Controlling the structure of transparent Langmuir-Blodgett films for nonlinear optical applications Geoffrey J. Ashwell,*' Paul D. Jackson,' Gary Jefferies,' Ian R. Gentleb and Colin H. L. Kennard* 'Centrefor Molecular Electronics, Cranjield University, Cranfeld, UK MK43 OAL bDepartment of Chemistry, The University of Queensland, Brisbane, Qld 4072, Australia Langmuir-Blodgett (LB) multilayers of (E)-N-alkyl-4-[ 24 4-docosyloxyphenyl)ethenyl]pyridinium bromide, C,H2,+ -Py+ -CH=CH-C6H,-O-C22H,,Br-(dye I), are centrosymmetric (Y-type) for n66 and non-centrosymmetric (Z-type) for 8 6n <20. The areas obtained from grazing incidence synchrotron X-ray diffraction indicate that the molecules adopt a 'stretched' rather than a 'U-shaped' configuration and, unlike conventional amphiphilic materials, the long chain homologues do not invert during deposition.The second-harmonic intensity from films of the higher homologues (n2 10) increases quadratically with the number of LB layers (120(N)=120(l)N2)whereas for the lower homologues (n<6) the intensity is negligible for even numbers of layers. The nonlinear optical properties of the sulfur (dye 11),selenium (dye 111) and quinolinium (dye IV) analogues are also reported and, in each case, quadratic enhancement of the second-harmonic intensity has been observed for suitable combinations of alkyl chain lengths. The second-order susceptibility, x(2)zzz, docosyloxyphenyl)ethenyl]quinolinium bromide (dye IV) is 20 pm V-'. of a 100 layer LB film of (E)-N-dodecyl-4-[ 2-(4- Interest in LB films for second-harmonic generation (SHG) stems from the requirement that the structure must be non- centrosymmetric and the fact that the LB technique offers control of the packing at the molecular level. However, as most conventional dyes form centrosymmetric Y-type struc- tures, in which the molecular layers pack head-to-head and tail-to-tail, it has been necessary to interleave with inactive For further improvement every layer should be active but to date, of the homomolecular films, only 2-docosyla- mino-5-nitropyridine, DCANP,' and the quinolinium zwit- terion, C16H33-Q3CNQ,6 have shown a quadratic increase in the SHG to thicknesses of a few hundred layers.Few optically nonlinear dyes are known to form Z-type (or X-type) films in which the molecular layers pack head-t~-tail.~-~ However, recent work at Cranfield has shown that unconventional materials with hydrophobic chains at opposite ends of a hydrophilic chromophore invariably form non-centrosym-metric Z-type structures providing that the alkyl groups are of an appropriate length.'0*'' For the majority of amphiphilic materials the layers tend to invert during deposition placing like ends together whereas this is resisted when each end is hydrophobic.In this work we report the nonlinear optical properties of four different dyes (I-IV) and show that Z-type film structures may be fabricated by careful consideration of the alkyl end-groups. Quadratic SHG enhancement has resulted and the applicability of the technique to different classes of dyes suggests that alternate-layer deposition is no longer necessary for the formation of non-centrosymmetric films. I IV Experimental Synthesis The optically nonlinear dye, (E)-N-alkyl-4-[ 2-( 4-docosyloxy- pheny1)ethenyll pyridinium bromide (dye I), was synthesised by the treatment of p-docosyloxybenzaldehyde (2 mmol) and the appropriate N-alkyl-4-picolinium bromide (2 mmol) in hot ethanol with piperidine (2 drops) as catalyst.The sulfur (dye 11) and selenium (dye 111) analogues and the quinolinium dye (IV) were synthesised in a similar manner by the treatment of the appropriate heterocyclic cation and a para-substituted benzaldehyde. The products were purified by column chroma- tography and the resultant pale yellow crystals characterised by their 'H NMR and 13C NMR spectra recorded on a Bruker AM300 spectrophotometer; J values are given inHz. The elemental data are consistent with incomplete combustion but an assessment of the I3C NMR spectra indicates purities better than 99.5%.