pubs.acs.org/Langmuir © 2009 American Chemical Society
Molecular Order in Langmuir-Blodgett Monolayers of Metal-Ligand Surfactants Probed by Sum Frequency Generation Himali D. Jayathilake,†,‡ Jeffery A. Driscoll,‡ Andrey N. Bordenyuk,‡ Libo Wu,§ Sandro R. P. da Rocha,§ Claudio N. Verani,‡ and Alexander V. Benderskii*,‡ ‡ Department of Chemistry, Wayne State University, Detroit, Michigan 48202, and §Department of Chemical Engineering, Wayne State University, Detroit, Michigan 48201. † Current address: Department of Chemistry, Mt. Holyoke College, South Hadley, Massachusetts 01075
Received January 14, 2009. Revised Manuscript Received March 16, 2009 Molecular organization of Langmuir-Blodgett (LB) monolayers of novel copper-containing metal-ligand surfactants was characterized by the surface-selective vibrational sum frequency generation (SFG) spectroscopy. The orientational and conformational order inferred from the SFG peak amplitudes and line shapes were correlated with the two-dimensional phases of the monolayers observed in the compression isotherms. The octadecyl-pyridin2-ylmethyl-amine (LPyC18) ligand by itself shows good amphiphilic properties, as indicated by the high monolayer collapse pressure at the air/water interface, but its LB films transferred onto fused silica exhibit a high degree of transgauche conformational disorder in the alkyl tails. Coordination of copper(II) ions to the chelating head group enhances the molecular alignment and reduces the fraction of gauche defects of the alkyl chains. Monolayers of single-tail (LPyC18CuIICl2) and double-tail [(LPyC18)2CuII]Cl2 metallosurfactants show distinctly different behavior of their molecular organization as a function of the area per molecule. Our observations suggest metal-ligand interactions as a pathway to induce molecular order in LB monolayer films.
Introduction Interest in metal-containing soft materials has increased because of their potential applications in magnetic and conductive films1 and molecular electronics.2,3 These materials are usually composed of a functional fragment attached to a ligand capable of coordinating metals. Rigid ligands with nitrogen donors have been used to append different functional moieties, thus forming building blocks for molecular transistors,4 heteroleptic *To whom correspondence should be addressed. E-mail: alex@chem. wayne.edu. (1) Talham, D. R. Chem. Rev. 2004, 104, 5479–5501. (2) Hjelm, J.; Handel, R. W.; Hagfeldt, A.; Constable, E. C.; Housecroft, C. E.; Forster, R. J. Inorg. Chem. 2005, 44, 1073–1081. (3) Fujigaya, T.; Jiang, D. L.; Aida, T. J. Am. Chem. Soc. 2003, 125, 14690–14691. (4) Park, J.; Pasupathy, A. N.; Goldsmith, J. I.; Chang, C.; Yaish, Y.; Petta, J. R.; Rinkoski, M.; Sethna, J. P.; Abruna, H. D.; McEuen, P. L.; Ralph, D. C. Nature 2002, 417, 722–725. (5) Wu, X.; Frasier, C. L. Macromolecules 2000, 33, 7776–7785. (6) Gohy, J. F.; Lohmeijer, B. G. G.; Schubert, U. S. Chem.;Eur. J. 2003, 9, 3472–3479. (7) Griffiths, P. C.; Fallis, I. A.; Willock, D. J.; Paul, A.; Barrie, C. L.; Griffiths, P. M.; Williams, G. M.; King, S. M.; Heenan, R. K.; Gorgl, R. Chem.;Eur. J. 2004, 10, 2022–2028. (8) Hayami, S.; Danjobara, K.; Shigeyoshi, Y.; Inoue, K.; Ogawa, Y.; Maeda, Y. Inorg. Chem. Commun. 2005, 8, 506–509. (9) Beck, J. B.; Rowan, S. J. J. Am. Chem. Soc. 2003, 125, 13922–13923. (10) Mandon, D.; Nopper, A.; Litrol, T.; Goetz, S. Inorg. Chem. 2001, 40, 4803–4806. (11) Storr, T.; Sugai, Y.; Barta, C. A.; Mikata, Y.; Adam, M. J.; Yano, S.; Orvig, C. Inorg. Chem. 2005, 44, 2698–2705. (12) Kirin, S. I.; Dubon, P.; Weyhermuller, T.; Bill, E.; Metzler-Nolte, N. Inorg. Chem. 2005, 44, 5405–5415. (13) Shakya, R.; Imbert, C.; Hratchian, H. P.; Lanznaster, M.; Heeg, M. J.; McGarvey, B. R.; Allard, M.; Schlegel, H. B.; Verani, C. N. Dalton Trans. 2006, 2517–2525. (14) Imbert, C.; Hratchian, H. P.; Lanznaster, M.; Heeg, M. J.; Hryhorczuk, L. M.; McGarvey, B. R.; Schlegel, H. B.; Verani, C. N. Inorg. Chem. 2005, 44, 7414– 7422. (15) Shakya, R.; Hindo, S. S.; Wu, L.; Allard, M. M.; Heeg, M. J.; Hratchian, H. P.; McGarvey, B. R.; da Rocha, S. R. P.; Verani, C. N. Inorg. Chem. 2007, 46, 9808–9818.
