Study On The Mechanism Of Excited State Intramolecular Proton Transfer And Aggregation-induced Luminescence Of Quinazolinone Derivatives

Photochemical reactions play an important role in both nature and human society,therefore,photochemistry(also known as excited-state chemistry)are of great practical significance.Due to its complexcity with respect to ground-state thermal chemistry,research on photochemistry remains a hot and challenging topic.It is experimentally known that combining excited-state intramolecular proton transfer(ESIPT)to aggregation-induced emission(AIE)brings some advantages,such as no self-quenching at high concentration and large Stokes shifts,therefore,such systems have a wide range of potential applications.Understanding the emission mechanism of these luminophores is essential for the molecular design and construction of functional system.In this thesis,we have employed the density functional theory(DFT),time-dependent density functional theory(TD-DFT),and high-level complete active space self-consistent field method(CASSCF),combined with the Quantum Mechanics/Molecular Mechanics(QM/MM)method,to explore the mechanism of fluorescence quenching in THF solution and AIE mechanism in crystal of a 2-(2-Hydroxy-phenyl)-4(3H)quinazolinon(HPQ),and clarified the role of ESIPT process in the mechanism of AIE.Furthermore,we have carried out on-the-fly trajectory surface-hopping(TSH)nonadiabatic molecular dynamics method to confirm the mechanism of fluorescence quenching of HPQ.The main contents include:1.In Chapter 3,we report a QM(MS-CASPT2//TD-DFT,MS-CASPT2//CASSCF)and ONIOM(QM:MM)studies on the fluorescence quenching and AIE mechanism of 2-(2-Hydroxy-phenyl)-4(3H)-quinazolinone(HPQ)with typical characteristics of AIE and ESIPT as an exemplar case.The computational results indicate that in THF solution,once being excited to the S1 state,the molecule tends to undergo an ultrafast,barrierless ESIPT from enol to keto tautomer,and then accesses a S1/S0 conical intersection in the vicinity of a C=C bond twisted intramolecular charge-transfer(TICT)intermediate,leading to nonradiative decay from excited to ground state.Hence,the TICT-induced nonadiabatic transition accounts for the fluorescence quenching in solution.Contrarily,in the solid state,the non-radiative relaxation pathway via C=C bond rotation is suppressed due to environmental hindrance,leaving the ESIPT-induced enol-keto tautomerization as the only excited-decay channel,thus the fluorescence is observably enhanced in crystal.2.In Chapter 4,we have employed the TSH molecular dynamics to confirm the fluorescent quenching mechanism of HPQ in gas phase.The potential energies and gradients are calculated on-the-fly at the TD-DFT level.Based on the analyses of total 188 trajectories started from the S1-state FC(Z-enol)geometry,we have found that all trajectories can undergo an ultrafast ESIPT process within 20.3 fs.Subsequently,the formed keto tautomers start to rotate around C5=C6 bond,and between 729.3 fs-2000.0 fs,96%of trajectories are maintained at regions where |?|=90.0°.It's worth noting that,within an average hopping time of 1234.8 fs,23%of trajectories switch from S1 state to S0 state.In addition,one can find that the structures of S1? S0 hopping points are similar to that of the S1/S0-CI optimized by electronic-structure calculation.The fluorescent quenching mechanism was well reproduced by the combined electronic structure calculations and dynamic simulations.