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 (tau) and unity photoluminescence quantum yields (Phi(PL)). Structural diversity of these compounds is largely limited by the small number of suitable carbene fragments possessing sufficient stability and pi-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 ((LE)-L-3) phosphoresce seen in sulfone-free structural analogues. Compounds are characterized using the Phi(PL) of up to 0.85 and outstandingly high radiative rates (k(r)) of up to 1.3 x 10(6) 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 (Delta E-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% and a maximum luminance of 26 000 cd m(-2).