Synthesis of novel triarylamine-based dendrimers with N4,N 6-dibutyl-1,3,5-triazine-4,6-diamine probe for electron/energy transfers in H-bonded donor-acceptor-donor triads and as efficient Cu 2+ sensors

Article abstract of DOI:10.1039/c2jm00112h

Two novel highly soluble triarylamine dendrimers TPAD1 and TPAD2 with N⁴,N⁶-dibutyl-1,3,5-triazine-4,6-diamine probe were synthesized via normal synthetic routes. Both dendrimers (TPAD1 and TPAD2) form H-bonded donor-acceptor-donor (D-A-D) supramolecular triads TPAD1-PBI-TPAD1 and TPAD2-PBI-TPAD2 with 3,4,9,10-perylene tetra carboxylic diimide derivative (PBI). The presence of multiple H-bonds in the solution state was elucidated by ¹H NMR titrations and IR spectral studies. J-aggregations and electron/energy transfers provided by both dendrimers were verified by UV-Vis and photoluminescence (PL) titrations with PBI and the particle sizes of supramolecular triads were calculated by X-ray diffraction (XRD) analysis. Similarly, both dendrimers also showed sensitivities towards Cu²⁺ in comparison with 19 interfering metal ions, which were evidenced via UV-Vis and PL titraions in both single and dual metal systems. The maximum detection limit of Cu²⁺ ions was determined to be 20 ppm from PL titrations for both dendrimers, and the 1:2 stoichiometry of the complexes formed by both dendrimers (TPAD1-Cu²⁺ and TPAD2-Cu²⁺) were calculated by Job plots based on UV-Vis absorption titrations. More importantly, the binding mechanism of the 1,3,5-triazine-4,6-diamine probe of both dendrimers was well characterized by ¹H and ¹³C NMR titrations ([D ₈]THF:D₂O = 2:1 in vol.) and supported by the fluorescence reversibility by adding metal ions and PMDTA sequentially.

Full text of DOI:10.1039/c2jm00112h

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Materials Chemistry
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PAPER
4
6
Synthesis of novel triarylamine-based dendrimers with N ,N -dibutyl-1,3,5-
triazine-4,6-diamine probe for electron/energy transfers in H-bonded
2+
donor–acceptor–donor triads and as efficient Cu sensors†
Muthaiah Shellaiah, Yesudoss Christu Rajan and Hong-Cheu Lin*
Received 6th January 2012, Accepted 5th March 2012
DOI: 10.1039/c2jm00112h
4
6
Two novel highly soluble triarylamine dendrimers TPAD1 and TPAD2 with N ,N -dibutyl-1,3,5-
triazine-4,6-diamine probe were synthesized via normal synthetic routes. Both dendrimers (TPAD1 and
TPAD2) form H-bonded donor–acceptor–donor (D–A–D) supramolecular triads TPAD1-PBI-
TPAD1 and TPAD2-PBI-TPAD2 with 3,4,9,10-perylene tetra carboxylic diimide derivative (PBI). The
1
presence of multiple H-bonds in the solution state was elucidated by H NMR titrations and IR spectral
studies. J-aggregations and electron/energy transfers provided by both dendrimers were verified by
UV–Vis and photoluminescence (PL) titrations with PBI and the particle sizes of supramolecular triads
were calculated by X-ray diffraction (XRD) analysis. Similarly, both dendrimers also showed
2+
sensitivities towards Cu in comparison with 19 interfering metal ions, which were evidenced via UV–
2+
Vis and PL titraions in both single and dual metal systems. The maximum detection limit of Cu ions
was determined to be 20 ppm from PL titrations for both dendrimers, and the 1 : 2 stoichiometry of the
2+
2+
complexes formed by both dendrimers (TPAD1-Cu and TPAD2-Cu ) were calculated by Job plots
based on UV–Vis absorption titrations. More importantly, the binding mechanism of the 1,3,5-triazine-
1 13
,6-diamine probe of both dendrimers was well characterized by H and C NMR titrations ([D
4
8
]
THF : D
2
O ¼ 2 : 1 in vol.) and supported by the fluorescence reversibility by adding metal ions and
PMDTA sequentially.
Introduction
derived from non-covalent interactions such as hydrogen bonds,
van der Waals forces, p–p stackings, and dipole–dipole inter-
6
actions. Among these interactions, hydrogen bonds are more
Due to their highly branched and symmetric structures, den-
drimers have attracted great attention in the past few decades,
which can be utilized for various applications including guest–
specific and highly directional in both solutions and surfaces, so
they are widely used in the construction of three-dimensional
supramolecular architectures in both chemical and biological
1
2
3
host chemistry, analytical chemistry, optical data storage and
4
medical applications. In addition, dendrimers have been
7
systems. Supramolecular controls over dye arrangements are
designed as light-harvesting (LH) antennas in artificial photo-
synthesis because of their capability of directional excitation
5
energy transfer (EET). It is easy to imagine that dendrimers are
important for both improving the performance of existing
optoelectronic devices and creating new dye-based materials.
Perylene bisimides (PBI) are widely applied as red dyes for
industrial purposes owing to their favorable chemical, light and
also interesting scaffolds for photoinduced charge transfer from
the core to the periphery, the number of units from the core to the
periphery is at least doubled with each higher generation. At the
same time, the supramolecular concept also attracted increasing
attention in macromolecular science, as one can design new
supramolecular architectures with interesting morphologies
8
thermal stabilities. In addition, several reports reveal that per-
ylene bisimide forms H and J type aggregations with various
9
triazine based donors.
