J O U R N A L O F C H E M I S T R Y Materials A comment upon the aggregation of squaraine dyes GeoVrey J. Ashwell Centre for Molecular Electronics, Cranfield University, Cranfield, UK MK43 0AL The electrospray ionisation mass spectrum (ESI-MS) of 2,4-bis[4-(N-methyl-N-butylamino)phenyl]squaraine shows peaks which correspond to the fragmentation pattern of the dimeric aggregate (e.g. m/z=810 [2M+2H]+) and, although the molecule is centrosymmetric, solid solutions of the dye exhibit second-harmonic generation (SHG).In order to satisfy the structural requirement the nonlinear optical behaviour is attributed to the dimeric aggregate which must be non-centrosymmetric. In contrast, solid solutions of the tetrahydroxy analogue, 2,4-bis[4-(N-methyl-N-butylamino)-2,6-dihydroxyphenyl]squaraine, are SHG-inactive and the ESI-MS data are consistent with the formation of a heptameric aggregate (m/z=819 [1.75M]+O[7M]4+).The aggregation number is compound specific and an extensive study of fourteen anilino squaraines has shown direct correlation between the type of aggregate and the occurrence or absence of SHG. Squaraine dyes have properties which may be applied to electrophotographic devices,1 optical recording,2 third-order nonlinear optics3 and surprisingly, the frequency-doubling of light.4 The donor–acceptor–donor structure is centric but, in spite of this, SHG has been observed from Langmuir–Blodgett (LB) films of a variety of analogues with anilino4–7 and heterocyclic8,9 donors.The SHG is not inherent to the molecule and, furthermore, the intensity is too strong to be associated with the glass|LB interface.Instead, it is attributed to the aggregate which must be non-centrosymmetric. It is assumed that the molecules adopt a T arrangement and that the SHG arises from intermolecular charge transfer between the acceptor and donor moieties. There is evidence that the squaraine aggregates persist in solution and that the properties are dependent upon the type of association and the extent of the intermolecular donor– acceptor interaction.4,6 For example, Chen et al.10 have reported the inclusion of anilino squaraines as monomers in aqueous solutions of b-cyclodextrin and as dimers in the larger cavity of c-cyclodextrin, the absorption maximum being hypsochromically- shifted from 650 to 594 nm.McKerrrow et al.11 have observed two types of aggregate, J and H, by arrested crystallis- In this work, the aggregation of dye 1 has been reinvestigated ation from dimethyl sulfoxide–water mixtures. Using a Bernesi– and, in addition, the electrospray mass spectra and nonlinear Hildebrand analysis, Chen et al.12 have disclosed that the optical properties of several anilino squaraines are reported.supramolecular unit of the anilino squaraine is tetrameric and The results of Langley et al.13 have been reproduced by doping has a cyclic chiral structure. Furthermore, Langley et al.13 have with Na+ (or K+ to give the potassiated species) but importreported that the electrospray mass spectra of the 2,4-bis[4- antly, for solutions of the pure dye, none of the spectra shows (N,N-dialkylamino)phenyl]squaraines (dye 1) show aggregate evidence of metallation.Furthermore, the Cranfield ESI-MS peaks which may be assigned to [nM+Na]+ where 2n is an data reveal m/z values which may be assigned to dimeric (or integer and 42n9. They have suggested that the sodiation multiples of two for z>1), hexameric and heptameric aggreis a ‘true reflection of the behaviour of these species in solution’ gates.The association number is compound specific and there which, if