Centrosymmetric Chromophores: Second-Harmonic Generation from

2 × 10-5 mol dm-3) with m/z values which may be assigned to dimeric aggregates for dye II. ..... 50 Å2 molecule-1 at π = 0 and these decrease to ca...
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Langmuir 1998, 14, 2850-2856

Centrosymmetric Chromophores: Second-Harmonic Generation from Langmuir-Blodgett and Spun-Coated Films of Hydroxy-Substituted Squaraines Geoffrey J. Ashwell,* Gary Jefferies, Nicholas D. Rees, and Patricia C. Williamson 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 November 17, 1997 The linear and nonlinear optical properties of 2,4-bis[4-(N-methyl-N-alkylamino)-2,6-dihydroxyphenyl]squaraine (I), 2,4-bis[4-(N,N-dialkylamino)-2-hydroxyphenyl]squaraine (II), and 2,4-bis(8-hydroxy-9julolidinyl)squaraine (III) are reported. Electrospray ionization mass spectrometry (ESI-MS) has provided evidence of association in dilute solution (ca. 2 × 10-5 mol dm-3) with m/z values which may be assigned to dimeric aggregates for dye II. Solid solutions of this dye in poly(vinyl acetate) show second-harmonic generation (SHG), albeit very weak, and therefore indicate that the dimeric aggregate is noncentrosymmetric. In contrast, dyes I and III form heptameric aggregates with their solid solutions being SHG-inactive. There is some correlation between the type of aggregate from ESI-MS and the Langmuir film spectra, but van der Waals interactions between the hydrophobic groups play a more significant role in determining the structure at the air-water interface. The absorption maxima of the Langmuir and deposited LangmuirBlodgett films vary from 530 nm to the near-infrared, but only those films with a maximum at 650-700 nm, or a significant shoulder in this range, are SHG-active. The second-order behavior is not an inherent property of the molecule itself but, instead, is a result of intermolecular effects and is dependent upon the noncentrosymmetry of the aggregate and the film.

Introduction The past decade has seen intense activity in the design of donor-(π-bridge)-acceptor materials and in their noncentrosymmetric alignment to preserve the molecular nonlinearities as a bulk property for second-harmonic generation (SHG).1 The frequency doubling of light occurs in structures which lack inversion symmetry, resulting in the generally accepted assumption that the smallest unit, the molecule, should satisfy the criterion. This notion was dispelled in 1995 when we reported strong SHG from LB films of the centric dye, 2,4-bis[4-(N-hexyl-N-methylamino)phenyl]squaraine, even though the molecular nonlinearity (β) is effectively zero.2 The squaraines are not obvious candidates, but at Cranfield, we have since observed SHG from a variety of different analogues.3-8 The second-harmonic intensity is too strong to be at* To whom correspondence should be addressed. Fax: (44)-01234750875. E-mail: [email protected]. (1) Bosshard, C.; Sutter, K.; Preˆtre, P.; Hulliger, J.; Flo¨rsheimer, M.; Kaatz, P.; Gu¨nter, P. Organic Nonlinear Optical Materials; Garito, A. F., Kajzar, F., Eds.; Advances in Optics; Gordon and Breach: Basel, Switzerland, 1995; Vol. 1. (2) (a) 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. (b) Ashwell, G. J.; Roberts, M. P. S.; Rees, N. D.; Bahra, G. S.; Brown, C. R.; Langmuir, submitted for publication. (3) 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. (4) Ashwell, G. J. Adv. Mater. (Res. News) 1996, 8, 248. (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.; Leeson, P. in Electrical and Related properties of Organic Solids; Munn, R., Miniewicz, A., Kuchta, B., Eds.; NATO ASI Series; 1997; Kluwer Academic: Dordrecht, Vol. 24, p 297. (8) Ashwell, G. J.; Handa, T.; Leeson, P.; Skjonnemand, K.; Jefferies, G.; Green, A. J. Mater. Chem. 1998, 8, 377.

