Why Ligand Doping Increases the Fluorescence of an Anthracene-Based Metal-Organic Framework

Metal-organic frameworks (MOFs)built from fluorescent ligandsfrequently exhibit enhanced fluorescence when doped with inert ligands.This study focuses on a MOF of the UiO-68 structure, which is builtfrom a fluorescent dibenzoate-anthracene ligand doped with a dibenzoate-benzeneligand. Our investigation aims to understand the mechanism behindthe doping-enhanced emission of this MOF. We rule out several possiblemechanisms, including exciton coupling, electron transfer betweenligand and metal center, and ligand intersystem crossing induced bythe metal center. Inhibition of the interligand charge transfer isconsidered a possible way to enhance emission. Furthermore, we proposethat the conformational change of the anthracene-based ligand in theMOF cavity is also a way for enhancement. Our molecular dynamics simulationsof the MOF structure filled with solvents reveal that the steric crowdingin the cavity induces a conformational change at different dopinglevels, affecting the rate of intersystem crossing of the ligand. The fluorescence quantum yield of UiO-MOFwith anthracene-basedligand was increased from 39 to 93% by the ligand-doping strategy.The inhibition of the interligand charge transfer and/or conformationalchange of the anthracene-based ligand in the MOF cavity is thoughtto be responsible for the enhanced emission.