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2,4-Bis[4-(N-methyl-N-alkylamino)phenyl]squaraines: Structure-Property Relationship and Dependence of the Type of Aggregate upon the Hydrophobicity of the Alkyl Group Geoffrey J. Ashwell,* Michael P. S. Roberts, and Nicholas D. Rees Centre for Molecular Electronics, Cranfield University, Cranfield MK43 0AL, U.K.
Gurmit S. Bahra and Christopher R. Brown Defence Evaluation Research Agency, Fort Halstead, Sevenoaks, Kent TN14 7BP, U.K. Received May 11, 1998. In Final Form: June 30, 1998 The molecular packing in Langmuir-Blodgett (LB) films of the title compound, 2,4-bis[4-(N-methylN-alkylamino)phenyl]squaraine, is influenced by chromophore-dominated interactions when the alkyl chains are short and by van der Waals interactions when they are sufficiently hydrophobic. The butyl to dodecyl analogues pack parallel to the substrate with interlayer spacings of 8.2-8.9 Å from grazing incidence X-ray synchrotron diffraction (GIXD). In contrast, the tetradecyl to docosyl analogues adopt a “U-shaped” configuration, with the chromophore parallel to the substrate, and the d spacing increases from 19.8 to 24.1 Å. The transition is manifested by an abrupt change in the molecular area in contact with the substrate and by subtle differences in the linear and second-order nonlinear optical properties. Freshly deposited monolayers have a broad absorption maximum at 660-695 nm and exhibit second-harmonic generation, but with time, films of the higher alkyl analogues, tetradecyl to docosyl, alter to an H-aggregate phase (λmax ) 530 nm) with loss of SHG. Long-term stability has been observed for films of 2,4-bis[4(N-methyl-N-hexylamino)phenyl]squaraine: the LB monolayer has an optimum susceptibility of χ(2)zzz ≈ 710 pm V-1 at 1.064 µm, a thickness from atomic force microscopy (AFM) and surface plasmon resonance (SPR) of 5.0 ( 0.5 Å (cf. 8.5 ( 1.5 Å layer-1 in the bulk film), and real and imaginary components of the dielectric permittivity of r ) 3.0 ( 0.1 and i ) 0.7 ( 0.1 respectively at 532 nm.
Introduction The 2,4-bis[4-(N,N-dialkylamino)phenyl]squaraines have a donor-acceptor-donor chromophore which is centric: the two donors (D ) anilino group) and acceptor (A ) C4O2) are coplanar, and from X-ray crystallographic analysis,1-3 the dimensions of half of the molecule are symmetry generated across an inversion center by the coordinates of the other. Despite this, SHG has been realized from LB films of the anilino squaraines4-7 as well as from solid solutions of the dyes in poly(vinyl acetate).5,8 The phenomenon is identified with noncentrosymmetry and consequently the second-order nonlinear optical behavior is attributable to the aggregate structure and not the molecule itself. Squaraines readily associate, even in dilute solution, and electrospray ionization mass spectrometry (ESI-MS) has provided evidence with high mass/charge peaks which conform to the dimeric species * To whom correspondence should be addressed. Fax: (44)01234-750875. Telephone: (44)-01234-754224. E-mail: g.j.ashwell@ cranfield.ac.uk. (1) Ashwell, G. J.; Bahra, G. S.; Brown, C. R.; Hamilton, D. G.; Kennard, C. H. L.; Lynch, D. E. J. Mater. Chem. 1996, 6, 23. (2) Bernstein, J.; Goldstein, E. Mol. Cryst. Liq. Cryst. 1988, 164, 213. (3) Dirk, C. W.; Herndon, W. C.; Cervantes-Lee, F.; Selnau, H.; Martinez, S.; Kalamegham, P.; Tan, A.; Campos, G.; Velez, M.; Zyss, J.; Ledoux, I.; Cheng, L.-T. J. Am. Chem. Soc. 1995, 117, 2214. (4) Ashwell, G. J.; Jefferies, G.; Hamilton, D. G.; Lynch, D. E.; Roberts, M. P. S.; Bahra, G. S.; Brown, C. R. Nature 1995, 375, 385. (5) Ashwell, G. J.; Wong, G. M. S.; Bucknall, D. G.; Bahra, G. S.; Brown, C. R. Langmuir 1997, 13, 1629. (6) Ashwell, G. J.; Leeson, P.; Bahra, G. S.; Brown, C. R. J. Opt. Soc. Am. B 1998, 15, 484. (7) Ashwell, G. J.; Jefferies, G.; Rees, N. D.; Williamson, P. C.; Bahra, G. S.; Brown, C. R. Langmuir 1998, 14, 2850. (8) Ashwell, G. J. J. Mater. Chem. 1998, 8, 373.
