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.

Platinum-Centered Oligoynes Capped by Boron Difluoride Formazanate Dyes and Their Thin-Film Properties.

Since the Nobel prize winning discovery that polyacetylene could act as a semiconductor, there has been tremendous efforts dedicated to understanding and harnessing the unusual properties of 𝜋-conjugated polymers. Much of this research has focused on the preparation of oligoynes and polyynes with well-defined numbers of repeating alkyne units as models for carbyne. These studies are usually hampered by a structure-property relationship where the stability of the resulting materials decrease with the incorporation of additional alkyne units. Here, we describe a series of oligoynes, with up to 12 alkyne units, where electron-rich [Pt(PBu3)2]2+ units are incorporated into the center of oligoyne backbones which are capped by electron-poor BF2 formazanate dyes. These compounds exhibit excellent stability and solubility, panchromatic absorption, and redox activity characteristic of their structural components. These traits facilitated thin-film studies of extended oligoyne materials, where it is shown that incorporating [Pt(PBu3)2]2+ units leads to smoother films, decreased conductivity on the microscale, and increased conductivity on the nanoscale when compared to metal-free analogs. Remarkably, our oligoynes have superior conductivity compared to the ubiquitous poly(3-hexylthiophene) semiconductor.

Glaser-Hay-Coupled Random Copolymers Containing Boron Difluoride Formazanate Dyes.

𝜋-Conjugated polymers, including those based on acetylenic repeating units, are an exciting class of materials that offer narrow optical band gaps and tunable frontier orbital energies that lead to their use in organic electronics. This work expands the knowledge of structure-property relationships of acetylenic polymers through the synthesis and characterization of a series of Glaser-Hay-coupled model compounds and random copolymers comprised of BF2 formazanate, fluorene, and/or bis(alkoxy)benzene units. The model compounds and copolymers synthesized exhibit redox activity associated with the reversible reduction of the BF2 formazanate units and the irreversible reduction of the fluorene and bis(alkoxy)benzene units. The copolymers exhibit absorption profiles characteristic or intermediate of their respective models and homopolymers, leading to broad absorption of UV-vis light. The alkyne linkages of the model compounds and copolymers are reacted with [Co2(CO)8] to convert the alkyne functional groups into cobalt carbonyl clusters. This transformation leads to blue-shifted absorption profiles due to a decrease in π-conjugation, demonstrating the ability to tune the properties of these materials through post-polymerization functionalization. The redox activity and broad absorption bands of the polymers reported make them excellent candidates for use in photovoltaics and other light-harvesting applications.

Organic Room Temperature Phosphorescence from BN-Substituted Xanthene Derivatives.

Emissive organic materials are predominantly fluorescent and there is significant interest in realizing and understanding examples that defy this paradigm and exhibit phosphorescence under ambient conditions. Organic room temperature phosphorescence (ORTP) offers the long-lived excited states and bathochromically-shifted emission maxima of phosphorescence without the use of potentially toxic and expensive transition metals. Most ORTP materials rely on well-studied structural motifs that include aryl carbonyls, sulfones, and heavy main group elements. We report the unexpected ORTP of a series of heavy atom-free BN-substituted xanthene derivatives. The creation of heteroatom-rich scaffolds, combined with stabilizing C-H···F interactions in the solid-state, resulted in oxygen-tolerant heavy atom-free organic phosphorescence without relying on the use of cryogenic temperatures, polymer matrices, or host-guest interactions. The observation of ORTP in these simple systems sets a blueprint for the further development of heavy atom-free organic phosphors.

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.

Multicomponent Synthesis of Poly(α-aminophosphine chalcogenide)s and Subsequent Depolymerization.

