Stephanie M. Barbon

Substituent-Dependent Optical and Electrochemical Properties of Triarylformazanate Boron Difluoride Complexes

The straightforward synthesis and detailed characterization of nine substituted triarylformazanate boron difluoride complexes is reported. The effect of electron-donating (p-anisole) and electron-withdrawing (p-benzonitrile) substituents on optical and electrochemical properties, relative to phenyl substituents, was studied at two different positions within the formazanate ligand framework. Each of the BF2 complexes was characterized by (1)H, (13)C, (11)B, and (19)F NMR spectroscopy, cyclic voltammetry, infrared spectroscopy, UV-vis absorption and emission spectroscopy, mass spectrometry, and elemental analysis. Select examples were studied by X-ray crystallography, revealing highly delocalized structures in the solid state. The complexes were reversibly reduced in two steps electrochemically to their radical anion and dianion forms. The complexes also exhibited substituent-dependent absorption and emission properties, accompanied by significant Stokes shifts, with the aryl substituents at the 1,5-positions of the formazanate backbone having a greater influence on these properties than aryl substituents at the 3-position. Breaking the symmetry in three different complexes resulted in a modest increase in emission intensity relative to that of symmetrically substituted derivatives.

Effect of Extended π Conjugation on the Spectroscopic and Electrochemical Properties of Boron Difluoride Formazanate Complexes

The effect of extended π conjugation on the spectroscopic and electrochemical properties of boron difluoride (BF2) formazanate complexes was studied by the systematic comparison of phenyl- and naphthyl-substituted derivatives. Each of the BF2 complexes described was characterized by (1)H, (13)C, (11)B, and (19)F NMR spectroscopy, cyclic voltammetry, infrared spectroscopy, UV-vis absorption and emission spectroscopy, and mass spectrometry. X-ray crystallography and electronic structure calculations were used to rationalize the trends observed, including direct comparison of 3-cyano-, 3-nitro-, and 3-phenyl-substituted BF2 formazanate complexes. In all cases, the wavelengths of maximum absorption and emission were red-shifted as π conjugation was systematically extended (by replacing phenyl with naphthyl), fluorescence quantum yields increased (up to 10-fold), and electrochemical conversion of the formazanate complexes to their radical anion and dianion forms occurred at less negative potentials (easier to reduce).

Structurally Tunable 3‐Cyanoformazanate Boron Difluoride Dyes

The straightforward synthesis of a series of 3-cyanoformazanate boron difluoride dyes is reported. Phenyl, 4-methoxyphenyl and 4-cyanophenyl N-substituted derivatives were isolated and characterized by single-crystal X-ray crystallography, cyclic voltammetry, and UV/Vis spectroscopy. The compounds were demonstrated to possess tunable, substituent-dependent absorption, emission, and electrochemical properties, which were rationalized through electronic structure calculations.

Synthesis and characterization of conjugated/cross-conjugated benzene-bridged boron difluoride formazanate dimers

One of the most common strategies for the production of molecular materials with optical properties in the far-red/near-IR regions of the electromagnetic spectrum is their incorporation into dimeric architectures. In this paper, we describe the synthesis and characterization (1H, 11B, 13C and 19F NMR spectroscopy, IR and UV-vis absorption and emission spectroscopy, mass spectrometry and X-ray crystallography) of the first examples of boron difluoride (BF2) formazanate dimers. Specifically, the properties of meta- and para-substituted benzene-bridged dimers p-10 and m-10 were compared to closely related boron difluoride triphenyl formazanate complex 11 in order to assess the effect of electronic conjugation and cross conjugation on their light absorption/emission and electrochemical properties. While the properties of cross-conjugated dimer m-10 did not differ significantly from those of monomer 11, conjugated dimer p-10 exhibited red-shifted absorption and emission maxima and was easier to reduce electrochemically to its bis radical anion and bis dianion form compared to monomer 11. Both dimers are weakly emissive in the far-red/near-IR and exhibited large Stokes shifts (>110 nm, 3318 cm−1). Unlike a closely related para-substituted benzene-bridged boron dipyrromethene (BODIPY) dimer, the emission quantum yields measured for the BF2 formazanate dimers exceeded those observed for monomeric analogues.

Side-chain boron difluoride formazanate polymers via ring-opening metathesis polymerization

The synthesis, characterization, and ring-opening metathesis polymerization (ROMP) of a novel norbornene-based boron difluoride (BF2) formazante monomer are described in detail. The polymerization studies confirmed ROMP to occur in the presence of BF2 formazanates, and also demonstrated the controlled nature of the polymerization. The polymers retained many of the unique characteristics of the monomers in dichloromethane, including absorption and emission at maximum wavelengths of 518 and 645 nm, large Stokes shifts (υST = 127 nm, 3800 cm−1), and the ability to act as electron reservoirs to form borataverdazyl-based poly(radical anions) (). Furthermore, the results described in this paper demonstrate the potential of these and related polymers based on BF2 formazanates as redox-active, light-harvesting materials.

