SYNTHETIC CONTROL OF EXCITED-STATE PROPERTIES IN LIGAND-BRIDGED COMPLEXES OF RHENIUM(I) - INTRAMOLECULAR ENERGY-TRANSFER BY AN ELECTRON-TRANSFER ENERGY-TRANSFER CASCADE

In the series of complexes [(4,4′-(X)2-2,2′-bpy)(CO)3ReI(4,4′-bpy)ReI(CO)3(4,4′-(Y)2-2,2′-bpy)]2+ (X, Y = H, CH3, NH2, CO2Et), the ultimate site of the excited electron following metal-to-ligand charge-transfer (MLCT) excitation has been studied by transient absorbance and emission spectroscopies in polar organic solvents. The electron-donating groups NH2 and Me increase the energy of the π* levels of the 2,2′-bpy ligand. They lead to localization of the excited electron on the bridging 4,4′-bpy ligand, as shown by the appearance of an absorption feature at 570-610 nm in CH3CN in transient absorbance difference spectra. Electron- withdrawing C02Et groups lower the energy of the π* levels and lead to localization of the excited electron on the 2,2′-bpy ligand, as shown by the appearance of a narrow transient absorption feature at 380-385 nm in CH3CN. With X = Y = H, a solvent-dependent equilibrium exists between the 2,2′-bpy and 4,4′-bpy states. In the asymmetrical complexes with X = NH2 or H and Y = CO2Et, rapid (k > 2 × 108 s−1) intramolecular energy transfer occurs following Re → 4,4′-(X)2-bpy MLCT excitation in CH3CN. Intramolecular energy transfer continues to occur in a 4:1 (v/v) ethanol/methanol glass at 77 K. With 3,3′-(Me)2-4,4′-bpy as the bridging ligand, the rate constant for energy transfer is far slower (k < 4 × 106 s−l), suggesting that energy transfer in the 4,4′-bpy bridged complex may occur by an electron-transfer/energy-transfer pathway. © 1990, American Chemical Society. All rights reserved.