Study of the aggregation behavior of a π-conjugated dendrimer with a twisted core

Article abstract of DOI:10.1016/j.tetlet.2012.03.095

A dendrimer having phenylene vinylene and phenylene ethynylene moieties, a twisted core, and eighteen chiral centers on the periphery has been prepared in high yield by using Sonogashira and Horner-Wadsworth-Emmons reactions. UV-visible and fluorescence spectra and circular dichroism measurements have been envisaged to study the aggregation behavior.

Full text of DOI:10.1016/j.tetlet.2012.03.095

Tetrahedron Letters 53 (2012) 2752–2755
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Tetrahedron Letters
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Study of the aggregation behavior of a
twisted core
p-conjugated dendrimer with a
Joaquín C. García-Martínez a, Enrique Díez-Barra b, Julián Rodríguez-López b,
⇑
a Facultad de Farmacia, Universidad de Castilla-La Mancha, 02071 Albacete, Spain
b Facultad de Química-IRICA, Universidad de Castilla-La Mancha, 13071 Ciudad Real, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
A dendrimer having phenylene vinylene and phenylene ethynylene moieties, a twisted core, and eighteen
chiral centers on the periphery has been prepared in high yield by using Sonogashira and Horner–
Wadsworth–Emmons reactions. UV–visible and fluorescence spectra and circular dichroism
measurements have been envisaged to study the aggregation behavior.
Received 1 March 2012
Revised 20 March 2012
Accepted 23 March 2012
Available online 29 March 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Dendrimers
Aggregation
p-Stacking
p-Conjugation
Optical properties
Dendrimers1 have evolved as a powerful class of materials with
a daily increased presence in a large number of interesting fields.2
They possess a highly branched, monodisperse, and precise pri-
mary structure, which can be appropriately designed to generate
self-assembling into functional supramolecular architectures with
numerous technological applications, in analogy with biological
macromolecules.3
86% yield (Scheme 1). Purification could be performed by column
chromatography and also verified by GPC.8 On the other hand, aryl
iodide 3 was prepared by Horner–Wadsworth–Emmons reaction of
1-iodo-3,5-bis(diethoxyphosphorylmethyl)benzene9 with two
molecules of 3,4,5-tris{(S)-3,7-dimethyloctyloxy}benzaldehyde.10
Both new compounds were fully characterized by a variety of
analytical techniques. The trans-stereochemistry of the double
bonds was unequivocally established on the basis of the coupling
constant for the vinylic protons in the 1H NMR spectra (J ꢀ 16 Hz).
The mass spectra, NMR data, and elemental analysis indicate that
both dendritic structures were obtained with high purity, also
verified by GPC (see Supplementary data).
As it is known, UV–vis and photoluminescence (PL) spectros-
copy are useful tools to get information about molecular aggrega-
tion.4a Hence, we recorded the absorption and emission spectra of
the target molecule 1 and the intermediate dendron 3 in heptane
at room temperature and different concentrations. Spectra in
dichloromethane and methylcyclohexane were also taken for den-
drimer 1 (Fig. 1).
Dendrimers with a
p-conjugated, shape persistent backbone
have shown particularly attractive in this context. They are able
to self-organize into supramolecular entities that enormously
influence the macroscopic optical and electronic properties.4
Our experience in the synthesis of dendritic compounds with a
p
-conjugated framework5 prompted us to prepare a new molecule,
1, with the aim of obtaining well ordered supramolecular
assemblies (Scheme 1). The structure of 1 contains promising fea-
tures for aggregation, that is, a twisted
peripheral branches bearing both ether functionalities and chiral
centers. The twisted -core might allow stacking, while the
p-system as core and
p
p–p
chiral centers might induce a preference on the sign of the helicoi-
dal organization.
Both molecules are almost colorless and are absolutely trans-
parent above 400 nm and show strong absorption bands with max-
ima at 321 nm. This is indicative that the incorporation of
dendrons to the twisted core does not modify the conjugation
and as one would expect, the meta arrangement through which
the dendrons are linked causes the absorption spectra to consist
essentially of a simple superposition of the absorptions due to dif-
ferent chromophores, stilbene, and tolane moieties.
Following a convergent methodology previously described by
us,6 the target molecule 1 was synthesized by Sonogashira reaction
of the 1,3,5-(o-ethynylphenyl)benzene,7 2, and the dendron 3, in
⇑
Corresponding author.
E-mail address: julian.rodriguez@uclm.es (J. Rodríguez-López).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.03.095