In a similar manner, chemosensors based on ion-induced
fluorescence changes become increasingly popular due to their
10
relative simplicity, sensitivity and selectivity. Among the
2+
metals, Cu is a significant metal pollutant due to its widespread
12
Department of Materials Science and Engineering, National Chiao Tung
University, Hsinchu 30049, Taiwan, ROC. E-mail: linhc@mail.nctu.edu.
tw; Fax: +8863-5724727; Tel: +8863-5712121 ext.55305
11
use, but it is also required as a cofactor in nearly 20 enzymes
and an essential micro-nutrient for all known life forms. In
addition, most of the copper-selective sensors suffer from the
†
characterizations of dendrimers, H NMR titrations of supramolecular
Electronic supplementary information (ESI) available: Synthesis and
1
13
2+
2+
2+
3+
interfering effect of cations, such as Zn , Hg , Pb , Fe and
1
13
triads and H and C NMR titrations of sensor response, UV–Vis and
PL spectra of dendrimers and sensor reversibilities. See DOI:
+
Ag . Therefore, the development of a highly selective fluores-
14
cence probes for copper ions in the presence of a variety of
10.1039/c2jm00112h
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3
2,33
other metal ions has attracted great interest. Fluorescence
the literature,
and compounds 9 and 11 were obtained by
2+
sensors of Cu ions are utilized to clarify the physiological
reacting compound 8 with compounds 5 and 10, respectively, to
reflux in toluene for 72 h in the presence of catalytic amounts
of tri-tert-butylphosphine, palladium acetate, and cesium
carbonate. Dendrons 10 and 12 were obtained from compounds
15
roles in vivo as well as to monitor their concentrations in the
16
metal-contaminated sources due to the high detection sensi-
tivity and intrinsic operation simplicity of fluorescence sensors.
Since the sulfur and nitrogen donor atoms can be coordinated
with transition-metal ions to form metal complexes, small size
33
9 and 11, respectively, by Claisen’s base treatments. Then,
compounds 10 and 12 were reacted with cyanuric chloride 13 in
1
7
18
19
ꢀ
thiacrown ethers, non-cyclic neutral ionophores, calixarenes
THF at 0 C in the presence of catalytic amounts of diisopro-
20
and Schiff bases have been used for ion carrier sensor appli-
pylethylamine (DIPEA). After this reaction, the crude product
was refluxed with butylamine, using sodium bicarbonate as
a catalyst and dioxane as a solvent to provide the final den-
drimers (TPAD1 and TPAD2) with high purities.
21
cations. There are many polymeric materials reported to be
utilized as transition metal sensors, and also very recently non-
conjugated dendritic molecules, such as poly(amidoamine) and
poly(propylenimine) dendrimers, are developed as metal ion
22
sensors. However, to the best of our knowledge there is still
(a) Formation of (D–A–D) supramolecular triads
only a few reports available in the literature for the sensor
IR spectra. As shown in Fig. 2, the attenuated total reflection
Fourier transform infrared (ATR-FTIR) spectra of TPAD1,
23
applications of conjugated dendritic molecules via energy
transfers from fluorescence sensors (to be quenched) to metal
ions. The energy transfers in fluorescent dendrimers prevailed in
2 2
TPAD2 and PBI in CH Cl showed free NH stretching bands at
ꢁ1
3428, 3410 and 3170 cm , respectively. The NH stretching bands
2
4
nano-structural materials.
Triarylamine dendrimers have
of supramolecular triads TPAD1-PBI-TPAD1 and TPAD2-PBI-
25
important roles in the applications of organic LEDs and
2 2
TPAD2 (2 : 1 stoichiometry) in CH Cl were observed at 3317
26
ꢁ1
organic solar cells, because of their excellent thermal and
electrochemical stabilities, electron donating and charge-trans-
porting abilities, and transparent properties, as well as three-
and 3318 cm , respectively, which confirmed the formation of
multiple hydrogen bonds between the H-bonded donors and
34
acceptors.
27
28
dimensional propeller structures. The triazine core derivatives
also enhance energy transfer processes in various supramolecular
architectures, and the greater affinities of the triazine units
demonstrate that they may act as useful spacers as well as
selective receptors for sensors and supramolecules.
1
H NMR titrations. To reconfirm the presence of multiple
1
hydrogen bonds in triad complexes, we performed the H NMR
titrations as shown in Fig. S21 and S22 (ESI),† in which the PBI
concentration was kept constant at 10 mM and the concentra-
tions of dendrimers TPAD1 and TPAD2 increased up to 2.5
equivalents with an equal span of 0.5 equivalent. When
increasing the cocentrations from 0 to 2.5 equivalents, the NH
protons of PBI showed downfield shifts in both supramolecular
triads from 8.44 to 9.66 ppm and from 8.44 to 9.75 ppm for
TPAD1-PBI-TPAD1 and TPAD2-PBI-TPAD2, respectively.
The down-field shifts of PBI-NH protons were also evidence for
the formation of multiple hydrogen bonds in the supramolecular
29
Literature reports of triarylamine derivatives containing
conjugated structures also proved to be excellent fluorescence
sources, and hence provide enough flexibility to construct novel
fluorescent triarylamine dendritic moieties with a triazine core
for energy transfer in supramolecules as well as for metal sensor
applications.
