Molecular Arrangements and Photoisomerization of Amphiphilic Azobenzene Derivatives in Mono- and Multilayers

For long-chain azobenzene derivatives containing different polar head groups and the ehro-mophore in various positions of normal hydrocarbon chain (CnAzoCmX, n and ni: carbon numbers of the chains, X: polar group), the monolayer characteristics in the surface pressure-area isotherms and spectroscopic behaviors in the UV, visible and IR spectra of LB films have been studied to clarify the molecular arrangements in the films and their correlation with molecular structures and temperature. In the case of X= -COOH and -NHCH2CH2NH2, stable condensed monolayers are formed and the azobenzene chromophores are closely packed with the long axis oriented almost vertically in the films. For X=-N+(CH3) 2CH2CH2OH, the molecular orientation and packing are significantly dependent on the carbon numbers temperature, surface pressure and the composition of the aqueous subphase. For example, the compounds of [6,10] and [12,5] give expanded monolayers at lower temperatures and the molecular long axes tend to incline. In addition, photoisomerization of the azobenzene derivatives in the mono- and multilayers has been examined. The cis'trans isomerization occurs predominantly, whereas the reverse reaction accompanied with an increase in the molecular area is significantly retarded due to the close packing of the chromophores. In the expanded monolayer at lower pressures, the reversible photochromism can be caused to some extent.