Using light of longer wavelengths additional leads to the enrichment of the thermally much more stable E configured closed isomer. In the region between the isosbestic points of open 1 and closed Z-1 at 271 nm and 330 nm, the absorption of open 1 is stronger with greatest difference at about 300 nm in MeOH. This is key for selective addressability of open 1 with 300 nm in this three-state photoswitch. Stronger absorption for closed Z-1 is obtained beyond 330 nm, which allows addressing this isomer with suitable wavelengths of light >300 nm. At Compared to closed Z-1 isomer, a reduced absorption of closed E-1 between 400 and 550 nm is visible (Figure ). 1 H NMR irradiation experiments with 365 nm light at -40 °C in MeOD-d4 supported the UV/vis observations. At first, the closed Z-1 species is enriched, after which the light induced double bond isomerization to closed E-1 occurs (Figure ). Either prolonged irradiation with light of 340 nm or 365 nm at UV/vis concentrations (Figure ), or multiple T-type irradiation cycles of open 1 with light of 340 nm and 365 nm led to strong enrichment of closed E-1 (Figure ). Already after the first 365 nm irradiation step at UV/vis concentrations, the thermal recovery of open 1 at 23 °C was not complete anymore due to the accumulation of closed E-1. After eight light irradiation cycles of each 1 min, solely closed E-1 was present in solution, which is stable at ambient temperature in MeOH solution (Figure ). This observation is important, as aggregation of closed Z-1 in the presence of closed E-1 was observed at higher concentrations in NMR experiments, after a certain threshold of closed E-1 enrichment. Making use of the wavelength dependence of either closed Z-1 or closed E-1 enrichment, we first accumulated the closed Z-1 species to 94% with light of 300 nm at -78 °C in MeOD-d4, followed by irradiation with light of 365 nm at -40 °C leading to the enrichment of seemingly only closed E-1 (Figure ). However, after annealing the sample to 23 °C, a mixture containing 46% open 1 and 54% closed E-1 was obtained (Figure , spectrum to open 1 could be achieved for our first generation aza-diarylethene. To our knowledge, no reversible photoswitching mode with a reversible C=N bond is known so far. A related C-N single bond opening in a thiazole-based diarylethene has been reported just very recently by the Kobatake group. Therefore, we irradiated a sample containing 90% closed Z-1, 3% closed E-1, and 7% open 1 with light of 520 nm until the closed Z-1 species was fully consumed in MeOD-d4 solution. To avoid any interfering thermal isomerization processes, we conducted the experiment at -40 °C and -60 °C with in-situ irradiation within the NMR spectrometer. This experiment revealed photoisomerization reactions from closed Z-1 to open 1 and from closed Z-1 to closed E-1 (Figure ). The measured photoisomerization rate from closed Z-1 to open 1 (gradient: 0.24 % min -1 within the first 35 min of light irradiation) is dominant compared to that from closed Z-1 to closed E-1 (0.05 % min -1 ) at 520 nm irradiation under these conditions (Supporting Figure ). After a certain threshold of closed E-1 enrichment in solution is reached, a competing aggregation process of closed E-1 with closed Z-1 leads to an apparent speeding up of closed Z-1 depletion (0.28 % min -1 within 305 min of irradiation) and relative closed E-1 enrichment (0.17 % min -1 within 305 min of irradiation) in solution (Supporting The differentiation between the double bond configuration of the two photoproducts (closed Z-1 and closed E-1) can be obtained clearly from 1 H-1 H NOE/ROE experiments. Selective excitation of protons H11', and H9' of closed Z-1 were executed (for proton numbering see Figure and). ROE cross signals were obtained between H11'/H12' (2.5 Å), H11'/H9' (2.4 Å), and a very minor correlation between H9'/H15' (3.3 Å) (Figure , Supporting Figure and 13). The values in brackets depict the distances in Å as obtained from optimized geometries on the B3LYP-GD3BJ/6-311++G(d,p)-IEFPCM (methanol) level of theory, matching the experimental 1 H-1 H ROE correlations. There is no ROE correlation between H11' and H15' and only a very weak correlation between H9' and H15' as expected for the Z-configuration and in stark contrast to the correlations of the V-shaped closed E-1 structure. For closed E-1, selective excitation of protons H9'', H11'', H15'', and H21'' was performed, resulting in 1 H-1 H NOE correlations between H15''/H9'' (3.4 Å), H9''/H11'' (4.7 Å), H15''/H11'' (4.0 Å), H14''/H11'' (4.7 Å), H15''/H21'' (2.1 Å), and H20''/H21'' (4.4 Å), which prove Econfiguration of the photoisomerizable double bond (Figure , Supporting Figure ). The correlations of H21''' additionally evidence the light-induced build-up of the pyrido[1,2a]pyrimidin-5-ium scaffold. Values in brackets depict the distances in Å as obtained from the optimized geometries on the B3LYP-GD3BJ/6-311++G(d,p)-IEFPCM (methanol) level of theory, matching the experimental 1 H-1 H NOE correlations.