Fluorene dyes are the third generation of pressure/thermal dyes. They have the advantages of high sensitivity, high color density, and complete chromatography, and are widely used in carbonless copy paper, temperature indicating materials, textile printing, and analytical testing. Fluorene dyes are colorless or light-colored powder, but in the presence of color developer, the lactone ring will be opened, the central carbon atom will be changed from sp3 hybridization to sp2 hybridization, the molecular co-planarity will be increased, the conjugation degree of the system will be increased, the formation of large π-bonds and the absorption will be red-shifted to reveal the color. Therefore, the choice of color developer is crucial, requiring good whiteness, fast color development, high color concentration, low price, easy manufacturing, and good water resistance.
So far, there are two major directions of research on fluorescent dyes. One direction is the synthesis of new fluorescent compounds and their intermediates, the core of which is to change the two substituents attached to the amino group on the parent ring of fluorescent dyes. The other main research direction is to develop new applications for fluorene dyes in new areas of research. For example, new attempts in the field of inks, inks and writing materials.
The earliest pressure-sensitive paper with black coloring was made by laminating green, red and yellow dyes, but due to the different color development speed and light resistance of various dyes, the hue changes during color development and preservation. Fluorescent dyes are capable of producing black color alone, and the recording materials made of black dyes have excellent electrostatic re-replication performance, and can be used for both pressure-sensitive paper and thermal paper, so fluorescent dyes are the most widely used, most diverse and most promising type of pressure/thermal color-changing dyes internationally.