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

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, T ₁ → S _exc ) and intrinsic DF. Benefiting from greatly enhanced SOC and reduced Δ E _ST , the proof-of-concept emitter Np-2CzB exhibited k _RISC up to 6.5 × 10 ⁵ 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 _{S 1 →S 0} < 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.
Competing Interests: The authors declare no competing financial interest.
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