(Invited) Theoretical Study on the Environmental Effects on Photoexcited Properties of Donor-Acceptor-Linked Molecules

Abstract

Photoinduced charge separation process plays important roles in various fields such as natural and artificial photosynthesis, solar cell, and light-emitting diode. Toward an understanding of the molecular mechanism of photoinduced charge separation, we have been studying the excited and charge-separated states of donor-acceptor-linked molecules with theoretical methods such as QM/MM method and molecular dynamics (MD) simulation. In particular, we have focused on the environmental effects on photoexcited properties. In this presentation, we will present recent progress of our research. For example, we will talk about theoretical analysis of acceptor-donor-acceptor type non-fullerene acceptor called TACIC. The excited state of TACIC is known to have a longer lifetime in the film state than in solution. This is in contrast to a similar acceptor-donor-acceptor type non-fullerene acceptor molecule, ITIC, which has a shorter lifetime in the film state than in solution. However, the molecular mechanism is unknown. We analyzed the structures and excited states of the TACIC and ITIC by using quantum chemical calculations and MD simulation. First, it is found that the excited-state potential energy surface of TACIC monomer is similar to that of ITIC monomer, which is consistent with the experimental result that both have similar lifetimes in solution. Next, the packing structure of TACIC in the film state is much different from that of ITIC. Analysis of the excited states of dimer obtained from the film state structure demonstrated that the transition dipole of TACIC dimer is much smaller than that of ITIC dimer. These results could be related to the difference in lifetime of the excited state between TACIC and ITIC in the film state. Further analysis is currently in progress. We will also talk about the photoexcited properties of porphyrin-fullerene linked molecules.