Langmuir 2002, 18, 5971-5973
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Formation of Conjugated Azomethine Oligomers on Quartz and Silicon Oxide Surfaces Jose Amado M. Dinglasan, Ehtesham Baig, and Al-Amin Dhirani* Department of Chemistry, University of Toronto, Toronto, Ontario, Canada M5S 3H6 Received January 30, 2002. In Final Form: May 31, 2002 We describe a solution phase method to generate conjugated oligoazomethine assemblies on quartz and silicon oxide surfaces. The method is based on a controlled iminization reaction in which aromatic aldehydes and amines (1,4-terephthaldicarboxaldehyde, TPDA, and 1,4-phenylenediamine, PPDA) are sequentially assembled onto amino-functionalized substrates. Polarized attenuated total reflectance Fourier transform infrared spectroscopy and spectroscopic ellipsometry confirm monomer assembly on oxidized silicon surfaces. Using UV-visible spectroscopy with quartz substrates, we explored the roles of temperature, acid catalyst, and monomer concentration on rates of film formation. Arrhenius plots of observed rate constants yielded values for activation energies of 28 ( 7, 14 ( 7, and 17 ( 8 kJ mol-1 for uncatalyzed TPDA, catalyzed TPDA, and catalyzed PPDA assembly, respectively. Observed rate constants for TPDA assembly grew linearly with monomer concentration, while those for PPDA assembly were independent of monomer concentration. Fully formed films were found to be remarkably stable to repeated sonication and exposure to solvents at elevated temperatures.
Organic assemblies may be used to tune properties of solid surfaces and interfaces and may find applications in a number of technologically important areas. These include corrosion inhibition,1 development of novel lithographic strategies,2,3 chemical control over electrical properties of devices such as Schottky barriers,4,5 and metal-insulatormetal tunnel junctions,6-9 as well as development of new electronic devices such as molecular-scale transistors,10,11 rectifiers,12-16 and switches.8,17 In this paper, we explore the use of functionalized organic assemblies to implement a controlled liquid-phase iminization reaction between aromatic aldehydes and amines, in particular 1,4-terephthaldicarboxaldehyde (TPDA) and 1,4-phenylenediamine (PPDA). The reaction can be used to produce surfacebound aromatic azomethine oligomers, which are isolelectronic with p-polyphenylenevinylene (PPV) and exhibit * To whom correspondence should be addressed. Electronic mail:
[email protected]. (1) Zamborini, F. P.; Crooks, R. M. Langmuir 1997, 13, 122. (2) Whidden, T. K.; Ferry, D. K.; Kozicki, M. N.; Kim, E.; Kumar, A.; Wilbur, J.; Whitesides, G. M. Nanotechnology 1996, 7, 447. (3) Huck, W. T. S.; Yan, L.; Stroock, A.; Haag, R.; Whitesides, G. M. Langmuir 1999, 15, 6862. (4) Campbell, I. H.; Rubin, S.; Zawodzinski, T. A.; Kress, J. D.; Martin, R. L.; Smith, D. L.; Barashkov, N. N.; Ferraris, J. P. Phys. Rev. B 1996, 54, 14321. (5) Campbell, I. H.; Rubin, S.; Zawodzinski, T. A.; Kress, J. D.; Martin, R. L.; Smith, D. L.; Barashkov, N. N.; Ferraris, J. P. Appl. Phys. Lett 1997, 71, 3528. (6) Tour, J. M. Acc. Chem. Res. 2000, 33, 791. (7) Wong, E. W.; Collier, C. P.; Begloradsky, M.; Raymo, F. M.; Stoddart, J. F.; Heath, J. R. J. Am. Chem. Soc. 2000, 122, 5831. (8) Collier, C. P.; Jeppesen, J. O.; Luo, Y.; Perkins, J.; Wong, E. W.; Heath, J. R.; Stoddart, J. F. J. Am. Chem. Soc. (9) Chen, J.; Calvet, L. C.; Reed, M. A.; Carr, D. W.; Grubisha, D. S.; Bennett, D. W. Chem. Phys. Lett. 1999, 313, 741. (10) Scho¨n, J. H.; Meng, H.; Bao, Z. Nature 413, 713. (11) Garnier, F. Philos. Trans. R. Soc. London, Ser. A 1997, 355, 815. (12) Dhirani, A.; Lin, P.-H.; Guyot-Sionnest, P.; Zehner, R. E.; Sita, L. R. J. Chem. Phys. 1997, 106, 5249. (13) Sentein, C.; Fiorini, C.; Lorin, A.; Sicot, L.; Nunzi, J.-M. Opt. Mater. 1998, 9, 316. (14) Martin, A. S.; Sambles, J. R.; Ashwell, G. J. Phys. Rev. Lett. 1993, 70, 218. (15) Zhou, C.; Deshpande, M. R.; Reed, M. A.; Jones, L., II; Tour, J. M. Appl. Phys. Lett. 1997, 71, 611. (16) Li, D. Q.; Bishop, A.; Gim, Y.; Shi, X. B.; Fitzsimmons, M. R.; Jia, Q. X. Appl. Phys. Lett. 1998, 73, 2645. (17) Gittins, D. I.; Bethel, D.; Schiffrin, D. J.; Nichols, R. J. Nature 2000, 408, 67.
