Luminescence quenching of Re(i) molecular rectangles by quinones

The rhenium-based rectangles [{Re(CO)3(μ-bpy)Br}{Re(CO)3(μ-L)Br}]2 (I, L = 4,4′-dipyridylacetylene (dpa); II, L = 4,4′-dipyridylbutadiyne (dpb); III, L = 1,4-bis(4′-pyridylethynyl)benzene (bpeb); bpy = 4,4′-bipyridine) are emissive in solution at room temperature. The presence of extended π conjugation leads to an increase in electron delocalization, which, in turn, results in improved luminescence and lower nuclear reorganization energy. These rectangles, upon electronic excitation, undergo facile electron transfer (ET) reactions with quinones and both the dynamic and static quenching contribute to the reaction. Spectral and electrochemical measurements show that quinone 7,7,8,8-tetracyanoquinodimethane (TCNQ) binds strongly to rectangle I. The driving force dependence of ket, deduced from the luminescence quenching of rectangles with quinones, can be well accounted for within the context of the Marcus theory of electron transfer.