Stimuli-Responsive Photoacid Polyelectrolytes

Spiropyran-based polymers are widely studied on account of their multifaceted stimuli-responsive behaviors. However, their photoacid characteristics in aqueous environments remain overlooked. Here, we synthesized a series of poly(3-sulfopropyl methacrylate) copolymers containing spiropyran units in their photoacidic, protonated merocyanine form. We studied the behavior of these copolymer polyelectrolytes in water and found that polymer constitution significantly enhances the hydrolytic stability of protonated merocyanines, whereas polymer composition can be used as a tool to program their photoacidity - i.e., the acidity difference between their ground state and metastable state. Exposure to 500 nm light enables these polyelectrolytes to reversibly dissociate protons, causing metastable fluctuations in bulk pH as large as 3 pH units. The results presented here hold promise in the development of robust light-to-protonic energy conversion devices.