interesting optoelectronic behavior arising from their high degree of conjugation.18-21 Yoshimura and co-workers have shown that physisorbed polyazomethines, prepared by chemical vapor deposition of TPDA and PPDA on quartz, exhibit exitonic behavior and optical nonlinearities.22-25 Using an aminothiophenol primer layer and the iminization rection, Rosink and co-wokers have assembled oligoazomethines onto gold substrates.26,27a Their scanning tunneling spectroscopy studies revealed current voltage characteristics with oligomer length dependent asymmetries and nonlinearities.28 Controlled iminization is a straightforward and versatile approach for modifying surface and interfacial properties as it can be used to generate surface-bound oiligoazomethines on a variety of surfaces and a range of reagents are readily available. We describe herein an extension of the approach to the chemisorption of oiligoazomethines onto oxide surfaces, namely, silicon oxide and quartz. To guide optimal growth of such films, the roles of reaction parameters, such as temperature, monomer concentration, and the presence of acid catalyst, are discussed quantiatively. The reaction scheme we followed is illustrated in Figure 1. Initially, substratesscommercial grade fused quartz slides (Quartz Plus, Inc., Brookline, NH), a 50 mm × 10 mm × 3 mm Si prism (Spectra-Tech Inc., Shelton, CT), (18) Lee, T. S.; Kim, J.; Kumar, J.; Tripathy, S. Macromol. Chem. Phys. 1998, 199, 1445. (19) Yang, C.-J.; Jenekhe, S. A. Macromolecules 1995, 28, 1180. (20) Yang, C.-J.; Jenekhe, S. A. Chem. Mater. 1994, 6, 196. (21) Yang, C.-J.; Jenekhe, S. A. Chem. Mater. 1991, 3, 878. (22) Yoshimura, T.; Tatsuura, S.; Sotoyama, W. Appl. Phys. Lett. 1991, 59, 482. (23) Yoshimura, T.; Tatsuura, S.; Sotoyama, W.; Matsuura, A.; Hayano, T. Appl. Phys. Lett. 1992, 60, 298. (24) McElvain, J.; Tatsuura, S.; Wudl, F.; Heeger, A. J. Synth. Met. 1998, 101. (25) Fischer, W.; Stelzer, F.; Meghdadi, F.; Leising, G. Synth. Met. 1996, 76, 201. (26) Rosink, J. J. W. M.; Blauw, M. A.; Geerligs, L. J.; van der Drift, E.; Rousseeuw, B. A. C.; Radelaar, S. Mater. Sci. Eng., C 1999, 8-9, 267. (27) (a) Rosink, J. J. W. M.; Blauw, M. A.; Geerligs, L. J.; van der Drift, E.; Rousseeuw, B. A. C.; Radelaar, S.; Sloof, W. G.; Fakkeldij, E. J. M. Langmuir 2000, 16, 4547. (b) Moon, J. H.; Shin, J. W.; Kim, S. Y.; Park, J. W. Langmuir 1996, 4621. (28) Rosink, J. J. W. M.; Blauw, M. A.; Geerligs, L. J.; van der Drift, E.; Rousseeuw, B. A. C.; Radelaar, S. Phys. Rev. B 2000, 62, 10459.
