Open 1 possesses absorption in the UV range and therefore was irradiated with 285 nm or 365 nm light (Figure ). Unexpectedly, we did not observe the photoproduct of a classical 6-πelectrocyclic diarylethene reaction, which was recently reported to take place in thiazolediarylethene. Instead, irradiation leads to the electrocyclic formation of a positively charged aromatic pyrido[1,2-a]pyrimidin-5-ium ring in conjunction with reversible C-S bond breaking and ring opening of the adjacent thiophene. Overall, a zwitterionic thiolate species is obtained in up to 94% yield (Figure , 1c, 2c), which is colored and shows rare negative solvatochromism, typical for charged ground state species. This unusual C=N bond forming photoreaction was elucidated fully by a comprehensive UV/Vis and NMR analysis in solution (Supporting Information and Supporting Figures ) in conjunction with theoretical modeling at the TPSS0-D3BJ/def2-TZVPP-CPCM level of theory (see Supporting Information for details). Only one signal set of a closed 1 species is observed in the NMR experiments in MeOH solution, which matches well with the theoretically predicted and significantly more stable Z isomeric thiolate. It does not match at all with the theoretical predictions for the conventional 6π-electrocyclization product. In the following, key-observations of this analysis are highlighted. A 1 H-15 N HMBC NMR experiment directly established covalent bond connectivity between C8' of the former thiophene ring (C8) with one pyrimidine nitrogen atom via cross signals to the adjacent methyl protons. The observed aromatic chemical shift of the particular carbon C8' within the newly formed C=N bond corresponds to a charged iminium species, which is fully supported by the theoretical calculations. In case of a conventional 6πelectrocyclization product, the C8' would possess aminothiol character with a distinct aliphatic chemical shift, which is not observed. Further HMBC and NOE experiments as well as chemical shift comparisons between experiment and theory fully support the zwitterionic character and Z configuration of the closed 1 species (see Supporting Information for the full analysis).