Synthesis of novel platinum complex core as a selective Ag+ sensor and its H-bonded tetrads self-assembled with triarylamine dendrimers for electron/energy transfers

Article abstract of DOI:10.1039/c4ta04231j

A novel platinum complex PtC with a tri-armed uracil hydrogen-bonded (H-bonded) unit was synthesized via a modular synthetic approach and characterized by ¹H, ¹³C NMR and MALDI-TOF mass spectroscopies. Two H-bonded tetrads, PtC-(TPAD1)₃ and PtC-(TPAD2)₃, based on a metal core PtC complexed with two generations of triarylamine dendrimers, TPAD1 and TPAD2 (with electron-donating nature), were successfully constructed with improved organic solubility via a classical H-bonded self-assembly approach. Supramolecular H-bonding in solution and solid state was elucidated by ¹H NMR titrations, IR spectral studies and time resolved photoluminescence (TRPL) measurements. The electron/energy transfers, as well as the self-assemblies of supramolecular tetrads, were established by UV-Vis and PL titrations and AFM morphological studies. Furthermore, metal complex core PtC showed selective sensitivity towards Ag⁺ ions through fluorescence turn-off responses without any interference from other common metal ions. The 1:1 binding stoichiometry and complexation mechanism between the probe and Ag⁺ ion was established by ¹H NMR titration. Moreover, PL reversibility of PtC + Ag⁺ could be achieved on addition of PMDTA.

Full text of DOI:10.1039/c4ta04231j

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Materials Chemistry A
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PAPER
Synthesis of novel platinum complex core as a
selective Ag+ sensor and its H-bonded tetrads self-
assembled with triarylamine dendrimers for
electron/energy transfers†
Cite this: J. Mater. Chem. A, 2014, 2,
17463
Muthaiah Shellaiah, Mandapati V. Ramakrishnam Raju, Ashutosh Singh,
*
Hsin-Chieh Lin, Kung-Hwa Wei and Hong-Cheu Lin
A novel platinum complex PtC with a tri-armed uracil hydrogen-bonded (H-bonded) unit was synthesized
via a modular synthetic approach and characterized by 1H, 13C NMR and MALDI-TOF mass spectroscopies.
Two H-bonded tetrads, PtC–(TPAD1)3 and PtC–(TPAD2)3, based on a metal core PtC complexed with two
generations of triarylamine dendrimers, TPAD1 and TPAD2 (with electron-donating nature), were
successfully constructed with improved organic solubility via a classical H-bonded self-assembly
approach. Supramolecular H-bonding in solution and solid state was elucidated by 1H NMR titrations, IR
spectral studies and time resolved photoluminescence (TRPL) measurements. The electron/energy
transfers, as well as the self-assemblies of supramolecular tetrads, were established by UV-Vis and PL
titrations and AFM morphological studies. Furthermore, metal complex core PtC showed selective
sensitivity towards Ag+ ions through fluorescence turn-off responses without any interference from
other common metal ions. The 1 : 1 binding stoichiometry and complexation mechanism between the
Received 15th August 2014
Accepted 20th August 2014
DOI: 10.1039/c4ta04231j
1
probe and Ag+ ion was established by H NMR titration. Moreover, PL reversibility of PtC + Ag+ could be
www.rsc.org/MaterialsA
achieved on addition of PMDTA.
exible systems are tolerant of physical deformations, such as
cellular lipid bilayer membranes.4–6
Introduction
In the realm of modern supramolecular architectures, metal
centered assemblies (MCs) governed by non-covalent interactions
are promising candidates in designing functional electronic
materials.7,8 Molecular organizations of these complexes regulated
by a wide variety of non-covalent interactions are oen ubiquitous
in nature and play pivotal roles in many elds such as oxygen
transport, gene activation, pharmaceuticals and catalysis.9,10
However, such functional supramolecular entities constituted
with hydrogen-bonds and metal complexes are more specic and
highly directional in both solutions and surfaces, and so are
widely exploited in construction of three-dimensional supramo-
lecular architectures in both chemical and biological systems.11
However, both hydrogen-bonded and metal complexed entities
have some advantages such as (1) convenient approaches towards
precisely control of the geometries of electron donor–acceptor
assemblies via hydrogen-bonded (H-bonded) interactions;12a,b (2)
2.5-fold enhancement in photocurrent of photo-electrochemical
devices incorporating components possessing complementary
H-bonded units;12c,d (3) enhancement of electron and hole trans-
port as well as stability and even colors (pure colors to white light)
potentially achieved in metal complexes as active layers;12e,f (4)
high thermal stabilities as well as greater charge transporting
properties such as MLCT provided by metal complexes.12g,h
The design of functional macromolecular structures and
supramolecular metal complexes, by exploitation of self-
assembly of complementary components via spontaneous and
reversible supramolecular interactions, has the potential to
create a myriad of fascinating molecular architectures.1–3
Supramolecular interactions have many advantages, such as (1)
nanoscale ordering morphology can be controlled effectively by
supramolecular interactions of active layers to move beyond the
trial and error tuning of the device; (2) long-range order can be
introduced to restore high charge-carrier mobilities for exi-
bility of devices, which offers the benet of amorphous mate-
rials and is potentially cost effective in manufacture; (3) a direct
method for assembling large numbers of molecules into
structures via supramolecular organization can bridge length
scales from nanometers to macroscopic dimensions for effi-
cient long distance charge transport; (4) precise architectures
can be controlled by the type and number of interactions (e.g.
the photosynthetic reaction center in bacteria), where more
Department of Materials Science and Engineering, National Chiao Tung University,
Hsinchu 30049, Taiwan, ROC. E-mail: linhc@mail.nctu.edu.tw
† Electronic supplementary information (ESI) available: 1H and 13C NMR,
MALDI-TOF Spectra of PtC and characterizations of sensor complex. See DOI:
10.1039/c4ta04231j
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Transition metal-based supramolecular MCs have recently titrations, TRPL and AFM morphological studies. For the rst
emerged as a special area in supramolecular chemistry as they time we evaluated the sensing ability of a metal complex core
have crucial roles in catalysis, molecular-based electronic (PtC) with three uracil units in specic sensing of various metal
devices and well dened nanostructures at a molecular level.2b,13 ions, in which the PtC core selectively recognized Ag+ ions via
Among these intriguing metals, platinum is in daily use owing binding with imidazole –NH unit, as veried by NMR-titrations.
