Synthesis and characterization of poly(benzyl ether) dendrimer containing fluorene as a core chromophore

Article abstract of DOI:10.1080/15421400802329624

The convergent synthetic strategy for the emissive poly(benzyl ether) dendrimers having the chromophore at core via the copper-catalyzed 1,3-dipolar cycloaddition reaction between alkyne and azide was described. 2,7-Diazido-9H-fluorene, designed to serve

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Synthesis and Characterization
of Poly(benzyl ether)
Dendrimer Containing Fluorene
as a Core Chromophore
Jae Wook Lee a , Byoung-Ki Kim a , Seung Choul Han
a , Un Yup Lee a , Ji Hyeon Kim b , Jinho Oh c & Sung-
Ho Jin d
a Department of Chemistry and Department of
Medical Bioscience, Dong-A University, Busan, Korea
b Department of Chemical & Bio Engineering,
Kyungwon University, Seonynam, Korea
c Institute for Gifted Students, Korea Advanced
Institute of Science and Technology, Daejeon, Korea
d Department of Chemistry Education & Center
for Plastic Information System, Pusan National
University, Busan, Korea
Version of record first published: 22 Sep 2010.
To cite this article: Jae Wook Lee , Byoung-Ki Kim , Seung Choul Han , Un Yup Lee ,
Ji Hyeon Kim , Jinho Oh & Sung-Ho Jin (2008): Synthesis and Characterization of
Poly(benzyl ether) Dendrimer Containing Fluorene as a Core Chromophore, Molecular
Crystals and Liquid Crystals, 491:1, 164-172
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Mol. Cryst. Liq. Cryst., Vol. 491, pp. 164–172, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400802329624
Synthesis and Characterization of Poly(benzyl ether)
Dendrimer Containing Fluorene as a
Core Chromophore
Jae Wook Lee1, Byoung-Ki Kim1, Seung Choul Han1,
Un Yup Lee1, Ji Hyeon Kim2, Jinho Oh3, and Sung-Ho Jin4
1Department of Chemistry and Department of Medical Bioscience,
Dong-A University, Busan, Korea
2Department of Chemical & Bio Engineering, Kyungwon University,
Seonynam, Korea
3Institute for Gifted Students, Korea Advanced Institute of Science and
Technology, Daejeon, Korea
4Department of Chemistry Education & Center for Plastic Information
System, Pusan National University, Busan, Korea
The convergent synthetic strategy for the emissive poly(benzyl ether) dendrimers
having the chromophore at core via the copper-catalyzed 1,3-dipolar cycloaddi-
tion reaction between alkyne and azide was described. 2,7-Diazido-9H-fluorene,
designed to serve as the core in dendrimer, was stitched with the alkyne-functio-
nalized poly(benzyl ether) dendrons by the click chemistry leading to the forma-
tion of fluorescent poly(benzyl ether) dendrimers in high yields. The absorption
and emission of the dendrimers were investigated. Their photophysical studies
indicated that 2,7-diazido-9H-fluorene showed no fluorescence due to the quench-
ing effect from the electron-rich a-nitrogen of the azido group but the dendrimers
fluoresced due to the elimination of the quenching through the formation of the
triazole ring.
Keywords: 1,3-dipolar cycloaddition; click chemistry; dendrimer; fluorene
This work was supported by the Ministry of Information & Communications, Korea,
under the Information Technology Research Center (ITRC) Support Program.
Address correspondence to Jae Wook Lee, Department of Chemistry, Dong-A Univer-
sity, Busan 604-714, Korea. E-mail: jlee@donga.ac.kr and Sung-Ho Jin, Department
of Chemistry Education and Center for Plastic Information System, Pusan National
University, Busan 609-735, Korea. E-mail: shjin@pusan.ac.kr
164

