Oxoborane Formation Turns on Formazanate‐Based Photoluminescence

Abstract

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 hasr eceivedminimal attention. We report an oxoborane (B=O) complex that is electronicallys tabilized by af ormazanate ligand in the absence of significant steric bulk and, unlikethe common BX 2 (X = F, Cl) formazanate adducts,e xhibitsi ntense photoluminescence. The latter propertyw as rationalized throughd ensity-functional calculations whichi ndicated that the B=Ob ond 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 BCl 2 formazanate complex and subsequently converted to ar edox-activeb oroxine. Each of these species may also serve as ap recursor to functional materials.The discoveryo fm ain-group compounds with unusuals tructure and bonding is am ajor focuso fi nterest in synthetic chemistry. [1,2] In this context, the preparation of stablec ompounds containing multiple bonds to boron presentsaparticular challenged ue to the electron-deficient character and compactness of the valence orbitals of the Ba tom. To address these challenges, early efforts by the Berndt, Power,a nd Nçth groups utilized strong reducing agents to access anionic diboron speciesw ith boron-boron multiple-bond character. [3] More recent strategies of stabilizing double and triple bonds to boron have relied upon Lewis bases and/ors terically bulky substituents, as exemplified by compounds 1-7. [4] The first example of an eutrald iborene, compound 1,w as reported by the Robinsong roup. [4a] This molecule is stabilized by two sterically bulky and strongly s-donating N-heterocyclic carbene ligands. Notablef eatures of 1 include the short B=B bond of 1.560(18) and its trans-bent molecular structure. More recently,B raunschweig and co-workers [4d] isolated di-boryne 2,t he first example of ab oron-boront ripleb ond. This molecule has an effectively linear geometry of the core atoms (C!BB ! C) and as hort BBb ond length of 1.449(3) .C ompounds with boron-chalcogen multiple bonds [5, 6] have been isolated through the combination of strongly donating and sterically bulky ligands, as in compound 3,w hich wasprepared by insertion of elemental Te into aB = Mn bond. [4f] The Cui group [4c] reported the anionic b-diketiminate oxoborane complex 4,s tabilized through ah ydrogen-bonding interaction, whereas the Aldridge group has recently reported an acid-free, anionic oxoborane 5. [7] Thioxoborane 6 is the first cationic, electron-precise species containing aB =Sb ond. [4e] The Braunschweig group has also prepared compound 7, [4b] which containsthe first example of ab oron-oxygen triple bond.Apart from their intrinsic appeal from af undamentalp erspective, [8,9] species with multiple bond character at boron show promise as reagents that allow one to carry out difficult che...

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

“…We must therefore conclude that the t =0 μs excitation is to a different and higher‐energy state, however, it also does not have a clear 0–0 transition and additionally, each of the two states that we identify from the excitation spectra gives rise to the same emission spectrum (Figures B,D and A). Therefore, in Figure , we have shown the planar excited‐state relaxed geometry for S 1 * that has been previously reported as the emitting state . To assess the possible emission maximum from this relaxed state, we calculated the absorption spectrum from S 0 to S 1 * with a fixed planar geometry.…”

“…A major proposed difference between the geometries of the electronic states of BODIPY dyes and BF 2 formazanate dyes is that the excited state geometry of the BF 2 formazanate dyes changes from the folded ground state to a planar, emissive excited state . In this paper, we examine the electronic origins of both the absorption and the emission spectral data for a prototypical BF 2 formazanate dye, 2 , which exhibits both a high molar absorptivity and emission quantum yield.…”

Acid–Base Free Main Group Carbonyl Analogues

“…reported a formazanate‐based neutral boracarbonyl 6 stabilized by AlCl 3 (Scheme 3). [21f] Remarkably, examination of the photoluminescent properties revealed that 6 exhibits a small Stokes shift (50 nm) and has a photoluminescence intensity enhancement of more than 36‐fold ( Φ PL : 36 %), in comparison with P6 , which has large Stokes shift (174 nm) and was essentially found to be non‐emissive in solution ( Φ PL : <1 %). DFT analysis revealed that P6 is highly bent in the ground state and perfectly planar in the excited state, whereas the molecular geometry of photoexcited 6 undergoes little structural distortion and resembles its ground state.…”

Formazanate coordination compounds: synthesis, reactivity, and applications

“…A second non-radiative decay pathway was uncovered when the Lewis-acid-supported oxoborane (BQO) complex 64 was isolated (Scheme 26). 77 Access to this species required a halide exchange reaction between BF 2 triarylformazanate 59b (Ar 1 /Ar 5 = p-tolyl; R 3 = Ph) and BCl 3 to generate BCl 2 complex 63. The BCl 2 unit in compound 63 was subsequently converted to the oxoborane by treatment with one equivalent of AlCl 3 and H 2 O.…”

“…The photoluminescence characteristics of oxoborane 64 support this hypothesis, as the formation of the BQO unit turns on photoluminescence (l PL = 636 nm, F PL = 36%). 77 The origin of this behavior lies in the fact that both the ground and excited states of compound 64 adopt a planar structure as the result of sp 2 hybridization at boron, thus limiting structural reorganization upon photoexcitation and attenuating non-radiative decay. Based on these studies, it is reasonable to assume that 62,64,65,70,73 Compound 78 it may not be surprising that BF 2 formazanates have been shown to be biocompatible during their use as cell-imaging agents (Fig.…”