Layer-by-Layer-Assembled Multilayer Films of Polyelectrolyte

The stable incorporation of pyrene into multilayer films was confirmed by quartz ... which was developed by Decher and co-workers in the early 1990s,(...
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12986

Langmuir 2008, 24, 12986-12989

Layer-by-Layer-Assembled Multilayer Films of Polyelectrolyte-Stabilized Surfactant Micelles for the Incorporation of Noncharged Organic Dyes Xiaokong Liu, Lu Zhou, Wei Geng, and Junqi Sun* State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin UniVersity, Changchun, P. R. China 130012 ReceiVed August 15, 2008. ReVised Manuscript ReceiVed September 12, 2008 Noncharged pyrene molecules were incorporated into multilayer films by first loading pyrene into poly(acrylic acid) (PAA)-stabilized cetyltrimethylammonium bromide (CTAB) micelles (noted as PAA&(Py@CTAB)) and then layerby-layer (LbL) assembled with poly(diallyldimethylammonium chloride) (PDDA). The stable incorporation of pyrene into multilayer films was confirmed by quartz crystal microbalance (QCM) measurements and UV-vis absorption spectroscopy. The resultant PAA&(Py@CTAB)/PDDA multilayer films show an exponential growth behavior because of the increased surface roughness with increasing number of film deposition cycles. The present study will open a general and cost-effective avenue for the incorporation of noncharged species, such as organic molecules, nanoparticles, and so forth, into LbL-assembled multilayer films by using polyelectrolyte-stabilized surfactant micelles as carriers.

Introduction Layer-by-layer (LbL) assembly technique, which was developed by Decher and co-workers in the early 1990s,1 has emerged as a group of versatile and convenient methods for the construction of advanced functional layered composite films with precise control of film thickness and composition.2 On the basis of electrostatic interaction as the driving force, various water-soluble charged species, including biomacromolecules,3 oligo-charged dyes,4 colloids,5 vesicles,6 and so forth, have been successfully assembled into LbL-assembled multilayer films. Meanwhile, some of the noncharged polymers can be assembled into multilayer films employing hydrogen-bond,7 halogen-bond,8 coordination* To whom correspondence should be addressed. Fax: 0086-43185193421. E-mail: [email protected]. (1) (a) Decher, G. Science 1997, 277, 1232. (b) Decher, G.; Hong, J. D. Makromol. Chem., Macromol. Symp. 1991, 46, 321. (c) Decher, G.; Hong, J. D. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 1430. (2) (a) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319. (b) Hammond, P. T. AdV. Mater. 2004, 16, 1271. (c) Tang, Z. Y.; Wang, Y.; Podsiadlo, P.; Kotov, N. A. AdV. Mater. 2006, 18, 3203. (d) Quinn, J. F.; Johnston, A. P. R.; Such, G. K.; Zelikin, A. N.; Caruso, F. Chem. Soc. ReV. 2007, 36, 707. (e) Zhang, X.; Chen, H.; Zhang, H. Y. Chem. Commun. 2007, 1395. (3) (a) Kong, W.; Zhang, X.; Gao, M. L.; Zhou, H.; Li, W.; Shen, J. C. Macromol. Rapid Commun. 1994, 15, 405. (b) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. J. Am. Chem. Soc. 1995, 117, 6117. (c) Lvov, Y.; Lu, Z.; Schenkman, J. B.; Zu, X.; Rusling, J. F. J. Am. Chem. Soc. 1998, 120, 4073. (d) Picart, C.; Lavalle, Ph.; Hubert, P.; Cuisinier, F. J. G.; Decher, G.; Schaaf, P.; Voegel, J.-C. Langmuir 2001, 17, 7414. (e) Serizawa, T.; Yamaguchi, M.; Akashi, M. Macromolecules 2002, 35, 8656. (f) Johnston, A. P. R.; Read, E. S.; Caruso, F. Nano Lett. 2005, 5, 953. (g) Yu, A.; Liang, Z.; Caruso, F. Chem. Mater. 2005, 17, 171. (4) (a) Zhang, X.; Gao, M. L.; Kong, X. X.; Sun, Y. P.; Shen, J. C. Chem. Commun. 1994, 1055. (b) Ariga, K.; Lvov, Y.; Kunitake, T. J. Am. Chem. Soc. 1997, 119, 2224. (c) Saremi, F.; Tieke, B. AdV. Mater. 1998, 10, 388. (d) Tedeschi, C.; Caruso, F.; Mo¨hwald, H.; Kirstein, S. J. Am. Chem. Soc. 2000, 122, 5841. (e) Advincula, R. C.; Fells, E.; Park, M. K. Chem. Mater. 2001, 13, 2870. (5) (a) Gao, M. Y.; Gao, M. L.; Zhang, X.; Yang, Y.; Yang, B.; Shen, J. C. Chem. Commun. 1994, 2777. (b) Schmitt, J.; Decher, G. AdV. Mater. 1997, 9, 61. (c) Mamedov, A. A.; Belov, A.; Giersig, M.; Mamedova, N. N.; Kotov, N. A. J. Am. Chem. Soc. 2001, 123, 7738. (6) (a) Michel, M.; Vautier, D.; Voegel, J.-C.; Schaaf, P.; Ball, V. Langmuir 2004, 20, 4835. (b) Michel, M.; Arntz, Y.; Fleith, G.; Toquant, J.; Haikel, Y.; Voegel, J.-C.; Schaaf, P.; Ball, V. Langmuir 2006, 22, 2358. (7) (a) Wang, L. Y.; Wang, Z. Q.; Zhang, X.; Shen, J. C.; Chi, L. F.; Fuchs, H. Macromol. Rapid Commun. 1997, 18, 509. (b) Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 2717. (c) Wang, L.; Cui, S.; Wang, Z.; Zhang, X.; Jiang, M.; Chi, L.; Fuchs, H. Langmuir 2000, 16, 10490. (d) Fu, Y.; Chen, H.; Qiu, D. L.; Wang, Z. Q.; Zhang, X. Langmuir 2002, 18, 4989.

