Achieving Deep‐Red TADF with Negative‐Thermal Quenching in [Cu(N^N)(P^P)]⁺ Emitters Through a Perchlorination Strategy

Abstract A perchlorination strategy is introduced as a novel approach for designing Cu(I)‐based deep‐red TADF materials exhibiting large negative thermal quenching (NTQ). Replacing the 1,10‐phenanthroline (phen) ligand in conventional [Cu(phen)(P^P)]⁺ complexes with octachloro‐1,10‐phenanthroline (phenCl 8 ) unexpectedly induces a strong NTQ effect while simultaneously triggering a giant bathochromic shift in absorption, excitation, and emission. Whereas the parent complex [Cu(phen)(DPEPhos)]PF 6 emits at λ max = 576 nm with nearly temperature‐independent emission intensity in the 77–300K range ( I 300 K / I 77 K = 1.1), its perchlorinated analog [Cu(phenCl 8 )(DPEPhos)]PF 6 exhibits deep‐red emission (λ max = 680 nm) and a remarkable NTQ effect with the impressively high ratio I 300 K / I 77 K of 12.3. Detailed investigations reveal that the NTQ behavior stems from perchlorination‐induced spin‐orbit coupling enhancement, which facilitates non‐radiative T 1 → S 0 decays at low temperatures. Additionally, perchlorination boosts X‐ray luminescence intensity, further highlighting its potential for optoelectronic applications.