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)(3)Re(mu-QL)Re(CO)(3)}(2)(mu-bpy)(2)] (1, QL = 6,7-dimethyl 1,4-dioxido-9,10-anthraquinone (QL(1)); 2, QL = 1,4-dioxido-9,10-anthraquinone (QL(2)), 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 K(c) = 4.18 x 10(8) for complex 1 and 4.08 x 10(8) for 2. In spite of the high K(c) values, no obvious intervalence charge transfer bands were detected in either the vis, NIR, or IR regions, suggesting very weak electronic coupling between the ligand centers in the mixed-valent intermediates. In the mixed-valent intermediate, the overlap between donor and acceptor orbitals of the two bpy ligands engendered weak electronic coupling associated with distance that exceeded van der Waals ligand/ligand distances and created a class I fully isolated, non-interacting, valence-localized situation. Furthermore, unusual ligand-to-metal-to-ligand charge-transfer (LMLCT) transitions of complexes 1 and 2 at 298 K were observed in the visible region. Molecule 2 exhibited multiple emissions from the triplet-centered pi-pi* intraligand ((3)IL), metal-to-ligand charge-transfer ((3)MLCT) and triplet ligand-ligand charge transfer ((3)LLCT) levels and showed biexponential decay. By contrast, in complex 1, (3)IL emission was absent and only single-exponential decay was observed. These results reveal the different nature of the electronically excited states between 1 and 2. The mechanisms of the photophysical deactivation processes in these systems can be explained in terms of the electronic characteristics of the quininone molecules and possible geometrical differences of the excited states involved. In addition, the energies, characteristics, and molecular structures of the ground and lowest triplet excited state were calculated using the density functional theory method.