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copolymers,5,6 metallomicelles,7 liquid crystals,8 and responsive plastics.9 More flexible alkylpyridyl ligands have been used for sensing purposes,10-12 and asymmetric ligands have been recently explored.13-16 One of the main difficulties in creating materials for molecular electronics is related to the fact that high intermolecular organization of conjugated cores is necessary to achieve the desired electronic properties. One possible approach to create such molecular order is amphiphilic self-assembly into a monolayer on an aqueous subphase, followed by LangmuirBlodgett (LB) transfer.1,17-19 A strategy that can be envisioned for creating highly ordered monolayer materials with the desired electronic properties is to combine amphiphilicity for ordered monolayer formation and coordination of metal ions to affect the ground and excited electronic states of the precursor molecules and, consequently, tune the electronic properties of the resulting material. If successful, such monolayers may prove to be catalyst in promoting the use of organic systems as electronic and magnetic materials. It has been demonstrated that the inclusion of transition-metal ions to thermothropic liquid crystals20 can induce molecular organization and promote unique electronic, optical, and magnetic properties in such materials. A series of novel surfactants with a pyridine-based metalchelating head group have been developed recently and shown to exhibit amphiphilic behavior and film pattern formation.21 (16) Hindo, S. S.; Shakya, R.; Rannulu, N. S.; Allard, M. M.; Heeg, M. J.; Rodgers, M. T.; da Rocha, S. R. P.; Verani, C. N. Inorg. Chem. 2008, 47, 3119–3127. (17) Paloheimo, J.; Kuivalainen, P.; Stubb, H.; Vuorimaa, E.; Ylilahti, P. Appl. Phys. Lett. 1990, 56, 1157–1159. (18) Wang, G. M.; Swensen, J.; Moses, D.; Heeger, A. J. J. Appl. Phys. 2003, 93, 6137–6141. (19) Saxena, V.; Malhotra, B. D. Curr. Appl. Phys. 2003, 3, 293–305. (20) Shakya, R.; Keyes, P. H.; Heeg, M. J.; Moussawel, A.; Heiney, P. A.; Verani, C. N. Inorg. Chem. 2006, 45, 7587–7589. (21) Driscoll, J. A.; Allard, M. M.; Wu, L.; Hegg, M. J.; da Rocha, S. R. P.; Verani, C. N. Chem.;Eur. J. 2008, 14, 9665–9674.
Published on Web 04/21/2009
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In this paper, we focus on the molecular organization in the LB monolayers of the chelating surfactant octadecyl-pyridin2-ylmethyl-amine (LPyC18), 1, its singly coordinated copper complex (LPyC18CuIICl2), 2, and doubly coordinated copper complex [(LPyC18)2CuII]Cl2, 3 [two ligands coordinated to one copper(II) ion], depicted in Scheme 1. Using vibrational sum frequency generation (SFG) spectroscopy, we demonstrate the enhancement of molecular order in copper-containing complexes 2 and 3, when compared to the nonmetalated surfactant 1. Vibrational SFG, a highly surface-selective nonlinear optical technique, has been demonstrated as a direct and sensitive probe in analyzing surfaces and interfaces.22-36 In particular, it has been demonstrated to provide detailed information on the molecular orientation and conformational state, such as trans-gauche disorder of surfactant molecules at interfaces.22,23,33,35-43 Here, we use SFG spectroscopy to characterize the orientational and conformational order of the surfactant molecules in the metalligand LB monolayers transferred onto glass substrates as a function of the surface pressure, i.e., two-dimensional phase of the monolayer. The molecular-level information from SFG vibrational spectra is correlated with the macroscopic thermodynamic data on the monolayer phases observed in the compression isotherm. The results show that coordination of copper(II) ions to the head group of the ligand surfactant 1, especially in singly coordinated geometry, significantly enhances the monolayer packing and conformational order.
