Langmuir 1998, 14, 1525-1527
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Improved Second-Harmonic Generation from Langmuir-Blodgett Films G. J. Ashwell,* T. W. Walker, and P. Leeson Centre for Molecular Electronics, Cranfield University, Cranfield MK43 0AL, U.K.
U.-W. Grummt and F. Lehmann Institut fu¨ r Physikalische Chemie, Friedrich Schiller Universita¨ t, Lessingstrasse 10, 07743 Jena, Germany Received December 2, 1997 The second-harmonic intensity from alternate-layer Langmuir-Blodgett films of a coumarin dye, 4-[2(7-diethylamino-2-oxo-2H-benzopyran-3-yl)ethenyl]-1-octadecylquinolinium bromide, and an inert spacer, 1-octadecyl-4-methylquinolinium bromide, increases quadratically with the number of active layers. The optimum effective second-order susceptibility, albeit resonantly enhanced, is the highest to date for a thick alternate-layer film with χeff(2) ) 190 ( 30 pm V-1 at 1.064 µm. The thickness and the real and imaginary components of the dielectric permittivity from the surface plasmon resonance at 532 nm are l ) 4.6 ( 0.4 nm bilayer-1, r ) 2.4 ( 0.2 and i ) 0.8 ( 0.2.
Introduction Interest in Langmuir-Blodgett (LB) films for secondharmonic generation (SHG)1-5 stems from the spontaneous alignment of hydrophobically substituted chromophores at the air-water interface and the control of the structure at the molecular level.6-14 The SHG-active films must lack inversion symmetry, and thus, it is necessary to suppress the formation of centrosymmetric Y-type structures in which the molecular layers pack head-to-head (hydrophilic) and tail-to-tail (hydrophobic). This may be achieved by rendering each end of the chromophore hydrophobic,6,8 thereby moderating the tendency of the molecules to invert during deposition, or interleaving the active LB layers with compatible spacer materials.9-14 Noncentrosymmetric LB structures are now readily available and the main focus has shifted to the design of optically nonlinear donor-(π-bridge)-acceptor chromophores with improved properties. Such materials already possess * To whom correspondence should be addressed. (1) Bosshard, Ch.; Flo¨rsheimer, M.; Ku¨pfer, M.; Gu¨nter, P. Opt Commun. 1991, 85, 247. (2) Bosshard, Ch.; Otomo, A.; Stegeman, G. I.; Ku¨pfer, M.; Flo¨rsheimer, M.; Gu¨nter, P. Appl. Phys. Lett. 1994, 64, 2076. (3) Asai, N.; Tamada, H.; Fujiwara, I.; Seto, J. J. Appl. Phys. 1992, 72, 4521. (4) Clays, K.; Armstrong, N. J.; Penner, T. L. J. Opt. Soc. Am. B 1993, 10, 886. (5) Penner, T. L.; Motschman, H. R.; Armstrong, N. J.; Ezenyilimba, M. C.; Williams, D. J. Nature 1994, 367, 49. (6) Ashwell, G. J.; Jackson, P. D.; Crossland, W. A. Nature 1994, 368, 438. (7) Ashwell, G. J.; Jefferies, G.; George, C. D.; Ranjan, R.; Charters, R. B.; Tatam, R. P. J. Mater. Chem. 1996, 6, 131. (8) Ashwell, G. J.; Jackson, P. D.; Jefferies, G.; Gentle, I. R.; Kennard, C. H. L. J. Mater. Chem. 1996, 6, 137. (9) Era, M.; Nakamura, K.; Tsutsui, T.; Saito, S.; Niino, H.; Takehara, K.; Isomura, K.; Taniguchi, H. Jpn. J. Appl. Phys. 1990, 29, L2261. (10) Penner, T. L.; Armstrong, N. J.; Willand, C. S.; Schildkraut, J. S.; Robello, D. R. SPIE Int. Soc. Opt. Eng. 1991, 1360, 377. (11) Hodge, P.; Ali-Adib, Z.; West, D.; King, T. A. Macromolecules 1993, 26, 1789. (12) Ashwell, G. J.; Dawnay, E. J. C.; Kuczynski, A. P.; Martin, P. J. SPIE Int. Soc. Opt. Eng. 1991, 1361, 589. (13) Ashwell, G. J.; Jackson, P. D.; Lochun, D.; Thompson, P. A.; Crossland, W. A.; Bahra, G. S.; Brown, C. R.; Jasper, C. Proc. R. Soc. London A 1994, 445, 385. (14) Ashwell, G. J.; Walker, T. W.; Gentle, I. R.; Foran, G. J.; Bahra, G. S.; Brown, C. R. J. Mater. Chem. 1996, 6, 969.
