Boron-containing monopyrrolo-annelated tetra thiafulvalene compounds: Synthesis and absorption spectral/electrochemical responsiveness toward fluoride Ion

Article abstract of DOI:10.1021/jo1007306

(Figure presented) Two new boron-based conjugated compounds 1 and 2 containing one and three monopyrrolo-annelated tetrathiafulvalene (TTF) unit(s) were synthesized and characterized. They exhibit typical ICT absorptions which can be modulated after addit

Full text of DOI:10.1021/jo1007306

pubs.acs.org/joc
ting materials for organic light-emitting devices (OLEDs).2
Boron-Containing Monopyrrolo-Annelated Tetra
thiafulvalene Compounds: Synthesis and Absorption
Spectral/Electrochemical Responsiveness toward
Fluoride Ion
It has also been reported that π-conjugated systems with
boron show strong affinity toward fluoride ion.3 The binding
of fluoride ion to the boron center disrupts or perturbs the
p-π conjugation between the boron center and the aromatic
chromophore, leading to absorption and fluorescent spectral
changes. By taking advantage of boron-containing π-con-
jugated molecules, new selective and sensitive sensors for
fluoride are developed.4-6
In typical boron-containing π-conjugated molecules, elec-
tron-donor units such as anthracene2c and substituted
amine4d,e were linked to boron either directly or through
conjugated spacers. These conjugated molecules exhibit in-
tramolecular charge-transfer (ICT) absorption and ICT
fluorescence in some cases. It is expected that the ICT
interaction within the boron-containing π-conjugated mole-
cules would be modulated if a stronger electron donor such
as tetrathiafulvalene (TTF) is incorporated. It should be
noted that TTF and its derivatives have been intensively
investigated as strong electron donors for conducting mate-
rials, molecular machines, redox-fluorescence switches, and
chemical sensors.7-9 A number of TTF-based electron donor
(D)-acceptor (A) dyads or triads have been studied for
intramolecular CT interactions and photoinduced electron
Jing Li,†,‡ Guanxin Zhang,*,† Deqing Zhang,*,†
Renhui Zheng,† Qiang Shi,† and Daoben Zhu†
†Beijing National Laboratory for Molecular Sciences,
Organic Solids Laboratory, Institute of Chemistry, Beijing
100190, China, and ‡Graduate School of Chinese Academy of
Sciences, Beijing 100049, China
dqzhang@iccas.ac.cn; gxzhang@iccas.ac.cn
Received April 15, 2010
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Two new boron-based conjugated compounds 1 and 2
containing one and three monopyrrolo-annelated tet-
rathiafulvalene (TTF) unit(s) were synthesized and
characterized. They exhibit typical ICT absorptions
which can be modulated after addition of fluoride ion.
In addition, the oxidation potentials of 1 and 2 are
shifted to low potential region in the presence of
fluoride ion. Such absorption spectral and electro-
chemical responsiveness of 1 and 2 toward fluoride
ion is due to the binding of the boron units with fluo-
ride ion.
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Published on Web 07/13/2010
DOI: 10.1021/jo1007306
r
2010 American Chemical Society

Li et al.
JOCNote
SCHEME 1. Synthetic Approach to Compounds 1 and 2
FIGURE 1. Absorption spectra of 1 (1.0 ꢀ 10-5 M in THF) (a) and
2 (1.0 ꢀ 10-5 M in THF) (b) upon addition of different amounts of
n-Bu4NF.
As will be discussed below, compounds 1 and 2 exhibit ICT
absorptions which can be modulated upon binding with
fluoride ion.
transfer processes.8h,10-12 A TTF-based D-A compound with
a boronic acid unit was synthesized as saccharide sensor.13
However, to the best of our knowledge, no boron-containing
conjugated molecules with TTF units have been reported. In
this paper, we describe two new boron-based conjugated
compounds 1and 2(Scheme 1) containing one and three mono-
pyrrolo-annelated tetrathiafulvalene (TTF) unit(s), respectively.
The synthetic approach to compounds 1 and 2 was out-
lined in Scheme 1. Compound 6, the monopyrrolo-annelated
tetrathiafulvalene, was synthesized by the cross-coupling of
compounds 312d and 4,14 followed by removal of the tosylate
group in compound 5 with NaOCH3 (see the Supporting
Information). Compounds 1 and 2 were obtained by N-
arylation reaction of the pyrrole unit in 6 with (4-iodophenyl)-
dimesitylborane (7) and tris(4-bromo-2,6-dimethylphenyl)-
borane (9) (see the Supporting Information), respectively, in
acceptable yields.15 Compounds 73 and 816 were synthesized
by following the procedures reported previously. For com-
parison, compound 10 was prepared via the coupling reaction
between 6 and iodobenzene (see the Supporting Information).
Figure 1 shows the absorption spectra of compounds 1
and 2 in THF. Compared to those of compound 10 and
compound 7 (or 9), which exhibit no strong absorptions
above 350 nm (see Figure S1, Supporting Information), com-
pounds 1 and 2 show new absorptions around 400 nm. This
should be due to the ICT absorptions from th TTF units to
the boron centers in 1 and 2. Moreover, the ICT absorption
maximum of compound 2 is red-shifted by 20 nm compared
to that of compound 1. As discussed below, this is in agree-
ment with the corresponding HOMO-LUMO energy gaps
of 1 and 2 which are estimated theoretically. It is interesting
to note that the absorption spectra of compounds 1 and 2 can
be modulated by addition of fluoride ion. The ICT absorp-
tions of 1 and 2 around 400 nm became progressively weaker as
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J. Org. Chem. Vol. 75, No. 15, 2010 5331