Multicomponent reactions of primary phosphines (R-PH2), diimines (R'-N═C(H)-R-(H)C═N-R'), and chalcogens (O2, S8) generate poly(α-aminophosphine chalcogenide)s (4-7) through step-growth polymerization. Characterization of the linear polymers using 31P{1H} diffusion-ordered NMR spectroscopy (DOSY) experiments aided in determining the molecular weight (Mw) of the material. Subjecting the polyphosphine oxide or sulfide to reducing conditions in the presence of a Lewis acid resulted in complete depolymerization of the polymers, quantitatively releasing the 1° phosphine and diimine (2) starting materials, with concomitant reduction of diimine to diamine (9).

Reactivity of Primary Phosphines and Primary Phosphine Sulfides towards Imines.

Reactivity of primary phosphines with two stoichiometric equivalents of imine results in the formation of bis-α-aminophosphines (2 a-e), which can be subsequently oxidized in the presence of S8 or H2 O2 to generate air stable bis-α-aminophosphine sulfides (2 b-m(S/O)). To elucidate the mechanism of this three-component reaction, Hammett analysis, kinetic isotope effect (KIE), and trapping experiments were performed. Ultimately a P(V)-P(III) tautomerization is invoked, followed by nucleophilic attack by the P(III) species to generate the desired products.

Heterocyclic Phosphenium Cations and Their Divergent Coordination Chemistry.

While they are often encountered as reaction intermediates, phosphenium cations are not commonly incorporated into π-conjugated systems. We report the synthesis and characterization of donor-stabilized phosphenium cations supported by pyridylhydrazonide ligands. The preparation of these cations relies on precise control of ligand E-Z isomerism. The heterocycles were treated with a variety of transition metals, with [Rh(COD)Cl]2 yielding the only well-defined organometallic products. The optoelectronic properties of the phosphenium heterocycles and their transition-metal complexes were examined using UV-vis absorption spectroscopy, cyclic voltammetry, and modeling by density functional theory (DFT). Computations support the description of these compounds as phosphenium cations and corroborate our observation of a weak P-Npyridine bond, which was manifested experimentally as the Rh adducts undergo selective insertion of Rh into the P-Npyridine bond, depending on the substituent at phosphorus. The reported compounds provide a framework for further study of π-conjugated, N,N'-chelated phosphenium cations and their transition-metal adducts.

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.

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.

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.

A Boron Difluoride Hydrazone (BODIHY) Polymer Exhibits Aggregation-Induced Emission.

Polymers that exhibit aggregation-induced emission (AIE) find use, for example, as cell-imaging agents and as fluorometric sensors due to their unique optical properties. However, the structural diversity of AIE-active polymers has not necessarily advanced at the same rate as their applications. In this work, ring-opening metathesis polymerization is used to synthesize the first example of a polymer (Mn  = 61,600 g mol-1 , D = 1.32) containing boron difluoride hydrazone (BODIHY) heterocycles in its repeating unit. The BODIHY monomer and polymer described absorb and emit in the visible region in solution (λabs  = 428 and 429 nm, λem  = 528 and 526 nm) and as thin films (λabs  = 443 and 440 nm, λem  = 535 and 534 nm). Monomer (ΦFilm  = 10%) and polymer (ΦFilm  = 6%) exhibit enhanced emission as thin films compared to solution (ΦSoln  ≤ 1%) as well as AIE upon the addition of water to DMF solutions as a result of restriction of intramolecular motion. Enhancement factors for the monomer and polymer are determined to be 58 and 15, respectively. The title BODIHY polymer exhibited an earlier onset of AIE and enhanced sensitivity to solution viscosity when compared to the parent monomer.

Formazanate coordination compounds: synthesis, reactivity, and applications.

