Langmuir 1999, 15, 3627-3631
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Diminished Second-Harmonic Generation from Oligomeric Squaraine Derivatives of N-Octadecylpyrrole Relative to the Intensity from Films of the Centric Anilinosquaraines Geoffrey J. Ashwell,* Antony N. Dyer, and Andrew Green Centre for Molecular Electronics, Cranfield University, Cranfield MK43 0AL, U.K. Received October 19, 1998. In Final Form: February 23, 1999 Langmuir-Blodgett (LB) monolayers of oligomeric dyes (I), obtained from the reaction of Noctadecylpyrrole and squaric acid, exhibit second-harmonic generation (SHG). This arises from the inherent noncentrosymmetry but, importantly, the intensity is 4 orders of magnitude weaker than the optimum signal from LB films of 2,4-bis[4-(N-methyl-N-hexylamino)phenyl]squaraine (II). The anilinosquaraine is centric, but solid solutions of II in poly(vinyl acetate) are SHG-active. The intensity increases quadratically with the concentration of dye but is extinguished by the incorporation of electron acceptors into the film. The second-order properties are attributed to a noncentrosymmetric aggregate structure rather than the molecule itself or the interface between the film and the underlying glass substrate. The latter is ruled out by the direct observation of SHG from a free-standing film of the solid solution.
Introduction Prior to our initial discovery of second-harmonic generation (SHG) from Langmuir-Blodgett (LB) films of anilinosquaraines,1 it was assumed that second-order effects require the molecule, as well as the bulk film, to be noncentrosymmetric. The chromophore is both linear and planar and, from X-ray structural analysis, the dimensions of half are symmetry-generated across an inversion center by the atomic coordinates of the other.2-4 Nonetheless, LB films of 2,4-bis[4-(N-methyl-N-hexylamino)phenyl]squaraine (II) have a high optimum susceptibility of χ(2)zzz ≈ 710 pm V-1 at 1.064 µm,5 but the behavior is dependent upon the phase. The SHG-active films have an absorption maximum at ca. 695 nm but become inactive when converted to the H aggregate, with the absorption being blue-shifted to ca. 530 nm. Furthermore, solid solutions of the dye in poly(vinyl acetate) exhibit SHG,5 and thus to satisfy the structural requirement, the behavior is attributed to an association of the squaraine molecules even in dilute solution. Evidence has been provided by electrospray ionization mass spectrometry, which shows m/z values which conform to the dimeric species.6,7 The molecules must adopt an acentric SHGactive arrangement, and the persistent aggregation infers intermolecular interaction between the terminal donor (anilino group) and central acceptor (C4O2) moieties. Thus, within the dimeric aggregate, there is most likely a T arrangement, with the second-order properties being dependent upon the extent of the charge-transfer interac* To whom correspondence should be addressed. Tel: (44)-01234-754224. Fax: (44)-01234-750875. E-mail: g.j.ashwell@ cranfield.ac.uk. (1) Ashwell, G. J.; Jefferies, G.; Hamilton, D. G.; Lynch, D. G.; Roberts, M. P. S.; Bahra, G. S.; Brown, C. R. Nature 1995, 375, 385. (2) Ashwell, G. J.; Bahra, G. S.; Brown, C. R.; Hamilton, D. G.; Lynch, D. G.; Kennard, C. H. L. J. Mater. Chem. 1996, 6, 23. (3) Bernstein, J.; Goldstein, E. Mol. Cryst. Liq. Cryst. 1988, 164, 213. (4) Dirk, C. W.; Herndon, W. C.; Cervantes-Lee, F.; Selnau, H.; Martinez, S.; Kalmegham, P.; Tan, A.; Campos, G.; Velez, M.; Zyss, J.; Ledoux, I.; Cheng, L. J. Am. Chem. Soc. 1995, 117, 2214. (5) Ashwell, G. J.; Roberts, M. P. S.; Rees, N. D.; Bahra, G. S.; Brown, C. R. Langmuir 1998, 14, 5279. (6) Ashwell, G. J. J. Mater. Chem. 1998, 8, 373. (7) Ashwell, G. J.; Williamson, P. C.; Green, A.; Bahra, G. S.; Brown, C. R. Aust. J. Chem. 1998, 51, 599.
