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Philosophical Transactions of the Royal Society A Mathematical Physical and Engineering Sciences

The interaction of solutes with their environments

Graham R. Fleming, Sean A. Passino, Yutaka Nagasawa

Published Feb 15, 1998• 46 citations
Publisher

Abstract

The origins of electronic spectral broadening in solutions, glasses and proteins are discussed via three pulse stimulated echo peak–shift measurements. Spectral densities for dye molecules in polar solutions are presented and discussed in terms of molecular models for the dynamics. The breakdown of the harmonic bath picture at low frequencies is presented. Finally, echo measurements on light harvesting complexes of purple bacteria are presented and it is shown that the energy transfer timescale can be obtained from peak–shift measurements.

DOI
10.1098/rsta.1998.0172

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The origins of electronic spectral broadening in solutions, glasses and proteins are discussed via three pulse stimulated echo peak–shift measurements. Spectral densities for dye molecules in polar solutions are presented and discussed in terms of molecular models for the dynamics. The breakdown of the harmonic bath picture at low frequencies is presented. Finally, echo measurements on light harvesting complexes of purple bacteria are presented and it is shown that the energy transfer timescale can be obtained from peak–shift measurements.
The optical properties of molecules dissolved in glasses and ligated to proteins are very similar. The technique of three pulse stimulated photon echo peak shift is used to explore these properties. For dilute chromophores in a glass, a model of linear coupling to a harmonic bath combined with temperature-independent inhomogeneous broadening works well from 30 to 300 K. For concentrated chromophores, such as exist in photosynthetic light harvesting complexes, energy transfer changes the appearance of the peak shift data. The timescale of energy transfer can be extracted from the peak shift data, along with the coupling to the protein and the width of the inhomogeneous distribution.
The signals generated by various third-order nonlinear spectroscopies (transient absorption, transient grating, and three-pulse photon echo peak shift) on energy transfer systems are investigated by carrying out model calculations. Focusing on the understanding of basic features, we employ a simple model system consisting of pairs of energy donors and acceptors. We assume that energy transfer occurs via the Förster mechanism (weak electronic coupling). Static disorder in the transition energies of the chromophores induces inhomogeneities in the rate of energy transfer and in the optical response. The interplay between these two processes is discussed. We show that the peak shift experiment has advantages over the other types of experiment in clarifying the nature of disorder which affects the mechanism and time scale of energy transfer. For example, the peak shift technique can reveal the extent to which disorder is correlated in individual chromophore assemblies, and the extent to which energy transfer can correlate the energies of donors and acceptors. Finally the ability of the peak shift method to distinguish and quantify diagonal and off-diagonal disorder in energy transfer systems is discussed.
Light harvesting 2 (LH2) complex is a primary component of the photosynthetic unit of purple bacteria that is responsible for harvesting and relaying excitons. The electronic absorption line shape of LH2 contains two major bands at 800 nm and 850 nm wavelength regions. Under low light condition, some species of purple bacteria replace LH2 with LH3, a variant form with almost the same structure as the former but with distinctively different spectral features. The major difference between the absorption line shapes of LH2 and LH3 is the shift of the 850 nm band of the former to a new 820 nm region. The microscopic origin of this difference has been subject to some theoretical/computational investigations. However, the genuine molecular level source of such difference is not clearly understood yet. This work reports a comprehensive computational study of LH2 and LH3 complexes so as to clarify different molecular level features of LH2 and LH3 complexes and to construct simple exciton-bath models with a common form. All-atomistic molecular dynamics (MD) simulations of both LH2 and LH3 complexes provide detailed molecular level structural differences of BChls in the two complexes, in particular, in their patterns of hydrogen bonding (HB) and torsional angles of the acetyl group. Time-dependent density functional theory calculation of the excitation energies of BChls for structures sampled from the MD simulations, suggests that the observed differences in HB and torsional angles cannot fully account for the experimentally observed spectral shift of LH3. Potential sources that can explain the actual spectral shift of LH3 are discussed, and their magnitudes are assessed through fitting of experimental line shapes.
We present a novel approach to molecular spectroscopy with light pulses of arbitrary strength and duration. The key quantity is the frequency-resolved net energy transfer dE/dω which reveals at which frequencies energy is transferred from the field to the molecule (absorption) or from the molecule to the field (stimulated emission). It is shown that dE/dω can be expressed as the Fourier transform of the cross-correlation function of the molecular polarization P(t) and the time derivative of the applied field, dℰ/dt. In this sense, it is formally equivalent to the absorption cross section under weak-field conditions which, as commonly known, can be represented as the Fourier transform of the autocorrelation function S(t). The time-dependent polarization P(t) is determined by exact integration of the time-dependent Schrödinger equation including the light-matter interaction to all orders. It is shown that under weak-field conditions the expression for dE/dω reduces to the well-known cross section formula in the time-dependent picture of spectroscopy, multiplied by the spectral intensity of the light pulse. Therefore, we consider the expression for the frequency-resolved energy transfer, which is valid for arbitrary electric fields, as the natural extension of the absorption cross section in the weak-field limit. Furthermore, dE/dω is shown to be formally equivalent to the change of the spectral intensity, ΔI(ω), of an optical pulse after transmission through a sample, the latter being derived by solving Maxwell’s equations under well-known approximations. The theory is applied to a simple one-dimensional model with two electronic states and the frequency-resolved energy transfer is investigated as a function of the field strength. For sufficiently strong fields, dE/dω exhibits transitions between essentially all vibrational levels in the ground and all states in the excited electronic manifold. The new expression distinguishes between absorption and emission and that is clearly seen in the spectra.