Experimental Section Synthesis and characterization of the amphiphilic ligand 1 and its singly and doubly coordinated copper complexes 2 and 3 have been presented elsewhere, including X-ray diffraction (XRD) and ESI+ mass analysis.21,44,45 Note that the structures in Scheme 1 are for illustration purposes only and do not reproduce exactly the (22) Guyot-Sionnest, P.; Hunt, J. H.; Shen, Y. R. Phys. Rev. Lett. 1987, 59, 1597–1600. (23) Bain, C. D. J. Chem. Soc., Faraday Trans. 1995, 91, 1281–1296. (24) Eisenthal, K. B. Chem. Rev. 1996, 96, 1343–1360. (25) Chen, Z.; Shen, Y. R.; Somorjai, G. A. Annu. Rev. Phys. Chem. 2002, 53, 437–465. (26) Richmond, G. L. Chem. Rev. 2002, 102, 2693–2724. (27) Voges, A. B.; Al-Abadleh, H. A.; Musorrariti, M. J.; Bertin, P. A.; Nguyen, S. T.; Geiger, F. M. J. Phys. Chem. B 2004, 108, 18675–18682. (28) Raschke, M. B.; Shen, Y. R. Curr. Opin. Solid State Mater. Sci. 2004, 8, 343–352. (29) Bordenyuk, A. N.; Jayathilake, H.; Benderskii, A. V. J. Phys. Chem. B 2005, 109, 15941–15949. (30) Jayathilake, H. D.; Zhu, M. H.; Rosenblatt, C.; Bordenyuk, A. N.; Weeraman, C.; Benderskii, A. V. J. Chem. Phys. 2006, 125, 1–9. (31) Liu, J.; Conboy, J. C. J. Am. Chem. Soc. 2004, 126, 8894–8895. (32) Ma, G.; Allen, H. C. Langmuir 2006, 22, 5341–5349. (33) Himmelhaus, M.; Eisert, F.; Buck, M.; Grunze, M. J. Phys. Chem. B 2000, 104, 576–584. (34) Watry, M. R.; Tarbuck, T. L.; Richmond, G. I. J. Phys. Chem. B 2003, 107, 512–518. (35) Can, S. Z.; Mago, D. D.; Esenturk, O.; Walker, R. A. J. Phys. Chem. C 2007, 111, 8739–8748. (36) Can, S. Z.; Mago, D. D.; Walker, R. A. Langmuir 2006, 22, 8043–8049. (37) Ye, S.; Noda, H.; Nishida, T.; Morita, S.; Osawa, M. Langmuir 2004, 20, 357–365. (38) Hirose, C.; Yamamoto, H.; Akamatsu, N.; Domen, K. J. Phys. Chem. 1993, 97, 10064–10069. (39) Weeraman, C.; Yatawara, A.; Bordenyuk, A. N.; Benderskii, A. V. J. Am. Chem. Soc. 2006, 128, 14244–14245. (40) Bordenyuk, A. N.; Weeraman, C.; Yatawara, A.; Jayathilake, H. D.; Stiopkin, I.; Liu, Y.; Benderskii, A. V. J. Phys. Chem. C 2007, 111, 8925–8933. (41) Wang, H. F.; Gan, W.; Lu, R.; Rao, Y.; Wu, B. H. Int. Rev. Phys. Chem. 2005, 24, 191–256. (42) Chen, X. Y.; Boughton, A. P.; Tesmer, J. J. G.; Chen, Z. J. Am. Chem. Soc. 2007, 129, 12658–12659. (43) Moad, A. J.; Simpson, G. J. J. Phys. Chem. B 2004, 108, 3548–3562. (44) Driscoll, J. A. Ph.D. Thesis, Department of Chemistry, Wayne State University, Detroit, MI, 2008. (45) Driscoll, J. A.; Keyes, P. H.; Heeg, M. J.; Heiney, P. A.; Verani, C. N. Inorg. Chem. 2008, 47, 7225–7232.
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Scheme 1. Molecular Structures of the Surfactants Used in This Studya
a Note that labile Cl- counterions for 3 are not shown. The scheme does not reflect the X-ray crystal structures of the compounds.
