Bidirectional Photoswitching via Alternating NIR and UV Irradiation on a Core–Shell UCNP–SCO Nanosphere

Bidirectional photoswitching of molecular materials under ambient condition is of significant importance. Herein, we present for the first time that a core–shell UCNP–SCO nanosphere (UCNP = upconversion nanophosphor, SCO = spin crossover), which was composed of a UCNP core (NaYF4: 20 mol % Yb3+, 1 mol % Er3+) and an SCO iron(II) shell ([Fe(H2Bpz)2(bipy-COOH)], H2Bpz = dihydrobis(1-pyrazolyl)borate, bipy-COOH = 4,4′-dicarboxy-2,2′-bipyridine), can be reversibly photoswitched between the high-spin and low-spin states at room temperature in the solid state, via alternating irradiation with near-infrared (λ = 980 nm) and ultraviolet (λ = 310 nm) light. What’s more, this reversible spin-state switching was accompanied by a variation of fluorescent spectrum and dielectric constants. The strategy here, that is, integrating the SCO iron(II) complex into a UCNP–SCO nanosphere for molecular photoswitching, may open a new area in the development of photocontrolled molecular devices.