Correlated Exciton Fluctuations in Cylindrical Molecular Aggregates

Abstract

Femtosecond electronic relaxation dynamics of a cylindrical molecular aggregate are measured with transient grating (TG) and two-dimensional Fourier transform photon echo (PE) spectroscopies. The aggregates are double-walled cylindrical structures formed by self-assembly of amphiphilic cyanine dye molecules in water. The diameters of the inner and outer cylinders are approximately 6 and 10 nm. The linear absorption spectrum of the aggregate exhibits four spectrally resolved single exciton transitions corresponding to excited states localized on particular regions of the structure: (1) an excited state localized on the inner cylinder corresponds to the lowest energy transition at 16670 cm(-1); (2) a transition at 17150 cm(-1) represents a state localized on the outer cylinder, (3) whereas an overlapping peak found at 17330 cm(-1) is more closely associated with the inner cylinder; (4) an excited state delocalized between the inner and outer cylinder is assigned to a transition in the linear absorption spectrum at 17860 cm(-1). TG spectra show a series of resonances reflecting the electronic structure of both the single and double exciton manifolds. In addition, PE spectra reveal coherent modulation of both diagonal and cross-peak amplitudes persisting for 100 fs, where the coherence frequency matches the energy gap between transitions 1 and 4 in the linear absorption spectrum. PE line shapes suggest correlated energy level fluctuations for the exciton states associated with these two transitions, which is consistent with this fairly long-lasting coherence at room temperature in aqueous solution. The impact of these correlations on Forster energy transfer efficiency is discussed. The observations imply fairly long-range correlations between the molecular sites (>0.6 nm), which in turn reflects the length scale of the environmental motion inducing the fluctuations. We suggest that this environmental motion is most likely associated with water confined inside the cylinder and/or fluctuations of the dye's aliphatic functional groups.

Coherent and incoherent dynamics in excitonic energy transfer: Correlated fluctuations and off-resonance effects

“…As we shall see, in this situation it is relatively straightforward to derive a strict criterion governing whether or not we expect the energy transfer dynamics to be able to display signatures of coherence. 11,51 Hence, resonant conditions provide a natural situation in which to begin to understand, for example, the role of bath spatial correlations 9,11,33,53,[56][57][58][59][60] or the range of the bath frequency distribution in determining the nature of the energy transfer process.…”

“…A fascinating series of recent experiments demonstrating signatures of quantum coherence in the energy transfer dynamics of a variety of systems [1][2][3][4][5][6][7][8][9] has sparked renewed interest in modeling excitation energy transfer beyond standard methods. [10][11][12][13][14][15][16][17][18][19] This process, which occurs when energy absorbed at one site (the donor) is transferred to another nearby site (the acceptor) via a virtual photon, 20 is often considered to be incoherent; the result of weak donor-acceptor interactions, treated perturbatively using Fermi's golden rule.…”

“…In the resonant case (no energy mismatch) we define a strict crossover temperature above which the energy transfer dynamics ceases to be coherent. 11 Of particular practical interest is the role played by correlations between the donor and acceptor environmental fluctuations, suggested as one mechanism by which quantum coherence may survive in the energy transfer process under otherwise adverse conditions 1,4,9,11,53,[56][57][58][59][60] (though see, for example, Refs. 61 and 62 for alternatives).…”

Ultrafast dynamics of solvation: the story so far

“…41 Diffractive-optic elements were particularly helpful in the design of phase-stable, two-dimensional PE interferometers. [42][43][44][45][46] The connection between solvation dynamics and the time-dependent PE signal is based on the fluctuationdissipation theorem as derived by Kubo, 47 which relates solvent-induced shifts in the solute's electronic energy spectrum to equilibrium solvent fluctuations. The fluctuation-dissipation theorem formed the basis for the correlation function description of spectroscopy, a powerful formalism connecting fundamental timescales of molecular motion to spectroscopic observables.…”

Exact quantum master equation for a molecular aggregate coupled to a harmonic bath

“…Fundamentally, the master equation offers a plausible explanation of the origins of long-lived coherent optical responses of molecular aggregates in dissipative environments [32][33][34][35][36][37][38][39][40][41].…”