In Situ Infrared Study of 4,4′-Bipyridine Adsorption on Thin Gold Films Th. Wandlowski,* K. Ataka, and D. Mayer Institute for Thin Films and Interfaces ISG3, Research Center Ju ¨ lich, 52425 Ju ¨ lich, Germany Received January 28, 2002. In Final Form: March 11, 2002 The phase formation of 4,4′-bipyridine (4,4′-BP) and its coadsorption with interfacial water on quasi- Au(111) film electrodes (20 nm) from 0.05 M KClO4 has been studied employing in-situ surface enhanced infrared reflection adsorption spectroscopy (SEIRAS). Organic molecules form, dependent on the electrode potential and in the absence of Faradaic reactions, three monolayers of distincly different orientation. The high coverage adlayer I is composed of perpendicularly oriented 4,4′-BP molecules coordinated with one nitrogen atom to the underlying positively charged electrodes (C2v symmetry). Changing the electrode potential toward negative values causes two first-order phase transitions giving rise to two low-coverage organic adlayers. These transitions are accompanied by an in-plane tilting of the N-coordinated molecule. The conclusions on the interfacial orientation of 4,4′-BP are supported by a comparative analysis of in situ SEIRAS, surface enhanced Raman spectroscopy, and sum frequency generation spectra. SEIRAS spectra also indicate that 4,4′-BP is coadsorbed with water molecules and modifies the interfacial hydrogen- bonded network of the later. The nature of these interactions and their consequences for the creation of functionalized adlayers on metal electrodes are compared with related N-heterocyclic molecules. 1. Introduction The electrochemical adsorption of organic molecules is characterized by (1) interactions with the electrode through image charges and/or substrate-adsorbate co- ordination, (2) displacement of previously adsorbed solvent molecules and/or electrolyte ions, (3) hydrophobic and hydrophilic interactions with remaining solvent molecules, and (4) interactions between the adsorbate species. The latter may involve dispersion and dipole-dipole coupling, π-stacking, and/or hydrogen bonding. 1 Vibrational spec- troscopy, such as infrared reflection adsorption spectros- copy (IRAS), 2 sum frequency generation (SFG) spectros- copy, 3 surface-enhanced Raman spectroscopy (SERS), 4 and/or surface-enhanced infrared adsorption spectroscopy (SEIRAS) 5 are powerful techniques for in situ investiga- tions on structure and reactivity aspects of complex interfacial processes under steady-state as well as under dynamic conditions. The combination of SEIRAS and/or SERS with an attenuated total reflection (ATR) config- uration provided new insight on the identity and reactivity of interfacial species, such as water, organic molecules, or ions, under potential controlled conditions at defined interfaces. 6-10 In 1980 Hartstein et al. reported for the first time that the IR absorption of organic molecules, deposited on thin gold and silver films, is remarkably enhanced employing the so-called Kretschmann configuration. 11 Osawa et al. pioneered the application of this technique for equilibrium and time-resolved studies at electrochemical interfaces. 5 The surface-enhanced infrared adsorption effect is at- tributed to (1) a long-range electromagnetic mechanism (dominant) that involves the excitation of local surface plasmons within thin films of coinage metals and the polarizability perturbation of the metal by the adsorbate species 12 and (2) a short-range chemical contribution similar to the charge-transfer mechanism in SERS. 13,14 The magnitude of the enhanced IR signal, I, depends critically on island structure (size, shape, interparticle spacing, mass thickness) and chemical composition of the metal film. 15 The surface selection rule of SEIRAS states that only molecular vibrations with dipole changes perpendicular to the surface can be observed, which implies that the exciting electric field is normal to the local surfaces of the metal islands at any point and that s-polarized light does not generate any detectable reso- nance. 5,15 The surface selection rule is represented by the mathematical expression 16 where Γ is the surface concentration and θ is the angle between the dipole moment derivative of the vibrational mode (dμ/dQ) 2 and the electric filed that excites the molecule |E 2 |. Consequently, the observed spectral features can be directly ascribed to a specific interfacial orientation of the respective adsorbate. The application of ATR- SEIRAS experiments at solid/liquid interfaces with well- * To whom correspondence may be addressed: e-mail, th.wandlowski@fz-juelich.de; fax, 49 2461 61 3462; telephone, 49 2461 61 3907. (1) Guidelli, R. In Adsorption of Organic Molecules; Lipkowski, J., Ross, P. N., Eds.; VCH: New York, 1992; p 1. (2) Iwasita, T.; Nart, F. C. Prog. Surf. Sci. 1997, 55, 271. (3) Tadjeddine, A.; Le Rille, A.; Pluchery, O.; Vidal, F.; Zheng, W. Q.; Peremans, A. Phys. Status Solidi A 1999, 175, 89. (4) Tian, Z. Q.; Ren, B. In Encyclopedia of Analytical Chemistry; Meyers, R. A., Ed.; J. Wiley & Sons: New York, 2001; p 9162. (5) Osawa, M. Bull. Chem. Soc. Jpn. 1997, 70, 2861. (6) Ataka, K.; Osawa, M. Langmuir 1998, 14, 951. (7) Watanabe, M.; Zhu, Y.; Uchida, H. J. Phys. Chem. B 2000, 104, 1762. (8) Noda, H.; Wan, L. J.; Osawa, M. Phys. Chem. Chem. Phys. 2001, 3, 3336. (9) Bruckenbauer, A.; Otto, A. J. Raman Spectrosc. 1998, 29, 665. (10) Futamata, M.; Bruckenbauer, A. Chem. Phys. Lett. 2001, 341, 425. (11) Hartstein, A.; Kirtley, J. R.; Tsang, J. C. Phys. Rev. Lett. 1980, 45, 201. (12) Osawa, M.; Ataka, K.; Yoshi, K.; Nishikawa, Y. Appl. Spectrosc. 1993, 47, 1497. (13) Siemes, C.; Bruckbauer, A.; Goussev, A.; Otto, A.; Sinther, M.; Pucci, A. J. Raman Spectrosc. 2001, 32, 231. (14) Merklin, G. T.; Griffith, P. R. Langmuir 1997, 13, 6159. (15) Suetaka, W. Surface Infrared and Raman SpectroscopysMethods and Applications; Plenum Press: New York, 1995. (16) Cai, W. B.; Wan, L. J.; Noda, H.; Hibino, Y.; Ataka, K.; Osawa, M. Langmuir 1998, 14, 6992. I ∼ Γ(dμ/dQ) 2 |E 2 | cos 2 θ ∼ Γ cos 2 θ 4331 Langmuir 2002, 18, 4331-4341 10.1021/la025585k CCC: $22.00 © 2002 American Chemical Society Published on Web 04/26/2002