Binary Ligand‐Mediated Photochromic Tuning of Lanthanide‐Doped Upconversion Nanoparticle Conjugates

Abstract The ability to tune photochromic features of nanoparticle (NP)‐ligand conjugates can vastly extend the utility of dynamic materials in the fields of drug delivery, bioimaging, and data storage. Herein, the photochromic tuning of lanthanide‐doped upconversion nanoparticle (UCNP)‐spiropyran (SP) conjugates through a binary ligand strategy is reported. It is found that the content of SP ligand on the surface of UCNPs can be rationally controlled by an SP‐to‐oleate ligand exchange reaction, allowing for regulating the steric congestion in the ligand layer. A highly congested SP layer not only leads to an ultralong lifetime for the purple open‐ring state but also results in improved red fluorescence under UV excitation. The enhancement in steric congestion further enables increased activation energy for the ring‐opening reaction for the bound SP ligand, permitting the generation of high‐resolution self‐erasing patterns in a UCNP‐SP conjugate‐doped polydimethylsiloxane film by local irradiation with either UV or near‐infrared light. The findings provide new insights into the optical tuning of nanoconjugates by controlling the conformational dynamics of their mixed ligand layers.