Facilitating intrinsic delayed fluorescence of conjugated emitters by inter-chromophore interaction

Abstract

Delayed fluorescence (DF) is a unique emitting phenomenon of great interest for important applications in organic optoelectronics. In general, DF requires well-separated frontier orbitals, inherently corresponding to charge transfer (CT)-type emitters. However, facilitating intrinsic DF for local excited (LE)-type conjugated emitters remains very challenging. Aiming to overcome this obstacle, we demonstrate a new molecular design strategy with a DF-inactive B,N-multiple resonance (MR) emitter as a model system. Without the necessity of doping with heavy atoms, we synthesized a co-facial dimer in which an excimer-like state (S_exc) was expected to facilitate efficient reverse intersystem crossing (RISC, T₁ → S_exc) and intrinsic DF. Benefiting from greatly enhanced SOC and reduced ΔE_ST, the proof-of-concept emitter Np-2CzB exhibited k_RISC up to 6.5 × 10⁵ s⁻¹ and intrinsic DF with >35% contribution (Φ_DF/Φ_F) in dilute solution. Further investigation indicated that S_exc state formation relies on an optimized co-facial distance (d = ∼4.7 Å), strong inter-chromophore interaction (J_coul > 450 cm⁻¹) and a rigid structure (Γ_S1→S0 < 350 cm⁻¹). Although our strategy was demonstrated with a B,N-MR emitter, it can be applicable to many LE-type conjugated emitters without intrinsic DF. By triggering potential DF emission, many classic emitters might play a more important role in optoelectronics.