Synthesis, optical and thermal studies of dendritic architectures with chalcone surface groups

Article abstract of DOI:10.1016/j.tetlet.2008.09.005

Dendritic architectures bearing three, six, and twelve chalcone units in the periphery are synthesized through convergent approaches. Absorption, emission, photoisomerization, and thermal properties of the chalcone-based dendritic architectures are also s

Full text of DOI:10.1016/j.tetlet.2008.09.005

Tetrahedron Letters 49 (2008) 6539–6542
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis, optical and thermal studies of dendritic
architectures with chalcone surface groups
*
Perumal Rajakumar , Sebastian Raja
Department of Organic Chemistry, University of Madras, Guindy Campus, Chennai 600 025, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 July 2008
Revised 30 August 2008
Accepted 2 September 2008
Available online 5 September 2008
Dendritic architectures bearing three, six, and twelve chalcone units in the periphery are synthesized
through convergent approaches. Absorption, emission, photoisomerization, and thermal properties of
the chalcone-based dendritic architectures are also studied.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Chalcones
Dendrimers
Optical
Photoisomerization
Thermal properties
Dendrimers are highly branched monodisperse synthetic organ-
ic spherical macromolecules. Due to their multifunctional nature,
dendrimers offer innumerable applications in biological and
material sciences.1,2 Photocrosslinkable,3–5 photoswitchable,6,7
of 621 in the presence of K2CO3 in CH3CN under reflux for two days
afforded dendron 8 in 83% yield.
However, in a two-step procedure, dibromide 5 was reacted
with 2.1 equiv of 6 under similar conditions as mentioned earlier,
but for 8 h, to give the acyl dendron 7 in 88% yield which was then
deprotected using 2.0 equiv of KOH in ethanol under refluxing con-
ditions for 3 h to give dendron 8 in 65% yield (Scheme 1). The 1H
NMR spectrum of dendron 8 displayed three sharp singlets at d
5.18, d 6.86, and d 9.67 for two O-methylene, two ortho protons
of the phenolic moiety, and one OH proton in addition to olefinic
and aromatic protons. In the 13C NMR spectrum, the O-methylene
and carbonyl carbon appeared at d 69.3 and d 187.3 in addition to
14 aromatic carbons. The structure of dendron 8 was confirmed
based on spectral and analytical data.22
In order to synthesize the dendrimers 1 and 2, the tribromide
923 was reacted with 3.0 equiv of 6 or 8 in the presence of K2CO3
in CH3CN under refluxing conditions for two days to give dendri-
mers 1 and 2 in 95% and 73% yields, respectively (Scheme 2).
The 1H NMR spectrum of 1 displayed two sharp singlets at d
2.50 and d 5.23 for three identical sets of methyl and O-methylene
protons in addition to olefinic and aromatic protons. In the 13C
NMR spectrum, the methyl, O-methylene, and carbonyl carbons
appeared at d 16.0, d 65.2, and d 188.7 in addition to the aromatic
carbons.
and water soluble8 dendrimers have been widely reported.
a,b-
Unsaturated compounds are important precursors for many
potentially bioactive compounds, for example, hydroxy chalcones
are the most important nucleus abundantly present in plants.9
Chalcone derivatives are known to exhibit excellent cytotoxic,10
antioxidant,11 antibiotic,12 and anticancer13 activities. Similarly,
chalcone core units are used in redox activity14 and for the synthe-
sis of nonlinear optical materials15 and fluorescent dyes.16
Among many promising photosensitive groups, chalcones are
often used for synthesizing photocrosslinkable polymers due to
their high sensitivity to UV radiation and high cross-linking effi-
ciency.17 Though a few reports on the synthesis of dendritic archi-
tectures with chalcone moieties are available in the literature,18 as
well as from our laboratory,19 extensive studies have not been car-
ried out. In view of the emerging importance of chalcone dendri-
mers with respect to photoresponsive behavior, we report herein
our synthesis, absorption, emission, photoisomerization, and ther-
mal studies of aryl ether dendritic architectures 1 and 2 with three
and six chalcone units and also dendritic architectures 3 and 4 con-
taining six and twelve chalcone units, respectively.
Dendron 8 could be synthesized by either a one-step or a two-
Similarly, dendrimers 3 and 4 were synthesized in 83% and 69%
yields, respectively, from hexakis(bromomethyl)benzene 1023 and
6.0 equiv of 6 and 8 (Scheme 3). The 1H NMR spectrum of 3 showed
a sharp singlet at d 5.34 for the twelve O-methylene protons
in addition to olefinic and aromatic protons. In the 13C NMR
step procedure. Reaction of 1 equiv of dibromide 520 with 2.1 equiv
* Corresponding author. Tel.: +91 044 22351269x213; fax: +91 44 22300488.
E-mail address: perumalrajakumar@hotmail.com (P. Rajakumar).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.09.005

