2,4-Bis [4-(N,NOdibutylamino) phenyl] squaraine: X-ray crystal structure of a centrosymmetric dye and the second-order non-linear optical properties of its non-centrosymmetric Langmuir-Blodgett films Geoffrey J. Ashwell,*" Gurmit S. Bahra,b Christopher R. Brown,b Darren G. Hamilton,a Colin H. L. Kennard' and Daniel E. Lynch" aCentrefor Molecular Electronics, Cranjield University, Cranjield, UK MK43 OAL bDefence Research Agency, Fort Halstead, Sevenoaks, Kent, UK TN14 7BP 'Department of Chemistry, The University of Queensland, Brisbane, Australia Qld 4072 Crystals of the title compound exist as a green monoclinic phase [space group P21/c with u=9.046( l), b= 19.615(2), c =9.055( 1) A, p =116.107(5)", 2=21 and a purple triclinic phase. The chromophore is both planar and centrosymmetric and its dimensions indicate a tendency towards a quinoidal structure with extensive delocalisation.The Langmuir-Blodgett (LB) films show two types of aggregation with absorption maxima at 655 nm and 540 nm. These monolayers also exhibit strong second harmonic generation (SHG) comparable to the intensity from films of hemicyanine dyes. The anomalous non-linear optical properties are attributed to a serendipitous non-centrosymmetric packing arrangement within the films and to an intermolecular charge transfer contribution to the bulk second-order susceptibility. Current interest in organic materials for non-linear optics'-' stems from the high molecular hyperpolarizabilities of donor-(z-bridge)-acceptor molecules and the potential appli- cations of such materials in the electronics and communications industries.The non-linearities arise from the dependence of the polarization on the electric field of the incident radiation: p =aE +PE2+yE3 + . . . (1) P =EO {x"'E +x'~'E~ + . . . 1 (2)+x'~'E~ x is the bulk susceptibility of the material and, hitherto, has been attributed to the tensor sum of the individual molecular components (a, p, y) corrected for orientation and local field effects. As significant SHG can only occur in non-centrosym- metric media the studies to date have mainly concerned dipolar molecules and their alignment by an electric field' or LB dep~sition.~.' Thus, it has been widely accepted that the molecule, the smallest building block of any organic structure, should be acentric but this assumes that intermolecular inter- actions are weak.Contrary to this belief, we have recently discovered that centrosymmetric squaraine? dyes, previously studied as third-order materials,*-" can give rise to strong second-order effects when deposited as LB films.'' The molecu- lar hyperpolarizability (p) is zero but SHG can arise if there is an intermolecular charge transfer contribution to the suscep- tibility, x(~),and if the molecules aggregate in a manner that satisfies the symmetry requirements. This recent discovery" (reviewed by Bredas and MeyersI2) has focused attention on the influence of intermolecular effects on the second-order properties. As part of our continuing work, we have observed SHG from anilinium" and indolinium13 derivatives of the squaraine dyes and, in this paper, we report the synthesis and X-ray crystal structure of 2,4-bis [4-(N,N-dibutylamino)-phenyl] squaraine (Fig.1) and the non-linear optical properties of its LB films. Experimenta1 Synthesis The method employed in the squaraine synthesis is that developed by Law and Bailey14 for their syntheses of unsym- t Squaraine =cyclobutene-1,3-dione. metrical squaraine derivatives. We have found this method to be superior to the original general proced~re'~ which employs a Dean-Stark apparatus to drive the condensation process. Thus, 2,4-bis[4-(N,N-dibutylamino)phenyl]squaraine was obtained from