Thermal Modulation of Photoisomerization in Double-Azobenzene-Chain Liposomes

Cationic double-azobenzene-chain lipid 5 undergoes efficient trans → cis photoisomerization in MeCN solution and in 1:9 coliposomes with bisether lipid 2 at temperatures above 20 °C. However, in 1:1 5 / 2 coaggregates, or in pure liposomal 5, the extent of photoisomerization is very sensitive to temperature; for example, it is limited to ≤10% below 40 °C. Dispersed 1:9 coliposomes of 5 and 2 spontaneously segregate into domains rich in the individual lipids. Thermal cis → trans isomerization of 5 in MeCN solution and in 1:9 5 / 2 coliposomes proceeds readily between 20 and 60 °C with activation energies of 19−20 kcal/mol. The photoisomerization of 5 in pure liposomes or in 1:1 5 / 2 coliposomes is less efficient than that of its single-azobenzene-chain analogue, 1, under comparable conditions.