Poly(benzyl ether) Dendrimer Containing Fluorene as a Core Chromophore
165
INTRODUCTION
Dendrimers, which are prepared by repetition of a given set of reac-
tions using either divergent or convergent strategies, are highly
branched and regular macromolecules with well-defined structures
and have served as functional objects in nanotechnology and
nanoscience [1]. Although recent research emphasis seems to shift
from the synthesis of novel dendrimers to their properties and poten-
tial applications, future more expanded applications of dendrimers
rely on efficient and practical synthetic procedures. Recently, click
chemistry, the copper-catalyzed 1,3-dipolar cycloaddition reaction
between an alkyne and an azide,has attracted much attention [2].
This type of reaction, characterized by high yields, mild and simple
reaction conditions, oxygen and water tolerance, and simple work-up
of products, is highly chemoselective in the formation of the desired
1,4-disubstituted 1,2,3-triazole even presence of a large variety of
functional groups.
Polymer chemists have employed click chemistry to construct
dendritic and linear macromolecules [2b]. However, the synthesis of
emissive dendrimers by 1,3-cycloadditions has been conspicuously
absent. We have been interested in the construction of new pathway
for synthesis of novel fluorene-based dendrimers via click reaction,
which will be soluble in common organic solvents and easily spin-coated
with high quality optical thin films. In our previous work, we have
´
synthesized various types of convergent Frechet- and PAMAM-type den-
drimers using click chemistry [3–5]. Overall, this method was found to
be a straightforward strategy for the synthesis of triazole-based dendri-
mers. Taking advantage of this fact, herein we report the first general and
´
efficient strategy for the synthesis of Frechet-type poly(benzyl ether)
dendrimers having a chromophore at core.
EXPERIMENTAL
General Methods
1H- and 13C-NMR spectra were recorded on a Bruker AM-500 spec-
trometer. Mass spectra were obtained from Korea Basic Science
Institute (KBSI) in Daegu or Daejeon and POSTECH. Flash chroma-
tography was performed with 37–75 mm silica gel. UV-visible spectra
were recorded with a Shimadzu UV-3100 spectrophotometer with a
baseline correction and normalization carried out using Microsoft
Excel software. The emission spectra for dilute solutions were determ-
ined using a Hitachi F-4500 fluorescence spectrophotometer. Solid

166
J. W. Lee et al.
state emission measurements were carried out by supporting each film
on a quartz substrate that ꢀwas mounted to receive front-face excitation
at an angle of less than 45 . The polymer films were prepared through
spin casting of polymer solution containing 0.1 wt% chloroform. The
polydispersity of the dendrimer was determined by gel permeation
chromatography (GPC) using PLgel 5 mm MIXED-C column on an Agi-
lent 1100 series liquid chromatography system with THF as an eluent
and calibration with polystyrene standards.
´
General Procedure for the Synthesis of Frechet-type poly(benzyl
ether) Dendrimers 3-Gm. A mixture of alkyne-functionalized poly
(benzyl ether) dendrons 1-Dm (0.22 mmol) and 2,7-diazido-9H-fluorene
2
(0.10 mmol) in DMF-H2O (4:1, 1 mL) in the presence of
10 mol% CuSO4 ꢁ 5H2O with 20 mol% sodium ascorbate was stirred
at 50ꢀC for ꢂ 30 min. The reaction mixture was poured into brine
(20 mL) and the resulting solution was extracted with EtOAc
(20 mL ꢃ 3). The combined organic phase was dried with sodium sul-
fate, concentrated, and purified by column chromatography to afford
the desired product 3-Gm.
Compound 3-G1. Rf 0.4 (EtOAc=hexane ¼ 3:2); 94% yield. A yellow
solid; IR: 3016, 2962, 2862, 1720, 1597, 1462, 1215, 1153, 1041 cmꢄ1
;
1H NMR (500 MHz, CDCl3) d 3.79 (s, 12H), 4.07 (s, 2H), 4.61 (s, 4H),
4.78 (s, 4H), 6.40 (s, 2H), 6.56 (d, J ¼ 1.5 Hz, 4H), 7.75 (d, J ¼ 8.1 Hz,
2H), 7.91 (d, J ¼ 7.9 Hz, 4H), 7.96 (s, 2H), 8.05 (s, 2H); 13C NMR
(125 MHz, CDCl3) d 37.6, 55.8, 64.1, 73.1, 100.3, 106.1, 118.1, 120.2,
121.3, 121.6, 136.6, 140.5, 141.4, 145.5, 146.5, 161.4; MS (FAB): m=z
660.4 [Mþ], 661.4 [Mþ þ H]; HRMS (FAB) Calcd for C37H36N6O6:
660.2696. Found: 661.2772 [Mþ þ H]. PDI: 1.01.
Compound 3-G2. Rf 0.45 (EtOAc=hexane ¼ 3:2); 94% yield; A
yellowish oil; IR 3000, 2931, 2839, 1720, 1597, 1458, 1203, 1153,
1
1045 cmꢄ1; H NMR (500 MHz, CDCl3) d 3.78 (s, 24H), 4.07 (s, 2H),
4.61 (s, 4H), 4.77 (s, 4H), 4.98 (s, 8H), 6.40 (s, 4H), 6.55 (s, 2H), 6.57
(d, J ¼ 1.8 Hz, 8H), 6.65 (d, J ¼ 1.7 Hz, 4H), 7.76 (d, J ¼ 7.6 Hz, 2H),
7.91 (d, J ¼ 8.2 Hz, 2H), 7.97 (s, 2H), 8.02 (s, 2H); 13C NMR (125 MHz,
MHz, CDCl3) d 37.6, 55.8, 64.1, 70.5, 73.1, 100.3, 102.0, 105.7, 107.3,
118.0, 120.1, 121.3, 121.6, 136.6, 139.6, 140.7, 141.4, 145.5, 146.4,
160.5, 161.4; MS (FAB): m=z 1205.1 [Mþ þ H]; HRMS (FAB) Calcd
for C69H68N6O14: 1204.4794. Found: 1205.4877 [Mþ þ H]. PDI: 1.01.
Compound 3-G3. Rf 0.35 (EtOAc=hexane ¼ 3:2); 95% yield; A
yellowish oil; IR 3012, 2935, 2839, 1735, 1597, 1458, 1203, 1157,
1
1049 cmꢄ1; H NMR (500 MHz, CDCl3) d 3.77 (s, 48H), 3.99 (s, 2H),
4.62 (s, 4H), 4.76 (s, 4H), 4.96 (s, 16H), 4.98 (s, 8H), 6.39 (s, 8H),

