Abstract
Vibrational-electronic resonance in photosynthetic pigment-protein complexes invalidates Förster's adiabatic framework for interpreting spectra and energy transfer, thus complicating determination of how the surrounding protein affects pigment properties. This paper considers the combined effects of vibrational-electronic resonance and inhomogeneous variations in the electronic excitation energies of pigments at different sites on absorption, emission, circular dichroism, and hole-burning spectra for a non-degenerate homodimer. The non-degenerate homodimer has identical pigments in different sites that generate differences in electronic energies, with parameters loosely based on bacteriochlorophyll a pigments in the Fenna-Matthews-Olson antenna protein. To explain the intensity borrowing, the excited state vibrational-electronic eigenvectors are discussed in terms of the vibrational basis localized on the individual pigments, as well as the correlated/anti-correlated vibrational basis delocalized over both pigments. Compared to those in the isolated pigment, vibrational satellites for the correlated vibration have the same frequency and precisely a factor of 2 intensity reduction through vibrational delocalization in both absorption and emission. Vibrational satellites for anti-correlated vibrations have their relaxed emission intensity reduced by over a factor 2 through vibrational and excitonic delocalization. In absorption, anti-correlated vibrational satellites borrow excitonic intensity but can be broadened away by the combination of vibronic resonance and site inhomogeneity; in parallel, their vibronically resonant excitonic partners are also broadened away. These considerations are consistent with photosynthetic antenna hole-burning spectra, where sharp vibrational and excitonic satellites are absent. Vibrational-excitonic resonance barely alters the inhomogeneously broadened linear absorption, emission, and circular dichroism spectra from those for a purely electronic excitonic coupling model. Energy transfer can leave excess energy behind as vibration on the electronic ground state of the donor, allowing vibrational relaxation on the donor's ground electronic state to make energy transfer permanent by removing excess energy from the excited electronic state of the dimer.
Highlights
Tiwari et al argued that low temperature absorption spectra of a photosynthetic light harvesting antenna and the Raman spectra of the isolated pigments imply a vibrational-electronic resonance between singly excited electronic states of the antenna and showed that reported 2D spectroscopic signatures are consistent with such a resonance for a model dimer with one vibration per pigment
This paper uses that Hamiltonian to investigate the role of additional pigment and protein vibrations and how absorption, emission, circular dichroism, and hole-burning spectra are modified by vibrational-electronic resonance for parameters appropriate to chlorophylls in photosynthetic light harvesting antennas
Vibrational-excitonic resonance can alter the intensities of vibrational-electronic transitions so that they do not reflect the Franck-Condon vibrational overlap factors given by the displacement of vibrational equilibrium between electronic states
Summary
The remarkable efficiency of photosynthetic light harvesting has been a mystery since the 1930s.1 Recently, Tiwari et al argued that low temperature absorption spectra of a photosynthetic light harvesting antenna and the Raman spectra of the isolated pigments imply a vibrational-electronic resonance between singly excited electronic states of the antenna and showed that reported 2D spectroscopic signatures are consistent with such a resonance for a model dimer with one vibration per pigment. For this model, the delocalized anti-correlated linear combination of intramolecular vibrations causes a breakdown of Forster’s adiabatic framework for energy transfer. Tiwari et al argued that low temperature absorption spectra of a photosynthetic light harvesting antenna and the Raman spectra of the isolated pigments imply a vibrational-electronic resonance between singly excited electronic states of the antenna and showed that reported 2D spectroscopic signatures are consistent with such a resonance for a model dimer with one vibration per pigment.. Tiwari et al argued that low temperature absorption spectra of a photosynthetic light harvesting antenna and the Raman spectra of the isolated pigments imply a vibrational-electronic resonance between singly excited electronic states of the antenna and showed that reported 2D spectroscopic signatures are consistent with such a resonance for a model dimer with one vibration per pigment.2 For this model, the delocalized anti-correlated linear combination of intramolecular vibrations causes a breakdown of Forster’s adiabatic framework for energy transfer. As in the pioneering investigations of photosynthetic electron transfer by Friesner and co-workers, the results obtained here indicate that vibronic complications in spectra of the antenna are such that the spectra of the isolated monomers, in combination with the linear absorption and circular dichroism spectra of the antenna, provide sound initial estimates for the underlying parameters of a vibrational-excitonic resonance model for energy transfer
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