The rich photophysical properties of the Cu(I) cluster Cu{sub 4}I{sub 4}-(py){sub 4} (I) (py = pyridine) have been the subject of recent attention in this laboratory. In room temperature solutions, the luminescence spectrum of I displays an intense band at {lambda}{sub max} = 690 nm with a relatively long lifetime ({tau} = 3{minus}10 {mu}s in different solvents), assigned as emission from a {open_quotes}cluster centered{close_quotes} (CC) excited state (I*) having mixed iodide-to-copper charge transfer (XMCT) and d-s character. The energy of I* has been conservatively estimated as 1.66 {mu}m{sup {minus}1} and the potential for the ES half cell I{sup +} + e{sup {minus}} {yields} I* (E{sub {1/2}}I{sup +}/I*) as {minus}1.78 V. I* can be quenched in CH {sub 2}Cl{sub 2} solution by nitrobenzenes and by tris({beta}-dionato)chromium(III) complexes CrL{sub 3}. The former quenchers (Q) function by an electron transfer mechanism demonstrated by correspondence of rate constants k{sub q} with the reduction potentials (E{sub 1/2}Q/Q{sup {minus}}). The CrL{sub 3} complexes quench by competitive energy transfer and electron transfer mechanisms. A surprising feature of these results was the requirement of very large potentials ({minus}{Delta}G{sub el}{degrees} = E{sub {1/2}}(Q/Q{sup {minus}}) {minus} E{sub {1/2}}(I{sup +}/I*)){sup 6} before electron transfer quenching becomes significantly competitive with radiative andmore » nonradiative deactivation of I* and, for the Cr(III) complexes, with energy transfer quenching. The present study was initiated to extend the range of quencher properties and to examine the origin of the above kinetics properties.« less