After elucidation of the photochemistry, the thermal behavior of the two photoproducts closed Z-1 and closed E-1 was investigated in more detail. In closed Z-1 the highly nucleophilic thiolate sulfur is in close proximity to the ring fused α-carbon C7' and to nitrogen N22' (Figure ). The nucleophilic attack of the thiolate sulfur at C7' leads to a reopening of the positively To also quantify the thermal stability of closed E-1, a sample containing 54% open 1 and 46% closed E-1 in MeOD-d4 solution was heated to 50 °C for over two months (Figure ). The amount of closed E-1 was reduced over this period, but not because of an isomerization reaction to open 1. Instead thermal degradation (yellow marked signals in Figure ) was observed under these conditions. Thus, a conservative estimate for the lower limit of the thermal isomerization To gain deeper insights, we also conducted a theoretical investigation at the B3LYP-GD3BJ/6-311++G(d,p)-IEFPCM(methanol) level of theory. According to our calculations (Supporting chapter 10), the intrinsic energies of closed Z-1 and closed E-1 species in MeOH solution is 10.7 kcal mol -1 and 10.3 kcal mol -1 higher than that of open 1. Controlled release of this photochemically stored energy in the form of heat would match exactly the requirements for Molecular Solar Thermal Systems (MOST). However, to be effective in this avenue, the stored energy must be stable at ambient and elevated temperatures and a triggered release mechanism must be implemented in our system. Clearly thermal lability of closed Z-1 precluded its direct use for MOST applications, but closed E-1 would be a very good candidate. We thus set out to establish a release mechanism for the stored energy in closed E-1. For this purpose, we enriched 69% closed E-1 in MeOD-d4 and evaporated the solvent under reduced pressure. For a further application of aza-diarylethene 1 we investigated its capability as photoinitiator for polymerization reactions. Previously we have reported on such application for our firstgeneration aza-diarylethene. There we achieved a radical polymerization of methyl acrylate (MA) within 2 h 40 min or 29 min using 365 nm light (260 mW or 1.5 W). Upon simultaneous irradiation with light of 365 nm (260 mW or 1.5 W) and 450 nm (400 mW) polymerization occurs within 1 h 5 min or 24 min, respectively. Our second generation aza-diarylethene 1 offers a more pronounced conjugated system enhancing the stabilization of radicals. Therefore, a significantly reduced time for photoinduced polymerization could be achieved with 1.