Structure, Spectroscopy, and Microscopic Model of Tubular Carbocyanine Dye Aggregates Ca ˇ ta ˇ lin Didraga, ² Audrius Pugz ˇ lys, ² P. Ralph Hania, ² Hans von Berlepsch, ‡ Koos Duppen, ² and Jasper Knoester* ,² Materials Science Centre, UniVersity of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands, and Forschungszentrum fu ¨r Elektronenmikroskopie, Freie UniVersita ¨t Berlin, Fabeckstrasse 36a, D-14195 Berlin, Germany ReceiVed: April 20, 2004; In Final Form: June 30, 2004 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. I. Introduction The preparation of low-dimensional supramolecular systems with controlled structure and optical dynamics is a topic of considerable current interest. 1,2 Such systems can serve as artificial light harvestors, in analogy with natural antenna complexes in photosynthetic systems, 3-5 and as transport wires of electronic excitation energy. In addition, they can be used to create devices with tunable and strongly anisotropic optical properties, such as a strong circular dichroism based on a possible chirality in the supramolecular arrangement. In search of such new materials, the family of substituted 5,5′,6,6′- tetrachlorobenzimidacarbocyanine dyes (Figure 1) recently has stirred special interest. 6 This is due to the possibility of tuning the morphology of the aggregates formed from these chro- mophores by (small) changes in their side chains and their environment. Thus, aggregates with planar, spherical, and cylindrical (tubular) morphologies have been prepared. 7,8 These different shapes give rise to strong variations in the optical properties. These variations do not arise from changes in the electronic structure of the individual chromophores; the side chains and environment hardly affect the π system of a single chromophore. 9 Rather, these variations result from the influence of the side chains and the solvent on the packing of the dye molecules in the aggregates. By influencing the intermolecular interactions and the spatial arrangement of dipole orientations, the packing has a strong effect on the collective optical properties of the molecules within an aggregate. Thus far, it has been found to be difficult to relate the changes in the optical properties directly to changes in the microscopic molecular arrangement. The reason is that, although changes in aggregate morphology can be readily observed using cryo- transmission electron microscopy (cryo-TEM), 7,8,10 this tech- nique lacks the spatial resolution to provide insight into the aggregate structure at the molecular scale. It is the aim of this paper to present the first detailed exploration of the relationship between morphology, structure, and optical properties for one type of tetrachlorobenzimidacarbocyanine aggregate, namely, for the tubular aggregates formed in the case of 1,1′-dioctyl and 3,3′-bis(3-sulfopropyl) subsituents (abbreviated by C8S3 in Figure 1). The reason for our special interest in these tubular aggregates is that they closely resemble the rod elements in the light-harvesting chlorosomes of green bacteria 11-14 and exhibit a rigid wire structure that might be suitable for energy transport. To appreciate the variation of morphologies and related optical properties, it is useful to briefly review recent studies on the aggregates of several of the derivatives of the 5,5′,6,6′- * Corresponding author. Fax: 31-50-3634947. E-mail: knoester@ phys.rug.nl. ² University of Groningen. ‡ Freie Universita ¨t Berlin. Figure 1. Chemical structure of the 5,5′,6,6′-tetrachlorobenzimida- carbocyanine dyes with their abbreviated names. 14976 J. Phys. Chem. B 2004, 108, 14976-14985 10.1021/jp048288s CCC: $27.50 © 2004 American Chemical Society Published on Web 08/26/2004