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

Both reducing singlet-triplet energy gaps (ΔE_S1T1) and enhancing spin-orbit couplings (SOCs) are key to improving reverse intersystem crossing rates (k_RISC) in thermally activated delayed fluorescence (TADF) materials. While considerable efforts have focused on reducing ΔE_S1T1, 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 k_RISC 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.