Li et al.
JOCNote
FIGURE 3. Partial 1H NMR spectra of 2 (5.0 ꢀ 10-3 M in THF-d8)
after addition of different amounts of n-Bu4NF.
oxidation waves with E1/2(ox1) = 0.70 V and E1/2(ox2) =
0.91 V were observed for compound 1, corresponding to the
formation of the radical cation (TTF•þ) and dication
(TTF2þ) of the TTF unit, respectively, according to previous
studies.14 Similarly, the oxidation potentials of compound 2
were measured to be E1/2(ox1) = 0.67 V and E1/2(ox2) = 0.93V.
For comparison, the cyclic voltammogram of compound 10
was recorded, and the corresponding oxidation potentials
were determined to be E1/2(ox1) = 0.65 V and E1/2(ox2) =
0.87 V (see Figure S7, Supporting Information). It is obvious
that the oxidation potentials of compounds 1 and 2 are
slightly more positive compared to those of compound 10.
This is probably due to the electron-withdrawing effects of
boron units in 1 and 2.
The cyclic voltammograms of 1 and 2 were changed after
the addition of fluoride ion as demonstrated in Figure 2. The
first oxidation wave of compound 1 was progressively shifted
to low-potential region. No further shifts were detected for
the oxidation potentials of 1 after the addition of more than
1.0 equiv of fluoride ion (see Figure S6, Supporting Infor-
mation).17 The first oxidation peak potential of 1 was shifted
from 0.78 to 0.64 V in the presence of 1.0 equiv of fluoride
ion. The second oxidation wave was also slightly shifted to
low-potential region. Similar variation was detected for
compound 2. The first oxidation peak potential of 2 was
shifted from 0.79 to 0.73 V when 1.0 equiv of fluoride ion was
added, but the oxidation waves were not changed further
after addition of more than 1.0 equiv of fluoride ion (see
Figure S6, Supporting Information). Such oxidation poten-
tial shifts can be interpreted by considering the fact that the
binding of the boron centers in 1 and 2 with fluoride ion will
weaken the electron-withdrawing effects of boron units in 1
and 2. The oxidation potential shift upon addition of fluoride
ion for compound 2 is smaller than that for compound 1. This
may be due to the fact the electron-withdrawing capacity of
the boron unit in 2 is weaker because of the presence of three
TTF units compared to that of the boron unit in 1, and as a
result, the binding of the boron unit in 2 with fluoride will not
significantly modify the electronic structure of 2.
FIGURE 2. Cyclic voltammograms of compounds 1 (1.0 ꢀ 10-3
M) (a) and 2 (1.0 ꢀ 10-3 M) (b) in THF at 25 °C with n-Bu4NPF6
(0.1 M) as the supporting electrolyte and Ag/AgCl as reference
electrode in the presence of increasing amounts of n-Bu4NF.
demonstrated in Figure 1. The color of solutions of 1 and 2
changed from yellow to colorless after introduction of the
fluoride ion. Such absorption spectral variation of 1 and 2
after addition of fluoride ion can be understood by consider-
ing the fact that the electron-withdrawing capacities of the
boron units in 1 and 2 would be largely weakened upon
binding with fluoride ion. Such absorption spectral change
in the presence of fluoride is relevant to a LUMO-centered
process.
The absorption spectra of 1 and 2 in the presence of diff-
erent amounts of fluoride ion (n-Bu4Nþ as the countercation)
were measured. The corresponding Job plots (see Figure S2,
Supporting Information) indicated that the binding of fluor-
ide ion with compounds 1 and 2 exhibited a 1:1 stoichiom-
etry. By fitting the absorbance variation at 390 and 408 nm
for compounds 1 and 2, respectively (see the Supporting
Information), the binding constants of 1 and 2 with fluoride
ion were estimated to be (3.65 ( 0.13) ꢀ 104 M-1 and (1.65 (
0.06) ꢀ 104 M-1, respectively. Therefore, compound 1 binds
fluoride ion more strongly than compound 2. The presence
of three TTF units in 2 would weaken the electron-with-
drawing capacity of the boron unit in 2, thus reducing the
binding with fluoride. Additionally, the boron center in 2 is
surrounded by three bulky groups, which would make the
binding of fluoride ion less favorable.
The absorption spectra of compounds 1 and 2 were also
examined in the presence of other anions including AcO-,
Cl-, Br-, I-, NO3-, HSO4-, PF6-, and ClO4-. The absorp-
tion spectra of 1 and 2 were almost unaltered in the presence
of these anions (see Figures S3 and S4, Supporting Informa-
tion). Therefore, it may be concluded that compounds 1 and
2 show selective binding with fluoride ion.
The binding of compounds 1 and 2 with fluoride ion was
also investigated with 1H NMR spectroscopy. Figure 3 shows
The electrochemical studies also confirmed the respon-
siveness of compounds 1 and 2 toward fluoride ion. Figure 2
shows the cyclic voltammograms of compounds 1 and 2 and
those after additions of fluoride ion. Two quasireversible
(17) The control cyclic voltammetric experiment with fluoride ion (see
Figure S6, Supporting Information) indicates that the slight variation of the
oxidation waves of 1 after the addition of more than 1.0 equiv of fluoride ion
is due to the excess fluoride ion.
5332 J. Org. Chem. Vol. 75, No. 15, 2010

