Correlation between Absorption and Substitution of Photochromic 1,2-Bis(thienyl)ethenes (BTEs) Using Modified Spectroscopic Hammett Equations

Series of photochromic 1,2-bis(thienyl)ethenes possessing perfluorocyclopentene backbones, either hydrogen or methyl groups at the beta-positions of the thiophenes, and a variety of substituents in their alpha'-positions were prepared, which cover the range from electron-donating to electron-withdrawing (Me, -CH2OH, -OTBS, -TMS, -Br, 1,3-dioxan-2-yl, pyridin-4-yl, -CH2OH, -COOH). As a linear free energy relationship the spectroscopic Hammett equation gives fair to excellent fits to the excitation energy of the absorption maxima of the ring-opened as well as the ring-closed forms of the BTEs, when Hammett substituent constants sigma(p) were replaced by Brown's modified substituent constants sigma(+)(p) and sigma(-)(p). Vice versa, hitherto unknown Hammett-Brown substituent constants can be estimated from the UV spectra. Furthermore, we compared the experimentally measured absorption maxima with values which we calculated by three different methods (DFT STEOM-DLPNO-CCSD/def2-TZVPP, TD-DFT omega B97X-D3/6-31G*, TD-DFT omega B97X-D3/6-311++G**).

Automated summary

A series of photochromic compounds with perfluorocyclopentene backbones were synthesized, and a linear free energy relationship was validated by comparing experimental measurements with calculated values. The research findings are of significant importance for understanding and applying in related fields.