Fluorescent Organic π‐Radicals Stabilized with Boron: Featuring a SOMO–LUMO Electronic Transition

Abstract

We report on the fluorescence properties of a new class of emissive and stable π-radicals that contain a boron atom at a position distant from the radical center. A fully planarized derivative exhibited an intense red fluorescence with high fluorescence quantum yields (Φ F > 0.67) even in polar solvents. To elucidate the origin of this phenomenon, we synthesized another boron-stabilized radical that contains a bulky aryl group on the boron atom. A comparison of these derivatives, as well as with conventional donor-π-acceptor (D-π-A)-type emissive π-radicals, unveiled several characteristic features in their photophysical properties. A theoretical analysis revealed that the SOMO-LUMO electronic transition generates an emissive D 1 state. Unlike conventional D-π-A-type π-radicals, this state does not undergo significant structural relaxation. The boronstabilized π-radicals demonstrated promising potential for organic light-emitting diodes as an emitting material.

The simplest structure of a stable radical showing high fluorescence efficiency in solution: benzene donors with triarylmethyl radicals

“…In particular, Mes 2 F 2 PyBTM shows high fluorescence efficiency above 60% in dichloromethane, chloroform, and PMMA at room temperature in ambient conditions. Although the PLQY of 69% in chloroform is not the highest among the luminescent today, 34,35,29 this value is comparable to famous fluorescent dyes such as rhodamine B ( f = 0.70 in methanol, 44 Fig. S7).…”

“…As far as we know, a higher PLQY of the uorescent radical in a liquid solution has only been reported for pyridoindole donor-TTM acceptor systems in 2020 34 and 2022 35 and the p-radical stabilized with boron in 2022. 29 The major structural difference between pyridoindole donor-TTM acceptor systems 34,35 and Mes 2 F 2 PyBTM is that the former has two nitrogen atoms in the donor and the latter has one nitrogen atom in the radical. In contrast, the TTM radical is made of a carbon skeleton and the mesityl group is a hydrocarbon.…”

“…24 It is noteworthy that efficient uorescence in more polar solvents has recently been reported in completely different systems: pyrene-dithiadiazolyl radical (F f = 50% in acetonitrile) 28 and the p-radical stabilized with boron (F f = 67% in DMF). 29 Here, we report that simple aromatic hydrocarbons, mesityl groups, work as donors, and signicantly enhance the uorescence efficiencies of PyBTM and F 2 PyBTM in dichloromethane and chloroform solutions. We explain the reasons for the high efficiencies of these nonplanar p-electron systems by photophysical theories and calculations using DFT and TD-DFT.…”

Enhancement of fluorescence and photostability of luminescent radicals by quadruple addition of phenyl groups

“…As a result, the shapes of the β-HOMOs of 2 and 3 were similar. In both the ground (D 0 ) and lowest excited (D 1 ) states, the electronic structures of these radicals are classified as donor–π–acceptor (D–π–A)-type, 17 in which the added o -diphenylphenyl group works as a donor and the radical (unpaired orbital) works as an acceptor.…”

The combination of skeleton-engineering and periphery-engineering: a design strategy for organic doublet emitters

“…The introduction of the boron (B) atom is conducive to the spin-delocalization of radical-centered carbon (α-C) and plays a crucial role in the high stability of radicals in solution which has been reported. 8 In addition, the periphery-engineering strategy, as another basic molecular regulation method, also provides an important clue for improving the luminescence efficiency of radical luminescent materials. For instance, different donor fragments such as carbazole and its derivatives can be added to the periphery of TAM radicals to optimize the luminescent intensities by tuning the hybridized locally excited (LE) and charge transfer (CT) excited state (HLCT) characteristics.…”