Coupling one electron photoprocesses to multielectron catalysts: Towards a photoelectrocatalytic system Matthew T. Rawls, Justin Johnson, Brian A. Gregg ⇑ National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, CO, United States article info Article history: Received 23 June 2010 Received in revised form 16 September 2010 Accepted 19 September 2010 Available online 14 October 2010 Keywords: Energy conversion Redox chemistry Molecular devices Photooxidation Synthesis design abstract We investigate a new approach to artificial photosynthetic systems that couples a one-electron organic photovoltaic material to a two-electron catalyst. This approach allows direct photo-production of fuels without requiring sacrificial reagents. The synthesis and characterization of the first such photoelectro- catalytic (PECat) system is described. A series of perylene diimides with appended polypyridyl ligands are synthesized. One compound is complexed to a ruthenium terpyridine catalyst. Illumination in dimethyl formamide solution results in the formation of the perylene diimide anion and irreversible oxidation of the solvent (or residual impurities). In the solid state, the PECat material is electroactive at both semicon- ductor and catalyst sites throughout the film. The films electrocatalyze the two-electron oxidation of iso- propanol to acetone in the dark. A small PECat effect is observed in the solid films. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction The development of a carbon free renewable energy supply is a vital goal. Solar energy is the ideal renewable source [1] if the problems of efficient photoconversion and energy storage can be addressed [2–5]. Conversion of solar power directly into a flux of fuel amenable to storage, transport, and consumption is perhaps the ideal process: for example, efficient photo-production of hydrogen and oxygen directly from water [6]. Direct photo-production of fuels from H 2 O, CO 2 ,N 2 , NAD + , etc. requires: (1) light absorption and photoelectron transfer to sepa- rate charges and (2) delivery of multiple electrochemical equiva- lents via a catalyst to the feed stock [7]. Many elegant model systems have been synthesized that mimic the manner by which nature utilizes electron transfer to accomplish step 1 [8–10]. For example, light absorbers were attached via a bridge to an electron (or sometimes hole) acceptor and the rates of the various electron transfer (ET) reactions were studied and compared to theory [11,12]. Energy transfer has been studied via a different set of mul- tipartite molecular structures [13]. Some of these super-molecules have been vectorially inserted into bio-like cells and shown to transform photons into reduced biological species [8,14]. Research into this ‘‘first generation” of photosynthetic mimics has been highly productive. But a fundamental design change, from homo- geneous solution systems to lower-dimensional heterogeneous systems may be required before reactants can be continuously sep- arated from products, thus enabling sustained photoconversion. Another requirement for efficient continuous production of fuels is the ability to transfer two electrons per reaction step (step 2) [15–17]. This avoids progression through high energy and very reactive one-electron radical species. Except for biological enzymes, however, we know little about two-electron catalysis. Many catalyzed electron transfer (ET) reactions in nature involve the approximately simultaneous transfer of two electrons and two protons, or a hydride ion [16,18]. The mechanism is thought to proceed via sequential but strongly coupled single ET reactions [19]. Because both reactions occur almost simultaneously, it is pos- sible to tunnel through the one-electron energy barrier rather than going over it. Photoelectron transfer reactions and two-electron catalysis have been studied separately for decades, but few examples [8,20] exist where as in nature they are effectively integrated. Some examples of systems based on inorganic semiconductors, usually TiO 2 , have been reported recently [8,21]. The goal of the re- search reported here is to couple these two steps in a novel and efficient manner in an organic semiconductor-based material—that is to couple single photon absorption processes to multielectron, multiproton catalysts to drive fuel forming reactions. We refer to this type of system as a photoelectrocatalytic (PECat) device. The geometry of an ideal PECat system (Fig. 1) attempts to solve some of the problems of earlier photosynthetic model systems, most importantly, their inability to sustain photoconversion with- out using sacrificial reagents and their inability to couple light har- vesting and photoelectron transfer to the necessary catalytic 1572-6657/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2010.09.016 ⇑ Corresponding author. Tel.: +1 303 384 6635; fax: +1 303 384 6490. E-mail address: brian.gregg@nrel.gov (B.A. Gregg). Journal of Electroanalytical Chemistry 650 (2010) 10–15 Contents lists available at ScienceDirect Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem