Synthesis and photoluminescence study of di-dendron dendrimers derived from mono-Boc-protected ethylenediamine cores

Article abstract of DOI:10.1002/bio.1223

This work is focused on the synthesis and optical properties of cone-shaped structural feature di-dendron polyamidoamine dendrimers up to the third generation with mono-Boc-protected ethylenediamine (EDA) as a core. Strong UV absorbance spectra and fluorescence spectra from di-dendron dendrimers with different terminal groups (-NH₂, -COOCH₃) were studied under different conditions by varying experimental parameters such as concentration and pH. The optical density and fluorescence intensities increased when di-dendron dendrimers generation number increased from 0.5 to 3.0. It was confirmed that the concentration of di-dendron dendrimers plays an important role in fluorescence intensity. The increase in fluorescence intensity was linear in low concentration regions, but the intensity increased slowly in high concentration regions. The results also showed a rapid increase in fluorescence intensity at low pH. The formation of a fluorescence-emitting moiety had a close relationship to protonated tertiary amine groups in di-dendron dendrimers derived from mono-Boc-protected EDA cores. Furthermore, the formation of fluorescent chemical species was irreversible. Copyright

Full text of DOI:10.1002/bio.1223

Research article
Received: 08 January 2010,
Revised: 06 April 2010,
Accepted: 19 May 2010,
Published online in Wiley Online Library: 16 July 2010
(wileyonlinelibrary.com) DOI 10.1002/bio.1223
Synthesis and photoluminescence study of
di-dendron dendrimers derived from
mono-Boc-protected ethylenediamine cores
Yili Zhao,a Sen Liu,b Wei Jiang,c Yulei Chang,a Yapeng Li,a* Xuexun Fang,b
Jingyuan Wanga
ABSTRACT: This work is focused on the synthesis and optical properties of cone-shaped structural feature di-dendron polya-
midoamine dendrimers up to the third generation with mono-Boc-protected ethylenediamine (EDA) as a core. Strong UV
absorbance spectra and fluorescence spectra from di-dendron dendrimers with different terminal groups (-NH2, -COOCH3)
were studied under different conditions by varying experimental parameters such as concentration and pH.The optical density
and fluorescence intensities increased when di-dendron dendrimers generation number increased from 0.5 to 3.0. It was
confirmed that the concentration of di-dendron dendrimers plays an important role in fluorescence intensity. The increase in
fluorescence intensity was linear in low concentration regions, but the intensity increased slowly in high concentration regions.
The results also showed a rapid increase in fluorescence intensity at low pH. The formation of a fluorescence-emitting moiety
had a close relationship to protonated tertiary amine groups in di-dendron dendrimers derived from mono-Boc-protected EDA
cores. Furthermore, the formation of fluorescent chemical species was irreversible. Copyright © 2010 John Wiley & Sons, Ltd.
Keywords: di-dendron dendrimers; absorbance spectra; fluorescence spectra; concentration; pH
observed from NH2-terminated PAMAM dendrimers (21,22),
Introduction
where the backbone of the dendrimer seemed to play a key role
There has been growing interest among researchers in dendritic
in forming the novel fluorescent center.The di-dendron dendrim-
polymers, including dendrimers (1–5), dendrons (6–9), dendri-
ers have the same backbone as the tetra-dendron PAMAM den-
grafts (10,11) and hyperbranched polymers (12,13). Dendrimers,
drimer, in which was also observed weak fluorescence emission.
with explicit symmetry and high density of functional terminal
To the best of our knowledge, this is the first demonstration of
groups, have applications in various fields. Typical are polyami-
the optical characteristics of di-dendron dendrimers with various
doamine (PAMAM) dendrimers (14), poly(propyleneimine) (PPI)
terminal groups such as amine, carboxyl and hydroxyl.
dendrimers (15) and poly(ethyleneimine) (PEI) dendrimers (16).
The properties of dendrimers, such as chemical structure (17),
solvent penetration (18,19), shape (20), optical characteristics
(21,22) and guest doping (23), have been reported.
