The Role of Chromogenic Aryl Group Orientation in Regioisomeric Oxazine Mechanochromism

The molecular optical response can be rationally controlled through external mechanical force by covalently attaching chromophores to stretchable polymers, which results in the mechanochromism technique. Among the various chromophores, oxazine-based chromophores have been attractive candidates due to their rapid and reversible mechanoresponsive properties. The design of such mechanochromic systems primarily focused on suitable mechanophores and linkages. In this work, we employed the quantum chemical method at the first-principles level to locate the ring-opening conditions, which is essential for mechano-induced optical response in the oxazine regioisomers. The theoretical results reveal that, for the oxazine regioisomer with the linkage attached at the 5-position, the conformation of the side chain determines whether the oxazine ring opens as designed. By qualitatively characterizing the changes in individual chemical bonds via Laplacian bond order analysis during this process, we find that the side chain plays a key role by altering the bond order of the target C–O bond in the oxazine ring – and thereby its response to external force – through conjugative effects. The theoretical calculations further suggest that, in oxazine-based mechanochromic systems, in-situ structural changes under applied force can be monitored by infrared spectroscopy. These findings provide an alternative route for the mechanical activation of polymer materials and shed light on the design of next-generation, more efficient mechanoresponsive materials.