(m/z ≡ [2M + nH]+) for the SHG-active dyes.8,9 The assumption of z ) 1, applicable to most earlier methods of generating gas-phase ions, is not necessarily appropriate to the electrospray technique. However, the uncertainty has been eliminated by the simultaneous analysis of two squaraines in solution:9 the spectra exhibit the expected aggregate peaks of the separate components and, in addition, a single heteromolecular peak conforming to [M + M′ + nH]+ whereas, for z > 1, satellite peaks would be expected for molecular ratios other than 1:1. The SHGactive solution aggregate is dimeric, and therefore, to satisfy the structural requirement, it is necessary for the D-A-D chromophores to adopt a nonparallel arrangement; the electron donor of one is probably directed toward the central electron acceptor of the other, and it is assumed that there is an acentric T motif.4 The aggregation of anilino squaraines in solution has been extensively studied. They exist as monomers in aqueous solutions of β-cyclodextrin (λmax ) 650 nm) and as dimers in the larger cavity of the γ-form (λmax ) 594 nm).10 When not restricted in this manner, the association number is dependent upon the ring substituents and dialkylamino groups. For example, Chen et al.11 have reported a tetrameric “unit aggregate” from a BernesiHildebrand analysis of the spectra of the arrested crystallites in aqueous dimethyl sulfoxide and attributed its blue-shifted absorption (λmax ) 530 nm) to a cyclic chiral (9) Ashwell, G. J.; Williamson, P. C.; Green, A.; Bahra, G. S.; Brown, C. R. Aust. J. Chem. 1998, 51, 599. (10) Chen, H.; Herkstroeter, W. G.; Perlstein, J.; Law, K. Y.; Whitten, D. G. J. Phys. Chem. 1994, 98, 5138. (11) Chen, H.; Law, K. Y.; Perlstein, J.; Whitten, D. G. J. Am. Chem. Soc. 1995, 117, 7257.
S0743-7463(98)00559-9 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/08/1998
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tron diffraction, surface plasmon resonance, atomic force microscopy, and quartz crystal gravimetric techniques. Experimental Section Figure 1. Molecular structure of 2,4-bis[4-(N-methyl-Nalkylamino)phenyl]squaraine.
structure.11,12 Furthermore, the ESI-MS data of 2,4-bis(8-hydroxy-9-julolidinyl)squaraine and some related hydroxy-substituted derivatives7 conform to the heptameric aggregate (m/z ≡ [1.75M]+ ≡ [7M]4+) and, once again, the arrested crystallites exhibit the frequently observed absorption band at 530 nm. Consistent with the findings of Chen et al.,11 the heptameric species may indicate a bicyclic arrangement where two tetrameric rings are fused. However, the electrospray data of most squaraines studied to date, including those with anilino8,9 and heterocyclic6,13 donor groups, conform to the dimeric species. A comparison of the ESI-MS data and optical properties illustrates a structure-property relationship:8 (a) dyes which form the higher aggregates show no discernible SHG when cast as solid solutions and their arrested crystallites exhibit a blue-shifted absorption relative to the solution spectrum; (b) solid solutions of dyes which form dimeric aggregates tend to be SHG-active, and the absorption spectra of the crystallites are red-shifted. The aggregate structure may be modified in the LB film, but as a simple indicator, SHG-activity is usually associated with a broad absorption maximum or an intense shoulder at 650-730 nm. Furthermore, the SHG from the LB monolayer4-7 is comparable with the intensity from films of conventional donor-(π-bridge)-acceptor materials.14 To account for these unusual properties, an intermolecular charge-transfer contribution to the bulk second-order susceptibility has been assumed.4 The Langmuir and LB films are polymorphic and, in the absence of phase coexistence, the anilino squaraines show sharp absorption maxima at ca. 520-530 nm (Haggregate phase)12 and ca. 750-770 nm (J-aggregate phase),7,12,15 this limiting behavior being dependent upon the overlap and relative orientations of chromophores. Chen et al.12 have reported the correlation between the spectra and the nature of the intermolecular interaction: the H-aggregate corresponds to a “cardpack” arrangement and the J-aggregate to a “slipped stack” arrangement with overlap of the neighboring donor and