Experimental investigations of superexchange-mediated ET

It was this widespread, but misplaced, belief which caused the controversy over the interpretation of the observed rapid intra-molecular ET rates (= 107 s-1) in the radical anion of the semi-flexible traw.v-dinaphthyl-cyclohexane system 3(7), where the number in parentheses refers to the number of C-C bonds in one of the relays of the hydrocarbon bridge connecting the pair of chromo-phores.50 The observed rapid ET rate was tacitly attributed to a TS mechanism, rather than to a TB mechanism, even though the two naphthalene rings are about 7 A apart (edge-to-edge). A TS mechanism was likewise advanced to account for the observed rapid intra-molecular ET occurring in the radical anion of 4(6).51 

Photophysical studies on 9(3) and 10(5) revealed that extremely rapid photo-induced ET took place in both systems, from the locally excited methoxybenzene donor to the DCV acceptor.59,60 Particularly noteworthy was the observation of CT fluorescence in 10(5) which was the first documented example of exciplex-like emission from a rigid D-B-A system with a donor-acceptor separation exceeding three C-C bonds. These data clearly point to TB-mediated ET processes in these molecules. 

The emerging investigations of biological ET made the need to obtain an unambiguous distance dependence of ET rates mediated by saturated hydrocarbon bridges even more urgent, not only because of the need to calibrate the accuracy of 

Our first step towards determining, unequivocally, the strength and the distance dependence of TB-mediated electronic coupling within saturated hydrocarbon bridges stemmed from our PES and ETS studies on the first three members of the series of totally rigid, symmetrical, polynorbomane-dienes 15(4)-15(6), where the numbers in parentheses refer to the numbers of C-C bridge bonds connecting the two double bonds (Fig. 12). The experimental studies were supplemented with 

There is overall fairly good agreement between the computed and experimental damping factors (cf. equations (11)—(14)), and the average values from these two methods are /3e (vr) = 0.89 bond-1 and /3el(7r ) =1.15 bond-1. 

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