Sulfonyl-functionalized benzo[d]imidazo[5,1-b]thiazole-based carbenes as building blocks for two-coordinate Cu(i) complexes exhibiting fast and efficient thermally activated delayed fluorescence

Linear two-coordinate carbene-metal-amide (CMA) complexes are an emerging class of luminescent materials that can provide efficient emission from triplet excited states through the thermally activated delayed fluorescence (TADF) mechanism, characterized by sub-microsecond lifetimes (τ) and unity photoluminescence quantum yields (Φ_PL). Structural diversity of these compounds is largely limited by the small number of suitable carbene fragments possessing sufficient stability and π-accepting property. By applying rational design principles, we have modified the tricyclic benzo[d]imidazo[5,1-b]thiazolium carbene precursor by the introduction of a strongly accepting sulfonyl group through the oxidation of the heterocyclic sulphur atom. Thus, an efficient TADF process is enabled in the corresponding Cu(i)-based CMA emitters in comparison to long-lived (³LE) phosphoresce seen in sulfone-free structural analogues. Compounds are characterized using the Φ_PL of up to 0.85 and outstandingly high radiative rates (k_r) of up to 1.3 × 10⁶ s⁻¹. The rapid emissive process is attributed to the unusually small overlap integral of 0.118 between the highest occupied and lowest unoccupied molecular orbitals (HOMO, LUMO), leading to a small singlet-triplet energy gap (ΔE_ST) of 0.05 eV. A vacuum-deposited green OLED incorporating the synthesized emitter was prepared showing an external quantum efficiency (EQE) of 8.6%