crystal structures from XRD. Surface pressure versus area per molecule (π-A) compression isotherms of the Langmuir monolayers of surfactants 1-3 were measured using an automated minitrough equipped with a Wilhelmy plate (KSV Instruments). Chloroform solutions (Fisher Scientific, spectral analyzed grade) of surfactants 1-3 with known concentrations (1 mg mL-1) were prepared, and a known quantity (30 μL) was spread on a pure aqueous subphase with a microsyringe. The film was allowed to equilibrate for 30 min before monolayer compression to allow for evaporation of chloroform and spreading of a homogeneous film on the water surface. The compression rate was 5 mm/min. After the target pressure was obtained, the films were allowed to equilibrate for 20 min for pressure stabilization and relaxation of the monolayer before the transfer. The monolayers of surfactants 1-3 were vertically transferred onto IR-grade fused silica windows (25.4 mm in diameter and 1.1 mm thick) while keeping constant pressure. The elevation speed was 0.2 mm/min for all of the samples. The depositions were conducted at 23.5 ( 0.5 °C. Ultra-pure water (Milli-Q, 18.2 MΩ/cm) was used for all of the cleaning purposes and for the water bath of the LB trough. The pH of the aqueous subphase in the compression isotherm measurements and LB transfer was close to neutral (pH ≈ 7). The nonmetalated ligand 1 is therefore most likely charge neutral (while no measurements have been performed regarding pKa values, but pyridine, the main component of the head group, is known to be mildly basic, pKa ∼ 5.21 for the conjugate acid). This basicity is due to the presence of an N-localized lone pair (not delocalized in the aromatic π framework, such as in a phenol). Pyridine can be protonated by acids but not by water. It can be expected that the hydrogen of the aminomethyl group attached to the ortho position of the pyridine ring is weakly hydrogen-bonded to the Npyridine. The presence of water could disrupt this weak interaction. We also note that the pKa values for surfactant head groups at the surface are known to sometimes differ from the bulkphase values.46 We did not conduct measurements in this study to investigate the interfacial pKa values of the three surfactants. (46) Zhao, X.; Subrahmanyan, S.; Eisenthal, K. B. Chem. Phys. Lett. 1990, 171, 558–562.
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The LB monolayer samples of 1-3 were prepared at different transfer pressures (marked by circles in Figure 1), to represent different phases on their respective compression isotherms. The films of surfactant 1 were transferred at 35, 30, 25, 20, 15, and 5 mN/m; the films of surfactant 2 were transferred at 40, 18, 12, and 5 mN/m; and the films of surfactant 3 were transferred at 40, 25, 15, and 5 mN/m. These surface pressure values were selected such that we can make a comparison of the molecular organization of monolayers in the different 2D phases. Vibrational SFG spectra of the transferred monolayers were measured using the broad-band (BB)-SFG scheme.29,47,48 The technique involves three optical fields (input infrared, visible, and output SFG), with the IR input beam providing resonant enhancement by spectrally overlapping with the interfacial vibrational modes of the surfactant molecules in the monolayer. A detailed description of our BB-SFG setup is presented in the Supporting Information. Here, we summarize a few experimental parameters relevant to the analysis of the results and discussion. The energies of the input IR and visible beams and their spot sizes at the sample surface were selected to minimize heating and not cause damage to the sample, as determined previously for other organic LB and polymer films.29,30 The IR beam was 4 μJ/pulse at the sample, with spot size at 150 μm, and the visible beam was 20 μJ/pulse at the sample, with spot size at 400 μm. To achieve adequate spectral resolution for the obtained BB-SFG spectra, a zero-chirp 4-f design pulse stretcher48 was also used to narrow the spectrum of the visible pulse to full width at half-maximum of 10 cm-1. The IR frequencies are calculated by subtracting the central frequency of the narrow visible pulse measured using the same monochromator from the SFG frequency, ωIR = ωSFG - ωovis. We also calibrate the IR frequency scale using a wellknown SFG surface spectrum of dimethyl sulfoxide (DMSO).49 The estimated IR frequency calibration accuracy is (3 cm-1. Polarization of the visible beam is controlled by a zero-order half-wave plate, while the IR beam polarization can be changed using the three-mirror polarization rotator50 before the sample. We use an analyzer after the sample to select the polarization of the SFG signal. In this experiment, we used SSP (S-SFG, S-visible, and P-IR) and PPP (P-SFG, P-visible, and P-IR) polarization combinations, where P denotes polarization in the plane of incidence and S denotes polarization perpendicular to the plane of incidence. All SFG spectra are normalized with respect to the nonresonant signal coming from a reference gold substrate.
Results The π-A compression isotherms in Figure 1 indicate that the ligand surfactant 1 and the metal-ligand complexes 2 and 3 form stable Langmuir monolayers at the air/water interface, with collapse pressures above 40 mN m-1. The collapse pressure, between 45 and 55 mN m-1, is significantly greater than that reported for other metal surfactant complexes.9,10 The minimum area per molecule Ac ∼ 20 A˚2/molecule observed for the pyridinecontaining ligand 1 is close to the maximum packing reported for Langmuir monolayers of surfactants with small head groups, such as carboxylic acids, with a similar number of carbon atoms in the hydrophobic tail.51 The pyridine head group of the ligand apparently does not prevent close packing of the monolayer of 1. The isotherms of the metal-ligand complex surfactant 2 are more expanded than 1. This is expected because of the addition of a bulky and charged copper cation to the head group. Because of its (47) Richter, L. J.; Petralli-Mallow, T. P.; Stephenson, J. C. Opt. Lett. 1998, 23, 1594–1596. (48) Bordenyuk, A. N.; Benderskii, A. V. J. Chem. Phys. 2005, 122, 1–11. (49) Allen, H. C.; Gragson, D. E.; Richmond, G. L. J. Phys. Chem. B 1999, 103, 660–666. (50) Johnston, L. H. Appl. Opt. 1977, 16, 1082–1084. (51) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience Publishers, J. Wiley and Sons, Inc.: New York, 1966.
