Triplet-state decay kinetics of hydrated chlorophyll complexes

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Abstract
The triplet-state lifetimes of Chl a⋅H2O, (Chl a⋅H2O)2, and (Chl a⋅2H2O)n aggregates were investigated by fluorometric techniques. A kinetic model is developed to account for the experimental observations. Evidence for the suitability of a four-level system is provided to include possible two-photon excitations involving an excited triplet state T2. In contrast to the dramatic dependence of the singlet-state lifetime of hydrated Chl a on the aggregate size, the triplet-state lifetimes of monomeric, dimeric, and polymeric Chl a aggregates in nonpolar hydrocarbon solvents are all found to be about 1.0–1.1 ms at temperatures <120 K. The experimental observations support a localized Chl a aggregate triplet-state interpretation for negligibly small triplet exciton interactions. Annihilation effects observed in (Chl a⋅2H2O)n are shown to be accompanied by shortening of the triplet-state lifetime, consistent with a four-level kinetic model.

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