Poly(benzyl ether) Dendrimer Containing Fluorene as a Core Chromophore
167
6.56 (br m, 22H), 6.64 (s, 4H), 6.68 (s, 8H), 7.72 (d, J ¼ 8.2 Hz, 2H),
7.85 (d, J ¼ 8.2 Hz, 2H), 7.90 (s, 2H), 7.97 (s, 2H); 13C NMR (125 MHz,
CDCl3) d 37.6, 55.8, 64.1, 70.4, 70.5, 73.1, 100.3, 102.0, 105.7, 106.8,
107.2, 118.0, 120.1, 121.3, 121.5, 136.6, 139.5, 139.8, 140.7, 141.3,
145.5, 146.4, 160.4, 160.5, 161.4; MS (MALDI): Calcd for
C
133H132N6O30: 2292.8988. Found: 2315.9252 [Mþ þ Na]. PDI: 1.01.
Compound 3-G4. Rf 0.2 (EtOAc= hexane= dichloromethane ¼ 1:2:6);
91% yield; A yellowish oil; IR 3008, 2939, 2839, 1728, 1593, 1454, 1203,
1149, 1049 cmꢄ1; H NMR (500 MHz, CDCl3) d 3.75 (s, 96H), 3.90 (s,
1
2H), 4.60 (s, 4H), 4.72 (s, 4H), 4.93 (s, 48H), 4.98 (s, 8H), 6.38 (s,
16H), 6.53–6.55 (br m, 46H), 6.65–6.66 (br m, 28H), 7.66 (d, J ¼ 8.0 Hz,
2H), 7.85 (d, J ¼ 7.8 Hz, 2H), 7.84 (s, 2H), 7.93 (s, 2H); 13C NMR
(125 MHz, CDCl3) d 37.0, 55.3, 63.6, 70.0, 72.6, 99.88, 99.92, 101.6,
105.2, 106.4, 106.8, 106.9, 117.4, 119.5, 120.8, 121.0, 136.0, 139.1,
139.2, 139.3, 140.2, 140.8, 145.1, 145.8, 159.98, 160.02, 160.9; MS
(MALDI): Calcd for C261H260N6O62: 4469.7377. Found: 4492.7363
[Mþ þ Na]. PDI: 1.01.
RESULTS AND DISCUSSION
The convergent methodology in the synthesis of dendrimers installs
the core in the final step and allows for a large degree of chemical
diversity such that functional groups can be incorporated at nearly
central position in the dendritic architecture [6]. Because of the high
yields and lack of byproducts provided by the click chemistry for
stitching together dendrons and core unit, the various dendrimers
having functional building block at core could be obtained easily
and shown the characteristic behaviors. 2,7-Diazido-9H-fluorene 2
was designed to serve as an emissive building block and the azide
functionalities for dendrimer growth via click reactions with the
alkyne-dendrons. Compound 2 was prepared from the reaction of
2,7-diaminofluorene with triflyl azide (TfN3) in the presence of triethy-
lamine and copper(II) sulfate according to previously reported
procedure [7].
´
The synthetic strategy for emissive Frechet-type poly(benzyl ether)
dendrimers, linked by the triazole units, utilized a convergent method
´
using the alkyne-functionalized Frechet-type poly(benzyl ether) den-
drons 1-Dm and 2,7-diazido-9H-fluorene 2 (Scheme 1). The propargyl-
´
functionalized Frechet-type poly(benzyl ether) dendrons 1-Dm
(m ¼ 1 ꢂ 4: generation of dendron) were synthesized by the propargyla-
tion of the corresponding dendritic benzyl alcohols with propargyl
bromide [3e]. To efficiently connect the propargylfocal point

