DYNAMIC ROLE OF TRIPLET STATES IN PHOTOSYNTHESIS

Open Access
Publisher Website
Google Scholar
Abstract
[long dash]The possible importance to photosynthesis of triplet state quenching, triplet energy transfer, triplet-triplet annihilation, and triplet energy multiplication is pointed out. The more restricted role of triplets in photosynthesis discussed by Franck and others can now be broadened to include these more efficient processes. The presence of triplet mechanisms in the photosynthetic apparatus would not, of course, rule out quenching, energy transfer, annihilation and energy multiplication steps via either the triplet or singlet states to produce charge separation with further energy migration or chemical processes involving the charge carriers. It would not be surprising if 2 or more of these types of energy transfer mechanisms combine in the primary photochemical steps. Such might be anticipated from Emerson enhancement effects which indicate that chemical cooperation on a relatively long time scale ([image] 5 seconds) takes place between the products of primary light absorption at different wavelengths. Thus, 2-quantum physical mechanisms may combine in a "2-quantum" chemical process in over-all photosynthesis. The presence of shallow energy sinks in the photosynthetic apparatus seems assured by the existence of electronically similar but slightly different pigments. These energy sinks are chlorophyll-a molecules which appear to be in very special chemical environments or in very special states of aggregation. One might speculate that at room temperature efficient energy transfer and annihilation may occur between the energy sinks themselves, provided the energy sinks are appropriately coupled through the nearby states of their own accessory pigment system. However, at fairly low temperatures the transfer of energy may be retarded. At such temperatures where triplet energy transfer is relatively slow and where photochemistry has been quenched, triplet-triplet annihilation may reveal itself as delayed fluorescence having a nonexponential decay of similar half life as the triplet state. That low temperature emission of this kind has already been in fact observed perhaps may be direct evidence for a dynamic rather than a static role of triplet states in photosynthesis.