Browsing by Author "Therien, Michael J"

Developing organic, light-weight materials that possess electrical conductivitieswhich rival those seen in inorganic semiconductors would revolutionize solar cells. As the market for indoor and building-integrated photovoltaics expands, the versatility of organic conducting materials becomes promising due to the abundance and flexibility of small-molecule and polymer-based systems. One key technological hurdle is the need to evolve organic photovoltaic components in which photogenerated excitons readily dissociate into free charge carriers, as this process plays a central role in photovoltaic devices. Towards these goals my dissertation research focuses on developing highly conjugated porphyrin materials that enable facile exciton dissociation and probing how electronic structure impacts: i) the spatial distribution of electronically excited singlet and triplet states, ii) exciton migration, iii) electron-hole pair separation, and iv) electron transfer dynamics. My doctoral research has focused on engineering chromophores with highly tunable excited states from molecular design, spectroscopic, and excited-state dynamical standpoints. Specifically, Chapter One serves as an introduction to (porphinato)zinc(II) chromophores as an ideal benchmark from which to engineer highly conjugated supermolecules to study the fundamental photophysics that drive contemporary technological advances in solar energy conversion. Chapter Two describes the synthesis v and photophysical characterization of electron-deficient (porphinato)zinc(II) chromophores (RfPZn) which feature incredible oxidative stability, long-lived singlet lifetimes, and high quantum yields. Chapter Three discusses how, through the careful synthesis of RfPZn monomers, we can outline a framework for modulation of singlettriplet energy gaps via reducing the exchange and coulomb interactions. Chapter Four reports the photophysical and computational analysis of three new families of proquinoidal-integrated PZn arrays which facilitate high quantum yield extending through the NIR. In-depth solvent-dependent ultrafast transient dynamical studies are described in Chapter Five, which facilitate the development of design principles for augmenting radiative rate constants while selectively minimizing the non-radiative internal conversion and intersystem crossing rates in highly conjugated fluorophores based on proquinoidal connection motif. Chapter Six presents the design of a novel donor-acceptor porphyrin motif in which electron rich and electron poor (porphinato)zinc(II) arrays are covalently linked, acting as a single molecule p-n junction. These molecules represent a rare example of ultrafast photoinduced charge separation in a highly conjugated array yielding highly delocalized electrons and holes residing on opposite ends of the system. Chapter Seven details progress towards synthesizing (porphinato)zinc(II)-iron(II) supermolecules. This chapter highlights our strategy of pairing highly electron deficient PZn macrocycles with N-heterocycle carbene coordinated Fe(II) complexes towards the goal of stabilizing the triplet metal-tovi ligand charge transfer excited state and destabilizing the triplet and quintet metalcentered states that dominate the decay pathways of conventional polypyridylsubstituted Fe(II) transition metal complexes. Finally, Chapter Eight extends the ideas of molecular design in electron transfer to the development of (porphinato)zinc(II)-based systems for spin information transfer. Herein I discuss the synthesis and photophysics of covalently linked PZn and nitroxide stable radical chromophores.

Highly absorptive photosensitizers are in great demand for solar energy conversion applications that include dye-sensitized solar cells, photoelectrochemical cells, and photo-redox catalysis. In this dissertation, the design, synthesis and spectroscopy of a series of different highly absorptive chromophoric systems based on the bis(tridentate)metal-ethyne-(porphinato)metal molecular framework are introduced and discussed. Apart from the substantially more intense absorption in the solar spectral range comparing to traditional photosensitizers, each chromophoric system discussed here further possesses other novel properties and design characteristics, which provides interesting perspectives in resolving existing problems in the energy conversion fields as well as inspire future advancement of photosensitizers and solar energy conversion devices.

