Tetrathiafulvalene derivatives bearing a crown ether with intramolecular charge transfer properties: Synthesis and cation binding studies

Article abstract of DOI:10.1039/b817892e

Two new TTF crown ether derivatives, TNC and TBC, have been synthesized and well characterized. The interaction between the TTF and crown ether moieties through a triple bond has been demonstrated to show a remarkable sensing ability towards Pb²⁺

Full text of DOI:10.1039/b817892e

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Tetrathiafulvalene derivatives bearing a crown ether with intramolecular
charge transfer properties: synthesis and cation binding studiesw
Yao-Peng Zhao, Xiao-Jun Wang, Jing-Jing Wang, Gang Si, Yan Liu,
Chen-Ho Tung and Li-Zhu Wu*
Received (in Montpellier, France) 13th October 2008, Accepted 2nd December 2008
First published as an Advance Article on the web 21st January 2009
DOI: 10.1039/b817892e
Two new TTF crown ether derivatives, TNC and TBC, have been synthesized and well
characterized. The interaction between the TTF and crown ether moieties through a triple bond
has been demonstrated to show a remarkable sensing ability towards Pb2+ over the other cations.
responses significantly depend on the electrostatic interaction
between the TTF moiety and cations, both in models A and B,
when TTF macrocycles bind a metal ion via a crown ether.
Introduction
As a well known organic p-donor, tetrathiafulvalene (TTF)
has aroused much interest over the past 30 years. Large
Spatial proximity to the TTF moiety is an important factor
numbers of TTF compounds with elaborate structures have
governing the interaction of a guest cation with the TTF unit.
been constructed and have shown promising applications in
Since the biggest HOMO coefficient of the TTF moiety is
supramolecular systems.1,2 Specifically, TTF macrocyclic sys-
tems containing a host unit capable of cation binding are
mainly located on the central S2CQCS2 fragment,10 model B
would be more appropriate for enhancing the through-space
attracting widespread attention because the complexation of
electrostatic interactions between metal ions and the TTF unit.
the host with an ionic guest can induce a change in their
physicochemical response (e.g., spectral or electrochemical).3–5
Indeed, Salle and co-workers have reported a model B
bis(pyrrolo)-TTF crown with more selective binding toward
Pb2+ and Ba2+ in its neutral state.11
In general, upon metal ion binding in TTF macrocyclic
systems, the first oxidation potential (E1ox) increases, while
the second (E2ox) remains unchanged. This is rationalized by
the expulsion of the cation after the first oxidation.4 Therefore,
the E1 shift upon metal complexation has been extensively
ox
used for sensing purposes.6 The groups of Becher and Salle
have dedicated themselves to this field. Salle et al. have
classified TTF macrocycles into model A or model B families
according to their grafting sites, in which the crown moieties
Other than direct interactions through space, however, we
are directly linked to TTF subunits.7
are not aware of reports concerning the interaction of the TTF
moiety with a guest cation through bonds in TTF crown ether
derivatives (model C). More recently, we reported that the
electronic communication between the TTF moiety and
acceptors can be effectively optimized by a p-conjugated
bond.12 In this context, we are interested in the incorpora-
tion of a crown ether functionality into the TTF moiety
via a p-conjugated triple bond for studying through-bond
In earlier reports, Becher et al. took the lead in developing
TTF crown derivatives of model A for recognition events.8
Salle et al. have made a number of TTF-based macrocycles of
model B for redox-selective metal ion sensors.9 It was estab-
lished that the association constants and physicochemical
Key Laboratory of Photochemical Conversion and Optoelectronic
Materials, Technical Institute of Physics and Chemistry,
Chinese Academy of Sciences, Beijing 100190, PR China.
E-mail: lzwu@mail.ipc.ac.cn; Fax: +86 10 8254 3580;
Tel: +86 10 8254 3580
w Electronic supplementary information (ESI) available: Additional
experimental details and data. CCDC reference number 644421. For
ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/b817892e
Scheme 1 The synthetic route to TNC and TBC, and references TN
and TB.