tion.5,7 This concept has been verified by the theoretical analysis of Bre´das and Brouye`re8 and independently corroborated by Honeybourne;9 their work provides a theoretical basis for the second-order activity of the centric molecules and clearly demonstrates that strong secondorder coefficients can result from a dimeric T arrangement. In contrast, Lynch et al.10 have alluded to the properties being induced by surface effects and have assumed a common mechanism for both the pyrrol-2-yl- and anilinosquaraines. Their argument is based on the premise, albeit erroneous, that only monolayers deposited on hydrophilic substrates exhibit SHG and on the fact that aggregation cannot explain the origin of weak SHG in LB films of the oligomeric derivative (Ib; Figure 1) which they contend is centric.10,11 However, their published analytical data indicate an asymmetrically substituted squarate, and it is unclear why the inherent second-order properties of this component have been overlooked. We have reinvestigated the pyrrol-2-yl dyes reported by Lynch et al.10,11 and, in this work, compare the secondorder nonlinear optical properties with those of the extensively studied anilinosquaraines.5-7,12,13 LB films of the oligomer exhibit weak SHG, barely distinguishable from the background noise, and this is inherent to its noncentrosymmetric structure. The behavior contrasts with the strong SHG from LB films of the centric anilino dyes5,7 and, as an extension of this work, we report the second-order properties of 2,4-bis[4-(N-methyl-N-hexylamino)phenyl]squaraine dispersed in poly(vinyl acetate). The second-harmonic intensity increases quadratically with the concentration of dye and, furthermore, SHG has been observed from a free-standing film of the solid (8) Bre´das, J. L.; Brouye`re, E., private communication. Brouye`re, E. Ph.D. Thesis, Universite´ de Mons-Hainaut, Mons, 1997. (9) Honeybourne, C. L., private communication. (10) Lynch, D. E.; Peterson, I. R.; Floersheimer, M.; Essing, D.; Chi, L. F.; Fuchs, H.; Calos, N. J.; Wood, B.; Kennard, C. H. L.; Langley, G. J. J. Chem. Soc., Perkin Trans. 2 1998, 779. (11) Lynch, D. E.; Geissler, U.; Peterson, I. R.; Floersheimer, M.; Terbrack, R.; Chi, L. F.; Fuchs, H.; Calos, N. J.; Wood, B.; Kennard, C. H. L.; Langley, G. J. J. Chem. Soc., Perkin Trans. 2 1997, 827. (12) Ashwell, G. J.; Leeson, P.; Bahra, G. S.; Brown, C. R. J. Opt. Soc. Am. B 1998, 15, 484. (13) Ashwell, G. J.; Jefferies, G.; Rees, N. D.; Williamson, P. C.; Bahra, G. S.; Brown, C. R. Langmuir 1998, 14, 2856.
10.1021/la981463+ CCC: $18.00 © 1999 American Chemical Society Published on Web 04/17/1999
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Ashwell et al. Table 1. Analytical Data for the Polymer and Oligomeric Fractions IR (KBr) fraction
mp (°C)
vis (CHCl3): λmax (nm)
νCH (cm-1)
νCO (cm-1)
νSq (cm-1)
1a 2b 3 4 5b 6 7
380 85-87 59-63 49-53 40-43 34-38 ?
546 552 563 579 590 624
2920 2918 2921 2924 2920 2923 2925
1747 1742 1748 1743 1750 1748 1742
1618 1626 1628 1634 1618 1640 1634
a
Figure 1. Molecular structures: assigned in this work (Ia), by Lynch et al.11 (Ib), and the oligomeric 2,4-squarate (Ic).
