Blocking π-Conjugation via Fused Heteroatom Engineering for Narrowband Multi-Resonance PO/N TADF Emitters.

The strategic development of multiresonance thermally activated delayed fluorescence (MR-TADF) emitters based on electron-deficient motifs represents a pivotal frontier in the pursuit of next-generation ultrahigh-definition displays. While carbonyl groups have proven to be promising alternatives to boron, the phosphine oxide (P═O) group has recently emerged as another competitive electron acceptor. Nevertheless, the potential of PO/N-type MR-TADF materials remains largely unexplored, with a pronounced scarcity of systematic studies addressing how heteroatom integration precisely modulates the excited-state distribution and emission bandwidth. To bridge this gap, we herein report the rational design and synthesis of a series of rigid PO/N aromatic emitters, wherein a "heteroatom toolkit" (sulfoxide, carbonyl, C═C) is embedded into the π-fused periphery to precisely fine-tune their MR properties. Photophysical and theoretical studies reveal a clear structure-property relationship: the sulfoxide-incorporated SOPO, with its strongest π-conjugation blocking at the sulfoxide ring, exhibits the most confined electron delocalization, yielding optimal MR characteristics and the narrowest full width at half-maximum (fwhm) of 31 nm. In contrast, the C═C-bridged CCPO suffers from extended electronic delocalization and complete loss of MR features, resulting in a broad emission with a fwhm of 92 nm. The carbonyl-incorporated COPO achieves an intermediate balance. This work establishes π-conjugation blocking as a decisive factor in regulating the PO/N-type MR-TADF properties. Importantly, SOPO delivered record-high electroluminescence performance, ranking among the highest reported for MR-TADF-based UV-OLEDs to date. This establishes a clear design paradigm for future high-performance, narrowband emitters.