The formation of compact and large-scale self-assem bled monolayers (SAMs) adsorbed on a rnica surface has been achieved by insertion of alkyl chains on azobeilzene derivatives, leading to strong intermolecular van der Waals interactions and hydrogen bonding. The reversible photoswitching of monolayers was investigated by monitoring the variation of the thickness of the SAMs during the cis-trans isomerization of the azobenzene cores with an atornic force microscope (AFM). The absence of covalent bonds between molecules and substrate induces a molecular diffusion which leads to the complete isomerization of the molecules constituting the SAMs.