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Figure 1. Pressure-area (π-A) isotherms of surfactants 1-3 on water subphase measured at 23.5 ( 0.5 °C.
double alkyl tail, surfactant 3 occupies an even larger area per molecule but not twice as much as 2. Thus, for surfactants 2 and 3, the packing is determined by the effective size and charge of the metal-ligand head group and not the alkyl tail. Note that, for 2, the chloride counterions are thought to be strongly bound as a part of the coordination shell, while for 3, the counterions are presumably labile.21,45 Some dissociation for 2 can be expected because of the ionic nature of the interaction between the couterion, such as X = ClO4 or Cl, and the discrete covalent cation (CuL2X)+. Similarly, some degree of dissociation seems reasonable, taking into account species, such as (CuL2)2+ + Xand to a lesser degree, (CuL)2+ + L. However, such dissociations will be present in any metallosurfactant because of simple Pearson (HSAB) arguments. The chelate effect exerted by the bidentate ligands should, however, prevail. The compression isotherm for 3 is reminiscent of isotherms of many surfactants with ionic head groups and labile counterions. The changes in slope of the compression isotherms indicate phase transitions between different mesophases of the monolayers. This behavior is in agreement with the Brewster angle microscopy (BAM) observations showing coexistence of mesophases as micrometer-size domains.21 Figures 2 and 3 show the SFG vibrational spectra in the C-H stretch region recorded for SSP (S for the SFG signal, S for the visible input beam, and P for the IR input beam) and PPP (P-SFG, P-visible, and P-IR) polarization combinations (where P is parallel and S is perpendicular to the plane of incidence of the laser beams) of LB films of surfactants 1-3 transferred at different surface pressures to represent different 2D mesophases of the monolayers. The three surfactants clearly show different behavior in the onset of the molecular order in the monolayers as a function of the increasing surface pressure, i.e., decreasing area per molecule creating a more close-packed monolayer. Qualitatively, the higher amplitude and narrower width of the vibrational bands in the SFG spectra correspond to a higher degree of molecular order in the monolayer (a film with no preferential molecular orientation would result in zero SFG signal). Thus, films transferred at 5 mN/m (largest area per molecule, most disordered) generally produced very weak SFG bands barely above the noise level under our experimental conditions. Spectra of 5 mN/m monolayers of 2 (SSP and PPP) and 3 (PPP) are not shown. All spectra of each of the three surfactants have the same SFG intensity scale (i.e., all spectra in Figure 2A, all spectra in Figure 2B, etc.), but the SFG transition intensities are not comparable between different surfactants. For a given surfactant, clear differences in the absolute intensities of the bands as well as the relative intensities of the CH-stretch peaks in the vibrational SFG spectra are observed for different monolayer mesophases, indicating changes in Langmuir 2009, 25(12), 6880–6886
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Figure 2. SFG spectra of LB monolayers of surfactants (A) 1, (B) 2, and (C) 3 measured with SSP polarization combination. Surface pressure values (mN/m) at which LB transfers were made are indicated next to each spectrum. Spectra are vertically offset for clarity.
Figure 3. SFG spectra of surfactants (A) 1, (B) 2, and (C) 3 measured with PPP polarization combination. Surface pressure values (mN/m) at which the depositions were made are indicated next to each spectrum. Spectra are vertically offset for clarity.
ð2Þ
vibrational resonances with Bloch-type dephasing, with amplitudes of Bj, line widths of Γj, and transition frequencies of ωj. The fits are shown as solid lines passing through the experimental data points in Figures 1 and 2. The fitting results for all of the spectra are presented in Tables S1-S3 in the Supporting Information. The spectra cover the region from 2800 to 3000 cm-1, which corresponds to the saturated CH-stretch vibrational modes. The following transitions of the alkyl chains of the surfactants are readily assigned: the CH2 symmetric stretch (d+) at ∼2850 cm-1, CH3 symmetric stretch (r+) at ∼2875 cm-1, CH2 antisymmetric stretch (d-) at ∼2917 cm-1, Fermi resonance of CH3 symmetric -1 stretch with CH3 bend overtone (r+ FR) at ∼2940 cm , and CH3 asymmetric stretch (r-) around ∼2970 cm-1.22,23,29,30,33,52-54 The region around 3000 cm-1 in the PPP spectra (Figure 3) shows interference with the aromatic CH-stretch vibrational modes of the pyridine ligand(s)55 resulting in “dips” in the spectra. Analysis of the fitting results for the CH-stretch modes is presented in the Discussion.
