Isomer Effect of Double-Spiro Mechanophores in Polymer Mechanochemistry

Abstract

Easy construction of rhodamine spiro rings and diversity of naphthalenediol enable various isomers of the naphthalene-fused double rhodamine structure and provide the possibility to study the structure–performance relationship of double-spiro mechanophores. Herein, we for the first time report the isomer effect of double-spiro mechanophores in polymer mechanochemistry. Three double rhodamine structures (BiRho-1, BiRho-2, BiRho-3) fused with different naphthalenediols were designed and synthesized. Intriguingly, three mechanophores showed their own unique characteristics of photochromism and mechanochromism in polyurethane elastomers. BiRho-1 and BiRho-2 undergo totally different ring-opening mechanisms for mechanochromism and photochromism. BiRho-1 undergoes simultaneous opening of both spiro rings during mechanochromism, whereas under UV irradiation, the second spiro ring undergoes only limited opening upon prolonged irradiation. For BiRho-2, only a single spiro ring was opened under UV irradiation, but double spiro rings ruptured simultaneously under pressure. Whether under pressure or UV irradiation, BiRho-3 mainly underwent single-spiro-ring opening, and its OO state was not readily generated under the examined conditions. Moreover, a multiple-network elastomer was fabricated for BiRho-3, but its OO state still cannot be detected upon compression despite the pre-stretching effect. CoGEF simulation confirmed that the first ring-opening and the second ring-opening of BiRho-1 and BiRho-2 required forces of approximately the same magnitude, but for BiRho-3, the second ring-opening demanded a much larger force than the first one. The recovery processes of these double-spiro mechanophores were also studied, showing different ring-closing processes. This work gives a clear picture of the structure–performance of double-spiro mechanophores in polymer mechanochemistry.