to its anticancer chemotherapy applications.14 Despite the Importantly, the sensing mechanism of the metal complex core
neurotoxicity of platinum drugs, a third-generation platinum was completely reversible on addition of a tridentate ligand
drug oxaliplatin was approved for treatment of metastatic 1,1,4,7,7-pentamethyl-diethylenetriamine (PMDTA) solution.
colorectal cancer by the US Food and Drug Administration
(FDA).15 Apart from use in chemotherapy, platinum-based
supramolecular MCs are reliable sources for next-generation
Results and discussion
molecular devices. Xing et al. reported an amphiphilic cationic
Molecular design
platinum(II) terpyridyl ferrocene complex that formed a host–
guest driven self-assembled vesicle structure.16 Shirakawa et al.
demonstrated the formation of a one-dimensional structure
using p–p and solvophobic interactions of an 8-quinolinol
Fine tuning of counter H-bonded units with a central metal core
and triple terminal dendritic units could be an ideal choice for
realization of entrenched electron/energy transfers via branch-
ing units offered by tetrapod structures. Derivative 12 contain-
ing both terminal alkyne and uracil H-bonded units was
synthesized from compound 1 using a modular multi-step
synthetic approach as shown in Scheme 1. 2-(4-Bromo-2,5-bis-
octyloxy-phenyl)-1H-1,3,7,8-tetraaza cyclopenta[l]phenanthrene
platinum dichloride (derivative 16) was prepared starting from
aldehyde 13 and phenonthroline diketone 14 using an acid
catalyzed imidazole formation, which was coupled with PtCl2-
platinum(II) chelate derivative with
benzoylamide unit.17
a 3,4,5(n-dodecyloxy)-
Dendrimers have long been known as ideal at-forms in
designing multidimensional and multifunctional supramolec-
ular hierarchical nano-architectures, because of their high
branching points with easily achievable molecular symmetry at
the nanoscale.18 The inherent directional excitation energy
transfer (EET) capabilities of dendrimers have been used to
prepare light-harvesting (LH) antennas in articial photosyn-
thesis. Dendritic nano-structured self-assemblies are prime
constituents of novel nano-technological electronic devices.19
Moreover, new bottom-up synthetic approaches for dendritic
MC nanostructures with control over sizes, surface properties,
solubilities and compositions are highly desirable for formation
of hierarchical MC-based self-assemblies. Shelnutt et al. repor-
ted the rst controlled photo-catalytic approach for synthe-
sizing sheet-like nanodendrites or foam-like nano-sheets of
platinum.20 However, to the best of our knowledge a platinum
complex core containing three peripheral uracil H-bonded units
for creation of MC-based self-assemblies to engineer electron/
energy transfer features has not yet been explored.
Metal-based organic complexes have long been used in
molecule sensing applications because of their exible energy
transfer characteristics.21 However, despite their conventional
applications in various elds, transition metals have posed a
great threat to human health and environmental pollution
over recent years. Silver ions can bind various metabolites,
including carboxyl, amine, imidazole and sulydryl
enzymes.22 Extreme bioaccumulation and ghastly toxic effects
(NH2)2 under reux in DMSO solution (see Scheme 2).25
A
central platinum core (PtC) containing peripheral uracil H-
bonded units was synthesized using compounds 12 and 16 via
sonogashira coupling strategy as depicted in Scheme 3. The
platinum complex core was completely characterized by 1H and
13C NMR spectroscopies. Moreover, the abundant molecular ion
peak in MALDI-TOF analysis (as shown in Fig. S1†) conrmed
the formation of PtC. The exceptional nano-material properties
of dendrimers and supramolecular self-assemblies and our
previous success in designing novel supramolecular architec-
tures prompted us to perform the aforementioned task.26 Den-
drimers TPAD1 and TPAD2 were prepared according to our
previous synthetic protocols.27 With two counter H-bonded
building moieties in hand to further appreciate self-assembly
properties, we fabricated supramolecular H-bonded tetrads
PtC–(TPAD1)3 and PtC–(TPAD2)3 via a classical supramolecular
non-covalent H-bonded approach by our previous report, as
shown in Scheme 4.27
IR spectral studies
of silver ions make them intriguing targets to detect selec- To probe the multiple H-bonded interactions between the metal
tively.23 Many small molecule-, polymer- and nanoparticle- complex core (PtC) and dendritic H-bonded termini (TPAD1 and
based probes have been reported as silver ion detectors,24 but TPAD2), attenuated total reection Fourier transform infrared
oen with low sensitivity. Thus, it is imperative to develop new (ATR-FTIR) measurements were carried out. As shown in Fig. 2,
metal complex-based probes for selective and sensitive detec- ATR-FTIR spectra of TPAD1, TPAD2 and PtC in CH2Cl2 showed
tion of silver ions.
free NH stretching bands at 3438, 3420 and 3176 cmꢀ1
,
Herein, as shown in Fig. 1, we developed two H-bonded respectively. The NH stretching bands of supramolecular
tetrads, PtC–(TPAD1)3 and PtC–(TPAD2)3, containing a central tetrads PtC–(TPAD1)3 and PtC–(TPAD2)3 (1 : 3 stoichiometry) in
platinum complex (PtC) and two generations of terminal tri- CH2Cl2 were observed at 3327 and 3328 cmꢀ1, respectively,
arylamine dendrimers (TPAD1 and TPAD2 with electron- which conrmed the formation of multiple H-bonding between
donating nature). We also studied the electron/energy transfers the complementary H-bonded core (PtC) and dendrimers
and self-assemblies of H-bonded tetrads by UV/Vis, PL, 1H NMR (TPAD1 and TPAD2),28 as noticed in Fig. 3.
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Fig. 1 Schematic representation of PtC sensor selectivity towards Ag+ and chemical structures of supramolecular tetrads PtC–(TPAD1)3 and
PtC–(TPAD2)3 containing a platinum complex core (PtC) and two generations of three triarylamine dendrimers (TPAD1 and TPAD2).
