Multi-scale Theoretical Simulations For Electronic Spectra Of Functional Molecules

Electronic spectroscopy is one of the most important techniques to study the electronic structures of molecules,and is always used for the research of functional melecules that have many important applications in the fields of physics,chemistry,material,biology and so on.Meanwhile,with the development and improvement of quantum chemistry methods and the huge advancement of computer technology,theoretical calculations have not only achieved great success in the field of ground-state properties of functional melecules,but also are playing an increasingly important role in the study of electronic spectra and excited-state properties for them,which is of great significance for understanding the structures and optical properties of them and rationally designing novel functional melecules.Density functional theory?DFT?and time-dependent density functional theory?TDDFT?are one of the most popular methods for dealing with the ground state and excited state calculations,respectively.However,there are still many challenges in the theoretical study of electronic spectroscopy,for example,how to reasonably simulate vibrationally-resolved electronic spectroscopy of small-sized molecule so as to better reflect the influence of nuclear vibration on the spectrum;how to accurately and efficently predict the electronic spectroscopy for medium-or large-sized functional molecule;how to correctly reflect the effects of environment on the electronic spectroscopy for functional molecule.Therefore,to address these challenges mentioned above,this dissertation focuses on the theoretical characterization of the electronic spectra for several functional molecules whose spatial scales are from small to large,exploring and developing suitable theoretical models and methods that mainly based on the DFT and TDDFT,and combining with molecular dynamics simulation or nuclear ensemble approach,which means the"multi-scale"in terms of molecular systems and methods,and also reprensents three common external environments in calculations,namely,gas phase,implicit and explicit solvent environments.On the one hand,the research results can explain the experimental spectra and then understand the relationship between the electronic structures and properties for functional molecules,which can provide theoretical guidance for the syntheses and designs of novel functional molecules;on the other hand,the experimental results can in turn verify the reliability of the theoretical methods,so that the research methods described in this dissertation can also be applied to study electronic spectra of other similar molecular systems.The dissertation consists of six chapters and the content is as follows:In Chapter 1,an introduction of the background,significance and main contents of research in this dissertation are given.In addition,a brief introduction of the functional molecules and the electronic spectroscopy are also provided.In Chapter 2,an overview of computational chemistry,including quantum chemistry,molecular dynamics and nuclear ensemble approach are given,especially for the development and current status of DFT.In Chapter 3,the negative ion photoelectron spectra in the gas phase of small-sized functional molecules?NaS₅^-and P₂N₃^-?are theoretically studied based on TDDFT and nuclear ensemble approach.The innovation in this chapter is that the nuclear ensemble approach is introduced to represent the nuclear vibrational effects on the photoelectron spectroscopy and the ionization intensities are charactered by Dyson orbitals calculated from TDDFT.The theoretical photoelectron spectra for NaS₅^-and P₂N₃^-anions agree well with the experimental spectra.The errors of ionization potentials are less than 0.2eV,and the calculated ionization intensities match experimental data.Moreover,it is found that the nuclear vibrational effects are more important to the photoelectron spectroscopy for the rigid P₂N₃^-molecule.In Chapter 4,the electronic absorption spectra for medium-sized fluorescent ligands and large-sized supramolecular metallacycles through self-assembly driven by metal-ligand coordination interaction are theoretically studied by TDDFT.The errors of absorption wavelength are about 10 nm,and the similarity factors are over 0.97,which indicates the well agreement between the experimental spectra and theoretical spectra that computed using optimally-tuned range-seperated functional?LC-?PBE*?.This work provides a reliable and efficent theoretical tool for the simulations of electronic spectra for supramolecular coordination complexes.In Chapter 5,the electronic spectra and luminescence characteristics of two novel fluorophores for biological imaging in the second near-infrared window using a multi-scale simulation method combining with molecular dynamics simulation and TDDFT are studied.The effects of the solvent?water or toluene?on the geometries of the fluorophores can be reflected by molecular dynamics simulation,and the calculated absorption spectra based on the multi-frame geometries from molecular dynamics simulation are in good agreement with the experimental spectra,which can correctly predict the red shift of the spectra in water solvent.The exploring of multi-scale simulation in this work can provide helpful guidance for the study of electronic spectra for other complex molecular systems in the future.Finally,Chapter 6 summarizes all the previous work and gives some directions for subsequent work.