On the Nature of Energy Transfer at Low Temperatures in the BChl a Pigment−Protein Complex of Green Sulfur Bacteria

Abstract

The technique of accumulated photon echoes was used to study optical dephasing of the lowest QY transitions of the bacteriochlorophyll a complex of the green sulfur bacteria Prosthecochloris aestuarii. Variations in the photon echo decay as a function of wavelength are interpreted in terms of (phonon-assisted) downward relaxation within the QY manifold as a mechanism of energy transfer at the lowest temperatures. A temperature dependence study revealed contributions to the total dephasing both from transitions within so-called two level systems (TLS) of the host as well as activation of transitions to higher electronic levels within the QY manifold at temperatures above 5 K. Conclusions are drawn about the nature of energy transfer at low temperatures.

The nature of the low energy band of the Fenna-Matthews-Olson complex: Vibronic signatures

“…With this model, the linewidths, say γ 0 , γ 1 , γ 2 , of the ZPL, the phonon sideband, and overtone, respectively, show a difference that persists for the whole range of temperatures measured as shown in the inset of this figure. This is in complete agreement with photon echo experiments (1.8-50 K, 17 ). The broadening can be understood from transitions involving n-th vibrational states, that become more populated with increasing temperature and whose thermalization rate is proportional to the excitation number L 2(3) |n n| ∝ n. The model also captures two important facts found in hole burning and photon echo experiments.…”

“…In that work, two important results were exposed. First, a greater homogeneous linewidth was found with greater energies within the B825, and confirmed the trend found by accumulated photon echo, of higher dephasing rates as the involved pulses were blue-shifted 17 . Second, a very rich structure of satellite low energy holes regarded as intramolecular vibrations (with the biggest components having energies of 36 and 72 cm −1 ) in the B825 was first observed.…”

“…It has not been possible to date, to uniquely assign its nature to electronic contributions. In this manuscript we propose a model for the lowest energy band in the FMO that includes explicitly the most significant resonances of its environment spectral density, which is consistent with the available experimental evidence achieved through hole burning 11,12,15 , fluorescence line-narrowing 12,16 , and accumulated photon echo 17 . We explore the possibility to detect the coupling of intra-molecular low energy vibrations of chromophores, to exciton dynamics, and shed light on the nature of the lowest energy band of the FMO (to be referred as B825 from its absorbance maximum).…”

“…The homogeneous linewidth of the resonances will primarily depend on the different processes inducing decoherence beyond the ensemble average. Accumulated photon echo 17 fitted the dephasing rate in 1.8-50 K range with the expression:…”

“…The second term, is attributed to dynamics of tunneling among energy minima of different structural configurations in the protein, and characterized by a rate that obeys a power law temperature dependence ∝ T 1.3 , typical of glassy hosts. The third terms involves thermally activated transitions not specified in 17 , that accounts here for the thermal equilibration of the vibrational manifold. Note that eq.…”