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P. Rajakumar, S. Raja / Tetrahedron Letters 49 (2008) 6539–6542
O
O
O
O
O
O
O
O
O
O
H3C
O
CH3
O
O
H3C
O
CH3
O
O
CH3
O
CH3
O
O
O
O
1
2
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
3
4
spectrum, the O-methylene and carbonyl carbons appeared at d
63.9 and d 188.5 and the aromatic carbons at d 114.4 to d 161.8.
The structures of dendritic architectures 1, 2, 3, and 4 were con-
firmed from spectroscopic and analytical data.24–27
The absorption and emission wavelengths of dendrimers 1, 2, 3
and 4 are given in Table 1. All the dendrimers exhibited an intense
absorption band between 312 and 319 nm due to the presence of
the chalcone chromophores. Dendrimer 4 showed a stronger
absorbance than the dendritic architectures 1, 2, and 3 as the
absorbance intensity is found to increase as the number of chal-
cone units in the dendritic architectures increases (Fig. 1).
The fluorescence properties of dendrimers 1, 2, 3, and 4 parallel
the absorption spectra. All the dendrimers showed fluorescence
maxima at 450 nm (kexc). Unlike in the absorption spectra, the
emission spectra showed fluorescence quenching with increasing
number of chalcone units28 (Fig. 2), which was further confirmed
from the fluorescence quantum yield
(UF). The fluorescence
quantum yields of all the dendrimers were recorded using anthra-
cene (0.2) in ethanol as an internal standard. The quantum yields of
1, 2, 3, and 4 are 0.007, 0.005, 0.006, and 0.003, respectively, and
indicate that the fluorescence intensity of the dendritic architec-
tures decreases with increasing chalcone units (Table 1). Though

P. Rajakumar, S. Raja / Tetrahedron Letters 49 (2008) 6539–6542
6541
O
CH3
O
O
CH3
O
O
O
HO
+
O
O
O
i
7
Br
Br
6
5
iii
ii
OH
O
O
O
O
8
Scheme 1. Reagents and conditions: (i) K2CO3, CH3CN, 8 h, reflux, 7 (88%); (ii) KOH, EtOH, 3 h, reflux 8 (65%); (iii) K2CO3, CH3CN, 48 h, reflux, 8 (83%).
Br
H3C
Br
CH3
i
ii
2
1
CH3 Br
9
Scheme 2. Reagents and conditions: (i) 3.0 equiv of 6, K2CO3, CH3CN, 48 h, reflux, 1
(95%); (ii) 3.0 equiv of 8, K2CO3, CH3CN, 48 h, reflux, 2 (73%).
Br
Br
ii
i
Br
4
3
Br
Br
Br
Figure 1. Absorption spectra of dendrimers 1, 2, 3, and 4 in CHCl3 (1 Â 10À6 M).
10
Scheme 3. Reagents and conditions: (i) 6.0 equiv of 6, K2CO3, CH3CN, 48 h, reflux, 3
(83%); (ii) 6.0 equiv of 8, K2CO3, CH3CN, 48 h, reflux, 4 (69%).
Table 1
Absorption and emission data and quantum yields of dendrimers 1, 2, 3, and 4
Dendrimer
UV–visa kmax (nm)
Emissiona kmax (nm)
UF
1
2
3
4
312
316
315
319
430
440
446
450
0.007
0.005
0.006
0.003
a
All spectra were recorded at 1 Â 10À6 M in CHCl3.
dendrimer 4 has the highest number of chalcone units, the quan-
tum yield is a lot less among the four dendrimers.
The photoisomerization behavior of dendrimers 1, 2, 3, and 4
were investigated by UV–vis spectrophotometry. The dendri-
mers (1 Â 10À6 M in CHCl3) were irradiated at 365 nm to induce
Figure 2. Emission spectra of dendrimers 1, 2, 3, and 4 in CHCl3 (1 Â 10À6 M).

