Harnessing Negative Photochromism in Styryl Cyanines for Light-Modulated Proton Transport

Efficient photoswitches capable of complete conversion to their metastable isomer are not so common, yet highly desirable for applications in smart materials and devices. Here, we report the photochromic behavior of a series of styryl cyanine photoswitches, all demonstrating high switching efficiency, with some achieving full conversion to the metastable form. Despite structural similarities to spiropyran photoswitches, we demonstrate that the photochemistry of styryl cyanines is fundamentally different. Unlike classical photoswitches that rely on double-bond rotation, these molecules undergo substantial geometric changes via the formation of a spiro carbon. This transformation disrupts conjugation, causing a desirable blue shift in absorbance ideal for creating responsive materials and devices. We further show that the switching kinetics can be finely tuned through electronic effects of various substituents or the choice of surrounding medium. These photoswitches exhibit excellent fatigue resistance and can be easily shifted into the visible region via their acidochromic properties. Taking advantage of their high switching efficiency and affinity for acidic polymers, we finally propose their use as smart dopants to develop light-responsive materials with tunable proton transport properties under visible light irradiation.