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Spectroscopic Characterization of Azo Dye Aggregation on Dendrimer Surfaces Kerry K. Karukstis,* Loren A. Perelman, and Wun K. Wong Department of Chemistry, Harvey Mudd College, Claremont, California 91711 Received June 17, 2002. In Final Form: September 11, 2002 The spectral properties of azo dye aggregates have been used to determine the influence of the dendritic surface of Starburst poly(amidoamine) dendrimers on the alignment of electrostatically bound azo dyes. The surface structure and morphology of dendrimers provide enhanced opportunities for the ordered assembly of azo dyes. In particular, the spacing of dendrimer surface groups is influenced by dendrimer generation and dictates the proximity of attached azo dyes and the ability of intermolecular forces between dyes to operate. Absorption spectra reveal the formation of highly ordered, spectrally distinct dye aggregates. Multivariate spectral analysis techniques enable a more complete characterization of the dye aggregation on the dendrimer surface and demonstrate the influential control of the dendrimer surface on the orientation of anchored azo dyes. Spectra are attributed to both H-aggregation (face-to-face arrangements of the aromatic rings attached to the azo functionality yielding blue-shifted spectra) and J-aggregation (sideby-side orientations of the aromatic rings inducing red-shifted spectra). In particular, J-aggregation was observed with azo dyes with (1) unsubstituted phenyl linking rings and hydrophilic, hydrogen-bonding substituents in the 4-position on the terminal ring or (2) hydroxy substituents on a linking phenyl or naphthyl ring and sulfonate and azo groups oriented at 1,4- or 1,3-positions on the linking ring. Azo dyes with unsubstituted naphthyl linking rings or hydrophobic substituents on a terminal phenyl ring most effectively promoted H-aggregation.
Introduction Supramolecular chemistry focuses on intermolecular noncovalent interactions between two or more molecules to create novel, organized molecular associations and sophisticated structures. The ideal supramolecular species spontaneously self-organizes when the component molecules are mixed. To enhance self-assembly to create a desired structure, the building blocks and thus the nature of the intermolecular interactions can be carefully controlled. The selected molecular components contain information in the form of molecular recognition features that dictate assembly and thereby control the architecture and functionality of the assembled supramolecular species.1,2 The well-defined molecular composition and highly branched constitution of dendrimers make them versatile building blocks for a wide variety of supramolecular applications. These polymeric macromolecules have a welldefined treelike branching structure that radiates from a central core. While the scaffolding of the dendrimer creates internal microenvironments of varying characteristics that can act as sites for guest molecules, the functional groups on the dendrimer exterior also serve as points of attachment for guest species.3 Indeed, dendrimers are ideal templates for reversible supramolecular assembly via electrostatic interactions. The dendritic surface plays a central role in the construction of a nanostructure involving Coulombic attractions. For example, the Starburst poly(amidoamine) dendrimers typically have an * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: (909) 607-3225. Fax: (909) 6077577. (1) Fuhrhop, J.-H.; Koning, J. Membranes and Molecular Assemblies: The Synkinetic Approach; The Royal Society of Chemistry: Cambridge, United Kingdom, 1994. (2) Lehn, J.-M. Supramolecular Chemistry: Concepts and Perspectives; VCH: Weinheim, FRG, 1995. (3) Naylor, A. M.; Goddard, W. A.; Kiefer, G. E.; Tomalia, D. A. J. Am. Chem. Soc. 1989, 111, 2339.
ammonia or ethylenediamine core and repeating tertiary amine/amide branching units. For full-generation dendrimers these branches terminate in a primary amine surface4 with a generation-dependent pKa in the range 7-95 that is easily protonated in Millipore-filtered water at pH 4-5.6 For the internal tertiary amines, a generationdependent pKa range of 3-65 provides possible additional protonated sites for electrostatic interactions with ionic substrates. The smaller the generation, the higher the pKa of the tertiary amine group and the more likely the internal amine is protonated.7 In addition to internal 3° amine groups for ligand interaction, half-generation dendrimers also possess anionic surface carboxylate groups at pH levels g 4.0.8 Thus, with ionic functional groups present on the dendrimer surface, the dendritic macromolecule may participate in electrostatic interactions with ionic substrates present in solution. The electrostatic binding of organic dye molecules to supramolecular species and to macromolecules has been extensively studied by spectral techniques. Of particular interest are those situations where the formation of highly ordered, spectrally distinct dye aggregates is attributed to interaction with the host surface. This templating action to control the structure of a dye aggregate has been observed for such varied hosts as AOT micelles,9 SiO2 and SnO2 nanocrystallites,10 Langmuir-Blodgett films,11,12 peptides,13 and double-helical DNA.14 J- and H-aggre(4) Tomalia, D. A.; Naylor, A. M.; Goddard, W. A. Angew. Chem., Int. Ed. Engl. 1990, 29, 138. (5) Tomalia, D. A.; Baker, H.; Dewald, J.; Hall, M.; Kallos, G.; Martin, S.; Roeck, J.; Ryder, J.; Smith, P. Polym. J. 1985, 17, 117. (6) Kabanov, V. A.; Zezin, A. B.; Rogacheva, V. B.; Gulyaeva, Z. G.; Zansochova, M. F.; Joosten, J. G. H.; Brackman, J. Macromolecules 1999, 32, 1904. (7) Kleinman, M. H.; Flory, J. H.; Tomalia, D. A.; Turro, N. J. J. Phys. Chem. 2000, 104, 11472. (8) Ottaviani, M. F.; Cossu, E.; Turro, N. J.; Tomalia, D. A. J. Am. Chem. Soc. 1995, 117, 4387. (9) Das, S.; Kamat, P. V. J. Phys. Chem. B 1999, 103, 209. (10) Nasr, C.; Liu, D.; Hotchandani, S.; Kamat, P. V. J. Phys. Chem. 1996, 100, 11054.
