The absorbance at 365 nm increases linearly, showing its constant encapsulation within the bilayer (Figure ). Moreover, the size of the liposomes increased with the E-CyP-OBu concentration, from 85 nm at 0% content to 95 nm at 5%, to finally reach a plateau at 165 nm for concentrations of photo-switch ranging from 10% to 20% (Figures ). The efficiency and rate of encapsulation were determined by UV-Vis spectroscopy. To simplify data analysis, we hypothesized that the molar absorption coefficients of E-CyP-OBu solubilized in 100% ethanol or in an ethanol/water mixture (9/1) are similar, which allows us to estimate the encapsulation efficiency around 60%. As expected, the encapsulation rate increases linearly with the molar ratio of photoswitch of the feed solution from 2 to 11% (Table ). To study the impact of the confinement on the direct and reverse photoisomerization of CyP-OBu, a liposomal solution containing 10 mol% photoswitch was irradiated at both 375 and 300 nm. The evolution of the absorption over time revealed that the photoisomerization is dependent on the laser power. Indeed, at a low irradiation power, equivalent to the one used in organic solvents (28mW), the PSS is reached in several hours (Figure , blue curve). Moreover, it was observed that after this prolonged irradiation at 375 nm the total absorbance of the solution was still decreasing, in a way reminiscing of a dilution effect, until it reaches a plateau within several days (Figure , red curve). Coherently with the FEP obtained from MD simulations, this phenomenon can be assigned to the slower release of the membrane-bound Z-CyP-OBu in water bulk. On the other side, when using the maximum irradiation power at 375 nm (1.5W) the PPS state is reached within few minutes, without a noticeable expulsion of the photoswitch, which happens on much longer timescales (Figure ). Interestingly the rate constant of the direct photoisomerization for encapsulated switches drops from 26.8 x 10 -3 s -1 in CHCl 3 to 0.2 x 10 -3 s -1 , this is most probably due to the more hindered environment and can again be seen as a confirmation of the formation of stable aggregates with the membrane. On the other hand, the rate constant for the expulsion of the Z-isomer is evaluated at 0.01 x 10 -3 s -1 , coherently with the formation of a metastable state. Interestingly, upon irradiation at 375 nm, the size of the liposomes decreases from 165 to 95 nm, which may be caused not only by the expulsion of the Z-isomer but also by the establishment of a more space-saving packing between the chromophores and DPPC (Figure ). Deeper insights on those phenomena may be obtained from lipid monolayers and Langmuir film approaches. First, we examined the interfacial behaviour of DPPC, Z-and E-CyP-OBu, as well as their mixtures, in particular the corresponding surface pressure -area (Π-A) which are shown in Figure .