tributed to interfacial effects, and furthermore, SHG has been observed with organic buffer layers between the glass substrate and the LB film.7,8 Instead, the second-order effects may be assigned to an aggregate structure with intermolecular charge transfer and/or a charge density distortion contributing to the bulk susceptibility. The smallest acentric unit capable of SHG is dimeric with, for example, close intermolecular contacts between the terminal donor and central acceptor. The structural requirement is satisfied if the molecules adopt a nonparallel configuration (e.g., “T-shaped”) and if this motif is repeated in a manner which allows the aggregate nonlinearities to be aligned. The aggregation of anilino squaraines has been extensively studied in a variety of media: (a) in spun-coated films,5 (b) in single crystals9,10 and LB films,2-6,11-13 (c) in DMSO-H2O mixtures where the type of aggregate or arrested crystallite is dependent upon the solvent ratio,13-15 and (d) in cyclodextrins where the molecule is included as a monomer in the β-form and as a dimer in the larger (9) (a) Tristani-Kendra, M.; Eckhardt, C. J.; Bernstein, J.; Goldstein, E. Chem. Phys. Lett. 1983, 98, 57. (b) Tristani-Kendra, M.; Eckhardt, C. J. J. Chem. Phys. 1984, 81, 1160. (c) Bernstein, J.; Goldstein, E. Mol. Cryst. Liq. Cryst. 1988, 164, 213. (10) Wingard, R. E. IEEE Ind. Appl. 1982, 1251. (11) (a) Kim, S.; Furuki, M.; Pu, L. S.; Nakahara, H.; Fukuda, K. J. Chem. Soc., Chem. Commun. 1987, 1201. (b) Kim, S.; Furuki, M.; Pu, L. S.; Nakahara, H.; Fukuda, K. Thin Solid Films 1988, 159, 337. (12) Law, K. Y.; Chen, C. C. J. Phys. Chem. 1989, 93, 2533. (13) Liang, K.; Law, K. Y.; Whitten, D. G. J. Phys. Chem. 1994, 98, 13379. (14) (a) Chen, H.; Law, K. Y.; Perlstein, J.; Whitten, D. G. J. Am. Chem. Soc. 1995, 117, 7257. (b) Chen, H.; Farahat, M. S.; Law, K. Y.; Whitten, D. G. J. Am. Chem. Soc. 1996, 118, 2584. (15) (a) Buncel, E.; McKerrow, A. J.; Kazmaier, P. M. J. Chem. Soc., Chem. Commun. 1992, 1242. (b) McKerrow, A.; Buncel, E.; Kazmaier, P. M. Can. J. Chem. 1995, 73, 1605.

S0743-7463(97)01254-7 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/24/1998

Centrosymmetric Chromophore

Langmuir, Vol. 14, No. 10, 1998 2851 Table 1. Selected Visible, Infrared, 1H NMR, and MS Data dye

vis (CHCl3) λmax (nm)

IR (KBr) νCO (cm-1)

Ia Ib Ic IIa IIb III

644 644 644 640 644 665

1631 1637 1624 1615 1614 1615

NMR (CDCl3) δOH (ppm)

MS m/z

10.99 10.99 10.99 11.40, 12.11 11.38, 12.09 11.42, 12.06

469 525 861 437 521 457

cavity of the γ-form.16 Additionally, the electrospray mass spectra of unsubstituted squaraines have corroborated the existence of dimeric aggregates in solution.5 In contrast, electrospray data for the tetrahydroxy-substituted analogue (I) and 2,4-bis(8-hydroxy-9-julolidinyl)squaraine (III), described herein, have provided evidence of the heptameric species. In solution, monomers of anilino squaraines typically display intense absorption bands at ca. 640 nm whereas, for deposited LB films, two types of behavior are commonly observed: an SHG-active form with an absorption maximum at 660-730 nm (red-shifted compared with solution)2,5 and an SHG-inactive phase with a single maximum at ca. 530 nm (blue-shifted).5 The principal intermolecular interaction in the red-shifted phase has previously been assigned to the overlapping donor (anilino group) and acceptor (C4O2) groups, and from this assignment, we have attributed the unorthodox second-order behavior to intermolecular charge transfer and a noncentrosymmetric packing arrangement. The criterion for SHG can only be realized if there is a nonparallel alignment of molecules; we previously suggested that this may be attained by a “T” or a distorted “T” arrangement within the dimeric aggregate2,3 and the concept has since been confirmed by the theoretical study of Bre´das and Brouye`re.17

