Force-induced fluorescence spectrum shift of spiropyran-based polymer for mechano-response sensing

As spiropyran exhibits mechanophore response, i.e., becomes vibrantly fluorescent under an applied force, its change has been measured primarily by fluorescence intensity. However, fluorescence intensity, which is an extensive property, is strongly affected by specimen and set-up parameters, and the corresponding measurements therefore require laborious and assumptive corrections. Herein, spiropyran is covalently linked to poly(dimethylsiloxane) and an in-situ opto-mechanical set-up is developed to monitor fluorescence spectrum evolution under deformation. Peak wavelength, which is an intensive property, is shown to be a superior mechano-response indicator that offers high accuracy/reliability, allows one to establish a 1:1 correspondence to strain (or stress) without corrections, and is independent of size. To the best of our knowledge, this is the first report of peak wavelength as mechano-response indicator for SP and its first systematic in-situ study. In summary, peak wavelength is shown to successfully reflect the force applied to polymer specimens without the need for corrections and exhibit considerable resistance to cycling-induced fatigue, while analysis of the finite extensibility limit reveals that the change of peak wavelength is closely related to the forces exerted directly on the polymer chain.