Saving paper with switchable ink

An oxazine auxochrome and a carbazole chromophore can be integrated within the same covalent skeleton to generate a halochromic molecular switch. Upon addition of acid, the oxazine ring opens to bring the carbazole fragment in conjugation with a 3H-indolium cation. This structural transformation shifts the main absorption of the carbazole chromophore from the ultraviolet to the visible region and, as a result, is accompanied by the appearance of an intense red color. This species can be formulated into an ink to print colored patterns on conventional paper. Upon treatment with base, however, the oxazine ring closes to restore the initial colorless state and erase the printed pattern. In fact, the very same sheet of paper can be recycled for multiple printing and erasing steps. Thus, this structural design for switchable inks can evolve into viable operating principles to enable innovative printing technologies and reduce drastically paper consumption.