Interexciton-state relaxation and exciton localization in allophycocyanin trimers

We have employed dynamic absorption, transient hole-burning and discrete two-color pump-probe femtosecond spectroscopic methods to obtain a time-resolved view of exciton dynamics in trimers of allophycocyanin, the major component of the core of the phycobilisome in cyanobacteria. Allophycocyanin trimers contain a C-3-symmetric array of phycocyanobilin (open-chain tetrapyrrole) dimers. The femtosecond time-resolved pump-probe spectra and single-wavelength transients observed in allophycocyanin preparations were interpreted with the aid of a series of calculated spectra, which are based on an assignment of the ground-state absorption spectrum in terms of exciton-coupled chromophore dimers. The observed and calculated time-resolved pump-probe spectra indicate that interexciton-state relaxation, a radiationless transfer of population between dimer exciton states, is responsible for a red shift of the spectra on the similar to 30-fs time scale. The time evolution of the pump-probe spectra is inconsistent with a model accounting for the groundstate absorption spectrum in terms of pairs of uncoupled chromophores linked by subpicosecond Forster energy-transfer paths. On a slower time scale, extending from the 300-fs to ps delay range, the time-resolved spectra evolve in a manner consistent with localization of an exciton on one of the chromophores in a dimer. These results are to be compared with those of two-color anisotropy experiments that we previously described [J. Phys. Chem. 1995, 99, 15699-15704], which suggest time constants for interexciton-state relaxation and exciton-state dephasing of 10-30 fs and 280 fs, respectively. The present results suggest that exciton localization and exciton-state dephasing occur on similar time scales. We suggest that interexciton-state relaxation and exciton localization in allophycocyanin trimers provide a potent mechanism for directed energy transfer that arises from the energy-transfer coherence properties of chromophore dimers.