Electron transfer saturated hydrocarbon bridges

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). 

A particularly interesting contribution to the study of electron transfer is described in the published version of a lecture by Verhoeven . This discusses electron transport through saturated hydrocarbon bridges and a resulting "exciplex" emission from flexible, rigid, and semiflexible bichromophoies. 

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). 

In the bridged EDA molecule I (Figure 7) where the donor dimethylaniline and the acceptor anthracene are bridged by a saturated hydrocarbon chain -(CH2),-, the features of the electron-transfer reaction are known to be strongly dependent on the chain length n. Mataga and co-workers demonstrated that in the condensed phase bridged EDA molecules with = 0, 1 and 2 formed the CT state only in polar solvents. In the case of = 3, the CT state was formed even in non-polar solvents [86]. The rise time of the CT emission in 2-propanol was 54 and 93 ps for n = 1 and 2, respectively. In -hexane solvent, a rise time of 2.6 ns was reported for the = 3 molecule. 

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