1H NMR titrations
showed isosbestic points at 372 and 393 nm, respectively, which
arose from the formation of multiple H-bonded tetrads in
To further corroborate the presence of multiple H-bonds in
chloroform solution. Simultaneously, the PL titrations of both
tetrads, we performed 1H NMR titrations as shown in Fig. 4, in
supramolecular tetrads (Fig. 5c and d) showed bathochromic
which PtC concentration was kept constant at 10 mM and the
concentrations of dendrimers TPAD1 and TPAD2 were
increased up to 3.5 equivalents with an equal span of 0.5
equivalent. While increasing the dendrimer concentrations in
the range of 0–3.5 equivalents, the NH protons of PtC showed
upeld shis in both supramolecular tetrads from 11.08 to 9.71
ppm and from 11.08 to 9.65 ppm for PtC–(TPAD1)3 and PtC–
shis in PtC–(TPAD1)3 (from 461 nm to 466 nm) and PtC–
(TPAD2)3 (from 461 nm to 468 nm). Both results of UV-Vis and
PL titrations veried the metal complex core aggregations of PtC
in both supramolecular tetrads. Similar to conventional
J-aggregated dyes,30 our supramolecular tetrads showed
enhanced PL titrations during the formation of H-bonded
structures, where the PL emission intensities of PtC were
(TPAD2)3, respectively. As noticed in Fig. 4, downeld shis of
enhanced (lex ¼ 405 nm) by adding both dendrimers (TPAD1
PtC–NH protons were also well evidenced for the formation of
multiple H-bonds in the supramolecular complexes as per the
and TPAD2 with electron-donating units). This consequence
was rationalized by an electron/energy-transfer process from the
literature.29 To further assess the multiple H-bonded interac-
dendrimers TPAD1 and TPAD2 to the platinum complex core
tions and the electron/energy transfer features between counter
H-bonded moieties, we performed UV-Vis and PL titrations in
chloroform solvent medium.
PtC. The observed UV-Vis and PL titration trends further
conrmed a molar ratio of 1 : 3 as PtC H-bonded to both TPAD1
and TPAD2 to form supramolecular tetrads PtC–(TPAD1)3 and
PtC–(TPAD2)3, respectively.
UV-Vis and PL titrations
TRPL spectra of energy transfers in H-bonded tetrads
UV-Vis and PL titration experiments in chloroform were per-
formed to study the interactions of both dendrimers (TPAD1 To further evaluate the characteristics of energy transfers
and TPAD2) with the metal complex core PtC. Upon adding between H-bonded tetrads, we conducted TRPL measurements
aliquots of TPAD1 to PtC, a bathochromic shi (Fig. 5a) from for both tetrads PtC–(TPAD1)3 and PtC–(TPAD2)3. Previous
408 to 413 nm was observed for the platinum complex absorp- studies of metal-centered complexes revealed that energy
tion maximum corresponding to the S0–S1 electronic transition transfers arose from branched ends to metal centers.31 Ventura
of PtC–(TPAD1)3. A similar red-shi in PtC–(TPAD2)3, i.e. from et al. reported the energy transfer dynamics in multi-
408 to 415 nm (Fig. 5b), was observed during the UV-Vis titra- chromophoric arrays engineered from phosphorescent Pt(II)/
tions. During UV-Vis titrations, dendrimers TPAD1 and TPAD2 Ru(II)/Os(II) centers linked to a central truxene platform.32 They
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Scheme 1 Synthetic procedure of compound 12.
Scheme 2 Synthetic procedure of compound 16.
reported that the electronic properties of the Ru and Os truxene and 17.5 ms for the bis ethynyl-bipy substituted
subunits in the arrays were unaffected by the presence of the truxene). Therefore, the direct linking of uracil units in PtC
truxene core, whereas direct linking of the Pt subunit to the through alkyne linkage would enhance energy transfers as
truxene via the s-alkyne bond markedly inuenced the spec- evidenced by TRPL spectral studies. Similar to metal organic
troscopic behavior of the Pt center as evidenced by the TRPL frameworks, multiple H-bonded assemblies demonstrated
studies (lifetime of 4.3 s for the mono ethynyl-bipy substituted energy transfers as illustrated in previous reports.33 Zhang et al.
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CH2Cl2 solution at room temperature. Therefore, the combi-
nation of both organic metal framework and H-bonded
assemblies would provide greater energy transfers as reported
previously.35 Hence we carried out TRPL studies as illustrated in
Fig. 6a and b. The energy transfers occurred during tetrad
formations of metal complex core PtC with various molar ratios
of dendrimers (TPAD1 and TPAD2) through TRPL spectral
changes. As summarized in Table S1,† we found that the uo-
rescence decay constants (s) were affected typically by the energy
transfers in both tetrads [PtC–(TPAD1)3 and PtC–(TPAD2)3]. The
average decay constants (savg) of PtC were enhanced from 3.70
to 6.43 and 8.11 ns for PtC–(TPAD1)3 and PtC–(TPAD2)3,
respectively. The longer decay time values of H-bonded tetrads
were attributed to the energy transfer contribution from
complementary dendrimers, which appeared in the results of
bi-exponential decay ttings. Similar to the changes of average
decay constants (savg), the faster decay component (A1) of PtC
(28.3%) was increased to 45.2% and 47.1%, respectively, for
both tetrads [PtC–(TPAD1)3 and PtC–(TPAD2)3]. On the other
hand, the longer decay component (A2) of PtC (71.7%)
was reduced to 54.8% and 52.9%, respectively, for both tetrads
[PtC–(TPAD1)3 and PtC–(TPAD2)3], which also occurred in
chelation of Cu2+ and Ni2+ ions to our previous sensor
polymers.36 As a result of J-aggregated assembly of both tetrads
[PtC–(TPAD1)3 and PtC–(TPAD2)3], rapid changes in the lifetime
of PtC were evidenced from the experimental data, which
conrmed electron/energy transfers from triarylamine
dendrimers (TPAD1 and TPAD2) to the platinum complex core
(PtC) as shown in Fig. 1 and 3. These results show that
H-bonded assemblies consisting of different kinds of building
blocks, including a platinum complex core (PtC) and two
generations of triarylamine dendrimers (TPAD1 and
TPAD2), are attractive for construction of supramolecular
metallo-dendrimers by the self-assembly technique.
Scheme 3 Synthetic procedure of PtC.
