Borderline class II/III ligand-centered mixed valency in a porphyrinic molecular rectangle

A molecular rectangle of the form ([Re(CO)₃]₂BiBzlm) ₂-μ,μ′-(LL)2, where BiBzlm is 2,2′- bisbenzimidazolate and LL are cofacially aligned [5,15-bis(4-ethynylpyridyl)-10, 20-bis(n-hexyl)-porphyrinato]zinc ligands, has been examined via electrochemical, spectroelectrochemical, and electronic Stark effect methods. The rectangle displays three electrochemically accessible reductions assigned as LL based. The singly reduced rectangles are part of an unusual class of mixed-valence complexes where cofacial ligands, in this case porphyrins, comprise the degenerate redox centers. Absorption spectra for the singly reduced rectangle show two intense and narrow absorption bands in the near-infrared (NIR) region; the lower energy band is assigned as an intervalence transition. Time-dependent density functional theory electronic structure calculations support the assignment. Curiously, the transition displays a non-Marcus-type solvent dependence. NIR region electroabsorbance measurements of the singly reduced rectangle reveal a small but readily measurable dipole moment change of 0.56 ± 0.05 eÅ. On the basis of spectroelectrochemical and electroabsorption measurements, the singly reduced rectangle is assigned as a borderline class II/class III mixed-valence species.