Triplet excitons in natural photosynthetic and artificial light harvesting systems: Measurement and modeling
Under full sunlight, unprotected (Bacterio)Chlorophyll ((B)Chl) molecules photodegrade in a matter of minutes. This is the result of the generation of highly reactive singlet oxygen (1O2) by energy transfer from the (B)Chl triplet state (3(B)Chl) to the oxygen ground state. Natural photosynthetic systems must protect themselves from 1O2, typically done by positioning carotenoids within a few angstroms of each (B)Chl molecule to quench 3(B)Chl states. Using phosphorescence spectroscopy and computational modeling, we investigated alternative, carotenoid independent, mechanisms which nature may employ to prevent 1O2 sensitization by lowering the energy of 3(B)Chl below that of 1O2. The two proposed triplet lowering mechanisms investigated were: triplet state lowering by strong pigment-pigment interactions (i.e. triplet exciton formation) and triplet state lowering by pigment-protein interactions. Possible natural examples employing these mechanisms are two structures found in green sulfur bacteria: the chlorosome (an antenna containing ~100000 coupled BChl c, d, or e molecules with unexpectedly high photostability) and the Fenna-Matthews-Olson (FMO) complex (an auxiliary antenna containing eight seemingly unprotected BChl a molecules). Measurements performed on linear aggregates of the dye perylene diimide (PDI) show that triplet exciton formation does reduce the triplet state energy. However, direct measurement of triplet state energies for the chlorosome and FMO complex proved experimentally difficult, thus an alternative approach was used to calculate these energies using empirical and excitonic models. Since the use of excitonic modeling requires knowledge of both the pigment site energies and the pigment-pigment interactions (i.e. couplings), work was performed to catalog the monomeric singlet and triplet state energies of all known natural (B)Chl pigments by direct measurement or computational modeling and to characterize the triplet-triplet (T-T) coupling in artificial (B)Chl and porphyrin dimers by experimental and computational methods. This data set obtained allowed for the development of an empirical model for prediction of the triplet state site energy from a given singlet site energy and for development and calibration of a T-T coupling model. Use of these models shows that triplet state lowering by pigment-protein interaction provides photoprotection to the FMO complex, while triplet state lowering by triplet exciton formation is insufficient to provide protection to the chlorosome antenna. Additionally, the T-T coupling model shows that in dimers and other aggregates, the coupling is highly sensitive to relative monomer orientation and position, contrary to what was previously assumed. The simple exponential models used to estimate T-T couplings miss this orientation sensitivity, thus in systems with significant contact between adjacent monomers a more accurate approach is required.
Savikhin, Purdue University.
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