Rational design of bright near and shortwave infrared (NIR: 700-1000 SWIR: 1000-2000 nm) molecular and nanoscale emitters is fundamental for applications ranging from deep tissue imaging to telecommunications. However, currently all reported organic chromophores with energy gaps in the SWIR have suboptimal properties due to their especially low fluorescent quantum yields. This thesis introduces the main questions pertaining to molecular absorption and emission in Chapter 1, and in Chapter 2 demonstrates the fundamental principles that govern photophysical properties of SWIR dyes, explicating the precipitous decline of quantum yield with longer wavelengths. I demonstrate that for polymethine dyes such declines are consistent with the combination of decreased radiative rates due to shrinking singlet energy gaps and increased nonradiative deactivation via high frequency vibrations. Through simplifying and combining earlier energy gap laws, I develop an energy gap independent parameter that enables comparison of quantum yields among NIR/SWIR chromophores independent of the differences between each molecule’s energy gaps. Applying the energy gap independent parameter, I provide predictions on whether specific synthetic modifications of a dye would lead to improvements in photophysical properties. In Chapter 3, I recontextualize my work in terms of brightness, which is the quantum yield weighted by the dye’s absorptivity, a paramount chromophore property for imaging applications. Here the analysis expands to a greater survey of other dye scaffolds. In Chapter 4, I show preliminary experimental work that tests temperature dependence, heavy atom effects, and other dye scaffolds. In Chapter 5, I discuss potential synthetic and photophysical experiments to further understand the molecular parameters important to bright SWIR chromophores. Modelling both molecular and solvation impacts of fluorescence lifetimes and quantum yield will give both synthetic and physical chemists a handle on key variables to help develop better organic SWIR imaging systems. 

Molecular systems are ideal platforms for future quantum technologies because molecules can be fine-tuned by synthetic chemistry to provide atomic precision, tunability, and reproducibility. Molecules possess electronic and nuclear degrees of freedom that provide a wide range of transition energies across optical, infrared, and microwave frequencies. In this thesis, I present my studies of the physical and electronic structures of lanthanide organometallic complexes and transition metal chalcogenide nanoplatelets. First, I present my investigation into the spectral properties of Yb(III) complexes, which exhibit an ultra-narrow absorption linewidth in solution at room temperature. The ultra-narrow absorption offers unique applications in magnetic sensing. I demonstrate three different magnetometry techniques that we developed, based on magnetic circular dichroism of the narrow absorption feature. Secondly, I discuss the photophysical properties of highly bound and mobile excitons in colloidal nanoplatelets, which have atomically precise monolayer thicknesses that can be easily tuned for specific absorption and emission properties. I utilize two concepts (the Elliott model and dielectric screening) commonly applied to 2D materials to investigate the absorption of cadmium and mercury chalcogenide nanoplatelets, reporting exciton binding energies for several nanoplatelets for the first time. The investigations highlighted in these chapters emphasize the importance of chemistry in the rational design of quantum technologies. The last chapter depicts the chemical education research conducted in the remote implementation of general and organic chemistry courses during the COVID-19 pandemic, emphasizing the importance of providing structure to online discussion sections to increase student engagement.

Semiconductor nanoplatelets (NPLs) are nanocrystals with quantum confinement only in the thickness dimension and precise monolayer thicknesses. They have excellent monodispersity giving them narrow linewidths, high absorption cross-sections, and high photoluminescence quantum yields. In addition to their exceptional photophysical properties, they can have lateral extents that exceed 1000 nm, making them analogous to two-dimensional semiconductors such as transition metal dichalcogenides with the advantage of colloidal preparation. In this dissertation I explore how interesting photophysical properties can arise within these materials as a result of their anisotropic structure. Chapter 1 provides the foundation for understanding the electronic and physical structures of NPLs, briefly summarizes the current understandings in the field, and presents open questions that this dissertation aims to address.

In Chapter 2, we develop a new type of heterostructure through cation exchange from CdTe to HgTe NPLs. HgTe NPLs are promising candidates for optoelectronic applications in the short-wave infrared (SWIR), but their absorption and photoluminescence has limited tunability. By taking advantage of their large, exposed surfaces following cation exchange, we grow HgTe quantum dots (QDs) on the lateral faces and establish a heterostructure capable of energy transfer from 2D NPL to 0D QD. We extend the tunability of photoluminescence through the energy transfer by controlling the confinement within the quantum dot, and observe high quantum yields across the SWIR.