solution. This demonstrates that the previously observed second-order effects are indeed not a property of the interface between film and substrate. Therefore, we dismiss the argument of Lynch et al.10 regarding the anilinosquaraines and, for the pyrrol-2-yl dye, refute their claim of the first example of SHG from a centrosymmetric polymer chain of alternating donors and acceptors. The molecular structure, as previously depicted by Lynch et al.10 (Ib) and described herein (Ia), is indisputably acentric. In addition, their sample11 is impure and may contain as many as six different oligomeric forms. These have been isolated in this work. Experimental Section Synthesis. Polymeric and oligomeric samples were obtained from the condensation of squaric acid (200 mg, 1.75 mmol) and N-octadecylpyrrole (560 mg, 1.75 mmol) in methanol (50 cm3), by adapting the general procedures of Triebs and Jacob14,15 and Yu et al.16 When refluxed for 4 h, the solution turned initially red and then blue before yielding a black, insoluble, flocculent product (Ia). The precipitate was collected by filtration and repeatedly washed with hot chloroform to remove the soluble oligomeric derivatives. Yield 60%. Mp 380 °C (dec). Found: C, 79.5; H, 10.8; N, 3.8. (C48H79N2O2)m(C26H39NO2)n requires C, 79.47; H, 10.47; N, 3.71 for n ) 2m. Infrared (KBr): νSq 1618 cm-1 (2,4-squarate); νCO 1747 cm-1 (CO group of trisubstituted ring); νCH 2920 cm-1 (C18H37). The reaction of squaric acid and pyrroles (Py), substituted at one of the two R sites, is known to yield a trisubstituted analogue and a 2,4-squarate,15 i.e., the isolated units of Ia. When both R sites are active, as in this work, the elemental data are ambiguous (14) Treibs, A.; Jacob, K. Angew. Chem., Int. Ed. Engl. 1965, 4, 694. (15) Treibs, A.; Jacob, K. Liebigs Ann. Chem. 1966, 699, 153. (16) Yu, L. P.; Chen, M.; Dalton, L. R.; Cao, X. F.; Jiang, J. P.; Hellwarth, R. W. Mater. Res. Soc. Symp. Proc. 1990, 173, 607.
Insoluble polymer. b Samples used for LB deposition.
and conform to the polymer, AmBn, where m e n e 2m (Ia) and oligomeric 2,4-squarate, Py-(B)n-H, where n is 2 or 3 (Ic). However, the polymer is inferred from the infrared data: (i) the peak at 1747 cm-1 corresponds to the carbonyl group of the trisubstituted unit and not the 2,4-squarate; (ii) the absorbance ratio of 1.3 for νCH/νSq, cf. 1.2 ( 0.2 for 2,4-bis(N-octadecyl-3,5dimethylpyrrol-2-yl)squaraine and related dyes, suggests that the donor and C4O2 units are present in a ratio of ca. 2:1. Thus, the data are representative of Ia where m:n ) 1:2. In addition, soluble oligomeric derivatives were obtained from the filtrate and purified by thin-layer preparative plate (silica) chromatography using CHCl3 as the eluent, with the fractions being repeatedly subjected to this technique. Different products with discrete absorption maxima at 546-624 nm were isolated (Table 1), but yields suitable for deposition were only obtained for reddish-purple (λmax 546 nm, hwhm 12 nm) and blue (λmax 579 nm, hwhm 30 nm) forms. Their respective infrared absorbance ratios of ca. 1.4 and 4, as defined above for νCH/νSq, correspond to stoichiometries of ca. 1:2 and 3:1 (m:n). However, the assignment should be treated as informative rather than quantitative. The anilinosquaraine, 2,4-bis[4-(N-methyl-N-hexylamino)phenyl]squaraine (II), was obtained as previously described, and the analytical data are reported in ref 5. Deposition. The pyrrol-2-yl dyes were spread from dilute chloroform solutions (ca. 0.1 mg mL-1) onto the pure water subphase of an LB trough (Nima Technology model 622), left for 5 min at ca. 24 °C, and then compressed at 0.5 cm2 s-1 (ca. 0.1% s-1 of total area). Monolayers were deposited on the upstroke by passing a hydrophilically treated glass substrate (for SHG) or a gold-coated substrate [for surface plasmon resonance (SPR)] through the floating monolayers at a rate of 30 µm s-1 and a surface pressure of 20 mN m-1. Spin-coated films of the anilino dye were obtained from dilute chloroform solutions of 2,4-bis[4-(N-methyl-N-hexylamino)phenyl]squaraine and poly(vinyl acetate) in ratios of 0.1-5% by mass. Films were fabricated under identical conditions to maintain a constant thickness. Optical Characterization. SPR studies were carried out using an attenuated total reflection geometry in the Kretschmann configuration.17 Gold was vacuum deposited to a thickness of ca. 45 nm onto the face of a glass substrate which was index matched to a 60° BK7 crown glass prism. Reflectivity data were collected as a function of the incident angle using a p-polarized HeNe laser (λ ) 632.8 nm) and subsequently corrected for reflections at the entrance and exit faces of the prism prior to analysis using the Fresnel reflection formulas.18,19 The real (r) and imaginary (i) components of the dielectric permittivity and thickness (l) obtained for the gold film were used in the subsequent analysis of the data obtained for the glass/Au/monolayer structures. SHG measurements on the deposited monolayers were performed in transmission with the p-polarized laser beam (Nd: YAG, λ ) 1.064 µm) incident at an angle of 45° to the LB film, with there being no discernible signal at normal incidence. The data were calibrated against the Maker fringes of a Y-cut quartz reference plate (d11 ) 0.5 pm V-1). (17) Kretschmann, E. Z. Phys. 1971, 241, 438. (18) Barnes, W. L.; Sambles, J. R. Surf. Sci. 1986, 177, 399. (19) Barnes, W. L.; Sambles, J. R. Surf. Sci. 1987, 183, 189.
