Substituents affect the mechanism of photochemical E-Z isomerization of diarylethene triazoles via adiabatic singlet excited state pathway or via triplet excited state

Photochemical reactivity in the Z-E isomerization for two heterostilbene derivatives containing 1,2,3-triazole unit were investigated theoretically and experimentally by irradiation experiments, fluorescence and laser flash photolysis (LFP). The molecules were designed to probe the effect of the para-nitro group in 1 on the photochemical E-Z pathways, as well as to investigate the steric effect of the ortho-methyl group in 2. The quantum yield for the Z -> E isomerization for both cis-isomers is 0.42, and for the E -> Z is somewhat lower 0.16 and 0.12, respectively. Furthermore, fluorescence measurements for the ortho-methyl derivative indicated that the Z -> E isomerization takes place in an adiabatic reaction on the potential energy surface of the S1 state. On the contrary, the para-nitro derivative undergoes the Z -> E isomerization via a triplet excited state, which was detected by LFP. For both cis- and trans-isomers of the nitro derivative a transient was detected absorbing with a maximum at 520 nm, which was assigned to the triplet excited state of the trans-isomer. All experimental observations were corroborated by computations. The stationary points were computed at the PBE50/6-31++G** level of theory, whereas potential energy surfaces were obtained by linear interpolation and computations at the SF-TDDFT/PBE50/6-31++G** level of theory. The mechanistic investigation presented gives insight in the fundamental and simple Z -> E isomerization and provides new findings which are important in the rational design of different photoreactive diarylethene derivatives used in different fields of science.

Automated summary

The study investigated the photochemical reactivity in Z-E isomerization for two heterostilbene derivatives containing 1,2,3-triazole unit. Different quantum yields and reaction mechanisms were found for cis- and trans-isomers, providing insights into the fundamental Z-E isomerization.