NMR studies on the dihydroxy analogues have previously shown19 two isomeric forms, an acentric cis form with OH groups on the same side of the molecule and a centric trans form with the OH groups on opposites sides. In this study, the ortho H and OH groups of dye II show different isomeric chemical shifts: ortho H, 7.87-7.89 ppm (trans) and 8.02-8.03 ppm (cis); OH group, 11.38-11.40 ppm (cis) and 12.09-12.11 ppm (trans). The trans form is predominant, and from the ratio of the integrated resonances, the composition is ca. 20 ( 5% cis and 80 ( 5% trans. The composition is supported by an X-ray structural analysis of the related N,N-diethylamino analogue9 which gives an OH site occupancy of 9:91 for the two ortho positions of the asymmetric unit. If the data are attributed to a random distribution of the isomeric forms, rather than to alternative orientations of the trans isomer, the ratio of cis:trans in the crystal is approximately 16:84. ESI-MS. Positive ion electrospray mass spectrometry data were obtained at the Michael Barber Centre for Mass Spectrometry using a VG Quattro quadrupole mass spectrometer (upgraded to Quattro II specifications) and MassLynx data system (VG Organic, Altringham). Experiments were performed with the electrospray source high-voltage lens at 0.32 kV and the source sampling cone voltage at 25 V. The data were collected in the mass range 200-2500 Da for solutions of the dye (10 µg cm-3; ca. 2 × 10-5 mol dm-3) in pure CH3CN or 4:1 CH3CN:H2O, with and without 0.1% formic acid. Spectra were averaged over 15 scans with a scan rate of 100 amu s-1 and a flow rate of 5 µL min-1. The spectrometer was calibrated using poly(ethylene glycol). Langmuir and LB Films. Spectroscopic studies of the Langmuir film were achieved by spreading the dyes from dilute chloroform solution (0.01-0.1 mg cm-3) onto the pure water subphase of the trough (Nima Technology, model 611) with a sapphire window in the base. Spectra of the floating monolayers were obtained in transmission as a function of surface pressure by using a fiber-optic diode array spectrometer. Deposited films were obtained using a model 622 LB trough (Nima Technology); solutions were spread as above, 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 obtained by passing a hydrophilically treated glass slide through the floating monolayer to deposit on the upstroke at 80 µm s-1. SHG. Studies were performed in transmission. The angle of the laser beam (Nd:YAG, λ ) 1.064 µm), relative to the normal to the film, was altered from 0° to 70° and the polarization rotated using a half-wave plate. The second-harmonic intensity was calibrated against the first Maker fringe of a Y-cut quartz reference plate (d11 ) 0.5 pm V-1) and, at 45° incidence, compared with the mean SHG from an LB monolayer of the extensively studied hemicyanine dye, (E)-4-[2-(4-dimethylaminophenyl)ethenyl]-N-docosylpyridinium bromide.

Experimental Section

Results and Discussion

Materials. The hydroxy-substituted dyes (Figure 1) were obtained from the condensation of squaric acid and a substituted aniline using the general procedures previously described.18 Satisfactory analytical results were obtained in each case, and selected 1H NMR and MS data are listed in Table 1. Proton

The properties of the unsubstituted dyes, 2,4-bis[4-(N,Ndialkylamino)phenyl]squaraine, have been reported previously and formed the focus of our first reports of SHG from centrosymmetric molecules.2-5 The N-methyl-Nhexylamino derivative packs with its chromophore and short alkyl chains parallel to the substrate: the thickness

Figure 1. Molecular structures: 2,4-bis[4-(N-methyl-N-alkylamino)-2,6-dihydroxyphenyl]squaraine (R ) butyl, Ia; hexyl, Ib; octadecyl, Ic); 2,4-bis[4-(N,N-dialkylamino)-2-hydroxyphenyl]squaraine (R ) methyl and R1 ) butyl, IIa; R ) R1 ) butyl, IIb); 2,4-bis(8-hydroxy-9-julolidinyl)squaraine (III).

