Correlation between Steric Substituent Constants and Thermal Cycloreversion Reactivity of Diarylethene Closed-Ring Isomers

Abstract

Abstract Two kinds of diarylperfluorocyclopentenes having 5-phenyl-2-propyl-3-thienyl and 2-isobutyl-5-phenyl-3-thienyl group as the aryl groups were newly synthesized, and the thermal stability of their closed-ring isomers was investigated. The bulkiness of the substituent (R) at the reacting positions in photochromic diarylethenes plays an important role in the thermal cycloreversion reactivity of the photogenerated closed-ring isomers. The steric substituent constant was introduced to investigate the relationship between the steric hindrance of the substituent R and the thermal cycloreversion reactivity. We employed three steric substituent constants, Es(R), Esc(R), and v(R) values. We found that there is a good correlation between the Esc(R) value and the thermal cycloreversion reactivity. Furthermore, the Es(CH2R) value has also a good correlation with the thermal cycloreversion reactivity, while Es(R) has no correlation. These results indicate that the steric hindrance has an effect on the thermal cycloreversion and there is no hyperconjugation of the α-hydrogen on the 2-substituent on the thiophene ring. Moreover, the activation energy was also found to be related to the steric substituent constant. These findings would give a new useful strategy to design novel diarylethenes having desired thermal stability.