Crystal structure and spectroscopic properties of the CT complex of methyl viologen dication and o-dimethoxybenzene included in a polycyano-polycadmate host and electrostatic effects on its CT absorption

Using the polycyano-polycadmate host that is a negative-charged Cd cyanide complex with a framework structure built with Cd2+ ions and bridging cyanide ligands, we synthesized a clathrate including methyl viologen dication (MV2+) and o-dimethoxybenzene (ODMB) as guests. The color of the clathrate was reddish-brown, and its origin was considered to be a charge transfer (CT) interaction of a CT complex formed with MV2+ as an acceptor and ODMB as a donor in the host. The wavelength at the CT absorption maximum was largely red shifted as compared with that of the CT complex in an acetonitrile solution. The single-crystal X-ray diffraction analysis revealed a pi-pi stacking structure of the CT complex and a one-dimensional arrangement of the CT complexes in a channel-like cavity of the framework host. In the array of the CT complexes, MV2+ ions are adjacent to each other and their separation is short. The direction of the transition moment of the CT absorption determined from the structural information and the spectrum measured on a crystalline sample agreed with that derived from ab initio calculations at the CIS/6-31+G* level. Our calculations also showed that the red shift of the CT absorption is mainly due to electrostatic effects between the CT complexes, not due to the shortening of the distance between MV2+ and ODMB, which is generally believed to be the reason. The negative-charged host stabilizes the ground state of the CT complex, but the electrostatic interaction between the CT complexes heightens the ground state and lowers the excited state. In this clathrate, due to the CT complex array with the short separation, the effect from the guests overcomes that from the host, and the red shift appears.