Azobenzene dye-coupled quadruply hydrogenbonding modules as colorimetric indicators for supramolecular interactions

Article abstract of DOI:10.3762/bjoc.8.55

The facile coupling of azobenzene dyes to the quadruply hydrogen-bonding modules 2,7-diamido-1,8-naphthyridine (DAN) and 7-deazaguanine urea (DeUG) is described. The coupling of azobenzene dye 2 to mono-amido DAN units 4, 7, and 9 was effected by classic

Full text of DOI:10.3762/bjoc.8.55

Azobenzene dye-coupled quadruply hydrogen-
bonding modules as colorimetric indicators for
supramolecular interactions
Yagang Zhang and Steven C. Zimmerman*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2012, 8, 486–495.
Department of Chemistry, 600 South Mathews Avenue, University of
Illinois, Urbana, IL 61801, USA
doi:10.3762/bjoc.8.55
Received: 04 January 2012
Accepted: 08 March 2012
Published: 02 April 2012
Email:
Steven C. Zimmerman* - sczimmer@illinois.edu
* Corresponding author
This article is part of the Thematic Series "Supramolecular chemistry II".
Guest Editor: C. A. Schalley
Keywords:
azobenzene dye; colorimetric indicators; 7-deazaguanine urea
(DeUG); 2,7-diamido-1,8-naphthyridine (DAN); polymer; quadruply
hydrogen bonding; supramolecular
© 2012 Zhang and Zimmerman; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
The facile coupling of azobenzene dyes to the quadruply hydrogen-bonding modules 2,7-diamido-1,8-naphthyridine (DAN) and
7-deazaguanine urea (DeUG) is described. The coupling of azobenzene dye 2 to mono-amido DAN units 4, 7, and 9 was effected
by classic 4-(dimethylamino)pyridine (DMAP)-catalyzed peptide synthesis with N-(3-dimethylaminopropyl)-N’-ethyl carbodi-
imide hydrochloride (EDC) as activating agent, affording the respective amide products 5, 8, and 10 in 60–71% yield. The amide
linkage was formed through either the aliphatic or aromatic ester group of 2, allowing both the flexibility and absorption maximum
to be tuned. Azobenzene dye 1 was coupled to the DeUG unit 11 by Steglich esterification to afford the product amide 12 in 35%
yield. Alternatively, azobenzene dye 16 underwent a room-temperature copper-catalyzed azide–alkyne Huisgen cycloaddition with
DeUG alkyne 17 to give triazole 18 in 71% yield. Azobenzene coupled DAN modules 5, 8, and 10 are bright orange–red in color,
and azobenzene coupled DeUG modules 12 and 18 are orange–yellow in color. Azobenzene coupled DAN and DeUG modules
were successfully used as colorimetric indicators for specific DAN–DeUG and DAN–UPy (2-ureido-4(1H)-pyrimidone) quadruply
hydrogen-bonding interactions.
Introduction
Lehn’s pioneering studies [1] have advanced supramolecular trostatic interactions, π–π stacking and metal coordination have
chemistry to the point where complex hierarchical self-assem- been used alone and in concert to assemble a broad range of
bled [2] and even dynamically assembled structures are building blocks, from small molecules [4-7] to polymers [8,9]
routinely described [3]. Noncovalent hydrogen bonding, elec- including dendrimers [10,11]. Among these noncovalent inter-
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Beilstein J. Org. Chem. 2012, 8, 486–495.
actions, hydrogen bonding is especially useful, not only because block copolymer with a high propensity for alteration [38], and
of its predictability both in terms of strength and geometry, but (3) a supramolecular ABC triblock polymer [39]. Further, a
also because of its intrinsically dynamic and reversible nature. structure–property relationship has been developed for
Of the hydrogen-bonding units developed for assembly, those DAN·UG-based supramolecular-network polymer blends [40],
that feature multiple hydrogen bonding sites are particularly and a redox-active eDAN unit was described wherein a
desirable because they usually pair with high affinity and high >104-fold drop in binding affinity occurred upon reversible oxi-
fidelity [12]. High-affinity hydrogen-bonding units have found dation [41].
