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

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 (theta) between the donor and acceptor units. This theta fundamentally modulates the energy difference between the lowest singlet state (S-1) and the high-lying triplet state (T-2), Delta ES1-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 T-2.