168
J. W. Lee et al.
SCHEME 1 Synthesis of dendrimers containing fluorene.
´
Frechet-type dendrons with 2,7-diazido-9H-fluorene 2, the synthetic
approach selected is based on the click condition using Cu (I)
species [8].
The efficiency of the click reaction between the alkyne-dendrons
and bis(azides) was evaluated by performing the model coupling reac-
tions between alkyne-dendron 1-D1 and compound 2. Click reaction
was carried out in a 4:1 solvent ratio of DMF to H2O using 5 mol%
CuSO4 ꢁ 5H2O with 10 mol% sodium ascorbate with respect to alkyne
as the in situ reducing agent to generate the active Cu(I) species.
The reaction of 2,7-diazido-9H-fluorene 2 and 2.2 equiv of alkyne-
dendron 1-D1 afforded the desired product 3-G1 in a yield of 94%
after 10 min at 50ꢀC which was separated by column chromatography.
The disappearance of bis(azides) as well as generation and disappear-
ance of the mono-triazole derivative were monitored by TLC runs
of the reaction mixture. Given the success in the synthesis of first-
generation dendrimer, therefore we expanded this reaction to get
higher-generation dendrimers. The reactions of 2,7-diazido-9H-
fluorene 2 and 2.2 equiv of alkyne-dendrons 1-D2 and 1-D3 afforded
the dendrimers 3-G2 and 3-G3 in yields of 94 and 95%, respectively,
after 30 min at 50ꢀC. Finally, reaction of 2,7-diazido-9H-fluorene 2
and 2.2 equiv of alkyne-dendron 1-D4 gave the dnedrimer 3-G4 in a