bond,9 and charge-transfer interaction10 as the driving force. However, the LbL assembly of noncharged organic molecules without necessary functional groups into multilayer films remains a challenge. As one of the successful and frequently used approaches, the organic molecules were first encapsulated into block copolymer micelles, and then the resultant micelles were further alternatively deposited with a partner polyelectrolyte to produce multilayer films.11-14 Kataoka and co-workers encapsulated noncharged pyrene (Py) into reactive micelles of poly(ethylene glycol)-poly(D,L-lactide) (PEG-PLA) bearing an acetal group at the PEG end. Py-incorporated multilayer films were fabricated by alternatively depositing the Py-loaded micelles and polyallyamine in the presence of a reducing reagent.11 Without using the troublesome interfacial reaction, Zhang and co-workers loaded Py into negatively charged poly(styrene-b-acrylic acid) copolymer micelles and fabricated Py-containing multilayer films by LbL deposition of the negatively charged micelles with a partner polycation.12 Recently, LbL-assembled block copolymer micelles have been fabricated for use as photochromic films13 and controlled releasing coatings.14 Although the noncharged organic molecules can be incorporated into LbL-assembled multilayer films by employing copolymer micelles as carriers, block copolymers are usually expensive and difficult to synthesize. Therefore, facile methods without the use of expensive block copolymers are still highly desirable to realize the incorporation of noncharged species into LbL-assembled multilayer films. Herein, polyelectrolytestabilized surfactant micelles were used as carriers for noncharged (8) Wang, F.; Ma, N.; Chen, Q.; Wang, W.; Wang, L. Langmuir 2007, 23, 9540. (9) Xiong, H. M.; Cheng, M. H.; Zhou, Z.; Zhang, X.; Shen, J. C. AdV. Mater. 1998, 10, 529. (10) Shimazaki, Y.; Mitsuishi, M.; Ito, S.; Yamamoto, M. Langmuir 1997, 13, 1385. (11) (a) Emoto, K.; Nagasaki, Y.; Kataoka, K. Langmuir 2000, 16, 5738. (b) Emoto, K.; Iijima, M.; Nagasaki, Y.; Kataoka, K. J. Am. Chem. Soc. 2000, 122, 2653. (12) Ma, N.; Zhang, H.; Song, B.; Wang, Z.; Zhang, X. Chem. Mater. 2005, 17, 5065–5069. (13) (a) Ma, N.; Wang, Y.; Wang, Z.; Zhang, X. Langmuir 2006, 22, 3906. (b) Cho, J.; Hong, J.; Char, K.; Caruso, F. J. Am. Chem. Soc. 2006, 128, 9935. (14) (a) Qi, B.; Tong, X.; Zhao, Y. Macromolecules 2006, 39, 5714. (b) Nguyen, P. M.; Zacharia, N. S.; Verploegen, E.; Hammond, P. T. Chem. Mater. 2007, 19, 5524.