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Table 2. Preparation Times, Melting Points, and 1H NMR Data for Oligomeric Samples with Absorption Maxima at 546 nm in CHCl3 and IR Absorbance Ratios of 1.4 for νCH/νSq reflux (h)
mp (°C)
1 4 48
32-36 85-87 85-89
1H
7.85 (d, J ) 2 Hz) 7.86 (d, J ) 2 Hz) 7.85 (d, J ) 2 Hz)
NMRa
7.32 (s) 7.32 (s) 7.32 (s)
6.49 (q, J ) 4 Hz) 6.49 (q, J ) 4 Hz) 6.49 (q, J ) 4 Hz)
a Octadecyl groups (300 MHz, CDCl ): δ 0.88 (t, J ) 7 Hz, 3 H CH3); 1.26 (br s, CH2); 1.5-1.9 (m, CH2CH2N); 3.2-3.9 (br, CH2N).
Figure 2. Surface pressure vs area isotherms of the oligomeric analogues: reddish-purple form (solid line) and blue form (broken line). At higher surface pressures, the films begin to collapse at the corners of the trough and, consequently, the collapse points (not shown) are ill-defined. Please note that the molecular unit as defined here corresponds to the N-octadecylpyrrole group and its associated substituents.
Results and Discussion Pyrrole Dyes. The polymer reported by Lynch et al.10,11 has absorption maxima at 562 and 574 nm in CHCl3 and an unusually broad melting range of 29-89 °C. Structure Ib was assigned, and the infrared data were attributed to centric 2,4-squarate (1620 cm-1) and acentric 1,2squarate (1750 cm-1) units. In contrast, our polymeric product is insoluble and decomposes without melting at ca. 380 °C. Thus, the previously reported sample11 probably corresponds to the oligomeric fractions listed in Table 1. They have discrete visible-range absorption maxima and, in addition, sharp melting points. These are encompassed by the broad melting range of Lynch et al.,11 with the upper (85-87 °C) and lower (34-38 °C) values approximating to the limits of this range. The data suggest that the previously reported sample is a mixture of the six oligomeric forms isolated in this work by preparative plate chromatography. However, the melting points are also dependent upon the reaction time and, thus, the relative molecular mass which may vary from batch to batch (Table 2). In this work, we report the LB deposition and properties of the predominant reddish-purple and blue forms with absorption maxima at 546 and 579 nm, respectively, when dissolved in CHCl3. The pressure-area isotherms, shown in Figure 2, are very different from the data published by Lynch et al.11 For example, the previously reported limiting area of ca. 0.16 nm2 unit-1 at π ) 0 is significantly less than the van der Waals cross section, suggesting clustering or premature collapse. The corresponding value shown in
Figure 3. Visible spectra of LB monolayers of the oligomeric analogues: (a) reddish-purple form; (b) blue form.
