Dihydroanthracene bridged bis-naphthopyrans: A multimodal chromophore with mechano- and photo-chromic properties

Abstract

Mechanochromophores based on bichromic molecular switches, such as bis-naphthopyanes, allow multimodal mechanochromic behavior beyond the typical binary response from single chromophores, which is important for distinguishing between multiple stress states through discrete changes in color. Spontaneously generated persistent and distinguishable multi-colors from activated bis-naphthopyanes remain challenging. And the versatility of bis-mechanophore design for advanced optical molecular systems and the fundamental insights into the corresponding mechano-reactivity are not enough. Here, we identify a dihydroanthracene bridged bis-naphthopyrans as a multimodal mechanochromophore in polymers. Bridging two pyrans with the sterically constrained dihydroanthracene is helpful to control the steric effect for the favorable formation of a distinctly appreciable bis-merocyanine (bis-MC) product. By varying the length of the polymer chains, the force delivered to the mechanophore is modulated, resulting in a gradient change in the relative distribution of two distinctly colored MC products and a multicolor mechanochromism. Mechanical activation of this bis-naphthopyanes proceeds via a mechanistically distinct pathway compared to the photochemical process. In addition, the bulk films can also achieve pronounced color changes when subjected to mechanical force. This study thus further expands the molecular diversity of mechanochromophores and tune the multimodal switch properties of bis-naphthopyrans based polymers.