Spiropyran\nPhotoisomerization Dynamics in Multiresponsive\nHydrogels

Light-responsive,\nspiropyran-functionalized hydrogels have been\nused to create reversibly photoactuated structures for applications\nranging from microfluidics to nonlinear optics. Tailoring a spiropyran-functionalized\nhydrogel system for a particular application requires an understanding\nof how co-monomer composition affects the switching dynamics of the\nspiropyran chromophore. Such gels are frequently designed to be responsive\nto different stimuli such as light, temperature, and pH. The coupling\nof these influences can significantly alter spiropyran behavior in\nways not currently well understood. To better understand the influence\nof responsive co-monomers on the spiropyran isomerization dynamics,\nwe use UV–vis spectroscopy and time-dependent fluorescence\nintensity measurements to study spiropyran-modified hydrogels polymerized\nfrom four common hydrogel precursors of different pH and temperature\nresponsivity: acrylamide, acrylic acid, <i>N-</i>isopropylacrylamide,\nand 2-(dimethylamino)­ethyl methacrylate. In acidic and neutral gels,\nwe observe unusual nonmonotonic, triexponential fluorescence dynamics\nunder 405 nm irradiation that cannot be explicated by either the established\nspiropyran–merocyanine interconversion model or hydrolysis.\nTo explain these results, we introduce an analytical model of spiropyran\ninterconversions that includes H-aggregated merocyanine and its light-triggered\ndisaggregation under 405 nm irradiation. This model provides an excellent\nfit to the observed fluorescence dynamics and elucidates exactly how\ncreating an acidic internal gel environment promotes the fast and\ncomplete conversion of the hydrophilic merocyanine speciesto the hydrophobic\nspiropyran form, which is desired in most light-sensitive hydrogel\nactuators. This can be achieved by incorporating acrylic acid monomers\nand by minimizing the aggregate concentration. Beyond spiropyran-functionalized\ngel actuators, these conclusions are particularly critical for nonlinear\noptical computing applications.