Band-Gap Tuning in Acceptor-Donor-Acceptor Boron Difluoride Formazanates

π-Conjugated molecules with acceptor-donor-acceptor (A-D-A) electronic structures are an important class of materials due to their tunable optoelectronic properties and applications in, for example, organic light-emitting diodes, nonlinear optical devices, and organic solar cells. The frontier molecular orbital energies, and thus band gaps, of these materials can be tuned by varying the donor and acceptor traits and π-electron counts of the structural components. Herein, we report the synthesis and characterization of a series of A-D-A compounds consisting of BF2 formazanates as electron acceptors bridged by a variety of π-conjugated donors. The results, which are supported by DFT calculations, demonstrate rational control of optoelectronic properties and the ability to tune the corresponding band gaps. The narrowest band gaps (EgOpt= 1.38 eV and EgCV= 1.21 eV) were observed when BF2 formazanates and benzodithiophene units were combined. This study provides significant insight into the band-gap engineering of materials derived from BF2 formazanates and will inform their future development as semiconductors for use in organic electronics

A π-conjugated inorganic polymer constructed from boron difluoride formazanates and platinum(II) diynes

The first example of a π-conjugated polymer incorporating boron difluoride (BF2) formazanates is introduced. The film-forming properties, controllable reduction chemistry, and low optical band gap (ca. 1.4 eV) of the polymer make it an excellent candidate for use as a light-harvesting n-type semiconductor in organic electronics. Comparison of the polymer to model compounds confirmed that its unique optoelectronic properties can be directly attributed to the presence of the BF2 formazanate repeat unit and that the [Pt(PBu3)2]2+ unit must also be present to achieve the narrow band gaps observed

Exploring the Optical and Redox Properties of π-Conjugated Boron Difluoride Formazanate Dyes

This thesis describes the synthesis of π-conjugated small molecules and polymers consisting of boron difluoride (BF2) formazanate dyes. It utilizes different strategies such as extended π-conjugation, planarity of the backbone, donor-acceptor interactions, etc. to modulate the frontier molecular orbitals and band gaps of these systems. The resulting materials are redox-active and exhibit rich optical properties including high molar absorptivities and low band gaps. Chapter 2 describes a series of acceptor-donor-acceptor compounds consisting of BF2 formazanates as electron acceptors bridged by a variety of π-conjugated donors. The work in this chapter demonstrates that the optoelectronic properties and the corresponding band gaps can be tuned by varying the electron donating spacers. The resulting compounds exhibited low optical band gaps (1.73−1.38 eV). Chapter 3 describes the synthesis and characterization of a series of oligoynes with up to 10 alkyne units using BF2 formazanates as end-caps. Not only did BF2 formazanates stabilize the oligoynes, but they also introduced unique optical and redox properties. The resulting compounds exhibited a blend of properties that cannot be achieved by either oligoyne or BF2 formazanates individually (e.g., panchromatic absorption, multiple redox waves). Chapter 4 describes the first example of a π-conjugated polymer incorporating BF2 formazanate and Pt(II)-acetylide units. This polymer exhibited reversible redox waves, low optical band gap (1.4 eV), and film forming properties. The reversible electron accepting ability of the polymer was demonstrated by chemically reducing the BF2 formazanate units using cobaltocene and subsequently oxidizing it back to the neutral form in air. The work described in Chapter 5 is a follow-up to the work in Chapter 4. It describes a series of small molecules incorporating BF2 formazanate and Pt(II)-acetylides to thoroughly examine the effect of Pt(II)-acetylide conjugation on the optoelectronic properties of BF2 formazanates. The results presented in this chapter demonstrate improved redox properties and red-shifted absorption and emissions bands compared to parent BF2 formazanates. Taken together, the work described in this thesis demonstrates the readily tunable optoelectronic properties of BF2 formazanate dyes. Furthermore, owing to their low band gaps, π-conjugated materials described in this thesis are promising candidates for use in the organic electronic arena