Herein, we report the synthesis of novel triarylamine-based
4
dendrimers TPAD1 and TPAD2 with a N ,N -dibutyl-1,3,5-
6
30
35
triazine-4,6-diamine probe for efficient electron/energy trans-
fers in perylene bisimide (PBI)-based donor–acceptor–donor
H-bonded nano-sized J-aggregation of supramolecular triads.
For the first time, their sensing capabilities towards various metal
ions were investigated by UV–Vis and fluorescence spectros-
copies. Both dendrimers TPAD1 and TPAD2 show high selec-
complexes as per the literatures. Fig. 3 illustrated the changes in
the chemical shifts of NH (PBI) with respect to the concentra-
tions of both dendrimers (TPAD1 and TPAD2). During these
titrations, some up-field shifts in triazine NH protons of TPAD1
and TPAD2 were found, but the triazine NH protons were not
distinguishable because of the greater crowding in their alkyl
chain regions. The presence of multiple hydrogen bonds and
energy transfers as well as aggregation types were well estab-
lished by UV/PL titrations in low polarity solvents, such as
methylcyclohexane (MCH), which is shown in the next section.
2+
tivity towards Cu ions via the turn-off fluorescence mechanism
Fig. 1) in the presence of other transition and non-transition
(
metal ions in semi-aqueous media (THF : H O ¼ 2 : 1 in vol.) at
2
ꢀ
pH ¼ 7.0 (at 25 C).
UV–Vis and PL titrations. UV–Vis and PL titration experi-
ments in methylcyclohexane (MCH) were performed to study the
interactions of both dendrimers (TPAD1 and TPAD2) with PBI.
Upon adding aliquots of TPAD1 to PBI, a bathochromic shift
(Fig. 4a) from 562 to 567 nm was observed in the perylene
Results and discussion
Synthesis
The 3,4,9,10-perylene tetracarboxylic diimide derivative (PBI)
31
was synthesized according to the previous literature with
required purity and the synthetic procedures of dendrimers
TPAD1 and TPAD2 were shown in Schemes 1–3, where the
introduction of hexyloxy groups in compounds 2 and 4 provides
good solubilities to the dendritic molecules. Compounds 2 and 4
reacted in the presence of L-proline and CuI as catalysts to afford
compound 5. Compounds 7 and 8 were synthesized according to
0 1
absorption maximum of the S –S electronic transition for
TPAD1-PBI-TPAD1. However, this shift was larger in TPAD2-
PBI-TPAD2, i.e., from 562 to 570 nm (Fig. 4b). Similar to the
titrations of absorption spectra, the PL titrations of fluorescence
spectra in both supramolecular triads (Fig. 4c and 4d) showed
bathochromic shifts in TPAD1-PBI-TPAD1 (from 591 nm to
596 nm) and TPAD2-PBI-TPAD2 (from 591 nm to 599 nm).
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Fig. 1 Structures of triarylamine-based dendrimers (TPAD1 and TPAD2): (a) schematic representation of electron/energy transfer in supramolecular
2+
triads of PBI with TPAD1 and TPAD2; (b) schematic representation of Cu sensor responses provided by TPAD1 and TPAD2.
Both results of UV–Vis and PL titrations verified that the per-
ylenes pack tightly in a J-type aggregation in supramolecular
triads. Ordinary J-aggregated dyes showed intense fluorescence
during the titration of supramolecular formation. Here the PL
energy/electron transfers by multiple H-bonded complexes in
MCH solutions. The titration analysis of both supramolecular
triads (TPAD1-PBI-TPAD1 and TPAD2-PBI-TPAD2) in Fig. 4
showed the formation of 2 : 1 H-bonded triads as reported
36
titrations of the perylene was also enhanced (lex ¼ 575 nm, l
¼
previously. Since the formation of multiple H-bonds in the
em
5
91 nm) by adding both dendrimers (TPAD1 and TPAD2). This
supramolecular triads were well supported by UV–Vis, PL and
1
H NMR titrations along with ATR-FTIR spectra, further
consequence was rationalized by an electron/energy-transfer
process from the dendrimers (TPAD1 and TPAD2) to the
perylene chromophore (PBI), and the optimal enhanced fluo-
rescence was obtained as PBI was fully H-bonded to TPAD1 (or
TPAD2) at a ratio of 1 : 2.
investigations of particle size of both supramolecular triads were
carried out via X-ray diffraction (XRD) analysis of powder
samples on glass substrates.
The electron/energy transfers as well as the presence of
J-aggregation in both supramolecular triads were well demon-
strated by UV–Vis and PL titration experiments. During UV–Vis
titrations, dendrimers TPAD1 and TPAD2 showed isosbestic
points at 368 and 398 nm, respectively, which arose from the
X-ray diffraction (XRD) analysis. As evidenced in Fig. 5, the
powder XRD patterns of TPAD1, TPAD2, PBI and supramo-
lecular triads (see the experimental procedure in the ESI† for the
preparation of triads TPAD1-PBI-TPAD1 and TPAD2-PBI-
TPAD2) were taken on glass substrates at room temperature.
ꢀ ꢀ
2 2 3 2 2 3 2 5 2 3
Scheme 1 (a) Me CO/K CO , 90%; (b) (i) Me CO/K CO , (ii) C H OH: HCl (6 : 4), 100 C, 92%; (c) DMSO/K CO , CuI, L-proline, 90 C,
24–30 h, 84%.