J. C. García-Martínez et al. / Tetrahedron Letters 53 (2012) 2752–2755
2753
While no changes were observed in kmax, suggesting the ab-
sence of aggregation, the observed slight increase of the molar
extinction coefficient, when concentration increases, does propose
the existence of aggregation.
In the same way points the PL spectra registered in heptane
(Fig. 2). The PL spectrum of dendrimer 1 shows, in addition to
the expected blue response for hybrid phenylene vinylene and
phenylene ethynylene dendrimers at kmax = 394 nm,6 a shoulder
I
OR
OR
+
3
RO
RO
OR
OR
2
3
86%
i
OR
3.0
2.7
2.4
2.1
1.8
1.5
1.2
0.9
0.6
0.3
0.0
600
500
400
300
200
100
0
OR
OR
HOL3
HOL4
OR
RO
RO
OR
OR
OR
OR
RO
RO
CH3
RO
OR
CH3
CH3
OR
200
300
400
500
600
R =
wavelength (nm)
1
RO
Figure 2. UV–vis and PL spectra for dendron 3 (dotted, c = 1.17 Â 10À5 M) and
dendrimer 1 (solid, c = 1.22 Â 10À5 M) in heptane at room temperature.
OR
OR
Scheme 1. Preparation of target dendrimer 1. Reagents and conditions: (i)
PdCl2(PPh3)2, CuI, PPh3, Et3N/DMF, 60 °C.
2.0
1.5
1.0
0.5
0.0
-0.5
1.2x105
1.0x105
1.17x10-5 M in heptane
8.0x104
1.17x10-4 M in heptane
6.0x104
4.0x104
1.17x10-5 M in heptane
-1.0
1.17x10-4 M in heptane
2.0x104
-1.5
-2.0
0.0
200
200
250
300
350
400
450
500
250
300
350
400
450
500
wavelength (nm)
wavelength (nm)
1.22x10-5 M in heptane
2
1.14x10-4 M in heptane
0
-2
1.22x10-4 M in methylcyclohexane
1.13x10-3 M in heptane
3.5x105
-4
3.0x105
2.5x105
2.0x105
1.5x105
1.0x105
1.05x10-3 M in methylcyclohexane
1.13x10-3 M in dichloromethane
-6
-8
1.13x10-3 M in heptane
-10
-12
-14
-16
-18
-20
-22
1.05x10-3 M in methylcyclohexane
1.05x10-3 M in methylcyclohexane, 60 °C
1.13x10-3 M in dichloromethane
5.0x104
0.0
200
250
300
350
400
450
500
200
250
300
350
400
450
500
wavelength (nm)
wavelength (nm)
Figure 1. UV–vis spectra for dendron 3 (top) and dendrimer 1 (bottom) at room
Figure 3. Circular dichroism measurements for dendron 3 (top) and dendrimer 1
temperature.
(bottom).

