Regiochemical Effects on Mechanophore Activation in Bulk Materials

Abstract

Mechanochromic force probes, including spiropyran derivatives, have proven to be useful in visualizing the stress/strain distribution and fracture behavior in polymeric materials. Here, we report the macroscopic response of silicone elastomers including cross-links made up of three spiropyran (SP) regioisomers. The SP derivatives SP­(o), SP­(m), and SP­(p) are connected to the network through an identical attachment point on the indoline fragment and regioisomeric attachments ortho, meta, and para to the spirocyclic C–O bond on the benzaldehyde fragment, respectively. The relative colorimetric response of these regioisomers under quasi-static uniaxial tensile load is SP­(o) > SP­(m) > SP­(p), consistent with the expected mechanical sensitivity of the regioisomers obtained from molecular modeling. The extrapolated strain onset for detectable activation of all three regioisomers, however, is indistinguishable and occurs at ∼90% uniaxial strain. Finally, the ratiometric response of the three isomers is constant across the strains investigated (90–135% uniaxial strain), in contrast to expectations based on simulations of strained intact polymer networks.

Rotaxane-Based Mechanophores Enable Polymers with Mechanically Switchable White Photoluminescence

“…We note that the fact that the mechanoresponse of the three polymers is comparable does not necessarily reflect that the molecular activation energies of the three rotaxanes are indeed the same, as mechanophores with different molecular response may result in very similar macroscopic responses when incorporated into polymers. 22,91 However, the data unambiguously demonstrate that our molecular design strategy based on the shuttling function of rotaxane structures is robust and versatile.…”

“…First, the activation requires a relatively high activation energy, as the process involves the cleavage of covalent chemical bonds that are supposed to be stable in the absence of mechanical force. 4,11,16,21,22 Second, the process is usually irreversible, 5,7,10 or the return reaction requires activation energy or is slow, 9,15−17 which results in limited reversibility. Third, the mechanical stimuli-induced cleavage reaction can typically also be caused by thermal treatment or light irradiation, which makes the process unspecific.…”

Cephalopod‐Inspired High Dynamic Range Mechano‐Imaging in Polymeric Materials

“…Separately, spiropyran cross‐linker is incorporated in a PDMS elastomer (see Experimental Section and Supporting Information); the spiropyran is functionalized with two alkene groups, which allow it to be covalently coupled into the PDMS network via hydrosilylation. [ 33–36 ] The photonic film is then positioned on top of the cured spiropyran‐PDMS. To reduce diffuse scattering from the photonic film which makes the color appear pale, a separately cured film of PDMS containing 5 wt% carbon black nanoparticles is placed on top of the photonic film; alternatively, 0.5 wt% carbon black nanoparticles can be added to the PEGPEA/silica pre‐polymer mixture prior to photo‐curing.…”

Force-Modulated Reductive Elimination from Platinum(II) Diaryl Complexes

“…Over the last decade or so, coupled mechanical forces have been used to drive a range of targeted covalent responses in isolated polymers and in bulk polymeric materials (covalent polymer mechanochemistry). 1,2 Mechanochemical strategies continue to evolve, including in very recent years their use in biasing and probing reaction pathways, 3,4 the release of small molecules and protons, 5,6 stress reporting, [7][8][9][10][11] stress strengthening, 12,13 degradable polymers, 14,15 and fundamental studies of polymer behavior under load. 16 In organic reactions, mechanochemical coupling has been investigated in simple bond dissociation reactions [17][18][19] and in a wide variety of reaction classes with respect to regiochemistry, [20][21][22][23][24] orbital symmetry, 20,[25][26][27] stereochemistry, 28,29 supramolecular architecture, [30][31][32] dynamic effects, 33,34 and the alignment and/or loading of scissile bonds with applied tension.…”