Formazans (Ar1-NH-N[double bond, length as m-dash]CR3-N[double bond, length as m-dash]N-Ar5), a class of nitrogen-rich and highly colored compounds, have been known since the late 1800s and studied more closely since the early 1940s. Their intense color has led to their widespread use as dyes, especially in cell biology where they are most often used to quantitatively assess cell-viability. Despite structural similarities to well-known ligand classes such as ß-diketiminates, the deprotonated form of formazans, formazanates, have received relatively little attention in the transition metal and main group coordination chemistry arenas. Formazanate ligands benefit from tunable properties via structural variation, rich optoelectronic properties owing to their highly delocalized π-systems, low-lying frontier orbitals that stabilize otherwise highly reactive species such as radicals, and redox activity and coordinative flexibility that may have significant implications in their future use in catalysis. Here, we review progress in the coordination chemistry of formazanate ligands over the past two decades, with emphasis on the reactivity and applications of the subsequent complexes.

Cationic Boron Formazanate Dyes*.

Incorporation of cationic boron atoms into molecular frameworks is an established strategy for creating chemical species with unusual bonding and reactivity but is rarely thought of as a way of enhancing molecular optoelectronic properties. Using boron formazanate dyes as examples, we demonstrate that the wavelengths, intensities, and type of the first electronic transitions in BN heterocycles can be modulated by varying the charge, coordination number, and supporting ligands at the cationic boron atom. UV-vis absorption spectroscopy measurements and density-functional (DFT) calculations show that these modulations are caused by changes in the geometry and extent of π-conjugation of the boron formazanate ring. These findings suggest a new strategy for designing optoelectronic materials based on π-conjugated heterocycles containing boron and other main-group elements.

Optoelectronic Properties of Carbon-Bound Boron Difluoride Hydrazone Dimers.

The creation of dimeric boron difluoride complexes of chelating N-donor ligands is a proven strategy for the enhancement of the optoelectronic properties of fluorescent dyes. We report dimers based on the boron difluoride hydrazone (BODIHY) framework, which offer unique and sometimes unexpected substituent-dependent absorption, emission, and electrochemical properties. BODIHY dimers have low-energy absorption bands (λmax =421 to 479 nm, ϵ=17 200 to 39 900 m-1 cm-1 ) that are red-shifted relative to monomeric analogues. THF solutions of these dimers exhibit aggregation-induced emission upon addition of water, with emission enhancement factors ranging from 5 to 18. Thin films of BODIHY dimers are weakly emissive as a result of the inner-filter effect, attributed to intermolecular π-type interactions. BODIHY dimers are redox-active and display two one-electron oxidation and two one-electron reduction waves that strongly depend on the N-aryl substituents. These properties are rationalized using density-functional theory calculations and X-ray crystallography experiments.

Optoelectronic, Aggregation, and Redox Properties of Double-Rotor Boron Difluoride Hydrazone Dyes.

We develop the chemistry of boron difluoride hydrazone dyes (BODIHYs) bearing two aryl substituents and explore their properties. The low-energy absorption bands (λmax =427-464 nm) of these dyes depend on the nature of the N-aryl groups appended to the BODIHY framework. Electron-donating and extended π-conjugated groups cause a redshift, whereas electron-withdrawing groups result in a blueshift. The title compounds were weakly photoluminescent in solution and strongly photoluminescent as thin films (λPL =525-578 nm) with quantum yields of up to 18 % and lifetimes of 1.1-1.7 ns, consistent with the dominant radiative decay through fluorescence. Addition of water to THF solutions of the BODIHYs studied causes molecular aggregation which restricts intramolecular motion and thereby enhances photoluminescence. The observed photoluminescence of BODIHY thin films is likely facilitated by a similar molecular packing effect. Finally, cyclic voltammetry studies confirmed that BODIHY derivatives bearing para-substituted N-aryl groups could be reversibly oxidized (Eox1 =0.62-1.02 V vs. Fc/Fc+ ) to their radical cation forms. Chemical oxidation studies confirmed that para-substituents at the N-aryl groups are required to circumvent radical decomposition pathways. Our findings provide new opportunities and guiding principles for the design of sought-after multifunctional boron difluoride complexes that are photoluminescent in the solid state.

Oxoborane Formation Turns on Formazanate-Based Photoluminescence.

