Reversible Optical Control of Monolayers on Water through Photoswitchable Lipids

We have obtained molecular insights into a monolayer of azobenzene-based photoswitchable lipids self-assembled on water, using the surface sensitive technique vibrational sum-frequency generation spectroscopy in combination with surface pressure measurements. The photolipids can undergo wavelength-dependent, light-triggered cis/trans and trans/cis isomerization, allowing for reversible control of the surface pressure and the molecular ordering of the lipids in the monolayer. If the photoswitchable lipid is embedded in a layer with conventional phospholipids, such as 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), we show that the surface pressure and molecular ordering of DPPC can be influenced by switching the azobenzene-based lipid between its two states. Remarkably, the state with the higher surface pressure (cis-state) is characterized by a lower degree of molecular order. This counterintuitive result can be understood by noting that the azobenzene moiety in the cis state has a higher dipole moment and therefore favors interaction with water. The surface free energy of the system is lowered (increase of surface pressure) by electrostatic interactions with the lipid headgroups at the interface, resulting in a loop formation of the lipid tail with the cis-azobenzene. This disorder in the tail of the photoswitchable lipid perturbs as well the ordering of DPPC.