Theoretical Studies On The Structure-property Relationship And Exciton Dynamics Of Multi-scaled Systems

In this thesis, I present my major works during the period of my PhD study. In the past years, I focus my research on developing methods to calculate the electronic structure and spectral properties of systems with different sizes. This thesis just gives a description of these methods. For small molecules, we can employ high accurate ab initio or first-principles methods to gain the information needed to calculate the spectra; While for aggregates, wavefunction-based methods or density functional the-ories are too expensive, so we have to employ effective model Hamiltonians, where the microscopic parameters can be determined through high accurate quantum chemistry calculations whatever the Hamiltonians are. The following is a brief introduction to these methods:1. For small molecules, the displaced harmonic oscillator model and the anhar-monic oscillator model are employed to treat the intramolecular harmonic vibrational modes and anharmonic ones, respectively. In the first model, the optimal geometries and vibrational frequencies of the ground and excited states needed as input are pro-duced by ab initio methods(HF/CIS) or first-principles methods(DFT/TDDFT); In the second model, two time-independent Schrodinger equations are solved numerically, where the anhamonic potential energy curves of the ground and excited states are from quantum chemistry calculations as well.2. When the intermolecular distance is large or the intermolecular interactions are weak, Frenkel exciton(FE) model alone is efficient to simulate the spectra of oligomers. And the parameters related to Frenkel excitons themselves and the couplings among the Frenkel excitons are gained through quantum chemistry calculations.3. When the intermolecular distance is small, the intermolecular electron ex-change effect can not be neglected, and thus the effective model Hamiltonians should include Frenkel excitons and charge-transfer excitons(CTEs). Similarly, the micro-scopic parameters related to the excitons themselves and the couplings among the ex-citons can be determined via quantum chemistry methods.The above methods are employed to study the electronic structure and spectral properties of different systems, and the main results are summarized as follows:1. The displaced harmonic oscillator model is employed to investigate the spectra and dynamics upon photo-excitation of molecule AIP together with the anharmonic os- cillator model. The results show that the effect of anharmonic potential for the torsional motion plays a dominant role on the mirror-image symmetry breakage of absorption and emission line shapes in solution at room temperature, while the emission spectra at 77K possess well-resolved vibronic features because the anharmonic torsional motion is forbidden in the methanol glass matrix at low temperature. Besides, we also propose that the dynamical procedure on the first excited state is dominated by a relaxation pro-cess along the low-frequency torsional motion by investigating the PES of this state. The relation between the energy transfer rate from the AN moiety to the IP moiety and the bulk viscosity of the solvents can be well explained by the solution of the Langevin equation for the torsional motion.2. We present a theoretical study on the temperature-dependent absorption and photoluminescence spectroscopy of rubrene multichromophores by combining the time-dependent long-range-corrected density functional theory with the Frenkel exciton model. It is found that the spectral behavior of rubrene aggregates is very much de-pendent on aggregation details. Two types of aggregates up to heptamers are studied. Aggregates along b direction of the rubrene crystal are H-aggregated, and the 0-0 tran-sition in the emission spectra can not be observed at low temperature, while it appears gradually when the temperature increases; Aggregates along c direction of the rubrene crystal are J-aggregated, and the intensity of 0-0 transition decrease as the temperature increases. We take dimers as examples to demonstrate that the above spectral behavior is induced by the symmetry of the excitonic states.3. We characterize the nature of the low-lying excited states in H-aggregated pery-lene bisimide(PBI) dyes in terms of the recently proposed long-rage-corrected density functional theory(LRC-DFT) and the mixed FE-CTE model, and find that the experi-mentally observed low-energy absorption peak near 2.3 eV should be a mixed excitonic state. By involving four to five PBI chromophores, we have succeeded in reproducing the experimental trends of optical properties in PBI stacking, such as the large blue shift of the absorption band and the the large Stokes shift of emission spectra.