(16) Chen, H.; Herkstroeter, W. G.; Perlstein, J.; Law, K. Y.; Whitten, D. G. J. Phys. Chem. 1994, 98, 5138. (17) Bre´das, J.-L.; Brouye`re, E. Private communication. (18) Law, K. Y.; Bailey, F. C. J. Org. Chem. 1992, 57, 3278.

(19) Kazmaier, P. M.; Hamer, G. K.; Burt, R. A. Can. J. Chem. 1990, 68, 530.

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Ashwell et al. Table 2. Electrospray Ionization MS Data dye

RMM

monomer (m/z) [M + nH]+

Ia Ib IIa IIb III

468 524 436 520 456

469 525 437 521 457

aggregates (m/z) [1.50M]+ [1.75M]+ [2M + H]+ 702 786

819 917 873 1041

676a

783a

a For dye III: m/z ) 676 [1.5M - 1/ O]+; m/z ) 783 [1.75M - O 2 + H]+.

Figure 2. Visible spectrum of dye Ia in acetonitrile.

and real and imaginary components of the dielectric permittivity, derived from the surface plasmon resonance (SPR), are l ) 4.9 Å, r ) 3.0 and i ) 0.68 at 532 nm for the LB monolayer2 and ∆l ) 8.5 ( 1.5 Å layer-1, r ) 2.9 ( 0.2 and i ) 0.8 ( 0.2 for the bulk LB film (four to six layers). There is often a discrepancy between the thickness of the layer adjacent to the substrate and all subsequent layers, and in this case, we note that the values derived from SPR have since been corroborated by atomic force microscopy and X-ray synchrotron diffraction. The linear and nonlinear optical properties are dependent upon the deposition pressure as π is increased from 5 to 19 mN m-1: the absorption maximum progressively shifts from 660 to 695 nm and the SHG displays a 3-fold increase. The optimum susceptibility of the monolayer at 1.064 µm -1 for l ) 4.9 Å (ref 2b). Furthermore, is χ(2) zzz ) 710 pm V the spectra and nonlinear optical behavior of films with λmax ≈ 695 nm have remained unaltered for more than a year when protected by an LB overlay. In contrast, other alkyl analogues in this series have shown a gradual blue shift of the absorption band to λmax ≈ 530 nm with total loss of SHG.5 As an extension of this work we now report some hydroxy-substituted analogues which have been found to be SHG-active when deposited as LB films. Their properties are dependent upon the hydrophobic group and also the number of hydroxy substituents. For the tetrahydroxy analogue (dye I), we have again observed relatively stable SHG from LB films of a centrosymmetric dye. However, for the dihydroxy analogues, II and III, it should be noted that the trans-hydroxy form is centric but the minority cis-hydroxy form is not. This probably has a minor effect on the second-order behavior, but the observed SHG is too strong to be the exclusive mechanism. Thus, as for dye I, the relatively strong SHG is attributed to a noncentrosymmetric LB structure. Solution Aggregates. The solution spectra of I-III are similar and, for example, in acetonitrile have a sharp absorption band at 630-665 nm with a well defined shoulder at ca. 570 nm (Figure 2). This asymmetry is a familiar property of squaraine dyes and may be attributed to aggregation, even at low concentrations, and confirmation has been obtained from positive ion electrospray mass spectrometry.20,21 A recent paper by Langley et al.20 reported that sodiated aggregation is prevalent: their m/z values corresponded to [nM + Na]+ where 4 e 2n e 9 and they suggested that this is a “true reflection of the (20) Langley, G. J.; Hecquet, E.; Morris, I. P.; Hamilton, D. G. Rapid Commun. Mass Spectrom. 1997, 11, 165. (21) Ashwell, G. J. J. Mater. Chem. 1998, 8, 373.