acceptor groups. The spectrum of the SHG-active phase8 falls between the above limits. In this case, it is assumed that the proposed T-motif of the dimeric aggregate in solution, verified by ESI-MS, is retained within the crystalline film and packs so as not to cancel the asymmetry in the bulk structure. In this work we report the structural properties of LB films of 2,4-bis[4-(N-methyl-N-alkylamino)phenyl]squaraine (Figure 1), where the alkyl group is butyl to docosyl, and demonstrate two distinctive packing arrangements. The film structure has been probed by using the following methods: grazing incidence X-ray synchro(12) Chen, H.; Law, K. Y.; Whitten, D. G. J. Phys. Chem. 1996, 100, 5949. (13) Ashwell, G. J.; Handa, T.; Leeson, P.; Skjonnemand, K.; Jefferies, G.; Green, A. J. Mater. Chem. 1998, 8, 377. (14) Bosshard, C.; Sutter, K.; Preˆtre, P.; Hulliger, J.; Flo¨rsheimer, M.; Kaatz, P.; Gu¨nter, P. In Organic Nonlinear Optical Materials; Garito, A. F., Kajzar, F., Eds.; Advances in Optics 1; Gordon and Breach: Basel, Switzerland, 1995. (15) (a) Iwamoto, M.; Majima, Y.; Hirayama, F.; Furuki, M.; Pu, L. S. Chem. Phys. Lett. 1992, 195, 45. (b) Kim, S.; Furuki, M.; Pu, L. S.; Nakahara, H.; Fukuda, K. J. Chem. Soc., Chem. Commun. 1987, 1201.
Synthesis. The 2,4-bis[4-(N-methyl-N-alkylamino)phenyl]squaraines, where the alkyl group is butyl to docosyl, were synthesized using the general procedures reported in ref 16. For example, a mixture of squaric acid (2 mmol), N-methyl-Nhexylaniline (4 mmol), and tributyl orthoformate (3 cm3) in propan-2-ol (30 cm3) was heated at reflux for ca. 5 h under nitrogen. Upon cooling, a microcrystalline product was collected by filtration and recrystallized from chloroform. Several batches were synthesized: typical yields ca. 50%; mp 162-164 °C. IR (KBr): νCO, 1605 cm-1. UV/vis (CHCl3): λmax, 632 nm; hwhm, 14 nm. 1H NMR (250 MHz, CDCl3, TMS, J/Hz): δH 8.38 (4 H, d, J ) 9, ortho H), 6.75 (4 H, d, J ) 9, meta H), 3.44 (4 H, q, J ) 7, NCH2), 3.18 (6 H, t, J ) 2, NCH3), 1.65 (4 H, broad t, NCH2CH2), 1.34 (12 H, broad s, CH2), 0.90 (6 H, t, J ) 7, CH3) ppm. 13C NMR (63 MHz, CDCl3): δC 183.44 (0), 154.22 (0), 133.23 (1), 119.78 (0), 112.25 (1), 52.84 (2), 38.65 (3), 31.54 (2), 27.22 (2), 26.61 (2), 22.55 (2), 13.96 (3) ppm. ESI-MS: m/z ) 461 [M + H]+. Anal. Calcd for C30H40N2O2: C, 78.22; H, 8.75; N, 6.08. Found: C, 78.15; H, 8.95, N, 6.03. Satisfactory analytical data (FTIR, UV/vis, 1H NMR) were obtained for each of the alkyl analogues: νCO (KBr), 1605-1620 cm-1; λmax (CHCl3), 630-635 nm; δH (CDCl3), 8.4-8.5 and 6.76.9 ppm, respectively, for chromophore hydrogens in ortho and meta positions relative to the four-membered ring. LB Deposition. The dyes were spread from dilute chloroform solutions (0.1 mg cm-3) onto the pure water subphase of an LB trough (Nima Technology, model 622), left for 10 min at ca. 20 °C and then compressed at 0.5 cm2 s-1 (ca. 0.1% s-1 of compartment area). Films were deposited on the upstroke by passing a glass substrate (for SHG), a silver coated substrate (for surface plasmon resonance), a silicon wafer (for X-ray diffraction) and a 10 MHz quartz crystal (for gravimetric studies) through the floating monolayer at 80 µm s-1. X-ray Diffraction. Grazing incidence X-ray synchrotron diffraction (GIXD) studies were performed at the Australian National Beamline Facility at the Photon Factory (Tsukuba, Japan) using imaging plate detection with a sagitally focused beam, λ ) 1.7389 Å, at an incident angle of 0.18°. Diffraction data from LB films of the butyl to docosyl analogues, typically 20 LB layers deposited onto silicon wafers at 15 mN m-1, were collected using the BIGDIFF facility at BL20B. Sets of {00l} reflections (qz scan at qx ) 0) were measured and the interlayer spacings determined using the experimental configuration and procedure described by Foran et al.17 Surface Plasmon Resonance. SPR studies were performed on glass|Ag structures by attenuated total reflection using the Kretschmann geometry.18 Reflectivities were obtained as a function of the angle of incidence, relative to the film, of a p-polarized frequency-doubled Nd:YAG laser beam (λ ) 532 nm). The data were corrected for reflections at the entrance and exit faces of a BK7 glass