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 (DBNCzNH 2 ) and phosphine oxide (P═O) units (TPONH 2 ) with an aliphatic dianhydride (HBPDA), yielding a set of semi‐aromatic polyimides TPOBN X % ( 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 (Δ E 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.