Experimental
In comparison, the properties of dendrons, which is the term
used for a dendritic wedge without a core, have not been
Materials
Di-tert-butyl dicarbonate were purchased from Aldrich Chemical
reported. For stabilizing quantum dots as the surface ligands,
Co. and used as received. Ethylenediamine (EDA), methyl acrylate
dendrimers are not as ideal as dendrons (24–28). The cone-
(MA) and 2-aminoethanol were dried by calcium hydroxide over-
shaped structural feature and the single binding site of a
night, and then distilled under reduced pressure before
dendron ligand should provide better packing in the ligand
use. Methanol and CH2Cl2 were used directly without further
shell and an unambiguous orientation of the ligands on the
surface of quantum dots, respectively. There is some back-
purification.
ground fluorescence when dendrons are used as ligands for
coating quantum dots. Therefore it is important to study the
properties, especially the optical characteristics, of cone-shaped
dendrons.
* Correspondence to: Yapeng Li, Alan G. MacDiarmid Institute of Jilin Univer-
sity, Changchun 130023, China. liyapeng@jlu.edu.cn
In this work, di-dendron dendrimers were synthesized with
different methods (29,30) and their optical properties were also
examined.We selected a divergent procedure from a di-tert-butyl
a
Alan G. MacDiarmid Institute of Jilin University, Changchun 130023, China
dicarbonate protected ethylenediamine (Boc-EDA) through the
exhaustive Michael addition reaction and the exhaustive amida-
tion reaction, the same as for the synthesis of tetra-dendron
PAMAM dendrimer. Recently, fluorescence emission was
b
Key Laboratory for Molecular Enzymology and Engineering, the Ministry of
Education of Jilin University, Changchun 130021, China
c
Adhesive Technologies of Henkel Corporation, Shanghai 200131, China
Copyright © 2010 John Wiley & Sons, Ltd.
Luminescence 2011; 26: 264–270

Synthesis and photoluminescence study of di-dendron dendrimers
1
Synthesis of N-tert-butoxycarbonylaminoethylamine
Boc-G2.5: H NMR (500 MHz, CDCl3, 25°C, TMS): 1.42 (s, 9H,
3CH3), 2.36 (t, 12H, 6CH2CO), 2.45 (t, 16H, 8CH2CO), 2.50 (t, 6H,
3NHCH2CH2N>), 2.55 (t, 8H, 4NHCH2CH2N>), 2.77 (t, 28H, 14
<NCH2CH2CO), 3.28 (t, 12H, 6CH2NH), 3.44 (t, 2H, CH2NH), 3.68 (s,
28H, 8COOCH3). FT-IR (KBr): u (NH) = 3380 cm-1, u (s, CH2) =
2830 cm-1, u (as, CH2) = 2950 cm-1, u (C=O) = 1740 cm-1, u (C–N) =
1170 cm-1, u (CONH) = 1650, 1520 cm-1, u (C–O) = 1250 cm-1, d
(CH2) = 1440 cm-1.
A 4 g (0.018 mol) aliquot of di-tert-butyl dicarbonate dissolved in
CH2Cl2 (20 mL) was added dropwise to a solution of EDA (5 mL,
0.075 mol) in CH2Cl2 (40 mL) at 0°C over 3 h with stirring. The
mixture was stirred for 16 h at room temperature. When the reac-
tion was completed, the solvent was removed by vacuum distil-
lation, and the residue was washed with water. Then the white
deposition (di-substituted Boc-EDA-Boc) was filtered. NaCl was
added to the filtrate until the solution was saturated and mono-
Boc-protected EDA was extracted by ethyl acetate three times.
Colorless oil was obtained after ethyl acetate was removed by
vacuum distillation. CHCl3 was added and NaCl was removed by
filtration. Finally, CHCl3 was removed by vacuum distillation and
the residual product was dried in vacuum.
Synthesis of ‘full-generation’ di-dendron dendrimers with
amine-terminated groups (Boc-G1.0, Boc-G2.0 and
Boc-3.0). To a 250 mL round-bottomed flask fitted with a mag-
netic stirrer and pressure equalizing dropping funnel, containing
a stirred solution of EDA (0.750 mol, 50 mL) in methanol (60 mL)
was added a solution of Boc-G0.5 (0.015 mol, 4.996 g) in metha-
nol (40 mL) dropwise over 3 h in an ice/salt bath (-10–0°C). The
mixture was stirred at room temperature for 24 h.The solvent was
removed by rotary evaporation and the residual product dried in
vacuum for 24 h to give a yellowish oil Boc-G1.0. Boc-G2.0 and
Boc-G3.0 were synthesized using the same process.
1H NMR (500 MHz, CDCl3, 25°C, TMS): 1.49 (s, 2H, NH2), 1.45 (s,
9H, 3CH3), 2.80 (t, 2H, CH2NH2), 3.18 (t, 2H, CH2NH). FT-IR (KBr): u
(NH2) = 3434 cm-1, u (s, CH2) = 2860 cm-1, u(as, CH2) = 2930 cm-1,
u (C=O) = 1740 cm-1, u (C–N) = 1170 cm-1, u (C–O) = 1250 cm-1.
1
Boc-G1.0: H NMR (500 MHz, CDCl3, 25°C, TMS): 1.28 (s, 9H,
Synthesis of cone-shaped structural feature di-dendron
dendrimers {(core: mono-Boc-EDA); (G = 1–3); [dendri-
poly(amidoamine)-Z2-8 (PAMAM) dendrimers]}
3CH3), 2.20 (s, 4H, 2CH2NH2), 2.37 (t, 4H, 2CH2CO), 2.58 (t, 4H,
2CH2NH2), 2.65 (t, 4H, 2COCH2CH2N>), 2.75 (t, 2H, NHCH2CH2N>),
3.00 (t, 2H, CH2NH), 3.13 (t, 4H, 2CH2NH). FT-IR (KBr): u (NH2) =
3280, 3070 cm-1, u (s, CH2) = 2860 cm-1, u (as, CH2) = 2970 cm-1, u
(CONH) = 1650, 1550 cm-1, u (C–N) = 1170 cm-1, d (CH2) =
1450 cm-1.
The synthesis of cone-shaped structural feature di-dendron den-
drimers includes a two-step interactive sequence, with the same
methods as for tetra-dendron PAMAM dendrimers, to produce
either ester- or amine-terminated structures. Iterative sequenc-
ing involves alkylation with MA followed by amidation with
excess EDA. The alkylation step produces ester-terminated (sub-
shells) that are referred to as ‘half-generation’ di-dendron den-
drimers. The second step involved amidation of the ester
terminated intermediates with a large excess of EDA to produce
amine terminated, ‘full-generation’ di-dendron dendrimers. The
detail reaction is as follows.
1
Boc-G2.0: H NMR (500 MHz, CDCl3, 25°C, TMS): 1.43 (s, 9H,
3CH3), 1.64 (s, 8H, 4CH2NH2), 2.38 (t, 8H, 4CH2CO), 2.54 (t, 4H,
2CH2CO), 2.66 (t, 6H, 3NHCH2CH2N>), 2.75 (t, 12H, 6COCH2CH2N>),
2.83 (t, 8H,4CH2NH2), 3.16 (t, 2H, CH2NH), 3.28 (t, 12H, 6CH2NH).
FT-IR (KBr): u (NH2) = 3290, 3080 cm-1, u (s, CH2) = 2850 cm-1, u (as,
CH2) = 2950 cm-1, u (CONH) = 1640, 1550 cm-1, u (C–N) =
1160 cm-1, d (CH2) = 1450 cm-1.
1
Boc-G3.0: H NMR (500 MHz, CDCl3, 25°C, TMS): 1.42 (s, 9H,
3CH3), 1.65 (s, 16H, 8CH2NH2), 2.35 (t, 16H, 8CH2CO), 2.42
(t, 4H, 2CH2CO), 2.52 (t, 8H, 4CH2CO), 2.66 (t, 14H, 7NHCH2CH2N>),
2.75 (t, 28H, 14COCH2CH2N>), 2.81 (t, 16H, 8CH2NH2), 3.27 (t, 28H,
14CH2NH), 3.44 (t, 2H, CH2NH). FT-IR (KBr): u (NH2) = 3280,
3060 cm-1, u (s, CH2) = 2850 cm-1, u (as, CH2) = 2930 cm-1, u
(CONH) = 1650, 1550 cm-1, u (C–N) = 1140 cm-1, d (CH2) =
1450 cm-1.
