PHOTOELECTROCHEMISTRY OF LANGMUIR-BLODGETT-FILMS OF CAROTENOID-PIGMENTS ON ITO ELECTRODES

Langmuir-Blodgett (LB) films of an amphipathic carotenoid, 7'-apo-7'-( 4-carboxyphenyl)-beta-carotene (ACC), were deposited on semiconducting transparent ITO electrodes which were then immersed in electrolyte co ntaining benzoquinone (Q) or hydroquinone (QH(2)) Photocurrents were m easured over the spectral range 350-700 nm. The action spectra implica te the excited carotenoid pigment, possibly in an aggregated form, as the photoactive species in the photoinduced electron transfer process. The photocurrents (up to 2 nA/cm(2)) were proportional to the light i ntensity and the number of deposited layers. The quantum yield of elec tron transfer was found to be independent of the number of layers, whi ch implies efficient charge transfer between the layers, This result i s unique to these carotenoid films. The polyenic carboxylic acid behav es as a photoconductor, in contrast to saturated long-chain acids whic h act as insulators in similar experiments. The direction of the photo current was investigated as a function of the bias potential, as well as the concentration of QH(2) and Q in the surrounding electrolyte. In the presence of a large excess of QH(2) the photocurrent was anodic, but it was cathodic in the presence of a large excess of Q.