8
978 | J. Mater. Chem., 2012, 22, 8976–8987
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ꢀ
ꢀ ꢀ
2 2 2 2 3 2 3
, CH Cl , 40 C, 90%; (c) Pd(OAc) , P(t-Bu) , Cs CO , toluene, 110 C, 72h, 72%/64%
Scheme 2 (a) Benzoyl chloride, pyridine, 0 C, 92%; (b) Br
9/11); (d) Claisen’s base, THF–H O, reflux, 12 h, 99% (10 and 12).
(
2
The donors (TPAD1 and TPAD2) and acceptor (PBI) showed
strong Bragg diffractions, but significantly decreased in intensi-
ties with broader peaks after the formation of multiple H-bonds
between dendrimers and PBI. The XRD peak values of supra-
peak and Bragg diffraction angle, respectively, at room temper-
ature. The diffraction angles (q), FWHM values and their
calculated lattice d spacings of supramolecular triads (TPAD1-
PBI-TPAD1 and TPAD2-PBI-TPAD2) are revealed in Table 1.
The average particle sizes of TPAD1-PBI-TPAD1 and TPAD2-
ꢀ
ꢀ
molecular triads were 17.60 and 25.69 for TPAD1-PBI-TPAD1
ꢀ
ꢀ
ꢀ
PBI-TPAD2 are 9.72 and 10.45 A, respectively. Attempts to
and 15.58 and 25.09 for TPAD2-PBI-TPAD2. The particle
sizes of supramolecular triads TPAD1-PBI-TPAD1 and TPAD2-
observe the self-assemblies of microscopic images and to measure
the photocurrent efficiencies of thin film transistors for both
supramolecular triads were in vain due to their poor film
qualities.
37
PBI-TPAD2 were calculated using the Scherrer formula.
Particle size (D) ¼ 0.9l/bcosq, where l, b and q represent the X-
ꢀ
ray wavelength (1.5406 A), FWHM (radian) value of diffraction
ꢀ
ꢀ
Scheme 3 (a) (i) Diisopropyl ethylamine, THF, 0 C, (ii) BuNH
2
, NaHCO
3
, dioxane, 100 C, 85%/78% (TPAD1/TPAD2).
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as THF : water ¼ 4 : 1 (vol.) for the sensor titrations of both
dendrimers (TPAD1 and TPAD2), and the final solvent
composition was maintained as THF : water ¼ 2 : 1 (vol.) after
titrations for both dendrimers. The quantum yields (Table 2) of
both dendrimers also showed that the above solvent system
was highly suitable for the sensor system. Hence, we performed
UV–Vis/PL titrations of both dendrimers in THF : H
2
O (4 : 1 in
] THF by adding metal
2
O, respectively.
1
13
vol.) and H, C NMR titrations in [D
ions in pure H O and D
8
2
Fluorescence titrations on metal ions. To prove the specific
selectivity of this dendritic sensory system, almost 20 metal ions
from their respective water soluble sources were chosen and the
titrations performed as shown in Fig. 6. Upon addition of 2
equiv. of aqueous solutions (40 mM in H
ions to the solutions (20 mM in THF : H
dendrimers TPAD1 and TPAD2, emission maxima at 398 and
27 nm (with an excitation at 308 and 314 nm) were seen,
2
O) with different metal
2
O ¼ 4 : 1 in vol.) of
4
2+
respectively. It shows that Cu has a relatively higher fluores-
cence quenching capability among the coexisting transition
+
+
2+
2+
2+
d-block) metal ions (such as Cu , Ag , Co , Fe , Mn , Zn ,
2+
(
2+
2+
Ni , Hg , Ag and Fe ), and non transition (s and p blocks)
2+
metal ions (such as Li , Na , K , Cs , Ba , Ca , Pb , Mg and
2+
3+
+
+
+
+
2+
2+
2+
3+
Al ). The above observations also support that the sensitivities
2+
of both dendrimers to Cu ions were much better even in the
2+
Fig. 2 ATR-FTIR spectra of (D–A–D) supramolecular triads in
3
+
2+
presence of interfering metal ions, such as Fe , Zn , Hg , Pb
2+
CH
2
Cl
2
with 2 : 1 ratio of (a) TPAD1 and PBI; (b) TPAD2 and PBI.
+
and Ag . However, TPAD1 has a slightly lower selectivity
2+
towards Cu ions than TPAD2, which was clearly revealed by
the lower fluorescence intensity of TPAD1 with a shorter
dendritic conjugation length.
The better sensing capabilities of TPAD1 and TPAD2 toward
2+
Cu ions were also verified in Fig. 7, where dendrimer TPAD2
shows three times higher sensitivity than TPAD1, because of its
38
higher generation structure and hence greater energy transfer in
contrast to TPAD1. This can be explained by the fact that as the
dendritic generation increases, the p–p conjugation increases to
improve the electron or energy transfer. Selectivities of both
2+
dendrimers (TPAD1 and TPAD2) towards Cu ions possibly
39
arise from the ligand to metal electron/energy transfer but it
might be essential to estimate the concentration limits of copper
ion detections by these dendritic molecules, which can be per-
formed by the UV–Vis/PL titrations. Moreover, the binding site
1
Fig. 3 H NMR shifts of N–H protons (in PBI) for supramolecular
triads formed by complexation of PBI with TPAD1 and TPAD2,
respectively, where the PBI concentration was fixed at 10 mM.
