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

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, E-s(R), E-s(c)(R), and v(R) values. We found that there is a good correlation between the E-s(c)(R) value and the thermal cycloreversion reactivity. Furthermore, the E-s(CH2R) value has also a good correlation with the thermal cycloreversion reactivity, while E-s(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 alpha-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.