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)

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 Phi(t-->c) and Phi(c-->t). Total quantum yields in n-hexane vary from Phi(total) = 0.72 for a bis-acceptor substituted TEE to Q(total) = 0.015 for a four-fold, bis-donor, bis-acceptor substituted TEE derivative. A strong relationship between Phi(total) and solvent polarity as well as a strong dependence of Phi(t-->c) and Phi(c-->t) on the wavelength of excitation lambda(exc) has been observed. The temperature dependence of the photoisomerisation has been investigated for a bis-acceptor-substituted DEE and shows no changes in Phi(total) over the temperature range 6.5-65 degrees C. None of the compounds studied undergoes thermal isomerisation at 27 degrees 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.