The triplet exciton of chlorophyll a and carotenoid in solution and in photosynthetic antenna proteins

We have observed the development and decay of triplet excitons formed in the ‘antenna’ chlorophyll ab protein complex by high-intensity laser excitation. The carotenoid triplet (3Car) appeared 5 ns after excitation in the protein isolation, commonly termed CP-II; the risetime in a larger antenna particle, called LHC (light-harvesting complex) was 12 ns. The quantum yield of 3Car in CP-II decreased 11-fold as intensity was increased from 1016 to 2 · 1017 photons/cm2 per pulse. The effect is attributed to exciton annihilation during the initial period of triplet formation. Above 5 · 1016 photons/cm2 per s, the 3Car lifetime decreases substantially from its low intensity value of 8.7 μs. A comparison of the transient absorption spectrum of CP-II with those of chlorophyll and carotenoid in vitro indicates that ‘trapped’ chlorophyll triplets formed at high intensities. We present a simple model of destructive interaction between 3Car and chlorophyll triplets which is compatible with the observed increased rate of 3Car decay. Indirect evidence suggests similar effects occur in LHC.