correct, would have far reaching consequences on the is a direct correlation between the type of aggregate and the control of the aggregation for commercially relevant appli- SHG activity for dyes 1 to 5. cations. However, it is more likely that the sodiation is a consequence of contamination.Experimental Squaraine dyes were synthesised using published procedures14 and characterised by 1H NMR, elemental analysis and ESIMS. A VG Quattro quadrupole mass spectrometer (upgraded to Quattro II specifications) and MassLynx data system (VG Organic, Altrincham) were used in the positive ion electrospray mass spectrometry study at the Michael Barber Centre (UMIST).Experiments were performed with the electrospray source high-voltage lens at 0.32 kV and the source sampling cone voltage at 25 V. Electrospray data were collected in the mass range m/z 100–2500 for dilute solutions of the dye (10 mg cm-3) in (a) CH3CN, (b) a 451 mixture of CH3CN–H2O J. Mater. Chem., 1998, 8(2), 373–376 373and (c) a 451 mixture of CH3CN–H2O containing 0.1% formic acid.The spectra were averaged over 15 scans with a scan rate of 100 amu s-1 and a flow rate of 5 ml min-1. The spectrometer was calibrated using polyethylene glycol. Spun-coated films were obtained by spreading ca. 500 ml of a dilute chloroform solution of the dye and poly(vinyl acetate) onto a glass substrate and rotating the substrate at 500 rpm using a Headway spin-coater.SHG measurements were performed in transmission with the p-polarised laser beam (Nd5YAG, l=1064 nm) at an angle of 45° to the film. Results and Discussion Structure–property relationship LB films of the anilino squaraines exhibit strong SHG4–9 and the second-order properties result from the aggregate structure Fig. 2 Electrospray mass spectrum of 2,4-bis[6-(N-butyl-2,3,4-triwhich must be non-centrosymmetric and not from the centric hydroquinoloyl)]squaraine 4.Monomer: m/z=457 [M+H]+. molecule itself. Spun-coated films of some but not all of the Aggregate: m/z=913 [2M+H]+. squaraines in poly(vinyl acetate) are also SHG-active. The second-harmonic intensity increases with the concentration of dye but is weak in comparison with the signal from the LB monolayer.Nonetheless, its observation corroborates the persistence of the aggregates in solid solution and the noncentrosymmetry, necessary for second-order eVects, probably results from a partial orientation of the acentric species in the spinning process. The SHG-active films include several alkyl analogues of the anilino squaraine (1b–1g), the dihydroxy substituted derivatives (2a–2c), and the heterocyclic analogues (4 and 5).A common feature of their electrospray mass spectra is a high mass peak, or fragmentation pattern, which may be assigned to the dimeric aggregate (Fig. 1–3). An assignment of the m/z values for 2,4-bis[4-(N-methyl-Nhexylamino) phenyl]squaraine (dye 1c: R1=CH3, R2=C6H13) is given in the legend to Fig. 1. The monomeric species is observed as a dihydrogenated ion (m/z=462, [M+2H]+), this being previously reported by Law and co-workers15 for other Fig. 3 Electrospray mass spectrum of 2,4-bis(9-julolidinyl)squaraine anilino squaraines. Furthermore, a peak corresponding to the 5. Monomer: m/z=425 [M+H]+. Aggregate: m/z=850 [2M+2H]+. trihydrogenated dimeric aggregate (m/z=923 [2M+3H]+) is shown and the main fragmentation results from the progressive Table 1 Positive ion electrospray MS dataa loss of one to four (CH2)5 units.The N-methyl-N-butylamino to N-methyl-N-decylamino analogues of dye 1 also form aggregates (m/z) amino group dimeric aggregates (Table 1) but for the higher analogues, dye R1MNMR2 RMM [1.5M]+ [1.75M]+ [2M+nH]+ dodecyl to docosyl, fragmentation of the alkyl groups makes it diYcult to unambiguously assign the MS data.In contrast, 1a CH3MNMCH3 320 480 — — the spectra of the heterocyclic analogues (4 and 5) show a 1b CH3MNMC4H9 404 — — 811 single high mass peak which may be assigned to [2M+nH]+ 1c CH3MNMC6H13 460 — — 923 (Fig. 2 and 3). 