Self-assembled cylindrical aggregates of amphiphilic carbocyanine dye molecules are interesting candidates for synthetic light-harvesting systems and electronic energy transport wires. To be able to optimize the properties of such systems, detailed information on the molecular structure as well as the static and dynamic optical properties is required. We report cryo-transmission electron microscopy (cryo-TEM) experiments on 3,3‘-bis(3-sulfopropyl)-5,5‘,6,6‘-tetrachloro-1,1‘-dioctylbenzimidacarbocyanine (C8S3) aggregates that reveal a double-layer tubular structure. By combining these results with information from both isotropic and polarized spectral responses, a detailed molecular picture of these aggregates is obtained. The basis of our theoretical analysis of the spectroscopic data is the formation of the inner and outer cylinders by rolling cyanine sheets with a brick-layer structure onto cylindrical surfaces with diameters of 11 and 16 nm. This model very well reproduces the spectral properties of the excitonic transitions of the C8S3 aggregates. The combination of experimental and theoretical techniques for the first time provides detailed insight into the molecular arrangement inside these aggregates.
Using a Frenkel exciton model, we study the optical absorption spectrum and linear and circular dichroism (CD) spectra of cylindrical molecular aggregates. We demonstrate that such aggregates can always be described as a stack of molecular rings with nearest-neighbor rings rotated relative to each other by a helical angle γ. For homogeneous aggregates, the cylindrical symmetry allows for a decomposition of the Hamiltonian into a set of effective one-dimensional Hamiltonians, which are characterized by a transverse wavenumber k 2 . The helical nature of the cylinder renders these Hamiltonians complex and noninversion symmetrical in general. Only the bands with k 2 ) 0 and k 2 ) (1 are dipole-allowed and yield contributions to the various linear spectra studied. The k 2 decomposition also allows for a convenient separation of the CD into ring and helical contributions, which in turn allows us to explain the strong sensitivity of this spectrum to various system parameters, such as the molecular orientations and the ratio of cylinder length and circumference. The latter is explicitly demonstrated by numerically studying the size dependence of the spectra for chlorosomes of green bacteria. The results suggest that the strong variation of the CD as reported experimentally may result from size variations. We also present analytical results valid for long cylinders. In this case, we find three superradiant states to be responsible for the complete linear optical response: one at total wave vector k ) 0 and the other two (degenerate) at wave vectors determined by the circumference and the helical angle γ.
We report temperature-dependent steady-state and time-resolved fluorescence studies to probe the exciton dynamics in double-wall tubular J-aggregates formed by self-assembly of the dye 3,3'-bis(3-sulfopropyl)-5,5',6,6'-tetrachloro-1,1'-dioctylbenzimidacarbocyanine. We focus on the lowest energy fluorescence band, originating from the inner cylindrical wall. At low temperatures, the experiments reveal a nonexponential decay of the fluorescence, with a typical time scale that depends on the emission wavelength. At these temperatures we also find a dynamic Stokes shift of the fluorescence spectrum and its nonmonotonic dependence on temperature under steady-state conditions. All these data indicate that below about 20 K the excitons in the lowest fluorescence band do not reach thermal equilibrium before emission occurs, while above about 60 K thermalization on this time scale is complete. By comparing the two lowest fluorescence bands, we also find indications for fast energy transfer from the outer to the inner wall. We show that the Frenkel exciton model with diagonal disorder, which previously has been proposed to explain the absorption and linear dichroism spectra of these aggregates, yields a quantitative explanation to the observed dynamics. To this end, we extend the model to account for weak phonon-induced scattering of the localized exciton states; the spectral dynamics are then described by solving a Pauli master equation for the exciton populations.
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