TTFAQ-cored D/A ensembles: synthesis, electronic properties, and redox responses to transition metal ions

Article abstract of DOI:10.1016/j.tetlet.2010.03.099

Two anthraquinone-type π-extended tetrathiafulvalene (TTFAQ)-cored D/A triads were synthesized and characterized by UV-vis absorption and cyclic voltammetric analyses. Electronic substituent effects were unraveled by making a comparison with analogous TTFAQ derivatives previously reported. Electrochemical titrations of the two TTFAQ D/A triads with selected transition metal cations (Ag⁺ and Cu²⁺) were examined by cyclic voltammetry, and the results suggest potential application of such TTFAQ derivatives as electrochemical sensors for transition metal ions.

Full text of DOI:10.1016/j.tetlet.2010.03.099

Tetrahedron Letters 51 (2010) 2892–2895
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
TTFAQ-cored D/A ensembles: synthesis, electronic properties, and redox
responses to transition metal ions
*
Min Shao, Yuming Zhao
Department of Chemistry, Memorial University of Newfoundland, St. John’s, Newfoundland, Canada A1B 3X7
a r t i c l e i n f o
a b s t r a c t
Article history:
Two anthraquinone-type p-extended tetrathiafulvalene (TTFAQ)-cored D/A triads were synthesized and
Received 15 February 2010
Revised 19 March 2010
Accepted 23 March 2010
Available online 27 March 2010
characterized by UV–vis absorption and cyclic voltammetric analyses. Electronic substituent effects were
unraveled by making a comparison with analogous TTFAQ derivatives previously reported. Electrochem-
ical titrations of the two TTFAQ D/A triads with selected transition metal cations (Ag+ and Cu2+) were
examined by cyclic voltammetry, and the results suggest potential application of such TTFAQ derivatives
as electrochemical sensors for transition metal ions.
Keywords:
Ó 2010 Elsevier Ltd. All rights reserved.
Tetrathiafulvalene
Substituent effects
Horner–Wittig reaction
Sonogashira coupling
Electrochemical sensor
p
-Extended analogues of tetrathiafulvalenes (exTTFs) have re-
detectable photophysical or electrochemical responses at the re-
porter moiety.10a,b
ceived considerable attention in the current TTF chemistry owing
to their wide ranging application as redox-active building blocks
for molecular-based electronic and optoelectronic devices and
materials.1 In particular, the exTTFs bearing an anthraquinone
(AQ)-type p-spacer, generally referred to as TTFAQs, present a class
of robust two-electron donors frequently employed in the con-
struction of complex donor/acceptor (D/A) molecular arrays with
other electroactive or chromophoric groups.1b,2–8
Our group has recently developed a 2,6-dialkynylation strategy
that allows for generating various
(e.g., 1a–f, Scheme 1) in a facile and modular manner.7c,d,9 The
acetylenic -bridge in these compounds has been found to effec-
p-extended TTFAQ derivatives
p
tively mediate the electronic interactions between TTFAQ and dif-
ferent endgroups, hence enabling electronic and redox properties
to be flexibly modulated and finely tuned. This feature could be
advantageous in constructing functional molecular devices oper-
ated or controlled by electrochemical stimuli or signals; particu-
larly, in the field of electrochemical sensing and molecular
recognition.10 Indeed, molecule systems consist of a receptor
(binding) and a reporter (signaling) units have become a motif
widely employed in the design of chemosensors. For an efficient
chemosensor, the recognition of particular analytes, such as cat-
ions, anions, and neutral compounds, is achieved by selective com-
plexation with the receptor unit, while the binding events result in
* Corresponding author. Tel.: +1 709 737 8747; fax: +1 709 737 3702.
E-mail address: yuming@mun.ca (Y. Zhao).
Scheme 1. TTFAQ-cored D/A ensembles previously synthesized via the 2,-dialky-
nylation strategy.
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.03.099

