Mechanochemical activation of a metal-organic framework embedded within a thermoplastic polyurethane matrix : probing fluorogenic stress-sensing

Abstract

Here, a mechanochemically triggered Cu(I) bis(N-heterocyclic carbene [NHC))-based metal–organic framwork (MOF) embedded into a thermoplastic polyurethane (TPU)-matrix is reported. The induced fluorogenic copper-catalyzed azide–alkyne cycloaddition (CuAAC) reveals the stressed parts within the thermoplastic PU via a simple optical detection. As determined via oscillating tensile rheology on dumbbell-shaped samples of TPU, a MOF, containing 4.66% copper, catalyzes the fluorogenic CuAAC between the nonfluorescent precursor dyes, 8-azidonaphthalen-2-ol, and 3-hydroxyphenylacetylene. After mechanical activation of the MOF situated inside the TPU, the fluorescent 8-(4-(3-hydroxyphenyl)-1,2,3-triazol-1-yl)naphthalen-2-ol dye is formed. Monitoring the formation of the dye inside the TPU via fluorescence spectrometry at λex = 458 nm shows an increase of the fluorescence intensity up to 60–70%. It is demonstrated that a dumbbell-shaped TPU, subjected to higher stress, displays higher fluorescence than the surrounding other areas, thus effectively functioning as a three-in-one stress-sensor system.