(E)-N-Dodecyl-4[ 24 4docosyloxyphenyl) ethenyl ]pyridinium bromide (dye I, n= 12) 6,(360 MHz; CDC13) 9.04 (2 H, d, J 7), 8.05 (2 H, d, J 7), 7.72 (1 H, d, J 16), 7.60 (2 H, d, J 9), 7.01 (1 H, d, J 16), 6.90 (2 H, d, J 9), 4.68 (2 H, t, J 7), 3.96 (2 H, t, J 7), 1.28-1.19 (60 H, br m), 0.88-0.83 (6 H, m); 6,(91 MHz; CDC13) 161.78 (0), 153.85 (0), 144.01 (l), 142.10 (l), 130.46 (l), 127.19 (0), 123.80 (l), 119.61 (1), J. Muter.Chem., 1996, 6(2), 137-141 137 115.20 (l), 68.38 (2), 60.77 (2), 31.97 (2), 31.73 (2), 29.76 (2), 29.65 (2), 29.57 (2), 29.50 (2), 29.41 (2), 29.23 (2), 29.15 (2), 26.18 (2), 26.09 (2), 22.74 (2), 14.17 (3). (E)-N-Dodecyl-4-[ 2-(4-docosylsulfanylphenyl)ethenyl]-pyridinium bromide (dye 11) 6,(360 MHz; CDC1,) 9.08 (2 H, d, J 7), 8.08 (2 H, d, J 7), 7.73 (1 H, d, J 16), 7.55 (2 H, d, J 9), 7.35 (2 H, d, J 9), 7.13 (1 H, d, J 16), 4.73 (2 H, t, J 7), 2.96 (2 H, t, J 7), 1.29-1.20 (60 H, br m), 0.88-0.83 (6 H, m); 6,(91 MHz; CDC13) 153.57 (0), 144.18 (l), 142.62 (0), 141.71 (l), 131.28 (0), 128.96 (l), 127.26 (l), 124.13 (l), 121.20 (l), 60.97 (2), 39.34 (2), 32.36 (2), 32.02 (2), 31.99 (2), 31.80 (2), 29.79 (2), 29.69 (2),29.63 (2), 29.60 (2), 29.46 (2), 29.43 (2), 29.33 (2), 29.18 (2), 29.07 (2), 28.92 (2), 28.63 (2), 26.22 (2), 22.78 (2), 14.21 (3).(E)-N-Dodecyl-4-[244-docosylselanylphenyl)ethenyl]-pyridinium bromide (dye 111) 6,(360 MHz; CDC1,) 9.05 (2 H, d, J 7), 8.09 (2 H, d, J 7), 7.75 (1 H, d, J 16), 7.52 (2 H, d, J 9), 7.45 (2 H, d, J 9), 7.16 (1 H, d, J 16), 4.71 (2 H, t, J 7), 3.00 (2 H, t, J 7), 1.30-1.22 (60 H, br m), 0.89-0.84 (6 H, m); 6,(91 MHz; CDC13) 153.49 (0), 144.23 (l), 141.74 (l), 136.71 (0), 132.35 (0), 131.14 (l), 128.93 (l), 124.22 (l), 121.62 (l), 61.06 (2), 32.02 (2), 31.79 (2), 31.09 (2), 30.40 (2), 30.24 (2), 30.06 (2), 30.01 (2), 29.80 (2), 29.71 (2), 29.63 (2), 29.46 (2), 29.25 (2), 29.19 (2), 27.39 (2), 26.22 (2), 22.79 (2), 14.21 (3).(E)-N-Dodecyl-4-[2-(4-docosyloxyphenyl)ethenyl]-quinolinium bromide (dye IV ) 6, (360 MHz; CDCl,) 10.09 (1 H, d, J 7), 8.66 (1 H, d, J 9), 8.33 (1 H, d, J 7), 8.11 (2 H, m), 7.91 (1 H, m), 7.76 (2 H, d, J 17), 7.70 (2 H, d, J 9), 6.93 (2 H, d, J 9), 5.07 (2 H, t, J 8), 3.99 (2 H, t, J 7), 1.31-1.19 (60 H, br m), 0.87-0.82 (6 H, m); 6, (91 MHz; CDCl,) 161.98 (0), 153.33 (0), 148.78 (l), 143.92 (l), 137.78 (0), 134.89 (l), 130.80 (l), 129.13 (l), 127.64 (0), 126.97 (l), 126.61 (0), 118.31 (l), 116.79 (l), 116.07 (l), 115.26 (l), 68.44 (2), 57.30 (2), 31.98 (2), 31.95 (2), 30.16 (2), 29.76 (2), 29.64 (2), 29.57 (2), 29.47 (2), 29.44 (2), 29.41 (2), 29.37 (2), 29.24 (2), 26.62 (2), 26.09 (2), 22.73 (2), 14.16 (3). LB film deposition Dilute chloroform solutions of the two-legged dyes were spread onto the pure water subphase of one compartment of an alternate-layer LB trough (Nima Technology, model 622), left for 10 min and then compressed at 0.5 cm2 s-' (ca.0.1% s-' of compartment area). LB films were obtained by cycling a silicon wafer (for X-ray) or a glass substrate (for SHG) at 5 mm min-l via the second compartment of the LB trough to deposit on the upstroke only. Details of the deposition press- ures are provided in the legends to the Figures of the various dyes. SHG measurements The SHG was measured in transmission with the laser beam (Nd:YAG, A= 1064nm) at an angle of 45"to the LB film. The polarization of the fundamental beam was rotated using a half-wave plate and the p-polarized second-harmonic intensity was calibrated against the Maker fringes of a Y-cut quartz reference (dll =0.5 pm V-'). The data were analysed using the method described previously.2 Results and Discussion Pyridinium dyes The film-forming properties of dye I, CnH2n+1-Py' -CH=CH-C6H,-O-C22H,, Br-, are