the condensation of N,N-dibutylaniline ( 1.80 g, 2.00 cm3, 8.8 mmol) with squaric acid (0.5 g, 4.4 mmol) in a refluxing mixture of propan-2-01 (35 cm3) and tributyl ortho- formate (3.00 cm').After 4 h the intense blue solution was cooled depositing green crystals with a metallic sheen. This material was collected by filtration and the filtrate reduced in volume and cooled to afford a further crop of crystals. After drying in vucuo a total yield of 1.90 g (88% yield) was obtained, mp 188-19OoC, (Found C, 78.9; H, 9.1; N, 5.6. C32H44N202 requires: C, 78.64; H, 9.07; N, 5.73%); GH(CDCl,; J/H,) 1.00 (12 H, t, J 7), 1.40 (8 H, sextet, J 7), 1.64 (8 H, quintet, J 7), 3.42 (8 H, t, J 7), 6.70 (4 H, d, J 8), 8.46 (4 H, d, J 8); Gc(CDC13) 14.04 (3), 20.40 (2), 29.74 (2), 51.34 (2), 112.40 (l), 119.55 (0), 133.38 (l), 153.52 (0), 183.52 (0), 187.47 (0); m/z (FAB; NBA) 488.The 'H NMR (300 MHz) and I3C NMR (90 MHz) spectra were recorded on a Bruker AM300 spectrophotometer and the FAB mass spectrum on a VG Analytical 70-250-SE double- focusing mass spectrometer. X-Ray crystallographic analysis Single crystals were obtained by partial evaporation of a dilute solution of the dye in ethanol and, as previously reported16 for anilinium squaraines, two crystal polymorphs were observed. Crystals of a green monoclinic phase were separated and employed in the structural determination whereas those of the purple triclinic phase were deemed unsuitable for X-ray structural analysis. 9-6' Fig. 1 Molecular structure of 2,4-bis[4-(N,N-dibutylamino)phenyl]-squaraine J.Muter. Chem., 1996, 6( l), 23-26 X-Ray intensity data were collected on an Enraf-Nonius four-circle diffractometer at 298 K using graphite crystal mono- chromated Mo-Ka radiation (1=0.71073 A) to 28 =50". Unit cell parameters were determined from the least-squares refinement for 25 reflections with 28 <25". 2536 independent reflections with I >241) were used for the structural analysis. Data were corrected for both Lorentz and polarization effects and empirical absorption correction was applied. The structure was solved by using SHELXS-86,17 and refined to residuals R1=0.0395 and wR, =0.1234 (SHELXL-93l8) using full-matrix least-squares with anisotropic thermal parameters for all non- hydrogens.Hydrogens were located by difference methods and both positional and thermal parameters refined. The crystallographic data and atomic coordinates are listed in Tables 1 and 2 while selected bond lengths are given in Fig. 2. Bond distances and angles, anisotropic thermal param- eters, hydrogen atom coordinates, and observed and calculated Table 1 Crystallographic data for 2,4-bis(4-[N,N-dibutylamino)-phenyl] squarainea empirical formula C32H44N202 formula weight 488.69 temperature 298(2) K wavelength 0.71073 A crystal system monoclinic space group P2Jc unit cell dimensions u =9.0460( 10)oA b =19.615(2)AD c =9.0550( 10) A /?=116.107(5)" volume 1442.8( 3) A3 z 2 density (calc) 1.125 Mg mP3 absorption coefficient 0.069 mm -' F(000) 532 crystal size 0.40 x 0.32 x 0.20 mm3 8 range for data collection 2.08 to 24.96' index ranges O<h<lO, O<k<23, -10<1<9 reflections collected 2702 independent reflections 2536 [R(int)=0.0376] absorption correction Psi scan refinement method full-matrix least-squares on F2 data/restraints/parameters 2 5 3 61012 52 goodness-of-fit on F2 0.676 final R indices [I >2a(1)] R, =0.0395, wR, =0.1234 R indices (all data) R1 =0.1042, wR2=0.?350 largest diff. peak and hole 0.124 and -0.139 e Ae3 Residuals R, and wR, are defined as R, =X(IF, I -IF, l)/X IF, 1 and wR, =[Xw(FO2-F,2)2/Cw(F,z)z]"2.Weighting scheme w = [0~(F,)~+(0.2084P)~+ 1.85P]-' where P= [max(FO2,0)+2F:]/3. Table 2 Atomic coordin+es (x lo4)and