result is dramatically different for the previous conclusion that the fluorescence emission of 503nm is attributed to the proton transferred PTI-Enol.To illustrate this confusion,the solvent effect,specifically intermolecular hydrogen bonds(HBs)between PTI and water have been considered.The formation of intermolecular HBs induces the absorption peak of PTI red-shift about 46 nm(from 362 nm to 408 nm),which is agreement with the experimental data(390 nm).After excitation to the fluorescent state,the strengthening of intermolecular HBs hinders the formation of intramolecular HB and the occurrence of proton transfer of PTI-Keto,inducing the red-shift of fluorescence emission to 489 nm.This peak is attributed to the experimentally observed fluorescence emission in 503 nm.It is confirmed that experimental Stokes shift of 113nm(from 390 nm to 503 nm)is caused by the intermolecular hydrogen bonding while not the proposed reason of ESIPT process.This work provides a reasonable explanation for the detection mechanism of CORM3-green and experimental fluorescence phenomenon.