Quantum Chemical Calculation of Excited-State Properties of Porphyrin-Fullerene Linked Systems

Abstract

Electron transfer (ET) is a fundamental process in many fields. Especially, photoinduced ET is crucial for natural and artificial photosynthesis because long-lived charge separation (CS) states are needed for the high efficiency of the chemical reaction and light-to-electricity conversion. To investigate the photoinduced ET properties accurately, the donor-acceptor linked systems have gathered more attention. Porphyrin-fullerene linked systems, such as ZnP-xyn-C60 (n=1-5), are often experimentally investigated because of their excellent ET properties. In the ZnP-xyn-C60 systems, zinc porphyrin, as an electron donor, and fullerene, as an electron acceptor, are connected with one or more oligo-p-xylene groups, as a linked moiety. Because the linked moiety is a rectilinear rigid bridge, the systems exhibit a length-independent electronic structure for local excited states of porphyrin and fullerene. Therefore, these systems are suitable for evaluating distance-dependent CS properties. The photorelaxation pathways of ZnP-xyn-C60 in benzonitrile (PhCN) solution were characterized by the time-resolved transient absorption (TRTA) spectroscopy. The final CS efficiency is changed by the length of the linked moiety and has a maximum value at n=2. The reason is not well known, and for a detailed analysis of the photoinduced ET process, it is necessary first of all to understand the excited-state properties. However, it is difficult to quantitatively evaluate these excited states, especially the CS states, with theoretical methods. The relaxations of both system structure and solvation shell can greatly affect the stability of the CS state, and thus a proper treatment is required. The purpose of this study is to examine the excited-state properties of donor-acceptor linked molecules, ZnP-xy2-C60, with the largest final CS efficiency in PhCN solution. To reduce the computational cost, long side chains of porphyrin are replaced with smaller ones. We perform the time-dependent density functional theory (TDDFT) calculation at LC-ωHPBE and ωB97XD/6-31G(d) level. We apply the Tamm-Dancoff approximation to avoid negative excitation energies in triplet states. For the solvent effect, we first employ the linear response SMD polarizable continuum model, in which solvation is in equilibrium to the ground-state solute electronic structure, and the solvation effect at the excited state is taken into account within the linear response theory. It is found that ωB97XD is more suitable for the evaluations of local excited states. However, the calculated CS-state energies are much higher than the local excited states, even if the relaxations of both system structure and solvation are considered, indicating that the linear response solvation model is insufficient. In addition, we apply , in which solvation response to the excited-state solute electronic structure is partly included. Nevertheless, the results are only slightly improved. Therefore, it is considered that a more sophisticated method which treats solvent molecules explicitly, such as the QM/MM method, is essential for the proper description of CS states. Further analyses are presented in the poster session.