Plasmonic metal nanostructures are expected to function as optical antennas that can promote photon molecule interactions. Despite recent advancements in the field of plasmonics, it is still difficult to achieve plasmonic enhancement of the overall efficiency of photochemical reactions. Here, we compare the photocurrent generation efficiency with and without optical antennas on a same porphyrin-based molecular monolayer. The overall reaction efficiency is strongly affected by the electrochemical properties of the optical antennas. This result indicates that plasmonic enhancement of photoelectrochemical reactions must be considered with respect to both the electromagnetic design of optical antennas for the efficient capture of photon energy and electrochemical design for the suppression of undesirable reverse electron transfer. (C) 2013 Elsevier Ltd. All rights reserved.