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interactions between the TTF moiety and guest cations. Here,
we wish to report the synthesis of two novel TTF crown ether
compounds, TNC and TBC (Scheme 1). Although the spatial
distance is longer than that in both models A and B, we
demonstrate that the interaction between the TTF unit and the
crown ether through a triple bond has a remarkable sensing
ability towards Pb2+ over the other cations.
UV-vis spectral changes of TNC and TBC toward cations
The absorption spectra of TNC and TBC, together with those
of TN and TB, in acetonitrile solution were investigated and
are shown in Fig. S1.w Similar to TN and TB, TNC and TBC
display two absorption bands at o370 nm and 370–500 nm
(Table 1). The absorption spectral properties follow Beer’s
Law at concentrations below 1 Â 10À4 M, suggesting the
absence of aggregation. With reference to our previous work
on triple bond-bridged TTF-p-A compounds,12a the intense
absorption band at o370 nm is assigned to a local transition
in the TTF moiety, while the low energy absorption band
arises from intramolecular charge transfer (ICT). The triple
bond linkage modulates the electron distribution between the
TTF core and the crown group in TNC and TBC. As indicated
in Table 1, the ICT absorption bands of TBC (lmax = 407 nm)
and TB (lmax = 403 nm) are lower in energy than those of
TNC (lmax = 395 nm) and TN (lmax = 396 nm). This may be
rationalized by the fact that the higher electron density of the
N centre in TNC and TN leads to a weaker electron accepting
ability compared to the benzo-15-crown-5 and methoxy
substituents in TBC and TB.
Results and discussion
Synthesis
The synthesis of target molecules TNC and TBC was carried
out under the typical Sonogashira coupling conditions shown
in Scheme 1.13 The starting materials 2-iodotetrathiafulvalene
(I-TTF) and HCRCR were prepared according to literature
methods.14 The reaction of I-TTF with 1 equiv. HCRCR in a
mixed solvent of NEt3 and THF, catalyzed by CuI and
Pd(PPh3)4, for 6 h at room temperature afforded TNC and
TBC as orange-yellow solids with yields of 70 and 60%,
respectively. 1H and 13C NMR spectroscopy, MS spectro-
metry, and satisfactory elemental analyses confirmed the
identities of all the compounds. A single crystal of TBC was
obtained and structurally characterised. Compared with the
synthetic procedures for model A and B compounds, this
reaction could be performed under mild conditions with
a relatively shorter reaction time and in higher yield.
Compounds TN and TB were also prepared as references for
TNC and TBC, respectively.
Taking advantage of the recognition ability of crown ethers
for cations, we investigated the complexation behaviors of
TNC and TBC toward metal ions, particularly Pb2+. Fig. 2
shows the UV-vis absorption spectral changes of TNC and
TBC in acetonitrile as a function of added Pb(ClO4)2.
Evidently, the interaction between the crown ether and Pb2+
leads to significant changes in the absorption spectra of TNC
and TBC. The maximum of the local absorption band of TNC
blue-shifts from 338 to 307 nm, while the ICT band red-shifts
from 395 to 413 nm, respectively, with increasing Pb2+ con-
centration. Throughout the titration, well-defined isosbestic
points at 317 and 433 nm were clearly observed, indicating the
presence of only two absorption species in solution: the free
TNC and the complex TNCÁPb2+. The observed red shift in
the low energy ICT absorption band should be a result of the
complexation of metal ions to the azo-crown ether moiety in
TNC, which would enhance the electron accepting ability of
the azo-crown ether moiety, thereby improving the extent of
ICT from the TTF unit to the azo-crown moiety through the
triple bond. Similar results were observed upon adding Pb2+
to a solution of TBC in acetonitrile, only with smaller spectral
changes than those of TNC (Fig. 2(b)). At the same time, no
spectral changes were observed in the control experiments; TN
and TB providing evidence of the binding of Pb2+ to the
crown ether moiety. Therefore, it can be deduced that Pb2+
can interact with the crown ether moiety, inducing by a
through-bond response from the TTF unit in TNC and
TBC. The smaller spectral change of TBC indicates that the
electronic communication between the TTF unit and the
X-Ray crystal structure determination of TBC
Single crystals of TBC were obtained by diffusion of hexane
into a CH2Cl2 solution of TBC and determined by X-ray
diffraction.15 The molecular structure of TBC is depicted in
Fig. 1. A noticeable feature of the TTF moiety lies in two of its
bond lengths, 1.722(3)
A (S(4)–C(5)) and 1.332(4) A
(C(5)–C(6)). All the other S–C bonds in TBC (between
1.737(3) and 1.768(3) A) are much longer than that of
S(4)–C(5). The shorter p-conjugated C(1)–C(2) bond
(1.316(4) A) than that of C(5)–C(6) indicates that the triple
bond-bridged TBC molecule is significantly conjugated.