Specifically, Chapter One serves as an introduction about common solar energy conversion applications and chromophores related to the discussed topics. Chapter Two describes an electron-deficient perfluoroalkyl-substituted bis(terpyridyl)Ru(II)-ethyne- (porphinato)Zn(II) chromophore that is endowed with intense panchromatic absorptivity, and long-lived, highly oxidizing singlet and triplet charge-transfer (CT) excited states. This study provides a design strategy to engineer high-potential photo- oxidants to drive challenging photo-oxidation reactions. Chapter Three reports a series of bis(terpyridyl)Fe(II)-ethyne-(porphinato)Zn(II) based Fe(II) complexes that manifest intense panchromatic light absorption, tunable potentiometric metal-to-ligand CT (MLCT) band gaps and low-lying MLCT featured highly delocalized triplet excited states. Although their 3MLCT lifetimes are short (sub-picosecond time scale) for realistic applications, this study signifies a strategy to decouple the modulation of MLCT and MC state energy levels and paves the way for realizing Fe(II) complexes with long-lived 3MLCT states. Based on the design strategy reported in Chapter Three, the prototype Fe(II) complex described in Chapter Four features a refined bis(N-heterocyclic carbene)Fe(II)-ethyne-(porphinato)Zn(II) structure, which achieves an unprecedentedly long (sub-nanosecond) phosphorescent MLCT state. It represents a new class of earth- abundant iron based photosensitizer and is expected to promote the advancement of environment-friendly and low-cost solar energy conversion devices. Chapter Four reports an asymmetric bis(terpyridyl)Ru(II)-ethyne-(porphyrin)Zn(II) donor based D−A system and its excitation-wavelength dependent photo-induced electron transfer dynamics. It shows that undesired excited-state decay channels, such as intersystem crossing, can be eliminated by designing chromophores with opposite excited-state polarizations to maximize the yields of high-energy photoproducts.

Significant advances in understanding the fundamental photophysical behavior of single-walled carbon nanotubes (SWNTs) have been made possible by the development of ionic, conjugated aryleneethynylene polymers that helically wrap SWNTs with well-defined morphology. My contribution to this work was the design and synthesis of porphyrin-containing polymers and the photophysical investigation of the corresponding polymer-wrapped SWNTs. For these new constructs, the polymer acts as more than just a solubilization scaffold; such assemblies can provide benchmark data for evaluating spectroscopic signatures of energy and charge transfer events and lay the groundwork for further, rational development of polymers with precisely tuned redox properties and electronic coupling with the underlying SWNT. The first design to incorporate a zinc porphyrin into the polymer backbone, PNES-PZn, suffered from severe aggregation in solution and was redesigned to produce the porphyrin-containing polymer S-PBN-PZn. This polymer was utilized to helically wrap chirality-enriched (6,5) SWNTs, which resulted in significant quenching of the porphyrin-based fluorescence. Time-resolved spectroscopy revealed a simultaneous rise and decay of the porphyrin radical cation and SWNT electron polaron spectroscopic signatures indicative of photoinduced electron transfer. A new polymer, S-PBN(b)-Ph2PZn3, was then synthesized which incorporated a meso-ethyne linked zinc porphyrin trimer. By changing the absorption profile and electrochemical redox potentials of the polymer, the photophysical behavior of the corresponding polymer-wrapped (6,5)-SWNTs was dramatically changed, and the polymer-wrapped SWNTs no longer showed evidence for photoinduced electron transfer.

