(Invited) Theoretical Study on the Excited and Charge-Separated States 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 MD simulation. In this presentation, we will mainly introduce two topics of our research. The first one is porphyrin-fullerene linked molecules. It is experimentally known that the yields of charge-separated states of these molecules is not linear with the bridge length, but reaches a maximum at a specific length. It is also known that the energy of the singlet excited state of this compound is lower in benzonitrile solvent in the following order: local excited state of porphyrin, local excited state of fullerene, and charge separation state. However, by simply using the stable structure of the ground state in the theoretical calculations, the energies of each excited state are in the exact opposite order, and thus the experimental results could not be reproduced. In order to reproduce the experimental results, the structural relaxation of the molecule in the excited state and the response of the solvent are important. However, a simple polarization continuum model of the solvent is insufficient to stabilize the charge separation state. Therefore, we used the QM/MM RWFE-SCF method, which efficiently combines the QM/MM method with MD simulations to optimize the geometry of the solute molecule (QM region) with quantum chemical calculations including the effects of the thermal fluctuations of the surrounding environment such as the solvent molecules (MM region) treated by the molecular mechanics force field. The energy differences calculated with the QM/MM RWFE-SCF method are in good agreement with the experimental results. The charge-separated state is greatly stabilized by solvent molecules due to the large dipole moment. The next topic is 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 non-fullerene acceptor molecule, ITIC, which has a shorter lifetime in the film state. 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. It is found that the packing structure of TACIC in the film state is much different from that of ITIC, which could be related to the difference in lifetime of the excited state. Further analysis is currently in progress.