Photoelectrochemistry of a pigment used in artificial photosynthesis: An anilinocarotenoid

LB films of an amphiphatic carotenoid, 7′-apo-7′-(4-aminophenyl)-β-carotene (H₂APC), deposited on semiconducting transparent ITO electrodes, when immersed in electrolyte-containing benzoquinone (Q) and illuminated, produced photocurrents that were measured over the spectral range of 350-700 nm. The generated photocurrent was investigated as a function of different variables: bias potential, number of LB layers, and concentration of Q in the surrounding electrolyte. Only cathodic photocurrents were observed with H₂APC. The action spectrum shows that the absorption of light by the carotenoid pigment is responsible for the photocurrent effects. The photoresponse increases linearly with the number of LB layers deposited. The quantum yield of electron transfer is independent of the number of layers, indicating efficient charge and/or energy transfer between the layers. This result is unique to carotenoids films. Anodic photocurrents were not detected even in the presence of electron-donor couples. This behavior is in contrast to that observed with another amphipathic carotenoid, 7′-apo-7′(4-carboxyphenyl)-β-carotene (ACC), which generates photocurrents in either direction depending on the external couple.