Scheme
4 Synthesis of H-bonded tetrads PtC–(TPAD1)3 and
PtC–(TPAD2)3.
established the photoinduced electron transfer and charge-
recombination in 2-ureido-4[1H]-pyrimidinone quadruple
H-bonded porphyrin–fullerene assemblies, which were also well
recognized through time-resolved studies.34 In addition, steady-
state and time-resolved spectroscopies demonstrated that
upon excitation of the porphyrin, electron transfers to the
fullerene occurred with rate constants of 1.6 ꢁ 108 sꢀ1 (60%)
and 4.2 ꢁ 108 sꢀ1 (44%) for assemblies I and II, respectively, in a
AFM studies
Encouraged by the studies described above in deducing the
energy transfer features between counter H-bonded compo-
nents, we were further intrigued to investigate the
Fig. 2 ATR-FTIR spectra of H-bonded tetrads PtC–(TPAD1)3 and PtC–(TPAD2)3 in CH2Cl2 with 3 : 1 molar ratio of (a) TPAD1 and PtC (b) TPAD2 and PtC.
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Fig. 3 Chemical structures of supramolecular tetrads PtC–(TPAD1)3 and PtC–(TPAD2)3 containing a platinum complex core (PtC) and two
generations of H-bonded triarylamine dendrimers TPAD1 and TPAD2, respectively.
morphological changes of these tetrads to understand self- which were induced from photoisomerization of the azo-
assembly phenomena at the nanoscale. Multiple H-bonded benzene components within the self-assembled nano-
arrays have been shown to be versatile tools for preparation of structures.37a Similarly, the utility of an external structure-
functional supramolecular materials in solution. We report the directing agent to induce orthogonal H-bonding-mediated
results obtained by designing and engineering a novel library of programmed supramolecular-assembly via AFM analysis and
shape persistent molecular modules able to transfer their gelation of an n-type NDI chromophore was reported by Ghosh
geometrical information to the nal supramolecular tetrad et al.37b Additionally, there are many reports that the presence of
architectures through formation of PtC–(TPAD1)3 and multiple H-bonds has provided self-assemblies with nano-level
PtC–(TPAD2)3 complexes at the nanoscopic level. Previously, aggregations.37 As shown in Fig. 7, atomic force microscopy
Sun et al. reported the morphology and chirality of supercoils (AFM) images of metal complex core PtC and both generations
demonstrating photoresponsiveness through AFM studies, of tetrads were measured and investigated. Upon H-bonding
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Cr3+, Mg2+, Cu2+, Mn2+, Hg2+, Fe2+ and Ag2+) in H2O. As shown in
Fig. 8 and 9, PtC showed better selectivity towards Ag+, whereas
other metal ions had negligible sensitivities to PtC. Sensor
selectivity of PtC towards Ag+ was further conrmed by single
and dual metal analysis as follows: during single metal analysis
of PtC (Fig. 8), Ag+ provided a huge uorescence quenching in
PtC + Ag+, where the PL intensity (I ¼ 261 at 461 nm for PtC +
Ag+) was 36-fold reduced in contrast with PtC (I0 ¼ 9694 at
461 nm), and all other metal ions exhibited trivial responses.
Similarly, the greater selectivity of PtC towards Ag+ (1000 mM) in
the presence of other metal ions (1000 mM) was evidenced in the
background metal analysis of PtC (Fig. S2†). In both cases,
sensor selectivity of PtC towards Ag+ was further conrmed by
Stern–Volmer quenching constant values, as summarized in
Table S2.† Stern–Volmer quenching constant (KSV) of PtC + Ag+
was found to be 3.61 ꢁ 104 molꢀ1, and other metal ions were in
the range of 102 molꢀ1
.
Sensor titration of PtC with Ag+
Upon titration of PtC (0.1 mM) with Ag+ (0–1000 mM with an
equal span of 100 mM in H2O), the PL intensity of PtC at 461 nm
was quenched slowly (Fig. 10). Similarly, PL quenching of PtC
was well established in Fig. S3† and the detection limit of Ag+ by
PtC was calculated as 5.54 ꢁ 105 M by standard deviation and
linear ttings, as shown in Fig. S4.† The linear uorescent
quenching of PtC upon addition of Ag+ ions may arise from
binding of Ag+ with the imidazole unit of PtC. However to
conrm the above assumption, we extended our investigation
by NMR titration of PtC with Ag+ to establish the binding site as
detailed below.
Stoichiometry and binding site of PtC + Ag+
In general, stoichiometry tests of sensor complexes can give a
clear idea of their binding sites. However, because of the low
concentration of PtC (0.1 mM) in the sensor titration with higher
concentrations of Ag+ (0–1000 mM with an equal span of
100 mM), the stoichiometry of PtC + Ag+ was not calculated from
Fig. 4 1H NMR titrations of PtC with (a) TPAD1 and (b) TPAD2 (PtC was UV/PL titrations. Hence, to calculate the stoichiometry and
binding site of PtC + Ag+, the 1H NMR titration was carried out
fixed at 1 equivalent and the concentrations of TPAD1 and TPAD2 were
increased up to 3.5 equivalents with an equal span of 0.5 equivalent in
d-chloroform).
as shown in Fig. 11. The 1H NMR titration of PtC in d8-THF
(10 mmol) with Ag+ in D2O (0–10 mmol with an equal span of
2 mmol) established the stoichiometry and binding site towards
the sensor response. Interestingly, the –NH peak of the imid-
with metal complex core PtC, the roughnesses of tetrads
azole group in PtC diminished slowly with increasing concen-
PtC–(TPAD1)3 and PtC–(TPAD2)3 were reduced to different
extents because of their self-assemblies, where the roughnesses
1
tration of Ag+ in D2O. However, the remaining H NMR peaks
1
were not affected (Fig. 11). Moreover, the H NMR titration of
of PtC, PtC–(TPAD1)3 and PtC–(TPAD2)3 were around 123 nm,
PtC with Ag+ conrmed the 1 : 1 stoichiometry of PtC + Ag+ as
17 nm and 5.1 nm, respectively. Hence, this also conrmed that
well. As the –NH imidazole peak of PtC was not affected aer 0.8
self-assemblies provided different degrees of uniform aggrega-
equiv. addition of Ag+ in D2O, the 1 : 1 stoichiometry of PtC +
tions in two generations of tetrads to enhance the electron/
Ag+ was further proven. Similarly, the binding site of PtC + Ag+
energy transfers.
was further veried by the reversibility test of PtC + Ag+ through
the addition of few drops of PMDTA in THF.38 As shown in
Sensor properties of PtC
Fig. S5,† uorescence emission of PtC was restored on the
Because of its excellent photophysical properties, PtC (0.1 mM) addition of PMDTA, hence the dissociation of complex PtC + Ag+
in THF was initially investigated towards 1000 mM of metal ions was proven. The photograph of sensor reversibility of PtC + Ag+
(Li+, Ag+, K+, Na+, Cs+, Ni2+, Fe3+, Co2+, Zn2+, Cd2+,Pb2+, Ca2+, on addition of PMDTA also supports this idea (Fig. 12).