prism, in contact with the glass substrate using methyl benzoate as an index matching fluid, and analyzed by comparison to the Fresnel reflection formulas using the method of Barnes and Sambles.19 The derived thickness (l ≈ 450 Å) and the real and imaginary components of the dielectric permittivity (r ) -10.8 and i ) 0.49) of the freshly deposited silver film were then used in the subsequent analysis of the reflection data from the glass|Ag|LB structures. Gravimetric Studies. The change in frequency of a 10 MHz AT-cut quartz crystal plate, coated on each side with overlapping 0.195 cm2 gold electrodes, was determined following the sequential deposition of one to six LB layers at 15 mN m-1. An oscillator circuit was used to drive the crystal at its resonance frequency, and changes were monitored using a computercontrolled Hewlett-Packard HP53131A frequency counter. The data were utilized to determine the molecular areas in contact (16) Law, K. Y.; Bailey, F. C. J. Org. Chem. 1992, 57, 3278. (17) Foran, G. J.; Peng, J. B.; Steitz, R.; Barnes, G. T.; Gentle, I. R. Langmuir 1996, 12, 774. (18) Kretschmann, E. Z. Phys. 1971, 241, 313. (19) Barnes, W. L.; Sambles, J. R. Surf. Sci. 1987, 183, 189.
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Figure 2. Electrospray mass spectrum of 2,4-bis[4-(N-methylN-hexylamino)phenyl]squaraine depicting the fragmentation pattern of the dimeric aggregate: m/z ) 922.8 [2M + 2H]+. with the central gold electrodes of the crystal and, combined with layer thickness from GIXD, the molecular volume and film density. SHG Characterization. SHG studies on the deposited films were performed in transmission using a p-polarized Nd:YAG laser (λ ) 1.064 µm) with the beam incident at 45° to the film, there being no discernible signal when the beam was normal. The polarization was changed from p to s by rotating a half-wave plate through 45° and, for both polarization states, the secondharmonic intensity was calibrated by comparison with the first Maker fringe of a Y-cut quartz reference (d11 ) 0.5 pm V-1).
Figure 3. Electrospray mass spectrometry data for the monomer (m/z ≡ [M + nH]+) and aggregate (m/z ≡ [2M + nH]+) vs the number of carbon atoms in each of the two alkyl groups of 2,4-bis[4-(N-methyl-N-alkylamino)phenyl]squaraine.
Results and Discussion Solution Aggregates. The anilino squaraines readily associate, even in dilute solution, and electrospray mass spectrometry has been used to investigate the aggregation in aqueous acetonitrile (2 × 10-5 mol dm-3). The electrospray spectra show peaks which may be assigned to the monomeric species (m/z ≡ [M + nH]+) and, in addition, a series of peaks with m/z values greater than the relative molecular mass which may be assigned to the fragmentation pattern of the aggregate. This is clearly demonstrated for the hexyl analogue (Figure 2) where the fine structure results from the progressive loss of the methylene groups, the m/z values corresponding to [2M - (CH2)m + nH]+ where 0 e m e 20. Interestingly, in this case, the integrity of the dimeric aggregate is retained upon fragmentation of the hexyl chains. Using chemical ionization mass spectrometry, Law et al.20 reported the molecular aggregate of the methyl analogue, 2,4-bis[4-(N,N-dimethylamino)phenyl]squaraine, to be trimeric (m/z ) 661, [3M + H]+), and this has since been verified by ESI-MS.8 In contrast, the dimeric aggregate is a common feature of the butyl to dodecyl analogues (Figure 3) whereas, for all higher analogues, fragmentation makes it difficult to unambiguously assign the ESI-MS data. However, there may be a change in the type of aggregate and conspicuously there are distinct differences in the visible absorption spectra of the arrested crystallites obtained from aqueous acetonitrile: the lower analogues have a broad absorption maximum at ca. 690 nm whereas for longer alkyl groups, tetradecyl to docosyl, the principal absorption band is blue-shifted to ca. 520 nm (Figures 4 and 5). Interestingly, the crossover coincides with an abrupt change in the molecular packing within the LB films, and it is assumed that the type of solution aggregate influences the structure of the crystalline film. (20) Law, K. Y.; Bailey, F. C.; Bluett, L. J. Can. J. Chem. 1986, 64, 1607.
