Chromism

Chromism is the property of a material to change color in response to an external stimulus. The color change can be reversible or irreversible and the phenomenon encompasses several subtypes, including photochromism, thermochromism, electrochromism, mechanochromism, and solvatochromism, among others. The color arises from changes in light absorption due to alterations in electronic structure, conjugation, oxidation state, protonation, or lattice changes.

In molecules, absorption shifts result from changes in electronic transitions (such as pi to pi* or n to pi*) caused by protonation, oxidation, or photoinduced isomerization. In solids, color can arise from charge-transfer interactions, metal–ligand effects, or phase transitions (for example, vanadium dioxide undergoes a metal–insulator transition that affects color). Reversibility is common but not universal; irreversible changes occur when products are formed that do not revert to the original state.

Major forms with examples include: photochromism, where materials such as spiropyrans and diarylethenes change color under light; thermochromism, as seen in VO2, which changes color with temperature; electrochromism, used in WO3-based smart windows, where color depends on applied voltage; mechanochromism, where mechanical deformation (grinding, pressure) alters color; solvatochromism, where solvent polarity shifts color in dyes; and acid–base chromism, exemplified by pH indicators like litmus or phenolphthalein. Some materials exhibit multiple chromic effects.

Applications encompass chemical sensing, smart coatings and displays, optical data storage, and anti-counterfeiting inks. Limitations include fatigue from repeated cycling, bleaching, and sensitivity to environmental conditions.