10.1021/la802674h CCC: $40.75  2008 American Chemical Society Published on Web 10/18/2008

LbL Assembly of PAA&(Py@CTAB) Multilayer Films Scheme 1. Preparative Process of PAA-Stabilized Py@CTAB Micelles

species and further LbL assembled with oppositely charged partner polyelectrolyte to produce multilayer films. The present work provides a facile and inexpensive method to incorporate noncharged species into LbL-assembled multilayer films by employing easily available surfactant micelles. To demonstrate the feasibility of the present method, cetyltrimethylammonium bromide (CTAB) micelles stabilized by poly(acrylic acid, sodium salt) (PAA) were used as carriers and PAA-stabilized CTAB micelles loaded with Py were LbL-assembled with poly(diallyldimethylammonium chloride) (PDDA) to fabricate Pyincorporated polymeric multilayer films.

Experimental Section Materials. CTAB was analytical grade and purchased from Beijing Chemical Reagents Company. PDDA (Mw ca. 100 000-200 000), PAA (Mw ca. 15 000), sodium poly(styrene sulfonate) (PSS, Mw ca. 70 000), and Py were obtained from Sigma-Aldrich. Deionized water was used for all the experiments. Preparation of PAA-Stabilized CTAB Micelles Loaded with Py. CTAB can self-organize into micelles in water above its critical micelle concentration (cmc) because of its amphiphilic property. Hydrophobic guest molecules such as Py can be encapsulated into the hydrophobic cores of CTAB micelles. As shown in Scheme 1, CTAB micelle solution can be easily obtained by dissolving 0.15 g of CTAB into 40 mL of deionized water. Then 50 µL of 40 mg/mL dichloromethane solution of Py was injected into the as-prepared 40 mL CTAB micelle solution followed by 1 h of continuous sonication. After being left open overnight under continuous stirring for the dichloromethane to evaporate, the solution was filtered to eliminate the precipitated Py. In this way, Py-loaded CTAB micelles (noted as Py@CTAB) were obtained. Next, 10 mL of 15.5 mg/mL PAA aqueous solution was poured into the as-prepared 40 mL of Py@CTAB micelle solution under continuous stirring. Complexes (noted as PAA&(Py@CTAB)) formed between the negatively charged PAA and the positively charged Py@CTAB micelles because of the electrostatic interaction between them.15 The Py@CTAB micelles were thus stabilized by PAA, and PAA&(Py@CTAB) micelles formed. Finally, the pH of the dispersion of the PAA&(Py@CTAB) complexes was adjusted to 12.2 with 1 M NaOH. In this way, the feed molar ratio of PAA monomer to CTAB is 4:1 in the dispersion of PAA&(Py@CTAB) complexes. Treatment for Substrates. Quartz and silicon wafers were immersed in piranha solution (1:3 mixture of 30% H2O2 and 98% H2SO4) and heated until no bubbles were released. (Caution: Piranha solution reacts Violently with organic material and should be handled carefully.) After being rinsed with ample water and dried with nitrogen flow, the resulting substrates were modified by 2.5 bilayers of PDDA/ PAA precursor films with PDDA as the outmost layer. The PDDA/ PAA precursor films were fabricated by alternatively immersing the cleaned substrate in an aqueous solution of 1 mg/mL PDDA and 1 mg/mL PAA for 20 min, with intermediate water washing for 1 min followed by nitrogen drying. Ag-coated quartz crystal microbalance (15) (a) Zhou, S.; Chu, B. AdV. Mater. 2000, 12, 545. (b) Klotz, J.; Kosmella, S.; Beitz, T. Prog. Polym. Sci. 2001, 26, 1199. (c) Fundin, J.; Hansson, P.; Brown, W.; Lidegran, I. Macromolecules 1997, 30, 1118.