The first term in the eq 2 accounts for the nonresonant (instantaneous) part of the response with amplitude ANR and phase Φ with respect to the vibrationally resonant part of the response. The second term describes the addition of several
(52) Lu, R.; Gan, W.; Wu, B. H.; Zhang, Z.; Guo, Y.; Wang, H. F. J. Phys. Chem. B 2005, 109, 14118–14129. (53) Nishi, N.; Hobara, D.; Yamamoto, M.; Kakiuchi, T. J. Chem. Phys. 2003, 118, 1904–1911. (54) Wolfrum, K.; Laubereau, A. Chem. Phys. Lett. 1994, 228, 83–88. (55) Klots, T. D. Spectrochim. Acta, Part A 1998, 54, 1481–1498.
microscopic structure: molecular organization, orientation, and conformation. Note that, in the region of phase transitions, the SFG spectra likely represent an average over coexisting mesophases at a given pressure, because the beam size in our setup (>100 μm) is much larger than the characteristic domain size observed by BAM.21 To quantify the observed trends, all spectra ISFG(ωIR) were fitted using the standard multi-Lorentzian approximation with a constant nonresonant background29 ð2Þ
ISFG SSP, PPP ðωIR Þ ¼ jΧSSP, PPP j2 IIR ðωIR ÞIvis ðωvis Þ
ð1Þ
where IIR and Ivis are the intensities of the IR and visible pulses and X(2) SSP,PPP is the effective macroscopic second-order nonlinear susceptibility for the corresponding polarization combination ð2Þ
ΧSSP, PPP ðωIR ÞµANR eiΦ þ
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N X
B j Γj ðω -ω IR j Þ þ iΓj j ¼1
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Discussion The intensity of a VSFG band for a given vibrational mode depends upon (1) the number density N of molecules on the surface, i.e., the area per molecule A, and (2) the degree of orientational ordering (i.e., tilt angle and distribution width) of the chromophore moieties. We begin by plotting the intensities of the two most prominent bands in the SSP spectra, CH3 + 2 symmetric stretch r+ and the CH3 Fermi resonance r+ FR, |BSSP(r )| 2 and |BSSP(r+ )| , as a function of area per molecule A (Figure 4). FR If the molecular orientation would remain unchanged as the monolayer is compressed through different phases, the intensity would be µ1/A2; i.e., the intensity versus A dependence would be linear in the semi-log coordinates used in Figure 4. Instead, monolayers of surfactants 1 and 2 show an abrupt change (about 1 order of magnitude) in the intensities of both r+ and r+ FR modes at the values of A corresponding to the phase transitions observed in their compression isotherms in Figure 1. As discussed in detail below, the intensity of the CH3 symmetric stretch modes in SSPpolarized SFG spectra qualitatively indicates the more vertical alignment of the alkyl tails. The results therefore suggest that the phase transition in monolayers of surfactants 1 and 2 are accompanied with a significant change of the orientation of the alkyl chains from more horizontal for more expanded monolayers to more vertical and better orientationally ordered structure for surface pressures above the phase transition. In contrast, monolayers of the double-tail surfactant 3 show a gradual, nearly linear change of intensities with the area per molecule, in good correlation with the absence of sharp phase transitions in its compression isotherm (Figure 1). The appearance of CH2 vibrational modes (d+ and d-) in the SSP-polarized SFG spectra of monolayers containing alkyl chains is a qualitative yet fairly sensitive indicator of the formation of gauche conformational defects.22,23,29,37,39,56 Qualitatively, in alltrans alkane chains, the near inversion symmetry of the CH2 modes makes them SFG-forbidden, while the loss of inversion symmetry associated with gauche defects makes the CH2 modes SFG-allowed. The relative intensities of the CH2 (d+ and d-) versus CH3 (r+ and r+ FR) modes can therefore be used as a qualitative measure of the conformational order of the alkyl chain.39,40 The SFG spectra (Figure 2) clearly show that all three surfactants have decreasing fraction of gauche defects with increasing surface pressure. This is demonstrated in Figure 5, which plots the ratio of the d+/r+ transition amplitudes from the fitting results of SSP spectra versus surface pressure. The alkyl chains of all three surfactants have a high degree of trans-gauche conformational disorder for the films transferred at low surface pressure. As the pressure increases, the monolayer becomes more tightly packed, resulting in markedly better conformational order and lower d+/r+ amplitude ratio. This is a typical behavior for the monolayers of surfactants with alkyl tails.22,23,29,34 SSP-polarized SFG spectra of surfactant 2 (Figure 2B) are dominated by the transitions of the terminal methyl group and show relatively weak d+ and d- transitions that nearly vanish at the higher transfer pressure, especially for monolayers transferred at 40 mN/m. Significantly narrower vibrational line shapes of the copper-ligand complex surfactant 2, consistent with decreased inhomogeneous broadening, also suggest enhanced order in the monolayer. This behavior is typical for monolayers composed of straight alkane chains in nearly all-trans conformation,29 indicating that the surfactant 2 above the phase transition (π ∼ 17 mN/m) forms well-ordered films with few gauche defects. On the contrary,