behaviour of these species in solution”. However, they used a mixture of NaI/CsI as the calibrant, and this is a likely source of the contamination. It is therefore of interest that none of our ESI-MS data6,21 relate to the metalated species even though high aggregation numbers are inferred. The electrospray mass spectra of dye I exhibit aggregate mass peaks corresponding to fractional values of the relative molecular mass: m/z ) 819 [1.75M]+ for the N-methyl-N-butylamino analogue and m/z ) 917 [1.75M]+ for the N-methyl-N-hexylamino analogue (Table 2; Figure 3). The squaraine chromophore can undergo a twoelectron oxidation to the quinoidal dication, and the MS data may be assigned to heptameric aggregates if z ) 4. Furthermore, peaks with masses of 702 and 786 [1.5M]+ have been obtained for the butyl and hexyl analogues, respectively, and accordingly, these may be assigned to trimeric aggregates if z ) 2 and to hexameric aggregates if z ) 4. The spectrum of the 2,4-bis(8-hydroxy-9julolidinyl)squaraine (III) also shows aggregate masses which correspond to [1.5M]+ and [1.75M]+ but the intensity is weak. In this case, the m/z values of the more dominant fragments indicate the loss of the hydroxy groups (Table 2), and furthermore, it is acknowledged that the trimeric or hexameric unit, m/z ≡ [1.5M]+, could arise from the fragmentation of the heptameric aggregate. Interestingly, Chen et al.14 have reported a tetrameric aggregate of a related squaraine from a Bernesi-Hildebrand analysis of the solution spectra of arrested crystallites. They suggested that the supramolecular squaraine unit has a cyclic chiral structure with four “T” interactions and perhaps a weak face-to-face interaction. Therefore, if this is applicable to the tetrahydroxy analogues (dye I), the molecules of the heptameric aggregate may well adopt a bicyclic arrangement in which two tetrameric rings are fused. The ESI-MS data obtained for dye II indicate the formation of dimeric aggregates with m/z ) 873 [2M + H]+ for dye IIa and m/z ) 1041 [2M + H]+ for dye IIb (Table 2). In contrast, mixed solutions of the dyes show three aggregate peaks, two which correspond to the expected homomolecular aggregates and an intermediate peak which conforms to the heteromolecular aggregate. This suggests that the aggregate is dimeric (z ) 1) because intermediate satellite peaks would be expected for all higher aggregates (z > 1). Interestingly, solutions of II in poly(vinyl acetate) show weak SHG and, therefore, confirm that the aggregates persist in solid solution and that the dimeric species is noncentrosymmetric. In contrast, I and III form heptameric aggregates and their solid solutions are SHG-inactive, inferring that this species is centric. Langmuir Films of I. The pressure-area (π-A) isotherm of the octadecyl analogue of I is generally featureless and shows a limiting area of ca. 70 Å2 molecule-1, obtained by extrapolating the high-pressure region of the isotherm to π ) 0, which decreases to ca. 50

Centrosymmetric Chromophore

Langmuir, Vol. 14, No. 10, 1998 2853

Figure 3. Electrospray mass spectrum of dye Ia in the range 600-900 Da: m/z ) 469 [M + H]+ (not shown); 702 [1.5M]+; 819 [1.75M]+.

Figure 4. Surface pressure versus area isotherms of Ia (R ) butyl, solid line) and Ic (R ) octadecyl, broken line) at ca. 20 °C. For clarity the hexyl analogue has been omitted because its isotherm partially overlaps that of Ia.

Figure 5. A typical visible spectrum of Ia (R ) butyl) at the air-water interface for surface pressures of 5 e π e 20 mN m-1. The spectrum of the hexyl analogue is similar but the absorbance at 550 nm progressively increases from ca. 0.02 at 5 mN m-1 to 0.06 at 20 mN m-1.