Langmuir, Vol. 24, No. 22, 2008 12987 Scheme 2. Schematic Illustration of the LbL Deposition Process for Fabrication of PAA&(Py@CTAB)/PDDA Multilayer Films

(QCM) resonators were sonicated sequentially in ethanol and water and were dried by nitrogen flow. A 2.5-bilayer PDDA/PAA precursor film was also deposited as in the case for silicon modification. Film Preparation. As shown in Scheme 2, The procedure of the fabrication of the Py-incorporated multilayer films is as follows: A substrate precoated with a 2.5-bilayer of PDDA/PAA film was first immersed into an aqueous dispersion of PAA&(Py@CTAB) complexes for 20 min. After rinsing in two water baths (pH ) 12.5) for 1 min in each and drying under a nitrogen stream, the substrate was transferred into an aqueous solution of 1 mg/mL PDDA for 20 min followed by rinsing in two water baths for 1 min in each and drying under a nitrogen stream. Multilayer films of [PAA&(Py@CTAB)/PDDA]*n (note: n refers to the number of film deposition cycles) can be fabricated by repeating the deposition steps of PAA&(Py@CTAB) complexes and PDDA in a cyclic fashion. Characterization. UV-vis absorption spectra were recorded with a Shimadzu UV-2550 spectrophotometer. QCM measurements were taken with a KSV QCM-Z500 using quartz resonators with both sides coated with Ag (F0 ) 9 MHz). Photoluminescence spectra were collected on a Shimadzu RF-5301 PC spectrometer. The multilayer film-thickness determination was carried out on a XL30 ESEM FEG scanning electron microscope (SEM). All samples were coated with a thin layer of gold prior to imaging. Atomic force microscopy (AFM) images were taken with a Nanoscope IIIa AFM Multimode (Digital Instruments, Santa Barbara, CA) under ambient conditions. AFM was operated in the tapping mode with an optical readout using Si cantilevers. Dynamic light scattering (DLS) studies and ζ-potential measurements were carried out on a Malvern NanoZS Zetasizer at room temperature. The measurements were made at a scattering angle of θ ) 173° at 25 °C using a He-Ne laser with a wavelength of 633 nm.

Results and Discussion The dispersion of the PAA&(Py@CTAB) complexes was cloudy but homogeneous. As shown in Figure 1, DLS measurements reveal that the PAA&(Py@CTAB) complexes exhibit polydisperse but monomodal size distribution with an average hydrodynamic diameter of 237 nm. PAA&(Py@CTAB) complexes are negatively charged with a ζ-potential of -42.6 mV, further confirming that PAA locates on the outer surface of the

Figure1. HydrodynamicdiameterdistributioncurveofPAA&(Py@CTAB) complexes in deionized water with a pH of 12.2.

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Figure 3. Fluorescence spectrum of an as-prepared [PAA&(Py@CTAB)/ PDDA]*8 film.

Figure 2. (a) QCM frequency decrease (-∆F) of alternative deposition of PAA&(Py@CTAB) complexes (O) and PDDA (b). (b) UV-vis absorption spectra of [PAA&(Py@CTAB)/PDDA]*n films with n ranging from 1 to 8 from bottom to top. The UV-vis absorption spectra were modified by equalizing their absorbance at 350 nm.

