Langmuir 2006, 22, 7631-7638
7631
Self-Assembled Multilayers of Transition-Metal-Terpyridinyl Complexes; Formation, and Characterization Laura Kosbar,* Charan Srinivasan,† Ali Afzali, Teresita Graham, Matt Copel, and Lia Krusin-Elbaum IBM T.J. Watson Research Center, Yorktown Heights, New York 10598 ReceiVed July 15, 2005. In Final Form: June 16, 2006 Layer-by-layer (LBL) growth of terpyridinyl ligands with a range of metal ions is reported. Monolayers of mercaptophenyl terpyridine on gold were used to initiate LBL assembly by complexing the first layer of metal ions. Tetra-2-pyridinylpyrazine was used as a linking ligand between subsequent metal ion layers. The assembly of the terpyridines with 21 different metals was evaluated using UV absorbance spectroscopy, variable-angle spectroscopic ellipsometry, and atomic force microscopy. Successful LBL growth appears to depend on the ionic radius of the metal ion. Metals that formed multilayered LBL structures were primarily limited to a small range of effective ionic radii between 66 and 73 pm. Metal ions with smaller ionic radii usually formed initial layers but seldom exhibited consistent LBL growth, while ions with radii larger than 73 nm generally did not demonstrate any evidence of LBL growth.
Introduction Control of the composition, ordering, and structure of thin films on a molecular level is an active area of research. The ability to prepare molecular and supramolecular assemblies has implications for nanoelectronics,1,2 nonlinear optics and optoelectronics,3,4 biotechnology,5,6 and chemical sensors.7-9 In the field of nanoelectronics, for example, fabrication of molecular electronic devices requires precisely controlled molecular assemblies with conductive, semiconductive, and insulating properties1 for applications as molecular wires, gates, switches, and magnetic storage media.10 Self-assembled organic monolayers (SAM) and other selforganizing structures have been prime fields of investigation for over 50 years.11 A variety of chemical interactionssincluding hydrogen bonding, covalent bonding, electrostatic interactions, and coordination chemistryshave been explored to produce ordered molecular and supramolecular structures. Layer-by-layer (LBL) growth allows supramolecular structures to be built up from surfaces through the self-assembly of successive, interacting monolayers. Monolayer films with reactive terminal groups have been extended into supramolecular structures through covalent chemical reactions.12 Electrostatic interactions have been used to deposit alternating layers of polyionic polymers with metallic * Corresponding author. E-mail:
[email protected]. † Charan Srinivasan’s current address is Department of Electrical Engineering, Pennsylvania State University, State College, PA 16801. (1) Chen, J.; Lee, T.; Su, J.; Wang, W.; Reed, M.; Rawlett, A.; Kozaki, M.; Yao, Y.; Jagessar, R.; Dirk, S.; Price, D.; Tour, J.; Grubisha, D.; Bennett, D. In Molecular Nanoelectronics; Reed, M., Lee, T., Eds.; American Scientific Publishers: New York, 2003; Chapter 3. (2) Cui, T.; Hua, F.; Lvov, Y. IEEE Trans. Electron. DeV. 2004, 51, 503. (3) Li, D.; Ratner, M. A.; Marks, T. J.; Zhang, C.; Yang, J.; Wong, G. K. J. Am. Chem. Soc. 1990, 112, 7389. (4) Neff, G.; Helfrich, M.; Clifton, M.; Page, C. Chem Mater. 2000, 12, 2363. (5) Holden, M.; Cremer, P. J. Am. Chem. Soc. 2003, 125, 8074. (6) Morikawa, M.; Yoshihara, M.; Endo, T.; Kimizuka, N. J. Am. Chem. Soc. 1990, 112, S, 2005, 127, 1358. (7) Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1988, 332, 426. (8) Rong, D.; Kim, Y. I.; Hong, H.-G.; Krueger, J. S.; Mayer, J. E.; Mallouk, T. E. Coord. Chem. ReV. 1990, 97, 237. (9) Resendiz, M.;, Noveron, J.; Disteldorf, H.; Fischer, S.; Stang, P. Org. Lett. 2004, 6, 651. (10) Smith, R.; Weiss, P. In Molecular Nanoelectronics; Reed, M., Lee, T. Eds.; American Scientific Publishers: New York, 2003; Chapter 6. (11) Bigelow, W.; Pickett, D.; Zisman, W. J. Colloid Sci. 1946, 1, 513; Shafrin, E.; Zisman, W. J. Colloid Sci. 1949, 4, 571.
