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

Abstract

A study designed to probe the effect of electronic conjugation and cross-conjugation on the optical and electrochemical properties of benzene-bridged boron difluoride formazanate dimers is presented. 2 AbstractOne 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 ( 1 H, 11 B, 13C and 19 F NMR spectroscopy, IR and UV-Vis absorption and emission spectroscopy, mass spectrometry and X-ray crystallography) of the first examples of boron difluoride (BF 2 ) 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 crossconjugated 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 BF 2 formazanate dimers exceeded those observed for monomeric analogues.

Boron difluorides with formazanate ligands: redox-switchable fluorescent dyes with large stokes shifts

“…26 Gilroy and co-workers described similar electrochemical data for these compounds in dichloromethane solution. While the 2-electron reduction products appeared unstable in dichloromethane solution, 25 our data in THF suggest that in this solvent the products are stable and might be isolable (vide infra).…”

“…These compounds were recently reported by Gilroy and co-workers while this manuscript was in preparation. 25 The formation of complexes 1 was confirmed by NMR spectroscopy, which shows diagnostic 1 : 2 : 1 triplets in the 11 B NMR spectra for the 4-coordinate B centres (δ −0.7 to −2.3 ppm). 19 In the 19 F NMR spectra, 1 : 1 : 1 : 1 quartets between −145.6 and −159.4 ppm with J B-F = 20-30 Hz are observed that are consistent with the presence of a BF 2 moiety.…”

Aluminum Complexes of N2O23– Formazanate Ligands Supported by Phosphine Oxide Donors

“…The calculations also revealed that the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) were the dominant orbital pair involved in the lowest energy excitation (*). The HOMO and LUMO isosurfaces ( Figure 2c) are indicative of highly delocalized electronic structures and closely resemble those of related boron difluoride formazanate complexes, [62][63] with minor orbital contributions located on the oxygen atoms of the phosphine oxide donors. The lack of frontier orbital density on the phosphorus atoms and their substituents are consistent with the experimental observation that the phosphine oxide donors have a negligible effect on the low-energy absorption maxima of the aluminum complexes described here.…”

Optoelectronic Properties of Carbon‐Bound Boron Difluoride Hydrazone Dimers

“…Several examples of 1,2‐bis((1 H ‐pyrrol‐2‐yl)methylene)hydrazines (BOPHYs) 5 and 8‐imidazodipyrromethenes ( aza ‐BOIMPYs) 6 exhibit exceptional Φ em in dilute solutions due to their relatively planar and rigid molecular geometries. BF 2 formazanate dimers 7 a and 7 b undergo emission enhancement upon dimerization at both the meta and para positions of the phenylene linkers . BODIPY dimers (e.g., 8 and 9 ) exhibit twisted‐intramolecular charge transfer in polar solvents, which is responsible for a decrease in the emission quantum yield …”