Å2 molecule-1 at collapse (Figure 4). The nature of the hydroxy substitution is likely to cause significant hydrogen bonding to the water surface, and from the isotherm, it is apparent that the molecule adopts a “U-shaped” configuration with the chromophore initially residing on its long edge. In contrast, the butyl (Ia) and hexyl (Ib) analogues have limiting areas of ca. 50 Å2 molecule-1 at π ) 0 and these decrease to ca. 40 Å2 molecule-1 at π ) 20 mN m-1 (Figure 4). The van der Waals dimensions of the chromophore, including the first methylene attached to each of the nitrogen atoms, are ca. 19 (length) × 8 (width) × 3.4 (thickness) Å and as such the areas are smaller than expected at higher pressures. These data suggest that the chromophores are tilted upward or, if aggregated at the air-water interface, possibly overlap. If the latter applies, the Langmuir film may be better described as a mosaic of aggregates rather than a true monolayer, and indeed, ESI-MS has confirmed that dye I forms hexameric and/or heptameric aggregates in dilute solution (Table 2). The spectra of the Langmuir films at the air-water interface are also dependent upon the length of the alkyl substituents. The short-chain analogues, Ia and Ib, have absorption bands with maxima at ca. 550 nm and a well-

developed shoulder at ca. 660 nm (Figure 5). In contrast, the octadecyl analogue shows a very broad transition with λmax ) 600 nm and a very weak absorbance of 0.008 (Figure 6); in this case, it is assumed that van der Waals interactions between octadecyl groups dictate the structure, and as a result, the solution aggregates possibly disintegrate under compression at the air-water interface. Conversely, the film structure of the short-chain analogues is dominated by chromophore-induced interactions with the blue-shifted absorption maximum at ca. 550 nm conforming to the shoulder of the solution band in Figure 2. Therefore, it is assumed that the solution aggregates persist in the floating Langmuir layer and are retained, albeit modified, in the deposited LB film. Langmuir Films of II. The π-A isotherms of the N-methyl-N-butylamino (IIa) and N,N-dibutylamino (IIb) analogues are surprisingly very different (Figure 7). The former is similar to that of the corresponding tetrahydroxy analogue; it rises steeply without transition and has a limiting area of 50 Å2 molecule-1, obtained by extrapolation to π ) 0. Its Langmuir and LB spectra are similar to those of the tetrahydroxy analogues, Ia and Ib, but the blue-shifted H-aggregate band is less pronounced

2854 Langmuir, Vol. 14, No. 10, 1998

Ashwell et al.

Figure 6. A typical visible spectrum of Ic (R ) octadecyl) at the air-water interface for surface pressures of 5 e π e 25 mN m-1.

Figure 7. Surface pressure versus area isotherm of IIa (R ) methyl, R1 ) butyl; broken line) and IIb (R ) R1 ) butyl; solid line) at ca. 20 °C.

and there are separate maxima at 530 and 650 nm. In contrast, the π-A isotherm of the N,N-dibutylamino analogue (IIb) rises steeply before undergoing a structural change as represented by the broad plateau (Figure 7). The limiting area of 72 Å2 molecule-1 at π ) 0 is consistent with the chromophore residing on its long edge. However, at higher pressures the isotherm mimics that of IIa but with a greatly reduced area per molecule, and therefore, the connecting plateau probably indicates reorientation and partial collapse as the pressure is increased. The spectrum of the floating monolayer at 0 e π e 6 mN m-1 shows a sharp J-aggregate band at 770 nm with a halfwidth at half-maximum of 8 nm. However, at pressures corresponding to the steep section of the isotherm, above the plateau region, the spectrum is broadened with the principal band being blue-shifted to 530 nm and with a weaker maximum at 660 nm and another, weaker still, at 770 nm (Figure 8a). Furthermore, the LB film spectrum is different with a broad absorption maximum at 660 nm and shoulder at ca. 590 nm, the maximum being independent of the deposition pressure (Figure 8b). On the basis of the general assignment of Law and Chen12 the spectra of dye IIb, excluding that of the Langmuir film for π > 7 mN m-1, indicate the coexistence of different aggregate phases. The band at 530 nm has been assigned to a parallel face-to-face arrangement of chromophores, and for the unsubstituted squaraine analogues, we have previously found that such films are SHG-inactive if there is no shoulder at 650-700 nm.5 An absorption in this region is usually indicative of SHG-activity and, therefore, to satisfy the symmetry requirement, probably reflects a nonparallel molecular arrangement and inter-