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P. Rajakumar, S. Raja / Tetrahedron Letters 49 (2008) 6539–6542
dendritic architectures with various substituents through click
chemistry and their biological properties are currently under
investigation.
Acknowledgments
The authors thank UGC, New Delhi, for financial assistance and
DST-FIST for providing NMR facilities to the department. SR thanks
UGC for a fellowship.
References and notes
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2. Gillies, E. R.; Jonsson, T. B.; Frec´het, J. M. J. J. Am. Chem. Soc. 2004, 126, 11936.
3. Furumi, S.; Otomo, A.; Yokoyama, S.; Mashiko, S. Thin Solid Films 2003, 438, 85.
4. Zhang, W.; Xie, J.; Shi, W. Eur. Polym. J. 2007, 43, 2387.
5. Kwak, G.; Choi, J. U.; Seo, K. H.; Park, L. S.; Hyun, S. H.; Kim, W. S. Chem. Mater.
2007, 19, 2898.
6. Kay, K. Y.; Han, K. J.; Yu, Y. J.; Park, Y. D. Tetrahedron Lett. 2002, 43, 5053.
7. Matsuda, K.; Irie, M. J. Am. Chem. Soc. 2000, 122, 7195.
Figure 3. Photoisomerization of dendrimer 4 in CHCl3 (1 Â 10À5 M).
8. Liao, L. X.; Junge, D. M.; McGrath, D. V. Macromolecules 2002, 35, 319.
9. Stevens, J. F.; Taylor, A. W.; Nickerson, G. B.; Ivancic, M.; Henning, J.; Haunold,
A.; Deinzer, M. L. Phytochemistry 2000, 53, 759.
10. Dimmock, R. J.; Kandepu, N. M.; Hetherington, M.; Quail, J. W.; Pugazhenthi, U.;
Sudom, A. M.; Chamankhah, M.; Rose, P.; Pass, E.; Allen, T. M.; Halleran, S.;
Szydlowski, J.; Mutus, B.; Tannous, M.; Manavathu, E. K.; Myers, T. G.; Clercq, O.
E. D.; Balzarini, O. J. J. Med. Chem. 1998, 41, 1014.
11. Cioffi, G.; Escobar, L. M.; Braca, A.; De Tommasi, N. J. Nat. Prod. 2003, 66, 1061.
12. Nielson, S. F.; Larsen, M.; Boesen, T.; Schønning, K.; Kromann, H. J. Med. Chem.
2005, 48, 2667.
13. Akihisa, T. S.; Tokuda, H.; Hasegawa, D.; Ukiya, M.; Kimura, Y.; Enjo, F.; Suzuki,
T.; Nishino, H. J. Nat. Prod. 2006, 69, 38.
14. Cotelle, N.; Hapiot, P.; Pinson, J.; Ronaldo, C.; Vézin, H. J. Phys. Chem. B 2005,
109, 23720.
15. Tao, X. T.; Watanabe, T.; Kono, K.; Deguchi, T.; Nakayama, M.; Miyata, S. Chem.
Mater. 1996, 8, 1326.
16. Rurack, K.; Bricks, J. L.; Reck, G.; Radeglia, R.; Resch-Genger, U. J. Phys. Chem. A
2000, 104, 3087.
17. Subramanian, K.; Krishnasamy, V.; Nanjundan, S.; Rami Reddy, A. V. Eur. Polym.
J. 2000, 36, 2343.
18. Cho, M. J.; Kim, G. W.; Jun, W. G.; Lee, S. K.; Jin, J.; Choi, D. H. Thin Solid Films
2006, 500, 52.
19. Rajakumar, P.; Ganesan, K.; Jayavelu, S.; Murugesan, K. Synthesis 2006, 3, 528.
20. Liu, P.; Chen, Y.; Deng, J.; Tu, Y. Synthesis 2001, 14, 2078.
21. Wattanasin, S.; Murphy, W. S. Synthesis 1980, 8, 647.
Figure 4. DSC curves of dendritic structures 1, 2, 3, and 4.
22. Dendron 8: Yield 83%, 65%, mp 165 °C (dec); 1H NMR (300 MHz, DMSO-d6): d
5.18 (s, 4H); 6.86 (s, 2H); 7.16 (d, 4H, J = 8.7 Hz); 7.45 (m, 7H); 7.71 (d, 2H,
J = 15.6); 7.87 (d, 4H, J = 3.6 Hz); 7.94 (d, 2H, J = 15.6); 8.17 (d, 4H, J = 8.4 Hz);
9.67 (s, 1H): 13C NMR (75 MHz, DMSO-d6): d 69.3, 113.9, 114.8, 117.2, 122,
128.7, 128.8, 130.4, 130.6, 130.9, 134.8, 138.2, 143.1, 157.6, 162.3, 187.3; MS
(EI, 70 eV): m/z 566 (M+). Elemental Anal. Calcd for C38H30O5: C, 80.55; H, 5.34.
Found: C, 80.42; H, 5.21.
cis–trans isomerization. Figure 3 shows the UV–vis spectral
changes of dendrimer 4 on UV irradiation. This dendrimer under-
goes cis–trans isomerization as revealed by the decrease in the
absorbance28 and appearance of an isosbestic point at 285 nm. A
photostationary state was attained within 660 s during which time