Chapter 3 builds on our understanding of the HgTe NPL/QD heterostructure by exploring the mechanism and efficiency of energy transfer in greater depth. We compare the in-situ grown QD on NPL system to a system of mixed QDs and NPLs and demonstrate that the near unity energy transfer is only achieved using the in-situ method. We quantify this efficiency and find that the mixed case follows a F�rster Resonance Energy Transfer while the in-situ case is likely a near field energy transfer. Using our estimates, we also model the exciton diffusion within HgTe NPLs and find that the magnitude of the diffusion constant is comparable to other extended 2D semiconductors.

In Chapter 4 we turn our focus to synthetic procedures for CdTe NPLs to control lateral area. We develop a seeded growth mechanism that can be used to reproducibly extend NPLs from their small size after initial synthesis (100-500 nm) to mesoscale length scales (1-2 μm). We show that these methods can be applied to CdTe and CdSe, and cation exchange from these extended NPLs produces mesoscale HgTe and HgSe NPLs. Using in-situ monitoring, we examine how the seeded growth proceeds and show that the extension follows the understood mechanism of lateral ripening. Finally, we image the photoluminescence of the mesoscale NPLs using correlative microscopy and demonstrate that their large extents allow for spatial resolution of their photophysical properties.

Chapter 5 develops another synthetic procedure for controlling lateral extent, but without the need for seeded growth. By closely examining the parameters used during a slow injection procedure, we are able to understand how the precursor, temperature, and ligands play a role in NPL area and directly synthesize mesoscale NPLs in one step. We systematically show how each parameter affects the lateral size and monolayer thickness of the final NPL product. Using the largest mesoscale NPLs, we use transmission electron and atomic force microscopy to examine their propensity for stacking, as well as Raman spectroscopy to observe strain with spatial resolution.

Chapter 6 describes a handful of remaining experiments and projects that were not fully developed for publication. This also includes some techniques and lessons learned in sample handling which may be useful for future students.

Finally, Chapter 7 describes our efforts in chemical education research within the general chemistry laboratory course for life science majors at UCLA. In this chapter we examine how the style of pre-lab affects student performance when tasked with learning a new technique. A cohort of students critically evaluate a series of videos demonstrating exemplary technique and poor technique before attending their lab sections, and these students are compared to a cohort who simply watched the exemplary technique before starting the experiment. Our goal is to reduce cognitive load during the experiment by establishing the proper steps before attempting to practice the technique. We show that although the majority of the class is unaffected by the intervention, we are able to reduce the percentage of students performing as outliers. We also demonstrate that the intervention reduces the dependence of students on instructor feedback during the lab sessions which is advantageous for large classrooms.

Single photon counting has led to many innovations in the manner we observe physical properties of materials, molecules and biological substrates. This comes with the ability to also generate photon streams that record emissive events on the timescale of picoseconds. Using time correlated single photon counting (TCSPC) the events of light stimulation can be recorded with high precision. I leverage the use of TCSPC as a platform to develop complimentary methods to study and manipulate this collected photon stream. Within this I develop methodologies to probe systems across the spectrum from the UV-visible to the short-wave infrared or SWIR (1-2 �m). In Chapter 1 I describe the basis of using light to stimulate photophysical events, covering definitions and theory. There I will also introduce single photon counting along with the ability to resolve spectral signatures through interferometry. In Chapter 2, I cover the implementation of Mach-Zehnder interferometry to simultaneously take broadband spectra while retaining the timing information of photon events. We call this technique Decay Associated Fourier Spectroscopy (DAFS), a method that utilizes interferometry to associate photophysical lifetimes with spectral signatures. I demonstrate DAFS’ capability by recovering Forster resonance energy transfer (FRET) dynamics for a mixture of an organic dye and quantum dots. I then demonstrate the broadband resolution of DAFS by mixing a visible and SWIR quantum dot and recovering the spectra and lifetime data simultaneously over an octave spanning window. In Chapter 3, I introduce a follow-up method to the aforementioned DAFS. Taking advantage of the spectral coherence of emitted photon streams, I introduce interferometric or Fourier filtering to separate signals and term this method “Spectrally-selective Time-resolved Emission Filtering” or STEF. I demonstrate STEF in the context of several systems including laser scatter and emitter, mixed and emitters, and imaging biological systems. STEF allows me to bypass the use of conventional filters in the study of complex chromophore mixtures. In Chapter 4, I then develop another method that utilizes precision timing of multiple pulses to shelf and study short-time events such as fluorescence and long-time events such as phosphorescence simultaneously. Here we term the method fluorescent optical cycling or FOC. Using FOC, I can use pulse trains of on and off times to collect the “total photon economy” or all photon events to understand all radiative, nonradiative, and other energy dissipation pathways. Along with a simple kinetic model, I use FOC to measure these properties in dual emitting platinum complexes, highly phosphorescent anti-boron clusters, and SWIR-emitting singlet oxygen. To finish, I show that FOC and DAFS can be used in tandem to achieve spectral specificity. I will then finish with a perspective on preliminary efforts to establish measurement schemes that could complement the above novel methods. Within Chapter 5, I describe and demonstrate our ability to collect SWIR Fluorescent lifetime imaging microscopy (SW-FLIM), Fluorescent Correlation Spectroscopy (FCS), and dip probe studies. Alongside this, I will discuss experimental plans, to combine these techniques with DAFS, STEF, and FOC while offering perspective on future experiments.