Figure 8. Typical visible spectra of IIb (R ) R1 ) butyl): (a) at the air-water interface at 0 e π e 5 mN m-1 (λmax ) 770 nm) and at π ) 10 mN m-1 (λmax ) 530 nm), the spectra being similar throughout the high-pressure regime but with the peak absorbance at 530 nm increasing from ca. 0.05 at 7 mN m-1 to ca. 0.08 at 15 mN m-1; (b) an LB film deposited at 2 e π e 25 mN m-1.

Figure 9. Surface pressure versus area isotherm of III at ca. 20 °C.

molecular charge transfer between the terminal donor and central acceptor. Finally, the sharp J-aggregate band at 770 nm is also associated with charge transfer, but in this case, it is assumed that the molecules adopt a centrosymmetric parallel arrangement with identical donor-acceptor interactions at opposite ends of the molecules. Langmuir Films of III. The π-A isotherm of dye III is relatively steep and shows a limiting area of ca. 50 Å2 molecule-1 at π ) 0 (Figure 9). Similar to dye IIb, the Langmuir film spectra show a broad red-shifted J-

Centrosymmetric Chromophore

Langmuir, Vol. 14, No. 10, 1998 2855 Table 3. Deposition Pressure, Principal Absorption Band, and Second-Order Nonlinear Optical Properties of LB Monolayers of Dyes I to III (SHG Data Are Relative to the Mean Intensity from Monolayer Films of the Hemicyanine Dye, (E)-4-[2-(4-Dimethylaminophenyl)ethenyl]-N-docosylpyridinium Bromide)a dye

π (mN m-1)

λmax (nm)

absorbance (layer-1)

I2ω

Ia Ib Ic IIa IIb III

5-20 5-20 5-20 5-25 5-25 5-10

540 530 620 530, 650 650 750

0.02-0.03 0.02-0.06b 0.003-0.005 0.01-0.02 0.02-0.04 0.02-0.03

0.3-0.5 0.02-0.23b undetectable 1.0-1.5 0.3-2.5c 1.0-1.5

a The data correspond to I (p f p) whereas for I (s f p) the 2ω 2ω signals were undetectable. b The absorbance and SHG for Ib are strongly dependent upon the deposition pressure: A ≈ 0.02 layer-1, I2ω ) 0.02-0.06 for π < 10 mN m-1; A ≈ 0.06 layer-1, I2ω ) 0.130.23 for π > 10 mN m-1. c Dye IIb: I2ω ≈ 0.3 for 2 e π e 5 mN m-1; I2ω ) 1.0-2.5 for π g 10 mN m-1.

Figure 10. Visible spectra of III: (a) at the air-water interface showing a progressive increase in the absorbance as π increases from 0 to 10 mN m-1; (b) LB film deposited at 10 mN m-1.