most of the trans chalcone moieties were converted to the cis iso-
mer. Similarly, dendritic architectures 1, 2, and 3 also underwent
cis–trans isomerization.
The thermal behaviors of the dendritic structures 1, 2, 3, and 4
were investigated through differential scanning calorimetry (DSC),
and the results are shown in Figure 4. All the compounds were
found to possess high thermal stability and decomposed above
300 °C. The glass transition temperatures (Tg) improved from
63 °C to 99 °C with an increase in the number of chalcone units
in dendritic structures 1 and 2. The Tg values could not be obtained
clearly for dendrimers 3 and 4 due to the incorporation of more
chalcone units in the dendritic structures. Among the dendrimers,
only 2 showed an isotropic curve at 180.06 °C (Fig. 4), which indi-
cates that the compound might exhibit liquid crystalline (LC) prop-
23. Závada, J.; Pánková, M.; Holy´, P.; Tichy´, M. Synthesis 1994, 11, 1132.
24. Dendrimer 1: Yield 95%; mp 142 °C (dec); 1H NMR (300 MHz, CDCl3): d 2.50 (s,
9H); 5.23 (s, 6H); 7.14 (d, 6H, J = 8.4 Hz); 7.44 (m, 9H); 7.58 (d, 3H, J = 15.6 Hz);
7.67 (d, 6H, J = 3.9 Hz); 7.84 (d, 3H, J = 15.6 Hz); 8.11 (d, 6H, J = 8.1 Hz): 13C
NMR (75 MHz, CDCl3): d 16.0, 65.2, 114.5, 121.9, 128.4, 128.9, 130.4, 130.9,
131.4, 131.5, 135.1, 139.6, 144.1, 162.9, 188.7; MS (EI, 70 eV): m/z 828 (M+).
Elemental Anal. Calcd for C57H48O6: C, 82.58; H, 5.84. Found: C, 82.70; H, 5.97.
25. Dendrimer 2: Yield 73%; mp 125 °C (dec); 1H NMR (300 MHz, CDCl3): d 2.36 (s,
9H); 5.05 (s, 18H); 6.96 (m, 18H); 7.05 (s, 3H); 7.30 (s, 18H); 7.44 (d, 6H,
J = 15.6 Hz); 7.52 (s, 12H); 7.70 (d, 6H, J = 15.9 Hz); 7.94 (d, 12H, J = 7.8 Hz): 13C
NMR (75 MHz, CDCl3): d 16.0, 65.1, 69.8, 113.3, 114.7, 118.8, 121.8, 128.4,
128.9, 130.4, 130.8, 131.5, 131.6, 135.0, 138.5, 139.4, 144.1, 159.7, 162.4,
188.6; (FAB-MS): m/z 1854 (M+). Elemental Anal. Calcd for C126H102O15: C,
81.53; H, 5.54. Found: C, 81.44; H, 5.41.
26. Dendrimer 3: Yield 83%; mp 185 °C (dec); 1H NMR (300 MHz, CDCl3): d 5.34 (s,
12H); 6.97 (d, 12H, J = 8.7 Hz); 7.35-7.37 (m, 18H); 7.44 (d, 6H, J = 15.6 Hz);
7.54–7.57 (m, 12H); 7.73 (d, 6H, J = 15.6 Hz); 7.96 (d, 12H, J = 9.0 Hz): 13C NMR
(75 MHz, CDCl3): d 63.9, 114.4, 121.5, 128.4, 128.9, 130.4, 130.9, 132.0, 134.9,
137.8, 144.4, 161.8, 188.5; (FAB-MS): m/z 1494 (M+). Elemental Anal. Calcd for
C102H78O12: C, 81.91; H, 5.26. Found: C, 81.75; H, 5.13.
erties. Though dendrimer
4 has structural similarity with
dendrimer 2, an isotropic curve was not observed due to its decom-
position at high temperature.
27. Dendrimer 4: Yield 69%; mp 210 °C (dec); 1H NMR (300 MHz, CDCl3): d 4.86 (s,
24H); 5.24 (s, 12H); 6.81–6.88 (m, 36H); 6.98 (s, 6H); 7.27 (s, 36H); 7.37 (d,
12H, J = 15.6 Hz); 7.48 (s, 24H); 7.65 (d, 12H, J = 15.6 Hz); 7.86 (d, 24H,
J = 8.4 Hz): 13C NMR (75 MHz, CDCl3): d 64.1, 69.6, 113.5, 114.5, 119.4, 121.6,
128.4, 128.9, 130.4, 130.8, 131.5, 135.0, 138.0, 138.6, 144.2, 159.0, 162.2,
188.4; (FAB-MS): m/z 3548 (M+). Elemental Anal. Calcd for C240H186O30: C,
81.20; H, 5.28. Found: C, 81.44; H, 5.43.
In summary, we have synthesized several chalcone-based den-
dritic structures and have studied their optical and thermal prop-
erties. The incorporation of increased numbers of chalcone units
at the peripheries of the dendritic architectures had significant
effects on improving the morphology. Syntheses of chalcone
28. Rajakumar, P.; Dhanasekaran, M.; Selvam, S. Synthesis 2006, 8, 1257.