Cryptochromes are the colorless forms of some color-changing dyes. Cryptochromes produce color in response to pressure, temperature, or acid, and when these physicochemical conditions change, the resulting color is able to fade. Cryptochromic dyes are mainly used in the manufacture of facsimile paper, carbonless copy paper, etc. They are an important functional material. The important feature of occult dyes is that the color of the dye changes, and color development and fading can be achieved by a variety of conditions, so that occultants can be used in many high-tech applications.
The mechanism of color change of leuco dyes is that the π-conjugated system of dyes is expanded by controlling chemical and physical conditions, the molecules absorb in the visible region and the dyes are converted from colorless to vivid colors. Such π-conjugated systems are reactive and are particularly susceptible to reduction, oxidation and hydrolysis reactions. When the π-conjugation is interrupted, the conversion from colored to colorless type occurs.
The color development system of leuco dyes consists of a color-emitting agent (or color-forming agent) and a color-developing agent, as well as a medium (or solvent). Commonly used chromogenic agents include crystalline violet lactone, thermosensitive green, green pigment, etc., which act as electron donor in the reaction; commonly used chromogenic agents include acid white clay, bisphenol A, etc. For example, crystalline violet lactone is a color-changing system with recessive dyes and bisphenol A is a color developer. When the temperature rises to a certain level, bisphenol A emits protons and the crystalline violet lactone ring opens to show color.
There are seven main categories of widely used recessive dyes, which are described as follows:
（1）Phthalide Dyes This type of dye is widely used as a colorant in pressure-sensitive material copy paper and thermal recording paper. The representative compound is 3,3-bis(4-dimethylaminophenyl)-6-dimethylaminophthalide, commonly known as crystalline violet lactone (CVL), which is one of the most produced and representative pressure/thermal dyes in the world.CVL emits a vivid blue color and is the main colorant for common printed blue paper.
（2）Fluorene dyes Fluorene dyes, as the third generation of pressure/thermal dyes, are considered to be the most promising recessive dyes. Different types of groups can be attached to the parent ring of fluorescent alkane dyes to produce various colors of fluorescent alkanes. Fluorescent alkanes are the most used recessive dyes because of their sensitive color development, high intensity and stability in the color development reaction. Such compounds are mainly used in pressure/thermal recording paper, and there are many patent reports on these chemicals. One of the most used fluorescent alkane dyes is ODB-1, which can produce black color alone.
（3）Triarylmethane Dyes The triarylmethane dyes have high application value and most of these dyes are used in the manufacture of various types of reproduction paper. Triarylmethane dyes have strong dyeing ability and bright colors. However, they have poor light resistance and are easily discolored by acids and alkalis.
（4）Phenothiazines, Thiazines and Oxazines The discovery of this class of dyes allowed for a wider range of colors and shades in the original dye field.
（5）Quinones Few studies have been conducted on these cryptochromes. The most widespread application of quinone cryptochromes is fiber dyeing.
（6）Tetrazolium salts Tetrazolium salts are a very important and special class of occult dyes.
（7）Spirocyclopyrans This class of compounds is widely used in optical storage and decorative materials, etc.