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

Abstract

The synthesis of a series of (formazanate)boron difluorides and their 1-electron reduction products is described. The neutral compounds are fluorescent with large Stokes shifts. DFT calculations suggest that a large structural reorganization accompanies photoexictation and accounts for the large Stokes shift. Reduction of the neutral boron difluorides occurs at the ligand and generates the corresponding radical anions. These complexes are non-fluorescent, allowing switching of the emission by changing the ligand oxidation state.

Redox‐Switchable Cyan Fluorescence of a BODIPY Analog Inspired by Propentdyopent Pigments

“…of a reducing agent [e.g., Na(Hg), KC 8 , CoCp* 2 ] to solutions of (pdp)BF 2 at room temperature leads to the formation of a paramagnetic species featuring a ligand‐based radical. The continuous‐wave EPR spectrum of [(pdp)BF 2 ] · – shows an isotropic signal at g = 2.003 at room temperature (Figure S9) and is in line with previous reports on propentyopent complexes and other boron difluoride radical anions , …”

Fine‐Tuning the Fluorescence Gain of FRET‐Type (Bodipy)(Bodipy′)‐NHC‐Iridium Complexes for CO Detection with a Large Virtual Stokes Shift

“…To solve such problems, we decided to synthesize carbon monoxide responsive molecules, characterized by a large difference of excitation and emission wavelengths. However, only a few bodipy‐based dyes with large Stokes shifts are known, the synthesis of which requires significant synthetic modifications of the bodipy core or the construction of related boron‐locked heterocycles . An alternative strategy to realize a large difference between excitation and emission wavelength utilizes fluorescence resonance energy transfer (FRET) which in this context is termed virtual Stokes shift .…”

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

“…[24] These metrics confirm that chemical reduction occurs in as tepwise fashion and that the radical anion is localized on one formazanate ligand. Neutral dimer 10 has a l max at 569 nm and amolar absorptivity (e)of20500 m À1 cm À1 .Singly reduced 10C À absorbs strongly at 568 nm, as well as 687 and 477 nm, which is typical of verdazyl species [30] and shows that 10C À is made up of independent borataverdazyl and formazanate units.D oubly reduced species 10CC 2À absorbs minimally at 568 nm, but exhibits two absorption peaks typical of borataverdazyl anions with l max of Scheme 1. [30] These conclusions were corroborated by UV-vis absorption spectroscopy in CH 3 CN (Figure 1e,T able S1).…”

“…Both species were characterized by single-crystal X-ray diffraction analysis (Figure 1c,d, Table S2). [24,29,30] Thea verage NÀNb ond length for N5 to N8 is 1.315(3) ,typical of aformazanate adduct. In compound 10C À ,the average N À Nbond length for N1 to N4 is 1.360 (3) , suggesting the presence of ab orataverdazyl radical in which the additional electron occupies an orbital with antibonding NÀNc haracter.…”

“…[24,29,30] Thea verage NÀNb ond length for N5 to N8 is 1.315(3) ,typical of aformazanate adduct. [30] These conclusions were corroborated by UV-vis absorption spectroscopy in CH 3 CN (Figure 1e,T able S1). In the doubly reduced species 10CC 2À ,t he average N À Nbond length was 1.366 (3) ,which corresponds to NÀNs ingle bonds as expected for borataverdazyl radicals.…”