UCLA

Molecular aggregates are non-covalent self-assemblies of chromophores wherein excitations on individual molecules couple coherently over long distances. This leads to the formation of delocalized excitons with drastically altered photophysical properties from the monomers, including extreme blue or red shifts, narrow linewidths and high molar absorptivities. Kasha’s model relates the observed blue or red shifts to the underlying molecular arrangements in 1-dimensional systems, known as H- or J-aggregates, respectively. In this dissertation, I explore how to modulate the excitonic couplings within an aggregate via molecular packing and topology in order to explore new photophysical behaviors. Chapter 1 covers the basic definitions, describes the origins of the excitonic properties and the relevance towards various applications, and orients the readers to the overall goals of this dissertation.In Chapter 2, we extend Kasha’s model to 2-dimensional systems and show the unusual situation that arises from 2-dimensional (2D) transition dipole coupling in extended sheet-like aggregates. In addition to traditional H- and J-aggregation, we find a new case of ‘I-aggregation’ which shows intermediate characteristics of H- and J-aggregates. We demonstrate two examples of I-aggregates – extended 2D sheet aggregates of the dyes Cy7-Ph and Cy7-DPA, whose absorption spectra look quite like traditional J-aggregates but temperature dependence shows critical differences. This work shows the relative distance of the bright state from the band-edge can be tuned via relative slip between adjacent molecules, thus providing design principles to tune photophysical properties over a broader spectral range across visible (400 - 700 nm), near-infrared (NIR 700 - 1000 nm), and shortwave infrared (SWIR, 1000 - 2000 nm). Chapter 3 provides insights on controlling the self-assembly of aggregates, achieving selective stabilization of H- or J-aggregate morphologies. Independent control of solvation conditions allows us to access a large aggregation phase space which can be modelled using a three-component equilibrium model. We obtain new insights into the self-assembly of the 2D aggregates. Mainly, the large sizes make the self-assembly highly cooperative and charge screening is important for stabilizing large aggregate morphologies. Such insights translate into general guidelines for thermodynamically controlling the aggregate self-assembly. We demonstrate this by aggregating several cyanine dyes into 2D sheet-like morphologies with narrow red shifted absorption spectra, enabling a library of 2D aggregates with absorptions spanning the visible and shortwave infrared (SWIR) regions. In Chapter 4, we describe the subtle differences in the excitonic band structures across the aforementioned library of 2D aggregates. We show that various observables from absorption, emission and temperature dependent spectroscopy reveal the complex band structures of 2D aggregates, and thus form a comprehensive tool for probing excitonic band structures in general. With subtle control of geometric parameters such as length of the dye molecules and relative slip between adjacent molecules, we modify the excitonic band structure from mid-band I-aggregates that are weakly emissive to band-edge J-aggregates with high quantum yields and superradiance. The above work demonstrates the importance of supramolecular packings within the aggregates in modulating the excitonic properties. A high resolution structure of the supramolecular aggregates will, therefore, pave the way for precise chemical design with desired excitonic properties. In Chapter 5, we present the first high-resolution cryo-electron microscopy structure of a prototypical tubular J-aggregate - double-walled light harvesting nanotubes (LHNs) of amphiphilic cyanine dye C8S3. We employ a cryogenic fixation technique to preserve the native structure, followed by a single particle analysis using the Iterative Helical Real Space Reconstruction (IHRSR) algorithm and obtain density maps of the inner wall at 3.3 � resolution. Our structure shows a 3 dimer (6 monomers total) asymmetric unit with brick layer arrangement, as opposed to the previously thought herringbone arrangement. We uncover important structural features that were previously unknown, providing new pathways for chemical modulation of the supramolecular self-assemblies and thereby, the excitonic properties. Several fundamental aspects about the structures and excitonic couplings remain open questions. Some of these challenges and experimental plans to address them are discussed in Chapter 6. Overall, this work establishes molecular aggregation as a tunable avenue for accessing unusual photophysical properties such as extreme spectral shifts, high SWIR quantum yields and molar absorptivities, and narrow linewidths. Thereby, this work opens up organic chromophores to new functionalities including SWIR imaging, photonics, and telecommunications. Chemical modulation of exciton transport and practical applicability will be the main challenges to be addressed in the future. This work lays down the foundational principles that will enable researchers to approach such challenges.