ROLE OF NUCLEAR AND ELECTRONIC FACTORS IN THE ELECTRONIC ENERGY TRANSFER FROM THE 2EG EXCITED STATE OF TRIS(2,2′-BIPYRIDINE)CHROMIUM(III) TO COBALT(III) COMPLEXES

The quenching of the phosphorescence emission of (2EJCr(bpy)J+ (bpy = 2,Y-bipyridine) by &(NH3)%+, CO(NH~),(H,O)~+, C ~ ( e n ) ~ ~ + , Co(NH3),X2+ (X = F, C1, NCS, NO2, HCOO), cis-C~(en)~(NH~)Cl~+, cis-Co(en)2(H20)C12+, and cisand rrans-Co(en),(XY)+ (X = C1, NCS; Y = C1, NCS) has been studied in 0.1 M HzSO4 aqueous solution. The bimolecular quenching constants are lower than diffusion and span a factor of -400. On the basis of theoretical considerations it is shown that the only plausible quenching mechanism is electronic energy transfer. Calculations based on available spectroscopic data show that the intrinsic barriers to energy transfer are relatively high owing to the excited-state distortion of the Co(II1) complexes but are canceled out by sufficiently large exoergonicities. Thus, nuclear factors are not responsible for the lower than diffusion quenching constants. It is suggested that the energy-transfer rate is controlled by electronic factors which depend on the nephelauxetic ability of the ligands and on those parameters (size, electric charge, geometrical configuration) that determine the intimate characteristics of the encounter complex.