M. Shao, Y. Zhao / Tetrahedron Letters 51 (2010) 2892–2895
2893
Scheme 2. Synthesis of amino-terminated TTFAQ derivative 2.
Figure 1. UV–vis absorption spectra of compounds 2 and 3 measured in CHCl3 at
room temperature.
In the literature, a plethora of molecular systems capable of elec-
trochemically sensing and recognizing specific chemical species or
ions has been prepared based on redox-active organic electrophores,
such as ferrocene, metallocenes, and TTF derivatives.10,11 In this
study, we investigated the potential of TTFAQ as electrochemical re-
porter for the detection of transition metal ions. The swift and effi-
cient detection of various transition metal cations is of great
importance in analytical and environmental chemistry. Organic
functional groups containing heteroatoms such as amino and car-
boxyl groups can strongly interact with transition metal cations,
and the resulting Lewis acid–base complexes usually exhibit consid-
erably altered electronic nature in comparison with the free ligands.
Upon this consideration, two new TTFAQ-cored D/A ensembles (2
and 3), in which the terminal functionalities are amino or carboxylic
ester groups, were prepared and characterized. These two TTFAQ
derivatives not only add new members to the TTFAQ-cored D/A triad
family but were also expected to serve new TTFAQ-based redox-ac-
tive ligands for the detection of transition metal ions.
The synthesis of TTFAQ-cored D–D–D type triad 2 was imple-
mented through a divergent approach as described in Scheme 2.
Iodination of N,N-dimethylaniline (4) with molecular iodine in
the presence of piperidine afforded para-iodo-N,N-dimethylaniline
5 in a decent yield. Compound 5 was then doubly cross-coupled
with 2,6-diethnyl-TTFAQ 6 under the catalysis of Pd/Cu to give
product 2 in 55% yield.12 It is worth noting that initial efforts to
perform the Sonogashira reaction in a mixture solvent (THF/Et3N,
1:1), which is a common protocol successfully used for a wide
range of Sonogashira couplings, failed unexpectedly. After a num-
ber of trials, the use of pure piperidine in lieu of THF/Et3N was
found to be the best condition for coupling electron-rich phenylio-
dide with ethynylated TTFAQ 6.
shown in Scheme 3, para-aminobenzoic acid (7) was converted
into iodide 8 via a Sandmeyer reaction. Compound 8 underwent
a Fischer esterification reaction in refluxing MeOH and the pres-
ence of a catalytic amount of H2SO4 to give methyl para-iodoben-
zoate (9) in 80% yield. Cross-coupling of compound 8 and TTFAQ
6 under typical Sonogashira conditions then afforded the desired
product 3 in a moderate yield of 33%.13
The electronic properties of the two TTFAQ derivatives, 2 and 3,
were investigated by UV–vis absorption spectroscopy. Figure 1
shows the absorption spectra of 2 and 3 measured in CHCl3, and
detailed spectroscopic and electrochemical data are summarized
in Table 1.
From the UV–vis spectroscopic analysis, the lowest-energy
absorption band of compound 2 at 453 nm is found to blueshift
by 7 nm relative to that of 3 (460 nm). This observation signifies
a narrower HOMO–LUMO gap in 3 as a result of the electron
push–pull effects7c,d among the electron-donating TTFAQ moiety
and two electron-withdrawing carboxylic ester endgroups. The
amino (donor) attached TTFAQ 2 shows molar extinction coeffi-
cients (e) that are considerably greater than those of carboxylic es-
ter (acceptor) endcapped TTFAQ 3 in the low-energy region. To
further probe the D/A-substituent effects on the electronic spectro-
scopic behavior, the UV–vis spectral data of 2 and 3 were com-
pared with analogous D/A-substituted TTFAQs 1a–d, and the
details are listed in Table 2.
A clear trend can be established from Table 2 that the lowest-
energy absorption peak (kmax) that corresponds to HOMO to LUMO
transition energy is redshifted as the substituent increases with
The synthesis of A–D–A type TTFAQ-core triad 3 was carried out
adopting a similar cross-coupling strategy to that used for 2. As
acceptor strength (r
para).14 Conversely, the electron-donating sub-
stituent causes a blueshift of the kmax value in comparison with the
unsubstituted TTFAQ 1a. Of note is that the kmax of 1d is noticeably
redshifted by 3 nm relative to 1a. This result indicates that the
Table 1
UV–vis spectroscopic and electrochemical data of 2 and 3
Entry
kabs (nm)
e
(L molꢀ1 cmꢀ1
)
Epa (V)
Epc (V)
2
453
391 (sh)
350
295
259
4.1 ꢁ 104
4.5 ꢁ 104
7.2 ꢁ 104
5.4 ꢁ 104
5.3 ꢁ 104
+0.56
+0.92
ꢀ
3
460
396
325
305
2.9 ꢁ 104
2.1 ꢁ 104
5.6 ꢁ 104
6.0 ꢁ 104
+0.61
+0.47
Scheme 3. Synthesis of carboxylic ester-terminated TTFAQ derivative 3.