The synthesis of compounds containing multiple bonds to boron has challenged main-group chemists for decades. Despite significant progress, the possibility that the formation of such bonds can turn on photoluminescence has received minimal attention. We report an oxoborane (B=O) complex that is electronically stabilized by a formazanate ligand in the absence of significant steric bulk and, unlike the common BX2 (X=F, Cl) formazanate adducts, exhibits intense photoluminescence. The latter property was rationalized through density-functional calculations which indicated that the B=O bond enhances photoluminescence by drastically reducing differences between the ligand's geometries in the ground and excited states. The title oxoborane compound was synthesized from an air- and moisture-stable BCl2 formazanate complex and subsequently converted to a redox-active boroxine. Each of these species may also serve as a precursor to functional materials.

Dialkynylborane Complexes of Formazanate Ligands: Synthesis, Electronic Properties, and Reactivity.

Dialkynylborane complexes of N-donor ligands have received significant attention because of their application in biological imaging, as light-harvesting materials, and as the functional component of organic photovoltaics. Despite these advances, relatively few types of N-donor ligands have been explored in this context. To this end, we prepared a series of dialkynylborane complexes of formazanate ligands and explored their electronic properties and reactivity. In doing so, we demonstrated that (1) the nature of the alkynyl substituents has little influence over the UV-vis absorption properties of the title complexes, but does affect the potentials at which they are electrochemically oxidized and reduced, (2) dialkynylborane formazanate complexes can be converted to stable radical anions by chemical reduction with cobaltocene derivatives, and (3) copper-assisted alkyne-azide cycloaddition chemistry at the alkynyl substituents directly bound to boron can be used to elaborate structural diversity. These conclusions are likely to lead to the development of, and provide guiding principles for the design of, future examples of functional molecular materials based on boron complexes of N-donor ligands.

X-ray Absorption Near-Edge Structure Spectroscopy of a Stable 6-Oxoverdazyl Radical and Its Diamagnetic Precursor.

The electronic structure of 1,3,5-triphenyl-6-oxoverdazyl, a heteroatom-rich stable organic radical, and its diamagnetic 1,3,5-triphenyl-6-oxotetrazane precursor are probed using X-ray absorption near-edge structure (XANES) spectroscopy. The N K-edge XANES spectra of the 6-oxoverdazyl radical contain strong N 1s → π* resonances for each set of equivalent nitrogen atoms. The fact that these resonances are absent from the analogous spectra of the 6-oxotetrazane, whereas the O K-edge and C K-edge XANES spectra of both species are very similar, demonstrates that the unpaired electron of the radical is localized primarily on the N atoms of the 6-oxoverdazyl heterocycle. The O K-edge XANES spectra of both species contain strong O 1s → π* (C═O) peaks, but the peak of the radical is red-shifted by 0.5 eV relative to that of the 6-oxotetrazane, which indicates that the C═O bond in the radical is part of a larger π-conjugated system. The proposed interpretations of the XANES spectra are aided by density-functional calculations.

Unveiling the Hidden, Dark, and Short Life of a Vibronic State in a Boron Difluoride Formazanate Dye.

Boron difluoride (BF2 ) formazanate dyes are contenders for molecular species that exhibit a large Stokes shift and bright red emission. Excitation of 3-cyanoformazanate complexes with 10 µs wide pulses of specific wavelengths resulted in strong luminescence at 663 nm at both room temperature in solution and at 77 K in a frozen solution. Analysis of the short-lived excitation spectrum from this luminescence shows that it arises from a vibronic manifold of a higher-lying excited state. This dark state relaxes to the emitting state over 10 µs. TD-DFT calculations of the two lowest-energy excited states show that the relaxed geometries are planar for S1 but highly distorted in S2 . The specific time- and wavelength-dependence of the excitation profile provides a unique optical encryption capability through the comparison of emission intensities between adjacent vibronic bands only accessible in the 0-12 µs time domain.