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Fig. 5 UV-Vis and PL spectra of PtC upon titrations of (a and c) TPAD1 and (b and d) TPAD2 (PtC was fixed at 1 equivalent and the concentrations
of TPAD1 and TPAD2 were increased up to 3.6 equivalents with an equal span of 0.3 equivalent in chloroform).
Therefore, a possible binding mechanism of PtC towards Ag+
was proposed as shown in Fig. S6.† However, to further conrm
the imidazole binding site, we attempted to carry out sensor
titrations with compounds 15 and 16. However, because of
multiple binding sites in 15 (two phenonthroline nitrogens) and
low solubility of 16 in THF, we were unable to obtain their
sensor properties from both compounds. As reported in the
literature and from our previous results39 (Fig. S7†), we found
that the uorescence decay constants (s) were affected typically
by turn-off sensor responses, as summarized in Table S1.† From
the TRPL signals without any sensor responses, the uores-
cence lifetime values of PtC were reduced from 3.70 ns to 2.95
ns. During the sensing process of PtC + Ag+, a faster decay
component (A1) of PtC (28.3%) was increased to 76.2%, along
with 71.7% of a longer decay component (A2) reduced to 23.8%,
as shown in Table S1.†
Conclusions
In this study, we designed and synthesized a novel metal
complex PtC possessing a central platinum core and three
terminal uracil H-bonded units via a modular synthetic
Fig. 6 (a) and (b) TRPL spectra of PtC with increasing concentrations
approach. For the rst time, a PtC complex was successfully
of TPAD1 and TPAD2, respectively. (0–3.0 equivs. with an equal span
synthesized for use as a selective sensor probe towards Ag+
of 1 equiv.).
recognition and to be self-assembled with two complimentary
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Fig. 7 AFM height and 3D images of (a and d) PtC, (b and e) PtC–(TPAD1)3, (c and f) PtC–(TPAD2)3. Scale bar 5 mM.
Fig. 8 Sensor selectivity of PtC (i.e. responses of PL intensities at 461
nm) towards various metal ions; where PtC concentration is 0.1 mM and
the metal ion concentration is 1000 mM (lex ¼ 405 nm & lem ¼ 461 nm).
Fig. 9 PtC sensor selectivity towards metal ions (I0: initial PL intensity
of PtC; I : PL intensity of PtC with metal ions); where PtC concentra-
tion is 0.1 mM and the metal ion concentration is 1000 mM.
dendrimers, TPAD1 and TPAD2, to form novel H-bonded
tetrads PtC–(TPAD1)3 and PtC–(TPAD2)3, respectively. The
existence and preservation of multiple H-bonds in both tetrads self-assembly at the nanoscale. Exclusively, the PtC core showed
were demonstrated by 1H NMR titration, as well as IR and TRPL a better selective and sensitive metal recognition capability
spectral studies. Furthermore, electron/energy transfer features towards Ag+ without interference from commonly encountered
in these tetrads were established by UV-Vis and PL titrations of transition metals. The 1 : 1 stoichiometry of Ag+ complex
1
PtC with two generations of dendrimers, TPAD1 and TPAD2, formed during the sensing event was established by H NMR
with electron-donating units. Self-assembled nanostructural titration proles. Remarkably, on–off–on reversible etiquette
formation with smoother surface roughness in tetrads formation by Ag+ and PMDTA was successfully achieved. Thus,
compared with its precursors in AFM studies further affirmed we believe that the ndings in this study illuminate the eld of
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Experimental
Materials and methods
All anhydrous reactions were carried out by standard proce-
dures under nitrogen atmosphere to avoid moisture, and all
solvents were dried by distillation over appropriate drying
agents. Reactions were monitored by TLC plates and column
1
chromatography was generally performed on silica gel. H and
13C NMR spectra were recorded on a 300 MHz spectrometer.
The chemical shis (d) in ppm and coupling constants (J) in Hz
were obtained relative to TMS (0.00) and d-chloroform (7.26 &
1
77.0) for H and 13C NMR spectra (s, d, t, q, m and br mean
single, double, ternary, quadruple, multiple and broad single,
respectively). Mass spectra (MALDI and FAB) were acquired on
the respective mass spectrometers. Elemental analysis was
carried out by Elemental Vario EL. ATR-FTIR spectra were
measured using a Perkin Elmer spectrum 100 series spec-
trometer, and were collected at a resolution of 4 cmꢀ1 using a
deuterated triglycine sulfate detector by averaging four scans.
Absorption and uorescence spectra were measured on the
V-670 Spectrophotometer and F-4500 Fluorescence Spectro-
photometer, respectively. Identication and purity of the
intermediates were characterized by NMR (1H & 13C), mass
(MALDI and FAB) and melting point measurements. The
synthesis of nal dendrimers TPAD1 and TPAD2 was as repor-
ted previously.27 Compounds TPAD1, TPAD2 and PtC were
dissolved in CHCl3 at 1 ꢁ 10ꢀ5 M, for UV/Vis and PL titrations.
Both tetrads PtC–(TPAD1)3 and PtC–(TPAD2)3, obtained from
Scheme 4, were dissolved in CHCl3 and their solid lms were
prepared by spin coating and then measured. Time-resolved
photoluminescence (TRPL) spectra were measured using a
home-built single photon counting system. Excitation was
performed using a 450 nm diode laser (Picoquant PDL-200,
50 ps fwhm, 2 MHz). The signals collected at the excitonic
emissions of solutions were connected to a time-correlated
single photon counting card (TCSPC, Picoquant Timeharp 200).
