Exciton delocalization probed by excitation annihilation in the light-harvesting antenna LH2

Singlet-singlet annihilation is used to study exciton delocalization in the light harvesting antenna complex LH2 (B800-B850) from the photosynthetic purple bacterium Rhodobacter sphaeroides. The characteristic femtosecond decay constants of the high intensity isotropic and the low intensity anisotropy kinetics of the B850 ring are related to the hopping time tau (h) and the coherence length N-coh Of the exciton. Our analysis yields Ncoh = 2.8 +/- 0.4 and tau (h) = 0.27 +/- 0.05 ps. This approach can be seen as an extension to the concept of the spectroscopic ruler.