10.1021/la025590o CCC: $22.00 © 2002 American Chemical Society Published on Web 06/27/2002
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Figure 2. (Main panel) UV-vis absorption spectra of aminofunctionalized quartz slides after self-assembly of TPDA (solid line) and subsequently PPDA (dashed line). (Inset) Time dependence of the 270 nm absorption peak during TPDA selfassembly (b) and desorption (2) measurements. Assembly data were obtained using a 1 mM methanolic solution maintained at 46 °C. The solid line is a fit to an exponential curve of the form θ ) A[1 - exp(-kobst)]. Desorption measurements were obtained by immersing and sonicating a fully formed TPDA assembly in neat methanol at 46 °C. The dotted line serves a guide to the eye.
and a Si(111) wafer (phosporus-doped, 0.02-0.5 Ω‚cm, Virginia Semiconductor, VA) cut into 2 cm squares with a diamond scribeswere cleaned in hot (80-90 °C) piranha solution (7 parts sulfuric acid/3 parts 30% H2O2) for at least 30 min, rinsed with deionized water, and dried with prepurified nitrogen (Matheson). Amino-terminated monolayers were obtained using, with a slight modification, a procedure employed by Schimmel.29 Briefly, freshly cleaned substrates were immersed, along with approximately 2 g of molecular sieve (Aldrich, 4A, beads, 4-8 mesh), into a 4% (v/v) toluene solution of (3-aminopropyl)triethoxysilane (Aldrich, APTES 99%). The solution was maintained at 80 °C, and substrates were immersed for 10 min. Silanized substrates were rinsed and sonicated for 2 min in toluene and dried with N2. Films of i and ii were formed by immersing the amino-functionalized substrates into methanolic solutions of TPDA (Aldrich, 99%) and subsequently PPDA (Aldrich, 97%), rinsing and sonicating in methanol for 1 min to remove excess material, and then drying with N2. UV-visible spectra were obtained using an HP 4258-B diode array UV-visible spetrophotometer. Polarized FTIR spectra were taken with a Nicolet Dimension 470 FTIR spectrophotometer equipped with a liquid-nitrogen-cooled MCT detector, dry air purge system, wire grid ZnSe polarizer, and a variable-angle attenuated total reflectance (ATR) attachment (Spectra-Tech Inc., Shelton, CT) set at 45° for our measurements. Thickness measurements were conducted using a Sopra ES4G-ESVG ellipsometer in the 400-800 nm spectral range. The angle of incidence was set at 75°, and refractive indices of 1.41 and 1.55 were used to calculate film thicknesses of APTES and of i and ii, respectively.27 Figure 2 (main panel) shows UV-visible spectra of an amino-functionalized quartz slide after TPDA and sub-
sequent PPDA assembly. An absorption peak at 270 nm exhibited by the TPDA-treated sample is indicative of TPDA chemisorption and the formation of i. The absorption peak has also been observed by Yoshimura22,23 and is typical of phenyl derivatives on surfaces. The formation of ii is indicated by the appearance of a second red-shifted peak at about 400 nm. This peak is evidence of an increase in the electron delocalization as the oligomer chain length increases. Polarized ATR-FTIR spectroscopy and spectroscopic ellipsometry confirm the chemisorption of both TPDA and subsequently PPDA on amino-functionalized silicon oxide. Upon formation of i, ATR-FTIR reveals a band at 1670 cm-1 attributed to the CdN group,30 along with another distinct peak at around 1700 cm-1 attributed to a free CdO moiety. The 1700 cm-1 peak disappeared after subsequent exposure to PPDA. Ellipsometry yields thickness measurements of 12 ( 2, 18 ( 1, and 23 ( 1 Å for APTES, i, and ii, respectively. The formation and thermal stability of i and ii can be monitored using the heights of the U V-visible absorption peaks at 270 and 400 nm, respectively. Figure 2 (inset) shows the time evolution of the 270 nm peak height, determined from spectra obtained during the formation of i, along with a fit to a model of the form θ ) A[1 exp(-kobst)], where kobs is interpreted as an effective rate constant. Although the quality of the fit suggests that TPDA assembly is well described by a simple exponential growth model, further results discussed below indicate the underlying kinetics do not follow a simple Langmuirian model. The fit does, however, provide a reasonable estimate of the half-life of formation as ln 2/kobs, or 20 min using a 1 mM methanolic TPDA solution at 46 °C. The thermal stability of i and ii was also investigated by exposing fully formed films to solvent at 46 °C for 160 min and by sonicating the films in solvent for several minutes (see Figure 2, inset). Time evolution of the 270 nm peak height for i indicates that the formed film is remarkably stable. Combined, the data suggest that formation of these films can be optimally performed at elevated temperatures
(29) Siqueria Petri, D. F.; Wenz, G.; Schunk, P.; Schimmel, T. Langmuir 1999, 15, 4520.