dependent upon the second hydrophobic group.The surface pressure uersus surface area isotherms are featureless for II <6 but show anoma!ous transitions for rn 2 8 (Fig. 1); the areas at collapse, 24 f4 A2 molecule-', are consistent with the molecular cross- sectiFn whereas the limiting areas at zero pressure, 110 to 140A2 molecule-' for 1126, match the face area of the chromophore (Fig. 2). The transitions may be associated with a change in the molecular geometry from 'U-shaped' at low pressures, whereby both alkyl chains point in the same direc- tion, to 'stretched' at high pressures with the chains pointing in opposite directions. The 'stretched' configuration has been confirmed by X-ray diffraction studies on both Langmuir and LB films (Table 1) and also, previously, from the layer thickness obtained from ellipsometry studies on deposited films." GrazinG incidence synchrotron X-ray diffraction studies at A= 1.488A were performed on wiggler beamline 16A at the Photon Factory, Tsukuba (Japan), using the procedure and apparatus described by Matsushita et X-Ray data were obtained for a Langmuir film of the tetradecyl homologue at 20.7 "C and 35 mN m-' with the film balance mounted on the sample stage of the diffractometer. The data are similar to 50 40 7 I 30z E1$ 3 20 Q 10 0 0 50 100 150 area per molecule/A2 Fig.1 Dependence of the surface pressure versus area (n-A) isotherms on the number of carbon atoms in the second alkyl chain of dye I at ca. 20°C: (a) n=20; (b) n=18; (c) n= 14. In each case the films were deposited at 35 mN m-'. F I t 1 8-00000000000 I...,r....r..,.l,.. . I I I 0 5 10 15 20 25 number of carbon atoms Fig. 2 Dependence of the compression data and nonlinear optical properties on the number of carbon atoms in the second alkyl chain of dye I: areas per molecule at collapse (0)and at zero pressure (0); SHG from LB monolayers of dye I (x).The region between the vertical lines corresponds to the alkyl chain lengths which give Z-type films. To the left of these lines the bilayer structure is Y-type whereas to the right there is a tendency towards antiparallel alignment within the layers. 138 J. Mater. Chem., 1996, 6(2), 137-141 Table 1 Areas per molecule of (E)-N-alkyl-4-[ 2-(4-docosyloxyphenyl) ethenyl] pyridinium bromide (dye I) obtained from the compression isotherms and from grazing incidence sychrotron X-ray diffraction isotherm" X-rayb alkyl group A,/A2 &/A2 Ad IA2 tetradec yl 22 27 23.83 octadec yl 23 31 (21.27) " A, and Ad correspond to the areas per molecule at collapse and at the deposition pressure of 35mN m-' respectively.bThe value in parentheses corresponds to the area per molecule from a deposited LB film and, in this case, the discrepancy probably results from a slight contraction upon deposition. All of the above values indicate that the molecule adopts a 'stretched' configuration with the hydrophobic chains pointing in opposite directions. those obtained for other Langmuir film^'^.'^ and are attributed to a hexagonal lattice with two-dimensional Miller indices (1,O). The scan shows a single X-ray peak at Q,= 1.383 A-l with a half width at half maximum of 0.024 A-l and these relate to a d-spaciqg of 4.543 A (dl,=2n/Q,) and to a correlation length of 42 A [Fig. 3(a)]. For comparison, the d-spacing and correlation length obtained for an LB film of the octadecyl homologue, deposited at 35mN m-', are 4.292 and 9.2A respectively [Fig.3(b)]. The correlation lengths are an estimate of the lower limit but clearly the crystallites are small, as is usual for such materials. The d-spacings correspond to areas of 23.83 42 for the tetradecyl homologue (Langmuir film) and 21.27 A2 for the octadecyl homologue (LB film). The areas are consistent with those obtained from the isotherms and confirm that the 200 h2.-c -200Ot 3$I,,,,,,]2-400 a-.