Quantum chemical study on excited states and electronic coupling matrix element in a catechol-bridge-dicyanoethylene system

The excited states of a donor-bridge-acceptor (DBA) model system have been investigated using time-dependent density-functional theory (TD-DFT) in vacuo and in solution. It is found that the MPW1PW91 functional always gives higher excitation energies than those with a B3LYP functional. Results from both TD-B3LYP and TD-MPW1PW91 are found consistent with the experimental observations. The two most intense absorptions of the DBA system, one resulting from the local excitation of catechol moiety and the other from that of dicyanoethylene, possess the pi pi* transition feature. It seems that the solvent polarity does not remarkably influence the positions of absorption peaks. The spectroscopic properties of isolated donor, acceptor, and bridge and the donor-bridge compound have been investigated at the TD-B3LYP/6-31+G* and TD-MPW1PW91/6-31+G* levels. Results indicate that the donor and the acceptor are weakly coupled with the bridge. Therefore, it is more likely that the electron transfer takes place through a superexchange mechanism. In addition, we calculate the electronic coupling matrix elements according to the generalized Mulliken-Hush theory, and the detailed analyses also predict that the strong absorptions are due to the local excitation of the DBA system.