Charge recombination kinetics of giant dipoles in saturated hydrocarbon solvents

Distance Dependence of Singlet and Triplet Charge Recombination Pathways in a Series of Rigid Bichromophoric Systems

“…shown by the small temperature dependence of k cr in systems 1[n]. 15,17 The data compiled in Table 1 have been plotted in Figure 1. As noted earlier k cr appears to decrease exponentially with distance, and even though the rates are very strongly solvent dependent, the damping factor cr appears rather solvent independent with an average value of 0.95 ( 0.10 per σ bond ( cr ) 0.83 Å -1 ).…”

“…For 1 [12] in benzene this assumption is not valid, because repopulation of the local singlet state (k bet ) occurs, as was discussed previously. 15,16 Therefore the data for 1 [12] in benzene will be omitted from the following analysis.…”

“…1b-g, [15][16][17] The main technique employed has been time-resolved microwave conductivity (TRMC), which allows direct detection of the decay of the dipolar CT state. In addition, the decay of the weak CT fluorescence emerging from this state has been measured in these cases where it was sufficiently strong.…”

“…An exponential distance dependence was indeed found for both k cs and k cr in 1[n]. 1a-g, [13][14][15][16][17] As already noted above, however, for k cr the situation is complicated. In fact charge recombination can occur Via three distinctly different pathways.…”

“…This "back electron transfer" (k bet ) occurs only for n g 8 in low-polarity solvents, as revealed by the delayed donor fluorescence emerging from it. 15,16 While conditions can be chosen to avoid a contribution of k bet to k cr , still two discrete recombination pathways are always accessible, leading either to the ground state (k 1 cr ) or to the local triplet state of the DMN donor (k 3 cr ), which is always significantly lower in energy than the CT state (see Scheme 2). This implies that the k cr values determined previously are in fact the sum of k 1 cr and k 3 cr and that the cr values represent a weighed average of that for singlet and triplet recombination ( 1 cr and 3 cr ).…”