On-demand photoswitching and energy transfer by post-synthetically confining Eu³⁺-complex and dithienylethene ligand in Zr-MOF-808

Tailorable porous scaffolds like metal-organic frameworks (MOFs) are emerging as a versatile platform for confining photoresponsive molecules. Herein, we employed a highly stable nanoscale Zr-MOF-808, {[Zr6(μ3-O)4(μ3-OH)4(HCOO)6(BTC)2]} n (Zr-MOF), as a porous host for post-synthetic incorporation of a carboxylic acid functionalized photochromic dithienylethene (DTE) molecule through the targeted modification of the Zr6-cluster in the MOF. The post-modified Zr-DTE-MOF exhibits a reversible color switching between faint off-white and blue upon UV and visible light irradiation, respectively, owing to the photoisomerization of confined DTE molecules. Furthermore, the nanoscale particle size and excellent dispersibility of Zr-DTE-MOF in a suitable solvent enhance its processability as a photochromic ink. Leveraging the prominent spectral overlap between the absorption band of the DTE in closed form and the emission maximum of the Eu3+-based complex, we subsequently encapsulated a Eu3+-terpyridine-based complex within the MOF pore and constructed a pore-confined photoresponsive donor-acceptor system, named Zr-DTE-CTPY-Eu-MOF. The rational selection of suitable donor-acceptor systems for confinement and their close proximity result in fast emission quenching by the photochromic Förster resonance energy transfer (pcFRET) with an appreciable efficiency of 69.9%. Parallelly, the Zr-DTE-CTPY-Eu-MOF exhibited a photoluminescence quantum yield of 40% (in the open form), which decreased to 14% (in the closed form) after UV light illumination due to the feasible energy transfer from the Eu3+-CTPY complex to DTE. Based on the photoswitching ability of the developed photochromic Zr-DTE-CTPY-Eu-MOF, it was further employed to create patterns using photomasks, secret writing, and encryption-decryption of confidential information.