Electronically excited states in cylindrical molecular aggregates: Exciton delocalization, dynamics, and optical response

For almost 100 years molecular aggregates have attracted considerable scientific attention, because their electronically excited states feature interesting collective effects that result in photophysical properties that differ significantly from those of the monomeric building blocks. This concerns the delocalization of the excitation energy over many molecules in the aggregate, the redistribution of oscillator strength causing spectral shifts and changes of the fluorescence lifetimes, and changes of the spectral bandwidths of the electronic transitions. These effects result from the intermolecular interactions between the building blocks that lead to the formation of delocalized electronically excited states, commonly referred to as Frenkel excitons or molecular excitons, that can be considered as the elementary electronic excitations of molecular assemblies. Next to arousing scientific interest, these features made molecular aggregates interesting candidates for applications in the fields of sensing, light harvesting, and catalysis. Given the large body of work that addresses molecular aggregates and the information that has been accumulated in the course of time, this review attempts to provide a guide for the readers to follow the literature and to summarize the key results obtained on such systems. After recapitulating the generic photophysical properties of molecular aggregates for various geometrical arrangements, we restricted the illustrative examples to molecular aggregates that self-assemble into tubular structures. This particular choice is motivated by the fact that in nature the secondary structural elements in the most efficient photosynthetic light harvesting antenna systems feature predominantly structural motifs with cylindrical symmetry. This has boosted a wealth of research on biomimetic tubular aggregates that serve as model systems for the development of light-harvesting antenna structures for artificial photosynthesis. Since the strengths of the intermolecular interactions are imposed by the arrangement of the monomers with respect to each other, information about the morphology of the aggregates is encoded in the spectral signatures, which are in the focus of this contribution. The purpose of this review is to bring together the general results about cylindrical molecular aggregates of this large literature.