-1.2 1.3 1.h 1.5 I coo I 200 0 I---I -200 1 I I I 1 1.0 1.2 1.4 1.6 1.8 Q*/A-' Fig. 3 X-Ray diffraction data for (a) a Langmuir film of the tetradecyl homologue of dye I at 20.7 "C and 35 mN m-' and (b)a ten layer LB film of the iodide salt of the octadecyl homologue.The solid line is a Lorentzian fit to the experimental data of the Qx resolved scan. molecules adopt a 'stretched' configuration (Table 1).The alkyl chains point in opposite directions and therefore the molecules must align on the subphase with one of the hydrophobic chains adjacent to the water surface. It is assumed that the shorter of the two alkyl chains is oriented downwards and, based upon the SHG data in Fig. 2, optimum alignment is realised when the chains differ by four or more carbons. The SHG decreases abruptly for n320, and the behaviour can be explained by assuming that the hydrophobic ends need to be sufficiently different for the chromophores to adopt a particular orientation (dipole up or down).The SHG from films comprising an even number of LB layers of the lower alkyl homologues of dye I (nd6) is negligible and indicates a centrosymmetric Y-type bilayer arrangement. Interestingly, the SHG from films of the higher homologues (10dn <18) increases quadratically with the number of layers and therefore the packing is Z-type. The sulfur (dye 11) and selenium (dye III) analogues also show a quadratic SHG dependence but the intensity is slightly reduced (see Fig. 4). The equivalent monolayer intensities, Izo(N)/N2where N is the number of layers, range from ca. 2% (dye 111) to 10% (dye I) of the signal from LB monolayers of the hemicyanine dye, (E)-4-[2-(4-dimeth ylaminophen yl)e thenyl ]-N-docos ylp yridinium bromide, first reported by Girling et a1." However, films of this dye are strongly absorbing at the harmonic wavelength =532 nm) whereas those of I to I11 are nearly transparent.The absorption maxima of I to I11 are at 360+5 nm in each case but the LB absorption band of the alkoxy dye is broader and has a very slight absorbance of 5 x layer-' at the harmonic wavelength. Thus, the nonlinear optical properties are probably influenced by minor variations in the resonant enhancement as the absorption band tails off. Quinolinium dye Homomolecular films of the quinolinium dye (IV) are rigid and difficult to deposit, but when spread in a 1 :1 ratio with stearic acid they become more manageable. As with the pyridinium analogues the isotherm shows a transitional region, only far more pronounced, and the areas per dye molecule are consistent with a transformation from a 'U-shaped' configur- ation at low pressures to a 'stretched' configuration above 28 mN m-' (Fig.5). An alternative explanation of collapse is far less likely; the strong SHG from films deposited in the high pressure regime and its quadratic dependence on the number lo ' 00 0*o 0 00 00 0 0 0 4-8 5U 2 0 10 20 30 60 number of layers Fig. 4 Variation of the square root of the second-harmonic intensity with the number of LB layers of the dodecyl homologue of dye I (x), dye I1 (a)and dye I11 (0) J. Mater. Chem., 1996, 6(2), 137-141 139 CO -I E 30z 4 5 20 h 10 0 area per molecuie/A* Fig.5 Surface pressure versus area (n-A) isotherm of dye IV and stearic acid (1:1 molar ratio) at ca.20 "C.LB films were deposited in the high pressure regime at 30 mN m- '. of layers leads us to firmly dismiss this idea. Thus, as with the pyridinium analogues the molecules probably align with a hydrophobic chain adjacent to the subphase at higher press- ures. This is contrary to notions currently perceived by the LB community but, for the series of two-legged dyes, all of the evidence (isotherms, X-ray diffraction, layer thickness) points to such an arrangement. LB films of dye IV are pale yellow and only weakly absorb at the harmonic wavelength with A2, =7 x layer-' (Fig. 6). The SHG increases quadratically with the number of LB layers (Fig.7) and the normalised intensity, 12,/N2, is 30% of the mean signal from monolayer films of Girling's hemicyan- ine dye." The hemicyanine has been extensively studied and has the drawback that it is coloured and forms centrosymmetric Y-type structures unless interleaved. In contrast, the quinolin- ium dye is nearly transparent at 532nm and it forms non- centrosymmetric Z-type structures. There is considerable interest in the efficiency/transparency tradeoff for second-order dyes and we have found that the peak absorbance of donor-(n-bridge)-acceptor materials may be finely tuned by varying the donor-acceptor combi-nation."