Emission decay data were analyzed with biexponential kinetics
in which two decay components were derived. Lifetime values
(s1 and s2) and pre-exponential factors (A1 and A2) were deter-
mined and summarized. Li+, Ag+, K+, Na+, Cs+, Ni2+, Fe3+, Co2+,
Zn2+, Cd2+,Pb2+, Ca2+, Cr3+, Mg2+, Cu2+, Mn2+, Hg2+, Fe2+ and
Fig. 10 PL titration of PtC (0.1 mM) with Ag+ (0–1000 mM) by increasing
concentration with an equal span of 100 mM.
Fig. 11 1H NMR titration of PtC (10 mmol in d8-THF) with Ag+ by
increasing concentration (0–10 mmol) with an equal span of 2 mmol
in D2O.
Ag2+ metal ions were dissolved in water medium at 1 ꢁ 10ꢀ3
M
concentration from their respective chloro compounds and PtC
was dissolved in THF medium at 1 ꢁ 10ꢀ7 M concentration.
Synthesis of 1,4-bis-octyloxy-benzene (2). To a 500 mL ask
containing 10 g (9 mmol) of hydroquinone (1) in 200 mL of
DMF, 37 mL (22.5 mmol; 2.5 equiv.) of 1-bromohexane was
added and reuxed for 36 h under N2 atmosphere. Then, the
mixture was cooled to room temperature and water was added.
The precipitate was ltered and dried in vacuo for 3 h, then
recrystallized with ethanol to afford the pure compound. Yield:
28.86 g (95%); 1H NMR (300 MHz, CDCl3): 6.82 (s, 4H), 3.90
(t, J ¼ 6.0 Hz, 4H), 1.74–1.65 (m, 4H), 1.34–1.27 (m, 20H), 0.88
(t, J ¼ 6.6 Hz, 6H).
Fig. 12 Photographs of titrations of PtC with reversible (on–off–on)
fluorescence changes towards Ag+ and its reversibility on further
addition of PMDTA (under UV lamp with lex ¼ 365 nm & lem ¼ 461 nm).
metal complexed nano-self-assembled structures. Furthermore,
opto-electronic applications of these H-bonded tetrads are
currently under way in our laboratory.
Synthesis of 2-bromo-1,4-bis-octyloxy-benzene (3). 10 g (29.9
mmol) of compound 2 was dissolved in 100 mL of CH3CN in a
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Journal of Materials Chemistry A
250 mL ask under N2 atmosphere. 5.5 g (32.9 mmol; 1.1 equiv.) (300 MHz, CDCl3): 9.12 (br, 1H), 7.14 (d, J ¼ 9.0 Hz, 1H), 5.70 (d,
of N-bromosuccinimide (NBS) was added portionwise and stir- J ¼ 6.0 Hz, 1H), 3.71 (t, J ¼ 7.5 Hz, 2H), 1.68–1.64 (m, 2H), 1.30–
red for 24 h. Excess solvent was distilled out. The crude product 1.28 (m, 6H), 0.87 (t, J ¼ 6.6 Hz, 3H).
was puried by column (SiO2) by 99 : 1 hexane–EtOAc to afford a
Synthesis of 1-hexyl-6-iodouracil (9). At 78 ꢂC, LDA (20.4 mL
1
colorless oil. Yield: 10.26 g (83%); H NMR (300 MHz, CDCl3): of a 2.5 M solution, 51.0 mmol) was added dropwise to a solu-
7.12 (s, 1H), 6.80 (s, 2H), 3.95 (t, J ¼ 6.0 Hz, 2H), 3.88 (t, J ¼ 6.0 tion of 1-hexyluracil (2.0 g, 10.2 mmol) in THF (55 mL), and the
Hz, 2H), 1.83–1.72 (m, 4H), 1.30–1.49 (m, 20H), 0.88 (t, J ¼ 6.6 resulting solution was stirred under N2 for 2 h. I2 (12.9 g, 51.0
Hz, 6H).
mmol) was added and the reaction mixture was stirred for
Synthesis of 1-bromo-4-iodo-2,5-bis-octyloxy-benzene (4). 5 g another 2 h at the same temperature. Acetic acid (2.0 mL) was
(12.1 mol) of compound 3 was dissolved in a CH3COOH–H2O– added to react with stirring at room temperature overnight. The
conc. H2SO4 (95 : 3 : 2) mixture and heated to 80 ꢂC for organic phase was extracted with ethyl acetate and washed with
30 minutes, then cooled to 50 ꢂC and 1.3 g (0.78 mmol) of KI and saturated NaHCO3 (3 ꢁ 30.0 mL) and Na2SO3 (3 ꢁ 30 mL)
0.91 g (0.43 mmol) of KIO3 was added and reuxed at 80 ꢂC for solutions. Finally, the product was washed with brine (30 mL)
24 h. Aer cooling to room temperature, the mixture was and dried over Mg2SO4. The solvent was removed by rotary
quenched with Na2SO4 and then extracted with hexane. The evaporator and the crude product was puried by column
pure compound 4 was obtained by column chromatography chromatography silica (hexane–ethyl acetate ¼ 5 : 5) to afford
(SiO2) 99 : 1 hexane–EtOAc to afford a white solid. Yield: 5.94 g compound 9 (2.2 g, 67%).1H NMR (300 MHz, CDCl3): 9.48 (br,
(91%); 1H NMR (300 MHz, CDCl3): 7.26 (s, 1H), 6.96 (s, 1H), 3.92 1H), 6.41 (s, 1H), 4.0 (t, J ¼ 8.1 Hz, 2H), 1.69–1.64 (m, 2H), 1.37–
(t, J ¼ 6.0 Hz, 4H), 1.82–1.76 (m, 4H), 1.34–1.48 (m, 20H), 0.89 1.32 (m, 6H), 0.88 (t, J ¼ 6.9 Hz, 3H).
(t, J ¼ 6.0 Hz, 6H).
