Anomalous photochromism and mechanochromism of a linear naphthopyran enabled by a polarizing dialkylamine substituent
In contrast to common angular naphthopyrans that exhibit strong photochromic and mechanochromic behavior, constitutionally isomeric linear naphthopyrans are typically not photochromic, due to the putative instability of the completely dearomatized merocyanine product. The photochemistry of linear naphthopyrans is thus relatively understudied compared to angular naphthopyrans, while the mechanochromism of linear naphthopyrans remains completely unexplored. Here we demonstrate that the incorporation of a polarizing dialkylamine substituent enables photochromic and mechanochromic behavior from polymers containing a novel linear naphthopyran motif. In solution phase experiments, a Lewis acid trap was necessary to observe accumulation of the merocyanine product upon photochemical and ultrasound-induced mechanochemical activation. However, the same linear naphthopyran molecule incorporated as a crosslinker in polydimethylsiloxane elastomers renders the materials photochromic and mechanochromic without the addition of any trapping agent. This study provides insights into the photochromic and mechanochromic reactivity of linear naphthopyrans that have conventionally been considered functionally inert, adding a new class of naphthopyran molecular switches to the repertoire of stimuli-responsive polymers.
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Financial support from an NSF CAREER award (CHE-2145791) and the Rose Hills Foundation is gratefully acknowledged. M. E. M. was supported by an NSF Graduate Research Fellowship (DGE-1745301) and a Barbara J. Burger Graduate Fellowship from Caltech. S. K. O. acknowledges support from an Institute Fellowship at Caltech. We thank the Center for Catalysis and Chemical Synthesis of the Beckman Institute at Caltech for access to equipment. M. J. R. gratefully acknowledges the Alfred P. Sloan Foundation for a Sloan Research Fellowship and the Camille and Henry Dreyfus Foundation for a Camille Dreyfus Teacher-Scholar Award.
M. E. M. conceptualized the project. Y. S., M. E. M., and M. J. R. designed the research. Y. S. performed the experiments. Y. S., M. E. M., S. K. O., R. W. B., and M. J. R. analyzed the data. Y. S. and M. J. R. wrote the manuscript. M. J. R. provided guidance during all stages of the project.
Conflict of Interest
There are no conflicts to declare.