| Summary: | 碩士 === 國立臺灣大學 === 化學研究所 === 105 === Because of the promising potential in the high-efficiency solar cell,singlet exciton fission (SF), the molecular analog of multiexciton generation in which the absorption of one photon results in the generation of two triplet excitons, has attracted intensive research attention recently. Although the striking research are advancing, the mechanism of SF is still controversial, and some open questions remain, e.g., the key parameters manipulating the occurrence of SF and the excited state wavefunctions involved in it. From an electronic structure point of view, we construct an approximate diabatic basis to unambiguously interpret the character of the excited states, by applying the restricted active space equation of motion single and double approach to show the importance of considering multi-configurational effects in polyacene dimers. Using a three-state model, the effective hamiltonian shows that strong superexchange effective couplings depend on little on the choose of the basis set and the size of the active space while the diabatic state energies depend on a lot. Second order perturbation theory is introduced, the results show strong couplings and near degeneracy condition that explain the ultrafast SF mechanism, which shows striking the balance between static and dynamic correlation is also important in conjugated systems. Finally, we show that dominant effective couplings in a dimer system strongly correlated with the monomeric orbital overlap between two molecules. Hence, in a dimer system, the overlap between two monomers elucidate the dependence of the relative orientation of monomers on the efficiency of SF providing a criterion to the design principle of high SF efficiency dye-sensitized cells.
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