Figure 1. Molecular structure of the dye (a) and inert spacer (b).
large second-order coefficients compared with their inorganic counterparts, and furthermore, the molecular hyperpolarizability may be optimized by varying the donor-acceptor combination and the length of the π-electron bridge. In this Letter, we report greatly improved behavior by using a cationic coumarin dye with a heterocyclic acceptor and an amino donor. The secondharmonic intensity from the LB monolayer, relative to the extensively studied hemicyanine dye, first reported by Girling et al.,15 is ca. 40 times stronger, and in addition, the SHG from alternate-layer films of the dye and an inert spacer (Figure 1) increases quadratically with the number of active layers. The effective second-order susceptibility is the highest value reported to date for an LB multilayer. (15) Girling, I. R.; Cade, N. A.; Kolinski, P. V.; Jones, R. J.; Peterson, I. R.; Ahmad, M. M.; Neal, D. B.; Petty, M. C.; Roberts, G. G.; Feast, W. J. J. Opt. Soc. Am. B 1987, 4, 950.
S0743-7463(97)01316-4 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/13/1998
1526 Langmuir, Vol. 14, No. 7, 1998
Letters
Figure 2. Pressure-area isotherm of the dye at 20 °C. The upper condensed phase extrapolates to ca. 35 Å2 molecule-1 at π ) 0 in this case and to 40 ( 10 Å2 molecule-1 for the series of isotherms obtained.
Experimental Section The dye was synthesized using the general procedure reported previously.16 It was spread from dilute chloroform solution (0.1 mg cm-3) onto the pure water subphase of one compartment of the LB trough (Nima Technology, model 622), left for 5 min at ca. 20 °C and then compressed at 0.5 cm2 s-1 (equivalent to a surface area loss of 0.1% s-1). The spacer material, N-octadecyl4-methylquinolinium bromide, was also spread from chloroform14 onto the second compartment of the trough and alternate-layer films of the dye and spacer were obtained by passing a glass substrate (for SHG) or a silver-coated slide (for SPR) through the floating monolayers at a rate of 5 mm min-1, the dye being deposited on the upstroke at 10-45 mN m-1 and the spacer on the subsequent downstroke at 30 mN m-1. Multilayer LB films for SHG were obtained by initially depositing a bilayer of the spacer followed by the alternate-layer deposition of the dye and spacer.
Results and Discussion The pressure-area (π-A) isotherm of the coumarin dye shows a limiting area of 130 Å2 molecule-1 at π ) 0 and, above the transition, the steep region of the high-pressure condensed phase extrapolates to a corresponding area of ca. 40 Å2 molecule-1 (Figure 2). The values approximate to the van der Waals area of the chromophore and its molecular cross section, respectively. Thus, the plateau separating the two regions probably reflects a change in the orientation of the chromophore from horizontal to vertical at the air-water interface. SHG measurements were performed in transmission using a Nd:YAG laser (λ ) 1.064 µm) and the secondharmonic intensity was compared with that of the Maker fringe envelope of a Y-cut quartz reference (d11 ) 0.5 pm V-1). The second-harmonic intensity is dependent upon the deposition pressure, the ratio of intensities from films deposited at 45 and 10 mN m-1 being ca. 30:1 (Figure 3). In contrast, the chromophore tilt angle (φ), obtained from the SHG polarization dependence, I2ω(pfp)/I2ω(sfp), is almost independent of the deposition pressure. Using the method of Kajikawa et al.,17 the angle of tilt relative to the normal to the substrate is 35 ( 3° for films deposited at (16) Lehmann, F.; Mohr, G. J. M.; Czerney, P.; Grummt, U.-W. Dyes Pigm. 1995, 29, 85.
Figure 3. Variation of the square root of the second-harmonic intensity with (a) the deposition pressure for monolayer films of the dye and (b) the number of bilayers of the dye and spacer. The multilayer structure has been studied to 30 bilayers but there is no reason the long-range structural order should not be maintained in much thicker films.
10 e π e 25 mN m-1 and 32 ( 3° for films deposited at higher pressures, i.e., π g 30 mN m-1. The layer thickness may be assumed to be independent of the deposition conditions, but probably as a result of optimum packing, strong SHG was obtained for films deposited at 30-45 mN m-1. Therefore, the remainder of this work only concerns those films obtained from the upper region of the isotherm. Surface plasmon resonance (SPR) studies were performed on glass|Ag|LB structures using an attenuated total reflection geometry in the Kretschmann configuration.18 The reflectivities were obtained as a function of the angle of incidence of a p-polarized laser beam (Nd: YAG, 532 nm) as described previously14 and the data analyzed by comparison with the Fresnel reflection formulas using the method of Barnes and Sambles.19 The (17) Kajikawa, K.; Kigata, K.; Takezoe, H.; Fukuda, A. Mol. Cryst. Liq. Cryst. A 1990, 182, 91. (18) Kretschmann, E. Z. Phys. 1971, 241, 313.