equivalent isotropic displace- ment parameters (x lo3 A').U(eq) is defined as one third of the trace of the orthogonalized Uij tensor -1578(3) 9216( 1) 477(4) 5975(3) 7704( 1) 2156(4) -723(4) 9640(2) 220( 4) 934(4) 9645(2) 312(4) 2189(4) 9158(2) 728(4) 1994(4) 8496(2) 1236( 4) 3221(4) 8024( 2) 1681 (4) 4750(4) 8177( 2) 1674(4) 4941(4) 8841 (2) 1161(4) 3705(4) 9306(2) 706(4) 7584( 4) 7830(2) 2196(5) 8930( 5) 7978(2) 3897(5) 8718( 6) 8640( 2) 4620( 6) 10023( 10) 8758(5) 6379(8) 5712(5) 7009(2) 2594(4) 4721(5) 6566(2) 11 10( 5) 4458(7) 5854(2) 1582(5) 3281 (9) 5436( 3) 125(7) C(16) '(15)-x Fig. 2 Molecular conformation, atomic numbering scheme and selec- ted bond distances (A ) and angles (") of the chromophore.Squarate group: O(1)-C( 1) 1.226(4), C( 1)-C(2) 1.464( 5), C( 1')-C(2) 1.466(4); O(1)-C( 1)-C(2) 135.2(3), C(2)-C(l)-C(2') 89.7(3), C(l)-C(2)-C(l') 90.3(3). Six-membered ring: C(3)-C(4) 1.414(4), C(4)-C(5) 1.364(5), C(5)-C(6) 1.418(5), C(6)-C(7) 1.417(4), C(7)-C(8) 1.360(4), C(S)-C(3) 1.411(4); C(3)-C(4)-C(5) 121.3(3), C(4)-C(5)-C(6) 1213 3), C( 5)-C(6)-C(7) 117.2( 3), C(6)-C( 7)-C(S) 120.9(3), C(7)-C(8)-C(3) 122.2(3), C(S)-C(3)-C(4) 117.0(3). Exocyclic bonds and angles: C(2)-C(3) 1.402(4), C(6)-N(1) 1.361(4); C(l)-C(2)-C(3) 134.3(3), C( l')-C(2)-C(3) 135.4(3), C(2)-C(3)-C(4) 120.7(3), C(2)-C(3)-C(8) 122.2(3), C(5)-C(6)-N( 1) 120.8(3), C(7)-C(6)-N( 1) 122.1 (3). The symmetry transformation used to generate the equivalent atoms is -x, -y+2, -2.structure factors are provided as supplementary material. All data have been deposited at the Cambridge Crystallographic Data Centre (refer to Information for Authors, J. Muter. Chem., Issue No. 1). Film deposition The squaraine dye was spread from a 0.1 mg cmP3 chloroform solution onto the pure water subphase (MilliQ) of an LB trough (Nima Technology, Model 622), left for 10 min at cu. 25 "C and then compressed at 0.5 cm2 s-'. Films were deposited at 5 to 25 mNm-l by passing a hydrophilically treated glass slide vertically through the floating monolayer at 80 pm s-' on the upstroke. Second-harmonic generation SHG measurements were performed in transmission using a Nd:YAG laser (A= 1.064 pm) with the beam at 45" to the film.The apparatus is described in ref. 19. Results and Discussion X-Ray crystal structure The molecule is centrosymmetric and with the exception of the four butyl groups is essentially planar. The asymmetric unit consists of half the molecule with the other half being symmetry generated across an inversion centre, centralized in the four-membered ring. The bond lengths are similar to those reported for other anilinium squaraines'0,16 with a tendency towards a quinoidal structure with extensive delocalisatiop (Fig. 2). The edrocyclic C-C and C-N bonds are 1.402(4) A and 1.361(4)A, respectively, whereas the contracted C-$ bonds of the six-memberet ring are C(4)-C(5)= 1.364(4) A and C( 7)-C( 8)= 1.360(4)A. Thesc may be compared with averaged C-C lengths of 1,415(8) A for the remainder of the donor group and 1.465( 6) A, for the four-membered ring.The C-0 bond lengt!, 1.226(4)A, is significantly shorter than the 1.244A to 1.253 A reported for other anilinium squaraines,10.16 but the carbonyl length in these cases is probably influenced by neighbouring hydroxy groups in the ortho positions of the six-membered ring. The packing arrangement consists of parallel stacks inclined 24 J. Muter. Chem., 1996,6( l), 23-26 5 Fig. 3 Packing within the unit cell viewed down a to the ac plane and a herringbone-type arrangement parallel to the b axis (Fig. 3). Common to other anilinium squa- raines,'2*16 the parallel stacking shows the molecules overlap- ping with the amino group (donor) aligned over