UNCONSTRAINED CIS-TRANS ISOMERIZATION OF AZOBENZENE MOIETIES IN DESIGNED MIXED MONOLAYERS AT THE AIR-WATER-INTERFACE

A novel concept for monolayer organization is presented which allows one to obtain compact monolayers with high chromophore density. The unique features of the designed monolayer are the following: chromophores remain in the monomeric form at high chromophore densities, sufficient free space is available for unconstrained photochemical cis-trans isomerization in the densely packed monolayer state, and basic monolayer structure is maintained in the wake of the cis-trans photoisomerization. On the basis of this concept, mixed monolayers of an amphiphilic azobenzene derivative (A820Py) and the spacer molecule dimyristoylphosphatidic acid (DMPA) have been investigated at the air/water interface. In addition to appropriate dimensions, the spacer molecule DMPA has a negatively charged head group which ensures mixing with A820Py (positively charged head group) at the molecular level. It is shown that, in the tightly packed state, the surface pressure in the mixed monolayers of DMPA/A820Py, molar ratio 1:1, remains almost unaffected as a result of cis-trans isomerization, while reversible changes in the UV-vis absorption spectrum, surface potential, and Brewster angle reflectivity are observed. The isomerization process in the mixed monolayers is a reaction of first order. The new concept of chromophore organization is not limited to the specific system investigated here and can be used for developing designed molecular organizates.