1d CH3MNMC8H17 516 — — 1033 1e CH3MNMC10H21 572 — — 1146 1f C4H9MNMC4H9 488 — — 977 1g C5H11MNMC5H11 544 — — 1089 2a CH3MNMCH3 352 528 — — 2b CH3MNMC4H9 436 — — 873 2c C4H9MNMC4H9 520 — — 1041 3a CH3MNMC4H9 468 702 819 — 3b CH3MNMC6H13 524 786 917 — 4 — 456 — — 913 5 — 424 — — 850 aNone of the ESI-MS spectra showed significant peaks at m/z values greater than those listed.The tetrahydroxy substituted squaraines, 2,4-bis[4-(Nmethyl- N-alkylamino)-2,6-dihydroxyphenyl]squaraine (3), diVer in so far as their solid solutions do not exhibit SHG and the electrospray spectra show aggregate peaks which correspond to fractional values of the relative molecular mass: e.g.m/z=702 and 786 [1.5M]+ and 819 and 917 [1.75M]+ Fig. 1 Electrospray mass spectrum of 2,4-bis[4-(N-methyl-N-hexyl- respectively for the butyl and hexyl analogues (see Fig. 4). The amino)phenyl]squaraine 1c.Monomer: m/z=462 [M+2H]+. former may be assigned to the trimeric aggregate if z=2. Aggregate: m/z=923 [2M+3H]+; 867 [2M+3H-4CH2]+; 853 Alternatively, the peaks may be attributed to hexameric and [2M+3H-5CH2]+; 797 [2M+3H-9CH2]+; 783 [2M+3H- 10CH2]+; 726 [2M+2H-14CH2]+; 712 [2M+2H-15CH2]+. heptameric species respectively if z=4 and, although they may 374 J.Mater. Chem., 1998, 8(2), 373–376Table 2 Assignment of the aggregate fragmentation patterns of 2,4- bis[4-(N,N-dimethylamino)phenyl]squaraine (1a) and 2,4-bis[4-(N,Ndimethylamino)- 2-hydroxyphenyl]squaraine (2a) ESI-MS data (m/z) possible mass 1a 2a assignmenta (m/z) — 528 [6M]4+ 528 480 — [6M]4+ 480 456 455 [6M-7CH2]4+ 455.5 438 439 [6M-12CH2]4+ 438 424 — [6M-16CH2]4+ 424 415 414 [6M-6D]4+ 414 402 403 [6M-22CH2-7D]4+ 403 392 393 [6M-8D]4+ 392 370 368 [6M-10D]4+ 370 aThe two dyes show almost identical fragmentation patterns and, for simplicity, the listed MS fragments are for 1a; the fragments of dye 2a Fig. 4 Electrospray mass spectrum of 2,4-bis[4-(N-methyl-N-butyl- also exclude the oxygen atoms of the hydroxy groups. D= amino)-2,6-dihydroxyphenyl]squaraine 3a.Monomer: m/z=469 -N(CH3)2 (donor). [M+H]+. Aggregate: m/z=819 [1.75 M]+O[7M]4+; 702 [1.5M]+O[6M]4+. The peak at 818 [1.75M-H]+ is stronger and has been labelled instead of 819. abundant aggregate. However, using the method of McKerrow et al.,11 arrested crystallisation from solution has provided absorption maxima at ca. 650–700 nm for the SHG-active exist as separate entities, the lower aggregate may result from dyes, those which form dimeric aggregates, with the maxima the fragmentation of the heptamer.Furthermore, Chen et al.12 being hypsochromically shifted to 530–550 nm for the inactive have previously suggested the formation of a cyclic tetrameric species, the hexameric and heptameric aggregates. aggregate and, thus, it is feasible that the molecules of the Furthermore, correlation between the linear and nonlinear heptameric aggregate adopt a bicyclic arrangement in which optical properties and the aggregate structures of anilino two tetramers are fused.squaraines has been established for LB films.4–7 Significantly, The ESI-MS data for the N,N-dimethylamino analogues of transitions involving a hypsochromic shift of the LB absorption 1 and 2 also correspond to fractional values of the aggregate band invariably result in the loss of SHG6 and, typically, the mass (m/z=480 for 1a and 528 for 2a, [1.5M]+).The fragmen- SHG-inactive phase is associated with a narrow absorption at tation patterns are similar and may be assigned to the progressca. 