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

Three different perfluoroalkylated borafluorenes (^F Bf) were prepared and their electronic and photophysical properties were investigated. The systems have 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 a proton (^F Xyl^F Bf, ^F Xyl: 2,6-bis(trifluoromethyl)phenyl), a trifluoromethyl group (^F Mes^F Bf, ^F Mes: 2,4,6-tris(trifluoromethyl)phenyl) or a dimethylamino group (p-NMe₂ -^F Xyl^F Bf, p-NMe₂ -^F Xyl: 4-(dimethylamino)-2,6-bis(trifluoromethyl)phenyl), respectively. All derivatives exhibit extraordinarily low reduction potentials, comparable to those of perylenediimides. The most electron-deficient derivative ^F Mes^F Bf was also chemically reduced and its radical anion isolated and characterized. Furthermore, all compounds exhibit very long fluorescent lifetimes of about 250 ns up to 1.6 μs; however, the underlying mechanisms responsible for this differ. The donor-substituted derivative p-NMe₂ -^F Xyl^F Bf exhibits thermally activated delayed fluorescence (TADF) from a charge-transfer (CT) state, whereas the ^F Mes^F Bf and ^F Xyl^F Bf borafluorenes exhibit only weakly allowed locally excited (LE) transitions due to their symmetry and low transition-dipole moments.