Figure 4. Normalized absorption spectra of arrested crystallites of 2,4-bis[4-(N-methyl-N-alkylamino)phenyl]squaraine: hexyl analogue (dashed line); docosyl analogue (solid line).
Similar spectra have been reported from arrested crystallization studies on several related anilinosquaraines,21,22 and competing interactions, for example, van der Waals forces between the hydrophobic chains and intermolecular charge transfer between the donor and acceptor groups of the chromophores, are the cause of the different types of associative behavior.22 Langmuir Films. The surface pressure vs area (πA) isotherms of the hexyl4 and octadecyl23 analogues have been reported previously, and other members of this series show similar characteristics (Figure 6). The general shape is dependent upon the alkyl chain length and, with the exception of the first two members, the butyl and hexyl analogues, the areas just prior to collapse are ca. 60 ( 5 Å2 molecule-1. This relates to the van der Waals area of the long edge of the chromophore, the dimensions being 19 × 3.4 Å, and therefore suggests that the two-legged dyes adopt a U-shaped configuration at the air-water (21) McKerrow, A. J.; Buncel, E.; Kazmair, P. M. Can. J. Chem. 1995, 73, 1605. (22) Liang, K.; Law, K. Y.; Whitten, D. G. J. Phys. Chem. 1994, 98, 13379. (23) Law, K. Y.; Chen, C. C. J. Phys. Chem. 1989, 93, 2533.
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Figure 5. Wavelength of the principal absorption band of 2,4bis[4-(N-methyl-N-alkylamino)phenyl]squaraine, obtained by arrested crystallization, vs the number of carbon atoms in each of the two alkyl groups.
Ashwell et al.
Figure 7. LB film data: X-ray lattice spacing of 2,4-bis[4(N-methyl-N-alkylamino)phenyl]squaraine vs the number of carbon atoms in each of the two alkyl groups. Table 1. 2,4-Bis[4-(N-methyl-N-alkylamino)phenyl]squaraines: Properties of LB Films Deposited at 15 mN m-1 alkyl
d-spacing (Å)
λmax (nm)
χ(2)eff (pm V-1)
butyl hexyl octyl decyl dodecyl tetradecyl hexadecyl octadecyl icosyl docosyl
8.21 8.19 8.38 8.61 8.91 19.80 20.76 21.51 23.79 24.11
680 695b 683 680 670 673 670 672 673 671
245a 150b 120 95 58 32 26 42 34 26
a SHG rapidly deteriorates after deposition. b Deposited at 19 mN m-1.
Figure 6. Surface pressure vs area isotherms of 2,4-bis[4(N-methyl-N-alkylamino)phenyl]squaraine where the alkyl groups are octyl (dashed) and octadecyl (solid). The areas at collapse are typically ca. 60 Å2 molecule-1 for the octyl and higher analogues.
