COMPETITIVE BIMOLECULAR ELECTRON-TRANSFER AND ENERGY-TRANSFER QUENCHING OF THE EXCITED STATE(S) OF THE TETRANUCLEAR COPPER(I) CLUSTER CU4I4PY4 - EVIDENCE FOR LARGE REORGANIZATION ENERGIES IN AN EXCITED-STATE ELECTRON-TRANSFER

The quenching of emission from the cluster-centered (ds/XMCT) excited state of the copper(I) cluster Cu4I4py4 (I, py = pyridine) by tris(beta-dionato)chromium(III) complexes CrL3 and several organic substrates has been investigated in dichloromethane solution. The E00 energy of the excited state (I*) was estimated to be 1.66 mum-1 (2.06 V), and the reduction potential E1/2(I+/I*) was estimated as -1.78 V (vs the ferrocenium/ferrocene couple). Each of the CrL3 complexes (2E(g) is similar to 1.3 muM-1) is capable of energy transfer quenching, and the rate of this process is shown to be about 10(7.9) M-1 s-1. Contributions to the quenching by an apparent electron-transfer mechanism were evident for those substrates with reduction potentials E1/2(Q/Q-) less than 1.4 V, i.e. reaction driving forces (-DELTAG(el)degrees) greater than 0.4 V. The large driving force required can be attributed to a very slow I+/I* self-exchange rate and is indicative of large contributions from inner sphere terms to the total electron-transfer reorganization energy. Such contributions are a likely explanation of the substantially positive DELTAH(q)double dagger values (up to + 40 kJ mol-1) noted for organic quenchers with E1/2(Q/Q-) near 1.4 V. Pressure effect studies demonstrate that the activation volume (DELTAV(q)double dagger) for energy-transfer quenching of the CrL3 species is approximately 0 cm3 mol-1 while that of those quenchers which operate near the diffusion limit is approximately +7 cm3 mol-1, consistent with the expected effects on solvent viscosity. In contrast, for those systems with E1/2(Q/Q-) approximately 1.4 V, substantially negative DELTAV(q)double dagger values were observed, a feature reflective of the solvent reorganization owing to charge creation upon electron transfer between I* and Q.