FEMTOSECOND TRANSIENT HOLE-BURNING DETECTION OF INTEREXCITON-STATE RADIATIONLESS DECAY IN ALLOPHYCOCYANIN TRIMERS

In its trimeric aggregation state, the phycobiliprotein allophycocyani n contains the simplest possible system, an isolated pair of chromopho res, in which delocalized excited states can be studied. By moving the pump wavelength across allophycocyanin's ground-state absorption band , we have obtained femtosecond time-resolved pump-probe spectra that e vidence interexciton-state radiationless decay from the upper exciton state and vibrational relaxation in the lower exciton state. Direct ev idence for interexciton-state radiationless decay is taken from the in itial appearance of photobleaching/stimulated-emission holes displaced to the red with respect to the pump-pulse spectrum. The holes broaden on the 120-fs time scale owing to intramolecular vibrational redistri bution and transient solvation and shift to the red on the 230-fs time scale owing to vibrational equilibration and transient solvation. We show that vibrational relaxation in the lower exciton state contribute s to the decay of a shoulder in the 590-640-nm region on the 400-fs ti me scale when 620-nm excitation is employed but not when the pump spec trum is tuned farther to the red, away from the 0 --> 1 vibronic trans ition to the lower exciton state. In the absence of vibrational relaxa tion in the lower exciton state, we can now discern that exciton local ization contributes to the time evolution in the 590-640-nm region of the spectrum on the 1-ps time scale.