Promoting intersystem crossing by charge transfer state of dimers for persistent room temperature phosphorescence

Efficient intersystem crossing (ISC) is essential for phosphorescence emission. The charge-transfer (CT) state is traditionally constructed by highly twisted donor-acceptor type structures to facilitate the ISC process. However, the highly twisted structural properties limit the diversity of room-temperature phosphorescence (RTP) molecules. By establishing the CT state in the intermolecular dimers structure, the dependence on the distorted donor-acceptor in the single-molecule design can be well eliminated. Based on this strategy, three novel pure organic luminescent molecules with long afterglow and mechanochromic luminescence (MCL) property were designed and synthesized. All of those exhibited triple emission, including prompt fluorescence (PF), delayed fluorescence (DF) and persistent RTP (pRTP) emission. In particular, CzC4-PhCN showed great potential for application in the field of anti-counterfeiting with its ultralong phosphorescent lifetime of 862 ms. Single crystal analysis and theoretical calculation revealed that the formation of CT states of intermolecular dimers effectively reduced ΔEST and promoted ISC process. This study would be highly instructive for realizing efficient ISC process and high-efficiency pRTP emission in pure organic systems.