Blocking Orbital π-Conjugation to Boost Spin-Orbit Coupling in Carbonyl-Embedded Polycyclic Heteroaromatic Emitters.

Both reducing singlet-triplet energy gaps (ΔES1T1) and enhancing spin-orbit couplings (SOCs) are key to improving reverse intersystem crossing rates (kRISC) in thermally activated delayed fluorescence (TADF) materials. While considerable efforts have focused on reducing ΔES1T1, investigations on SOCs remain limited. Here, blocking π-conjugation in carbonyl-embedded polycyclic heteroaromatic (PHA) molecules as potential approach to elevate ππ* excitation energy, allowing its hybridization with nπ* excitation, thereby increasing SOCs is proposed. Two proof-of-concept isomers, DNDK-1 and DNDK-2 are synthesized, with different orientations of carbonyl units. DNDK-1 adopts a heavily twisted structure that hinders π-conjugation, while DNDK-2 remains quasi-planar, maintaining stronger π-conjugation. Experimental measurements reveals stark differences in their photophysical properties, with DNDK-1 exhibiting faster kRISC and much higher electroluminescence efficiency. The ab-initio calculations elucidate that hindered conjugation in DNDK-1 elevates ππ* excitation energy, enabling nπ*-ππ* mixing, thus significantly boosting SOCs. In contrast, smooth π-conjugation in DNDK-2 leads to marginal nπ*-ππ* mixing. In addition, utilizing groups composed of meta-arranged carbonyl-Ar-carbonyl and meta-arranged N-Ar-N units emerges as another approach to block π-conjugation and enhance SOCs. This joint experimental and theoretical work provides promising pathways to enhance SOCs by blocking π-conjugation, offering crucial insights for designing carbonyl-embedded PHA emitters with larger SOCs.