Synthesis of 6-(4-bromo-2,5-bis-octyloxy-phenylethynyl)-1-
Synthesis of 4-bromo-2,5-bis-octyloxy-phenylethynyl)-tri- hexyl-1H-pyrimidine-2,4-dione (10). To a mixture of compound
methyl-silane (5). To 5 g (9.3 mmol) of compound 4 in a 250 mL 9 (1.26 g, 3.9 mmol) in THF (15 mL), 1-bromo-4-ethynyl-2,5-bis-
ask purged with continuous N2 atmosphere, 39 mg (0.05 octyloxy-benzene (6) (2.0 g, 3.9 mmol), CuI (10 mg, 0.05 mmol)
mmol) of PdCl2(PPh3)2 and CuI (0.05 mmol) were added with and NEt3 (15 mL) were added. Then, [Pd(PPh3)2 Cl2] (4 mg, 0.034
constant stirring. Then, 5 mL of freshly distilled THF was added mmolꢂ) was added under N2 and the reaction mixture was heated
via syringe and stirred for 10 minutes. Thereaer, Et3N 45 mL to 50 C for 36 h. The crude product was extracted with DCM
was added and then heated to 50 ꢂC. 1.32 mL (9.3 mmol; 1 followed by brine wash and dried over Mg2SO4. The resulting
ꢂ
equiv.) of TMSA was added and reuxed at 50 C for 12 h and solution was concentrated by rotary evaporator, and puried by
monitored via TLC. Aer completion, the mixture was cooled to column chromatography using silica (hexane–ethyl acetate ¼
1
RT, and the excess solvent was quenched and distilled out. The 7 : 3) to give a yellow solid. (1.98 g, yield 58%). H NMR (300
product was puried through column (SiO2) 9 : 1 hexane–EtOAc MHz, CDCl3): 9.32 (br, 1H), 7.12 (s, 1H), 6.93 (s, 1H), 5.98 (s, 1H)
to afford a yellow solid. Yield: 4.63 g (98%); 1H NMR (300 MHz, 4.0 (t, J ¼ 6.1 Hz, 2H), 3.96 (t, J ¼ 7.5 Hz, 4H), 1.86–1.69 (m, 6H),
CDCl3): 7.08 (s, 1H), 6.94 (s, 1H), 3.94 (t, J ¼ 6.0 Hz, 4H), 1.81– 1.48–1.27 (m, 26H), 0.88 (t, J ¼ 6.9 Hz, 9H); 13C NMR (75 MHz,
1.74 (m, 4H), 1.49–1.29 (m, 20H), 0.89 (t, J ¼ 6.0 Hz, 6H), 0.25 CDCl3): 162.9, 155.2, 151.0, 149.6, 139.2, 117.7, 117.5, 116.9,
(s, 9H).
109.1, 106.5, 97.4, 84.8, 70.4, 69.7, 46.8, 32.0, 31.7, 29.5, 29.3,
Synthesis of 1-bromo-4-ethynyl-2,5-bis-octyloxy-benzene (6). 29.1, 26.5, 26.2, 26.1, 22.9, 22.7, 14.3, 14.2; (FAB) calculated:
4.5 g of compound 5 was dissolved in THF with two drops of 630.30; found: 630.30.
water with constant stirring, and then a few drops of tetrabutyl
Synthesis of 6-(2,5-bis-octyloxy-4-trimethylsilanylethynyl-phe-
ammonium uoride (TBAF) were added and stirred for 3 h. nylethynyl)-1-hexyl-1H-pyrimidine-2,4-dione (11). Compound 11
Monitored by TLC, upon completion of the reaction, the solvent was synthesized as per the synthetic procedure of compound 5
was distilled out and puried through column (SiO2) 9 : 1 with a nal 89% yield; 1H NMR (300 MHz, CDCl3): 9.32 (br, 1H),
hexane–EtOAc to afford a yellow solid, which was processed for 7.12 (s, 1H), 6.93 (s, 1H), 5.98 (s, 1H) 4.0 (t, J ¼ 6.1 Hz, 2H), 3.96 (t,
the next step immediately. Yield: 3.83 g (99%); 1H NMR J ¼ 7.5 Hz, 4H), 1.86–1.69 (m, 6H), 1.48–1.27 (m, 26H), 0.88 (t, J ¼
(300 MHz, CDCl3): 7.07 (s, 1H), 6.96 (s, 1H), 3.96 (t, J ¼ 6.0 Hz, 6.9 Hz, 9H), 0.12 (s, 9H); 13C NMR (75 MHz, CDCl3): 162.9, 155.2,
4H), 3.28 (s, 1H), 1.84–1.74 (m, 4H), 1.46–1.28 (m, 20H), 0.88 151.0, 149.6, 139.2, 117.7, 117.5, 116.9, 109.1, 106.5, 99.3, 99.0,
(t, J ¼ 6.0 Hz, 6H).
97.4, 84.8, 70.4, 69.7, 46.8, 32.0, 31.7, 29.5, 29.3, 29.1, 26.5, 26.2,
Synthesis of 1-hexyluracil (8). K2CO3 (14.80 g, 107.05 mmol) 26.1, 22.9, 22.7, 15.7, 14.3, 14.2, 0. 17; (FAB) calculated: 648.43;
was added to a suspension of uracil (10.0 g, 89.ꢂ21 mmol) in found: 648.43.
DMSO (150 mL) and stirred for 15–20 min at 45 C. 1-Bromo-
Synthesis of 6-(4-ethynyl-2,5-bis-octyloxy-phenylethynyl)-1-
hexane (3.5 mL, 25 mmol) was added and the reaction mixture hexyl-1H-pyrimidine-2,4-dione (12). Compound 12 was synthe-
was stirred for 48 h. The reaction was cooled to room temper- sized as per the synthetic procedure of compound 6 with a nal
ature and poured into cold water. The product was extracted 89% yield; 1H NMR (300 MHz, CDCl3): 9.32 (br, 1H), 7.12 (s, 1H),
three times with DCM, and washed with dilute HCl, water, 6.93 (s, 1H), 5.98 (s, 1H) 4.0 (t, J ¼ 6.1 Hz, 2H), 3.96 (t, J ¼ 7.5 Hz,
brine, and dried over Mg2SO4. The organic layer was concen- 4H), 3.31 (s, 1H), 1.84–1.74 (m, 6H), 1.46–1.28 (m, 26H), 0.88 (t, J
trated and poured into cold hexane with vigorous stirring. The ¼ 6.9 Hz, 9H); 13C NMR (75 MHz, CDCl3): 162.9, 155.2, 151.0,
resulting precipitate was ltered and washed with cold hexane 149.6, 139.2, 117.7, 117.5, 116.9, 109.1, 106.5, 99.3, 99.0, 97.4,
to afford compound 8 (17.88 g, 69%) as a white solid.1H NMR 84.8, 81.8, 79.5, 70.4, 69.7, 46.8, 32.0, 31.7, 29.5, 29.3, 29.1, 26.5,
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26.2, 26.1, 22.9, 22.8, 14.3, 14.2; (FAB) calculated: 576.39; found: 84.8, 81.8, 70.4, 69.7, 46.8, 32.0, 31.7, 29.5, 29.3, 29.1, 26.5, 26.2,
576.39; elemental analysis; calculated: C, 74.96; H, 9.09; N, 26.1, 22.9, 22.7, 14.3, 14.2; MALDI-TOF; calculated: 2473.24,
4.86%, found: C, 74.67; H, 9.06, N, 4.88%.
