Figure shows the computed transition structures and t 1/2 for 3-TS, 13-TS, 3-OMe-TS, and 13-OMe-TS, representing the strongest EWG-and EDG-substituents (NH 2 vs. NO 2 , respectively). Our calculations show approximately 10 orders of magnitude difference in t 1/2 . This increase in t 1/2 is observed for both the unsubstituted thiophene (3-TS and 13-TS) and 2,5dimethoxythiophenes (3-OMe-TS and 13-OMe-TS). Three of the four hemi-azothiophenes in Figure undergo inversion mechanisms (3, 13, and 3-OMe), while 13-OMe-TS undergoes rotation. 3-TS, 13-TS, and 3-OMe-TS in Figure are examples of inversion mechanisms, and 13-OMe-TS is an example of a rotation mechanism. These mechanisms are characterized by different NNC angles about the diazo bond. For the inversion mechanism, one of the NNC angles is linear, whereas the other is close to 120° (116° -118°). The rotation transition structures do not have a linear CNN angle about the diazo (128° and 118° for 13-OMe-TS for NNC Ph and NNC Th , respectively). We assessed the role of the EWGs and EWGs in delocalizing 𝜋 NN -electrons to explain the nearly 12 orders of magnitude range in t 1/2 for 1-13 and (1-13)-OMe. We plotted the ∆𝐺 ‡ for 20 of the 26 photoswitches against the Hammett (σ para ) parameters (there are no reported σ para values for the methoxy diene groups in 11-13 and (11-13)-OMe. The Hammett parameter quantifies how much a given functional group stabilizes a developing negative charge in a transition structure. Venkataramani and König showed that azoarene Z➝E thermal isomerization barriers decrease with the strength of EWGs at para-Ph positions. Figure . Scatterplots showing the relationship between Hammett parameter σ para and calculated activation barrier for unsubstituted (left) and dimethoxy (right) hemi-thiophene systems. The parent structures for each system are presented on the plots, along with the linear correlation equation and R 2 . The barriers are shown in kcal mol -1 . The "a" on both plots corresponds to the ketone functional group.