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.

Semiconductor nanoplatelets (NPLs) are a type of two-dimensional (2D) nanomaterials with quantum confinement in the dimension of thickness. Similar to quantum confined quantum dots (QDs), photophysical properties of NPLs depends strongly on their thickness, but their large size in the unconfined lateral dimensions give rise to their other unique properties. The special 2D geometry also requires anisotropic synthesis in the colloidal solution. In this thesis, I study the photophysical properties and anisotropic growth by combing experiments with a powerful tool of kinetic Monte Carlo methods. Chapter 1 provides a review of the development of quantum confined nanocrystals on including discussions on their photophysical properties and anisotropic growth, gives a brief introduction to the kinetic Monte Carlo methods, and proposed some challenges in the field this thesis aims to address. In chapter 2, we propose a quantitative model of fluorescence quenching based on energy transfer mechanism of Fӧrster Resonance Energy Transfer (FRET) for binary mixtures of nanocrystals in colloidal solutions in the short-wavelength infrared (SWIR) region. This model is used to explain the fluorescence quenching of long-lifetime lead sulfide (PbS) quantum dots (QDs) mixed with plasmonic covellite copper sulfide (CuS) nanodisks (NDs), which serve as perfect fluorescent quenchers. By applying kinetic Monte Carlo methods which consider particle distributions and diffusion we are able to quantitatively reproduce experimental data which shows significant quenching at very small concentrations of NDs. The high concentration case is examined by conducting similar mixing experiments of mercury telluride (HgTe) NPLs and QDs, where we specifically discuss the impact of geometry on the distance dependence of energy transfer efficiency. Chapter 3 deals with a classic yet still puzzling phenomena in single-nanocrystal: photoluminescence intermittency (blinking). We apply Marcus theory of electron transfer to the blinking model of carrier trapping and detrapping, with meticulous mathematical analysis and kinetic Monte Carlo modeling. With canonical distribution of trap state energy, we quantitatively explain the difference between on and off time statistic in blinking cadmium telluride (CdTe) NPLs, as well as the temperature dependence of their quantum yield. In chapter 4, we focus on the anisotropic growth of NPLs, more specifically the different behaviors between cadmium selenide (CdSe) and cadmium telluride (CdTe) NPLs in terms of thickness selectivity and lateral size. We propose a simple kinetic model with 3 most important energetic parameters, on which kinetic Monte Carlo simulations of lattice are based. Using population percentage and evolution of size distribution from correlation function analysis, we offer a reasonable answer to questions regarding the difference of CdSe and CdTe NPLs. Finally, in chapter 5 we describe some of our ongoing effort towards further understanding of NPLs and QDs of interest, including using machine learning (ML) method to assist the optimization of synthetic conditions of NPLs and an in-depth study of correlation growth/ripening mechanisms with size distribution of QDs.

