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.