Theoretical Studies On Vibrationally-Resolved Molecular Spectroscopies And ππ~*/nπ Decay Dynamics

Molecular spectroscopies are key characterization techniques in modern material science,have become one of the most powerful analytical tools to understand the molecular structures and properties.With the development of quantum chemistry methods,we can compute fine structures,analyze the potential energy surfaces(PESs),and assign the main vibronic bands,providing comparable results with the experiment.Theoretical studies can reveal the important motions hidden in electronic and even in nuclei’,which should be very helpful in analyze molecular structure and optical responses.Therefore molecular spectroscopies and fine structures are studied in this thesis,by employing first principles calculations.Both time-independent(TI)and time-dependent(TD)approaches are employed to calculate the vibrationally-resolved spectra of absorption(ABS),electronic circular dichroism(ECD),emission(EMI)and circularly polarized luminescence(CPL).In fect,TI and TD are complementary in theoretical spectroscopies studies,TI can provide useful information in understanding fine structures and molecular structures by permitting to identify the modes that most contribute to the spectral shape,but convergence problems challenge the TI strategy,while TD allows us to get fully converged spectra without the ability to assign the main vibronic bands.Vibrationally resolved spectra have been computed in harmonic approximation including Duschinsky effects and accounting for both Franck-Condon(FC)and Herzberg-Teller(HT)contributions.The relative importance of solvent inhomogeneous broadening,torsional flexibility,FC vibronic progressions,and inter-state couplings in determining spectral shapes has been discussed in details.Furthermore,an effective mixed quantum/classical method has been proposed to study the solvent dynamical effects on the ππ/nπ decay of Thymine in water,can deal with both solute and solvent dynamics at the same time.The details have been discussed in the following:1.Theoretical studies on molecular spectra of a large flexible compound.Multi-photon absorption(MPA)has attracted significant attention in various promising practical applications,due to its greater penetration depth which allows a high degree of spatial selectivity in three dimensions.Density functional response theory has been applied here to investigate in detail the one-,two-,and three-photon absorption properties of a symmetric carbazole derivative(named POCP).The relationship between molecular structure,charges transfer and molecular properties have been investigated in details.However,the study of POCP shows that a pure electronic approach fails to reproduce the complex shape of the linear absorption.Therefore we investigated in detail the vibronic contributions to the one-photon electronic spectrum of POCP.However,our results highlight that the investigated POCP with pronounced flexibility represents a big challenge for available methodologies due to the existence of many close-lying excited electronic states coupled by internal low-frequency and high-frequency motions and by solvent fluctuations.We employed a mixed quantum/classical(QC)approach to compute the contribution of the torsions to the spectrum.Our calculations allow us to obtain a non-phenomenological description of the low-energy part of spectrum in good agreement with experiment,accounting for FC vibronic progressions,torsional flexibility and solvent inhomogeneous broadening.2.Comparison of the line shapes of the full set of the ABS,ECD,EMI and CPL vibrationally resolved spectroscopies.With the development and innovation of theoretical methods,the calculation models of vibronic ABS,ECD and EMI have been well developed.However,there is a very limited experience in the calculation of vibronic CPL spectra and in the comparison of the line shapes of the full set of the ABS,ECD,EMI and CPL spectroscopies.Therefore,the vibronic spectra of ABS,ECD,EMI and CPL have been computed with AH(Adiabatic Hessian)and VH(Vertical Hessian)models.Both Duschinsky and HT effects were considered.Our results are in good agreement with experiment.It clearly shows that HT effects dominate the shape of ECD and CPL spectra and have a remarkable impact also on ABS and EMI.They are the responsible for the sign inversion observed in ECD and CPL,for the differences between EMI and CPL and between ABS and ECD spectral shapes and,finally,for the breaking of the mirror symmetry between ABS and EMI and between ECD and CPL.3.Solvent dynamical effects on internal conversionThe above calculations are computed in BOA(Born-Oppenheimer adiabatic approximation),by neglecting the nonadiabatic couplings.However,when the PESs are coupled(diabatic states),the nonadiabatic couplings will be of vital importance.Here we present an approximate yet fast and effective mixed quantum/classical approach,quantum dynamical treatment of the intramolecular vibrations of the solute undergoing the IC with the classical dynamics of the surrounding medium.Both implicit polarizable continuum model and explicit solvent models have been used to investigate the ππ*/nπ*decay of Thymine.The solvent effects on the IC dynamics have been discussed and analyzed in three ways:(1)average static effects;(2)static disorder;(3)true dynamical solvent effects.Our results highlight that the solvents have an effects on internal conversion processes,which indicates the importance of solvent dynamics studies on the study of IC processes.The present thesis is divided into 8 chapters.Chapter 1 is the review,with brief introductions on the linear and nonlinear optics and the related spectroscopies,introduction on the vibronically resolved spectra and solvent models are also given.Special attentions are paid to the purpose and significant of current research.Chapter 2 collects the basic quantum chemistry methods used in the thesis,including density functional theory,molecular dynamics,and mixed quantum classical model.Chapter 3 presents the methods to calculate the multi-photon absorption,vibronically resolved spectra,mixed quantum/classical(QC)approach,mixed quantum/classical dynamic(MQC)approach.Chapter 4 employs the first principles to study one-,two-,and three-photon absorption properties of POCP.Moreover,the broad one-photon absorption line-shape of flexible POCP has also been analyzed in details.Chapter 5 shows how the vibronic coupling explains the different shape of electronic circular dichroism and of circularly polarized luminescence spectra of hexahelicenes.An effective mixed quantum/classical method highlights femtosecond solvent dynamical effects on the ππ*/nπ*decay of thymine in water was proposed in Chaper6.Chapter 7 is the summary and perspective.