Force-Dependent Multicolor Mechanochromism from a Single Mechanophore

We report a bis-naphthopyran mechanophore that exhibits force-dependent changes in visible absorption. A series of polymers incorporating a chain centered bis-naphthopyran mechanophore was synthesized and mechanically activated using ultrasonication. By varying the length of the polymer chains, the force delivered to the mechanophore is modulated systematically. We demonstrate that the relative distribution of two distinctly colored merocyanine products is altered predictably with different magnitudes of applied force, resulting in gradient multicolor mechanochromism. The mechanochemical reactivity of bis-naphthopyran is supported by density functional theory calculations and described by a theoretical model that provides insight into the force color relationship.