." The greatly improved SHG from films of dye IV compared with the pyridinium dyes (I to 111) is attributed, in part, to increased resonant enhancement, the absorption maxi- mum being red-shifted from 360 to 410nm with the stronger quinolinium acceptor.Using the method of Kajikawa et the SHG polarization dependence, Iz,(p+p)/12,(s-+p), of the 100 layer film suggests that the charge transfer axis of the 0.4 Q) 0.3 as3 0.2 0.1 0 300 400 500 600 700 800 900 wavelengthhm Fig.6 Absorption spectrum of a 100 layer LB film of dye IV-stearic acid 140 J. Muter. Chew., 1996, 6(2), 137-141 number of layers Fig. 7 Variation of the square root of the second-harmonic intensity with the number of LB layers of dye IV-stearic acid molecule is inclined at an angle of 36" from the perpendicular to the substrate. However, as intermolecular charge transfer as well as intramolecular charge transfer can give rise to strong SHGi7 the validity of this technique is ambiguous.Nonetheless, the refractive index (at 670 nm) and thickness of the 100 layer film from ellipsometry are 1.42 and 0.46 pm respectively, the thickness being consistent with the value obtained from mol- ecular modelling when the chromophore tilt angle is taken into consideration. From these data the second-order suscepti- bility, x(2)zzz,of the 100 layer film is 20 pm V-l but significantly, the absorbance at the harmonic wavelength is very small. In addition, the SHG has shown no sign of deterioration through- out a period of about a year and we suggest that the Z-type film structure, in this case, is stabilised by the hydrophobic ends of the chromophore.Conclusions In this paper we have shown that two-legged molecules can adopt a 'stretched' configuration at the air-water interface and that Z-type structures may be obtained by careful consideration of the alkyl chain lengths. In fact, our investigations on chromophores of general formula: CnH2,,+ -A+ -(n-bridge)-Y -CmH2,+ where A+ is a heterocyclic cation (pyridinium, quinolinium, isoquinolinium or benzothiazolium) and Y is 0,S, Se, OC(O), N(H) or N(C,H,, + have resulted in Z-type structures."." Most combinations have shown SHG enhancement with film thickness and, in four cases, a quadratic dependence to 100 layers has been realised. In contrast, only two of the single- legged dyes, DCANP and C16H33-Q3CNQ, have shown quad- ratic enhancement to 100 although others have dem- onstrated such behaviour in alternate-layer fi1ms.I~~ Thus, we conclude that the addition of the second leg provides a convenient route to non-centrosymmetric LB structures for nonlinear optical applications.We acknowledge the EPSRC (UK) for funding the nonlinear optics programme at Cranfield, EPSRC and British Gas plc for providing a studentship to P.D.J., and the Australian Nuclear Science and Technology Organisation and the Australian National Beamline Facility for support. We also acknowledge Professor T. Matsushita for use of the facilities at Tsukuba. References 10 G. J. Ashwell, P. D. Jackson and W. A. Crossland, Nature, 1994, 1 2 G.J. Ashwell, E. J. C. Dawnay, A. P. Kuczynski and P. J. Martin, SPIE Int. Soc. Opt. Eng., 1991, 1361, 589. G. J. Ashwell, P. D. Jackson, D. Lochun, W. A. Crossland, 11 368,438. G. J. Ashwell, G. Yu, D. 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