Abstract

In its most general sense the term, piezochromism, is defined as the change in color of a solid under compression.

There are three aspects of the phenomenon: The color of a solid results from the absorption of light in selected regions of the visible spectrum by promotion of an electron from the ground electronic state to a higher level. If the two electronic energy levels are perturbed differently by pressure, compression results in a color change. This is the basic definition of pressure tuning spectroscopy.

A discontinuous change of color occurs when a crystalline solid undergoes a first-order phase transition from one crystal structure to another.

The phenomenon of most interest is change in color of a solid as a result of a change in the molecular geometry of the molecules that make up the solid.

The main piezochromism systems are organic molecules in crystals and polymer films (anils, conjugated polymers, polysilenes, heterocyclic compounds, betaine derivatives), coordination compounds and metal cluster compounds, and organometallic complexes of copper (II). Their behavior and physico–chemical properties are discussed.