Reversible photochemistry in a chlorophyll model system.

A reversible, endothermic photochemical redox reaction, sensitized by chlorophyll a and related compounds, has been demonstrated in a heterogeneous particulate system. The oxidant, 5,5'-dithiobis(2-nitrobenzoate) (DTNB), is photoreduced to the thiolate which remains primarily in an aqueous phase. Reductants are trisubstituted hydrazines, capable of oxidation to tetrazanes in the hydrocarbon particle phase. In the course of three days in the dark, thiolate and tetrazane react to regenerate DTNB in yields approaching 100%. A novelty of the present system is that photoreaction often takes place in discrete rate regimes, which are related to the presence of spectrally identifiable associations of chlorophyll pigments, Mg-containing and free bases. Among the associations that promote photochemical activity are those of chlorophyll and pheophytin with themselves and with each other. Perhaps more active are associations of a Mg rhodochlorin allomerization product of chlorophyll with its free base. Contributing to the associations is the stabilizing presence of amphiphiles that both ligate the Mg of chlorophyll strongly and hydrogen-bond to carbonyls: 2-tridecylimidazole, 2-tridecylimidazoline, and (2-aminoethyl)myristamide. Results of this work demonstrate the possibility of generating reaction center models in an artificial heterogeneous system, and of conducting reversible photochemical reactions with them.