Li et al.
JOCNote
the partial 1H NMR spectrum of 2 and those in the presence
of different amounts of fluoride ion. The protons of the
methyl groups linked to benzene rings and those of the
benzene and pyrrole rings show signals at 2.1, 7.1, and 7.3
ppm, respectively, before the addition of fluoride ion. New
signals at 1.8, 2.2, 6.6, 6.8, and 7.0 ppm gradually emerged,
and the original signals became weak after introduction of
fluoride ion to the solution. The signals due to protons of the
benzene and pyrrole rings were shifted upfield after the
addition of 1.5 equiv of fluoride ion. This is likely due to
the binding of the boron center in 2 with fluoride ion, which
will make the boron unit less electron-deficient. Two singlet
signals at 1.8 and 2.2 ppm were observed for the protons of
the methyl groups after addition of 1.5 equiv of fluoride ion.
Such variations of 1H NMR signals are induced by the
binding of boron center in 2 with fluoride ion; as a result,
the hybridization of boron center in 2 will be changed from
sp2 to sp3, leading to a configuration change for the whole
molecule. Consequently, the protons on the methyl groups
become nonequivalent. Similar 1H NMR spectral variation
was detected for compound 1 after the addition of fluoride
ion (see Figure S5, Supporting Information). This 1H NMR
spectral variation provides further evidence for the binding
of compounds 1 and 2 with fluoride ion.
Finally, theoretical calculations were carried out for com-
pounds 1 and 2 using B3LYP/6-31G* with Gaussian 03 (see
the Supporting Information). The calculation results indi-
cate that the boron centers in 1 and 2 exhibit three-coordi-
nated propeller-like conformations (see Figure S8, Supporting
Information). The HOMO and LUMO energies of 1 were
calculated to be -4.76 and -1.82 eV, and those of 2 were
-4.82 and -2.32 eV. The energy gaps were 2.94 and 2.50 eV
for 1 and 2, respectively. Compound 2 has a smaller energy
gap than compound 1. This is consistent with the fact the
maximum ICT absorption of 2 appears at 408 nm, longer
than that of compound 1 (390 nm). The HOMO orbitals
mainly reside on the TTF units, and the LUMO orbitals are
mainly localized on the boron units for both compound 1
and 2 as demonstrated in Figure S9, Supporting Informa-
tion, but the atoms in the pyrrole ring(s) of the TTF unit(s)
also contribute slightly to the LUMO orbitals of 1 and 2 (see
Figure S9, Supporting Information). These results indicate
that weak interactions between the TTF and boron units in 1
and 2 exist.
troscopy. These results indicate that such TTF-based con-
jugated D-A molecules are promising for selective chemical
sensors and even interesting for electronic materials. There-
fore, they deserve further investigations.
Experimental Section
Synthesis of Compound 1. A flamed-dried Schlenk reaction
tube charged with a mixture of 6 (77 mg, 0.16 mmol), 7 (146 mg,
0.32 mmol), K3PO4 (103 mg, 0.48 mmol), and CuI (9.2 mg, 0.048
mmol) was degassed with N2 for 10 min. After the addition of
racemic trans-diaminocyclohexane (10 μL) and freshly distilled
dioxane (2 mL) under N2, the reaction mixture was stirred with
heating at 110 °C for 24 h. After being cooled to room temp-
erature, the mixture was filtered. The filtrate was collected and
concentrated to obtain a dark green residue which was purified
with silica chromatography using CH2Cl2/petroleum ether
