Two-Way Photoswitching Using One Type of Near-Infrared Light, Upconverting Nanoparticles, and Changing Only the Light Intensity

Only one type of lanthanide-doped upconverting nanoparticle (UCNP) is needed to reversibly toggle photoresponsive organic compounds between their two unique optical, electronic, and structural states by modulating merely the intensity of the 980 nm excitation light. This reversible "remote-control" photoswitching employs an excitation wavelength not directly absorbed by the organic chromophores and takes advantage of the fact that designer core−shell−shell NaYF4 nanoparticles containing Er3+/Yb3+ and Tm3+/Yb3+ ions doped into separate layers change the type of light they emit when the power density of the near-infrared light is increased or decreased. At high power densities, the dominant emissions are ultraviolet and are appropriate to drive the ring-closing, forward reactions of dithienylethene (DTE) photoswitches. The visible light generated from the same core−shell−shell UCNPs at low power densities triggers the reverse, ring-opening reactions and regenerates the original photoisomers. The "remote-control" photoswitching using NIR light is as equally effective as the direct switching with UV and visible light, albeit the reaction rates are slower. This technology offers a highly convenient and versatile method to spatially and temporally regulate photochemical reactions using a single light source and changing either its power or its focal point.