A picosecond pulse train study of exciton dynamics in photosynthetic membranes.

The fluorescence decay time of spinach chloroplasts at 77 degrees K was determined at 735 nm (corresponding to the photosystem I emission) using a train of 10-ps laser pulses spaced 10 ns apart. The fluorescence lifetime is constant at congruent to 1.5 ns for up to the fourth pulse, but then decreases with increasing pulse number within the pulse train. This quenching is attributed to triplet excited states, and it is concluded that triplet excitons exhibit a time lag of about 50 ns in diffusing from light harvesting antenna pigments to photosystem I pigments. The diffusion coefficient of triplet excitons is a least 300--400 times slower than the diffusion coefficient of singlet excitons in chloroplast membranes.