Excited State Electron and Energy Transfer Reactions

Excited State Electron and Energy Transfer Reactions 

At more favorable free energy for the redox reaction, kq increases (the turning point being AG°j 0.2 V) consistent with the increasing importance of outer sphere electron transfer as a component of this quenching. Thus for Cr(tfac)3, (tfac is trifloroacetylacetonate), AG°[ = 0.23 V and kq = 4.2 x 10 s in dichloro- 

The third type of behavior for these systems is seen when the excited state electron transfer becomes much more favorable and kq approaches the diffusion limit. For example, [Cu(dpp)2] quenching by Cr(hfac)3 (hfac = hexafloroacetylaceto- 

Fetterolf and Offen [26] studied the effect of pressure on the reductive quenching of Ru(bpy) by several aromatic amines. Quenching by N,N-dimethylaniline (DMA) in CH3CN resulted in AV values between +1.3 and +2.9 cm mol while values of 9 to 13 cm mol were reported for the quenching by the free base benzidine and N,N,N, N -tetra-methylbenzidine (TMB) in CH3CN and n-BuOH as solvents. These pressure effects can be interpreted in terms of the following mechanism 

Oxidative quenching by dimethylviologen (MV ) (Eq. 6.36, kq = 9.5 x 10 in aqueous solution) has been reported to exhibit a modestly negative AV (—2.6 cm mol ) [24]. Interestingly, recent PAG experiments [75] indicate that the overall volume change for Eq. 6.36 is positive. Thus, it appears that the transition state of the electron transfer reaction is dominated by formation of an encounter complex with a smaller volume than the separated reactants most likely due to solvation effects. Separation of the encounter complex into products has a large, positive AV (Fig. 6.15). 

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