aggregate band, but in this case, the wavelength is pressure-dependent and increases from 730 nm at π ) 0 to 745 nm at π ) 10 mN m-1 (Figure 10). The half-widths at half-maximum of 23-30 nm (cf. 8 nm for IIb) may indicate some disorder within the structure. Furthermore, the spectrum of the deposited LB film is very broad with maxima at 755 and 830 nm and, in addition, shoulders which relate to the blue-shifted phase of I (λmax ≈ 530 nm) and the SHG-active phase of IIb (λmax ) 660 nm). This implies further disordering on transferral to the substrate, resulting in an LB film comprising a mosaic of different aggregates. Nonlinear Optical Properties of LB Films. The main interest concerns the nonlinear optical properties of the tetrahydroxy-substituted analogues, dye I, because the molecular structure is centric. The behavior is strongly influenced by the chain length with the butyl and hexyl analogues being SHG-active and the octadecyl analogue not (Table 3). This is attributed to the distinctive types of aggregation as indicated by significantly different spectra (Figures 5 and 6). The SHG from films of the butyl analogue has a slight dependence on the deposition pressure for 5 e π e 20 mN m-1 whereas the hexyl analogue shows a large variation with an optimum response when deposited at ca. 20 mN m-1. The SHG is strong and comparable with the intensity from monolayer films of conventional donor-(π-bridge)-acceptor materials.1 Furthermore, the intensity is relatively stable, and a moderately large signal persists for several weeks (Figures 11 and 12). The molecular structures of the dihydroxy analogues, II and III, are centric for the more abundant trans-hydroxy

Figure 11. Dependence of the second-harmonic intensity from monolayer films of Ia (R ) butyl) on the deposition pressure and variation of the signals with time: (O) 5 mN m-1; (0) 10 mN m-1; (4) 15 mN m-1.

Figure 12. Time dependence of the second-harmonic intensity from a monolayer film of Ib (R ) hexyl) deposited at 15 mN m-1.

isomer, and in line with the SHG obtained for dye I, it is unlikely that the minority cis-hydroxy isomer has any discernible effect on their nonlinear optical properties. They are SHG-active and the behavior probably arises from the predominantly noncentrosymmetric aggregation, although there may be a slight contribution from the asymmetric substitution. The SHG is much stronger than that obtained for the tetrahydroxy analogues, and the

2856 Langmuir, Vol. 14, No. 10, 1998

principal absorption maximum is red-shifted to 650 nm for II and 750 nm for III (Table 3). The layer thickness is unknown, and therefore, the second-order bulk susceptibility cannot be calculated. However, if the molecules align with their long axes parallel to the substrate, then the limiting thickness of IIa is ca. 8 Å layer-1. Such an arrangement has been confirmed by the X-ray lattice spacing of the corresponding unsubstituted analogue,2 and using the second-harmonic intensity data in Table 3, the susceptibility of IIa is -1 estimated to be within the range χ(2) zzz ) 440-540 pm V . The value may be compared with the optimum susceptibility for the unsubstituted analogue, 2,4-bis[4-(Nmethyl-N-hexylamino)phenyl]squaraine: χ(2) zzz ) 710 pm V-1 for l ) 4.9 Å, the monolayer thickness being obtained from surface plasmon resonance.2 It is assumed that the values relate to the dimeric aggregate which is the smallest “molecular unit” capable of SHG. Conclusion As part of a long-term study of SHG-active squaraines, we have investigated the second-order nonlinear optical behavior of donor-(C4O2)-donor dyes with anilino2-6 and heterocyclic6-9 donor groups and, in this work, report the

Ashwell et al.

properties of several hydroxy-substituted analogues. The electrospray mass spectra of these dyes have provided evidence of aggregation in dilute solution with high mass peaks which correspond to dimeric aggregates for dye II and heptameric aggregates for dyes I and III. Solid solutions of II are SHG-active, and therefore, the behavior is attributable to the acentric dimer rather than the molecule itself. To satisfy the structural requirement, the two molecules probably adopt a T or distorted T arrangement with an angle of 0 , Θ e 90° between the molecular axes. The second-order properties of the LB films then probably result from a noncentrosymmetric condensed phase packing of such aggregates, albeit modified for the heptameric aggregates of I and III. Acknowledgment. We are grateful to the EPSRC (U.K.) and Defence Evaluation Research Agency (Fort Halstead) for support of the nonlinear optics program at Cranfield and for the award of studentships to N.D.R. and P.C.W. We are also grateful to Dr. I. Fleet and Mrs. L. Yu for providing the mass spectrometry data and Mr. A. Reeve for the NMR analysis. LA971254+