Synthesis of ‘half-generation’ di-dendron dendrimers with
ester terminated groups (Boc-G0.5, Boc-G1.5 and
Boc-2.5). To a 250 mL round-bottomed flask cooled in an ice/
salt bath (-10–0°C), fitted with a pressure equalizing dropping
funnel, mechanically stirred, containing MA (0.442 mol, 40 mL) in
methanol (40 mL), a solution of Boc-EDA (0.030 mol, 4.767 g) in
methanol (50 mL) was added dropwise over 2 h with stirring.
Then the mixture was warmed to room temperature and stirred
overnight (24 h). The solvent was removed by rotary evaporation
and the residual product was dried in vacuum for 24 h to obtain
yellowish oil Boc-G0.5. Boc-G1.5 and Boc-G2.5 were synthesized
with the same process.
Synthesis of G3.0 hydroxyl-terminated di-dendron
dendrimers (Boc-G3.0-OH)
Boc-G2.5 was modified by one-step reactions with
2-aminoethanol to obtain Boc-G3.0-OH using a modified litera-
ture method (31,32). Typically, in a round-bottomed flask
equipped with dropping funnel and magnetic stirrer,
2-aminoethanol (0.668 mol, 40 mL) was dissolved in methanol
(100 mL), and then Boc-G2.5 (0.003 mol, 5.3 g) in methanol
(40 mL) was added dropwise under vigorous stirring at -5°C. The
mixture solution was stirred in the dark at room temperature for
48 h. After the reaction completed, the mixture was purified by
distillation under the vacuum at 40°C for 24 h.
1
Boc-G0.5: H NMR (500 MHz, CDCl3, 25°C, TMS): 1.45 (s, 9H,
3CH3), 2.44 (t, 4H, 2CH2CO), 2.53 (t, 2H, NHCH2CH2N>), 2.75 (t, 4H,
2 <NCH2CH2CO), 3.18 (t, 2H, CH2NH), 3.69 (s, 6H, 2COOCH3). FT-IR
(KBr): u (NH) = 3400 cm-1, u (s, CH2) = 2830 cm-1, u (as, CH2) =
2970 cm-1, u (C=O) = 1740 cm-1, u (C–N) = 1170 cm-1, u(C–O) =
1240 cm-1, d (CH2) = 1440 cm-1.
1
Boc-G1.5: H NMR (500 MHz, CDCl3, 25°C, TMS): 1.43 (s, 9H,
3CH3), 2.36 (t, 4H, 2CH2CO), 2.44 (t, 8H, 4CH2CO), 2.50 (t, 2H,
NHCH2CH2N>), 2.56 (t, 4H, 2NHCH2CH2N>), 2.77 (t, 12H, 6
<NCH2CH2CO), 3.19 (t, 2H, CH2NH), 3.31 (t, 4H, 2CH2NH), 3.67 (s,
12H, 4COOCH3). FT-IR (KBr): u (NH) = 3350 cm-1, u (s, CH2) =
2830 cm-1, u (as, CH2) = 2950 cm-1, u (C=O) = 1730 cm-1, u (C–N) =
1190 cm-1, u (CONH) = 1650, 1540 cm-1, u (C–O) = 1250 cm-1, d
(CH2) = 1430 cm-1.
1H NMR (500 MHz, D2O, 25°C, TMS): 1.4 (s, 9H, 3CH3), 2.5 (t, 28H,
14CH2CO), 4.1 (t, 32H, 16CH2OH), 2.9 (t, 90H, 45CH2N>), 3.3–3.7 (t,
32H, 16CH2NH). FT-IR (KBr): u (OH) = 3390 cm-1, u (NH2) =
3080 cm-1, u (s, CH2) = 2840 cm-1, u (as, CH2) = 2950 cm-1, u
(CONH) = 1650, 1540 cm-1, u (C–O) = 1250 cm-1, u (C–N) =
1150 cm-1, d (CH2) = 1450 cm-1.
Luminescence 2011; 26: 264–270
Copyright © 2010 John Wiley & Sons, Ltd.
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Y. Zhao et al.
