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 (ΦPL). 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 ΦPL of up to 0.85 and outstandingly high radiative rates (kr) 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 (ΔEST) 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.