Figure 2 is ca. 0.36 nm2 unit-1, where the repeat unit, as defined here, is N-octadecylpyrrole and its associated substituents. Unlike the anilinosquaraines, whose spectra are strongly dependent upon the type of aggregate,5,6 LB films of the oligomeric dyes show maxima which approach the solution values but absorption bands which are significantly broadened (Figure 3). Analysis of the SPR data gave a monolayer thickness and real and imaginary components of the dielectric permittivity of l ) 2.5 ( 0.2 nm, r ) 2.6 ( 0.2, and i ) 0.0 ( 0.2, respectively, for the reddishpurple phase (Figure 4). The imaginary component is effectively zero and mirrors the trivial absorbance of ca. 0.0001 for the LB monolayer at the excitatory wavelength (632.8 nm). Similar results have been obtained for the blue form (l ) 2.2 ( 0.2 nm, r ) 2.9 ( 0.2, and i ) 0.0 ( 0.2) but, in this case, there is a slight discrepancy between the imaginary components derived from the absorbance, i ) 0.24 at 632.8 nm, and SPR data (Figure 5). This reflects the proximity of the absorption edge to the excitatory wavelength and may be explained by the fact that the different substrates, glass and gold, probably affect the packing and spectra of the LB overlay. A comparison of the layer thicknesses with the van der Waals length of N-octadecylpyrrole, i.e., 2.87 nm, suggests that the hydrophobic tails are tilted from the normal by ca. 30°
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Ashwell et al.
Figure 6. Molecular structure of the anilinosquaraine (II).
Figure 4. Normalized reflectance vs incident angle from a glass/Au/monolayer structure at 632.8 nm showing the theoretical fits (solid lines) and experimental data for the reddishpurple form of Ia. The theoretical fit corresponds to l ) 42.0 nm, r ) -11.2, and i ) 2.2 for the gold layer and l ) 2.5 nm, r ) 2.6, and i ) 0.0 for the LB monolayer.
Figure 5. Normalized reflectance vs incident angle from a glass/Au/monolayer structure at 632.8 nm showing the theoretical fits (solid lines) and experimental data for the blue form of Ia. The theoretical fit corresponds to l ) 40.4 nm, r ) -11.2, and i ) 2.1 for the gold layer and l ) 2.2 nm, r ) 2.9, and i ) 0.0 for the LB monolayer.
for the reddish-purple form and ca. 40° for the blue form. Furthermore, in each case, the area in contact with the substrate, obtained by using the quartz crystal microbalance technique, is 0.21 ( 0.03 nm2 per N-octadecylpyrrole unit and substituent groups. The data suggest that the pyrroles, interconnected by the four-membered rings, probably align with their edges rather than their faces adjacent to the substrate and the hydrophobic groups point upward. The films are SHG-active, but the second-harmonic intensity is feeble and barely distinguishable from the background noise, with the signal being some 4 orders of magnitude weaker than the optimum signal previously reported for monolayer films of the anilinosquaraines.5 The effective second-order susceptibility at 45° incidence is ca. 0.5 pm V-1. This approximates to the value reported by Lynch et al.,11 but they also report10 the SHG to be similar to the intensity from films of the anilino squaraine, 2,4-bis[4-(N-methyl-N-hexylamino)phenyl]squaraine (II).
In fact, the optimum values are in a ratio of ca. 1:104, and it is assumed that the greatly reduced SHG reported by Lynch et al.10 relates to a residual signal from an LB film which predominantly conforms to the H aggregate. The anilinosquaraines have at least three LB phases: (a) a commonly observed SHG-inactive phase typified by a sharp blue-shifted absorption band at ca. 530 nm (H aggregate);5,20,21 (b) an SHG-active phase5,7,13 with optimum intensity arising from films which have a broad absorption at 695 nm;5 (c) a red-shifted phase with a sharp absorption band at ca. 770 nm (J aggregate).7,21,22 In their first paper, Lynch et al.11 attributed the observed nonlinearities to electric-quadrupole and magneticdipole interactions, whereby noncentrosymmetry is not required, whereas in subsequent work they have alluded to surface-induced effects.10 We have found that the SHG is suppressed when films of the oligomeric derivatives are deposited on an organic buffer layer and concur that the nonlinearity could result from a polarization of the chromophore by the underlying substrate. However, it is foolhardy to speculate on such mechanisms when the oligomer is noncentrosymmetric and inherently SHGactive. In addition, we refute the premise of Lynch et al.10 that interfacial effects may be responsible for the secondorder properties of the anilinosquaraines. The intensity is 4 orders of magnitude stronger, and the experimental5 and theoretical8,9 evidence