particular applications in supramolecular polymers [13,14], in
which the presence of fewer hydrogen bonds means that the Herein, we extend the chemistry of the heterocyclic hydrogen
desired assemblies are not achieved, or results in polymers of bonding units (DAN and DeUG) by their coupling to azoben-
lower molecular weight. The utility of these units is further zene dyes allowing them to serve as colorimetric indicators for
demonstrated in the broad range of supramolecular architec- supramolecular interactions. Beyond reporting the straightfor-
tures that have emerged and the materials to which these units ward coupling of DAN and DeUG to azobenzene dyes, we
have been attached, including dendrimers [15-17], polymer show that the dye-recognition-unit conjugates act as selective
chain-ends [18-20] and polymer side-chains [21,22], modified polymer colorants (Figure 2). The development of a recogni-
polymeric materials, functionalized nano-structures [23] and tion unit opens the possibility of their use as a colorimetric
surfaces [24].
handle for monitoring and investigating quadruply hydrogen-
bonding interactions at molecular levels and in materials appli-
Of the numerous supramolecular coupling agents developed cations.
those that pair using quadruply hydrogen bonding have
arguably received the most attention in the context of supra- Results and Discussion
molecular polymer chemistry. In particular, the highly stable The azobenzene units were chosen because they are widely
UPy dimers developed by Meijer and Sijbesma [9,14,25] and used as dyes and exhibit a range of vivid colors. Furthermore,
the high-stability and high-fidelity DAN·UG heterodimers the application of azobenzenes in chemistry is quite broad and
developed in our laboratory [26,27] are appealing because includes their use as switches [42], in nonlinear optics [43],
beyond the stable complexes that they form, they are syntheti- sensing devices [44], and in nanostructured films for optical
cally quite accessible (Figure 1) [28]. Indeed, several syntheses storage [45]. Although this work does not focus on switching,
of the DAN unit are now available [29-34]. The original UG the ability to synthesize these recognition units containing
unit contains a labile nucleoside unit, but a DeUG unit [35] and azobenzene units opens up the possibility to turn hydrogen
more recently a DeUG module bearing a range of synthetic bonding on and off. With regard to synthesis, aromatic azo
handles for further elaboration [36] were both reported with compounds are commonly prepared by an electrophilic substitu-
more scalable syntheses. With regard to applications, the tion reaction, the best partners being an electron-rich aromatic
DAN·UG (DeUG) heterodimer has been used to drive the for- ring and an aryl diazonium cation.
mation of: (1) polymer blends [37], (2) a supramolecular multi-
The synthesis of azobenzene-dye-coupled DAN 5 began with
4-aminobenzoic acid tert-butyl ester (Scheme 1). Diazotization
and coupling with phenol afforded 1 in 60% yield, which was
comparable to the reported synthesis [46]. Compound 1 was
alkylated with ethyl 4-bromobutyrate to give 2 in 72% yield
[47]. Acid-catalyzed deprotection of 2 afforded carboxylic acid
3 in 85% yield [48]. The coupling of 3 with the mono-
heptanamide of 2,7-diamino-1,8-naphthyridine (DAN 4)
was effected by using a standard peptide-coupling method.
Thus, 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide
hydrochloride (EDC) was used as an activating agent and 4-(di-
methylamino)pyridine (DMAP) was used as a catalyst [49].
Azobenzene–DAN conjugate 5 was obtained in 71% yield as an
orange–red solid following chromatography.
Figure 1: Chemical structures of UPy dimer and DAN complexes with
UG and DeUG.
Deprotection of azobenzene 2 under mildly basic conditions
was also investigated, which afforded compound 6 in 88% yield
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Beilstein J. Org. Chem. 2012, 8, 486–495.
