The optical properties of self-assembled nanotubular aggregates have attracted much attention over the past decade. These quasi-one-dimensional systems exhibit anomalous exciton localization properties, which give rise to strong polarization dependence of emission and absorption, and enhancement of the exciton transport. The chlorosome, the most efficient photosynthetic large-harvesting system in nature, contains tubular molecular aggregates, but also a variety of synthetic molecules self-assemble into nanotubular systems.<br/> <br/>In this talk, I will report on ensemble and single-particle optical spectroscopies of nanotubular aggregates and their interpretation in terms of the underlying Frenkel excitons. Both synthetic and biological aggregates will be discussed. Topics addressed are structure-function relations, effects of energetic and structural disorder on exciton localization and the optical response of individual nanotubes, size-effects, and exciton transport. Specifically, I will show how small substitutions in a synthetic cyanine molecule change the diameter of the nanotube it self-assembles into, thereby changing the size confinement of the excitons and, hence, their transition energies. The resulting changes in the absorption spectrum are consistent with our numerical simulations. Also, I will show how the combination of single-particle experiments, electron microscopy, and simulations, is needed to determine the structure of the chlorosomes of green bacteria. Our results explain the long-standing debate about the question whether these systems contain tubular aggregates, lamellae, or yet other structures.  For a semi-synthetic aggregate of Zn-chlorin molecules considerable variation is observed between single-aggregate spectra, which may be explained by small variations of the molecular orientations within the aggregates. This shows how, even for systems containing thousands of molecules, single-aggregate spectroscopy may unveil important information on inhomogeneity that is out of reach of ensemble experiments. Last, I will address the direct measurement of the exciton diffusion constant in tubular aggregates of porphyrin molecules.

Excitons in nanotubular molecular aggregates: Inhomogeneity, localization, confinement, and transport

256th National Meeting (2018)

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