Solvent Effects Controlled Excited-state Structural Dynamics And Decay Mechanism Of 2-cyclohexen-1-one And 2-pyridone

The conjugated carbonyl and carbon-carbon double bond structures contained by 2-pyridone,2-cyclohexen-1-one and their derivatives are the same as thymine and uracil,which are two types of nucleic acid bases.By study the photophysics and photochemistry behaviors of these kind of compounds helps us reveal and understand the ultra-fast internal conversion mechanism（or in other words,photostability）of nucleic acid bases,and provides a short-time structure dynamics support for futher research of damage mechanism of DNA in biological system.This thesis studies excited-state short-time structure dynamics and decay mechanism of 2-pyridone,2-cyclohexen-1-one and their methyl derivatives by employ resonance Raman experment technique combined complete-active-space self-consistent-field method（CASSCF）.We aimed to investigate the mechanism of how solvent effects and methyl substitution regulating excited-state potential energy surfaces and adiabatic/nonadiabatic dynamic of decay channels.Results are as follows.（A）.Vibrational spectra,electronic spectra and resonance Raman spectra of six research compounds in a series of solvents were acquisited and carefully analyzed.We obtianed ultraviolet absorption spectra,fluorescence emission spectra,Micro Raman spectra,Fourier transform Raman spectra（FT-Raman）,Fourier transform IR spectra（FT-IR）and most importantly,resonance Raman spectra of research compounds.We carried out Density Function Theory（DFT）calculations to get structures and theoretical vibrational spectra of ground electronic state.Then the assignment of experiment vibrational spectra been accomplished.Furthermore,we adopted the Time-dependent Density Function Theory（TD-DFT）calculations to simulate ultraviolet absorption spectrum and get transition property and oscillator strength.（B）.Short-time structural dynamics of 2-pyridone upon excitation to the light-absorbing Sπ2 state were explored.Time-dependent wave packet theory was used in the simulation of the absorption cross section and resonance Raman absolute cross section in 2-pyridone.We acquired dimensionless normal mode displacements of 9 representative resonance Raman peaks.The dimensionless normal mode displacements were further converted to short-time structural dynamics in terms of the internal coordinates.The minimum energy structures of Sπ2 state and curve-crossing points（Sπ2Sn2 and Sπ2Sn1）were optimized at the CASSCF level of theory.The obtained short-time structural dynamics in easy-to-visualize internal coordinates were then compared with the CASSCF-predicted structural-parameter changes of Sπ2Sn2,Sπ2Sn1,and Sπ2-min.Our results indicate that the initial population of 2-pyridone in the Sπ2 state bifurcates in or near the Franck–Condon region,leading to two predominant（Sπ2Sn2 and Sπ2Sn1）internal conversion pathways.Strong nonadiabatic coupling between Sπ2 state and Sn2 state exists in the Franck-Condon region.A small amount of excited 2-pyridone reach the Sπ2-min,and emit fluorescence.（C）.We explored the adjust mechanism of solvent effects on the excited states of 2-cyclohexen-1-one.We also studied methyl substitution effects on excited state decay mechanism.The intensity pattern of resonance Raman spectra of 2-cyclohexen-1-one,2-methyl-2-cyclohexen-1-one and 3-methyl-2-cyclohexen-1-one in acetonitrile,methanol and water solvent indicate that short-time structural dynamics upon excitation to the light-absorbing S₂ state are mainly along the C1=O7 and C2=C3 stretch.Discrepancy quantum yield dual-emission were discovered in steady-state emission spectra in different solvents.Two minimum energy structures of S₂ state and Three curve-crossing points S₂S₁ were optimized at the CASSCF level of theory.Minimum energy structures S₂（P1）and S₂（P2）are ponsible for the two emission bands,while the curve-crossing points are three nonradiative transition deay channels competing with radiative transition.Both experiments and theoretical calculations show clearly that solvent and methyl substitution controls the S₂ state decay mechanism jointly.