Ultrathin two-dimensional nanosheets meet upconverting nanoparticles: in situ near-infrared triggered molecular switching

Abstract

Combining the advantages of ultrathin two-dimensional (2D) nanosheets featuring stimuli-responsive properties with lanthanide-doped upconverting nanoparticles (UCNPs) represents a challenging goal in near infrared (NIR) light-triggered molecular switching under ambient conditions. Here, we present the fabrication of a 2D@UCNPs hybrid system comprising the integration of distinct features of atomically thin 2D spin-crossover nanosheets ({[Fe(1,3-bpp)₂(SCN)₂]₂}_n) and ytterbium ion (Yb³⁺)-sensitized hexagonal phase UCNPs [sodium yttrium fluoride (NaYF₄): 20 mol% Yb³⁺, 1 mol% erbium ions (Er³⁺)]. The UCNPs are hybridized with 2D nanosheets involving bare thiocyanate (SCN⁻) ligands on both sides of the basal surfaces of the nanosheets, and as a consequence, effective synergistic effects between the up-conversion luminescence (UCL) and spin-state interconversion were established. Upon NIR (980 nm) irradiation, the UCL weakened the metal-to-ligand charge-transfer (MLCT) and triggered high-spin (HS) to low-spin (LS) state transition. Upon subsequent heating (40 °C), the MLCT strengthened and the LS to HS state transition was achieved, suggesting a reversible molecular-switching had occurred which was triggered by altering NIR and thermal stimuli. In addition, the corresponding aqueous colloidal suspension is highly stable, nontoxic, environmentally and human friendly in nature, making it suitable for practical applications in photo-switching devices and fabrication of next-generation user-friendly and wearable, fashionable optoelectronic devices.