| Molecular spectroscopy is the study of molecular motion and interaction of radiation and matter.Contemporary spectroscopy technology has been widely used in various research fields in chemistry and physics,and is one of the most important tools to obtain information about analysis of materials and dynamic characteristics.Based on molecular absorption spectrum(ABS)and emission spectrum(EMI),the properties of the ground state and excited state of the molecule can be obtained,thereby giving various physical information of the system;for chiral molecules,circular dichroism(ECD)and circular polarization spectroscopy(CPL)can be used realizes the analysis of spectral signals and material structures;resonance Raman(RR)spectroscopy can simultaneously reveal the properties of the ground state and excited state,and is also an effective tool for studying the properties of molecular excited states.In order to better understand the dynamic information of the excited state,it is necessary to use dynamics for analysis.Due to the coupling between electrons and nuclei,quantum effects must be considered in order to accurately consider the coupling between excited state radiation and non-radiative transitions.That is,it is analyzed using the very challenging quantum dynamics(QD).However,due to the huge amount of calculation of QD,the current method is limited to small molecule systems.This paper combines the multi-layer multi-configuration time-dependent Hartree(ML-MCTDH)and considers all the vibration modes involved in the system to challenge the theoretical simulation of the non-adiabatic vRR of QDs.Based on density functional theory(DFT)and time-dependent density functional theory(TDDFT),this paper studies the vibrationally resolved electronic spectra of various aromatic compounds,reveals the properties of the ground state and excited state of molecules,and analyzes the structure and spectroscopic characteristics of substances.The non-adiabatic quantum dynamics method explores the time-dependent dynamic evolution of excited states.During the research process,it was found that when there is a weak coupling between electronic states,the Herzberg-Teller(HT)effect can give an ideal description result;and when there is a strong coupling between electronic states,it is necessary to consider the non-adiabatic coupling effect(theoretical difficulty).The thesis mainly includes the following five research parts:1.Theoretical Study on the Vibrationally Resolved Circular Dichroism Spectrum and Circular Polarization Emission Spectrum of Boron-Fused Double Helicene.Helicene compounds have helical chirality and good luminescent properties,so the synthesis and properties of helicene and its analogues have become one of the current research hotspots.In the first part of this paper,the vibrationally resolved ABS,ECD,EMI and CPL spectra of boron-fused double helicene were investigated based on the adiabatic(AH)and vertical Hessian(VH)theoretical models using timedependent(TD)and time-independent(TI)methods.Among them,TI can effectively assign important vibration transitions,but it may face difficulties in convergence.TD achieves short-term rapid convergence through Fourier transform and solves the problem of difficult convergence,but it cannot assign transitions.The main difference between the AH and VH models is that they use different approximations for the construction of the potential energy surface of the final state.During the research,it was found that both the AH and VH models can reproduce the fine vibration structure of the experimental spectrum very well,but the VH model shows better performance on the simulation of the spectral shape.In addition,in order to fully analyze the influence of vibrational coupling on the spectrum,this chapter comprehensively considers Duschinsky mixings,Franck-Condon(FC)effect and Herzberg-Teller(HT)contribution.The results show that the vibrational coupling effect plays an important role in reproducing the experimental spectrum,among which the HT effect is the main reason leading to the spectral shapes of EMI and CPL,which not only adjusts the relative heights of different vibrational peaks,but also improves the agreement with the experiment and the HT effect is the main reason for breaking the mirror symmetry between the ECD and CPL spectra.2.Photophysical Study of Maleimide-Substituted[5]Carbohelicene DerivativesThe first part of the study confirmed the importance of weak vibration coupling in the transition,and confirmed the feasibility of the theoretical method used,laying a solid foundation for studying the regulation of dopant atoms on spectral properties and the influence of substituents on spectral properties.[5]Carbohelicene derivatives are generally unable to show chirality.While stable chirality could be achieved by introducing bulky substituents.To understand the substituent effect on the optical properties,this chapter present a systematic study on the chiroptical properties of two[5]carbohelicene derivatives(Helilm and MeOHelilm)from pure electronic level to vibrational analysis.Electronic calculations show that the introduction of methoxy(MeO-Helilm)promotes the electron transfer and causes narrower Molecular Orbital(MO)gaps of key orbital pairs with respect to Helilm.This leads to the relative shift of the spectra between HeliIm and MeO-HeliIm.The vibrationally resolved spectra nicely captured the vibronic features and also correctly reproduced the different signs of chiroptical spectra,in nice agreement with the experiment.They show that Franck-Condon(FC)contribution is of vital importance in the reproduction of the experimental spectral shapes.However,Herzberg-Teller(HT)moderately improves the performance of the vibrationally resolved spectra.What is more,HT modes play a unique role in the analysis of the different optical properties.Finally,it is highlighted that