the central ring (acceptor) yith a mean perpendicular plane-to-plane spac- ing of ca.3.9( 1) A. However, the closest intermolecular contac!s are lateral interactions between O(1)..C(9) [3.382(4) A; 1+x,y, z], O(l)...H(9a) [2.479(4) A; 1+x,y, z] and O(1)*-H(7) [2.618(4) A; 1+x, y, z]. The crystal structure is centrosymmetric and thus, unlike the LB films below, the monoclinic crystals show no SHG. The same applies to the purple triclinic phase but we have been unsuccessful in growing crystals suitable for structural analysis. LB films The dye is an unconventional material for LB deposition as it has four short legs rather than a long hydrophobic chain. Nonetheless, the initial part of the FA isotherm rises steeply before undergoing a structural rearrangement, qr collapse, at 9 mN m-' with a corresponding area of ca.82 A2 molecule-' (Fig. 4). This is consistent with the chromophore residing on its long edge as the dimensions, including the first CH, of each of the four butyl groups, obtain:d from the structural analysis are approximately 19 x 7 x 4A3. A second transition is observed at 16 mN m-' with a corresponding area of ca. 50 A2 molecule-' and, once more, the isotherm rises steeply above 20 mNm-'. The chromophores may adopt a vertical align- ment at higher pressures although the reduction in area may also be attributed to partial collapse and bilayer formation. The LB film spectra are significantly shifted from the absorp- tion in chloroform (1,,,=633 nm, half width at half maxi- mum= 13 nm) and the film colour is dependent upon the deposition pressure.Deposition below 9 mN m-', the first transition pressure, gives rise to an absorption maximum at 665 to 670 nm whereas, at higher pressures, deposition results in the emergence of a second peak at ca. 540 nm (Fig. 5; Table 3). This is characteristic of the formation of different types of aggregate and this type of behaviour has been exten- sively documented for various squaraine Aggregation Fig.4 Surface pressure versus area isotherm of the dye spread from chloroform solution at ambient temperature (ca. 18 "C)and compressed at 0.5 cm2 s-' 400 500 600 700 800 wavelength/nm Fig. 5 Absorption spectra of LB monolayers deposited at 5 mN rn-' (lower spectrum) and 25 mN rn-l (upper spectrum) Table 3 Dependence of the spectra and non-linear optical properties of freshly prepared LB films on the deposition pressure n/mN m-' ,l,,/nm absorbance/layer - SHG"/au 5-8 665-670' 0.026 0.5 20-25 540 0.060 0.5 665 0.037 The SHG is relative to the intensity obtained from monolayer films of the hemicyanine dye, (15)-4-[2-(4-dimethylaminophenyl)ethenyi]-N-docosylpyridinium bromide.'Weak shoulder at ca. 540 nm. the spectral differences attributed to changes in the molecular orientation and intermolecular overlap. By comparison with the published work of Law and Chen23 the band at 540nm probably corresponds to a face-to-face arrangement with dipole-dipole interactions between the central groups of adjac- ent chromophores whereas the absorption at ca.665nm involves charge transfer between the donor and acceptor parts of neighbouring molecules. For films deposited above 16 mN m-', the second transition pressure, both bands result and the absorbance of each is stronger than those reported for similar dyes.23*24 There may be a mixture of phases and the possibility of multilayer formation in this high pressure regime. LB films of this centrosymmetric molecule have unusual non-linear optical properties in so far as they exhibit strong and SHG, similar in magnitude to that obtained from films of in LB films has been studied by Law and co-~orkers~~*~~ J. Muter. Chern., 1996, 6(l), 23-26 hemicyanine dyes.25 The second harmonic