image: (a) RTP emission of PSP@PVA-N (λex = 550 nm) (b) Structural transformation of BR, BPB, and TBPB in aqueous solution with differing pH values and their charge distribution in the ground state (S0). The certain color regions (orange, dark pink and blue) corresponded to the certain functional group of molecules.
The fluorescence dyes were dominant species of the near-infrared (NIR) dyes, but the energy gap of the NIR dyes between S1 state and S0 state is generally small to induce the ultrafast internal conversion dynamics to quench the NIR emission of the fluorescence dyes. Therefore, the quantum yield of the fluorescence NIR dyes is usually low. On the other hand, the organic dyes with room-temperature phosphorescence (RTP) in the NIR region could prevent the ultrafast internal conversion dynamics quenching because of the T1 state and S0 state the organic molecules are spin forbidden.
Recently, scientists in China reported a new assumption to construct efficient NIR materials based on the energy gap law on the National Science Review. In addition, a series of metal-free RTP films with NIR emission was constructed based on the assumption, and the first phosphorescence-based half-subtractor operation was also developed based on the stimuli-responsive properties of the dyes.
It could be an available strategy to construct efficient metal-free NIR materials by controlling the external environments of the organic molecules with red-light fluorescence to inhibit the non-radiation decay of the light-emitting molecules. The short lifetimes of the triplet state of the NIR RTP molecules could benefit their NIR emission because the other non-radiation decay processes of the long-lifetime triplet state of the NIR RTP molecules could offset the spin forbidden benefit factor of the IC to quench the NIR emission. This assumption could lower the threshold of the construction of the metal-free NIR materials. Therefore, phenolsulfonphthalein dyes with red-light emission (BR, BPB, TBPB) were chosen as NIR emitters in this research to demonstrate this assumption and polyvinyl alcohol (PVA) were chosen as rigid matrixes to inhibit the non-radiation relaxation process. In conclusion, all phenolsulfonphthalein dyes emitted moderate NIR RTP in the PVA matrix, where TBPB@PVA films shown the best performance (ΦRTP = 3.0%, λp = 819 nm). This research preliminary proved that the spin forbidden IC process of the T state and the ground state of the organic molecules could be favorable to construct effective NIR dyes, and this assumption might provide a new train of thought for the development of the organic materials with NIR emission.