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

Abstract

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.