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.