Ground and Excited Electronic States of Quininone-Containing Re(I)-Based Rectangles: a Comprehensive Study of Their Preparation, Electrochemistry, and Photophysics

The self-assembly of two rectangular compounds [{(CO)3Re(μ-QL)Re(CO)3}2(μ-bpy)2] (1, QL = 6,7-dimethyl 1,4-dioxido-9,10-anthraquinone (QL1); 2, QL = 1,4-dioxido-9,10-anthraquinone (QL2), bpy = 4,4′-bipyridine) via an orthogonal-bonding approach was achieved in high yields. Their structures were characterized by single-crystal X-ray diffraction analysis. The rectangles exhibited multielectron-redox properties. The introduction of a bridging quininone moiety made notable changes in two well-separated single-electron reductions of the bpy moiety, as compared with other 2,2′-bisbenzimidazolate (BiBzlm) or thiolate- or alkoxide-bridged rectangles, followed by quasi-reversible reduction of the quininone moiety to allow the existence of different redox states. Electrochemical assessment using cyclic voltammetry and UV−vis−NIR spectroelectrochemistry revealed reversibly accessible 0, 1−, and 2− redox states. The comproportionation constant of the successive reduction processes was Kc = 4.18 × 108 for complex 1 an...