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gates15 are the terms coined for one-dimensional arrangements of strongly coupled monomers that exhibit absorption spectra distinct from those of their monomeric forms. In the case of J-aggregates, the monomeric molecules are arranged in one dimension to achieve a parallel orientation of their transition moments with a zero angle between the transition moments and the line joining the molecular centers.15 For aromatic dyes, such an arrangement would yield a side-by-side positioning of aromatic rings. Strong coupling of several similar monomers results in a redshifted (often narrower16) absorption spectrum relative to that of the monomer. Intermolecular interactions involving substituents on the aromatic rings act to stabilize J-aggregates. In contrast, H-aggregates assemble strongly coupled monomeric dye molecules in one dimension to achieve a parallel orientation of their transition moments and a perpendicular alignment of the transition moments to the line of molecular centers.15 A face-to-face arrangement of aromatic rings would result for aromatic dyes. Such an arrangement is stabilized by the π-π interactions between aromatic rings. The dipolar coupling of monomers in H-aggregates leads to a blue shift in the absorption band. Azo dyes, the class of synthetic organic dyes containing the azo functionality, -NdN-, with attached aryl groups, are one example of a dye system whose organization can be characterized using the position of the π f π* absorption band of its chromophore.17 As the optical properties of dye aggregates vary with the relative orientation of the monomers and the size and structure of the molecular aggregate, precise control over these parameters is highly desirable. In solution the architecture of the aggregate is often difficult to regulate or characterize. Use of a solid support to stabilize dye aggregation to create defined nanoscopic structures is one approach to achieve better control.18 In our investigations in aqueous solution, we use the surface of polymeric dendrimers as a platform on which to anchor ionic dyes. Our studies provide compelling evidence of the role of dendrimers as molecular scaffolds in supramolecular complexes. We have investigated the interaction of a number of anionic azo dyes with the protonated amino groups on the surface of and within Starburst poly(amidoamine) full-generation dendrimers with an ethylenediamine core. In our investigations both dendrimer generation (G0-G4) and the extent of surface coverage (through the molar ratio of dye to dendrimer) have been varied. We use multivariate spectral analysis to characterize the dye aggregation on the dendrimer surface and demonstrate the influential control of the dendrimer surface on the orientation of anchored azo dyes. Experimental Section A. Materials. Aqueous stock solutions of the following azo dyes (obtained commercially and recrystallized as necessary) were prepared: R-naphthol orange (R-NO; 4-(4-hydroxy-1naphthylazo)benzenesulfonic acid, sodium salt; TCI); acid red 88 (AR88; 4-[(2-hydroxy-1-naphthyl)azo]-1-naphthalenesulfonic (11) Farahat, C. W.; Penner, T. L.; Ulman, A.; Whitten, D. G. J. Phys. Chem. 1996, 100, 12616. (12) Khazraji, A. C.; Hotchandani, S.; Das, S.; Kamat, P. V. J. Phys. Chem. B 1999, 103, 4693. (13) Cooper, T. M.; Stone, M. O. Langmuir 1998, 14, 6662. (14) Wang, M.; Silva, G. L.; Armitrage, B. A. J. Am. Chem. Soc. 2000, 122, 9977. (15) Bohn, P. W. Annu. Rev. Phys. Chem. 1993, 44, 37. (16) Mait, N. C.; Mazumdar, S.; Periasamy, N. J. Phys. Chem. B 1998, 102, 1528. (17) Jin, J.; Li, L. S.; Zhang, Y. J.; Tian, Y. Q.; Jiang, S.; Zhao, Y.; Bai, Y.; Li, T. J. Langmuir 1998, 14, 5231. (18) Place, I.; Perlstein, J.; Penner, T. L.; Whitten, D. G. Langmuir 2000, 16, 9042.