J-aggregates are comprised of conjugated organic molecules that, when given the correct solvation conditions, self-assemble into a slip-stacked arrangement. In this form, not only do their absorption and emission redshift due to dipolar coupling, but many exciting properties emerge from the coherent coupling of their excitations over long distances. Namely, they boast superradiant emission from their excitonic states, leading to extraordinarily fast energy transfer and amplified radiative rates. Yet, while the optical properties of J-aggregates are unparalleled, one of the greatest challenges preventing their widespread use remains controlling their self-assembly into highly ordered nanostructures. In this dissertation, I unpack how changes to a dye’s molecular structure can affect its ability to undergo J-aggregation, with regard to the design and improvement of the resulting aggregate’s optical properties. Chapter one contains a historical perspective on cyanine dyes and aggregates as colorants, as well as a short literature survey of how a dye’s structure determines the optical properties of its resulting J-aggregates.In chapter two, I explore the design of superradiant J-aggregates in different spectral windows. By elongating the polymethine chain of a well-studied cyanine dye (colloquially known as C8S3), I demonstrate the formation of tubular aggregates in the near-infrared regime. This work begins with synthesis of penta- and heptamethine derivatives of C8S3, followed by a careful screening of their J-aggregation via absorbance and cryo-electron microscopy (cryo-EM), and lastly a thorough spectroscopic characterization of each aggregate’s emissive properties. While the new aggregates were considerably redshifted, they displayed different kinetics of aggregation, ultimately leading to stabilization of a morphology with decreased superradiance. To better understand those changes in superradiance, we employed a computational screening that unraveled a correlation between tube radius and disorder. The above work opened many questions about why C8S3 organizes into highly-ordered superradiant J-aggregates while other cyanine dyes fall short. Around this time, our lab began a collaboration with Dr. Weili Zheng and Professor Edward Egelman (University of Virginia), who used cryo-EM to reconstruct the C8S3 nanotubes’ structure with near-atomic resolution. Chapter three summarizes that work, which vastly expanded our knowledge of molecular packing in these systems. Prior to this work, the molecular structure of C8S3 J-aggregates had been posited from spectroscopic evidence and low-resolution cryo-electron micrographs. This result, however, unambiguously settled the long-standing dispute regarding C8S3 nanotube structure and represents an enormous advancement to our knowledge about how an individual dye organizes into a nanostructure. I contributed polarization-dependent spectroscopy and two molecules to this effort, which primarily demonstrated how chemical modifications to the C8S3 monomer can obfuscate its self-assembly. This result inspired the full-length follow-up work, covered in chapter 4. Chapter four investigates the role of electrostatics in the C8S3 nanotube self-assembly. This work begins from the revelation that C8S3 nanotubes feature a unique motif for self-assembly wherein the anionic sulfonate groups coordinate to the delocalized positive charge from the cyanine backbone. We tested this mechanism for self-assembly by synthesizing C8S3 derivatives with a variable number of methylene groups between the cationic cyanine and anionic sulfonate. Depending on the number of -CH2 units, we found the dyes formed aggregates with different absorption lineshapes, and confirmed their differing morphologies using cryo-EM. Lastly, we synthesized dyes that have differently charged functional groups, thereby turning off the cation/anion interactions, and found them to exhibit significantly reduced J-aggregation. Based on these experiments, we believe that the electrostatic interaction between the alkylsulfonate and cyanine core are a significant driving force in the C8S3 self-assembly. In chapter five, I examine the effect of a dye’s steric bulk on its resulting J-aggregates. This work comprises a large-scale collaboration between many students in the Caram and Sletten labs, spanning the synthesis of new cyanines to computational screening of their self-assembly. We accomplished this goal by first synthesizing heptamethine benzothiazole dyes that differ in their 4’ substitution (ranging from a phenyl ring to a 3,5-ditertbutylphenyl group). By modifying this position, we change neither the electronics nor the solubility of the dye, allowing a rational investigation of steric bulk on aggregation. Based on absorbance screening and cryo-EM, we found that these dyes form at least three J-aggregates each, all of which are extended 2D sheets (with one tubular anomaly). The addition of steric bulk was found to redshift the aggregate absorbance, while also changing the kinetics of self-assembly and therefore leading to stabilization of different nanoscale morphologies. In addition to these efforts, I have contributed to several other projects and conducted unpublished experiments that may inspire further investigation. Chapter six outlines these beginnings and gives direction for their future. Specifically, I discuss the potential for nuclear magnetic resonance (NMR) to offer structural knowledge on molecular aggregates and preliminary development of extremely redshifted J-aggregates in the shortwave infrared (SWIR). Taken together, these results inform the design of next-generation J-aggregated optical materials. By systematically exploring changes to the monomer chromophore, I have deduced clear structure-property relationships that serve as guidelines for making J-aggregates for applications in bioimaging, solar energy harvesting, or telecommunications. While many unsolved questions remain in J-aggregate self-assembly, this work breaks ground as a foundation for others to build upon our knowledge of cyanine dyes, their molecular aggregates, and, ultimately, their optical properties.