Chiral, molecular and nanoscale assemblies are promising candidates for the development of spintronic-based devices, characterized by information processing using both electronic charge and spin, and are poised to give rise to superior computational efficiency relative to modern electronic architectures that only operate using processing of electronic charge. Essential to realizing such spintronic assemblies is the ability to isolate and engineer enantiopure, chiral nanoscale materials that feature highly-tunable and unique electronic structures. Congruent with such requirements, this work focuses on engineering molecular and nanoscale organic matter that interface with single-walled carbon nanotubes (SWNTs), and are capable of: (i) generating concentrated, enantioenriched SWNT-based chiral inks from racemic mixtures that aim to be amenable with current ink-jet printing designs for electronic device fabrication, or (ii) inducing SWNT lattice handedness in achiral SWNT platforms that depend on conjugated polymer electronic structure and polymer pitch length. Specifically, this work explores: (i) experimental and computational investigation of engineered chiral, binaphthalene-based surfactant frameworks that are able to disperse and resolve enantiomers of SWNTs via enthalpic and entropic differences in surfactant-SWNT interactions in aqueous solutions, and (ii) chiral, semiconducting aryleneethynylene-based polymers that helically wrap metallic single-walled carbon nanotubes (m-SWNTs) and give rise to m-SWNT band gap opening and a metallic to semiconducting phase transition in such assemblies. Exploitation of such unique designs will enable opportunities to develop exceptional chiro-optic and spintronic materials, and help elucidate critical structure-function relationships that broadly inform material design for such applications.

Interdisciplinary investigation at the interface of chemistry, engineering and medicine has triggered the development of self-assembled nanomaterials with novel biomaterial and optical properties. Targeted near infrared (NIR) emissive polymersomes can be used as agents of deep tissue fluorescent imaging and targeted drug delivery. For the development of these agents, my approaches were modification of polymersome surface with functional groups for protein conjugation, and optimization of NIR emissive fluorophores enhancing their organization homogeneity as well as increasing dispersion in polymersomes matrix. Results show successful protein conjugation to biodegradable polymersomes and thus a step forward to apply polymersomes for in vivo imaging and targeted drug delivery with low toxicity.

The goal of this review is to highlight the unique and interesting aspects of conjugated metalloporphyrins. I will show the fundamental importance of these conjugated porphyrin systems by reviewing the excited state dynamics of some of the key systems that have been studied to date. The spectroscopic aspects I will be focusing on are the electronic absorption and emission and the transient absorption and emission. It is necessary to compare the ground and excited states in order to understand the significance of the excited state dynamics. Also important to discuss are the EPR and electrochemical data to further elucidate the unique qualities of conjugated porphryin oligomers.

Nature is challenged to move charge efficiently over many length scales. From sub-nm to μm distances, electron-transfer proteins orchestrate energy conversion, storage, and release both inside and outside the cell. Uncovering the detailed mechanisms of biological electron-transfer reactions, which are often coupled to bond-breaking and bond-making events, is essential to designing durable, artificial energy conversion systems that mimic the specificity and efficiency of their natural counterparts. Here, we use theoretical modeling of long-distance charge hopping (Chapter 3), synthetic donor-bridge-acceptor molecules (Chapters 4, 5, and 6), and de novo protein design (Chapters 5 and 6) to investigate general principles that govern light-driven and electrochemically driven electron-transfer reactions in biology. We show that fast, μm-distance charge hopping along bacterial nanowires requires closely packed charge carriers with low reorganization energies (Chapter 3); singlet excited-state electronic polarization of supermolecular electron donors can attenuate intersystem crossing yields to lower-energy, oppositely polarized, donor triplet states (Chapter 4); the effective static dielectric constant of a small (~100 residue) de novo designed 4-helical protein bundle can change upon phototriggering an electron transfer event in the protein interior, providing a means to slow the charge-recombination reaction (Chapter 5); and a tightly-packed de novo designed 4-helix protein bundle can drastically alter charge-transfer driving forces of photo-induced amino acid radical formation in the bundle interior, effectively turning off a light-driven oxidation reaction that occurs in organic solvent (Chapter 6). This work leverages unique insights gleaned from proteins designed from scratch that bind synthetic donor-bridge-acceptor molecules that can also be studied in organic solvents, opening new avenues of exploration into the factors critical for protein control of charge flow in biology.