Table 1. Molecular characteristics of di-dendron PAMAM dendrimers
Sample
Molecular
formula
Molecular
weight (Da)
Number of
terminal groups
Terminal group
Number of
tertiary amines
Yield (%)
Boc-EDA
Boc-G0.5
Boc-G1.0
Boc-G1.5
Boc-G2.0
Boc-G2.5
Boc-G3.0
C7H16N2O2
160
332
388
732
844
1
2
2
4
4
8
8
–NH2
–COOCH3
–NH2
–COOCH3
–NH2
–COOCH3
–NH2
0
1
1
3
3
7
7
92.3
83.3
89.6
91.2
99.5
99.3
99.6
C15H28N2O6
C17H36N6O4
C33H60N6O12
C37H76N14O8
C69H124N14O24
C77H156N30O16
1532
1756
Measurements
dendrimers. The comparative FT-IR spectra of G0.5 to G3.0 are
shown in Fig. 2. The spectra of half-generations (Boc-G0.5, Boc-
G1.5 and Boc-G2.5) with ester-terminated di-dendron PAMAM
dendrimers have similar bands. Figure 2(a) reveals methylene
stretching bands at 2950 and 2830 cm-1, a methylene bending
band at 1440 cm-1 and the bands at 1740 and 1250 cm-1 corre-
sponding to the stretching vibrations of the C=O group and C–O.
Additional bands at 3400 cm-1 was assigned to stretching vibra-
tions of –NH–, 1650 and 1500 cm-1 were assigned to stretching
vibrations of –CONH–, and 1170 cm-1 was assigned to stretching
vibrations of C–N. In full-generation spectra (Boc-G1.0, Boc-G2.0
and Boc-G3.0) with amine-terminated di-dendron PAMAM den-
drimers (Fig. 2b), there were also bands from methylene groups at
2930, 2850 and 1450 cm-1 and bands of –CONH– and C–N from
repeating units at 1640, 1550 and 1160 cm-1.The stretching vibra-
tions of the C=O group and C–O at 1740 and 1250 cm-1 were not
observed.The spectra of full-generation di-dendron PAMAM den-
drimers displayed amine stretching bands at 3280 and 3070 cm-1.
Measurements of NMR spectra were conducted with a Bruker
ARX-500 NMR spectrometer (Switzerland). The IR spectra of the
polymers were recorded on a Nicolet Impact 410 at room tem-
perature. Ultraviolet–visible absorption spectra of di-dendron
PAMAM dendrimers were measured by a Varian Cary 5000
UV–vis–NIR spectrophotometer. Photoluminescence (PL) spectra
of di-dendron dendrimers were recorded on a Jobin Yvon Spex
Fluorolog 3 fluorescence spectrophotometer.
Results and discussion
Synthesis of di-dendron PAMAM dendrimers
Di-dendron PAMAM dendrimers were synthesized using EDA as a
starting material. One amino group of EDA was protected by
di-tert-butyl dicarbonate, and then the other amino group suc-
cessively reacted with MA and EDA. On the growth step of gen-
eration, the Michael addition reaction was used for alkylation to
produce ester-terminated di-dendron dendrimers and followed
by exhaustive amidation reaction to produce amine-terminated
di-dendron dendrimers. The theoretical properties of di-dendron
dendrimers are listed in Table 1.
As the di-dendron dendrimers grow, the different compart-
ments of the dendritic structure begin to show distinct features
which are amplified with increasing generation. The di-dendron
dendrimers structure may be divided into three parts (Fig. 1): the
terminal groups, the repeating unit and the center of the
di-dendron dendrimers. The three parts of the dendrons can spe-
cifically be tailored towards a desired molecular property or func-
tion of the di-dendron dendrimers.