ions,13 nanoparticles,14 or oppositely charged polymers;15 however, the ordering and lateral dimensional control of such systems is limited by the size and random nature of the polymer chains. Coordination chemistry provides a flexible avenue for creating organic/inorganic assemblies, including LBL structures,16-19 through the incorporation of metal ions with desired electrical, magnetic, or optoelectronic properties and organic ligands. Complexes of terpyridinyl ligands (Chart 1) with metal ions have received significant recent attention for the assembly of supramolecular structures in solution. Terpyridine acts as a terdentate ligand; thus two terpyridinyl groups can satisfy sixcoordinate octahedral symmetry for many metallic ions. A range of metal ions have been reported to chelate with bipyridine and terpyridine containing compounds, including Ru(II/III),20 Co(II),21 Fe(II),22 Os(II),21 Ni(II),22 Rh(III),23 Zn(II),24 Cu(I/II),24 Ag(I),24 and Pt(II).20 Terpyridinyl/metal coordination allows the formation of simple to complex molecular structures, including linear rodlike arrays,25 two-dimensional sheets,25 gridlike structures,26,27 dimeric,28 trimeric and tetrameric cycles,29 and supramolecular squares.30,31 (12) Sabapathy, R.; Crooks, R. Langmuir 2000, 16, 7783; Cui, T.; Hua, F.; Lvov, Y. IEEE Trans. Electron. DeV. 2004, 51, 503. (13) Lvov, Y.; Kamau, G.; Zhou, D.-L., Rusling, J. J. Colloid Sci. 1999, 212, 570. (14) Sun, S.; Anders, S.; Hamann, H.; Thiele, J.; Baglin, J.; Thomson, T.; Fullerton, E.; Murray, C.; Terris, B. J. Am. Chem. Soc. 2002, 124, 2884. (15) Decher, G. Science 1997, 277, 1232. (16) Fang, M.; Kaschak, D.; Sutorik, A.; Mallouk, T. J. Am. Chem. Soc. 1997, 119, 12184. (17) Lin, C.; Kagan, C. J. Am. Chem. Soc. 2003, 125, 336. (18) Ansell, M.; Cogan, E.; Page, C. Langmuir 2000, 16, 1172. (19) Hatzor, A.; man der Boom-Moav, T.; Yochelis, S.; Vaskevich, A.; Shanzer, A.; Rubinstein, I. Langmuir 2000, 16, 4420. (20) Hofmeier, H.; Schubert, U. Chem. Soc. ReV. 2004, 33, 373. (21) Maskus, M.; Abruna, H. Langmuir 1996, 12, 4455. (22) Arana, C.; Yan, S.; Keshavarz-K, M.; Potts, K.; Abruna, H. Inorg. Chem. 1992, 31, 3680. (23) Bera, J.; Campos-Fernandez, C.; Rodolphe, C.; Dunbar, K. Chem. Commun. 2002, 2536. (24) Norsten, T.; Frankamp, B.; Rotello, V. Nanoletters 2002, 2, 1345. (25) Alcock, N.; Barker, P.; Haider, J.; Hannon, M.; Painting, C.; Pikamenou, Z.; Plummer, E.; Rissanen, K.; Saarenketo, P. J. Chem. Soc., Dalton Trans. 2000, 9, 1447. (26) Baxter, P.; Lehn, J.-M.; Fischer, J.; Youinou, M.-T. Angew. Chem., Int. Ed. Engl. 1994, 33, 2284. (27) Ziener, U.; Breuning, E.; Lehn, J.-M.; Wegelius, E.; Rissanen, K.; Baum, G.; Fenske, D.; Vaughan, G. Chem.-Eur. J. 2000, 6, 4132. (28) Cornstable, E.; Schofield, E. Chem. Commun. 1998, 3, 403. (29) Ziessel, R. Synthesis 1999, 1839.