As silicon-based microelectronics approaches its fundamental physical limit, molecular electronics is emerging as a promising candidate for future ultra-dense electronic devices with individual molecules as active device components. The emerging of molecular spintronics, which exploits the spin-dependent charge transport through organic materials, further demonstrate the promising future of molecular electronics. This dissertation describes the charge transport and spin-spin electronic coupling properties of an extraordinary class of molecular wires, alkyne-bridged porphyrin arrays. Chapter one provides a general background of molecular electronics and molecular wires, as well as the basics of electron paramagnetic resonance (EPR). Chapter two describes utilizing highly conjugated (porphinato)metal-based oligomers (PMn structures) as molecular wire components of nanotransfer printed (nTP) molecular junctions; electrical characterization of these “bulk” nTP devices highlights device resistances that depend on PMn wire length. This study demonstrates the ability to fabricate “bulk” and scalable electronic devices in which function derives from the electronic properties of discrete single molecules, and underscores how a critical device function—wire resistance—may be straightforwardly engineered by PMn molecular composition. Chapter three describe the electronic exchange coupling between two unpaired spin on Cu(II) ions in meso-meso alkyne-bridged multi[copper(II) porphyrin] (mmPCu2). Spin and conformational dynamics in symmetric mmPCu2 have been studied in toluene solution at variable temperature using EPR spectroscopy. Comparison of the dimer EPR spectra to those of Cu porphyrin monomers shows clear evidence of an isotropic exchange interaction (Javg) in these biradicaloid structures, manifested by a significant line broadening in the dimer spectra. Comparison of ethyne and butadiyne alkyne bridges reveals a remarkable sensitivity to orbital interactions between the spacer and the metal, which is reflected in measurements of Javg as a function of temperature. The results suggest that orbital symmetry relationships may be more important than previously recognized in the design of optimized molecular spintronic devices. Chapter four reports a study of β-β linked bis[(porphinato)copper(II)] complexes (ββPCu2), which exhibit very different electronic structures compared to their mm linked analogs. By using electron paramagnetic resonance (EPR) spectroscopy, this study exhibits that a wide range (3 orders of magnitude) of the average electronic spin-spin exchange coupling can be achieved by varying the length of bridges and points of connections between the porphyrin rings. The pathways for mmPCu2 and ββPCu2 complexes were also investigated, with the ββPCu2 complexes exhibiting a dominant σ-type pathway and the mmPCu2 complexes showing a dominant π-type pathway.

Organic pi-conjugated materials are emerging as commercially relevant components in electronic applications that include transistors, light-emitting diodes, and solar cells. One requirement common to all of these functions is an aptitude for accepting and transmitting charges. It is generally agreed that the development of organic semiconductors that favor electrons as the majority carriers (n-type) lags behind the advances in hole transporting (p-type) materials. This shortcoming suggests that the design space for n-type materials is not yet well explored, presenting researchers with the opportunity to develop unconventional architectures. In this regard, it is worth noting that discrete molecular materials are demonstrating the potential to usurp the preeminent positions that π-conjugated polymers have held in these areas of organic electronics research.

This dissertation describes how an extraordinary class of molecules, meso-to-meso ethyne-bridged porphyrin arrays, has been bent to these new uses. Chapter one describes vis-NIR spectroscopic and magnetic resonance measurements revealing that these porphyrin arrays possess a remarkable aptitude for the delocalization of negative charge. In fact, the miniscule electron-lattice interactions exhibited in these rigid molecules allow them to host the most vast electron-polarons ever observed in a pi-conjugated material. Chapter two describes the development of an ethyne-bridged porphyrin-isoindigo hybrid chromophore that can take the place of fullerene derivatives in the conventional thin film solar cell architecture. Particularly noteworthy is the key role played by the 5,15-bis(heptafluoropropyl)porphyrin building block in the engineering of a chromophore that, gram for gram, is twice as absorptive as poly(3-hexyl)thiophene, exhibits a lower energy absorption onset than this polymer, and yet possesses a photoexcited singlet state sufficiently energetic to transfer a hole to this polymer. Chapter three describes synthetic efforts that expand the repertoire of readily available meso-heptafluoropropyl porphyrin building blocks. The findings suggest that the remaining challenges to the exploitation of these pigments will be overcome by a sufficiently firm grasp of their subtle electronic structures, and a willingness to eschew the customary strategies of chromophore assembly.