1
The comparative H NMR spectra of Boc-EDA, Boc-G0.5, Boc-
G1.0, Boc-G1.5, Boc-G2.0, Boc-G2.5 and Boc-G3.0 di-dendron
PAMAM dendrimers are given in the Supporting Information. A
spectrum of Boc-EDA displayed signals of –CH3 of a Boc– at
1.5 ppm, signals of –CH2NH2 and –CH2NH– of EDA at 2.8 and
3.2 ppm and signals of –NH– at 5.2 ppm. Similarly, 1H NMR spectra
of Boc-G0.5, Boc-G1.0, Boc-G1.5, Boc-G2.0, Boc-G2.5 and Boc-G3.0
di-dendron PAMAM dendrimers presented signals of –CH3 of a
Boc– at 1.5 ppm. The signals from –CH2NH– at 3.2 ppm, signals
from –CH2N> at 2.5–2.8 ppm and signals from –CH2COO– at 2.3–
2.5 ppm and signals at 2.7–2.9 ppm were ascribed to protons of
1
–CH2NH2. The results of IR and H NMR spectra confirmed the
successful synthesis of di-dendron PAMAM dendrimers.
Generation is common for all di-dendron dendrimers and the
hyperbranching when going from the center of the di-dendron
dendrimers towards the periphery, resulting in homo-structural
layers between the branching points. The number of branching
points when going from the center towards the di-dendron den-
drimers surface is the generation number (Fig. 1). That is, a
di-dendron dendrimers having three branching points when
going from the center to the periphery is denoted as the third-
generation di-dendron dendrimers. The centra’ part of the
di-dendron dendrimers is sometimes denoted generation ‘zero’,
or in the terminology presented here, ‘G0’. In di-dendron PAMAM
dendrimers, Boc-EDA represents the G0 center-structure. Inter-
mediates, ester-terminated di-dendron PAMAM dendrimers,
during the di-dendron dendrimers synthesis are denoted half-
generations.
Optical characterization
In our experiments, all solutions prepared were 0.001 mol/L in
distilled water. Prior to UV–vis spectra and fluorescence measure-
ment, solutions were kept at room temperature for 24 h. Figure 3
shows strong UV absorption spectra from di-dendron PAMAM
dendrimers with different terminal groups. From Boc-G0.5 to
Boc-G2.5 or from Boc-G1.0 to Boc-G3.0, the optical density (OD)
was increased. Figure 3 shows that the OD is similar between
Boc-G0.5 and Boc-G1.0, Boc-G1.5 and Boc-G2.0 or Boc-G2.5 and
Boc-G3.0. From Fig. 1, we can see that the backbones of Boc-G0.5
and Boc-G1.0, Boc-G1.5 and Boc-G2.0, Boc-G2.5 and Boc-G3.0 are
the same, the only difference being the terminal groups
(–COOCH3 for the half-generation, –NH2 for the full generation).
This observation suggests that the backbone of the di-dendron
PAMAM dendrimers rather than the terminal groups plays an
important role in the OD.
FT-IR spectroscopy is mainly used for the routine analysis of the
chemical transformations occurring at the surface of di-dendron
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Luminescence 2011; 26: 264–270

Synthesis and photoluminescence study of di-dendron dendrimers
Figure 1. Di-dendron PAMAM dendrimers with generation of shell depiction.
The class of di-dendron dendrimers, which are a half the tetra-
dendron dendrimers, has the same repeating units as tetra-
dendron dendrimers. Therefore, similar UV–vis absorption
spectra to tetra-dendron dendrimers were detected. Figure 4
shows the OD of di-dendron dendrimers to be significantly lower
than that of tetra-dendron dendrimers of the same generation. It
is because the steric hindrance of tetra-dendron dendrimers is
relatively larger than that of di-dendron dendrimers.
Figure 5 shows excitation and emission fluorescence spectra of
Boc-G0.5, Boc-G1.5 and Boc-G2.5 ester-terminated di-dendron
PAMAM dendrimers. All samples present one excitation band at
385 nm, and an emission band at 455 nm. Boc-G0.5, Boc-G1.5 and
Boc-G2.5 showed the same fluorescence bands in both excitation
spectra and emission spectra, but the intensities at the same
di-dendron dendrimers concentration (0.001 mol/L) increased as
the genenrations increased. These results indicated that the
di-dendron dendrimers structure or the microenvironment of the
functional group significantly influenced the fluorescence prop-
erties. It is known that lower-generation di-dendron PAMAM den-
drimers have an expanded or ‘open’ configuration, but as the
generation of di-dendron dendrimers grows, the terminal func-
tional groups become crowded and cause the di-dendron den-
drimers to take a densely packed structure.
there was no big difference in fluorescence intensity among the
di-dendron dendrimers with the same backbone and different
terminal groups (Fig. 6b). These results indicated that the forma-
tion of a fluorescence-emitting moiety had a close relation to the
backbone and the terminal groups, and the interiors rather than
the terminal groups played an important role in the fluorescence
intensity of di-dendron dendrimers.
