Stepwise Energy Transfer: Near‐Infrared Persistent Luminescence from Doped Polymeric Systems

Organic near infrared (NIR) persistent‐luminescence systems with bright and long‐lived emission are highly valuable for applications in communication, imaging, and sensors. However, realizing these materials (especially lifetime over 0.1 s) is a challenge, mainly because of non‐radiative quenching of their long‐lived excitons. Herein, a universal strategy of stepwise Förster resonance energy transfer (FRET) for a bright NIR system with remarkable persistent luminescence (up to 0.2 s at 810 nm) is presented, based on a new triphenylene‐dye‐doped polymer (triphenylene‐2‐ylboronic acid@poly(vinyl alcohol) (TP@PVA)) with a persistent blue phosphorescence of 3.29 s. This persistent NIR luminescence is demonstrated for application not only in NIR anti‐counterfeiting but also NIR bioimaging with penetrating a piece of skin as thick as 2.0 mm. By co‐doping a red dye (such as Nile red) and an NIR dye Cyanine 7 (Cy7) into this doped PVA film, the shortage of spectral overlap between TP emission and Cy7 absorbance is successfully solved, through a stepwise FRET process involving triplet to singlet (TS)‐FRET from TP to the intermediate red dye and then singlet to singlet (SS)‐FRET to Cy7. It is noted that the efficiency of the upper TS‐FRET is enhanced significantly by the lower SS‐FRET, leading to high efficiencies for the continuous FRETs. A stepwise Förster resonance energy transfer (FRET) process with an intermediate dye is presented, which provides a universal strategy for tuning the luminescence peak by a large margin. Therefore, bright NIR luminescence with a lifetime up to 0.2 s at 810 nm is achieved based on a new triphenylene‐dye‐doped polymer (TP@PVA) with a blue phosphorescence of 3.29 s.