Two-photon excited fluorescence microscopy (TPM) has become a standard biological imaging tool due to its simplicity and versatility. The fundamental contrast mechanism is derived from fluorescence of intrinsic or extrinsic markers via simultaneous two-photon absorption which provides inherent optical sectioning capabilities. The NIR-II wavelength window (1000–1350 nm), a new biological imaging window, is promising for TPM because tissue components scatter and absorb less at longer wavelengths, resulting in deeper imaging depths and better contrasts, compared to the conventional NIR-I imaging window (700–1000 nm). However, the further enhancement of TPM has been hindered by a lack of good two-photon fluorescent imaging markers in the NIR-II.

In this dissertation, we design and characterize novel two-photon imaging markers, optimized for NIR-II excitation. More specifically, the work in this dissertation includes the investigation of two-photon excited fluorescence of various highly conjugated porphyrin arrays in the NIR-II excitation window and the utilization of nanoscale polymersomes that disperse these highly conjugated porphyrin arrays in their hydrophobic layer in aqueous environment. The NIR-emissive polymersomes, highly conjugated porphyrins-dispersed polymersomes, possess superb two-photon excited brightness. The synthetic nature of polymersomes enables us to formulate fully biodegradable, non-toxic and surface-functionalized polymersomes of varying diameters, making them a promising and fully customizable multimodal diagnostic nano-structured soft-material for deep tissue imaging at high resolutions. We demonstrated key proof-of-principle experiments using NIR-emissive polymersomes for in vivo two-photon excited fluorescence imaging in mice, allowing visualization of blood vessel structure and identification of localized tumor tissue. In addition to spectroscopic characterization of the two-photon imaging agents and their imaging capabilities/applications, the effect of the laser setup (e.g., repetition rate of the laser, peak intensity, system geometry) on two-photon excited fluorescence measurements is explored to accurately measure two-photon absorption (TPA) cross-sections. A simple pulse train shaping technique is demonstrated to separate pure nonlinear processes from linear background signals, which hinders accurate quantification of TPA cross-sections.

Light-matter interactions are fundamental to many critical emerging technologies – such as photovoltaics, photonic sensing, and information transmission – that rely upon the efficient capture of light and its conversion to useful energetic states. However, to realize these technologies as a viable future we must first understand the fundamental processes which govern and dictate the energetic, spatial, and temporal identity of materials following photoexcitation. As is suggested by the term “light-matter” both the qualities of the light and the structural composition of the material will influence these characteristics resulting from their interaction. This dissertation investigates how photoexcitation conditions and material structure can be leveraged to modulate the energetic and charged states, and the dynamics thereof, which arise following photoexcitation of nanoscale and molecular systems. Employing ultrafast pump-probe transient absorption spectroscopy, this work characterizes the transient states which arise from photoexcitation of: (i) single-walled carbon nanotubes (SWNTs) wrapped by aryleneethynylene semiconducting polymers; (ii) covalently linked ethyne bridged porphyrin donor, rylene acceptor, molecular “ratchets” and (iii) rylene chromophores covalently linked to amino acid models. In nanoscale systems, this work highlights how the electronic structure of 1-dimensional SWNTs: (i) enable a complex interplay of excitation fluence dependent multi-body interactions, arising from the multitude of photogenerated energetic states, which may be harnessed to modulate the nature and lifetime of charge separated states and; (ii) give rise to a collection of heretofore ill-defined photoexcited-states with low energy optical transitions. At a molecular level, this work demonstrates how molecular structures can be engineered to: (i) utilize quantum coherence in a donor-acceptor “ratchet” which exhibit excitation frequency dependent uphill energy transfer, via vibronic mixing, to undergo electronically irreversible charge transfer and; (ii) selectively photooxidize amino acid analogues in biologically reminiscent photoreactions. These findings presented herein may be used to guide optoelectronic designs which efficiently guide and harness the charged and energetic species which arise from photoexcitation.