2894
M. Shao, Y. Zhao / Tetrahedron Letters 51 (2010) 2892–2895
Table 2
mentary data), a quasi-reversible wave pair is discernible at
+0.61 (anodic) and +0.47 V (cathodic), respectively, which is char-
acteristic of TTFAQ. The anodic potential is higher than that of 2,
which testifies to the electron-withdrawing effect of the carboxylic
ester groups.
Conceivably, the amino groups and carboxyl groups of 2 and 3
are Lewis bases that could function as ligands to coordinate with
transition metal ions. If such complexation occurs, the electronic
nature of the substituent should be considerably modified. In prin-
ciple, the TTFAQ core in the resulting complexes should be more
electron-deficient in comparison with their neutral states, hence
leading to anodically shifted oxidation potentials.
Substituent effects on the UV–vis absorption behavior of D/A-substituted TTFAQs 1–3
(L molꢀ1 cmꢀ1
)
D
kmax (nm) rpara
a
b
c
Entry Substituent kmax (nm)
e
1a
1b
1c
1d
2
H
NO2
CN
S(t-Bu)
NMe2
COOCH3
458
470
470
461
453
460
2.7 ꢁ 104
2.2 ꢁ 104
1.9 ꢁ 104
2.6 ꢁ 104
4.1 ꢁ 104
2.9 ꢁ 104
0
0
+12
+12
+3
+0.81
+0.70
ꢀ
ꢀ5
ꢀ0.83
+0.45
3
+2
a
b
c
Wavelength of the lowest-energy absorption peak.
Shift of kmax relative to unsubstituted TTFAQ 1a.
Substituent constant taken from Ref. 14.
In the experiments, aliquots of AgOTf or Cu(OTf)2 were titrated
to the solutions of TTFAQ derivatives 2 and 3, and the titration pro-
cesses were monitored by cyclic voltammetry. Worth noting is that
upon metal cation titrations (Ag+ and Cu2+), both the solutions of 2
and 3 changed from orange into dark brown color, which is sugges-
tive of significant metal–ligand complexation. The titration results
shown in Figure 2, surprisingly, appear to be more complex than
expected. In Figure 2B and D, the increasing addition of Cu2+ to 2
considerably reduced the current intensity of the second anodic
peak (E2pa ¼ þ0:92V), which corresponds to the oxidation of amino
moieties. This observation confirms that the amino groups of 2 are
the active binding sites for complexation with Cu2+ cation. The
addition of Cu2+ to 3 also reduced appreciably the current intensity
of the anodic peak of TTFAQ (Epa = +0.61 V), substantiating the
complexation between 3 and Cu2+. Besides, as the titration pro-
gressed, the redox wave due to Cu2+ at +1.16 V was observed to
shift cathodically. In contrast to the Cu2+ titration data, titrations
S(t-Bu) group in 1d should be deemed as weakly electron-with-
drawing as a result of its relatively stronger inductive effect which
prevails over its resonance (electron-donating) effect.
The redox properties of TTFAQs 2 and 3 as well as their electro-
chemical responses to selected transition metal ions (Ag+ and Cu2+
)
were investigated by cyclic voltammetry. In the cyclic voltammo-
gram of 2 (see Fig. S-5 in Supplementary data), two irreversible
anodic peaks were clearly seen at +0.56 and +0.92 V. The first peak
can be assigned to a simultaneous two-electron oxidation of the
central TTFAQ core,7c while the second peak is due to the oxidation
of the terminal amino groups. The irreversible redox behavior is in
stark contrast to other TTFAQ analogues, wherein oxidation of the
TTFAQ moiety usually gives rise to a quasi-reversible redox wave
pair. A plausible explanation for the irreversibility is that swift
chemical reaction(s) took place after the oxidation of the amino
groups. In the cyclic voltammogram of 3 (see Fig. S-5 in Supple-
Figure 2. Cyclic voltammetric titrations of (A) 2 (1.0
lM) and AgOTf; (B) 2 (1.0 lM) and Cu(OTf)2; (C) 3 (1.0 lM) and AgOTf; (D) 3 (1.0 lM) and Cu(OTf)2. Electrolyte: Bu4NBF4
(0.1 M); solvent: CH2Cl2/CH3CN (4:1); working electrode: glassy carbon; counter electrode: Pt; reference electrode: Ag/AgCl.