530 nm and no significant higher wavelength shoulder.The ive loss of the CH2 and amino groups and, for the dihydroxy molecules probably adopt a parallel face-to-face arrangement derivative, the loss of both oxygens (Fig. 5; Table 2). Close whereas, for the SHG-active LB films and solution aggregates, scrutiny of the spectra suggests that the dyes probably form the structural requirement can only be met if there is non- the hexameric aggregate ([1.5M]+O[6M]4+) rather than the parallel alignment.It is therefore assumed that the molecules trimeric species ([1.5M]+O[3M]2+), the peaks at ca. 455.5 of the dimeric aggregate adopt a T arrangement with intermol- corresponding to the loss of 7CH2 for z=4 and a non-integral ecular charge transfer between the donor (anilino) and acceptor number for z=2. Furthermore, solid solutions of these dyes (C4O2) moieties.This was suggested in a previous report4 and are SHG-inactive and it is assumed that the aggregate structure the hypothesis has since been corroborated by the theoretical is centrosymmetric. In fact, none of the materials with m/z analysis of Bre�das and Brouye`re.16 values corresponding to fractional values of the relative molecular mass, i.e.[1.5M]+ or [1.75M]+, have thus far exhibited EVect of doping with Na+ and K+ SHG when deposited as spun-coated solid solutions. The solution spectra of these dyes typically display an Langley et al.13 reported ESI-MS data for three of the anilino intense monomer peak at ca. 635–665 nm with half widths at squaraines listed in Table 1 (labelled 1b, 1f and 1g) and half maximum of 7–13 nm and, additionally, a broad shoulder assigned the aggregate peaks to [nM+Na]+ where 2n is an which may be assigned to the aggregate.It is not possible to integer and 42n9. They proposed that the sodiation is a relate the nonlinear optical behaviour to the spectra because true reflection of the aggregates in solution whereas, in this the monomer absorption, albeit sharp, masks that of the less work, the ESI-MS data for dye 1 and several related squaraines clearly show that it is not.However, the metallated species may be obtained by doping with NaBr (or KBr to give the potassiated aggregate). The results are listed in Table 3 and, interestingly, the fine structure of the fragmentation pattern of Table 3 ESI-MS data of undoped and lightly doped samples of 2,4- bis[4-(N-methyl-N-alkylamino)phenyl]squaraine where the alkyl group is butyl (dye 1b) and hexyl (dye 1c) undoped (m/z) Na+ doped (m/z) K+ doped (m/z) assignmenta 1b 1c 1b 1c 1b 1c monomer [M+2H]+ 406 462 406 462 406 462 [M+H+A]+ — — 428 484 444 500 aggregate [2M+3H]+ 810 923 811 923 811 923 [2M+2H+A]+ — — 833 945 849 962 Fig. 5 Electrospray mass spectrum of 2,4-bis[4-(N,N-dimethylamino)- aA=alkali metal (Na+ or K+).Higher aggregates of general formula [ xM+yH+zA]+ have been observed but their occurrence is 2-hydroxyphenyl]squaraine 2a. Monomer: m/z=353 [M+H]+. Aggregate: m/z=528 [1.5M]+O[3M]2+ or [6M]4+. dependent upon the concentration of dopant. J. Mater. Chem., 1998, 8(2), 373–376 375K. Y. Law, J. Imaging Sci., 1987, 31, 83; K. Y.Law, Chem. Rev. 2,4-bis[4-(N-methyl-N-hexylamino)phenyl]squaraine is mim- 1993, 93, 449. icked by the sodiated and potassiated peaks with the m/z 2 V. P. Jipson and C. R. Jones, J. Vac. Sci. T echnol., 1981, 18, 105; values being shifted by 22 and 38 respectively. It is relevant D. J. Gravesteijn, C. Steenbergen and J. van der Ween, Proc. SPIE that Langley et al.13 used a mixture of NaI and CsI as the Int.Soc. Opt. Eng., 1983, 420, 327; A. H. Sporer, Appl. Opt., 1984, calibrant and this is a likely source of contamination of their 23, 2738. 