Dynamic monolayer behavior of a photo-responsive azobenzene surfactant

Photo-responsive behavior of an azobenzene surfactant is observed at the air–water interface in terms of the surface pressure modulation with 360 and 420 nm wavelength light. The surface pressure response of the monolayer changes depends on the degree of compression of the Langmuir monolayer, expressed in terms of the area per molecule, as well as the light condition during the formation of the Langmuir layer. The azobenzene chromophore requires molecular space to undergo photoisomerization, which causes surface pressure changes at the air–water interface. Closely packed trans molecules do not fully convert to the cis state under 360 nm due to the steric hindrance and formation of H-aggregates. The bulky cis chromophores do convert to the trans state. Ultraviolet-visible spectra of Langmuir–Blodgett (LB) films are blue shifted 17 nm relative to the UV-visible spectra for the azobenzene surfactant in chloroform solutions, indicating formation of H aggregates. Langmuir–Blodgett films show different molecular conformation depending on the light wavelength during compression.