Abstract

This thesis presents the synthesis and optical studies of a photoswitching model system, which consists of the lanthanide-doped upconversion luminescent nanoparticles (UCNPs) and photochromic bis-spiropyrans (BSPs).
The thermal decomposition synthetic strategy was employed to prepare monodisperse upconversion luminescent LiYF4 (Y3+ 74.5 mol %, Tm3+ 0.5 mol %, and Yb3+ 25 mol %) nanoparticles (UCNPs), using the optimized conditions established; a reaction temperature of 315 ºC, reaction time of 90 minutes, and an injection rate of 1.5 mL/min for precursor addition. The as-prepared UCNPs were fully characterized using X-ray powder diffraction (XRPD), Fournier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), and upconversion (UC) fluorescence emission spectroscopy.
Following a ligand exchange process, bis-spiropyran (BSP) was grafted on the surface of the UCNPs by replacing the oleate molecules. The BSP-UCNPs construct was investigated and we demonstrated the photoswitching of bis-spiropyran in this new system. Photoswitching occurs via fluorescence resonance energy transfer (FRET) from the UCNPs to bis-spiropyran.
We also explored the energy transfer efficiency between the UCNPs and ring-closed bis-spiropyran molecule and carried out a kinetic study of the photoswitching from ring-closed colorless bis-spiropyran to ring-open colored bis-merocyanine, as well as the photodegradation of the bis-spiropyran functionalized UCNPs.
Taking advantage of the high penetration depth, reduced photodamage, and minimal autofluorescence of the NIR light excitation, this model system could be useful in biological applications related to photoswitching of photochromic compounds which requires UV irradiation to achieve the photochromic transformation.