Fast, reversible mechanochromism of regioisomeric oxazine mechanophores: Developing in situ responsive force probes for polymeric materials

To address hysteretic or irreversible mechanochromism found in most bond-scission-based mechanophores, we developed a new family of mechanophores based on the oxazine (OX) structural motif. Three OX regioisomers differing in their point of attachment to the indole ring show variable mechanochromic mechanoresponsiveness in a polydimethylsiloxane (PDMS) matrix. Constrained DFT simulations correlate the experimental findings to molecular scale events; two of the three regioisomers exhibit a force-activated rearrangement, while the third one is mechanochemically inactive. Most significantly, when compared with the spiropyran (SP) and the naphthopyran (NP), the OX mechanophores exhibit faster and reversible mechanochromism without any sign of fatigue. Such a rapid response to mechanical loads is rationalized by the difference between SP and OX scaffolds, given that the SP ring opening is accompanied by a trans-cis isomerization step. These fast-responsive mechanophores are anticipated to bring deeper understanding to a broad range of soft materials by in situ monitoring the local mechanics.