Two-Photon Excitation Spectrum of Light-Harvesting Complex II and Fluorescence Upconversion after One- and Two-Photon Excitation of the Carotenoids

Abstract

The two-photon excitation (TPE) spectrum of light-harvesting complex II (LHC II) has been measured in the spectral region of 1000−1600 nm, corresponding to one-photon wavelengths of 500−800 nm. We observed a band with an origin at ∼2 × 660 nm (ca. 15 100 ± 300 cm-1) and a maximum at ∼2 × 600 nm. The line shape and origin of this band strongly suggest that the observed signal is due to the two-photon-allowed S1 state of the energy-transferring carotenoids (Car) in LHC II. We also report the time dependence of the upconverted chlorophyll (Chl) fluorescence after TPE at the maximum of the observed band. Surprisingly, a fast rise of 250 ± 50 fs followed by a multiexponential decay on the picosecond time scale was observed. This result provides strong indication that there is a fast energy transfer even from the dipole-forbidden Car S1 state to the Chl's. The sub picosecond energy transfer from the Car S1 state is likely a consequence of the large number of energy-accepting Chls in van der Waals contact with the central Car's in LHC II. We also present upconversion data of the Car S2, Chl a, and Chl b fluorescence observed after one-photon excitation into the dipole-allowed Car S2 state. The lifetime of the Car S2 state is ∼120 ± 30 fs. With the observed time constants we are able to calculate quantum yields for the different possible pathways contributing to the overall Car to Chl energy transfer in LHC II.

Femtosecond dynamics of the forbidden carotenoid S 1 state in light-harvesting complexes of purple bacteria observed after two-photon excitation

“…1) (28). We obtained similar results in the light harvesting complex LHCII of higher plants and green algae (29).…”

“…1) (28). We obtained similar results in the light harvesting complex LHCII of higher plants and green algae (29).In this work we report fs time-resolved excited-state absorption (ESA) of the Car S 1 state in three light-harvesting complexes of purple bacteria observed after TPE of this state. We report resuts for the B800-B850 and B800-B820 LH2s of Rps.…”

“…In this work ET1 has to be multiplied with the quantum efficiency for Car S2 3 Car S1 internal conversion after excitation into S2, 21, to calculate the contribution of Car S1 3 BChl EET to OA, which is the definition of ET1 in ref. 29. Fig.…”

“…We † Note that this definition of ET1 is different from the definition of ET1 in our former work on LHCII of higher plants and green algae (29). In this work ET1 has to be multiplied with the quantum efficiency for Car S2 3 Car S1 internal conversion after excitation into S2, 21, to calculate the contribution of Car S1 3 BChl EET to OA, which is the definition of ET1 in ref.…”

Time-Resolved Two-Photon Spectroscopy of Photosystem I Determines Hidden Carotenoid Dark-State Dynamics

“…This explains why sometimes fast components are observed after Car S 2 state excitation in fluorescence upconversion experiments, e.g., for PS I 15 and LHC II, 10 whereas no fast components can be observed by measuring the transient absorption kinetics of Car S 1 of the same species. 14,17 However, if these components have small amplitudes in fluorescence measurements, they also can be strongly convoluted with other transfer processes, such as ps intraband dynamics in the acceptor fluorescence or the Car S 2 f Chl energy transfer.…”