In addition, there is
a
dihedral angle of 38.21
(C(9)–C(10)–C(5)–C(6)) between the phenyl ring of the phenyl-
acetylene and the plane of the TTF moiety. The TTF
moiety also has a slightly twisted cis-conformation, with
dihedral angles of 17.81 (C(5)–C(6)–C(4)–C(3)) and 13.41
(C(4)–C(3)–C(1)–C(2)), respectively.
Table 1 The absorption properties of TBC, TB, TNC and TN in
CH3CN solution (10À5 M) at room temperature
TBC
TB
TNC
TN
lmax (ec)
310 (0.859)
407 (0.121)
297 (1.057)
403 (0.143)
338 (1.176)
395 (0.229)
332 (1.480)
396 (0.270)
a
lmax (ec)
b
Fig. 1 ORTEP drawing of the molecular structure of TBC with
a
b
c
l o 370 nm. l = 370–500 nm. e/105 dmÀ3 molÀ1 cmÀ1
.
thermal ellipsoids at 30% probability.
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Fig. 3 The absorbance changes at 338 nm for TNC (1.4 Â 10À5 M) in
the presence of a 1.7 Â 10À5 M acetonitrile solution of various cations.
the presence of 1.2 equiv. of metal cation in an acetonitrile
solution is presented in Fig. 3. Obviously, TNC shows the
highest binding affinity towards Pb2+ compared to the other
metal ions. The N heteroatom in the azo-crown ether moiety
of TNC enhances its affinity towards softer Pb2+ ions, and
thus affords a high selectivity and sensitivity towards them.
As described for the previous TTF crown ether derivatives,
the UV-vis spectral responses resulting from the complexation
of model A or B compounds and metal ions were quite small,
in spite of dramatic changes observed in electrochemical
studies. However, the interaction between the crown ether
moiety and Pb2+ leads to significant absorption spectral
changes of the triple bond-bridged TTF crown derivatives
TNC and TBC. For TNC, the absorbance at l = 338 nm
decreased by more than three-fold (from 1.176 to 0.346) when
1.5 equiv. of Pb2+ was added (Fig. 2), which is the largest
UV-vis spectral change observed so far for TTF crown
derivatives. Evidently, the triple bond can serve as a bridge
to optimize the electronic communication between the TTF
moiety and the crown ether, resulting in remarkable changes
to the ICT by an external stimulus in the absorption spectra.
Fig. 2 Absorption spectral changes of TNC and TBC in CH3CN
solution with 0.1 M nBu4NPF6 as a function of Pb2+ concentration
(0–1.5 equiv.). (a) TNC (1.4 Â 10À5 M), (b) TBC (2.4 Â 10À5 M); the
insets show plots of A0/(A0 À A) vs. 1/[Pb2+] for TNC and TBC.
Electrochemical studies
benzo-15-crown-5 moiety is disturbed by the dihedral angle of
38.21 (C(9)–C(10)–C(5)–C(6)) observed in its crystal structure.