2+
of the present sensor dendritic system with Cu ions was
confirmed through the evidence of NMR spectral titrations, and
the reversibility of binding behaviour can be detected by the UV–
Vis and fluorescence titrations. Consequentially, the stoichiom-
etry of the metal complexes can be proven from the Job plots of
UV–Vis absorbance titrations for dendrimers TPAD1 and
TPAD2.
2+
(
b) Dendrimers as Cu sensors
Optical properties. Both dendrimers TPAD1 and TPAD2
exhibited absorption maxima (see Fig. S27 in ESI†) at 308 and
3
14 nm, respectively. The little red-shifted and broadening
absorption peak of TPAD2 were due to the extended conjugation
in the higher generation of the dendritic structure in contrast to
TPAD1. As shown in Fig. S28 of the ESI,† both dendrimers
TPAD1 and TPAD2 provided PL emission maxima at 398 and
2
+
Fluorescence titrations of Cu
ions. By increasing the
2+
concentrations of Cu (0–24 mM with an equal span of 2 mM in
2+
2
H O), the sensitivities of TPAD1 and TPAD2 towards Cu ions
4
27 nm, respectively, and their photophysical properties are
were clearly observed in Fig. 8. The fluorescence spectra of
dendrimers TPAD1 and TPAD2 were quenched rapidly, and
Fig. 8c and 8d clearly illustrate that their quenching effects were
shown in Table 2.
Both dendrimers showed great solubilities in THF : water ¼
2+
2
: 1, but the water content was increased further by adding metal
saturated at the concentration of 20 mM for Cu ions. The
saturation points identified here represent the complete
quenching of both dendrimers and hence the maximum detection
ion solutions (in water) to the dendrimer solutions (in
THF : water ¼ 2 : 1). Therefore, we chose the solvent condition
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Fig. 4 UV–Vis (a), (b), and PL (c), (d) titrations of TPAD1-PBI-TPAD1, and TPAD2-PBI-TPAD2 in methyl cyclohexane (MCH) (the concentration
of PBI was kept constant as 1 equivalent at 1 ꢂ 10ꢁ mol L ) upon the addition of TPAD1 and TPAD2 at different concentrations (0–2.4 equivalents
5
ꢁ1
with equal span of 0.2 equivalents).
Table 1 Diffraction angles (2q), FWHM values, lattice spacings and
particle sizes of TPAD1-PBI-TPAD1 and TPAD2-PBI-TPAD2
TPAD1-PBI-
TPAD1
TPAD2-PBI-
TPAD2
H-bonded complex
2q
1
2q
2
2q
1
2q
2
ꢀ
2q/
17.60
7.60
0.133
6.82
10.57
25.69
9.09
0.159
3.85
8.87
15.58
7.20
0.126
7.62
11.13
25.09
8.32
0.145
4.35
9.78
ꢀ
FWHM/
FWHM/radians
ꢀ
d spacing/A
ꢀ
Particle size/A
ꢀ
Average particle size/A
9.72
10.45
Table 2 Photophysical properties of TPAD1 and TPAD2
Compound
lmax/nm
lem/nm
F
a
b
c
a
b
c
TPAD1
308
314
398
427
0.273
0
0
.231
.253
TPAD2
0.481
0
0
.425
.469
a
b
THF. THF : water (2 : 1). THF : water (4 : 1) and 2-aminopyridine
c
Fig. 5 XRD patterns of compounds. (a) PBI, TPAD1 and TPAD1-PBI-
TPAD; (b) PBI, TPAD2 and TPAD2-PBI-TPAD2 on glass substrates at
room temperature.
in 0.1 M H
2
SO
4
(F ¼ 0.6) was used as a standard reference.
2
+
2+
aqueous solutions of Cu and other metal ions (20 mM + 20 mM)
2+
were combined, as well as the mixed aqueous solution of Cu
limit of Cu ions was justified as 20 ppm, as reported in the
2+
literature. The above statement was further confirmed via Cu
40
ion titrations in the presence of different background metal ions
as illustrated in Fig. 9. In this case, two equal amounts of
and all other metal ions (20 mM + 20 mM). Therefore, the
2+
sensitivities of TPAD1 and TPAD2 towards Cu ions can be well
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Fig. 7 Fluorescence responses of (a) TPAD1 (20 mM in THF : H
: 1 in vol.) and (b) TPAD2 (20 mM in THF : H
O ¼ 4 : 1 in vol.) in the
absence and presence of different metal ions (40 mM in H O) with exci-
tations at 308 and 314 nm, respectively. The final solvent composition
after titrations was THF : H
O ¼ 2 : 1 in vol.
2
O ¼
Fig. 6 Fluorescence spectra of (a) TPAD1 (20 mM in THF : H
in vol.) and (b) TPAD2 (20 mM in THF : H
O ¼ 4 : 1 in vol.) in the
absence and presence of different metal ions (40 mM in H O) with exci-
tations at 308 and 314 nm, respectively. The final solvent composition
after titrations was THF : H
2
O ¼ 4 : 1
2
2
2
2
2
2
2
O ¼ 2 : 1 in vol. (due to similar fluorescence
2+
intensities for all metal ions other than Cu , the average values were
taken for simplicity).
shifted from 3.28 to 3.17 ppm and from 3.27 to 3.16 ppm for
1
TPAD1 and TPAD2, respectively. This H NMR spectral
demonstrated, which explains the sensing abilities of TPAD1 and
TPAD2 in the presence of different metal ion backgrounds. In
both cases of TPAD1 and TPAD2, the sensitivities were little
affected by the presence of the other metal ions, but TPAD2
shows a greater sensitivity than TPAD1 because of the higher
generation and stability in TPAD2. This result was also well
supported by Stern–Volmer constants (KSV) shown in Table 3.
evidence also clearly signifies that the diamine probe attached to
the triazine core is involved in the detection of copper ions, and
justifies that triazine with diamine probe may act as a binding site
2+
2+
1
in both TPAD1-Cu and TPAD2-Cu complexes. The
H
NMR titration results of TPAD1 and TPAD2 supports the
working principle of dendritic sensors via dehydration in the
2
semi-aqueous medium THF : H O (2 : 1 in vol.).