found: 2473.24; elemental analysis; calculated: C, 69.42; H, 7.98;
Synthesis of 4-bromo-2,5-bis-octyloxy-benzaldehyde (13). 5 g N, 5.66%, found: C, 68.57; H, 7.76, N, 5.58%.
of compound 6 in 50 mL THF was stirred under N2 atm and
cooled to ꢀ78 ꢂC maintained for 1 h. To the solution, 1 equiv. of
Acknowledgements
n-BuLi was added, then temperature was raised to 0 ꢂC; 1 equiv.
of DMF was added and stirred for 12 h at RT. Excess n-BuLi was This project was nancially supported by the National Science
quenched with two drops of HCl and the solvent was distilled Council of Taiwan (ROC) through funding nos NSC 101-2113-M-
out. Finally, a pure compound was obtained via column (SiO2) 009-013-MY2 and NSC 102-2221-E-009-174.
9 : 1 hexane–EtOAc to afford a yellow solid. Yield: 3.03 g (74%);
1H NMR (300 MHz, CDCl3): 10.40 (s, 1H), 7.29 (s, 1H), 7.21
References
(s, 1H), 3.99 (t, J ¼ 6.0 Hz, 4H), 1.84–1.74 (m, 4H), 1.46–1.28
(m, 20H), 0.88 (t, J ¼ 6.0 Hz, 6H).
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Synthesis of 2-(4-bromo-2,5-bis-octyloxy-phenyl)-1H-1,3,7,8-
tetraaza-cyclopenta[l]-phenanthrene (15). A 1 : 1 mixture of
aldehyde (11) and phenonthroline diketone (14) was dissolved
completely in acetic acid. To that solution an excess (30 equiv.)
of NH4OAc was added and reuxed for 12 h. The product
formation was monitored by TLC. Aer completion the crude
product was recrystallized from absolute ethanol to afford
compound 15 with 94% yield; 1H NMR (300 MHz, CDCl3): 11.13
(s, 1H), 9.05 (s, 2H), 8.85 (d, 1H), 7.92 (s, 2H), 7.49–7.60 (m, 2H),
7.07 (s, 1H), 4.08 (t, J ¼ 6 Hz, 2H) 3.99 (t, J ¼ 6.1 Hz, 2H), 1.84–
1.74 (m, 4H), 1.46–1.28 (m, 20H), 0.88 (t, J ¼ 6.9 Hz, 6H); 13C
NMR (75 MHz, CDCl3): 150.5, 149.8, 148.7, 148.3, 148.2, 135.7,
130.5, 127.9, 124.7, 124.5, 123.3, 122.7, 118.7, 117.8, 117.1,
114.3, 112.7, 70.2, 60.9, 32.1, 29.8, 29.7, 29.6, 29.5, 26.7, 26.3,
22.9, 22.7, 14.4, 14.2; (FAB) calculated: 630.3; found: 630.3;
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found: C, 66.57; H, 6.81, N, 8.88%.
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tetraaza-cyclopenta[l]phenanthrene platinum dichloride (16). A
1 : 1 mixture of compound 15 and PtCl2(NH2)2 was dissolved
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ꢂ
completely in DMSO and reuxed at 100 C for 12 h to afford
yellow solid of compound 16. The solid was ltered off and
washed with excess water and dried over vacuum at 70 ꢂC for 6
h. Yield: 63%; 1H NMR (300 MHz, CDCl3): 11.13 (br, s, 1H), 9.02
(br, s, 2H), 8.78 (br, s, 2H), 8.49 (br, s, 2H), 7.31–7.55 (br, m, 2H),
6.76 (br, s, 1H), 4.08 (br, t, 2H) 3.96 (br, t, 2H), 1.79–1.26 (br, m,
24H), 0.91 (br, t, 6H); (FAB) calculated: 897.63; found: 897.63.
Synthesis of PtC. To a mixture of compound 16 (1 equiv.) in
THF (15 mL), 6-(4-ethynyl-2,5-bis-octyloxy-phenylethynyl)-1-
hexyl-1H-pyrimidine-2,4-dione (6) (10 equiv.), CuI (0.1 equiv.)
and NEt3 (15 mL) were added. Then, PdCl2(PPh3)2 (1 equiv.) was
added under N2 and the reaction mixture was stirred at room
temperature for 48 h. The crude product was extracted with
DCM followed by brine wash and dried over Mg2SO4. The
resulting solution was concentrated by rotary evaporator, and
puried by column chromatography using neutral alumina
1
(CH2Cl2–MeOH ¼ 9 : 1) to give a yellow solid. Yield: 72%; H
NMR (300 MHz, CDCl3): 11.08 (s, 3H(NH)), 7.12 (m, 7H), 6.83
(m, 7H), 6.61 (s, 1H (NH)), 5.98 (s, 3H) 4.09 (t, J ¼ 6.1 Hz, 6H),
3.96 (t, J ¼ 7.5 Hz, 16H), 1.84–1.74 (m, 24H), 1.46–1.28 (m, 96H),
0.88 (t, J ¼ 6.9 Hz, 33H); 13C NMR (75 MHz, CDCl3): 162.5, 155.2,
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Journal of Materials Chemistry A
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