suggests that the SHG is inherent to the aggregate. Furthermore, direct observation of SHG from a free-standing solid solution of the anilino dye (see below) has irrefutably demonstrated that the second-order properties are not induced by the substrate. Anilinosquaraine. The second-order properties of 2,4bis[4-(N-methyl-N-hexylamino)phenyl]squaraine (Figure 6) have been extensively studied,1,5 and on the basis of our initial hypothesis, Bre´das and Brouye`re8 have provided a theoretical basis for the SHG from centric molecules. They have demonstrated that strong nonlinearities can result from a dimeric T arrangement and our experimental data provide justification to dismiss interfacial effects: (a) the SHG from LB films is too strong5 and comparable with the signal from conventional donor-(π bridge)acceptor molecules; (b) the optical nonlinearity is conserved when the chromophore layer is separated from the glass substrate by an organic buffer23 but is suppressed when the films undergo a change to the H-aggregate phase;20 (c) the SHG from LB films is extinguished by the addition of an organic acceptor.24 In this work, we report the second-order properties of solid solutions of the anilino dye dispersed in poly(vinyl acetate). The films are SHG-active and, as predicted by theory, the intensity increases quadratically with con(20) Ashwell, G. J.; Wong, G. M. S.; Bahra, G. S.; Brown, C. R. Langmuir 1997, 13, 1629. (21) Chen, H.; Law, K. Y.; Whitten, D. G. J. Phys. Chem. 1996, 100, 5949. (22) Iwamoto, M.; Majima, Y.; Hirayama, F.; Furuki, M.; Pu, L. S. Chem. Phys. Lett. 1992, 195, 45. (23) Ashwell, G. J.; Williamson, P. C.; Bahra, G. S.; Brown, C. R. Aust. J. Chem. 1999, 52, 37. (24) Ashwell, G. J.; Romose, M.; Bahra, G. S.; Brown, C. R. Langmuir, submitted for publication.
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of the squarate oxygens with the underlying substrate surface. Conclusion
Figure 7. Variation of the square root of the second-harmonic intensity with the concentration of 2,4-bis[4-(N-methyl-Nhexylamino)phenyl]squaraine in poly(vinyl acetate). For comparison, the intensity from the 5% by mass solution is ca. 2000 times the signal from LB monolayers of the oligomeric derivatives (Ia).
centration (Figure 7). This indicates that squaraines associate, even in dilute solution, and electrospray ionization mass spectrometry has verified the existence of the dimeric aggregate.5 However, the SHG is extinguished when 7,7,8,8-tetracyano-p-quinodimethane is incorporated into the film. Thus, it is assumed that competing interactions between the electron acceptor and the donorC4O2-donor squaraine suppress self-association. This provides additional evidence that the second-order properties are a property of the aggregate. Furthermore, a previously unreported observation of SHG from a freestanding film of the solid solution demonstrates that the properties of the anilinosquaraines are not, as suggested by Lynch et al.,10 an artifact arising from the interaction
The second-harmonic intensity from LB films of the oligomer is ca. 10 000 times weaker than the optimum signal from the SHG-active phase of the anilinosquaraine, and different mechanisms apply. The weak SHG is inherent to the molecular asymmetry of the oligomer and, for the batch investigated by Lynch et al.,11 there is probably an impurity-induced contribution to the secondorder susceptibility. For example, the published melting range11 of 29-89 °C encompasses the sharp melting points of the different oligomeric fractions isolated in this work (Table 1) and demonstrates that their sample is extremely impure. Their data and conclusions are compromised.10,11 Furthermore, their contention11 that the polymer chain of alternating donor and acceptor moieties is centrosymmetric cannot be substantiated. The material is undeniably acentric. As a consequence, arguments by Lynch et al.10 relating to the SHG mechanism are rendered invalid. In this work, as discussed elsewhere,5-7 the optical nonlinearity of the centric anilinosquaraine is attributed to noncentrosymmetric aggregation. Additional evidence is provided by the quadratic enhancement of the secondharmonic intensity with dye concentration, its suppression by an electron acceptor, and the direct observation of SHG from a free-standing film. Acknowledgment. We are grateful to the EPSRC (U.K.), Defence Evaluation Research Agency and Ministry of Defence, for joint support of the nonlinear optics program at Cranfield and, in addition, the EPSRC and Nima Technology for an award of a Total Technology Ph.D. studentship (to A.N.D.). We also acknowledge P. Karkanas, N. D. Rees, M. Romose, and T. W. Walker for technical assistance. LA981463+
