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J. Phys. Chem. 1996, 100, 814-821
Photoelectrochemistry of Langmuir-Blodgett Films of Carotenoid Pigments on ITO Electrodes Leonides Sereno,† Juana J. Silber,† Luı´s Otero,† Marı´a del Valle Bohorquez,‡ Ana L. Moore,*,§ Thomas A. Moore,§ and Devens Gust§ Departamento de Quı´mica y Fı´sica, UniVersidad National de Rı´o Cuarto, Rı´o Cuarto, Argentina; Department of Chemistry , Drake UniVersity , Des Moines, Iowa 50311; and Department of Chemistry and Biochemistry and Center for the Study of Early EVents in Photosynthesis, Arizona State UniVersity, Tempe, Arizona 85287-1604 ReceiVed: August 2, 1995; In Final Form: October 13, 1995X
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.
Introduction Carotenoid polyenes are common pigments in living organisms where their intense absorption bands in the visible spectral region are responsible for coloration and the initiation of certain photobiological responses. In photosynthetic membranes they participate with chlorophylls in singlet and triplet energy transfer which result in an enhanced rate of light absorption and suppression of singlet oxygen sensitization, respectively.1,2 In addition to functioning as antennas in photosynthetic membranes, carotenoids are thought to mediate the dissipation of excitation energy during periods of excess light absorption.3 By selecting carotenoids for cryptic coloration, plants and animals have also taken advantage of their strong absorption of light over a wide spectral range and photochemical inertness. In view of the participation of carotenoids in photosynthetic energy transfer processes and the possibility of their photochemical involvement as photoreceptors, it is of interest to explore conditions under which photochemical reactions of carotenoids can be elicited. Carotenoid pigments in solution are generally not photochemically reactive, although they can undergo unimolecular isomerizations and act as quenchers of the triplet states of other molecules through energy transfer. Presumably, this lack of reactivity is due both to the very short lifetime of the S1 excited state (from 50 to
