Spiropyran-based advanced photoswitchable materials: A fascinating pathway to the future stimuli-responsive devices

Recent years have witnessed tremendous progress and developments of the photoswitchable spiropyran-based polymers, owing to distinctive and particular physicochemical properties of their isomers upon a variety of triggers, and especially light illumination. Light is a fascinating and green stimulus because of its remote control, micron- or submicron-sized focusing area with controllable wavelength and energy, non-invasiveness and non-destructive nature, precisely controlled direction, and availability. In this review, we have emphasized on and summarized the most recent observations and efforts in the progress of photoswitchable spiropyran-based materials and their applications as sensors for heavy metal cations, anions, pH, acid and base vapors, wettability and humidity. Other items include data recording and anticounterfeiting devices, photorheological fluids, optically reversible switching membranes, photoregulating surface plasmon resonance, photomodulation of ion conductivity and mechanoresponsive polymers. The bio-based field is another interesting subject that is discussed here and consists of reversible cell sheet engineering, photodynamic therapy, switchable fluorescence labeling, controlled drug delivery and biological ion channels. On the other hand, limited light penetration inside the living tissues and hazards of high-energy ultraviolet irradiation for initiating photochemical transformations have limited the use of such light-controlled systems in medicinal and therapeutic means. Those spiropyran-based materials which are susceptible to being triggered by low energy near IR (NIR) two-photon light irradiation and upconversion nanoparticles are recently under serious explorations and have been reviewed as a new perspective for their advanced applications.