intensity from freshly prepared films, deposited at 5-8 mN m-l, is reproduc- ible but decays to 50% of the original value within ca.4 h and to only 5% within 24 h. This is accompanied by a decrease in absorbance at 667nm with the gradual emergence of the second absorption band at 540nm. Thus, it is assumed that the optical properties are influenced by the formation of an alternative aggregate with loss of the non-centrosymmetric packing arrangement. Surprisingly, the SHG is independent of the deposition pressure although the spectra of films deposited at 20-25 mN m-' are very different (Fig. 5; Table 3). In all cases the second harmonic intensity diminishes with time whereas films of the previously reported N-hexyl-N-methyl- amino analogue have remained stable over a period of several months." The phenomenon of SHG has been traditionally associated with donor-(n-bridge)-acceptor materials1V2 but, as explained previously," the structural criteria for SHG may be satisfied if adjacent chromophores adopt a non-parallel arrangement and if the repeating dimer motif does not pack centrosymmet- rically.For conventional donor-(n-bridge)-acceptor materials the intramolecular charge transfer contribution to the molecu- lar hyperpolarizability (b)is larger than the sum of all other effects.26 Since our observed SHG is as strong as that obtained for conventional second-order materials it is clear that there must be a significant intermolecular charge transfer contri- bution to the bulk susceptibility and that dimers, or higher aggregates, are responsible for the unusual non-linear optical behaviour.Conclusions In this work we have reported the crystal structure of a centrosymmetric molecule and SHG from its LB films. These results are significant as they conclusively show that inter- molecular charge transfer has a profound effect on the second-order non-linear optical properties of organic dyes. Additionally, the magnitude of the SHG arising from such intermolecular charge transfer is comparable with that observed from films of donor-(n-bridge)-acceptor molecules. In calculations which utilise SHG data (the determination of the chromophore tilt angle from the polarization dependence and pfrom f2)) it has been widely assumed that the intermol- ecular contribution is insignificant compared with that of the intramolecular component.27 However, it is apparent that polar materials may be susceptible to strong intermolecular interactions and this should be taken into account.In addition, the existing design rules should be broadened to include intermolecular complexes as well as donor-(n-bridge)-acceptor molecules as promising candidates for SHG. This is particularly relevant to LB film structures where alternate layers of amphi- philic donors and amphiphilic acceptors may be deposited to give significant SHG at the layer boundaries. We are grateful to the EPSRC for support of the non-linear optics programme at Cranfield and to the Ramsay Memorial Fellowships Trust and Defence Research Agency for co-sponsoring a research fellowship (to D.G.H.).We also thank Mrs. J. Street and Dr. G. J. Langley (Southampton University) for performing the NMR and mass spectral analyses. References Nonlinear Optical Properties of Organic Molecules and Crystals, eds. D. S. Chemla and J. Zyss, Academic Press, Orlando, 1987. S. Allen, in Molecular Electronics, ed. G. J. Ashwell, Wiley, Research Studies Press, Taunton, 1992, pp. 207-265. R. G. Denning, in Spectroscopy of New Materials, ed. R. J. H. Clark and R. E. Hester, Wiley, 1993, pp. 1-60. Principles and Applications of Nonlinear Optical Materials, ed. R. W. Munn and C. N. 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