the typical vibration of outof-plane-deformation of methyl in MeO is responsible for causing the different transition processes.3.Theoretical Study on Some Carbohelicenes Dyes with Tunable Emission Wavelength:Optical Properties,Vibronic Effect,Quantum YieldMolecules in an excited state will transition from a high excited state to a low excited state by radiative and non-radiative transitions,and the non-radiative process plays a pivotal role in the deactivation of the excited state,especially in the research of organic light-emitting materials.The competition with non-radiation directly determines the optical properties of luminescent materials.Quantum yield(QY)is the most important photophysical parameter to characterize the luminescence of molecules and materials,and the current research on QY is basically limited to the experimental level,and the corresponding theoretical research is very rare,mainly because of internal conversion(IC)and intersystem crossing.(ISC)calculation process is very complicated.This chapter realizes the calculation of IC and ISC transition coefficients based on the Fermi Golden Rule(FGR),and the obtained quantum yield value is 0.52,which is very close to the experimental value(0.65),which proves the reliability and accuracy of the research method.The results show that the introduction of BMes2(7B-HC)increases kr and reduces kISC which finally leads to the increase of QY.We also show that the competition between kr and kISC is the dominant reason for the quenched QY of molecule 7B-HC by 35%.The large SOC and small energy gap between S1 and the lowest six triplet excited states are highlighted for the highly accelerated ISC processes.The nice agreement with the experiment indicates that the QY calculation method can be provided as an efficient approach for the prediction of promising fluorescent dyes.4.Nonadiabatic Vibrational Resonance Raman from Quantum Dynamics Propagations with LVC models.Application to ThymineIn order to fully cover the range of experimental spectra,this chapter carried out the study of multiple excited states.As the number of excited states considered increases,coupled electronic states becomes non-negligible.HT perturbative approach can well describe the weak coupling between electronic states,but it cannot reasonably describe the strong coupling between states,may lead to large artifacts.Therefore,the vRR considering the non-adiabatic coupling effect becomes a powerful and effective tool to study the properties of excited states.This chapter expands to the theoretical research on vRR.In this study,the Hamiltonian of the system is first expressed based on the linear vibronic coupling(LVC),then the calculation of the non-adiabatic coupling term is realized based on the program independently developed by our research group,and then combined with the calculation of the non-adiabatic dynamics to get the timedependent Evolution.Taking thymine as an example during the study,seven excited states are considered.The research results show that the vRR at different excitation wavelengths is in good agreement with the experimental data.Computed vRR at different excitation wavelengths are in good agreement with the available experimental data.Up to 250 nm the signal is dominated by the lowest HOMO→LUMO transition,whereas at 233 nm,in the valley between the two lowest energy absorption bands,the contributions of all the three bright states,and their interferences and couplings,are important.Inclusion of solvent(water)effects improves the agreement with experiment,reproducing the coalescence of vibrational bands due to CC and C=O stretchings.With our approach we disentangle and assess the effect of interferences between the contribution of different quasi-resonant states to the transition polarizability and the effect of interstate couplings.Our findings strongly suggest that in cases of close-lying and potentially coupled states a simple inclusion of interference effects is not sufficient,and a fully nonadiabatic computation should instead be performed.We also document that for systems with strong couplings and quasidegenerate states,the use of HT perturbative approach,not designed for these cases,may lead to large artifacts.5.The Resonance Raman Spectrum of Cytosine in Water:Analysis of the Effect of Specific Solute-Solvent Interactions and Non-Adiabatic CouplingsAn accurate description of solvent effects is critical to reproducing experimental spectra,yet accounting for solute-solvent interactions using quantum dynamics methods is challenging.In order to fully consider the solvent effect on vRR,this chapter investigate the effect of specific solute-solvent interactions on the vRR spectra,by considering a cluster of cytosine,hydrogen-bonded by six water molecules,and embedded in a polarizable continuum.In this study,based on two recently developed time-dependent approaches,based either on quantum dynamical numerical propagations of vibronic wavepackets on coupled PES or on analytical correlation functions for cases in which inter-state couplings were neglected,the vRR of cytosine(considering the quasi-resonance with the eight lowest-energy excited states and all vibrational coordinates)was studied.By comparing the results of the two methods,these effects are only moderate in the excitation energy range explored by experiments,where the spectral patterns can be rationalized from the simple analysis of displacements of the equilibrium positions along the different states.Conversely,at higher energies,interference and inter-state couplings play a major role,and the adoption of a fully non-adiabatic approach is strongly recommended.In addition,the agreement with the experiment was significantly improved by considering a cluster of cytosine by considering a cluster of cytosine,hydrogen-bonded by six water molecules,and embedded in a polarizable continuum. |