1
The H NMR titrations of both dendrimers only proved the
involvement of the diamine probe attached to the triazine core
in the sensing mechanism but not of just the triazine core.
Therefore, we tried to evaluate the participation of the triazine
4
1
42
Characterization of binding site. and reversibility. The
binding site of the sensor system can be evaluated by the NMR
titrations of both dendrimer solutions, in which TPAD1 and
ꢁ
3
2+
13
TPAD2 were dissolved in [D ] THF (1 ꢂ 10 M), with Cu ion
core in the sensing mechanism via C NMR titrations
8
ꢁ2
solutions in D
2
+
O (2 ꢂ 10 M). During the addition of 2 equiv-
alents of Cu ions to 1 equivalent of dendritic compounds
TPAD1/TPAD2), the spectra in Fig. 10 show complete disap-
8
(Fig. S23 and S24 in the ESI†) of TPAD1 and TPAD2 in [D ]
2
ꢁ3
2+
ꢁ2
THF (1 ꢂ 10 M) with Cu ions in D O (2 ꢂ 10 M). The
2
13
(
obtained results in C NMR titrations proved the involvement
of the triazine core in the sensing mechanism, and TPAD1
showed two new peaks (green circles) at 176.27 and 155.13 ppm
corresponding to the diminished peaks (red circles) at 168.41
and 147.66 ppm, respectively. Similarly, TPAD2 also exhibited
two new peaks (green circles) at 176.27 and 154.56 ppm, cor-
responding to the diminished peaks (red circles) at 168.45 and
pearance of NH proton peak at 2.51 and 2.44 ppm, for TPAD1
and TPAD2, respectively which may arise from the dehydration
2+
induced by Cu ions. However, the broadening of the NMR
2+
spectra also occurs due to the presence of Cu ions in the
complex systems. To confirm the above statement, we also per-
2+
formed the titrations without Cu ions (Fig. S25 and S26 in the
ESI†) and it was confirmed that the dehydration as well as the
1
broadening of H NMR spectra arose only from the presence of
147.81 ppm, respectively. Upon the addition of 2 equivalents of
2+
Cu ions in D O solutions to 1 equivalent of both dendrimers
2
2+
Cu ions. Due to the complex formation, small down-field shifts
were observed for NH protons in both cases. In addition, as
(TPAD1 and TPAD2) in THF [D
8
] solutions, totally different
C NMR spectra appeared in comparison with the original
ones.
13
shown in Fig. 10, the nearby CH
2
protons were also up-field
8
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Fig. 9 Fluorescence responses of (a) TPAD1 (20 mM in THF : H
: 1 in vol.) and (b) TPAD2 (20 mM THF : H
O ¼ 4 : 1 in vol.) in the
absence and presence of different metal ions (40 mM in H O) with exci-
tations at 308 and 314 nm, respectively. The final solvent composition
after titrations was THF : H
O ¼ 2 : 1 in vol.
2
O ¼
4
2
2
2
1
3d. In general, binding sites are confirmed through the
reversibility of the system; therefore, the reversible nature of the
2+
sensor complexes with Cu ions was further investigated
through UV–Vis and fluorescence spectroscopies by adding
44
pentamethyl ethylene triamine (PMDTA) dissolved in THF
1 ꢂ 10ꢁ M). In order to maintain the final solvent composition
5
(
Fig. 8 Fluorescence spectra of (a) TPAD1 (20 mM in THF : H
in vol.) and (b) TPAD2 (20 mM in THF : H
O ¼ 4 : 1 in vol.) upon
increasing concentrations of Cu (0–24 mM with an equal span of 2 mM
in H O), where the excitation wavelengths were 308 and 314 nm,
respectively. (c) and (d) fluorescence maximum intensities of TPAD1 and
2
O ¼ 4 : 1
of THF : H O ¼ 2 : 1 (vol.), we also used the same equivalent of
2
2
2+
2+
PMDTA as Cu ions. Upon the addition of PMDTA to the
2+
2+
2+
2
Cu complexes (TPAD1-Cu and TPAD2-Cu ) the reversible
nature of the both sensor systems are evidenced in Fig. S29 (see
ESI†). Both UV–Vis absorbance and fluorescence spectra of
TPAD1 (Fig. S29a and S29c in the ESI†) and TPAD2
(Fig. S29b and S29d in the ESI†) clearly indicate the sensor
reversibility. This evidence provides enough confidence for
reusing the sensor probes, and hence further investigations were
carried out to establish the number of reversible cycles. Both
dendrimers (TPAD1 and TPAD2) provided the sensor revers-
ibilities of ca. 90% up to 10 cycles (Fig. 12c and 12 d), so it was
evident that both dendrimers could be reused several times. In
Fig. 12a and 12b, photographs of visualized reversibilities in
TPAD1 and TPAD2 by the naked eye as well as by commercial
2+
TPAD2 as a function of Cu concentrations.
