Stimuli-chromism of photoswitches in smart polymers: Recent advances and applications as chemosensors

Light-responsive polymers, especially photochromic ones, are among the most interesting classes of stimuli-responsive materials. Photochromic polymers can show different stimuli-chromic properties depending on the applied stimuli, chemical structure, and the surrounding media. Such polymers are commonly prepared by incorporation of photochromic compounds in polymer matrices to exhibit stimuli-chromic behavior. Photochromic compounds used in advanced materials include spiropyran, spiroxazine, azobenzene, and diarylethenes, while the recent studies have focused on their response to various stimulants. Stimulants may be classified into three types, physical, chemical, or biochemical, according to their effects on stimuli-chromic materials. Stimuli-chromic polymers that change color on stimulation may responds to light (photochromism), pH (acidochromism and basochromism), temperature (thermochromism), stress (mechanochromism), electrical potential (electrochromism), gelation (gelochromism), solvation (solvatochromism), pressure (piezochromism), water (hydrochromism), and also two or more stimulants (multi-stimuli-chromism). Chemosensors based on stimuli-chromic materials have received considerable attention as they may be used to sense a wide range of chemicals and targets, such as polarity, pH, temperature, positive ions, negative ions, stress, pressure, water, and also biomolecules. Studies focused on stimuli-responsive polymers based on photochromic compounds and different stimuli-chromic phenomena are comprehensively reviewed in the following. Effects of a photochromic compound on its polymeric substrate, matrix or carrier, and the converse effects are carefully investigated to further understanding of different stimuli-chromism phenomena. In addition, the most frequently used synthesis methods to prepare photochromic compounds and their corresponding stimuli-chromic polymers are investigated in detail. Photochromic properties may considerably be influenced by interactions of a photochromic compound with its surrounding media. Incorporation of photochromophores to the polymeric or non-polymeric matrices without decrease of photochromic properties, photofatigue resistant, photoswitchability, and photostability, in addition to removal of negative photochromism are the most significant challenges. This review presents new strategies to overcome these problems and new applications as optical and non-optical chemosensors in the future for a large family of photochromic compounds and its related stimuli-chromic polymers.