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J. C. García-Martínez et al. / Tetrahedron Letters 53 (2012) 2752–2755
around kmax = 470 nm, a typical excimer emission that may indi-
cate again stacking of the molecules.11
in the fluorescence spectrum may indicate molecular aggregation;
however, the CD results suggest lack of organization. Changes in
the nature of the chiral peripheral chains seem crucial in order to
modulate the desired behavior.
In order to evaluate if this aggregation has some organization
induced by the presence of chiral end groups on the dendrimer, cir-
cular dichroism (CD) experiments were performed. A noticeable
Cotton effect would be a sign of a molecular organization.
Whereas dendron 3 did not show a CD effect in heptane, a small
Cotton effect was observed for 1 mM solutions of dendrimer 1 in
heptane, methylcyclohexane, and dichloromethane (Fig. 3, see also
Supplementary data). However, the intensity of the effect was not
dependent on temperature or the polarity of the solvent. Therefore,
this small Cotton effect should be attributed to the intrinsic chiral-
ity of the dendrons and not to aggregation behavior.
Acknowledgments
We thank Professor Bert Meijer for helpful discussion as well as
for GPC analysis and CD measurements. This work has been co-
funded by the Ministerio de Ciencia e Innovación (Spain)/FEDER
(European Union)—project BFU2011-30161-C02-02, and the Junta
de Comunidades de Castilla-La Mancha—project PEII11-0106-
5042.
On the other hand, we tried to extend this study to solvents of
higher polarity. Although dendrimer 1 is not soluble in solvents
such as methanol and acetonitrile, we could register the spectra
in a chloroform solution adding different amounts of methanol
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.03.
095.
(0–20%) and water. In all cases no shift in kmax or
e nor a Cotton
effect were observed (Fig. 4).
Furthermore, the sticky solid 1 was heated, and at approxi-
mately 150 °C it started flowing. Upon cooling (either slow or fast)
no birefringence was observed by polarization microscopy.
In summary, a new dendrimer composed of alternate phenylene
ethynylene and phenylene vinylene moieties, a twisted core, and
18 chiral centers on the periphery has been efficiently synthesized
in high yield using a convergent synthetic route that combine
References and notes
1. (a) Vögtle, F.; Richard, G.; Werner, N. Dendrimer Chemistry: Concepts, Syntheses,
Properties, Applications; Wiley-VCH: Weinheim, Germany, 2009; (b) Hourani,
R.; Kakkar, A. Macromol. Rapid Commun. 2010, 31, 947–974; (c)Designing
Dendrimers; Campagna, S., Ceroni, P., Puntoriero, F., Eds.; Wiley, 2011.
2. (a) Astruc, D.; Boisselier, E.; Ornelas, C. Chem. Rev. 2010, 110, 1857–1959; (b)
Dendrimers: Towards Catalytic Material and Biomedical Uses; Caminade, A.-M.,
Turrin, C.-D., Laurent, R., Ouali, A., Delavaux-Nicot, B., Eds.; Wiley: Chichester,
UK, 2011; (c) Röglin, L.; Lempens, E. H. M.; Meijer, E. W. Angew. Chem., Int. Ed.
2011, 50, 102–112; (d) Majoral, J.-P. New J. Chem. 2012, 36; (e) Khandare, J.;
Calderón, M.; Dagia, N. M.; Haag, R. Chem. Soc. Rev. 2012, 41, 2824–2848.
3. Rosen, B. M.; Wilson, C. J.; Wilson, D. A.; Peterca, M.; Imam, M. R.; Percec, V.
Chem. Rev. 2009, 109, 6275–6540.
4. (a) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A. P. H. J. Chem. Rev.
2005, 105, 1491–1546; (b) García, F.; Sánchez, L. Chem. Eur. J. 2010, 16, 3138–
3146.
5. García-Martínez, J. C.; Díez-Barra, E.; Rodríguez-López, J. Curr. Org. Synth. 2008,
5, 267–290.
Sonogashira
and
Horner–Wadsworth–Emmons
reactions.
Although it is not conclusive, the noticeable shoulder that appears
percentage CH3OH in CHCl3
0 %
2.0
1 %
5 %
10 %
20 %
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
6. Díez-Barra, E.; García-Martínez, J. C.; Rodríguez-López, J. J. Org. Chem. 2003, 68,
832–838.
7. Collins, S. K.; Yap, G. P. A.; Fallis, A. G. Org. Lett. 2002, 4, 11–14.
20 % + 3 drops water
8. Preparation of dendrimer 1: To
a
stirred solution of 1,3,5-(o-
ethynylphenyl)benzene, 2, (32 mg, 0.084 mmol) and the aryl iodide
3
(340 mg, 0.253 mmol) in anhydrous DMF/Et3N (1:1, 25 mL) heated at 60 °C
under argon, was added a catalyst mixture of PdCl2(PPh3)2 (9 mg, 0.013 mmol),
CuI (2.5 mg, 0.013 mmol) and PPh3 (7 mg, 0.025 mmol). The reaction was
stirred overnight at 60 °C. After evaporation of the solvent under reduced
pressure, CH2Cl2 was added. The solution was washed with saturated
ammonium chloride, brine, and then dried (MgSO4). The solvent was
evaporated under vacuum and the crude product purified by column
chromatography (silica gel, hexanes/EtAcO 40:1) to give 290 mg (86%) of 1 as
viscous yellow oil which became a sticky solid upon standing. Two extra
chromatographic purifications were performed. 1H NMR (CDCl3, 500 MHz): d
0.85 (d, 72H, J = 6.5 Hz, 24 Â CH3), 0.87 (d, 36H, J = 6.5 Hz, 12 Â CH3), 0.92 (d,
18H, J = 6.5 Hz, 6 Â CH3), 0.93 (d, 36H, J = 6.5 Hz, 12 Â CH3), 1.1–1.9 (m, 180H),
3.9–4.1 (m, 36H, 18 Â OCH2), 6.69 (s, 12H, 12 Â CH arom.), 6.86 (A of ABq, 6H,
J = 16.0 Hz, 6 Â CH@), 6.92 (B of ABq, 6H, J = 16.0 Hz, 6 Â CH@), 7.21–7.23 (m,
6H, 6 Â CH arom.), 7.31 (d, 6H, J = 1.5 Hz, 6 Â CH arom.), 7.47 (br s, 3H, 3 Â CH
arom.), 7.55–7.57 (m, 3H, 3 Â CH arom.), 7.63–7.65 (m, 3H, 3 Â CH arom.), 8.12
(s, 3H, 3 Â CH arom.). 13C NMR and DEPT (CDCl3, 125 MHz): d 153.3 (C), 143.6
(C), 139.9 (C), 138.4 (C), 137.8 (C), 132.8 (CH), 132.2 (C), 129.8 (CH), 129.7 (CH),
129.4 (CH), 128.9 (CH), 128.2 (CH), 127.3 (CH), 126.5 (CH), 124.2 (CH), 123.8
(C), 121.3 (C), 105.1 (CH), 92.7 (C„), 89.6 (C„), 71.8 (CH2), 67.3 (CH2), 39.4
(CH2), 39.3 (CH2), 37.6 (CH2), 37.4 (CH2), 37.4 (CH2), 36.5 (CH2), 29.8 (CH), 29.7
(CH), 28.0 (CH), 24.8 (CH2), 24.7 (CH2), 22.7 (CH3), 22.6 (CH3), 22.6 (CH3), 19.6
(CH3). MS (MALDI-TOF, sinapinic acid), m/z Calcd for C276H426O18: 4032.25.
Found: 4032.34. Anal. Calcd for C276H426O18: C, 82.21; H, 10.65. Found: C,
81.91; H, 10.43.
250
300
350
400
450
500
wavelength (nm)
percentage CH3OH in CHCl3
0 %
1 %
5 %
10 %
3
2
20 %
20 % + 3 drops water
1
0
-1
-2
-3
9. Preparation of dendron 3: To
a
stirred solution of 1-iodo-3,5-
bis(diethoxyphosphorylmethyl)benzene (504 mg, 1 mmol) and the aromatic
aldehyde (1.15 g, 2 mmol) in anhydrous THF (15 mL), under argon, was added
potassium tert-butoxide in small portions (675 mg, 6 mmol). The deeply
colored mixture was stirred at room temperature for 3 h. After hydrolysis with
water, the mixture was extracted with CH2Cl2 (3Â). The combined organic
layers were then dried (MgSO4), and the solvent evaporated under reduced
pressure. The crude product was purified by column chromatography (silica
gel, hexanes/EtAcO, 9:1) to give 1.05 g (78%) of 3 as a yellow oil. 1H NMR
(CDCl3, 500 MHz): d 0.87 (d, 12H, J = 7.0 Hz, 4 Â CH3), 0.87 (dd, 24H, J = 6.5 Hz,
250
300
350
400
450
500
wavelength (nm)
Figure 4. UV–vis and circular dichroism spectra for dendrimer 1 in chloroform/
methanol solutions. Effect of the solvent polarity.

