Highly Emissive 9‐Borafluorene Derivatives: Synthesis, Photophysical Properties and Device Fabrication

Abstract

A series of 9‐borafluorene derivatives, functionalised with electron‐donating groups, have been prepared. Some of these 9‐borafluorene compounds exhibit strong yellowish emission in solution and in the solid state with relatively high quantum yields (up to 73.6 % for FMesB‐Cz as a neat film). The results suggest that the highly twisted donor groups suppress charge transfer, but the intrinsic photophysical properties of the 9‐borafluorene systems remain. The new compounds showed enhanced stability towards the atmosphere, and exhibited excellent thermal stability, revealing their potential for application in materials science. Organic light‐emitting diode (OLED) devices were fabricated with two of the highly emissive compounds, and they exhibited strong yellow‐greenish electroluminescence, with a maximum luminance intensity of >22 000 cd m−2. These are the first two examples of 9‐borafluorene derivatives being used as light‐emitting materials in OLED devices, and they have enabled us to achieve a balance between maintaining their intrinsic properties while improving their stability.

Substituent Modulation for Highly Bright 9-Borafluorene Derivatives with Carbazole Pendant

“…After that, little work was reported about donor-functionalized 9-borafluorene until recently, when Marder and coworkers reported a trifluoromethylated borafluorene with a dimethylamino group on its exo -aryl moiety at the position para to boron, p -NMe 2 - F Xyl F Bf, which exhibits thermal-activated delayed fluorescence (TADF). Successively, Yin and Marder reported a joint work on the enhanced chemical and thermal stability of 9-borafluorenes by introducing nitrogen-based electron donors at the para position of the exoaryl group …”

“…In the case of the photoluminescence (PL) properties, two para-substituted compounds exhibit very strong emission with maximum wavelengths at ca. 520 (photoluminescence quantum yield (PLQY) = 94%) and 530 nm (PLQY = 69%) in the solid state and 500 (PLQY = 100%) and 540 nm (PLQY = 100%) in hexane for Mes*BF-pCz and F MesBF-pCz , respectively (see Figure a), exhibiting the most efficient PL properties among 9-borafluorene compounds so far. ,− (See Table S6 for details.) In contrast, meta-substituted compounds exhibited relatively weaker and red-shifted emission with maximum wavelengths at ca.…”

“…Two F Mes-functionalized compounds exhibited significantly blue-shifted fluorescence at ca. 368 nm (PLQY = 3%) for F MesBF-pCz and 377 nm (PLQY= 58%) for F MesBF-mCz in dimethylformamide (DMF), which could be due to the coordination to solvent molecules given less kinetic protection and the strong electron-withdrawing effect of the F Mes group. , Interestingly, the emission spectra of F MesBF-mCz also exhibit evidence of coordination to tetrahydrofuran (THF) and 2-methyltetrahydrofuran­(2-MeTHF) with dual-emission character at ca. 398 and 650 nm.…”

“…Successively, Yin and Marder reported a joint work on the enhanced chemical and thermal stability of 9-borafluorenes by introducing nitrogenbased electron donors at the para position of the exoaryl group. 10 In this work, we revisit the synthesis of the D−A system based on 9-borafluorene by introducing the electron donor to the para or meta position of the boron on the borafluorene skeleton (see Figure 1a), thereby shortening the distance between the electron donor and the electron acceptor, conforming a twisted D−A system; then, we explore the influence of the position of the substituent on the photophysical properties and Lewis acidity of the compounds.…”

Future prospects in boron chemistry: new boron compounds and Lewis acids for catalysis and materials science

“…These unprecedented boron compounds with new structures and reactivities will enable the observation of new coordination modes at boron, in the isolation of long sought after reaction intermediates and new classes of boron-mediated reactions in the near future. 1E] [26] , π-conjugated boron compounds embedded in polyaromatic antiaromatic architectures [Figure 1F] [27] , redox-active tetraanionic diborafluorenes [Figure 1G] [28] , boron-nitrogen embedded polycyclic aromatic hydrocarbons [Figure 1H] [29] , BN-doped cycloparaphenylenes [Figure 1I] [30] , highly emissive donor-acceptor arylamino-9-borafluorene [Figure 1J] [31] , bora subporphyrins [Figure 1K] [32] and thiophene-based polycyclic aromatic tricoordinate boron semiconductor [Figure 1L] [33] are all receiving increasing recognition for their outstanding optical, photophysical, electrochemical and photoresponsive properties.…”