Tailoring the excited-state energy landscape in supramolecular nanostructures

Abstract

Nature's photosynthetic machinery uses precisely arranged pigment-protein complexes, often representing superstructures, for efficient light-harvesting and transport of excitation energy (excitons) during the initial steps of photosynthesis. This function is achieved by defined electronic Coulomb interactions between the conjugated molecules resulting in tailored excited-state energy landscapes. While such complex natural structures are synthetically difficult to achieve, supramolecular chemistry is now on its advent to realize defined artificial supramolecular nanostructures with tailored functionalities via controlled self-assembly processes of small molecules. In this review, we focus on recent work reporting photophysical studies on self-assembled and hierarchical nanostructures as well as complex superstructures. We discuss how the resulting excited-state energy landscapes influence energy transport. Progress in the field of supramolecular chemistry allows for the realization of distinct kinds of H-or J-aggregates with well-defined morphologies on the mesoscale. Advances in the field of optical spectroscopy and microscopy have permitted to resolve the incoherent/coherent dynamics of exciton transport in such systems down to the level of single nanostructures. Although outstanding diffusion lengths of up to several μm were found in selected nanostructures, a full understanding of the underlying principles is still missing. In particular, the unavoidable structural and electronic disorder in these systems influences the excited-state energy landscapes and thus the transport characteristics, which can be exploited to refine the molecular design criteria of supramolecular nanostructures and complex superstructures. Despite the rapid progress in the field of functional supramolecular nanostructures, we believe that revealing the full potential of such systems is far from complete. In particular, criteria for tailored and optimized (hierarchical) supramolecular nanostructures in view of applications are not yet established. Finally, we outline current challenges and future perspectives for optical and optoelectronic applications of supramolecular nanostructures.

Disorder in P3HT Nanoparticles Probed by Optical Spectroscopy on P3HT-b-PEG Micelles

“… Here, I 01 and I 00 are the integrated intensities of the 0–1 and the 0–0 transitions in the PL spectra. For ideal, disorder-free H-aggregates at T = 0 K, the 0–0 PL transition is entirely forbidden ( I 00 = 0); hence, 1/ R em → ∞ since the wavefunction of the emitting exciton features perfect symmetry with alternating phase from chain to chain along the π-stack of an aggregate. , With increasing intra-aggregate disorder, the relative 0–0 peak intensity I 00 increases, thus 1/ R em decreases, until the limit of strong disorder is reached. In this limit, disorder is much larger than the electronic coupling between P3HT chains, and the emitting exciton becomes strongly localized on very few adjacent P3HT chains along a π-stack.…”

Near-Atomic Resolution Structure of J-aggregated Helical Light Harvesting Nanotubes

“…[8][9][10][11][12] Inspired by such naturally evolved photosynthetic antennas, many scientists have sought to engineer synthetic supramolecular analogs, which leverage exciton delocalization and energetic disorder to drive energy transport at the nanoscale. [13][14][15][16][17][18][19] The classic signatures of excitonic coupling are shifts and narrowing in optical absorption of chromophores upon concentration or in poor solvents. Jelley and Scheibe concurrently observed this phenomenon in 1936, where absorption of a pseudoisocyanine dye red-shifted and narrowed upon concentration, a phenomenon later named 'J-aggregation'.…”

Vibronic coherences in light harvesting nanotubes: unravelling the role of dark states

“…Light Harvesting Nanotubes (LHNs) made of strongly packed cyanine dyes, characterized by a high degree of order and exciton diffusion over long distances, are ideal model systems for designing artificial bio-inspired materials for energetic applications and for disentangling the complex exciton dynamics which are typical of photosynthetic systems. 9–12…”

“…Light Harvesting Nanotubes (LHNs) made of strongly packed cyanine dyes, characterized by a high degree of order and exciton diffusion over long distances, are ideal model systems for designing artificial bio-inspired materials for energetic applications and for disentangling the complex exciton dynamics which are typical of photosynthetic systems. [9][10][11][12] Typical features of these materials are narrow spectral lineshapes, high emission quantum yield and large absorption cross-section in the visible and near infrared region, desirable properties for photonic, optoelectronic and bioimaging applications. The reason for these outstanding properties lies in the balance between intermolecular van der Waals interactions and environment fluctuations, allowing for the formation of highly delocalized excitonic states, [13][14][15][16][17][18] capable of transporting energy throughout the system in an extremely fast and efficient manner.…”