Synthesis and photophysical properties of neutral luminescent rhenium-based molecular rectangles

A series of neutral luminescent molecular rectangles [{Re(CO) ₃(μ-bpy)Br}{Re(CO)₃(μ-L)Br}]₂ (1-4) having fac-Re(CO)₃Br as corners and 4,4′-bipyridine (bpy) as the bridging ligand on one side and other bipyridyl ligands of varying length (L) on the other side have been synthesized and characterized. The crystal structure of 1 shows a rectangular cavity with the dimensions of 11.44 x 7.21 Å. When the cavity size is tuned from 1 to 4, a dimension of 11.4 x 20.8 Å could be achieved, as revealed by the molecular modeling. These rectangles exhibit luminescence in solution at room temperature. In particular, compound 4 containing 1,4-bis(4′-pyridylethynyl)benzene (bpeb) as bridging ligand shows the excited-state lifetime of 495 ns. Fine-tuning of the cavity size of the rectangles improves their excited-state properties. These properties facilitate the study of excited-state electron-transfer reactions with electron acceptors and donors and host-guest binding. Crystallographic information: 1·6CH₃COCH₃ is monoclinic, P2 ₁/c, with a = 12.0890(2), b = 24.2982(2), and c = 12.8721(2) Å, β = 107.923(1)°, and Z = 2.