Chromic materials are defined as molecules, dyes, and pigments that exhibit a distinct color change when exposed to an external stimulus, especially when the change is reversible and controllable. The most significant chromic materials could be solvatochromic; photochromic; halochromic; thermochromic; electrochromic; and ionochromic, with the number of this phenomena possible to be increased further.

To describe the technological application of color-changing materials via a variety of various phenomena causing color-change, such as vibrations and simple excitations; ligand field effects; transition between molecular orbitals; transition between energy bands; geometrical and physical optics; photochemistry; and conformational changes, the following processes should be understood clearly: Reversible color change; absorption/emission/reflection of light; energy transfer; and manipulation of light.

In recent years, the development of smart chromic materials with various ‘self-functioning’ capabilities have been opened up increasingly with possibilities of innovative and technological applications, especially in textiles, glasses and lenses. They are engineered to sense and respond to external environmental conditions and stimuli. In this manner, chemical processes and structural manipulations taking place enables them to gain new functions.

As an example, solvent sensitive chromic materials show color-change by reacting with pure water, acid/base and various solutions. Reversibility in these systems can be provided by heating and adding selected types of compounds. These types of materials offer significant potential for developing new sensor systems, rewritable papers, and inkless printing due to their unique color-changing properties and characterizability. This phenomenon is especially governed by the physical and chemical properties of solvents.

Mechanochromic materials, meanwhile, change color through stress such as, high pressure; hydrostatic pressure; and sonification, etc. The reversible color-switching property associated with mechanochromic compounds have been widely used in stress detection; failure monitoring and prevention; and biomedical and healthcare applications. The solid-state mechanochromism property allows the materials to be applied in rewritable paper technology. Based on their response to mechanical stimuli, the mechanochromism process in materials can be explained by: rearrangement of bonds; formation or breaking of dye aggregates; conformational changes; phase transition from crystalline to amorphous; or between two crystals and variations in photonic path lengths.

Discovering a diverse range of chromic materials that offer customizable color changes based on specific requirements is a hot research topic, with the improvement of known chromic materials, synthesizing new functional materials for color-based technological applications and researching new phenomena that produce color is popular today. For this purpose, it is necessary to understand the structural behavior of materials in environments where they acquire chromic properties and the processes which controls the phenomena. Synthesizing a new chromic material, discovering a new chromic phenomenon and determining the processes that define the origin of a chromic phenomenon are the most important goals of this Research Topic.

This Research Topic aims to collect the results of cutting-edge research in the exciting scientific field of chromic materials and phenomena. Original and innovative research and review articles on the development, characterization and application of chromic materials are most welcome in this Research Topic.

This Research Topic welcomes, but is not limited to, the following themes:

• Chromism

• Color changing materials

• Structural/conformational changes

• Smart chromic materials

• Photochemistry of chromic materials

• Spectroscopy of chromic materials

• Design and synthesis of new species of chromic materials

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Frontiers in Chemistry

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