Highly Stable, Readily Reducible, Fluorescent, Trifluoromethylated 9‐Borafluorenes

Abstract

Three differentp erfluoroalkylated borafluorenes (F Bf)w ere prepared and their electronic and photophysical properties were investigated. The systemsh ave four trifluoromethyl moieties on the borafluorene moiety as well as two trifluoromethyl groups at the ortho positions of their exo-aryl moieties. They differ with regard to the para substituents on their exo-aryl moieties, being ap roton(F Xyl F Bf, F Xyl:2 ,6-bis(trifluoromethyl)phenyl), at rifluoromethyl group (F Mes F Bf, F Mes:2 ,4,6-tris(trifluoromethyl)phenyl) or ad imethylamino group (p-NMe 2-F Xyl F Bf, p-NMe 2-F Xyl:4-(dimethylamino)-2,6-bis(trifluoromethyl)phenyl), respectively.A ll derivatives exhibit extraordinarily low reduction potentials, comparable to those of perylenediimides. The most electron-deficient derivative F Mes F Bf wasa lso chemically reduced and its radicala nion isolated and characterized. Furthermore, all compounds exhibit very long fluorescent lifetimes of about 250 ns up to 1.6 ms; however, the underlying mechanismsresponsible for this differ.T he donor-substitutedd erivative p-NMe 2-F Xyl F Bf exhibitst hermally activated delayedf luorescence (TADF) from ac harge-transfer (CT) state, whereas the F Mes F Bf and F Xyl F Bf borafluorenese xhibit only weakly allowed locally excited (LE) transitions due to their symmetry and low transition-dipole moments.

Electronically Driven Regioselective Iridium‐Catalyzed C−H Borylation of Donor‐π‐Acceptor Chromophores Containing Triarylboron Acceptors

“…All compounds exhibit 1 H, 11 B, 19 F and 13 C NMR spectra in accordance with their structures. The 19 F NMR spectra exhibit strong signal broadening, indicative of a hindered rotation about the B−C bonds, as previously observed for similar boranes with ortho ‐trifluoromethyl groups …”

“…The C−N bond lengths of 1 and 1‐(Bpin) 2 show a similar behavior and exhibit two longer bonds (1.430(12)–1.440(3) Å) to the terminal phenyl moieties and one shorter bond (1.395(3) and 1.395(4) Å, respectively) to the phenylene bridge. As the steric demand of all three substituents at the N atom is equal, this can be attributed to a polarized ground state, which is well known for d ‐π‐A systems . This is further supported by the larger torsion angles of the terminal phenyl groups with respect to the NC 3 plane (41.86(9)–71.3(3)°), compared to those of the phenylene bridge (16.71(8) and 8.2(2)°).…”

“…This can be avoided by using bulky substituents or by fixing the boron center in a rigid scaffold . Only recently the use of ortho ‐trifluoromethylated aryls in triarylboranes has been established as a strategy to improve acceptor strength as well as stability . The improved acceptor strength can be attributed to the electron‐withdrawing nature of the trifluoromethyl groups, while the increase in stability is due to steric shielding of the boron center and its empty p z ‐orbital, as well as a direct interaction of the fluorine lone pairs with the empty p z ‐orbital on boron.…”

(Hetero)arene-fused boroles: a broad spectrum of applications

“…93 The s-donation from the uorine atom(s) of ortho-CF 3 group(s) to the vacant p z -orbital of boron was also observed in other boranes [94][95][96][97][98][99][100][101][102] and boroles. 45,68 The 2,4,6-tri-tert-butylphenyl (Mes*) group is the bulkiest substituent among these protecting groups and, thus, provides the most stable 9-borauorenes. 103 Compared to TipBf, which still exhibits reactivity towards the small F À anion and can be applied as a F À sensor, Mes*Bf is inert to F À .…”

Enhancing the Acceptor Character of Conjugated Organoborane Macrocycles: A Highly Electron‐Deficient Hexaboracyclophane

“…[19] The F Mes substituents serve to sterically shield the boron centers but, in contrast to the Mes* groups,a lso enhance the electron-deficient character. [20] TheLewis acidity was first assessed according to the Gutmann-Beckett method [21] by adding Et 3 PO to solutions of 2-F Mes and 4-F Mes in dry C 6 D 6 under inert atmosphere.The solutions were analyzed by 11 Band 31 PNMR spectroscopy,which indicated that no binding at all occurred even after extended periods of time.This is consistent with the lack of Et 3 PO binding to other sterically hindered boranes such as B(C 6 Cl 5 ) 3 . [22] Although highly electron-deficient, the Lewis acidity towards Et 3 PO appears to be strongly diminished due to the steric hindrance of the F Mes groups.W enext investigated the binding of small anions [6a,23] to the electrondeficient F Mes-substituted derivatives 2-F Mes, 4-F Mes,a nd B6-F Mes.A nion titrations for F Mes-substituted compounds were conducted with different anion sources,i ncluding (n-Bu 4 N)F (TBAF), (n-Bu 4 N)CN (TBACN), and Na[HBEt 3 ].…”