ELECTROCHEMISTRY OF CIS-AZOBENZENE CHROMOPHORE IN COULOMBICALLY LINKED SELF-ASSEMBLED MONOLAYER-LANGMUIR-BLODGETT COMPOSITE MONOLAYERS

A SAM-LB composite film containing azobenzene chromophore was fabricat ed on gold by combining the self-assembled monolayer (SAM) technique a nd the Langmuir-Blodgett (LB) technique. Reflection absorption FTIR st udies indicate that the composite film has an ionic bonding character at the SAM-LB interface, which has greatly improved the film stability and effectively prevented the destructive intermolecular aggregation, as evidenced by atomic force microscopy observations. The relatively loose packing structure provides enough free volume to undergo a rever sible trans-cis photoisomerization and thus enabled us to perform the electrochemical studies of the cis-azobenzene redox group in such orga nized monolayer assemblies. The cis-azobenzene SAM-LB composite film s hows a stable and clearcut Faradaic response within the pH range 3.0-9 .0, attributable to the 2e(-) and 2H(+) electrochemical reduction-oxid ation of cis-azobenzene/hydrazobenzene. Comparing with the trans-azobe nzene composite film, the LB films deposited on SnO2, and the azobenze ne-functionalized thiolate SAMs on gold, its reaction kinetics is more reversible as seen from the larger electron-transfer rate constant, s maller peak-to-peak splitting, and smaller half-peak width. The logari thmic rate constant shows a complicated but unique pH dependence, char acteristic of azobenzene redox kinetics, which is explained by the var ied sequences of electron transfers and protonations in different pH r anges.