Molecular photoswitches enable the translation of light energy into predictable molecular changes and therefore offer precise control over the states of matter and its behavior at the smallest scales. The field of molecular photoswitch research has grown immensely in the last decades and in addition to more traditional motives like stilbenes, azobenzenes, or diarylethenes, new motives, e.g. hydrazones, Stenhouse dyes, imidazole-biradicals, or indigoids have been developed more recently. In addition to this increasing diversification, classical photoswitches like azobenzenes and diarylethenes have been subject to steep developments themselves and are found in virtually every application driven field of chemical research. With the advent of facile synthetic access to nonsymmetric diarylethenes new and exciting opportunities have emerged for structural-functional diversification and unique applications of this class of photoswitches, e.g. in catalysis, dynamic covalent chemistry, molecular logic, chemical biology, or materials research. To our surprise however, no investigations had been made to examine the possibility of forming and breaking CN bonds during the electrocyclic photoreaction in diarylethenes when we started this investigation. Just very recently, the group of Kobatake has published the reversible C-N single bond formation in a two-state aza-diarylethene photoswitch, similar to the C-C bond electrocyclization reactions observed in the parent compound. In this work we disseminate the reversible C=N double bond formation and breaking of pyrimidine-substituted three-state aza-diarylethene photoswitch, which proceeds via a concomitant thiophene-ring opening and formation of an aromatized zwitterionic metastable state. The metastable state is strongly colored and fully reverts back to the colorless open ring aza-diarylethene. Because of the zwitterionic character in the ground state, the closed ring isomer shows a rare negative solvatochromism with red-shifted absorption in apolar solvents and blue-shifted absorption in polar media. The thermal stability of the metastable state is highly sensitive to pH and can be modulated from a dissipative behavior to a highly thermally stable one. This very unusual photoswitching mechanism and the resulting facile pH control over the T-type photochromism will open up exciting realms of applications for pyrimidinediarylethenes in e.g. sensing, materials, or biological research in the future.