AZOBENZENE-DERIVATIVE LANGMUIR-BLODGETT-FILMS DEPOSITED ON VARIOUS THIOL MONOLAYERS

Monolayers or bilayers of the azobenzene derivative 4-octyl-4'-((carbo xyltrimethylene)oxy)azobenzene (ABD) have been deposited by the Langmu ir-Blodgett (LB) method on gold substrates modified with thiol self-as sembled monolayers. Here the thiol monolayer serves both as a substrat e for the LB film and as a barrier for the electron transfer between t he azobenzene and the gold substrate. The electrochemical properties o f the ABD films depend on the length of the thiol molecule and the ori entation of the ABD molecule. They become more irreversible when the t hiol molecule is longer and when the ABD molecule is adsorbed on the t hiol with its hydrophobic terminal. Although energetically unstable ci s-ABD is reduced to the hydrazobenzene derivative (HBD) at a more anod ic potential than trans-ABD, the reduction of cis-ABD was not observed in the cyclic voltammetry of ABD films on 2-mercaptoethanol. Moreover , UV light irradiation of the trans film in a certain cathodic potenti al range causes cathodic and anodic spikelike currents when the light intensity is increased and decreased, respectively, at the cathodic bi as potential. These phenomena are explained by using the following mec hanism; i.e., the reduction of cis-ABD can occur at a more anodic pote ntial than the oxidation of HBD when the separation of the redox poten tials is relatively small for trans-ABD. If the above two opposite rea ctions take place much faster than the voltammetric potential scan, th e electron flows cancel each other out and no net faradaic current can be observed. However, the spiked current responses are generated when the amounts of reduction and oxidation currents become out of balance .