Figure 2: Illustration of the use of DeUG-Dye and DAN-Dye as colorimetric indicators for supramolecular interactions through specific quadruply
hydrogen-bonding interactions. Top: DeUG-Dye interacts with DAN modified polystyrene (PS). Bottom: DAN-Dye interacts with UPy modified poly-
butyl acrylate (PBA).
Scheme 1: Synthesis of azobenzene-dye-coupled DAN 5.
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Beilstein J. Org. Chem. 2012, 8, 486–495.
Scheme 2: Synthesis of azobenzene-dye-coupled DAN 8 and 10.
(Scheme 2). The coupling of monoamide derivatives of 2,7- optimize the coupling conditions; instead, attention was turned
diamido-1,8-naphthyridine (DAN), i.e., 7 and 9 with 6 under to the possibility of coupling the partners by using the copper-
similar coupling conditions, yielded 8 and 10 in 60% and 62% catalyzed azide–alkyne Huisgen cycloaddition (click reaction).
yield, respectively. All three azobenzene-coupled DAN
modules were vivid orange–red in color and had excellent solu- The click approach began with the readily available and inex-
bility in a range of nonpolar organic solvents.
pensive starting material, 4-aminobenzoic acid (Scheme 4),
which was diazotized and treated with phenol to afford 13 in
The initial attempt to couple DeUG unit 11 to azobenzene dye 1 65% yield, comparable to published procedures [52-54]. To
utilized the established Steglich esterification procedure [50] install the azide functionality, 13 was treated with 1,5-dibro-
with DMAP as the catalyst [51]. Thus, DeUG carboxylic acid mopentane and potassium carbonate to afford bromide 14 in
11 was coupled with 1 by using EDC and DMAP in methylene 62% yield. Because of its poor solubility, 14 was esterified to
chloride, and the orange–yellow product 12 was isolated in a afford ethyl ester 15 in 76% yield. Treatment of 15 with sodium
relatively low 35% yield (Scheme 3). No attempt was made to azide in the presence of tetrabutyl ammonium bromide as a
Scheme 3: Synthesis of azobenzene dye-coupled DeUG 12.
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Beilstein J. Org. Chem. 2012, 8, 486–495.
Scheme 4: Synthesis of azobenzene dye-coupled DeUG 18.
phase-transfer reagent produced crude 16, which was used
directly in the next step without purification. Coupling of azide
16 with the known 17 [34] was successfully effected under stan-
dard conditions for the copper-catalyzed azide–alkyne cyclo-
addition [55]. Azobenzene dye-coupled DeUG module 18 was
obtained as an orange–yellow solid in 71% yield. The product
was assigned as the 1,4-substituted triazole by analogy to that
seen in other such cycloaddition reactions [56].
Azobenzene-coupled DAN modules 5, 8, and 10 are bright
orange–red in color and azobenzene-coupled DeUG modules 12
and 18 are orange–yellow in color (Figure 3). All of the azoben-
zene coupled DAN and DeUG modules have excellent solu-
bility in nonpolar organic solvents, wherein their supra-
molecular recognition is most effective.
Figure 3: Solution (20 mmol) of azobenzene-dye-coupled DAN and
DeUG in CH2Cl2; a = compound 5, b = compound 8, c = compound 10,
d = compound 12, e = compound 18.
To study the ability of the DAN and DeUG-coupled azoben-
zene dyes to engage in quadruply hydrogen-bonding interac- polystyrene (PS) was used as a control polymer and purified by
tions and demonstrate the possibility of using these compounds dissolving in CH2Cl2 and precipitating out with MeOH.
as colorimetric indicators, two types of polymers bearing DAN
and UPy were used (Figure 4). The DAN modified polystyrene PS and PS-DAN were characterized by SEC with THF as
(PS-DAN) was a gift from Dr. Cyrus Anderson and its syn- eluent, with PS molecular weight standards. The loading of the
thesis will be published elsewhere. Commercially available DAN unit was determined by 1H NMR (Table 1). Poly(butyl
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Beilstein J. Org. Chem. 2012, 8, 486–495.
