Study Of Excited-State Potential Energy Surfaces Crossing Dynamics Of Conjugated Polyenal

Conical intersection(CI)plays a key role in the non-adiabatic decay dynamics of polyatomic molecules in the excited states,and they can lead to very fast electronic relaxation dynamics(internal conversion)on the femtosecond and picosecond timescales.With the development of femtosecond time-resolving technology and the improvement of computing level,great progress has been made in the field of CI.Most of the research work can give an accurate description of CI dynamics in simple system,and individual research work has also begun to explore the dynamic process of complex system,such as visual photochemical reactions,and clear evidence has been given of the photoisomerization phenomenon due to the twisting of double bonds that occurs in this process,but the research on the position of photoisomerization is relatively less.If we want to further understand the CI dynamic of complex systems,we need to use simplified model compounds to simulate actual molecular systems and reveal the reaction mechanism from the structure and structure-activity relationship.Therefore,the initial non-adiabatic decay dynamics of polyenal with different degrees of conjugation were studied using resonance Raman spectroscopy(RRs)and complete-active space self-consistent field(CASSCF)calculations,and to investigate the effects of conjugation degree and substituents on the potential energy surface crossing dynamics.Obtain the following results:(1)We obtain the ultraviolet absorption spectrum,Fourier infrared(FT-IR),Fourier Raman(FT-Raman)and resonance Raman spectra(RRs)at different excitation wavelengths of hexadienal(HAL),phenylacrolein(PAC)experimentally,and the UV absorption spectra and resonance Raman vibration modes of each molecule were assigned by time-dependent density functional theory(TD-DFT).The HAL A-band RRs is composed of the fundamental bands of 8 vibration modes,and its overtones and combination bands:v9,v10,v11,v12,v14,v16,v18,v19.The C4=C5 stretch mode v10,its overtones nv10(n=2,3),and their combination bands with other modes occupy the most Raman intensities of the spectra.This indicates that the initial structural dynamics of HAL in the FC region of the S2 state moves predominantly along the C4=Cs reaction coordinate.The PAC RRs is composed of the fundamental bands of 9 vibration modes,and its overtones and combination bands:v9,v10,v11,v12,v13,v16,v21,v23,v29.The C2=C3 stretch mode v10 has the highest strength,and the initial structural dynamics of PAC predominantly along the C2=C3 reaction coordinate.At the same time,the computed optimized geometric structures and computed excitation energies of the ground state,excited states and CIs points at CASPT2(8,7)/cc-pVTZ//CASSCF(8,7)/6-3 1 G(d)level.(2)The resonance Raman and the A-band absorption cross-sections in cyclohexane of HAL are simulated using a time-dependent wave-packet theory coupled with Brownian Oscillator solvent model.The seven main vibration simulations in the spectrum were selected,in the present case,there are 27=128 possibilities in the determination of the internal coordinate changes.It was screened based on the electronic transition characteristics to obtain the two most probable internal coordinate changes,which were compared with the structural parameter changes of the four S2S1 intersections predicted and calculated by CASSCF.Our results indicate that the initial population of HAL in the S2 state ramifies in or nearby the FC region,leading to five S2?Si internal conversion pathways due to the flexibility of the molecular chain and the different electronic resonant structures formed nearby FC of the S2 state.And the CI(S2S1)structure of H-CO=C ?-and-C ?=CH=CH2 perpendicular to each other makes HAL have a doorway for trans-cis isomerization reaction to occur in the S1 state after the S2?S1 internal conversion via the CI(S2S1)conical intersection.(3)The time-dependent wave packet theory was used to simulate the RRs of PAC in cyclohexane solvent at the excitation wavelengths of 273.9 and 282.4 nm.The displacement of the main vibrational modes is obtained,which is transformed into the internal vibration coordinate form of lOfs after the molecules are excited,and compared with the intersection structure calculated by CASSCF.Our results indicate that the excited state wave packet of PAC differentiates in two directions in the FC region:one direction is to go to the CI(S2S1)outside the FC region,and then enter the Si potential energy surface;the other direction is that the wave packet decays to S2 through the intersection of CI(S3S2)and emit fluorescence back to the ground state.(4)The effects of polyenal with different conjugation degrees on the dynamics of the potential energy surface crossing were systematically analyzed.The intensity pattern of the resonance Raman spectrum of the system molecule shows that the excited state of the conjugated polyenal moves predominantly along the C=C and C=O reaction coordinate.And with the increase of conjugation degree,the intensity ratio(peak area ratio)between C=O and C=C peaks decreases.In addition,more than one CI points with different geometric conformation or difference electronic resonance structure were revealed when C?=C? was substituted by aromatic ring.