10H‐Dibenzo[b,e][1,4]thiaborinine 5,5‐dioxide (SO2B)—a high triplet (T₁=3.05 eV) strongly electron‐accepting boracycle was successfully utilised in thermally activated delayed fluorescence (TADF) emitters PXZ‐Dipp‐SO2B and CZ‐Dipp‐SO2B. We demonstrate the near‐complete separation of highest occupied and lowest unoccupied molecular orbitals leading to a low oscillator strength of the S₁→S₀ CT transition, resulting in very long ca. 83 ns and 400 ns prompt fluorescence lifetimes for CZ‐Dipp‐SO2B and PXZ‐Dipp‐SO2B, respectively, but retaining near unity photoluminescence quantum yield. OLEDs using CZ‐Dipp‐SO2B as the luminescent dopant display high external quantum efficiency (EQE) of 23.3 % and maximum luminance of 18600 cd m⁻² with low efficiency roll off at high brightness. For CZ‐Dipp‐SO2B, reverse intersystem crossing (rISC) is mediated through the vibronic coupling of two charge transfer (CT) states, without involving the triplet local excited state (³LE), resulting in remarkable rISC rate invariance to environmental polarity and polarisability whilst giving high organic light‐emitting diode (OLED) efficiency. This new form of rISC allows stable OLED performance to be achieved in different host environments.

A series of 8 luminescent borafluorene complexes were extensively studied both experimentally and theoretically in order to elucidate the effect of organoboron moiety rigidification on the physicochemical properties of these compounds. Due to the spiro geometry of the boron atom, borafluorene and ligand units are perpendicularly aligned, which considerably affects the flexibility of the molecule as well as its solid-state structure. Through comparative analysis with close diphenyl analogues, we show how these structural features influence the thermal, photoluminescent and charge mobility behaviour of the studied compounds. Crystal structural analysis revealed that the molecules are connected mostly through C–H⋯O and C–H⋯ π interactions formed between perpendicularly aligned borafluorene and ligand moieties from neighboured molecules, serving as a complementary donor and acceptor of electron density, respectively. This also efficiently prevents molecules from engaging in unfavoured π-stacking contact. Furthermore, structural analysis suggests that borafluorene complexes possess a considerable degree of flexibility due to OBN heterocycle distortions and mutual borafluorene-ligand plane movements. The magnitude of these effects strictly depends on the ligand structure and may lead either to enhancing or lowering the quantum yield value with respect to BPh₂ analogues, while the absorption and emission wavelength are slightly affected. The measured photophysical parameters for solid-state samples showed that the studied complexes are much better emitters in their crystalline states that in amorphous films. The TD-DFT and NTO calculations revealed a significant change in frontier molecular distribution, with the HOMO localized on the borafluorene moiety. However, as the HOMO–LUMO transition is geometrically not favoured, excitation occurred from HOMO−1 localized on the ligand. Finally, aggregation effects were discussed based on supramolecular arrangements in crystal structures and charge transfer rates obtained from theoretical calculations in the framework of the Marcus–Hush approximation. They suggest that borafluorene complexes are much better electron carriers with respect to non-annulated BPh₂ complexes.