Table 1: Molecular weight, molecular weight distribution and mol %
loading of DAN modified PS and UPy modified PBA.
entry polymer
Mw (kDa) Mn (kDa) PDI
A0
A1
A2
B0
B1
B2
B3
PS
138
148
131
84
69
114
73
38
38
38
55
2.0
1.3
1.8
2.2
2.2
2.1
2.0
PS-DAN (2.0 mol %)
PS-DAN (5.0 mol %)
PBA
PBA-UPy (2.6 mol %) 84
PBA-UPy (4.1 mol %) 80
PBA-UPy (7.1 mol %) 110
DAN–DeUG heterocomplex was measured as Kassoc ≈ 108 M−1
[36], whereas for the DAN–UPy heterocomplex Kassoc =
106 M−1 [58-60].
Whether these complex stabilities allow for the selective
coloration of the functional polymers was examined by mixing
CH2Cl2 solutions of polymer and dye-coupled recognition
units, evaporation of the solvent, and repeated washing of the
Figure 4: Structure of DAN-modified PS and Upy-modified PBA.
acrylate) (PBA) and 2-ureido-4(1H)-pyrimidone (UPy) modi- residue with CH2Cl2–hexanes in an attempt to remove the
fied poly(butyl acrylate) (PBA-UPy) were prepared and charac- color. These conditions were selected because they were effec-
terized by THF SEC against PS standards according to a known tive at removing dye from the unfunctionalized polymer. Thus,
procedure [57]. The loading of the UPy units, determined by as seen in Figure 6 (left panel, vial A), PS did not retain DeUG-
1H NMR, is listed in Table 1.
Dye compound 12 after washing. However, PS-DAN (2 mol %)
retained both compound 12 and 18 (vial B and C, respectively),
PS and DAN-modified PS were obtained as white powders showing intense coloration. The designed supramolecular
whereas PBA and UPy-modified PBA ranged from viscous recognition (DAN–DeUG heterocomplex) was shown to be the
liquids to gels depending on the loading of UPy groups on the key factor by a control experiment featuring PS-DAN and
polymer backbone (Figure 5). The supramolecular coupling of azobenzene dyes 2 and 15, both of which lack the DeUG unit.
DAN and DeUG units and DAN and UPy units has been well As seen in Figure 6 (left panel, vial D and E), the dye was
studied. Thus, the association constant (Kassoc) for the washed away from the polymer. Not surprisingly the 5 mol %
Figure 5: Physical appearance of DAN-modified PS and UPy-modified PBA. Left: A0 = PS, A1 = PS-DAN 2.0 mol %, A2 = PS-DAN 5.0 mol %.
Right: B0 = PBA, B1 = PBA-UPy 2.6 mol %, B2 = PBA-UPy 4.1 mol %, B3 = PBA-UPy 7.1 mol %.
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Beilstein J. Org. Chem. 2012, 8, 486–495.
Figure 6: Color change after the interaction of azo-benzene dye-coupled DeUG modules with different DAN modified PS, followed by washing.
Left: (A) PS with compound 12, (B) PS-DAN (2.0 mol %) with compound 12, (C) PS-DAN (2.0 mol %) with compound 18, (D) PS-DAN (2.0 mol %)
with compound 2, (E) PS-DAN (2.0 mol %) with compound 15. Right: (A) PS with compound 18, (B) PS-DAN (5.0 mol %) with compound 12, (C)
PS-DAN (5.0 mol %) with compound 18, (D) PS-DAN (5.0 mol %) with compound 2, (E) PS-DAN (5.0 mol %) with compound 15.
