Pnictogen-Rich Heterocycles Derived from a Phosphadiazonium Cation.

Heterocycles that pair main group elements and nitrogen are extremely important within the π-conjugated heterocycles research community. Compared to the vast number of boron-nitrogen heterocycles, those that include phosphorus are less common. Furthermore, the use of phosphorus-nitrogen triple bonds of any type to prepare such compounds is unprecedented. Here, we pair pyridyl hydrazonide ligands with phosphadiazonium cations and demonstrate that the chelated Mes*NP group is directly implicated in the photophysical and redox properties observed for the resulting heterocycles. In doing so, we introduce a novel building block for the production of phosphorus-containing heterocycles that could find use in small molecule activation and catalysis or as the functional component of emerging organic electronics.

Dyads and Triads of Boron Difluoride Formazanate and Boron Difluoride Dipyrromethene Dyes.

Dye-dye conjugates have attracted significant interest for their utility in applications such as bioimaging, theranostics, and light-harvesting. Many classes of organic dyes have been employed in this regard; however, building blocks don't typically extend beyond small chromophores. This can lead to minor changes to the optoelectronic properties of the original dye. The exploration of dye-dye structures is impeded by long synthetic routes, incompatible synthetic conditions, or a mismatch of the desired properties. Here, we present the first-of-their-kind dye-dye conjugates of boron difluoride complexes of formazanate and dipyrromethene ligands. These conjugates exhibit dual photoluminescence bands that reach the near-infrared spectral region and implicate anti-Kasha processes. Cyclic voltammetry experiments revealed the generation of polyanionic species that can reversibly tolerate the uptake of up to 6 electrons. Ultimately, we demonstrate that BF2 formazanates can serve as a synthetically accessible platform to build upon new classes of dye-dye conjugates.

Tuning the Properties of Donor-Acceptor and Acceptor-Donor-Acceptor Boron Difluoride Hydrazones via Extended π-Conjugation.

Molecular materials with π-conjugated donor-acceptor (D-A) and acceptor-donor-acceptor (A-D-A) electronic structures have received significant attention due to their usage in organic photovoltaic materials, in organic light-emitting diodes, and as biological imaging agents. Boron-containing molecular materials have been explored as electron-accepting units in compounds with D-A and A-D-A properties as they often exhibit unique and tunable optoelectronic and redox properties. Here, we utilize Stille cross-coupling chemistry to prepare a series of compounds with boron difluoride hydrazones (BODIHYs) as acceptors and benzene, thiophene, or 9,9-dihexylfluorene as donors. BODIHYs with D-A and A-D-A properties exhibited multiple reversible redox waves, solid-state emission with photoluminescence quantum yields up to 10%, and aggregation-induced emission (AIE). Optical band gaps (or highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gaps) determined for these compounds (2.02-2.25 eV) agree well with those determined from cyclic voltammetry experiments (2.05-2.42 eV). The optoelectronic properties described herein are rationalized with density functional theory calculations that support the interpretation of the experimental findings. This work provides a foundation of understanding that will allow for the consideration of D-A and A-D-A BODIHYs to be incorporated into applications (e.g., organic electronics) where fine-tuning of band gaps is required.

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

Oligoynes and polyynes are 1D 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.

Band Gap Engineering in Acceptor-Donor-Acceptor Boron Difluoride Formazanates.

π-Conjugated molecules with acceptor-donor-acceptor (A-D-A) electronic structures make up 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 density functional theory 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.

Near-Infrared Boron Difluoride Formazanate Dyes.

Near-infrared (NIR) dyes are sought after for their utility in light harvesting, bioimaging, and light-mediated therapies. Since long-wavelength photoluminescence typically involves extensive π-conjugated systems of double bonds and aromatic rings, it is often assumed that NIR dyes have to be large molecules that require complex syntheses. We challenge this assumption by demonstrating that facile incorporation of tertiary amine groups into readily available 3-cyanoformazans affords efficient production of relatively simple NIR-active BF2 formazanate dyes (λabs =691-760 nm, λPL =834-904 nm in toluene). Cyclic voltammetry experiments on these compounds reveal multiple reversible redox waves linked to the interplay between the tertiary amine and BF2 formazanate moieties. Density-functional calculations indicate that the NIR electronic transitions in BF2 formazanates are of π→π*-type, but do not always involve strong charge transfer.

Exploring Structure-Property Relationships in a Bio-Inspired Family of Bipodal and Electronically-Coupled Bistriphenylamine Dyes for Dye-Sensitized Solar Cell Applications.

A bio-inspired family of organic dyes with bichromic-bipodal architectures were synthesized and tested in dye-sensitized solar cells (DSSC). These dyes are comprised of a D-π-D-A motif with two triphenylamine (TPA) units acting as donors (D) and two cyanoacetic acid acceptors (A) capable of binding to a titania semiconductor. The role of the thiophene π-spacer bridging the two TPA units was examined and the distal TPA (relative to TiO2) was modified with various substituents (-H, -OMe, -SMe, -OHex, -3-thienyl) and contrasted against benchmark L1. It was found that the two TPA donor units could be tuned independently, where π-spacers can tune the proximal TPA and R-substituents can tune the distal TPA. The highest performing DSSCs were those with -SMe, 3-thienyl, and -H substituents, and those with one spacer or no spacers. The donating abilities of R-substituents was important, but their interactions with the electrolyte was more significant in producing high performing DSSCs. The introduction of one π-spacer provided favourable electronic communication within the dye, but more than one was not advantageous.

Bipodal dyes with bichromic triphenylamine architectures for use in dye-sensitized solar cell applications.

A family of four bipodal triphenylamine-based dyes, three of which incorporate two triphenylamine (TPA) units, have been studied to understand their potential in light-harvesting applications. Compared to previously reported TPA-based dyes, these exhibit improved device performance. Theoretical calculations correlate excited state dipole moments to device efficiency.