Smart Organic Materials with Acidochromic Properties

Smart materials displaying changes in color and optical properties in response to acid stimuli are known as acidochromic materials. The recent progress and emerging trends in the field of smart organic materials with acidochromic properties, reported in the last seven years, are presented herein. The molecular design of acidochromic organic materials, the origin of the chromic and fluorochromic response to acid stimuli, and related mechanisms are also discussed. Materials and systems covered in the review are divided according to the presence of basic moiety undergoing reversible protonation/ deprotonation, such as pyridine, quinoline, quinoxaline, azole, amine derivatives, etc., in the molecules. Many donor-acceptor molecules displaying acidochromic behavior are cited. Alterations in visual color change and optical properties supporting acidochromism are discussed for each example. Mechanistic studies based on the theoretical calculations, single crystal X-ray diffraction analysis, and powder pattern diffraction analysis are also discussed here. The application of these acidochromic molecules as acid-base switches, sensor films, self-erasable and rewritable media, data security inks, data encryption, molecular logic gates, etc., are also reported. Thus, this review article aims at giving an insight into the design, characterization, mechanism, and applications of organic acidochromic materials, which will guide the researchers in designing and fine-tuning new acidochromic materials for desired applications.