PS-DAN performed at least as well in the same series of experi- modified PBA (Figure 7). Thus, after mixing of DAN-Dyes 5, 8
ments (Figure 6, right panel). The overall approach is meant to and 10 with UPy modified PBA (both 4.1 mol % and
illustrate the strength and specificity of the recognition process, 7.1 mol %) the polymers became highly colored. However, after
not to provide a quantitative method. Thus, although the being washed repeatedly with CH2Cl2–hexanes, unfunctional-
washing procedure with a mixed nonpolar solvent ized PBA lost its color (Figure 7, left and right panels, vial A)
(CH2Cl2–hexanes) proved to be effective in removing non- whereas PBA-UPy retained an intense orange color (Figure 7
specifically associated compounds, over 50% of the polymer left and right panels, vials B and C). Control studies with PBA-
was also lost during the multiple washings.
UPy (both 4.1 mol % and 7.1 mol %) mixed with azobenzene
dye 2 and 15 lacking a DAN-recognition unit, showed a
The generality of this recognition process was demonstrated by complete loss of color following extensive washing (Figure 7
observing the same trend with DAN-Dyes 5, 8 and 10 and UPy- left and right panels, vials D and E).
Figure 7: Color change after the interaction of azobenzene dye-coupled DAN modules with different UPy-modified PBA, followed by washing.
Left: (A) PBA with compound 5, (B) PBA-UPy (4.1 mol %) with compound 5, (C) PBA-UPy (4.1 mol %) with compound 8, (D) PBA-UPy (4.1 mol %)
with compound 2 (E) PBA-UPy (4.1 mol %) with compound 15. Right: (A) PBA with compound 5, (B) PBA-UPy (7.1 mol %) with compound 8, (C)
PBA-UPy (7.1 mol %) with compound 10, (D) PBA-UPy (7.1 mol %) with compound 2, (E) PBA-UPy (7.1 mol %) with compound 15.
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Beilstein J. Org. Chem. 2012, 8, 486–495.
Conclusion
was carried out in a 20 mL scintillation vial under stirring for
In conclusion, we have demonstrated straightforward and scal- 40 min. After settling of the polymer, the washing solvent was
able syntheses of azobenzene dye-coupled quadruply hydrogen- decanted and removed with a pipette. The final polymer was
bonding recognition modules. Specifically, the 2,7-diamido-1,8- transferred into a small vial and dried at room temperature.
naphthyridine (DAN) unit was linked to azobenzene dyes
through one of its amide groups, giving compounds 5, 8, and
Supporting Information
10, and the 7-deazaguanine urea (DeUG) unit was linked to an
azobenzene dye by a Steglich esterification, giving 12, or by the
Supporting Information File 1
copper-catalyzed azide–alkyne cycloaddition (click reaction) to
give 18. The synthesis provides access to highly colored recog-
nition units that may serve as useful probes of recognition
events. In this work we have successfully demonstrated that
these units colored two polymers, PBA and PS, but only when
they contain the complementary recognition units along their
backbone.
General experimental procedures, detailed synthetic
procedures and characterization data.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-8-55-S1.pdf]
Acknowledgements
The authors gratefully acknowledge support of this work by the
NSF (CHE-1012212) and compounds that were provided by
Dr. Cyrus A. Anderson and Dr. Darrell W. Kuykendall.
Experimental
General
With the exception of 1-(3-dimethylaminopropyl)-3-ethylcar-
bodiimide hydrochloride (EDC), which was purchased from
Advanced ChemTech and used as received, all other chemicals
were purchased from Sigma-Aldrich and used without further
purification. Compounds 4, 7, and 9 were synthesized according
to the published procedures [34], as were compounds 11 and 17
[36]. Solvents were reagent grade and used without further
purification except as follows: Tetrahydrofuran (THF) was
distilled from sodium benzophenone ketyl immediately prior to
use. Methylene chloride (CH2Cl2) was obtained from an
MB-SPS Solvent Purification System and stored over 4 Å mo-
lecular sieves. All reactions were carried out under a dry
nitrogen atmosphere, except for the preparation of compounds 1
and 13. Ambient or room temperature refers to 25 ± 3 °C.
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