J. C. García-Martínez et al. / Tetrahedron Letters 53 (2012) 2752–2755
2755
J = 1.0 Hz, 8 Â CH3), 0.93 (d, 6H, J = 6.5 Hz, 2 Â CH3), 0.96 (d, 12H, J = 6.5 Hz,
4 Â CH3), 1.1–2.0 (m, 60H), 3.9–4.1 (m, 12H, 6 Â OCH2), 6.72 (s, 4H, 4 Â CH
arom.), 6.87 (A of ABq, 2H, J = 16.2 Hz, 2 Â CH@), 7.03 (B of ABq, 2H, J = 16.2 Hz,
2 Â CH@), 7.52 (br s, 1H, CH arom.), 7.72 (d, 2H, J = 1.5 Hz, 2 Â CH arom.). 13C
NMR and DEPT (CDCl3, 125 MHz): d 153.3 (C), 139.7 (C), 138.6 (C), 133.8 (CH),
131.9 (C), 130.4 (CH), 125.8 (CH), 123.9 (CH), 105.2 (CH), 95.2 (C–I), 71.8 (CH2),
67.4 (CH2), 39.4 (CH2), 39.3 (CH2), 37.5 (CH2), 37.4 (CH2), 37.3 (CH2), 36.4 (CH2),
29.8 (CH), 29.7 (CH), 28.0 (CH), 24.8 (CH2), 24.7 (CH2), 22.7 (CH3), 22.6 (CH3),
19.6 (CH3), 19.6 (CH3). MS (MALDI-TOF, sinapinic acid), m/z Calcd for
82H137IO6: 1344.95. Found: 1344.86. Anal. Calcd for C82H137IO6: C, 73.18; H,
10.26. Found: C, 73.56; H, 10.32.
C
10. Kind present from Professor Bert Meijer. Prepared following: Li, W.-R.; Kao, K.-
C.; Yo, Y.-C.; Lai, C. K. Helv. Chim. Acta 1999, 82, 1400–1407.
11. Yagai, S.; Hamamura, S.; Wang, H.; Stepanenko, V.; Seki, T.; Unoike, K.;
Kikkawa, Y.; Karatsu, T.; Kitamura, A.; Würthner, F. Org. Biomol. Chem. 2009, 7,
3926–3929.