10.1021/la051922o CCC: $33.50 © 2006 American Chemical Society Published on Web 08/04/2006
7632 Langmuir, Vol. 22, No. 18, 2006 Chart 1. Terpyridinyl Ligands Used in This Study
Examples of surface-bound structures incorporating terpyridinyl groups are limited. Well-ordered films of terpyridylpendant dendrimers have been deposited on highly oriented pyrolytic graphite (HOPG); however the dendrimers were believed to form 1-D strands in solution which then deposited in an ordered fashion onto the surface.32 Bis(terpyridine)/Fe(II) compounds self-assembled on indium oxide nanowire FET’s were reported to influence the I-V curves of the devices,33 although the ordering and coverage of the terpyridine layer remains vague. Maskus and Abruna21 attempted to build LBL structures similar to those depicted in Scheme 1. They reported binding cobalt(II) to a terpyridine-terminated monolayer but were unsuccessful in building up additional layers. They succeeded in producing bilayers when Os(II)tppz was initially bound to the terpyridine monolayer and Co(II)tppz was subsequently added; however multilayer structures were not demonstrated with this process. Steric constraints during layer formation were proposed as a limiting factor. In this article, we report the LBL growth of metal ions and terpyridinyl ligands. The assembly of terpyridines with 21 different metals, including multiple oxidation states of several metals, was evaluated using spectroscopic, ellipsometric, and atomic force microscopy (AFM) measurements. Successful LBL growth appears to depend on the ionic radius of the metal ion. Ions with radii within a limited size range demonstrated consistent layered growth, while most ions that were outside this range exhibited no or inconsistent growth. Experimental Section Materials. Tetra-2-pyridinylpyrazine (tppz), 2,2′:6′,2′′-terpyridine (tpy), ammonium hexafluorophosphate, solvents, and most of the metal halides were obtained from Aldrich Chemicals. Tantalum(V) chloride and ruthenium(III) chloride hydrate were obtained from Strem Chemical, and cobalt(II) chloride was obtained from Fischer chemical. All chemicals were used as received. Ru(tppz)2(PF6)2, Mn(tpy)2(PF6)2, and mercaptophenyl terpyridine (MPTP) were synthesized based on known procedures.35,36 Preparation of Multilayers. Self-assembled layers were formed from dilute solutions (1-10 mM) of metal salts or terpyridine molecules in ambient air. Metal halides were dissolved in ethanol or ethanol/water mixtures except for titanium(IV) chloride, which (30) Sun, S.-S.; Lees, A. Inorg. Chem. 2001, 40, 3154. (31) Sun, S.-S.; Silva, S.; Brinn, I.; Lees, A. Inorg. Chem. 2000, 39, 1344. (32) Diaz, D.; Bernhard, S.; Storrier, G.; Abruna, H. J. Phys. Chem. 2001, 105, 8746. (33) Li, C.; Fan, W.; Straus, D.; Lei, B.; Asano, S.; Zhang, D.; Han, J.; Meyyappan, M.; Zhou, C. J. Am. Chem. Soc. 2004, 126, 7750. (34) Molecular dimensions estimated from MM2 energy minimization calculations of proposed structures using CambridgeSoft Chem 3D Pro computational chemistry software, version 5.0. (35) Thummel, R.; Chirayil, S. Inorg. Chim. Acta 1988, 154, 77. (36) Auditore, A.; Tuccitto, N.; Marzanni, G.; Quici, S.; Puntoriero, F.; Campagna, S.; Licciardello, A. Chem. Commun. 2003, 2494.
Kosbar et al. was purchased as a 1.0 M solution in toluene and diluted to 10 mM with toluene. Some solutions required gentle heating. MPTP was dissolved in a 3:1 mixture of toluene:ethanol, and tppz was dissolved in a 1:1 mixture of toluene:ethanol to produce 1 mM solutions. All substrates were cleaned of organic contaminants prior to monolayer assembly by exposure for 20 min to UV light and ozone in a Jelight UVO Cleaner (Model 144). Reflectance IR was used to confirm the removal of organic contaminants. Gold-coated samples were immersed in the MPTP solution overnight, followed by rinsing and sonication for one minute each in 1:1 toluene:ethanol and pure ethanol and dried under a nitrogen stream. To build LBL structures, MPTP-covered substrates were dipped alternately in metal halide and tppz solutions for 10 min each, rinsing and sonicating between dipping steps. Samples immersed in metal halide solutions were sonicated for two minutes in ethanol, while those dipped in tppz were sonicated for one minute each in 1:1 toluene:ethanol and pure ethanol. Samples were dried under a nitrogen stream. Evaluation of Film Formation. UV Spectra. UV measurements were taken on a Hewlett-Packard 8453 single-beam UV-visible spectrometer with a photodiode array detector. Spectra for pure bulk materials were obtained in acetonitrile or methanol. Substrates for UV measurements of films were cleaned 1-in. quartz disks coated with 20 Å of titanium followed by 50 Å of gold to produce samples which had a maximum initial UV absorbance of
