Part of the fluorescence of chlorophyll a may originate in excited triplet states

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