line shapes are much broader for the prominent r+ and r+ FR bands of ligand alone 1 and double-tail complex 3, even for samples transferred at higher surface pressure, indicating that these monolayers are not as well-ordered as the surfactant 2. PPP-polarized spectra of the three surfactants (Figure 3) show asymmetric CH3 stretch (r-) appearing around 2970 cm-1, which is the dominant feature for surfactants 2 and 3. The situation when SSP spectra show high-intensity symmetric CH3 stretch modes (r+ and r+ FR) and low-intensity asymmetric stretch mode (r-), while PPP spectra are dominated by the asymmetric CH3 stretch (r-) with low-intensity r+ and r+ FR, is indicative of monolayers with nearly vertical alkyl chains.23,29,35,36,38,41,57 Qualitatively, monolayers of 2 and 3 appear to have nearly vertical alignment even at moderate surface pressure, while the alkyl tails in monolayers of 1 remain tilted even at the highest transfer pressure. This somewhat surprising finding underscores that the tilt angle does not solely depend upon the area per molecule. Other factors, such as hydrophilicity of the head group, contribute to the molecular organization in the monolayers, in particular, molecular orientation. We also note that the orientational and conformational order (e.g., the fraction of gauche defects in the alkane chains) are connected: for example, a large amount of conformational disorder may lead to an apparently larger tilt of the terminal methyl groups. For the copper-containing complexes 2 and 3, orientation of the nearly all-trans alkyl chains can be estimated from a comparison of the SFG spectra recorded with SSP and PPP polarization
(56) Oh-e, M.; Lvovsky, A. I.; Wei, X.; Shen, Y. R. J. Chem. Phys. 2000, 113, 8827–8832.
(57) Fourkas, J. T.; Walker, R. A.; Can, S. Z.; Gershgoren, E. J. Phys. Chem. C 2007, 111, 8902–8915.
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Figure 4. Intensities of CH3 symmetric stretch (r+, O) and CH3
Fermi resonance stretch (r+ FR, 0) bands for LB monolayers of surfactants (A) 1, (B) 2, and (C) 3 as a function of the area per molecule. Vertical teal shadows in A and B indicate the approximate positions of the phase transitions (flat region of the compression isotherm, Figure 1) for surfactants 1 and 2.
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Article Table 1. Calculated Orientation of the Terminal CH3 Group for LB Monolayers of Surfactants 2 and 3
surfactant 2 3
transfer pressure (mN/m)
r+ SSP/rPPP intensity ratio |BSSP(r+)|2/ |BPPP(r-)|2
CH3 tilt angle θ
18 40 25 40
0.27 2.31 1.36 5.61
55 ( 5° 35 ( 5° 40 ( 5° 30 ( 5°
orientational distribution, better packing, and consequently, fewer gauche defects of the alkyl chains in the LB monolayers of the copper-coordinated complex 2 compared to the ligand surfactant 1. While these factors are at play for the double-tail surfactant 3, the bulky doubly coordinated head group prevents tight packing of the alkyl tails and the high degree of the molecular order is not achieved. Finally, we note that the laser beam spot size in our SFG measurements (a few hundred micrometers) is much larger than the typical domain size (a few micrometers) observed by BAM,21 such that the spectra shown above represent an average of the coexisting mesophases that form the patterns. Although it would be very interesting to record spatially resolved SFG spectra from the mesophases forming the micrometer-size domains, our present instrumentation capabilities do not allow us to study the heterogeneity in the molecular organization of the monolayers on a micrometer scale. However, sufficient spatial resolution for SFG imaging has recently been demonstrated.58,59 Figure 5. Amplitude ratio of the CH2 symmetric stretch (d+) and CH3 symmetric stretch (r+) peaks in the SSP SFG spectra for LB monolayers of surfactants (A) 1, (B) 2, and (C) 3 as a function of the area per molecule. Vertical teal shadows in A and B indicate the approximate positions of the phase transitions for surfactants 1 and 2. Solid lines passing through the data points are a guide to the eye only.
