Identification of excited-state energy transfer and relaxation pathways in the peridinin–chlorophyll complex: an ultrafast mid-infrared studyElectronic supplementary information (ESI) available: The molecular structure of peridinin; the absorbance spectrum of the PCP complex at high concentration; decay-associated difference spectrum in the mid-infrared of PCP; the kinetic model applied for target analysis of time-resolved mid-infrared data of PCP; concentration profiles of various transient states in PCP; a spectral fit to the mid-IR data of PCP; a table summarizing the polar residues in the vicinity of Chl ain PCP. See DOI: 10.1039/b923695c

The peridinin chlorophyll-aprotein (PCP) is a water–soluble, trimeric light harvesting complex found in marine dinoflagellates that binds peridinin and Chl-ain an unusual stoichiometric ratio of 4 : 1. In this paper, the pathways of excited-state energy transfer and relaxation in PCP were identified by means of femtosecond visible-pump, mid-infrared probe spectroscopy. In addition, excited-state relaxation of peridinin dissolved in organic solvent (CHCl3and MeOH) was investigated. For peridinin in solution, the transient IR signatures of the low-lying S1and intramolecular charge transfer (ICT) states were similar, in line with a previous ultrafast IR study. In PCP, excitation of the optically allowed S2state of peridinin results in ultrafast energy transfer to Chl-a, in competition with internal conversion to low-lying optically forbidden states of peridinin. After vibrational relaxation of the peridinin hot S1state in 150 fs, two separate low-lying peridinin singlet excited states are distinguished, assigned to an ICT state and to a slowly transferring, vibrationally relaxed S1state. These states exhibit different lactone bleaches, indicating that the ICT and S1states localize on distinct peridinins. Energy transfer from the peridinin ICT state to Chl-aconstitutes the dominant energy transfer channel and occurs with a time constant of 2 ps. The peridinin S1state mainly decays to the ground state through internal conversion, in competition with slow energy transfer to Chl-a. The singlet excited state of Chl-aundergoes intersystem crossing (ISC) to the triplet state on the nanosecond timescale, followed by rapid triplet excitation energy transfer (TEET) from Chl-ato peridinin, whereby no Chl-atriplet is observed but rather a direct rise of the peridinin triplet. The latter contains some Chl-afeatures due to excitonic coupling of the pigments. The peridinin triplet state shows a lactone bleach mode at 1748 cm−1, while that of the peridinin ICT state is located at 1745 cm−1, indicating that the main channels of singlet and triplet energy transfer in PCP proceed through distinct peridinins. Our results are consistent with an energy transfer scheme where the ICT state mainly localizes on Per621/611 and Per623/613, the S1state on Per622/612 and the triplet state on Per624/614. [ABSTRACT FROM AUTHOR]

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