(60 °C-90 °C) (v/v, 1/20) as the eluant to give 1 as a yellow
solid (74 mg, 57%): mp 134-136 °C; 1H NMR (400 MHz, THF-
d8) δ 0.90 (t, J = 6.1 Hz, 6H), 1.29 -1.35 (m, 8H), 1.38-1.49 (m,
4H), 1.63-1.68 (m, 4H), 2.01 (s, 12H), 2.27 (s, 6H), 2.85 (t, J=
6.7 Hz, 4H), 6.81 (s, 4H), 7.32 (s, 2H),7.44 (d, J = 7.6 Hz, 2H),
7.54 (d, J = 7.4 Hz, 2H); 13C NMR (150 MHz, THF-d8) δ 13.4,
20.3, 22.5, 22.7, 28.1, 29.7, 31.3, 35.8, 110.2, 110.3, 117.5, 118.8,
123.0, 127.6, 128.0, 128.1, 138.3, 138.4, 140.3, 141.4, 142.5,
142.7; MALDI-TOF (m/z) calcd for C44H54BNS6 799.27, found
799.3. Anal. Calcd for C44H54BNS6: C, 66.05; H, 6.80; N, 1.75;
S, 24.05. Found: C, 66.02; H, 7.00; N, 1.85; S, 24.21.
Synthesis of Compound 2. A flamed-dried Schlenk reaction
tube charged with a mixture of 6 (246 mg, 0.52 mmol), 9 (72 mg,
0.13 mmol), K3PO4 (329 mg, 1.56 mmol), and CuI (5 mg, 0.026
mmol) was degassed with N2 for 10 min. After the addition of
racemic trans-diaminocyclohexane (10 μL) and freshly distilled
dioxane (2 mL), the system was evacuated three times and
backfilled with N2. The reaction mixture was heated to 110 °C
and stirred for 24 h. After being cooled to room temperature, the
mixture was filtered and the solid washed with CH2Cl2. The
filtrate was collected and concentrated to obtain a dark green
residue which was purified with silica chromatography using
CH2Cl2/petroleum ether (60 °C-90 °C) (v/v, 1/15) as the eluant
1
to give 2 as an orange solid (110 mg, 49%): mp >280 °C; H
NMR (400 MHz, THF-d8) δ 0.91 (t, J = 6.5 Hz, 18H), 1.30-
1.33 (m, 24H), 1.43-1.47 (m, 12H), 1.62-1.67 (m, 12H), 2.13 (s,
18H), 2.86 (t, J = 7.1 Hz, 12H), 7.14 (s, 6H), 7.27 (s, 6H); 13
C
NMR (150 MHz, THF-d8) δ 14.5, 23.5, 25.8, 29.1, 30.8, 32.3,
36.9, 111.0, 111.2, 118.6, 120.1, 123.2, 128.6, 142.0, 143.7, 144.8;
MALDI-TOF (m/z) calcd for C84H108BN3S18 1745.36, found
1745.4. Anal. Calcd for C84H108BN3S18: C, 57.73; H, 6.23; N,
2.40; S, 33.02. Found: C, 57.82; H, 6.46; N, 2.44; S, 32.62.
In summary, two new boron-based compounds 1 and 2
containing one and three monopyrrolo-annelated tetrathia-
fulvalene (TTF) unit(s) were synthesized and characterized.
Compounds 1 and 2 exhibit typical ICT absorptions by
virtue of the electron-donating and -withdrawing character
of the TTF and boron units. Moreover, the maximum ICT
absorption of 2 appears at longer wavelength than that of 1,
which is in agreement with the theoretical calculations. It is
interesting to note that both ICT absorption and cyclic
voltammograms of 1 and 2 can be modulated after the
addition of fluoride ion. Thus, compounds 1 and 2 show
absorption spectral and electrochemical responsiveness to-
ward fluoride ion. Additionally, the binding of compounds 1
Acknowledgment. The present research was financially
supported by NSFC, Chinese Academy of Sciences, and
State Key Basic Research Program. This work was partially
supported by the NSFC-DFG joint project (TRR61). We
thank the anonymous reviewers for their comments and
suggestions.
Supporting Information Available: Synthesis and character-
ization of compounds 5, 6, 9, and 10; Job plots; absorption
1
spectra of 1 and 2 in the presence of other anions; H NMR
spectra of 1 in the presence of F-; theoretical computations; 1H
and 13C NMR spectra. This material is available free of charge
via the Internet at http://pubs.acs.org.
1
and 2 with fluoride ion were studied with H NMR spec-
J. Org. Chem. Vol. 75, No. 15, 2010 5333