Photoswitchable photon upconversion from turn-on mode fluorescent diarylethenes

Photon upconversion (UC) is one kind of anti-Stokes shift process, which generally requires a combination of two or more low-energy photons to produce one high-energy photon. However, another interesting anti-Stokes process, namely, single-photon-absorption-based upconversion (SPA-UC), can take place by exciting thermally excited vibrational-rotational energy levels of ground state to the first level of excited state. This phenomenon is less involved in organic systems due to the limitation of appropriate materials. Herein, for the first time, we have demonstrated a series of turn-on mode fluorescent diarylethenes, including Thiophene- and p-alkoxybromo-substituted phenyl-substituted derivatives (DAES and DAEC4), which exhibited SPA-UC phenomenon in the closed-ring state. An anti-Stokes shift of 0.36 eV was observed in these molecules, which was the highest value accessible in such UC systems. A relatively high quantum efficiency (13.5%) was evaluated through a relative method by using methylene blue as a standard. Phonon-assisted or hot band absorption was in charge of the achievement of SPA-UC. Besides, the upconverted emission could be switched on/off by cyclization and cycloreversion reaction, regulated by UV-visible light. Thus, this unique finding of the SPA-UC phenomena in photoswitchable diarylethenes might enrich the development of new molecular engineering strategies for designing photo-functional materials.