Modeling of excitonic properties in tubular molecular aggregates

Abstract

Molecular aggregates of dye molecules offer great possibilities to develop nanoscale functional materials for optical and electronic applications. When closely packed in aggregates, dye molecules work together to efficiently absorb light and redistribute its energy. Molecular aggregates can be found in Nature; a notable example is constituted by light-harvesting complexes of plants and bacteria. There, the process of photosynthesis starts with the absorption of solar energy by molecular aggregates of photosynthetic dyes. Such systems can also be produced artificially. The work described in this thesis is focused on the theoretical modeling of the excitonic properties, i.e., properties of the collective excited states, in tubular molecular aggregates. As a particular model system, tubular aggregates composed of cyanine dyes are studied. This system is especially interesting as it resembles the light-harvesting antennae of green sulfur bacteria—the most efficient photosynthetic system known. Like natural light-harvesting antennae, these synthetic nanotubes are formed by thousands of closely packed molecules organized in a tubular geometry. In this work, the structure-optical properties of the molecular aggregates are established. The multiscale approach is used to develop a detailed microscopic model and obtained insights into physical phenomena of the complex double-walled molecular aggregate. Moreover, the methods applicable to study excitation energy transport in such large systems are carefully examined. The obtained results can be used in further studies towards the design of materials with fine-tuned optical behavior for optoelectronic applications, such as artificial photosynthetic systems or energy transport nanowires.