interface. This observation is corroborated by the previously reported5 neutron reflection study on a deuterated octyl (C8D17) analogue, the layer thickness and area of the floating monolayer being 17.5 ( 0.5 Å and 72 ( 3 Å2 molecule-1 respectively at 18 mN m-1. However, the molecules change their orientation during deposition, and the interlayer spacings from grazing incidence X-ray synchrotron diffraction (GIXD), corroborated by the thickness derived surface plasmon resonance (SPR) and atomic force microscopy (AFM), clearly show that the butyl to dodecyl analogues pack with the chromophore and both alkyl chains parallel to the substrate. This is further supported by the areas obtained using the quartz crystal microbalance technique. LB Films. Analysis of the GIXD data for films, comprising 20 LB layers on Si wafers, show a distinctive structural change with increasing alkyl chain length (Figure 7, Table 1). Short interlayer spacings of 8.2-8.9 Å for the lower alkyl analogues, butyl to dodecyl, indicate that the chromophore and alkyl groups essentially pack parallel to the substrate. In contrast, the tetradecyl to docosyl analogues show reflections which relate to spacings of 19.8 to 24.1 Å, an increase of 0.7 Å per methylene group,
thereby indicating a transition to a U-shaped configuration with the alkyl chains pointing away from the substrate. The derived values are somewhat smaller than those obtained for LB films of the comparable fatty acids and their salts,24-27 for example, hexadecanoate (22.7-23.3 Å) to docosonoate (29.7-30.2 Å), where the layer thickness increases by ca. 1.16 Å per methylene (ref 28). However, the discrepancy is anticipated because the cross-sectional area of the squaraine chromophore, parallel to its long axis, greatly exceeds the combined cross-sections of the two hydrophobic groups, and therefore, space filling within the LB layer may be achieved by tilting. Corroboration of the interlayer spacing has been obtained for films representative of the two structural regions. For example, AFM and SPR studies have provided a monolayer thickness of ca. 22 Å for the N-methyl-N-hexadecylamino analogue (cf. 20.8 Å layer-1 from GIXD studies on the bulk film). The difference in the values obtained for the monolayer and all subsequent layers is not unexpected because the molecular tilt is dependent upon the nature of the underlying surface. However, for the lower alkyl analogues, the discrepancy is accentuated because the molecules pack parallel to the (24) Strivastava, V. K.; Verma, A. R. Solid State Commun. 1966, 4, 367. (25) Mann, B.; Kuhn, H. J. Appl. Phys. 1971, 42, 4398. (26) Matsuda, A.; Sugi, T.; Fukui, S.; Iizima, S.; Miyahara, M.; Otsubo, Y. J. Appl. Phys. 1977, 48, 771. (27) Peng, J. B.; Foran, G. J.; Barnes, G. T.; Gentle, I. R. Langmuir 1997, 13, 1602. (28) Petty, M. C. In Langmuir-Blodgett Films; Roberts, G. G., Ed.; Plenum Press: New York; 1990; p. 133.
2,4-Bis[4-(N-methyl-N-alkylamino)phenyl]squaraines
Figure 8. Normalized reflectance at 532 nm vs incident angle for glass|Ag|LB structures of 2,4-bis[4-(N-methyl-N-hexylamino)phenyl]squaraine: from left to right, four to six LB layers on a 450 Å thick silver film.
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Figure 9. LB film data: substrate area occupied by 2,4-bis[4-(N-methyl-N-alkylamino)phenyl]squaraine vs the number of carbon atoms in each of the two alkyl groups.
substrate and the interlayer spacing is greatly reduced: for example, the previously obtained monolayer thickness of 4.9 Å from SPR studies on monolayer films of the N-methyl-N-hexylamino analogue,4 has since been verified by AFM (l ) 5.0 ( 0.5 Å) but differs from the bulk value of 8.19 Å from GIXD. However, for films comprising from four to six LB layers, SPR analysis has provided a mean increase in thickness of ∆l ) 8.5 ( 1.5 Å layer-1 with r ) 2.9 ( 0.1 and i ) 0.7 ( 0.1 at 532 nm (Figure 8). The structural change which occurs with increasing alkyl chain length is reflected by a change in the molecular area in contact with the substrate. This was determined by monitoring the frequency change (∆F) of an AT-cut quartz crystal upon deposition and by using the Sauerbrey equation29 to determine the change in mass (∆m):
∆F ) -
2Fo2 A(Fµ)1/2
∆m
where Fo is the resonance frequency of the quartz crystal (10 MHz), F the density of (2.65 g cm-3), µ the shear modulus (2.95 × 1011 dyn cm-2), and A the electrode area (0.195 cm2). The molecular area in contact with the central gold electrodes, Ao ) 2AMr/∆mL, where Mr and L are the relative molecular mass and Avogadro’s number respectively, may then be obtained from
Ao ) -
4Fo2Mr ∆F(Fµ)1/2L
The area per molecule shows a linear increase, from ca. 80 to 120 Å2 molecule-1, as the alkyl group changes from butyl to dodecyl, but for all higher analogues, the area is constant at ca. 60 Å2 molecule-1 (Figure 9). The transition coincides with the abrupt change in the layer thickness from GIXD, but the molecular volume, from the product of area and thickness, simply increases by 24 Å3 per methylene group (Figure 10). The mean LB film density obtained from these data, F ) 1.02 ( 0.05 g cm-3, may be compared with the single-crystal value of 1.125 g cm-3 for 2,4-bis[4-(N,N-dialkylamino)phenyl]squaraine.1 The monolayer is less dense than the three-dimensional crystal, and this probably results from macroscopic voids in the film. (29) Sauerbrey, G. Z. Phys. 1959, 155, 206.