A divergent strategy for the synthesis of redox-active verdazyl radical polymers

Stable radical polymers have attracted significant attention due to their use as magnetic materials, (co)catalysts in organic synthesis, and as functional components of organic electronics including batteries and memory devices. Despite their importance, a relatively narrow range of synthetic strategies exist for their production. The most common examples involve the transition-metal catalyzed polymerization of radical-containing monomers or the sequential polymerization of monomers containing radical precursors and their post-polymerization conversion to radicals. Post-polymerization installation of stable radicals is not commonly reported. Copper-assisted azide-alkyne cycloaddition (CuAAC) chemistry has seen limited use in this regard, but only for the production of p-conjugated and particle-bound polymers. Here, we present a divergent synthetic strategy for the production of stable radical polymers, that takes advantage of the efficiency of atom transfer radical polymerization and copper-assisted alkyne-azide cycloaddition chemistry, and is anticipated as general to a wide range of stable radicals. We chose verdazyl radicals as a case study for this synthetic paradigm as they offer highly tunable properties based on minor structural changes, and thus well represent the need for the divergent synthetic strategy presented

Altering the optoelectronic properties of boron difluoride formazanate dyes via conjugation with platinum(II)-acetylides

The combination of π-conjugated organic compounds and Pt(II)-acetylides is a powerful strategy for the production of functional optoelectronic materials. The presence of the heavy element, Pt, in these compounds enhances electronic delocalization generally resulting in low-energy absorption and emission maxima and often leads to intersystem crossing, resulting in phosphorescence. When boron complexes of N-donor ligands, such as boron dipyrromethenes (BODIPYs), are involved the molecular and polymeric materials produced have properties that are advantageous for their use as oxygen-sensors, in triplet–triplet annihilation, and as the functional components of photovoltaics. Based on these exciting results, we endeavored to thoroughly examine the effect of Pt(II)-acetylide conjugation on the properties of BF2 formazanate dyes, which offer improved redox properties and red-shifted absorption and emission bands compared to many structurally related BODIPYs. The results showed that phosphine-supported Pt(II)-acetylide incorporation enhanced electronic conjugation, rendering the electrochemical reduction of the BF2 formazanate dyes more difficult, while also red-shifting their absorption and emission maxima. Unlike similar BODIPYs, the presence of Pt(II) did not facilitate phosphorescence, but rather quenched fluorescence. This study provides significant insights into structure–property relationships and guiding principles for the design of BF2 formazanate dyes, a rapidly emerging family of readily accessible optoelectronic materials

Near-IR Absorption and Photocurrent Generation Using a First-of-Its-Kind Boron Difluoride Formazanate Non-Fullerene Acceptor

Herein, we report the synthesis and characterization of the first non-fullerene acceptor (NFA) containing a boron difluoride formazanate (BF2fz) core end-capped with N-annulated perylene diimides (PDIs). Electronic coupling between the BF2fz core and the PDI endcaps enabled tuning of the lowest unoccupied molecular orbital, leading to near-panchromatic optical absorption. Post-deposition solvent vapor annealing of the new NFA resulted in a significant red-shift in the optical spectra, which stretched into the near-IR. Proof-of-concept organic photovoltaic (OPV) devices were constructed to demonstrate the potential of this new material as an NFA. SVA treatment of the active layer resulted in a 2-fold increase in power conversion efficiency (PCE), due mainly to increases in the BF2PDI2 generated photocurrent that extended into the near-IR

Blending the Optical and Redox Properties of Oligoynes and Boron Difluoride Formazanates

Oligoynes and polyynes are 1-D chains of conjugated sp-hybridized carbon atoms consisting of alternating single and triple bonds. Their stability rapidly decreases with increasing chain length beyond only a few repeating units. Design strategies, such as the use of bulky end-capping groups, allow for their characterization and isolation while not contributing significantly to their physical properties. In this study, we incorporate redox-active BF2 formazanate dyes (BF2) as end-caps to prepare symmetric (BF2−[C≡C]n−BF2) and asymmetric (BF2−[C≡C]n−Si(iPr)3) families of oligoynes containing up to 10 alkyne units. In doing so, we introduce stable oligoynes that possess a blend of optical and redox properties that cannot be achieved by either oligoynes or BF2 formazanates individually (e.g., panchromatic absorption, multiple and tunable reversible redox waves). This approach is transferable to other functional end-caps to facilitate the preparation of π-conjugated materials with utility in the organic electronics arena