Editorial: Photochromic Materials: Design and Applications

Abstract

Materials able to respond to external stimuli have garnered interest in a variety of research areas. Ordinarily referred to as “smart materials”, these sophisticated structures are capable of distinguishing and adjusting their responses to different triggers (Schnurbus et al., 2020; Qian et al., 2021). In this context, the chromic materials have been considerably focused for designing devices for multiple applications. Photochromism can be described as a reversible conversion of a chemical species between two isomeric forms caused by electromagnetic radiation absorption, resulting in a color change (Reis et al., 2020). Figure 1A shows the evolution of publications over the years considering the keyword photochromism on the Scopus website. Based on the recorded data, Figure 1B shows that the two areas with the highest number of publications include chemistry and materials science. Light is one of the most attractive external stimuli to promote changes in chromic materials, since it is easy to control and non-destructive with high spatial and temporal precision (Li et al., 2020). In addition to color changing, some properties of the photochromic molecules and their materials may be tuned by light irradiation, such as electronic spectra, electrochemical properties, dipole interactions, and capability to coordinate. Photochromism can be observed in both organic and inorganic compounds, as well as their hybridmaterials. This responsive behavior to light irradiation can be found in somemetal oxides, alkaline earth, metal halides, and some transition metals (Prado et al., 2011). The promising organic classes of photochromic molecules include dithienylethene, spiropyran, and azobenzene (Zhang et al., 2013). Among these different classes of photochromic materials, the organic and its hybrid organic-inorganic types have been shown to be fatigue resistant with structural diversities. Applications of these materials include security markings, ions and solvents probing, optical shutters, photo-switchable molecular devices, topographical change materials, and optical memory storage systems (Klajn, 2014). In this Research Topic, great examples of photochromic materials are described.Raymo et al. described the synthesis and spectroscopic characterization of four molecular switches with ratiometric fluorescence response in their paper entitled Switchable Coumarins for Ratiometric pH Sensing. Based on their data, the equilibria between closed and open rings are achieved by trifluoroacetic acid increments in dyes solutions. These results from electronic spectra are a consequence of the structural transformation of the carbon atom at the junction of the indole and either the oxazine or the oxazolidine heterocycle, increasing the extended electronic delocalization. The emission spectra data for all molecules reveals an intense band between 454 and 480 nm for the closed ring forms, however, upon acidification this emission band decreases in intensity with the concomitant appearance and growth of a new band at 647–666 nm. The chemical equilibria Edited and reviewed by: Weihua Li, University of Wollongong, Australia