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.

The properties of excitons in one-dimensional molecular aggregates, dissolved at room temperature in a liquid, were studied by means of femtosecond nonlinear optical experiments. Both the one-exciton band ͑i.e., Frenkel-excitons͒ and multiexciton bands contribute to the observed nonlinear optical response. The rapid motions in the liquid lead to ultrafast perturbations of the molecular energy levels. This localizes the excitons on limited sections of the chains of aggregated molecules. Ultrafast frequency-resolved pump-probe spectroscopy on the lowest two exciton bands was employed to determine the delocalization length of the optical excitations. The kinetics of the exciton populations was measured by ultrafast grating scattering experiments and time-resolved single photon counting. A model is described in which the multiexciton bands act as doorway states in the exciton-exciton annihilation process. These bands thereby determine the population decay of the Frenkel excitons at high excitation densities. Room temperature photon echo experiments show that stochastic perturbations of the exciton transition frequencies occur on two distinct time scales. In particular the slow components of the fluctuations are affected by motional narrowing, associated with the exciton delocalization length. It is therefore argued that the optical dephasing of excitons is directly related to the spatial extent of the excitation on the aggregate chain.

Low-frequency intermolecular dynamics in liquids is studied by ultrafast four-and six-wave mixing. The theory of these nonlinear optical processes is given for electronically nonresonant optical interactions up to fifth order in the electric field. The Born-Oppenheimer approximation is used to separate the motional part of the response functions from coordinate independent electronic hyperpolarizabilities. A large variety of experiments, involving far-infrared absorption, ordinary Rayleigh-Raman or hyper Rayleigh-Raman scattering is covered by this theory. The response in nonresonant six-wave mixing comprises four dynamically different processes. It is shown that one of the terms contains information on the time scale͑s͒ of intermolecular dynamics, that is not available from lower-order nonresonant experiments. For instance, homogenous and inhomogeneous contributions to line broadening can be distinguished. The optical response of harmonic nuclear motion is calculated for nonlinear coordinate dependence of the polarizabilities. Results for level-dependent and level-independent damping of the motion are compared. It is shown that level-dependent damping destroys the interference between different quantum mechanical pathways, yielding an extra contribution to the fifth-order response that has not been discussed before. When two or more nuclear modes determine the optical response, their relative contributions to the four-and six-wave mixing signals are in general different. These contributions are determined by the coordinate dependence of the electronic polarizability, which is usually not fully known. Model calculations are presented for the dynamic parameters of liquid CS 2. The theory of this paper will be employed in Part II, to analyze experimental results on femtosecond four-and six-wave mixing.

The switching behavior of 1,2-bis(5-phenyl-2-methylthien-3-yl)cyclopentene is studied by means of polarization selective nonlinear optical spectroscopy and time-dependent density functional theory. The combined information from the observed population and orientational dynamics together with the results of theoretical calculations show that on a subpicosecond time scale rapid mixing and relaxation of electronic states occur, before switching takes place. Such preswitching dynamics was not studied in detail in these systems before. Then, the switching process itself occurs by the formation of a C−C bond in the central cyclopentene ring with a time constant of 4.2 ps. Driven by the ring closure, the side groups of the switch molecules rotate to a nearly coplanar conformation with a time constant of about 8 ps. The switching process is completed by relaxation of the vibrationally hot ground state of the closed form of the molecule to thermal equilibrium.