Molecular conformation modulating luminescence switching between delayed fluorescence and room-temperature phosphorescence

Abstract

A new pure organic donor–acceptor molecule, (4-chlorophenyl)(5H-dibenzo[b,f]azepin-5-yl)methanone (IS-CBZ), was designed and synthesized, which demonstrates near ultraviolet (NUV) delayed fluorescence (DF) and dual emission of NUV DF and yellow room-temperature phosphorescence (RTP) in two crystals (B-crystal and Y-crystal), respectively. These two kinds of luminescence (DF and RTP) can be reversibly switched by external stimuli, such as grinding and heating/fuming, accompanied by reversible phase transition between two crystalline states. The experimental and theoretical studies reveal that this switchable luminescence between DF and RTP originates from a single-molecule conformational change in different molecular packings, mainly involving the twist angle (θ) between the donor and acceptor units. This θ fundamentally modulates the energy difference between the lowest singlet state (S₁) and the high-lying triplet state (T₂), ΔE_S1–T2, which determines the luminescence switching between DF and RTP. This work not only provides a new donor–acceptor architecture for pure organic multifunctional materials with mechanochromic luminescence (MCL) and mechanoluminescence (ML) properties, but also suggests a novel strategy to design stimuli-responsive materials with a unique luminescence switching between DF and RTP by tuning T₂.