3 C. Poga, T. M. Brown, M. G. Kuczyk and C. W. Dirk, J. Opt. Soc. ESI-MS data. The results of this study clearly indicate that Am B, 1995, 12, 531; J. H. Andrews, J. D. V. Khaydarov, K. D. the aggregates freely exist, even in dilute solution, and their Singer, D.L. Hull and K. C. Chuang, J. Opt. Soc. Am. B, 1995, 12, presence is manifested by the second-order properties. 2360; C. W. Dirk, W. C. Herndon, F. Cervantes-Lee, H. Selnau, S. Martinez, P. Kalamegham, A. Tan, G. Campos, M. Velez, J. Zyss, I. Ledoux and L. T. Cheng, J. Am. Chem. Soc., 1995, 117, 2214. Conclusion 4 G. J. Ashwell, G. JeVeries, D.G. Hamilton, D. E. Lynch, M. P. S. Squaraine dyes associate in solution and the ESI-MS analysis Roberts, G. S. Bahra and C. R. Brown, Nature, 1995, 375, 385; G. J. Ashwell, Adv.Mater. (Research News), 1996, 8, 248. has revealed m/z values which may be assigned to acentric 5 G. J. Ashwell, G. S. Bahra, C. R. Brown, D. G. Hamilton, D. E. dimeric aggregates (or multiples of two if z>1) which are Lynch and C.H. L. Kennard, J.Mater. Chem., 1996, 6, 23. SHG-active and to hexameric and/or heptameric aggregates 6 G. J. Ashwell, G. M. S. Wong, D. G. Bucknall, G. S. Bahra and which are not. The aggregation number is compound specific C. R. Brown, L angmuir, 1997, 13, 1629. and, for the anilino squarainesthe higher aggregates may be 7 G. J. Ashwell, G.JeVeries, N. D. Rees, P. C. Williamson, G. S. Bahra and C. R. Brown, L angmuir, submitted for publication. obtained by using hydroxy substituents and less sterically 8 G. J. Ashwell and P. Leeson, Electrical and Related Properties of hindered N,N-dialkylamino groups. It is assumed that hydro- Organic Solids (ed. R. W. Munn, A. Miniewicz and B. Kuchta), gen bonding and chromophore-dominated interactions play NATO ASI Series, 1997, 24, 297.an important role in determining the aggregate structure. 9 G. J. Ashwell, T. Handa, P. Leeson, K. Skjonnemand, G. JeVeries and A. Green, J.Mater. Chem., following paper. I am grateful to Gary JeVeries, Paul Leeson and Trish 10 H. Chen, W. G. Herkstroeter, J. Perlstein, K. Y. Law and D. G. Williamson (Cranfield) for technical assistance and Ian Fleet Whitten, J. Phys. Chem., 1994, 98, 5138. 11 A. J. McKerrow, E. Buncel and P. M. Kazmair, Can. J. Chem., and Lu Yu (UMIST) for providing the ESI-MS data. The 1995, 73, 1605. EPSRC (UK) and Defence Evaluation Research Agency (UK) 12 H. Chen, K. Y. Law, J. Perlstein and D. G. Whitten, J. Am. Chem. are also acknowledged for support of the nonlinear optics Soc., 1995, 117, 7257. programme on squaraine dyes. 13 G. J. Langley, E. Hecquet, I. P. Morris and D. G. Hamilton, Rapid Commun.Mass Spectrom., 1997, 11, 165. 14 K. Y. Law and F. C. Bailey, J. Org. Chem., 1992, 57, 3278. 15 K. Y. Law and F. C. Bailey, Can. J. Chem., 1993, 71, 494; K. Y. References Law, F. C. Bailey and L. J. Bluett, Can. J. Chem., 1986, 64, 1607. 16 J.-L. Bre�das and E. Brouye`re, personal communication. 1 R. J. Meiz, R. B. Champ, L. S. Chang, C. Chiou, G. S. Keller, L. C. Liclian, R. B. Neiman, M. D. Shattuck and W. J. Weiche, Photogr. Sci. Eng., 1977, 21, 73; A. C. Tam, Appl. Phys. L ett., 1980, 37, 978; Paper 7/05466A; Received 28th July, 1997 376 J. Mater. Chem., 1998, 8(2), 373–3