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Figure 1. Chemical structure of the azo dyes examined in this study. acid, sodium salt; Acros); chrome violet (CV; 4-hydroxy-3-(2hydroxy-1-naphthylazo)benzenesulfonic acid, sodium salt; TCI); ethyl orange (EO; 4-[4-(diethylamino)phenylazo]benzenesulfonic acid, sodium salt; Aldrich); methyl orange (MO, 4-[4-(dimethylamino)phenylazo]benzenesulfonic acid, sodium salt; Allied Chemical and Dye Corporation); orange II (O-II; 4-[(2-hydroxy-1-naphthyl)azo]benzenesulfonic acid, sodium salt; Aldrich); PHAP (phydroxyazophenyl-p′-sulfonate or 4-hydroxyazobenzene-4′-sulfonic acid, sodium salt; TCI); tropaeolin O (TO; 4-[2,4-dihydroxyphenylazo]benzenesulfonic acid, sodium salt; Aldrich); crocein orange G (COG; 6-hydroxy-5-phenylazo-2-naphthalenesulfonic acid, sodium salt; TCI); eriochrome blue black B (EBB; 3-hydroxy4-(1-hydroxy-2-naphthylazo)-1-naphthalenesulfonic acid, sodium salt; TCI); palatine chrome black 6BN (PCB; 3-hydroxy-4-(2hydroxy-1-naphthylazo)-1-naphthalenesulfonic acid, sodium salt; Aldrich). Figure 1 presents the chemical structures of these dyes. Azo dyes may be characterized by the size and substitution pattern of the aromatic rings joined to the azo (-NdN-) functionality. One of these aromatic rings is designated as the linking ring (i.e., the ring with the sulfonate substitutent), and the second aromatic ring is denoted the terminal ring. These dyes may vary in the positioning of the naphthalene ring with respect to the sulfonate group, in the number and position of substituents on the aromatic rings, and in the relative position of the sulfonate and azo groups on the aromatic ring (e.g., para vs meta). Table 3 summarizes the classification of linking and terminal rings, substituents, and position of the azo functionality for the azo dyes in this investigation. The following Starburst (PAMAM) dendrimer solutions were obtained from Aldrich and used without further purification: generation 1 (G1, 8 surface sites, 0.138 M), generation 2 (G2, 16 surface sites, 0.0608 M), generation 3 (G3, 32 surface sites, 0.0286 M), and generation 4 (G4, 64 surface sites, 0.0139 M). All aqueous solutions were prepared with 18.2 MΩ ultrapure water at pH 5
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Figure 3. Changes in the absorption spectrum of PHAP at 30 µM upon addition of generation 0-4 dendrimer solutions. Final concentrations of dye and dendrimer branches equal 30 and 40 µM, respectively.
Figure 2. Changes in TO spectral shape using Gaussian deconvolution. The absorption spectra of TO contain two major Gaussian components at 397 ( 2 and 440 ( 1 nm. (a) At low TO concentrations (here, 10 µM), the 397- and 440-nm Gaussians are equal in area. (b) At high [TO] (500 µM), a blue shift is observed in the absorption spectrum as the 397-nm Gaussian increases in area. (c) As dendrimer is added to low [TO] (10 µM TO + G1), a red shift in the absorption spectrum occurs as the 440-nm Gaussian gains in area. obtained from a Milli-Q Plus Millipore water filtration system. Under these conditions the primary amine surface of the Gn.0 poly(amidoamine) dendrimers is protonated6 and ensures that, for smaller-generation dendrimers ( 440-nm
red shift max. λ observed ) 430 ( 1 nm effect more pronounced with smaller generations at high surface coverage 440-nm Gaussian > 397-nm
R-naphthol orange
λmax ) 475 ( 1 nm Gaussians: 404 ( 3, 456 ( 2, 493 ( 2, and 536 ( 8 nm 456- and 493-nm Gaussians equal in area
no effect
red shift max. λ observed ) 511 ( 1 nm 490-nm Gaussian > 456-nm effect more pronounced with higher generation
orange II
λmax ) 484 ( 1 nm
no effect
no effect on λmax small secondary peak at 420-nm maximized with midgenerations
chrome violet
λmax ) 501 ( 1 nm Gaussians: 475, 500, 540, and 590 nm 500-nm major component
red shift max. λ observed ) 522 ( 1 nm
red shift max. λ observed ) 558 ( 3 nm effect more pronounced with G4 at low surface coverage 540- and 590-nm Gaussians equal in area
acid red 88
λmax ) 504 ( 1 nm Gaussians: 381 ( 1, 425 ( 1, 466 ( 3, 508 ( 2, and 553 ( 4 nm 508-nm major component
no effect
blue shift max. shift in λ observed ) 444 ( 1 nm effect more pronounced with G3 & G4 at low surface coverage areas of 425-nm and 508-nm Gaussians increase 425-nm major component
palatine chrome black
λmax ) 559 ( 2 nm Gaussians: 431 ( 2, 482 ( 3, 531 ( 3, and 572 ( 1 nm 531- and 572-nm Gaussians comparable in area
no effect
red shift max. λ observed ) 583 ( 3 nm effect more pronounced with higher generation