2+
The possible approaches of Cu ions towards the triazine
binding site are shown in Fig. 11. However, according to the
43
stability concern, symmetrical approaches were preferred to
13
unsymmetrical ones and the results of C NMR titrations also
verified the above statement. Hence, we propose a similar
2+
approach mechanism of Cu ions towards the triazine core as
2+
shown in Fig. 1 (schematic representation of Cu sensor
response) and these results are also well supported by the 1 : 2
2+
stoichiometry of the Cu complexes obtained in Fig. 13c and
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Table 3 Stern–Volmer constants (KSV) of different metal ions
a
TPAD1 (labs ¼ 308 nm; lem ¼ 398 nm) KSV (Mꢁ1
b
TPAD2 (labs ¼ 314 nm; lem ¼ 427 nm) KSV (Mꢁ1
b
)
Metal ions
)
+
3
3
3
3
3
3
3
3
3
3
3
3
5
3
3
3
3
3
3
3
3
Ag
2.05 ꢂ 10
2.30 ꢂ 10
1.95 ꢂ 10
2.33 ꢂ 10
2.79 ꢂ 10
2.70 ꢂ 10
1.35 ꢂ 10
1.60 ꢂ 10
2.86 ꢂ 10
2.54 ꢂ 10
2.26 ꢂ 10
2.91 ꢂ 10
4.10 ꢂ 10
2.28 ꢂ 10
2.41 ꢂ 10
2.26 ꢂ 10
2.01 ꢂ 10
2.43 ꢂ 10
1.98 ꢂ 10
2.44 ꢂ 10
9.93 ꢂ 10
+
3
Cs
Cu
7.61 ꢂ 10
+
4
2.35 ꢂ 10
+
4
K
Li
1.08 ꢂ 10
+
3
7.89 ꢂ 10
+
3
Na
7.14 ꢂ 10
3+
4
Fe
1.28 ꢂ 10
3+
4
Al
Ag
1.03 ꢂ 10
2
+
3
7.92 ꢂ 10
2+
3
Ba
Ca
Co
Cu
7.12 ꢂ 10
2
+
3
5.79 ꢂ 10
2
+
3
4.16 ꢂ 10
2
+
6
1.20 ꢂ 10
2+
3
Fe
7.54 ꢂ 10
2
+
3
Mg
Mn
7.14 ꢂ 10
2
+
3
8.43 ꢂ 10
2
+
3
Ni
Pb
Zn
5.54 ꢂ 10
2+
3
8.40 ꢂ 10
2+
3
3.96 ꢂ 10
2+
Hg
3
6.44 ꢂ 10
O from their respective aqueous solutions and dendrimer (TPAD1 or TPAD2) concentrations
a
All metal ion concentrations were taken as 20 mM in H
were kept constant at 20 mM in THF : H
2
b
2
O ¼ 4 : 1 in vol.
K
SV ¼ [(I
0
/I) ꢁ 1]/[Q]; [Q] ¼ quencher concentration (20 mM for all metal ions).
1
NH protons in H NMR titrations (Fig. 10) and the appearance
UV lamps (l ¼ 365 nm) were demonstrated. The formation of
pale green/blue color during the addition of Cu ions may be
2+
of two new peaks in conjunction with the complete disappear-
13
ance of corresponding peaks in C NMR titrations (Figs. S23
2+
2+
due to the formation of TPAD1-Cu
and TPAD2-Cu
2+
complexes.
and S24†) upon the addition of 2 equivalents of Cu ions in
D
2
O also support the 1 : 2 stoichiometry. According to these
4
5
2+
Stoichiometry calculation. by Job plots UV–Vis absorption
spectral titration results were shown in Fig. 13a and 13b. Both
concentrations of dendrimers (TPAD1 and TPAD2) were kept
results, the formation of dimeric complexes by Cu ions was
further confirmed, and hence the probable 1 : 2 stoichiometric
complexes for both dendrimers TPAD1 and TPAD2 were
constant (20 mM in THF : H O ¼ 4 : 1 vol/vol), and during the
proposed as shown in Fig. 11. Table 3 shows Stern–Volmer
46
2
2+
2+
addition of Cu ions in H O the formation of Cu complexes
quenching constants (KSV
)
of different metal ions, calculated
2
with dendrimers (TPAD1 and TPAD2) were confirmed by the
from KSV ¼ [(I /I) ꢁ 1]/[Q], where [Q] is the quencher concen-
0
2+
appearance of new peaks at 405 and 439 nm, respectively. Upon
tration. The detection limit of Cu ions was estimated as 20 mM
from both single and dual metal ion titrations. Therefore, the
quencher concentration for all metal ions was chosen as 20 mM.
2+
addition of Cu ions from 0 to 60 mM, the initial absorbance
maxima of both dendrimers at 308 and 314 nm were found to
have decreased and the peaks at 405 and 439 nm increases
slowly. However, the peaks at 405 and 439 nm start quenching
slowly upon addition of 30 mM and 38 mM aqueous solutions of
2+
Stern–Volmer quenching constants of Cu ions clearly reveal
the best selective sensitivities of both dendrimers (TPAD1 and
TPAD2) with the 1,3,5-triazine-4,6-diamine probe is towards
2+
2+
Cu ions to dendritic solutions of TPAD1 and TPAD2 (20 mM
in THF : H O ¼ 4 : 1 vol/vol), respectively. More interestingly,
Cu ions in comparison with other interfering metal ions, such
2+
2+
2+
3+
+
as Zn , Hg , Pb , Fe , and Ag . In addition, it was verified
that the sensitivity of dendrimer TPAD2 was relatively higher
than that of TPAD1. Quenching constant values of TPAD1 and
2
the appearances of two isosbestic points for TPAD1 (281 and
3
43 nm) and TPAD2 (281 and 376 nm) indicate the presence of
2+
2+
5
6
simple equilibrium in the Cu complexes. Further investigations
TPAD2 towards Cu ions were 4.10 ꢂ 10 and 1.20 ꢂ 10 ,
of Job plots occurred at 405 nm (TPAD1) and 439 nm (TPAD2)
2+
respectively, and found to be the best reported values in regards
2+
to Cu ion sensors based on triarylamine dendrimers so far.
