Crystal Structure and Spectroscopic Properties of the CT Complex of Methyl Viologen Dication and <i>o</i>-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 π−π 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.本研究采用由Cd²+离子与桥联氰根配体构筑骨架结构的聚氰基聚镉主体（polycyano−polycadmate host，该主体为带负电的氰化镉配合物），合成了以甲基紫精二阳离子（MV²+）与邻二甲氧基苯（o-dimethoxybenzene，ODMB）为客体的笼形包合物。该笼形包合物呈红棕色，其发色根源被认为是主体内以MV²+为受体、ODMB为供体形成的电荷转移配合物（charge transfer complex，CT配合物）所产生的电荷转移（charge transfer）相互作用。相较于乙腈溶液中的CT配合物，该包合物的CT吸收峰波长发生了显著红移。单晶X射线衍射分析结果显示，该CT配合物存在π-π堆积结构，且CT配合物在主体骨架的类通道空腔中呈一维排列。在该CT配合物阵列中，MV²+离子彼此相邻且间距极短。通过结构信息与晶体样品光谱测得的CT吸收跃迁矩（transition moment）方向，与在CIS/6-31+G*理论级别下开展的从头算（ab initio calculations）所得结果一致。本研究的计算结果还表明，CT吸收的红移主要源于CT配合物之间的静电相互作用，而非学界普遍认为的MV²+与ODMB之间距离缩短所带来的影响。带负电的主体可稳定CT配合物的基态（ground state），但CT配合物间的静电相互作用会同时抬升基态能级并降低激发态（excited state）能级。在本笼形包合物中，由于间距极短的CT配合物阵列效应，客体带来的影响超过了主体的作用，最终呈现出红移现象。