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.