Nano vs. bulk: surfactant-controlled photophysical and morphological features of luminescent lanthanide MOFs

Abstract

A surfactant-assisted bottom-up synthesis route provides access to nanoscale metal–organic frameworks (nMOFs) that exploit size-dependent property advantages. This includes an increased surface-to-volume ratio, improved dispersibility and superior morphological properties by a narrow size distribution compared to the bulk analogues. Photophysical properties such as photoluminescence are also influenced by particle size, surfactant components and post-synthetic modification. The series of the related organic linkers (H₂bdc, H₂bpdc and H₂bpydc) together with trivalent lanthanides (Ln³⁺ = Eu³⁺, Tb³⁺) in synthesis and post-synthetic modification together with surface-active agents (CTAB, PVP) were used to assemble luminescent nMOFs for three archetype MOFs: nLn³⁺-bdc, nDUT-5:Ln³⁺ and nMOF-253:Ln³⁺. Both, dispersibility and morphology benefit from the CTAB- and PVP-controlled bottom-up particle downsizing down to 35 nm. DLS confirms homogeneous, narrow particle size distributions down to ±5 nm, which is 21-times smaller than the bulk analogues. Moreover, excellent luminescence QYs of up to 78.1(3)% were determined for the Ln³⁺-containing nMOFs. Successful post-synthetic modification with trivalent lanthanide ions of nMOFs was accomplished showing an improved photoluminescence sensitization effect compared to the bulk MOFs and exhibiting increased Ln³⁺-to-linker emission intensity ratios. The amount and photophysical properties of surfactants encapsulating the nMOFs were further quantified by DTA/TG-MS and UV-Vis-DRS. Finally, this work aims to elaborate thoroughly on the previously mentioned properties of nMOFs by comparison with their bulk analogues. Since surfactants play a key role in this synthesis route, the pros and cons of this approach were also assessed concerning several nMOF features.