combinations. Table 1 summarizes the measured intensity ratios |BSSP(r+)|2/|BPPP(r-)|2 of the symmetric stretch r+ mode in the SSP spectrum to the asymmetric stretch r- mode in the PPP spectrum and the calculated average tilt angle θ of the C3v symmetry axis of the terminal methyl group. Details of the orientational analysis have been presented elsewhere.30 We estimate the tilt angle of the terminal methyl group to be 35 ( 5° and 30 ( 5° from normal for densely packed monolayers (transferred at 40 mN/m) of surfactants 2 and 3, respectively. Because the C3v symmetry axis of the CH3 group forms a ∼30° angle with the chain axis, this indicates that the alkyl chains of the copperligand complex 2 and 3 tend to be in all-trans conformation and align nearly vertical to the surface (within ∼10°) for monolayers prepared at higher pressures. On the contrary, monolayers of 1 remain conformationally disordered and, therefore, orientationally disordered as well, even at higher transfer pressures. The observed tendency of the copper coordination to improve molecular order and alignment of the ligand surfactant monolayers can be rationalized as follows. Coordination of copper to the ligand surfactant head group (i) reduces conformational flexibility of the organic scaffold ligand surfactant by arresting some of the hinge and twist motions in the head-group region (Scheme 1) and (ii) renders the head group more hydrophilic, pulling it deeper into the water subphase. Electronic structure calculations reported earlier showed a significant increase of the permanent dipole of the surfactant upon CuII coordination: ∼14D for 2 as opposed to ∼2D for the nonmetalated ligand 1.21 These effects may lead to more vertical alignment, narrower Langmuir 2009, 25(12), 6880–6886
Conclusions Molecular organization in the LB monolayers of ligand surfactant 1 and its copper-containing complexes 2 and 3 have been characterized by the vibrational SFG spectroscopy. SFG provides microscopic insight into the orientational and conformational order in the LB films, which can be correlated with the macroscopic (thermodynamic) measurements of the π-A compression isotherms showing phase transitions as the monolayers are compressed. The alkyl tails of the ligand surfactant 1 in the LB monolayers show a high degree of trans-gauche conformational disorder even at high transfer pressure. Metal-ligand complex 2, obtained by coordination of copper(II) ions to the chelating head group of 1, forms much better ordered monolayers, with few gauche defects and nearly vertically aligned alkyl chains. This is likely a result of the copper-ligand complex being a more rigid and more hydrophilic head group. Monolayers of 1 and 2 show abrupt changes in the degree of molecular alignment as a function of the area per molecule as they go through a phase transition. Double-tail metal-ligand surfactant 3 resulting from chelation of copper(II) by two ligand surfactants 1 also forms well-aligned monolayers; however, the degree of packing and conformational order is not as high as for the single-tail metal-ligand surfactant 2, likely because of its bulky head group. Compression isotherms of 3 do not show sharp phase transitions, and the SFG vibrational spectra correspondingly show smooth variation with the average area per molecule. The SFG spectroscopy in this study is used to provide microscopic information on the changes in molecular organization in the LB monolayers upon phase transitions, which is complementary to and in a sense completes the macroscopic picture provided by compression isotherms. Our observation of the enhancement of the molecular order in the copper-containing surfactant LB films suggests that metal-ligand interactions can (58) Cimatu, K.; Baldelli, S. J. Am. Chem. Soc. 2006, 128, 16016–16017. (59) Cimatu, K.; Baldelli, S. J. Am. Chem. Soc. 2008, 130, 8030–8037.
DOI: 10.1021/la900168p
6885
Article
Jayathilake et al.
be used as a part of a general strategy to achieve the high-order and desired molecular organization in monolayer materials of metal-containing surfactants.
(Grant 42575-G3). S.R.P.R. thanks the NSF (Grant 0553537) for partial support. We also thank the Nano@Wayne initiative (Fund-11E420) for supporting the initial stages of this project.
Acknowledgment. This work is supported by the National Science Foundation (NSF) CAREER Grant CHE-0449720. C.N. V. thanks the NSF (Grant CHE-0718470) and the Donors of the American Chemical Society (ACS) Petroleum Research Fund
Supporting Information Available: SFG spectroscopy setup, SFG spectral-fitting parameters, and SFG orientational analysis. This material is available free of charge via the Internet at http://pubs.acs.org.
6886 DOI: 10.1021/la900168p
Langmuir 2009, 25(12), 6880–6886