Letters
Figure 4. Normalized reflectance versus incident angle of the glass|Ag|LB structures showing the theoretical fits (solid lines) and experimental data for LB films deposited 30 mN m-1 onto an approximately 48 nm thick silver layer: dye monolayer (left); dye/spacer/dye trilayer (right).
experimental data and theoretical curves for the dye monolayer, deposited at 30 mN m-1, are shown in Figure 3. The derived thickness and the real and imaginary parts of the dielectric permittivity are l ) 2.7 ( 0.2 nm, r ) 2.8 ( 0.2 and i ) 1.3 ( 0.2. The dye forms centrosymmetric Y-type bilayers in which the chromophores pack headto-head and tail-to-tail, the thickness and dielectric permittivities of the bilayer being l ) 4.1 ( 0.2 nm, r ) 2.9 ( 0.2 and i ) 1.3 ( 0.2. The data suggest that the octadecyl chains interdigitate at the hydrophobic interface, the increase in thickness for the second layer being only 1.4 ( 0.4 nm. The cross-sectional areas of the hydrophilic chromophore and the aliphatic tail are in the ratio 2:1, and therefore, an interdigitating arrangement is not unusual. The behavior is reproducible and, furthermore, when the third layer is deposited onto the hydrophilic surface of the Y-type bilayer the increase (∆l ) 3.1 ( 0.4 nm) is similar to the thickness of the monolayer. However, when the layers are interleaved by N-octadecyl-4-methylquinolinium bromide, the corresponding results for the dye/spacer/dye trilayer are l ) 7.3 ( 0.2 nm, r ) 2.4 ( 0.2 and i ) 0.8 ( 0.2 (Figure 4). There is a slight mismatch in the cross sections of the dye and spacer, and in this case, the results do not indicate an interdigitating arrangement, the thickness of the repeating bilayer unit being 4.6 ( 0.4 nm by difference. The second-order susceptibility was calculated using the thickness and dielectric permittivities from SPR and the measured second-harmonic intensity relative to the (19) Barnes, W. L.; Sambles, J. R. Surf. Sci. 1987, 183, 189.
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quartz reference plate. Optimum SHG was obtained for films deposited at 45 mN m-1, the intensity being ca. 40 times stronger than that obtained from the extensively studied hemicyanine dye reported by Girling et al.15 with an effective susceptibility, χeff(2), of 340 ( 40 pm V-1 at 1.064 µm with the laser beam incident at 60° to the film. Analysis of the experimental data for LB films is often based upon a model whereby Kleinman’s symmetry is assumed and the susceptibility is reduced to two nonzero tensor elements. This is applicable if the dye is not strongly absorbing, whereas in this case, the monolayer has a broad absorption maximum at ca. 548 nm with an absorbance of 0.010-0.014. Nonetheless, the charge transfer band of such dyes may be finely tuned by varying the donor/acceptor combination and, for example, films of the N-octadecyl-4-pyridinium analogue, deposited at 35 mN m-1, have an absorption maximum at 480 nm and a high second-order susceptibility of χeff(2) ) 150 pm V-1 (χzzz(2) ) 250 pm V-1 and χzxx(2) ) 42 pm V-1 for φ ) 30°). The second-harmonic intensity from alternate-layer films of the dye and spacer shown in Figure 1 increases quadratically with film thickness, i.e., as I2ω(N) ) I2ω(1)N2, where N is the number of bilayers (Figure 3b). The normalized intensity, I2ω(N)/N2, is slightly weaker than the signal from the monolayer on glass but has a similar dependence upon the deposition pressure, and furthermore, the chromophore tilt angle is identical. By using the bilayer thickness and dielectric permittivities obtained from the SPR data in Figure 4, the second-order susceptibility, χeff(2), of the alternate-layer film is 90 ( 10 pm V-1 when the dye is deposited at 30 mN m-1 and 190 ( 30 pm V-1 when carefully deposited at 40-45 mN m-1. To date there have been few nonlinear optical studies on coumarin dyes: Moylan20 determined the hyperpolarizabilities of some simple amino-substituted analogues and Yankelvich et al.21 observed frequency doubling. However, the cationic coumarin, reported in this work, more closely resembles the hemicyanine dyes whose nonlinear optical properties have been extensively studied.11-15,22 Our current work adds to these studies and we report the highest susceptibility and strongest SHG, albeit resonantly enhanced, for a multilayer LB film. These are attributed, in part, to the extended conjugation of the optically nonlinear chromophore and, in part, to their favorable alignment within the alternate-layer structure. Acknowledgment. We are grateful to the EPSRC (U.K.) and the German Fonds der Chemischen Industrie and Deutsche Forschungsgemeinschaft for financial support. The EPSRC is also acknowledged for providing research studentships to P.L. and T.W.W. LA971316W (20) Moylan, C. R. J. Phys. Chem. 1994, 98, 13513. (21) Yankelvich, D. R.; Dienes, A.; Knoesen, A.; Schoenlein, R. W.; Shank, C. V. IEEE J. Quantum Electron. 1992, 28, 2398. (22) Ashwell, G. J.; Hargreaves, R. C.; Baldwin, C. E.; Bahra, G. S.; Brown, C. R. Nature 1992, 357, 393.
