PRESSURE EFFECTS ON THE COMPETITIVE ENERGY AND ELECTRON-TRANSFER QUENCHING OF THE MLCT EXCITED-STATE OF CU(DPP)2+ (DPP = 2,9-DIPHENYL-1,10-PHENANTHROLINE) BY CRL3 (L = BETA-DIONATO LIGANDS) AND OTHER QUENCHERS IN SOLUTION

The quenching of emission from the metal-to-ligand charge transfer (MLCT) state of the copper(I) complex Cu(dpp)2+ (dpp = 2,9-diphenyl-1,10-phenanthroline) by tris(beta-dionato)chromium(III) complexes CrL3 and several organic substrates has been investigated in dichloromethane solution as a function of hydrostatic pressure (0.1-250 MPa). For those quenchers having reduction potentials E1/2(Q) < -1.6 V, the principal bimolecular deactivation mode is energy transfer, and there is little effect of pressure on the quenching rate constant k(q) (DELTA-V double-ended-dagger q = approximately 0 cm3 mol-1). In contrast, substantially more positive E1/2(Q) values lead to k(q) values approaching diffusion control and significantly positive DELTA-V double-ended-dagger q values (up to +8 cm3 mol-1) reflecting the pressure-induced increases in solution viscosity. For E1/2(Q) approximately -1.6 V, the dominant pressure-dependent term is electron transfer within the outer-sphere precursor complex, and substantially negative DELTA-V double-ended-dagger q's were noted. These results are interpreted in terms of the McMillin proposal (J. Am. Chem. Soc. 1985, 107, 1141-1146) of competitive energy and electron transfer quenching for these complexes.