Hydrocarbon bridges saturated

We touch here on the distance dependence of the MMCT rate. This problem is widely under study at the moment [130]. This rate depends on the distance r according to exp( — fir). The value of p seems to be in between 1.1 and 1.4 A Elegant work has been performed by Hush, Paddon-Row and Verhoeven [131] who studied molecules in which the reactants are separated by rigid saturated hydrocarbon bridges of various lengths. 

Shephard MJ, Paddon-Row MN, Jordan KD (1993) Electronic coupling through saturated hydrocarbon bridges. Chem Phys 176 289-304... 

Concept The rates of long-range electron transfer (ET) and excitation energy transfer (EET) processes between a pair of chromo-phores (redox couple) may be strongly facilitated by the presence of an intervening non-conjugated medium, such as saturated hydrocarbon bridges, solvent molecules and n-stacks, e.g.,... 

Electron Transfer Mediated by Saturated Hydrocarbon Bridges... 

In summary, electron transfer dynamics, mediated through saturated hydrocarbon bridges and proteins, displays a surprisingly weak distance dependence behaviour (/J = 0.8-12 A 1), compared to that predicted for a pure through-space mechanism (P 3.0 A1). 

Fiereby, Vq refers to the maximal electronic coupling element and p is the decay coefficient factor (damping factor), which depends primarily on the nature of the bridging molecule. From the linear plot of In ETmax versus R the p value is obtained as 0.60 A [47]. This p value is located within the boundaries of nonadiabatic ET reactions for saturated hydrocarbon bridges (0.8-1.0 A ) and unsaturated phenylene bridges (0.4 A ) [1-4,54,55]. 

Returning to the photosynthetic system it now seems plausible that the explanation for the short charge-separation times in the primary steps must be found in the nature of the medium between the redox-sites involved. In this connection it is interesting to note that saturated hydrocarbon chains (i.e. phytyl sidechains) extend from the special pair and from the menaquinone towards the intermediate bacteriopheophytin (see Fig.l). At this moment it is not clear, however, whether in rhodopseudomonas viridis any of these phytyl sidechains plays the role of a molecular wire (see also Kuhn, 1986) that we attribute to the hydrocarbon bridges in l(n). For rhodopseudomonas sphaeroides a fivefold decrease in the rate of the reverse electron transfer from the quinone (ubiquinone) to the bacteriopheophytin was recently reported to result upon removal of the isoprenoid sidechain from the quinone (Gunner et al., 1986). 

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... 

The third, and possibly the most important conclusion is that the distance dependence of TB-mediated electronic coupling in 15(h) is much weaker than that predicted by the simple McConnell model. In that model, a /3el value of about 2.3 bond-1 is estimated, whereas the average /3el value of ca. 1.0 bond-1 for 15(/t) is 2.3 times smaller Thus, the attenuation in the ET rate, mediated by superexchange through a saturated hydrocarbon bridge, could be as small as ca. 2.5 bond-1, as opposed to ca. 10 bond-1, predicted from the simple McConnell model. 

From these extensive studies, we may conclude that the distance dependence of ET rates, mediated by a superexchange mechanism involving saturated hydrocarbon bridges, is largely independent of the nature of the hydrocarbon and has an average /3 value of 0.99 0.2 bond-1. 

In summary, we have demonstrated a proof of principle that long-lived triplet CS states in dwarf dyads - that is, dyads possessing short rigid saturated hydrocarbon bridges - may be achieved using the triplet sensitised approach. This sensitisation method offers at least two important advantages over other methods discussed above. These are ... 

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