Poly(benzyl ether) Dendrimer Containing Fluorene as a Core Chromophore
169
FIGURE 1 1H-NMR spectra for (a) 2, (b) 3-G1, (c) 3-G2, (d) 3-G3, and (e)
3-G4.
yield of 91% after 30 min at 50ꢀC. This result showed that the forma-
tion of triazole can be regarded as a new connector to stitch the den-
drons with a chromophore. Therefore this approach may provide new
methodological insight into introduction of various fluorescent cores
and would greatly contribute to researches on the application side.
Structural characterization of the dendrimers 3-Gm with 1H NMR,
13C NMR, and IR spectroscopy showed complete stitching of dendrons.
From the 1H NMR spectra (CDCl3), the peaks of the methylene
protons of fluorene moiety and the triazole proton in dendrimers
3-Gm were found at 4.07 and 8.05 ppm for 3-G1, 4.07 and 8.02 ppm
for 3-G2, 3.99 and 7.97 ppm for 3-G3, and 3.90 and 7.93 ppm for
3-G4, respectively (Fig. 1). As the dendrimer generation increased,
the peaks of the methylene protons of fluorene moiety and the triazole
proton shifted gradually to up-field which may be influenced by the
dendritic microenvironment effect [9]. In the 1H NMR spectra, all
the signals corresponding to the fluorene unit of the dendrimers
3-Gm shifted to lower field than that of compound 2. The IR spectra
showed that neither alkyne (ꢂ3285 cmꢄ1) nor azide (2116 cmꢄ1) resi-
dues remain in the final dendrimer. Their FAB or MALDI mass
spectra for dendrimer exhibited very good correlation with the calcu-
lated molecular masses. Analysis of the dendrimers by gel-permeation

170
J. W. Lee et al.
FIGURE 2 GPC diagrams of dendrimers 3-Gm obtained from THF eluent.
chromatography (GPC) shows very low polydispersity values,
PDI ¼ 1.01 for all dendrimers (Fig. 2).
The UV-visible absorption spectra of 3-Gm and 2 in chloroform
solutions were investigated. Compound 2 exhibited the maximum
absorption at 316 nm. Dendrimers showed absorption maxima at
320, 319, 321, and 321 nm for 3-G1–G4, respectively. Linkage of fluorene
unit to the dendrimer showed red shift relative to the corresponding
bands for compound 2 which may be caused by the conjugation effect
FIGURE 3 (a) UV-vis spectra in CHCl3 (conc. ¼ 2.5 ꢃ 10ꢄ5 M) and (b) PL
spectra in CHCl3 (kex ¼ 310 nm, conc. ¼ 3.7 ꢃ 10ꢄ6 M) for 2 (black), 3-G1
(red), 3-G2 (green), 3-G3 (blue), and 3-G4 (cyan).

Poly(benzyl ether) Dendrimer Containing Fluorene as a Core Chromophore
171
FIGURE 4 (a) UV-vis spectra and (b) PL spectra in film for 3-G1 (black),
3-G2 (red), 3-G3 (green), and 3-G4 (blue).
according to the formation of the triazole rings. The photoluminescence
(PL) spectra of compound 2 and the dendrimers 3-Gm in chloroform
solutions were obtained (Fig. 3). Compound 2 showed no fluorescence
due to the quenching effect from the electron-rich a-nitrogen of the
azido group [10]. The maximum emission peaks of dendrimershowed
at 356 nm, 356, 358, and 358 nm for 3-G1–G4, respectively, due to
the elimination of the quenching through the formation of the triazole
ring. And these dendrimers have shoulder emission peaks at 341, 341,
342, and 342 nm, respectively. The photoluminescence (PL) spectra of
compound 2 and the dendrimers 3-Gm in the film state were investi-
gated (Fig. 4). Compound 2 showed no emission and the PL efficiencies
of the dendrimers 3-Gm are increased with increasing the generation
number. There is a red shift in the PL emission maxima for films
compared to solutions of the dendrimers. The maximum emission
peaks of dendrimers showed at 389, 383, 378, and 367 nm for 3-G1–
G4, respectively. Our results demonstrate that potentially lumi-
nescent dendrimers can be obtained by incorporating a profluorophore
unit with dendrons.
CONCLUSION
We have successfully synthesized a series of novel soluble fluorene-
based functional dendrimers by copper-catalyzed 1,3-dipolar click
chemistry between azide and alkyne. The profluorophore-diazide
building block, designed to serve as the core in dendrimer, was
´
stitched together with the propargyl-functionalized Frechet-type
poly(benzyl ether) dendrons leading to the formation of fluorescent

172
J. W. Lee et al.
´
Frechet-type poly(benzyl ether) dendrimers in high yields. This strat-
egy will lead to the convenient synthesis of other fluorescent nanoma-
terials with specific structures and properties in conjunction with
their applications in nanotechnology.
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