From the classic viewpoint, the functional groups in
di-dendron dendrimers could not emit observed fluorescence.
Now, it is difficult to explain the ultimately novel fluorescence
phenomenon on the basis of our present experiments. Neverthe-
less, the phenomenon observed in the present work was highly
reproducible. We also tested a sample of Boc-G3-OH synthesized
in our laboratory (see the experiment section), and the obtained
fluorescence phenomena were similar to those from NH2-
terminated and COOCH3-terminated di-dendron dendrimers.
Figure 7 shows the emission intensity from aqueous solutions
of Boc-G3.0-OH di-dendron dendrimers with various di-dendron
dendrimer concentrations. The emission intensity increased
almost linearly in the low concentration regions, but in the high
concentration regions, the emission intensity increased slowly.
This result confirms that there is an effect of intermolecular inter-
action on the fluorescence properties of the tested di-dendron
dendrimers under the present experimental conditions. In
di-dendron dendrimers with flexible structures, the conforma-
tion is not only affected by small molecules like solvents, salts or
protons, but may also be sensitive to larger objects, such as other
di-dendron dendrimers or surfaces which have a great affect on
the molecular density and conformation of the di-dendron den-
drimers. The molecular conformation of di-dendron dendrimers
There was a small difference in fluorescence properties
between half-generation and full-generation di-dendron den-
drimers (Fig. 6). Importantly, it the fluorescent bands of full-
generation di-dendron dendrimers showed a strong emission
band at 430 nm, with the excitation band at 350 nm, which was
obviously different from half-generation di-dendron dendrimers
(Fig. 6a). Furthermore, under the same experimental conditions,
Luminescence 2011; 26: 264–270
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Y. Zhao et al.
Figure 4. UV–vis absorbance spectra of tetra-dendron dendrimers (G2.0 and G3.0)
and di-dendron dendrimers (Boc-G2.0 and Boc-G3.0).
Figure 2. IR spectra of di-dendron PAMAM dendrimers. (a) Half-generation (Boc-
G0.5, A; Boc-G1.5, B; and Boc-G2.5, C) with ester-terminated di-dendron dendrimers.
(b) Full-generation (Boc-G1.0, A1; Boc-G2.0, B1; and Boc-G3.0, C1) with ester-
terminated di-dendron dendrimers.
Figure 5. Fluorescence spectra of G0.5, G1.5 and G2.5 ester-terminated
di-dendron PAMAM dendrimers. (aex) Excitation spectra emitted at 455 nm of G0.5.
(aem) Emission spectra excited at 385 nm of G0.5. (bex) Excitation spectra emitted at
455 nm of G1.5. (bem) Emission spectra excited at 385 nm of G1.5. (cex) Excitation
spectra emitted at 455 nm of G2.5. (cem) Emission spectra excited at 385 nm of G2.5.
6 to 12, the emission intensity was very weak and had little
change. However, as the pH was lowered further, fluorescence
intensity displayed a rapid increase, which reached maximum
value at about pH 2.5, although the emission band position
slightly changed. The pH-dependent fluorescence properties can
be explained on the basis of the molecular dynamics to predict
the structural behavior of di-dendron PAMAM dendrimers as a
function of pH.
It is noteworthy that pH 6 correlates well with the pKa values
(33,34) of tertiary amines in di-dendron dendrimers, which is the
critical point in Fig. 8. For di-dendron dendrimers with interiors
containing tertiary amines, the low pH region (below pH 6) gen-
erally leads to extended conformations due to electrostatic repul-
sion between the positively charged ammonium groups. The
strength of the hydrogen bond in the di-dendron dendrimer is
enhanced under acidic conditions (35), and the strong charge–
charge repulsion makes the structure of the di-dendron PAMAM
dendrimer more rigid (36,37). It is reasonable to assume that the
formation of new fluorescent chemical species by the chemical
reaction of functional groups along di-dendron dendrimers
branches takes place under acidic conditions (38). Furthermore, if
the new fluorescent chemical species is formed by the chemical
reaction, the fluorescent properties cannot change when the pH
is altered after the fluorescent chemical species are formed. Two
methods were used to prove this assumption. The first was that
Figure 3. UV–vis spectra of di-dendron PAMAM dendrimers.
becomes increasingly contracted upon increasing concentration.
