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Excited state dynamics of chromophores in glasses and in photosynthetic proteins

Yutaka Nagasawa, Jae-Young Yu, Minhaeng Cho

Published Jan 1, 1997• 26 citations
Publisher

Abstract

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.

DOI
10.1039/a707668a

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The Journal of Physical Chemistry•Jan 1, 1996•276 citations
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 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 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.
The influence of coupling strength and temperature on the solute–solvent interaction of two chromophores in polymer glass is reported. The three-pulse photon echo peak shift method was used to study the dye IR144 in polyvinylformal (PVF) and the dye DTTCI (3,3′-diethylthiatricarbocyanine iodide) in polymethylmethacrylate (PMMA). IR144 is more strongly coupled (larger reorganization energy) to both its intramolecular modes and to the solvent than is DTTCI. Our results can be well described by the linearly coupled harmonic bath model over the range 300 to 30 K. In particular, the strikingly different temperature sensitivities of the long-time (asymptotic) peak shift are well described by the theory. Temperature-independent spectral densities and inhomogeneous widths suffice to quantitatively describe the peak shift data over this temperature range and a number of numerical predictions based on the theoretical model are experimentally confirmed. An ultrafast component corresponding to a decay of ∼100 fs time scale in the transition frequency correlation function is found in all cases, though the amplitude is small for the DTTCI solutions. This ultrafast response is assigned to the inertial response of the solvent. The inertial response measured for DTTCI in PMMA is very similar to our previous measurements of the inertial response for IR144 in PMMA, suggesting that a general characteristic of the solvent is being probed. The weaker coupling of DTTCI produces a drastic increase in the width of the photon echo signal as predicted by theory and strong vibrational quantum beats at 30 K. By contrast, such beats are not observed in the echo signal for IR144.
The third-order nonlinear optical response of energy transfer systems is theoretically investigated. A system composed of two chromophores having the same electronic transition energies is considered. The dynamics of energy transfer between the two chromophores is assumed to occur via a hopping (incoherent) mechanism. We introduce new types of pathways incorporating the hopping processes occurring while the system is in population states and reconstruct a third-order response function which is computationally viable. The nuclear propagators in the electronic population states are written as convolution integrals between those of the nonreactive two-state system weighted by some factors for the energy transfer. The response function is given by multitime correlation functions and these are analyzed by the cumulant expansion method. Based on this approach, the three-pulse photon echo peak shift for several models of energy transfer systems is discussed. It is shown that the rephasing capability of the induced signal is reduced by the memory loss due to resonant energy transfer. A previous model which incorporates resonant energy transfers in an intuitive way is reviewed and modified to supplement the loss of dynamic correlation of nuclear motion within the framework of the theory. The response function obtained by our new approach gives a more accurate description than the existing theory and a comparative discussion is given. The effect of inhomogeneity in rate constants on the third-order signal is discussed and the temperature dependence of the echo signal is examined.
We calculate the signal in a time-resolved, coherent light scattering experiment on a dilute solution of a nonpolar solute dissolved in a nonpolar solvent. We model the solute and solvent molecules as electronic two level systems, and assume that only the solute interacts with radiation. The electronic degree of freedom is treated perturbatively within the Born–Oppenheimer approximation. The nuclear degrees of freedom are treated classically, and their effects are modeled by a molecular dynamics simulation. We assume the electronic absorption spectrum is dominated by inhomogeneous broadening. The validity of this assumption for our model is verified through the calculation of the absorption line shape with and without the inhomogeneous broadening assumption. A quantitative measure of the homogeneous dephasing time is obtained from a calculation of the photon echo signal for our model. The time dependence of the light scattering signal is shown to reflect both dynamics of the fluid in its electronic ground state and in its electronic excited state. We analyze the relative importance of contributions to the signal from these two types of dynamics as a function of experimental time scale and laser frequency.
The time-integrated three-pulse photon echo is discussed in terms of the line-shape functions which are shown to be directly related to the solvation dynamics. By using the short-time approximation to the line-shape function, an algebraic form of the three-pulse photon echo signal S PE (τ, T ) is obtained as a function of two delay times, τ and T, between pulses. On the basis of the approximate form of the photon echo signal, the echo peak shift with respect to the second delay time (population evolution period) is derived and found to be linearly proportional to the time-dependent fluorescence Stokes shift function for times greater than the bath correlation time, τ c . In the case of intermediate inhomogeneous broadening, the asymptotic echo peak shift magnitude is found to be useful in estimating the static inhomogeneous width. To illustrate the theoretical results, an analysis of the echo peak shift measurement of IR144 in a room temperature glass, PMMA, is presented.