Photoisomerization processes in Langmuir-Blodgett films monitored by ellipsometry

We present ellipsometric measurements on photoisomerization processes, induced in Langmuir-Blodgett (LB) films of (co-PM-5AZ), a polymethacrylate copolymer with azobenzene sidegroups. Ellipsometry gives a value of n = 1.557 for the refractive index of the LB films and a film thickness of d = 1.58 nm per monolayer. This value is below the estimated value of dth = 2.3 nm per monolayer, indicating interdigitation of the sidechains. When exposed to light of appropriate wavelength, the azobenzene groups of the polymer undergo a reversible transcis photoisomerization process which influences the optical properties of the LB films, and hence can be monitored by ellipsometry. Whereas the obtained data do not exhibit any significant change in refractive index, we saw a small change in monolayer thickness of δd = 0.02 nm per monolayer which, owing to interdigitation of the sidegroups, is one magnitude below the theoretically expected value of δdth = 0.5 nm per monolayer. Our measurements demonstrate that ellipsometry is sensitive enough to monitor trans-cis photoisomerization processes in LB films down to bilayer thickness.