This molecular contraction may minimize the repulsive forces
between the di-dendron dendrimers molecules and increase the
ability of the di-dendron dendrimers to exhibit a tighter intermo-
lecular packing.
G2.5 di-dendron dendrimers with ester-terminal and G3.0
di-dendron dendrimers with amine-terminal showed a signifi-
cant pH-dependent profile of fluorescent intensity in the pH
range from 1 to 12 (as shown in Fig. 8). Within the pH range from
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Copyright © 2010 John Wiley & Sons, Ltd.
Luminescence 2011; 26: 264–270

Synthesis and photoluminescence study of di-dendron dendrimers
Figure 8. pH-dependent fluorescence emission intensities in the pH range from 1
to 12. (a) G2.5 estsr-terminated di-dendron dendrimers; (b) G3.0 NH2-terminated
di-dendron dendrimers.
Figure 6. The generation-dependent fluorescence bands of di-dendron PAMAM
dendrimers (a) and the generation-dependent fluorescence intensity of di-dendron
PAMAM dendrimers (b).
Figure 9. pH-dependent fluorescence intensities. (aex) Excitation spectra of G2.5
estsr-terminated di-dendron dendrimers directly adjust pH = 8; (bex) excitation
spectra of G2.5 estsr-terminated di-dendron dendrimers adjusted to pH = 8 with
the initial solution at pH 2; (aem) emission spectra of of G2.5 estsr-terminated
di-dendron dendrimers directly adjusted to pH = 8; (bem) emission spectra of G2.5
estsr-terminated di-dendron dendrimers adjusted to pH = 8 with the initial solu-
tion at pH 2.
Conclusion
In conclusion, we synthesized di-dendron PAMAM dendrimers
with different generations according to a procedure similar to the
synthesis of di-dendron dendrimers, and reported on the optical
properties of di-dendron dendrimers. Some groups have synthe-
sized the di-dendron dendrimers with different methods;
however, no report on the optical properties of di-dendron den-
drimers could be found in the literature. In this work, fluores-
cence emission was observed from different terminal groups of
di-dendron dendrimers under the same conditions. There was a
small difference in fluorescence properties between NH2-
terminated di-dendron dendrimers and COOCH3-terminated
di-dendron dendrimers. From the results, it can be assumed that
the backbone of the di-dendron dendrimers playw a key role in
the formation of novel fluorescent centers.
Figure 7. Emission intensity of the OH-terminal third generation of di-dendron
PAMAM dendrimers (Boc-G3.0-OH) with changing the concentration of solution
from 0.1 to 10 mmol/L.
G2.5 di-dendron dendrimers were dissolved in water and the
solution was adjusted to pH 2, and then sodium hydroxide was
used to transform the solution to pH = 8. The second was that the
solution of G2.5 di-dendron dendrimers was directly adjusted to
pH = 8 by sodium hydroxide. The fluorescence excitation spectra
and emission spectra were monitored. As shown in Fig. 9, at the
same pH, excitation and emission intensities of di-dendron den-
drimers solutions obtained by the first method were obviously
stronger than the solution obtained by the second method. That
is, the formation of fluorescent chemical species was irreversible.
Supporting information
1H NMR spectra of di-dendron dendrimers are supplied in the
Supporting Information, which can be found in the online
version of this article.
Luminescence 2011; 26: 264–270
Copyright © 2010 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/luminescence

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Acknowledgements
This work was supported by the Natural Science Foundation of
China (no. 20574028), the Natural Science Foundation of China
(no. 20904014), the Youth Foundation of Jilin Province (no.
20080119) and the Graduate Innovation Program of Jilin Univer-
sity (no. 20091010).
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Luminescence 2011; 26: 264–270