The quenching of emission from the cluster-centered (ds/XMCT) excited state of the copper(I) cluster Cu4I4py4 (I, py = pyridine) by tris(β-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 µm−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 (2Eg ∼ 1.3 µΜ−1) is capable of energy transfer quenching, and the rate of this process is shown to be about 107.9 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 (−ΔGel°) 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 ΔHq‡ 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 (ΔVq‡) for energy-transfer quenching of the CrL3 species is ∼0 cm3 mol−1 while that of those quenchers which operate near the diffusion limit is ∼+7 cm3 mol−1, consistent with the expected effects on solvent viscosity. In contrast, for those systems with E1/2(Q/Q−) ∼ 1.4 V, substantially negative ΔVq‡ values were observed, a feature reflective of the solvent reorganization owing to charge creation upon electron transfer between I* and Q. © 1993, American Chemical Society. All rights reserved.