Asymmetric Double-Spiro Mechanophore in Single-Network and Double-Network Elastomers

Asymmetric spiro systems hold promise for eliciting richer stimulus-specific responses, warranting further investigation. In this study, we reported an asymmetric double-spiro mechanophore (named as Fluo-Rh) with the fusion combination of rhodamine and fluorescein. Theoretically, Fluo-Rh has four states of closed–closed (Fluo_c-Rh_c), open–closed (Fluo_o-Rh_c), closed–open (Fluo_c-Rh_o), and open–open (Fluo_o-Rh_o) due to its asymmetry. The mechanophore Fluo-Rh was covalently linked into single-network and double-network elastomers with the help of two hydroxyls, and their mechanochromic and photochromic properties were studied. Interestingly, Fluo-Rh underwent ring opening of the fluorescein spiro ring under force but suffered sequential ring opening of both spiro rings (fluorescein spiro ring first) under UV irradiation in the single-network PU. The Fluo_o-Rh_c state featured the fluorescence band at 562 nm and the absorption bands at 500 and 530 nm. The photoinduced sequential ring opening of Fluo-Rh was reflected by the fluorescence band shift from 555 to 581 nm and the appearance of the absorption band at 432 nm. Only the fluorescein part of the closed ring during heating. However, in the polyurethane-polymethacrylate interpenetrating network (IPN) polymer, only the fluorescein spiro ring can be opened under compression; under UV irradiation, only the rhodamine spiro ring can be opened, showing that the second network inhibits photoinduced ring opening of the fluorescein spiro ring owing to low free volume in IPN. The CoGEF simulation further supported force-induced single ring opening of the fluorescein ring. Compared with the symmetric double-spiro mechanophore, the asymmetric double-spiro mechanophore Fluo-Rh manifested a totally different ring-opening mechanism upon force or UV irradiation. We believe that this work gives deep insight into the double-spiro mechanophore and expands the scope of polymer mechanochemistry.