The insets in Fig. 2 show plots of A0/(A0 À A) vs. 1/[Pb2+],
where A and A0 refer to the absorbance in the presence and
absence of Pb2+ at 395 and 407 nm for TNC and TBC,
respectively. Analysis of the plots using the Benesi–Hildebrand
equation reveals that the complexation of the crown ether
moiety and Pb2+ is in the ratio 1 : 1 (see Fig. S2w),16 with the
binding constant, log K, being 4.07 for TNC, and 4.60 for
TBC, respectively.
TNC and TBC, along with their references TN and TB, exhibit
two reversible one-electron oxidation waves (E1 and E2,
Table 3), which are derived from the oxidation of TTF to
TTF+ꢁ and TTF2+, respectively. In addition, a third oxidation
potential (E3) at 1.07 V for TNC and 1.03 V for TN,
respectively, corresponding to the oxidation of the N hetero-
atom, was detected. Progressive addition of Pb2+ to solutions
of TNC and TBC resulted in positive shifts of their E1 and E2
values (Table 3, Fig. S5w). In contrast to models A and B, the
positive shift of E1 in the presence of Pb2+ is small and the
oxidation of TTF up to its dication (E2) does not lead to
expulsion of the metal cation. This fact provides indirect
evidence that the metal cation and TTF2+ are well separated.
TNC and TBC associate with metal ions through the triple
bond. Since Pb2+ interacts with the N atom of the azo-crown
moiety directly, the E3 shift of TNC is much larger, close to
50 mV. Under identical conditions, no alteration could be
observed in the cyclic voltammograms when Pb2+ ions were
introduced into TN or TB solutions. The addition of other
TNC and TBC can also complex with other metal cations,
such as Li+, Na+, Mg2+, Ca2+ and Zn2+, available as their
perchlorate salts, resulting in similar absorption spectral
changes in TNC and TBC compared to those seen for Pb2+
.
However, the spectral changes are much less significant
(Fig. S3 and Fig. S4w) and a large excess of these other metal
cations need to be added. The binding constants of TNC and
TBC with these cations in acetonitrile were determined, and
are summarized in Table 2. To provide a clear picture, the
change of absorbance at a wavelength of 338 nm for TNC in
n
Table 2 The binding constants of TBC and TNC for various metal ions in acetonitrile with 0.1 M Bu4NPF6 at 298 K
Li+
Na+
Mg2+
Ca2+
Zn2+
Pb2+
TBC
TNC
—
2.40
—
2.16
4.13
2.48
4.18
3.47
3.92
2.57
4.60
4.07
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Table 3 The oxidative potentials of TNC, TN, TBC, TB and parent TTF in the presence and absence of Pb2+a
TTF
TNC
TNC + Pb2+
TN
TBC
TBC + Pb2+
TB
E11/2/V
E21/2/V
E31/2/V
0.320
0.700
—
0.407
0.771
1.12
0.424
0.804
1.17
0.414
0.762
1.09
0.422
0.803
—
0.439
0.819
—
0.432
0.790
—
a
n
The data are given vs. a saturated calomel electrode in CH3CN with Bu4NPF6 (0.1 M) as the supporting electrolyte (100 mV sÀ1).
effectively with metal cations through their triple bonds,
leading to remarkable changes in their UV-vis absorption,
1H NMR spectra and electrochemical properties.
Experimental
Synthesis
TBC. A mixture of I-TTF (110 mg, 0.33 mmol), 4-acetylene-
benzo-15-crown-5 (88 mg, 0.3 mmol), Pd(PPh3)4 (17 mg)
and CuI (6 mg) was added to a solution of anhydrous
THF (20 mL) and TEA (0.2 mL), and stirred for 12 h at
room temperature. After filtration, the filtrate was evapo-
Fig. 4 1H NMR spectra of TNC (2.0 Â 10À3 M) in CD3COCD3 as a
rated and purified by column chromatography over silica gel
(CH2Cl2/CH3OH) to afford TBC as a yellow solid (yield 60%;
m.p. 154 1C). 1H NMR (DMSO-d6, 400 MHz): d 7.05 (s, 1H),
function of Pb2+ concentration: (a) 0, (b) 0.3, (c) 0.6, (d) 1.0, (e) 1.5
and (f) 2.0 equiv.