Py@CTAB micelles. The negatively charged PAA&(Py@CTAB) complexes allow them to be used for LbL deposition with polycation PDDA to produce PAA&(Py@CTAB)/PDDA multilayer films. The deposition process of PAA&(Py@CTAB)/ PDDA multilayer films was characterized by QCM measurements and UV-vis absorption spectroscopy. As indicated in Figure 2a, successive QCM frequency decreases are detected with the alternative deposition of PAA&(Py@CTAB) complexes and PDDA, verifying the successful fabrication of PAA&(Py@CTAB)/ PDDA multilayer films. Meanwhile, the QCM measurements reveal that the as-prepared PAA&(Py@CTAB)/PDDA films show an exponential growth pattern. To investigate whether the Py molecules could be successfully loaded into the LbL-assembled PAA&(Py@CTAB)/PDDA multilayer films, UV-vis absorption spectra of PAA&(Py@CTAB)/PDDA films with different deposition cycles were measured. The spectra after deposition of each PDDA layer are presented in Figure 2b. The characteristic absorptions of Py regularly increase with increasing number of film deposition cycles, affirmatively confirming the successful incorporation of Py in the as-prepared PAA&(Py@CTAB)/PDDA films. It should be noted that, for clarity in comparison, the UV-vis absorption spectra were modified by equalizing their absorbance at 350 nm. The unmodified UV-vis absorption spectra are provided in the Supporting Information as Figure S1. The large surface roughness of the PAA&(Py@CTAB)/PDDA films, especially in the initial several deposition cycles, led to light scattering and made the spectra move irregularly. The UV-vis spectra after deposition of each PAA&(Py@CTAB) complex layer were also recorded. In the beginning four deposition cycles, the UV-vis absorbance of Py decreases after the deposition of the PDDA layer, indicating the release of Py during the immersion of the films into PDDA aqueous solution. From the fifth deposition cycle, no release of Py was detected because the UV-vis absorbance of Py showed no decrease after the PDDA layer deposition. The release of Py in the early four deposition cycles of PAA&(Py@CTAB)/PDDA films originates from the desorp-

tion of PAA&(Py@CTAB) complexes, as also revealed by the QCM measurements in Figure 2a. The desorption of PAA&(Py@CTAB) complexes is assumed to be related with their excessive deposition in the beginning four deposition cycles. Controlled experiments revealed that CTAB micelles could not be alternatively deposited with polyanions such as PSS to produce multilayer film because CTAB micelles disassembled during the LbL deposition process. Therefore, the PAA-stabilized CTAB micelles are essential for realizing the incorporation of Py into LbL-assembled films. The peak intensity ratio of bands I ([0,0] band) to III ([0,2] band) (II/IIII) of Py’s emission spectrum is very sensitive to the polarity of its microenvironment. A lower ratio means a more hydrophobic environment in which Py locates.16 The II/IIII ratios for Py dissolved in the CTAB micelle solution and water were reported to be 1.16 and 1.64, respectively.17 As shown in Figure 3, The fluorescence intensity ratio of the two peaks at 373 and 383 nm (II/IIII) for Py in the as-prepared [PAA&(Py@CTAB)/ PDDA]*8 film is around 0.99. Such a low II/IIII ratio confirms that Py molecules exist in the hydrophobic cores of CTAB micelles in PAA&(Py@CTAB)/PDDA multilayer films.16-18 This result also supports that the CTAB is still in the micellar structure in the as-prepared PAA&(Py@CTAB)/PDDA films. The lower II/ IIII ratio for Py in PAA&(Py@CTAB)/PDDA film than that in CTAB micelle solution originates from the lower water content in the solid PAA&(Py@CTAB)/PDDA film than in aqueous CTAB micelle solution because water can penetrate into micelles in solution.16 The thickness of (PAA&Py@CTAB)/PDDA films with different deposition cycles was determined from their crosssectional SEM images. As shown in Figure 4a, their thickness increases exponentially with increasing number of film deposition cycles. The thicknesses of the as-prepared film with 9 and 15 deposition cycles are ca. 404.3 ( 34.5 nm (Figure 4b) and 2 020.5 ( 181.5 nm (Figure 4c), respectively. The surface morphology of the as-prepared (PAA&Py@CTAB)/PDDA multilayer films weas investigated by AFM to understand the mechanism for their exponential growth behavior. The AFM images of the as-prepared PAA&(Py@CTAB)/PDDA films on silicon wafers with different deposition cycles are indicated in Figure 5. The root-mean-square (rms) roughness of the asprepared films with 3, 6, 9, 12, and 15 deposition cycles is ca. 24.3, 12.2, 19.1, 35.2, and 51.0 nm, respectively. The surface roughness of the film with the first three deposition cycles was higher than that of 6 because the substrate was not fully covered (16) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039. (17) Bhattacharya, S.; Haldar, J. Langmuir 2004, 20, 7940. (18) Cao, T.; Munk, P.; Ramireddy, C.; Tuzar, Z.; Webber, S. E. Macromolecules 1991, 24, 6300.

