Mechanoluminescence and aggregation-enhanced emission (AEE) of an In-MOF based on a 9,9′-diphenyl-9H-fluorene tetraacid linker

Abstract

A water-stable In-MOF, constructed based on a conformationally-flexible tetraacid linker, i.e., 2,7-bis(3,5-dicarboxyphenyl)-9,9′-diphenyl-9H-fluorene, i.e., H₄DPF, is shown to exhibit a significantly enhanced solid-state fluorescence quantum yield (ϕ_f) of 23% in comparison with that of the linker (ϕ_fca. 4%) as a consequence of rigidification of the latter by metalation. Application of external stimulus in the form of grinding of the In-MOF leads to a drastic enhancement by 29%, ϕ_f from 23 to 52%. Solid-state absorption and emission spectra show that the absorption in the region of 368–550 nm gets diminished with a concomitant change in the emission maximum with a blue shift upon grinding. Fluorescence enhancement with grinding is correlated with a gradual reduction in the size of the particles, as established by SEM analysis. MOF particle aggregation has been invoked to account for the observed fluorescence enhancement in addition to a subtle conformational change in the structure of the linker upon grinding. Intriguingly, the ground MOF particles exhibit aggregation behaviour in the DMF–water solvent system with the emission further increasing up to 75% for the increase in the water fraction (f_w) from 0 to 60%; hydrophobic aggregation of particles evidently leads to a change in the conformation of the linker and particle aggregation-enhanced emission (AEE). De-aggregation of particles ensues for f_w = 70–90%, as reflected by a gradual decrease in the emission intensity. It is shown that the suspension of ground In-MOF particles in water permits sensing of metal ions, in particular Al³⁺ ions, by fluorescence quenching with detection at a sub-ppb level. The observed results comprise first demonstration of both mechanoluminescence and AEE of MOF particles.