(30) Horton, R. C., Jr.; Herne, T. M.; Myles, D. C. J. Am. Chem. Soc. 1997, 119, 12980.
Figure 1. Formation of surface-bound π-conjugated azomethine oligomers by controlled iminization of aromatic dialdehydes and diamines onto an amino-functionalized substrate.
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Figure 3. Arrhenius plots for acid-catalyzed (9) and noncatalyzed (0) TPDA self-assembly and subsequent acidcatalyzed PPDA assembly (2). The plots have been offset for clarity. kobs* is obtained by normalizing kobs by RC + β, where C is monomer concentration used at a given temperature and R and β are determined from measurements of kobs vs C at fixed temperature. The solid lines are linear fits to the data. The negative slopes yield activation energies of 14 ( 7, 28 ( 7, and 17 ( 8 kJ mol-1 for (9), (0), and (2), respectively.
without the risk of desorption or thermal degredation. Similar results were obtained for films of ii. Effective rate constants extracted from absorption vs time data were used to monitor the effects of acid catalyst and temperature on the rates of formation of the azomethine films. Arrhenius plots for the formation of i (both with and without acid) and of ii (with acid) over a temperature range of 21-52 °C are shown in Figure 3. The temperature range was limited by the boiling point of the solvent used (methanol bp 64 °C). Slopes of the plots provide estimates of the activation energies for the assembly processes, that is, 28 ( 7 kJ mol-1 for uncatalyzed TPDA assembly, 14 ( 7 kJ mol-1 for catalyzed TPDA assembly, and 17 ( 8 kJ mol-1 for catalyzed PPDA assembly. Note that the activation energies for the catalyzed formation of i and ii are similar. However, activation energies for the catalyzed and uncatalyzed formation of i differ by a factor of 2, suggesting that a small amount of acid greatly facilitates film formation. Figure 4 shows the effect of TPDA concentration on the rate of formation of i. kobs exhibits a linear relationship with concentration in the 0.1-3 mM range, interestingly, with a statistically significant nonzero intercept. In a simple first-order reversible Langmuir model, a nonzero intercept can be attributed to a desorption process. However, if the experimentally determined intercept is interpreted in this fashion, the half-life of desorption is expected to be ∼ln 2/kobs or approximately 34 min, which is not consistent with the lack of desorption indicated by Figure 2. Indeed, kobs vs PPDA concentration measurements indicate a nonzero intercept and an essentially
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Figure 4. Dependence of kobs on concentration for TPDA selfassembly. The solid line is a linear fit to the data, with a slope of 0.014 ( 0.006 mM-1 min-1 and an intercept of 0.020 ( 0.003 min-1.
concentration independent growth for the same concentration range, while fully formed films are resistant to desorption. These results suggest that the formation of i and ii is not described by simple Langmuir kinetics, and further studies are required to elucidate the kinetics and growth mechanisms involved. It is possible, for instance, that physisorption of molecules plays an important role in the assembly process, as has been observed in alkanethiol self-assembly onto gold.31,32 In summary, we have described a simple and versatile method, based on a condensation reaction between aromatic aldehydes and amines, to produce conjugated oligomer films on oxide surfaces. We determined the roles of temperature, acid catalyst, and monomer concentration on film formation by measuring effective rate constants for film formation under various conditions. Although exact details of the assembly process are not fully understood, the data presented illustrate a general utility of controlled iminization to effectively modify oxide surfaces. Future work in our lab will explore electrical properties of these conjugated assemblies in different device configurations. Acknowledgment. This work was supported by the Natural Science and Engineering Council for Canada, the Canadian Foundation for Innovation, Ontario Innovation Trust, and the Connaught Fund. A.D. acknowledges support through the Premier’s Research Excellence Award of Ontario and the Nortel Institute. We are grateful to Edward H. Sargent and Qiying Chen for their assistance with ellipsometry measurements. LA025590O (31) Xu, S.; Cruchon-Dupeyrat, S. J. N.; Garno, J. C.; Liu, G.-Y.; Jennings, G. K.; Yong, T.-H.; Laibinis, P. E. J. Chem. Phys. 1998, 108, 5002. (32) Peterlinz, K. A.; Georgiadis, R. Langmuir 1996, 12, 4731.
