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

Abstract

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.

Formazanido complexes of heavier group 13 elements aluminium, gallium, and indium

“…These are air-and water-stable, purely organic radicals that can be isolated and stored over months without decomposition. 6 More recently, it was discovered by the groups of Hicks, Otten and Gilroy, [7][8][9] that the unique stability of these nitrogen-rich radicals can also be observed in isoelectronically related boron compounds and borataverdazyl radical anions (Scheme 3).…”

Oxoborane Formation Turns on Formazanate‐Based Photoluminescence

“…This behaviour is attributed to coulombic interactions that resemble those observed during the electrochemical oxidation of cyclic ferrocenyl dimethylsilane trimers. [17,25] In contrast, the absorption spectrum of oxoborane formazanate 10 contained two low-energy bands at 569 (e = 36 800) and 586 nm (e = 36 700 m À1 cm À1 )i ndicative of vibronic fine structure. Complexes 9 and 11 were strongly absorbing (e = 14 900-43 800 m À1 cm À1 )i n the visible region of the electromagnetic spectrum and had broad absorption bands with maximaa t5 24 (9)a nd 468 nm (11), consistent with other four-coordinate boron adducts of formazanates,i ncluding compound 8 (520 nm, e = 36 600 m À1 cm À1 ).…”

“…BF 2 formazanate 8 [17] underwent halide exchange with BCl 3 to afford BCl 2 formazanate 9 as an air-andm oisture-stable, dark-purple solid in 78 %y ield after an aqueous workup. BF 2 formazanate 8 [17] underwent halide exchange with BCl 3 to afford BCl 2 formazanate 9 as an air-andm oisture-stable, dark-purple solid in 78 %y ield after an aqueous workup.…”

“…[17] Replacement of the fluorides with chlorides in complex 9 resulted in irreversible electrochemical reduction( Figure S15, Supporting Information). [17] Replacement of the fluorides with chlorides in complex 9 resulted in irreversible electrochemical reduction( Figure S15, Supporting Information).…”

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“…These complexes can be synthesized in two straightforward and high yielding steps from commercial starting materials for just a few dollars per gram. 4,5 The spectroscopic and electrochemical properties of the complexes are very sensitive to the nature of the sub stituents appended to the formazanate backbone (Figure 2), which can be easily modified. We have demonstrated the efficient electrochemiluminescence 6 of these complexes and we are currently exploring their application as donor materials in solar cells and as fluorescence imaging agents.…”