ACTIVATION VOLUMES (DELTA-V Q) FOR ENERGY-TRANSFER AND ELECTRON-TRANSFER QUENCHING OF [CU(DPP)2(+)]ASTERISK (DPP = 2,9-DIPHENYL-1,10-PHENANTHROLINE)

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 the uncharged chromium(III) complexes CrL3 (L = acetylacetonate, 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate, 1,1,1-trifluoro-2,4-pentanedionate) and several nitrobenzene derivatives has been investigated in dichloromethane, chloroform, and tetrahydrofuran solutions as a function of hydrostatic pressure (0.1-250 MPa). The patterns in all three solvents were similar. For those quenchers having reduction potentials E1/2(Q) < -1.6 V (vs the ferrocenium/ferrocene, Fc+/Fc, couple in dichloromethane), the principal bimolecular deactivation mode is energy transfer. In such cases, there is little effect of pressure on the quenching rate constant k(q) (DELTAV(double dagger)q = approximately 0 cm3 mol-1). In contrast, substantially more positive E1/2(Q) values lead to k(q)'s approaching diffusion limits and significantly positive DELTAV(double 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 the resulting charge creation in the transition state leads to substantially negative DELTAV(double dagger)q values approaching -20 cm3 mol-1.