Fig. 3
Switching behavior of HTI tweezers 1 and 2. a Molar extinction coefficients of 1 (black) and 2 (red) in CHCl3. The Z isomers are shown in solid lines and E isomers in dashed lines. b Reversible absorption changes observed during irradiation of a 1:1 mixture of 1 and 2 in CHCl3 with 435 nm and 530 nm light. c Indicative section (signals of the methylene groups) of the 1H NMR spectra of 1 and 2 in CDCl3 solution at 27 °C after reaching the pss at two different wavelengths. 1. 86% E-1 are obtained at 435 nm. 2. 63% E-2 are obtained at 435 nm. 3. 80% Z-1 are obtained at 530 nm. 4. 84% Z-2 are obtained at 530 nm. 5. Essentially the same pss composition as for the individual tweezers is obtained for a 1:1 mixture of 1 and 2 at 435 nm. 6. Essentially the same pss composition as for the individual tweezers is obtained for a 1:1 mixture of 1 and 2 at 530 nm. d Free activation enthalpies for the thermal E to Z (solid bars) and Z to E (hollow bars) equilibrations of 1 and 2 in CDCl3 in the dark. Energy values range from 22.7 to 24.4 kcal mol−1