M. Shao, Y. Zhao / Tetrahedron Letters 51 (2010) 2892–2895
2895
of Ag+ to 2 and 3, respectively, did not cause any significant
changes in the redox features of TTFAQ, thus ruling out the possi-
bility of significant interactions between Ag+ and S atoms in 3. On
the other hand, the anodic peak at ca. +0.58 V due to Ag+ was ob-
served to move anodically with increasing titration, the exact rea-
son for such shift is unclear and awaits further exploration.
In conclusion, two new D/A-substituted TTFAQ derivatives 2 and 3
were prepared and added to the family of TTFAQ-cored D/A triads.
Availability of these compounds has allowed for a systematic exami-
nation of the D/A substitution effects on the electronic spectroscopic
properties of TTFAQ through acetylenic conjugation. Electrochemical
titration experiments on TTFAQ derivatives 2 and 3 with selected me-
tal ions have demonstrated good sensing selectivity for Cu2+ ions.
Application of such D/A TTFAQ triads as electrochemical sensors for
other transition metal ions (e.g., Pb2+, Cd2+, Hg2+, Zn2+, etc.) is under
investigation and will be reported in due course.
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Acknowledgments
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The authors thank NSERC, CFI, IRIF, and Memorial University for
financial support.
12. Characterization data for compound 2: An orange solid. Mp: 250–252 °C; IR
Supplementary data
(neat): 2918, 2852, 2197, 1609, 1593 cmꢀ1 1H NMR (500 MHz, CDCl3): d 7.64
;
(s, 4H), 7.51 (d, J = 7.9 Hz, 2H), 7.46–7.41 (m, 6H), 6.69 (d, J = 7.9 Hz, 4H), 3.00
(s, 12H), 2.41 (s, 12H); HSQC data see Supplementary data; HRMS (MALDI-TOF,
m/z) calcd for C44H38N2S8: 850.0801 [M]+, found: 850.0869.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2010.03.099.
13. Characterization data for compound 3: an orange solid. Mp: 135–136 °C; IR
;
(neat): 2948, 2920, 2205, 1724, 1605, 1527 cmꢀ1 1H NMR (500 MHz,
References and notes
CDCl3): d 8.04 (d, J = 8.4 Hz, 4H), 7.69 (d, J = 1.3 Hz, 2H), 7.63 (d, J = 8.4 Hz,
4H), 7.55 (d, J = 8.4 Hz, 2H), 7.48 (dd, J = 8.0, 1.5 Hz, 2H), 3.94 (s, 6H), 2.42
(s, 12H); 13C NMR (125 MHz, CDCl3): d 166.5, 134.8, 134.6, 131.6, 129.8,
129.58, 129.55, 128.2, 127.9, 126.7, 125.8, 125.5, 122.1, 120.7, 92.3, 89.5,
52.5, 19.42, 19.25 (one signal not observed due to coincidental overlap);
HRMS (MALDI-TOF, m/z) calcd for C44H32O4S8: 880.0066 [M]+, found:
880.0057.
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