7.00 (s, 1H), 6.98 (s, 1H), 6.87 (d, 1H), 6.68 (s, 2H), 3.98 (m,
metal cations (Li+, Na+, Mg2+, Ca2+ and Zn2+) caused no
significant modification in the cyclic voltammograms of TNC
or TBC, suggesting that the interaction between TTF and
crown ether moieties through the triple bond has a remarkable
sensing ability towards Pb2+ over the other cations.
4H), 3.67 (m, 4H) and 3.52 (m, 8H). 13C NMR (DMSO-d6,
100 MHz): d 149.6, 148.3, 126.3, 125.2, 120.1, 119.9, 116.0,
114.1, 113.5, 113.3, 112.8, 106.0, 93.8, 78.9, 70.5, 69.7, 69.6,
68.7, 68.6, 68.5 and 68.3. MS (EI): m/z = 494. Anal. for
C22H22O5S4: found (calc.): C, 53.26 (53.42); H, 4.49 (4.48)%.
1H NMR titration study
TNC. TNC was synthesized by a procedure similar to that
for TBC, except that 4-acetylene-phenyl-aza-15-crown-5 was
used in place of 4-acetylene-benzo-15-crown-5. This product
was obtained as an orange-yellow solid (yield 70%; m.p.
145 1C) after purification by column chromatography over
A 1H NMR titration study was also carried out to confirm the
specific sensing properties of TNC towards Pb2+. Fig. 4 shows
the family of 1H NMR spectra of TNC in CD3COCD3
solution taken during the course of the titration. A com-
1
1
silica gel (CH2Cl2/CH3OH). H NMR (CD3CN, 400 MHz):
parison of the H NMR spectra in the presence and absence
of Pb2+ reveals that the chemical shifts associated with the
phenyl and the crown ether protons are evidently shifted
downfield by the progressive addition of Pb(ClO4)2 to the
solution. In contrast, the addition of Pb2+ to a TN solution
under identical conditions caused no obvious changes in its
1H NMR spectrum, except for broadening of the signals of the
TTF moiety. Other metal cations did not interfere with the
detection of Pb2+. These observations clearly indicate that
TNC interacts with Pb2+ through the triple bond, and that
the whole TNC molecule is conjugated.
d 7.27 (d, 2H, J = 9.0 Hz), 6.68 (d, 2H, J = 9.0 Hz), 6.61
(s, 1H), 6.49 (s, 2H), 3.67 (m, 4H), 3.57 (m, 8H) and 3.54
(m, 8H). 13C NMR (DMSO-d6, 100 MHz): d 148.4, 131.7,
130.9, 126.5, 126.3, 125.2, 120.7, 119.9, 114.4, 109.8, 94.0, 71.3,
70.5, 70.2, 67.9 and 51.6. MS (EI): m/z = 522. Anal. for
C24H27NO4S4: found (calc.): C, 55.30 (55.25); H, 5.24 (5.22);
N, 2.67 (2.68)%.
Acknowledgements
We are grateful for financial support from the National
Science Foundation of China (20732007, 20728506 and
20672122), the Ministry of Science and Technology of China
Conclusions
(2006CB806105,
2007CB808004,
2007CB936001,
and
Two novel TTF crown ether derivatives, TNC and TBC, have
been synthesized by a straightforward Sonogashira coupling
reaction. The p-conjugated triple bond serves as a bridge to
optimize the electronic communication between the TTF unit
and the crown ether moiety. The resulting ICT of TNC and
TBC is effective for sensing purposes. In contrast to reported
models A and B, TTF crown derivatives TNC and TBC show a
remarkable sensing ability towards Pb2+. Despite being far
separated, the TTF units in TNC and TBC can communicate
2009CB22008) and the Bureau for Basic Research of the
Chinese Academy of Sciences.
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