The phenomenon of photochromism is the property of a chemical compound to reversibly change its visible light absorption spectrum from a first color to a second color when exposed to an energy radiation of a certain wavelength or wavelength range.  The energy radiation, as well as the changes in the absorption spectra, is usually in the ultraviolet, visible, or infrared regions.  Sometimes, the change in one direction is thermally induced. 
The technical and commercial interest in photochromic compounds and compositions including photochromic compounds have increased considerably.  Numerous photochromic compounds have been synthesized for ophthalmic lenses, optical data storage, sunglasses, copying devices, security marks on documents, and holography. 
Photochromic phenomena are observed in both organic compounds, such as anils, disulfoxides, hydrazones, oxazones, semicarbazones, stilbene derivatives, o-nitrobenzyl derivatives, spiro compounds, and in inorganic compounds, such as metal oxides, alkaline earth metal sulfides, titanates, mercury compounds, copper compounds, minerals, and transition metal compounds such as carbonyls. 
Unfortunately, to date, photochromic compounds found only relatively limited usefulness for printing purposes.  This seems to be due to the fact that photochromic compounds are generally highly reactive and normally undergo chemical and photochemical changes in the complex medium of printing inks. 
There is a need for printing inks useful for printing documents to carry marks normally invisible to the human eye but which become visible after exposure to an activating radiation, especially, for example, for producing security documents to detect or prevent forgery or counterfeiting; producing special patterns, text or pictures; for producing security printing of packages and labels; or for using prints as sensing “devices” for detecting product overexposure due to undesired conditions (e.g., over accepted threshold temperature or sunlight). 
There remains a need for a printing ink that reversibly changes color when exposed to UV light such as sunlight or UV black light, and methods of producing such ink compositions.