Part of the fluorescence of chlorophyll a may originate in excited tripletstates

Absorption and fluorescence spectra of chlorophyll a have been analyzed on the basis of an extended version of Kennard-Stepanov (KS) theory. It is pro posed that at least one new electronic state lies just below the normal S-1 - S-0 transition (Q(y)), borrowing approximately 2-4% of its strength and contributing to the fluorescence in the tail. The KS anomalies leading to t his hypothesis occur in a wide variety of cases, including chlorophyll a in solution and protein-bound chlorophyll a, suggesting that the phenomenon i s an intrinsic property of the molecule. Natural candidates for the new sta te(s) are the second and third triplet states. The relationship of the fluo rescence excitation spectrum to KS theory is investigated and applied to ex plain a red drop in yield.