LbL Assembly of PAA&(Py@CTAB) Multilayer Films

Figure 4. (a) Dependence of the thickness of the PAA&(Py@CTAB)/ PDDA films as a function of deposition cycles. (b,c) Cross-sectional SEMimagesofa[PAA&(Py@CTAB)/PDDA]*9(b)and[PAA&(Py@CTAB)/ PDDA]*15 film.

Langmuir, Vol. 24, No. 22, 2008 12989

Figure 6. UV-vis absorption spectra of a [PAA&(Py@CTAB)/ PDDA]*8 film in Figure 2b before (black line) and after (red line) being immersed into 0.9% normal saline for 4 h.

6.7 mg/mL, we fabricated PAA&(Py@CTAB)/PDDA films with an incorporation amount of Py being about 20% of the PAA&(Py@CTAB)/PDDA films in Figure 1b when both films have the same number of deposition cycles. The emission spectrum of the as-fabricated [PAA&(Py@CTAB)/PDDA]*8 film with lower amount of Py incorporated reveals that the Py incorporated in the films is less aggregated. UV-vis absorption spectrum of this [PAA&(Py@CTAB)/PDDA]*8 film after immersion in 0.9% normal saline for 4 h shows no difference from that of the freshly prepared one. This result confirms that Py molecules indeed can be firmly incorporated in the asfabricated PAA&(Py@CTAB)/PDDA films. The stability of the incorporated Py in PAA&(Py@CTAB)/PDDA films is independent of its concentration in the stabilized micelles. The robust incorporation of the water-insoluble molecules into LbL assembled multilayer films is essential for the fabrication of functional film materials for applications such as the optical and electronic devices, sensors and so forth.

Conclusions

Figure 5. AFM images of the [PAA&(Py@CTAB)/PDDA]*n films with n being 3(a), 6 (b), 9 (c), 12 (d), and 15 (e).

by the [PAA&(Py@CTAB)/PDDA]*3 film and islands form on the substrate. With more cycles of film deposition, the PAA&(Py@CTAB) complexes were glued together with PDDA, and the rms of the films increases with increasing number of film deposition cycles. The increased surface roughness permits more PAA&(Py@CTAB) complexes and PDDA to deposit than in the previous deposition cycle. In this way, the PAA&(Py@CTAB)/ PDDA films exhibit an exponential growth behavior. The high roughness of the film explains the irregular movement of the corresponding UV-vis absorption spectra in Figure 2b. For instance, the [PAA&(Py@CTAB)/PDDA]*3 film has an abruptly increased roughness, which leads to an elevation of the whole spectrum in the examined range. The stability of a [PAA&(Py@CTAB)/PDDA]*8 film was investigated by immersing the film in 0.9% normal saline for 4 h. UV-vis absorption spectrum of the film after immersion shows no difference from that of the freshly prepared one. By decreasing the concentration of Py added in the aqueous CTAB solution to

In summary, a novel methodology has been established for the stable incorporation of noncharged Py molecules into LbLassembled multilayer films by using PAA-stabilized CTAB micelles as carriers. Compared with the previous works of using block copolymer micelles as carriers, the surfactant micelles and polyelectrolytes are easily available and therefore provide a costeffective way to incorporate noncharged species into LbLassembled multilayer films. We believe that the present method can be extended to a wide range of polyelectroyte-stabilized surfactant micelles and will open a general and cost-effective avenue for the fabrication of advanced film materials containing noncharged species, such as organic molecules, nanoparticles and so forth by using LbL assembly technique. Acknowledgment. This work is supported by the National Basic Research Program (2007CB808000), the Foundation for the Author of National Excellent Doctoral Dissertation of P. R. China (FANEDD Grant No. 200323), the Program for New Century Excellent Talents in University (NCET), and the Jilin Provincial Science and Technology Bureau of Jilin Province (20070104). Supporting Information Available: Unmodified UV-vis absorption spectra of PAA&(Py@CTAB)/PDDA films, and UV-vis absorption spectra of LbL deposited films of CTAB micelle/PSS. This material is available free of charge via the Internet at http://pubs.acs.org. LA802674H