Figure 10. LB film data: molecular volume of 2,4-bis[4-(Nmethyl-N-alkylamino)phenyl]squaraine vs the number of carbon atoms in each of the two alkyl groups.
Nonlinear Optical Behavior. SHG from LB films of these centrosymmetric dyes was first discovered4 at Cranfield in 1995, and since then, similar behavior has been observed for hydroxy-substituted squaraines7,9 and analogues with heterocyclic donor groups.6,13,30 The second-order susceptibilities of monolayer LB films of 2,4-bis[4-(N-methyl-N-alkylamino)phenyl)squaraine, determined with the beam at 45° to the film, are listed in Table 1. The data represent the optimum values, derived using the lattice spacing from GIXD, and there are distinctive trends for the lower and higher alkyl analogues: (a) the effective susceptibility of the butyl to dodecyl analogues decreases with increasing alkyl chain length, from 250 to 60 pm V-1, and the behavior is in part representative of dilution of the optically nonlinear aggregates in films where the chromophores and alkyl groups are coplanar; (b) the susceptibilities of the tetradecyl to docosyl are almost independent of the alkyl group, χ(2)eff ) 34 ( 8 pm V-1, the chromophores having (30) Ashwell, G. J.; Leeson, P. In Electrical and Related Properties of Organic Solids; Munn, R., Miniewicz, A., Kuchta, B., Eds.; NATO ASI Series; Kluwer Academic Publishers: Dordrecht, The Netherlands 1997; Vol. 24, p 297.
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a similar environment and being separated, in the outof-plane direction, by the hydrophobic tails. The type of packing affects the film stability and, whereas the higher alkyl analogues readily revert to the H-aggregate phase with loss of SHG, films of the hexyl analogue have shown long-term stability without loss. However, the behavior is dependent upon the aggregate structure, and the better films tend to have a slightly red-shifted absorption maximum at ca. 695 nm compared with ca. 670 nm for the less stable examples. Furthermore, in contrast to films of the higher alkyl analogues, tetradecyl to docosyl, it is relevant that the SHG-active aggregates are partially separated by alkyl groups within the plane of the film, and therefore, the initial structure is less susceptible to chromophore-induced change. The susceptibilities of the monolayer LB films, listed in Table 1, have been calculated using the interlayer spacings from GIXD whereas the thickness of the first layer, adjacent to the glass substrate, often differs from the bulk value. This is not usually a problem, but in this case, the layers are exceptionally thin for the lower alkyl analogues and causes the discrepancy in the calculated susceptibility to be accentuated. Thus, the remainder of this discussion only concerns the hexyl analogue where the monolayer thickness (l ) 4.9 Å) and dielectric permittivities (r ) 2.9 and i ) 0.7 at 532 nm) have been obtained from an analysis
Ashwell et al.
of the SPR data at 532 nm. The reduced thickness, albeit obtained for the film on silver, gives an effective susceptibility of 250 pm V-1 for the p-polarized beam incident at 45° to the monolayer film. There is no discernible signal when the Nd:YAG laser beam is s-polarized and, from these data, the principal component of the susceptibility is χ(2)zzz ≈ 710 pm V-1. It is one of the highest values reported to date for an LB monolayer, and we should not lose sight of the fact that the molecule is centric. However, as demonstrated by the electrospray MS study, the squaraines readily associate in solution and the basic building block is the dimeric aggregate. Acknowledgment. The Engineering and Physical Sciences Research Council (EPSRC, U.K.) and Defence Evaluation Research Agency (DERA, U.K.) are acknowledged for support of this work and for providing research studentships to M.P.S.R and N.D.R. We are also grateful to Nicholas J. Calos and Colin H.L. Kennard (Queensland University) for analyzing the X-ray diffraction data, and the Australian Nuclear Science and Technology Organization for providing access to the Australian National Beamline Facility in Tsukuba. LA9805591