2
+
against the mole fractions (X
M
¼ [Cu ]/[Cu ] + [TPAD1] or
2+
2+
[
TPAD2]) of TPAD1-Cu
and TPAD2-Cu
complexes.
This new sensory probe was also known to be specific and
2+
Fig. 13c and 13d represent the Job plots of TPAD1 and TPAD2,
selective to Cu ions even when mixed with some interfering
metal ions, and the reversible nature of the sensor probes with
high quenching constant (KSV) values were also proven by the
addition of pentamethyl ethylene triamine (PMDTA). Finally, it
was well established that 1,3,5-triazine-4,6-diamine probe with
good fluorophores can act as selective and sensitive sensors
respectively, which indicate the formation of 1 : 2 stoichiometric
0 1 M
complexes by both dendrimers. Two plots of (A –A ) versus X
show that (A –A ) values go through a maximum at a molar
0
1
fraction of ca. 0.59 (TPAD1) and 0.65 (TPAD2) in Fig. 13c and
3d, respectively, indicating a 1 : 2 stoichiometric complex
1
1 13
formation. This was also supported by H and C NMR spec-
2+
towards Cu ions. Specific selectivities of both dendrimers to
2+
tral titrations of TPAD1 and TPAD2 upon addition of 2
Cu ions in the presence of 19 interfering metal ions were
confirmed by the Stern–Volmer quenching constants (KSV) of all
metal ions.
2
+
equivalents of Cu ions (in D
2
O) to 1 equivalent solution of
both dendrimers (in [D ] THF). The complete disappearance of
8
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1
Fig. 10 (a) and (b) H NMR spectra of 1 equiv. of TPAD1 and TPAD2
1 ꢂ 10ꢁ M in [D
3
2+
] THF) with 2 equiv. of Cu (2 ꢂ 10 M in D
ꢁ2
(
8
2
O);
the final solvent composition after titrations was [D
8
] THF : D
2
O ¼
2
: 1 in vol.
Fig. 12 Reversibility of (a) TPAD1 and (b) TPAD2 visualized by the
naked eye and UV-light irradiations. (c, d) Reversible sensor cycles of
TPAD1 and TPAD2, respectively.
first time they were utilized for dual applications such as electron/
energy transfers in H-bonded supramolecules as well as in che-
mosensors. Both dendrimers (TPAD1 and TPAD2) formed H-
bonded donor–acceptor–donor (D–A–D) supramolecular triads
(TPAD1-PBI-TPAD1and TPAD2-PBI-TPAD2) with PBI. The
presence of multiple H-bonds in solution state was elucidated by
2
+
Fig. 11 Possible approach mechanism of Cu ions towards the triazine
core.
1
H NMR titrations and IR spectral studies. J-aggregations and
electron/energy transfers provided by both dendrimers were
established by UV–Vis and PL titrations with PBI and the nano-
structural sizes of the supramolecular triads were evaluated by
X-ray diffraction (XRD) analysis. In similar manners, both
dendrimers also shows their best sensing selectivities towards
Conclusions
Two novel fluorescent triarylamine-based dendrimers TPAD1
4
and TPAD2 with a new N ,N -dibutyl-1,3,5-triazine-4,6-diamine
probe were synthesized via normal synthetic routes and for the
6
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2
+
ꢁ5
Cu ions among the 20 selected metal ions (at 10 M levels) in
semi-aqueous media (THF : H O ¼ 2 : 1 in vol.), and TPAD2
2
provides a higher sensitivity than TPAD1. Therefore, it confirms
that the higher generation of a dendritic structure with a larger
number of fluorescent units was required for the higher sensitivity
towards specific metal ions. In general, both dendrimers have
2+
specific selectivities towards Cu ions, even in the presence of
2
+
2+
2+
3+
other interfering metal ions, such as Zn , Hg , Pb , Fe , and
+ 2+
Ag . Moreover, the detection limit of Cu ions by these den-
drimers were estimated as 20 ppm by fluorescence titrations in
4
6
THF : H O ¼ 2 : 1 in vol., and the binding site deduced as N ,N -
2
1
13
dibutyl-1,3,5-triazine-4,6-diamine probe from H, C NMR
titrations in [D ] THF : D O ¼ 2 : 1 in vol. The stoichiometry of
8
2
2+
Cu complexes formed during the sensor process was established
as 1 : 2 from the Job plot method based on UV–Vis titrations and
the preferred complex formation mechanism was proposed.
Acknowledgements
We are grateful to the National Science Council of Taiwan
(
ROC) through NSC99-2113-M-009-006-MY2 for the financial
support. We also wish to thank R. Saminathan for his timely help
during the writing of the paper.
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This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 8976–8987 | 8987