Molecular Chirality and Charge Transfer through Self-Assembled Scaffold Monolayers J. J. Wei, C. Schafmeister, G. Bird, A. Paul, R. Naaman, and D. H. Waldeck* Chemistry Department, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15260, and Chemical Physics Department, Weizmann Institute of Science, RehoVot 76100 Israel ReceiVed: September 12, 2005; In Final Form: October 5, 2005 The effect of molecular chirality on electron transmission is explored by photoelectrochemistry. Thiol-terminated chiral scaffold molecules containing a porphyrin chromophore were self-assembled on gold surfaces to form a monolayer. Incorporation of the SAM-coated gold into an electrochemical cell and illumination with visible light generated a cathodic photocurrent. When using circularly polarized light, the photocurrent displayed an asymmetry (different magnitude of photocurrent for right versus left polarization) that changed with the molecular chirality (left- or right-handedness of the scaffold). A symmetry constraint on the electronic coupling between the porphyrin and the organic scaffold is proposed as a possible mechanism for the photocurrent asymmetry. The primary process of electron transfer underlies many chemical and biological reactions and is of primary importance in many technologies. Consequently, the nature of electron transfer (its dependence on energetics, nuclear degrees of freedom, and electronic coupling) has been under experimental and theoretical study for many years. 1,2 Despite these efforts, little attention has focused on the influence of molecular chirality on electron transfer. This work examines the effect of molecular chirality on the photocurrent of film coated electrodes. On a fundamental level, spin-polarized electrons have been used to perform chemistry and are implicated in the origin of chiral selectivity in biology. 3 On a technological level, molecular chirality can be used to introduce a new control parameter for spin-sensitive devices. Naaman reported the first investigation of spin dependent electron transmission through thin chiral films of stearoylysine 4 and more recently observed an asymmetry for electron transmis- sion through monolayers of L (or D) polyalanine films. 5 The magnitude of the effect is 10 3 to 10 4 times larger than the chiral selectivity found for the interaction of polarized electrons with molecules that are not organized into two-dimensional arrays. 6-17 In photoemission through an organic monolayer film, the electron wave function can be delocalized among many chiral molecules in the film, whereas tunneling electrons are more localized. Hence, it is interesting to ask if such large effects are possible for electron tunneling. Spin polarized tunneling has been observed in Metal-Oxide-GaAs (MOS) structures with an asymmetry of the order of 1%. 18 In those studies the polarized distribution of carriers is generated in the GaAs by circularly polarized light and tunneling occurs through a thin Al 2 O 3 film (2 to 20 nm) on Al. In recent work spin polarized electrons were selectively transmitted between two quantum dots through an organic molecule. 19 Those findings show that it is possible to create the polarized distribution of charge carriers and observe asymmetry in electron tunneling. This study investigates the photocurrent, induced by circular polarized light, through organic monolayer films on Au electrodes that are immersed in an electrochemical cell. The films are composed of a chiral scaffold molecule, which is linked to the Au by a cysteine moiety, and presents a porphyrin chromophore to the solution. Although related systems have been studied previously (e.g., Morita et al. 20 placed helical peptides containing a carbazolyl chromophore on gold elec- trodes), the effects of molecular chirality and light polarization were not explored. Under photoexcitation of the porphyrin, an electron is transferred to an acceptor (e.g., methyl viologen), and the resulting cation of the porphyrin is reduced by the gold electrode. By measuring the dependence of the photocurrent on the polarization of the light field and correlating it with the scaffold’s chirality, a preference for electron tunneling of one handedness is indicated. Experimental Section Reagents and Materials Preparation. Scheme 1 shows the chiral scaffold molecules (L-Cys-(pro4(2S4S)) 4 -Porph (S1), D-Cys-(pro4(2R4R)) 4 -Porph (R1)) with their covalently linked porphyrin chromophore. The compounds were prepared in the manner reported previously, 21 and details of the synthesis are provided in the Supporting Information. Solid phase synthesis was performed in a 1.5 mL disposable polypropylene reaction column, connected to a three-way valve equipped with vacuum and argon for mixing. Dichloromethane (DCM) used in coupling reactions was distilled over calcium hydride. Dry grade dimethylformamide (DMF) from Aldrich was used for coupling. Diisopropylamine (DIPEA) was distilled under nitrogen sequentially from ninhydrin and potassium hydroxide and stored over molecular sieves. O-(7-Azabenzo- triazol-1-yl)-N,N,N′,N′-tetramethyluronium hexafluorphospate (HATU) was obtained from Acros. All solid-phase reactions were mixed by bubbling argon up through the reactor, allowing for mixing and an inert atmosphere over the reaction. HPLC analysis was performed with a Hewlett-Packard Series 1050 instrument equipped with a Varian Chrompack Microsorb 100 C 18 column (5 μm packing, 4.6 mm × 250 mm) or a Hewlett- Packard Series 1100 instrument equipped with a Waters Xterra MS C 18 column (3.5 μm packing, 4.6 mm × 100 mm) and a diode-array detector. * Address correspodence to this author at the University of Pittsburgh. 1301 J. Phys. Chem. B 2006, 110, 1301-1308 10.1021/jp055145c CCC: $33.50 © 2006 American Chemical Society Published on Web 12/30/2005