Boron difluoride formazanate copolymers with 9,9-di-n-hexylfluorene prepared by copper-catalyzed alkyne–azide cycloaddition chemistry

The synthesis and characterization of copolymers based on boron difluoride formazanate (BF2L) and 9,9-di-n-hexylfluorene (hex2Fl) units are described. A series of model compounds [(BF2L)-(hex2Fl), (hex2Fl)-(BF2L)-(hex2Fl), and (BF2L)-(hex2Fl)-(BF2L)] were also studied in order to fully understand the spectroscopic properties of the title copolymers [(BF2L)-(hex2Fl)]n. The model compounds and copolymers, which were synthesized by copper catalyzed alkyne–azide cycloaddition chemistry, exhibited high molar absorptivities (25 700–54 900 M−1 cm−1), large Stokes shifts (123–143 nm, 3590–3880 cm−1), and tunable electrochemical behaviour (ca. −0.75 V and ca. −1.86 V vs. ferrocene/ferrocenium). The low-energy wavelength of maximum absorption and emission of the model compounds red-shifted relative to the BF2L repeating unit by ca. 30 nm per triazole ring formed, to maximum values of 557 nm and 700 nm in DMF, respectively. The low-energy absorption and emission properties of the copolymer were consistent with the model compound bearing two triazole rings [(hex2Fl)-(BF2L)-(hex2Fl)] and were not dependent on copolymer molecular weight. However, the title copolymers may show promise as light-harvesting materials based on their thin-film optical band gap of 1.67 eV.

Structurally Diverse Boron-Nitrogen Heterocycles from an N2O23− Formazanate Ligand

Five new compounds comprised of unprecedented boron-nitrogen heterocycles have been isolated from a single reaction of a potentially tetradentate N2 O23- formazanate ligand with BF3 ⋅OEt2 and NEt3 . Optimized yields for each product were obtained through variation of experimental conditions and rationalized in terms of relative Gibbs free energies of the products as determined by electronic structure calculations. Chemical reduction of two of these compounds resulted in the formation of a stable anion, radical anion, and diradical dianion. Structural and electronic properties of this new family of redox-active heterocycles were characterized using UV/vis absorption spectroscopy, cyclic voltammetry and X-ray crystallography.

Copper-assisted azide–alkyne cycloaddition chemistry as a tool for the production of emissive boron difluoride 3-cyanoformazanates

The synthesis and characterization of emissive boron difluoride (BF2) complexes of 3-cyanoformazanate ligands produced using copper-assisted azide–alkyne cycloaddition (CuAAC) chemistry is described. Detailed spectroscopic and electrochemical characterization of benzyl-functionalized complexes served as models and demonstrated that triazole formation at the N-aryl substituents of the formazanate ligand scaffold led to red-shifted absorption and emission maxima and more difficult electrochemical reduction compared to alkyne-substituted precursors. CuAAC chemistry was also used to append ferrocene and tetraethylene glycol substituents to the formazanate backbone. In the case of the ferrocene-substituted complexes, fluorescence was quenched and a reversible oxidation feature (in addition to the reduction features associated with formazanate complexes) was observed using cyclic voltammetry. Treatment with NOBF4 oxidized ferrocene to ferrocenium and resulted in the reestablishment of fluorescence. Tetraethylene glycol substitution produced the first water soluble BF2 formazanate, which was shown to distribute throughout the cytoplasm and nucleus of mouse fibroblast cells when studied as a fluorescence imaging agent.

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.

Boron Difluoride Adducts of a Flexidentate Pyridine-Substituted Formazanate Ligand: Property Modulation via Protonation and Coordination Chemistry

The synthesis and characterization of a flexidentate pyridine-substituted formazanate ligand and its boron difluoride adducts, formed via two different coordination modes of the title ligand, are described. The first adduct adopted a structure that was typical of other boron difluoride adducts of triarylformazanate ligands and contained a free pyridine subsituent, while the second was formed via the chelation of nitrogen atoms from the formazanate backbone and the pyridine substituent. Stepwise protonation of the pydridine-functionalized adduct, which is essentially nonemissive, resulted in a significant increase in the fluorescence quantum yield up to a maximum of 18%, prompting the study of this adduct as a pH sensor. The coordination chemistry of each adduct was explored through reactions with nickel(II) bromide [NiBr2(CH3CN)2], triflate [Ni(OTf)2], and 1,1,1,4,4,4-hexafluoroacetylacetonate [Ni(hfac)2(H2O)2] salts. Coordination to nickel(II) ions altered the physical properties of the boron difluoride formazanate adducts, including red-shifted absorption maxima and less negative reduction potentials. Together, these studies have demonstrated that the physical and electronic properties of boron difluoride adducts of formazanate ligands can be readily modulated through protonation and coordination chemistry.