eriochrome blue black B
λmax ) 533 ( 2 nm Gaussians: 418 ( 2, 475 ( 7, 524 ( 3, 572 ( 2, and 622 ( 1 nm major component: 524-nm
no effect
red shift max. λ observed ) 639 ( 2 nm effect more pronounced with higher generation and low surface coverage
crocein orange G
λmax ) 484 ( 1 nm
no effect
no effect
sulfonate and azo groups oriented at 1,4- or 1,3-positions (chrome violet, palatine chrome black, eriochrome blue black B), a red spectral shift is observed. In contrast, for azo dyes with hydroxy substituents on a linking phenyl or naphthyl ring and the sulfonate and azo groups on different rings (crocein orange G), no spectral changes are observed. From these results, we conclude the Jaggregates are forming via hydrogen bonding in the former set of azo dyes. The bulkiness of the ring arrangements in COG appears to limit both the encapsulation of the dye within the dendrimer core at the internal tertiary amines and the side-by-side interaction on the dendrimer surface. The results of the additional studies with chrome violet are also consistent with the interaction of this dye with surface amino groups only. Figure 8 indicates that the optimal formation of dye/dendrimer complexes with maximal J-aggregation occurs when the dye concentration is less than the concentration of dendritic surface amino
groups (e.g., compare the relative absorbance intensities at 590 and 500 nm in Figure 8a and d). Formation of dye aggregates on the dendrimer surface may also be further enhanced by increasing both the dye and dendrimer concentrations while maintaining a fixed ratio, as supported by all four graphs (a-d) in Figure 8. Conclusions These investigations reveal that the aggregation of azo dyes can be induced through the electrostatic interaction of the dyes with protonated amino groups on the surface of Starburst dendrimers. The orientation of the anchored azo dyes is primarily controlled by the dye structure. Our studies with these azo dyes reveal a number of patterns that relate azo dye structure to the formation of H- and J-aggregates on dendrimer surfaces or, to a limited extent, within dendritic cores. In particular, the size of the linking and/or terminal rings of azo dyes and the substitution
Azo Dye Aggregation on Dendrimer Surfaces
patterns on those rings determine the observed aggregate. Weak blue shifts in spectra are observed for azo dyes with no substituents on the linking phenyl ring and hydrophobic substituents on the terminal phenyl ring (e.g., methyl and ethyl orange). These dyes may be interacting with internal tertiary amine groups or aligning with dendritic branches to preclude dye-dye interaction. In contrast, strong blue shifts in spectra are observed for the azo dye studied with an unsubstituted naphthyl ring as the linking ring (acid red 88). This dye, likely anchored on the dendrimer surface due to the steric bulk of the linking ring, appears to exhibit H-aggregation with a face-to-face arrangement of the naphthyl rings. Red shifts in spectra are observed for azo dyes with no substituents on the linking phenyl ring and a hydroxy group in the 4-position on a terminal phenyl or naphthyl ring (TO, PHAP, and R-naphthol orange). Red shifts in spectra are also observed for azo dyes with hydroxy substituents on a linking phenyl or naphthyl ring and the sulfonate and azo groups oriented at 1,4- or 1,3positions (chrome violet, eriochrome blue black B, and crocein orange). These dyes are likely positioned on the dendritic surface and oriented radially outward to permit
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dye-dye interaction via hydrogen bonding. Thus, the investigations firmly demonstrate that the surface structure and morphology of polymeric dendrimers provide enhanced opportunities for the ordered assembly of azo dyes. We are currently pursuing investigations of these anionic azo dyes with half-generation dendrimers as well as studies with cationic azo dyes to further support our interpretation of dye locations on dendrimer surfaces and within dendritic cores. Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for the partial support of this research. This research was also supported in part by a grant from the National Science Foundation Research Experiences for Undergraduates Program (CHE-0097262). One of us (L.A.P.) gratefully acknowledges the financial support of a Beckman Scholars Award to Harvey Mudd College from the Arnold and Mabel Beckman Foundation. LA020558F
