Porphyrin based metal-organic frameworks with record sensitivity in optical oxygen sensing

03 August 2021, Version 1
This content is a preprint and has not undergone peer review at the time of posting.

Abstract

The optical oxygen sensing capabilities of the porphyrin-based metal-organic frameworks, PCN-224, Pt(II)PCN-224 and Pd(II)PCN-224 were investigated. The bimolecular quenching constants (kq) of 37000 (PCN-224), 6700 (Pd(II)PCN-224) and 3900 Pa-1s-1 (Pt(II)PCN-224) were found and reveal an exceptionally high oxygen-permeability for these materials. A fast gas transport within the network, large pore sizes, electronic and spatial isolation of the porphyrin indicator in the framework are held responsible for the unprecedentedly high kq values. PCN-224 shows 6.7 ns fluorescence lifetime and the fluorescence in air is quenched by 4.2-fold. The metal-organic frameworks based on phosphorescent Pt(II) and Pd(II) porphyrins possess significantly longer decay times of 18.6 and 390 µs, respectively, and are suited to detect oxygen in trace and ultra-trace ranges with limits of detection of 1 and 0.015 Pa, respectively. Apart from free-standing crystals, also metal-organic frameworks supported on different fibrous substrates (poly(acrylonitrile) nanofibers, glass fibres), and flat substrates (TLC silica-gel, poly(amide) filter) were prepared in order to provide oxygen sensor materials of practical use. Electrospun and thermally treated poly(acrylonitrile) nanofibers were proven to be particularly favourable and the resulting composite material exhibited the same sensitivity as the free crystals. All sensing materials show reversible cross-talk to humidity at levels up to 53 % relative humidity but demonstrate a drastic decrease of oxygen sensitivity at high humidity levels and when exposed to water.

Keywords

metal-organic frameworks
luminescence
platinum
palladium
porphyrin

Supplementary materials

Title
Description
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Title
Supporting information for porphyrin based metal-organic frameworks with record sensitivity in optical oxygen sensing
Description
The supporting information contains preparations of the precursors, a detailed description of the PXRD experiments and results, SEM images of free crystals and additional SEM images of the immobilized MOFs, PXRD data for PCN-224 grown on PAN-fibres, fit parameters for the Stern-Volmer plots obtained for different samples of PCN-224-based MOFs and Stern-Volmer plots of reference measurements.
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