Photoelectrochemistry of Langmuir-Blodgett films of carotenoid pigments on ITO electrodes

Langmuir-Blodgett (LB) films of an amphipathic carotenoid, 7′-apo-7′-(4-carboxyphenyl)-β-carotene (ACC), were deposited on semiconducting transparent ITO electrodes which were then immersed in electrolyte containing benzoquinone (Q) or hydroquinone (QH₂). Photocurrents were measured over the spectral range 350-700 nm. The action spectra implicate 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/cm²) were proportional to the light intensity and the number of deposited layers. The quantum yield of electron transfer was found to be independent of the number of layers, which implies efficient charge transfer between the layers. This result is unique to these carotenoid films. The polyenic carboxylic acid behaves as a photoconductor, in contrast to saturated long-chain acids which act as insulators in similar experiments. The direction of the photocurrent was investigated as a function of the bias potential, as well as the concentration of QH₂ and Q in the surrounding electrolyte. In the presence of a large excess of QH₂ the photocurrent was anodic, but it was cathodic in the presence of a large excess of Q.