Solution-Processable Semi-Aromatic Polyimide MR-TADF Materials for High-Efficiency Narrowband OLEDs

Solution-processed multi-resonance TADF (MR-TADF) polymer OLEDs face a persistent design conflict: conjugated backbones broaden and redshift emission via extended π-delocalization, whereas nonconjugated alternatives preserve narrow spectra but suffer from carrier-transport imbalance, historically capping external quantum efficiency (EQE) below 20%. Here, we circumvent this dilemma by copolymerizing diamines containing B/N-fused MR emitters (DBNCzNH₂) and phosphine oxide (P═O) units (TPONH₂) with an aliphatic dianhydride (HBPDA), yielding a set of semi-aromatic polyimides TPOBNX% (X = 1, 3, 5, and 10). Our design leverages two synergistic effects: the rigid nonconjugated aliphatic imide ring confines the MR chromophores in its intrinsic short-range charge-transfer regime, yielding narrow solution emission (full width at half maximum FWHM = 27-30 nm) and suppressing non-radiative decay via conformational restriction; simultaneously, the P═O units facilitate electron transport while their σ-bond insulation avoids perturbing the MR excited-state manifold. The optimal TPOBN5% exhibits a minimal singlet-triplet gap (ΔE_ST) of 17.5 meV in neat film, favoring reverse intersystem crossing, and an 87% photoluminescence quantum yield in doped film. Solution-processed OLEDs based on TPOBN5% achieve 25.3% EQE and 33 nm electroluminescence FWHM, representing record performance for nonconjugated MR-TADF polymers. The synergy of rigid nonconjugated skeletons and polar transport units establishes a generalizable platform to address the efficiency-color purity trade-off.