Photochemical trans–cis isomerisation of donor/acceptor-substituted (E)-hex-3-ene-1,5-diynes (1,2-diethynylethenes, DEEs) and 3,4-diethynylhex-3-ene-1,5-diynes (tetraethynylethenes, TEEs)

Abstract

The photochemically reversible trans–cis isomerisation of (E)-hex-3-ene-1,5-diynes (1,2-diethynylethenes, DEEs) and 3,4-diethynylhex-3-ene-1,5-diynes (tetraethynylethenes, TEEs) substituted with electron-donating (p-dialkylaminophenyl) and/or electron-accepting (p-nitrophenyl) groups has been examined. The type and degree of donor/acceptor (D/A) functionalisation has been found to drastically affect the partial quantum yields of isomerisation Φ_t→c and Φ_c→t. Total quantum yields in n-hexane vary from Φ_total = 0.72 for a bis-acceptor substituted TEE to Φ_total = 0.015 for a four-fold, bis-donor, bis-acceptor substituted TEE derivative. A strong relationship between Φ_total and solvent polarity as well as a strong dependence of Φ_t→c and Φ_c→t on the wavelength of excitation λ_exc has been observed. The temperature dependence of the photoisomerisation has been investigated for a bis-acceptor-substituted DEE and shows no changes in Φ_total over the temperature range 6.5–65 °C. None of the compounds studied undergoes thermal isomerisation at 27 °C in n-hexane. Further analysis of these compounds by theoretical investigations at the semiempirical level of theory reveals a significant reduction of the bond order of the central olefinic double bond in the D–D, D–A and A–A TEEs upon electronic excitation, thus ultimately facilitating photoisomerisation.