THE PHOTOPHYSICS OF IN VITRO CHLOROPHYLL-WATER AGGREGATES

Although the chlorophyll a (Chl a) molecule and its derivatives have been widely studied, little effort has been directed toward a controlled comparison of the photophysics of Chl a-water aggregates in vitro. This study is motivated by the observation that photogalvanic cells with Chl a/Pt versus Pt electrodes are capable of generating H(,2) and O(,2) from water with only a visible light driving force. The chlorophyll present on the platinum surface exists in several states of aggregation coresponding to those present in a homogeneous nonpolar solution: Chl a(.)H(,2)O, (CHl a(.)H(,2)O)(,2), and (Chl a(.)2H(,2)O)(,n). The redox and photophysical properties of the aggregates as well as the metal surface exert a combined influence on water splitting efficiency. In this study the singlet and triplet state lifetimes of the Chl a monomer, dimer, and dihydrate polymer were determined in solution and optical glass environments. Fluorimetric methods are fully developed for measuring the nonphosphorescent triplet lifetime of the Chl a species on a millisecond timescale. A versatile, reliable technique is also described for measuring the subnanosecond to nanosecond fluorescent lifetimes of these aggregates at wavelengths extending well beyond 700 nm. Although the triplet state lifetimes of the Chl a aggregates show no size dependence, the singlet state lifetimes decrease markedly with aggregate size. Evidence for singlet-triplet fusion in samples enriched in (Chl a(.)2H(,2)O)(,n) is provided