Understanding and leveraging light-matter interactions, broadly defined as the generation and decay of a material’s photo-excited states, is key to progress in many optical and optoelectronic technologies that include optical sensors, photovoltaics, and spintronics. As the size of a material is reduced to the low-dimensional regime, where an electron’s degrees of freedom are limited, distinct optical and electronic properties begin to emerge. To realize optimized properties at the macroscale it is necessary to understand how these properties present at the nano- and mesoscale.This work designs, synthesizes, and characterizes novel functional materials and assemblies based on : i) semiconducting polymers coupled with metallic single-walled carbon nanotubes (m-SWNTs) and semiconducting single-walled carbon nanotubes (s-SWNT), ii) ethyne bridged zinc (II) porphyrin arrays covalently linked to polypeptides, and iii) lanthanide doped nanocrystals. While these three systems are uniquely different, they share the common theme of this work in that small deliberate changes to their composition or morphology can have a drastic impact on their photophysical and electronic properties. We exploit these unique designs to develop exceptional optical, electro-optic, and spintronic materials, and elucidate critical structure-property relationships that broadly inform materials design for these applications.

As the shrinking size of silicon-based electronic devices approaches the physical limit hindered by quantum tunneling, molecular electronics has gained attention to be a promising candidate for the development of molecule-size electronics where the individual designed molecules can be assembled to serve the similar functionality of transitional components. In this regard, the introduction of molecular spintronics brings more versatility to superior efficiency in information processing. Spintronic devices sharply contrast with traditional electronics by exploiting the electron spin degree of freedom in addition to the charge and key to the realization of spintronics is elucidating molecular spintronic components. This dissertation illustrates electron spin polarization, propagation, and relaxation behaviors in perylene diimide derivatives, meso-to-meso acetylene-bridged multi[zinc(II) porphyrin] and [copper(II) porphyrin] oligomers. Chapter one discusses a background about molecular spintronics of highly conjugated molecular wires and perylene diimide derivatives, the chiral-induced spin selectivity effect as well as the basics of electron paramagnetic resonance. Chapter two provides the utilization of polyproline-porphyrin complexes as spintronics molecular wires to propagate spin-polarized current measured by spin-dependent Hall devices and mC-AFM. This study demonstrates the importance of employing achiral conjugated wires as low-resistance spintronic wires. Chapter three describes the design of chiral highly conjugated (porphinato)Zn arrays through chirality induction of chiral ligand. The spin polarization and propagation thus can be achieved in the high conductance porphyrinic wires conducted by Hall devices and mC-AFM. In addition, the strategy of ligand exchange in host-guest chemistry enables the opportunity to tune chirality as well spin polarization by replacing enantiomeric ligand in solution, paving the new avenue for the design of chirality-controllable spintronics wires. Chapter four reports the study of aggregation system alanine-based perylene diimide derivatives by conjugation systems through solvent control. By tuning the solubility of solvent, the aggregation can be altered to be nanofibers having orthogonal conjugation systems or nanodonuts having paralleled conjugation systems with the gold electrode. The spin-polarized current was measured in nanodonuts and demonstrate diode-like current-voltage responses. The mechanism may result from asymmetric molecule-electrode coupling. In chapter five, we report spin dynamics in symmetric, strongly π-conjugated bis[(porphinato)copper] (bis[PCu]) systems in which atom-specific macrocycle spin density, porphyrin-to-porphyrin linkage topology, and orbital symmetry play an important role on the magnitudes of electronic spin−spin couplings over substantial Cu−Cu distances.