Versatile strategy to access fully functionalized benzodifurans: Redox-active chromophores for the construction of extended φ-conjugated materials

Article abstract of DOI:10.1021/jo100323s

An efficient synthetic approach to construct a fully substituted benzo[1,2-b:4,5-b′]difuran (BDF) 2a via base-catalyzed double annulations is presented. Compound 2a can readily undergo Suzuki, Heck, and Sonogashira coupling reactions to afford in good yields a manifold of extended φ-conjugated BDF derivatives, e.g., with pyridine termini (4-6) and with different spacers. These are highly luminescent materials that undergo two reversible one-electron oxidations. Remarkably, their photophysical and electrochemical properties can be largely tuned by methylation or protonation. Consequently, they can function as pH-dependent fluorescence switches. Finally, the observed electronic properties are explained on the basis of density functional theory.

Full text of DOI:10.1021/jo100323s

pubs.acs.org/joc
Versatile Strategy To Access Fully Functionalized Benzodifurans:
Redox-Active Chromophores for the Construction of Extended
π-Conjugated Materials
Chenyi Yi,† Carmen Blum,† Mario Lehmann,† Stephan Keller,† Shi-Xia Liu,*,†
†
Gabriela Frei, Antonia Neels, Jurg Hauser, Stefan Schurch, and Silvio Decurtins
‡
†
†
†
€
€
†
€
Departement fu€r Chemie und Biochemie, Universitat Bern, Freiestrasse 3, CH-3012 Bern, Switzerland, and
‡XRD Application LAB, CSEM Centre Suisse d’Electronique et de Microtechnique SA, Jaquet-Droz 1,
^
Case postale, CH-2002 Neuchatel, Switzerland
liu@iac.unibe.ch
Received March 1, 2010
An efficient synthetic approach to construct a fully substituted benzo[1,2-b:4,5-b0]difuran
(BDF) 2a via base-catalyzed double annulations is presented. Compound 2a can readily undergo
Suzuki, Heck, and Sonogashira coupling reactions to afford in good yields a manifold of
extended π-conjugated BDF derivatives, e.g., with pyridine termini (4-6) and with different
spacers. These are highly luminescent materials that undergo two reversible one-electron
oxidations. Remarkably, their photophysical and electrochemical properties can be largely
tuned by methylation or protonation. Consequently, they can function as pH-dependent
fluorescence switches. Finally, the observed electronic properties are explained on the basis of
density functional theory.
Introduction
diodes (OLEDs), organic field-effect transistors (OFETs), and
photovoltaic cells. Specifically, the synergetic effect of the BDF
core and the different substituents which extend the π-conjuga-
tion of the BDF skeleton play a decisive role; this is an issue which
is directly relevant in the context of the present work. This class of
planar and highly conjugated heterocyclic molecules can also be
addressed as organic fluorophores3 which classifies them, alto-
gether, as multifunctional materials. However, only quite limited
reports on their synthesis appear in the literature. A common
approach involves the intramolecular double-cyclization forma-
tion of a difuran moiety starting from properly substituted
arenes.4 For example, various multisubstituted BDFs have been
Derivatives of the heterocyclic compounds benzofuran
and benzodifuran (BDF) are ubiquitous in nature, and they
show a wide range of pharmacological properties and bio-
logical activities which consequently put them into the focus
of longstanding and widespread interest in the fields of
bioorganic and medical research.1
Additionally, within the field of materials chemistry, BDF
derivatives proved also to be excellent compounds for high-
performance hole-transporting materials,2 which, for in-
stance, find applications in multilayer organic light-emitting
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Published on Web 04/26/2010
DOI: 10.1021/jo100323s
r
2010 American Chemical Society

Yi et al.
JOCArticle
chemical, and electrochromic materials8 and solar energy
conversion.9
Results and Discussion
Synthesis. Because of our interest in malononitrile
chemistry,10 we probed the reactivity of malononitrile with
iodo- or ethynyl-substituted 1,4-benzoquinones (Schemes 1
and 2), demonstrating that multifunctionalized BDF mole-
cules can be elaborated via a base-catalyzed intramolecular
cyclization. We assume that a plausible reaction mechanism
is analogous to the one proposed by Obushak et al.11 This
one-pot process involves a Michael reaction between the
carbanion of malononitrile and quinone, followed by an
intramolecular double cyclization. As shown in Scheme 1, 1a
is readily available from 2,5-diiodo-1,4-benzoquinone. How-
ever, it was observed that 1a could not be converted into 1b
(shown in Scheme 2) through the Sonogashira coupling
reaction, very probably because of its poor solubility.
FIGURE 1. Chemical structure of the fully functionalized benzo-
[1,2-b:4,5-b0]difuran (BDF) derivatives.
obtained in good yields by using palladium-catalyzed double
annulations of bis(allyloxy)bis(alkynyl)benzenes or bis(alkynyl)-
dihydroxybenzenes in the presence of allylic halides.4a
To create a novel series of molecules that could have
potentially interesting material properties, we embarked on
the design and synthesis of a range of fully functionalized
BDFs. Herein, we report a facile and efficient synthetic
protocol to a fully functionalized benzo[1,2-b:4,5-b0]difuran
(BDF) molecule 1 (Figure 1). This approach has a number of
advantages. First, two nitrile groups at the 3,7-positions
of the BDF core substantially extend the π-conjugated
BDF skeleton. Second, two amine functions located at
the 2,6-positions, when alkylated, render good solubility
and processability to the rigid backbone, which are pre-
requisite for the attainment of a variety of functional materials.
Finally, the 1,4-positions of the benzene core are occupied
either by an iodo functionality or by pending ethynyl
groups, which clearly renders a motif for employing Suzuki,
Heck, and Sonogashira coupling reactions to prepare a
broad range of functional molecules. For example, the
BDF derivatives 1 can act as precursors for the construction
of oligo(p-phenyleneethylene) (OPE)5-type compounds
which are of particular interest in organic molecular
(opto)electronics.
It is well-known that molecules with pyridyl substituents
at the terminal positions are expected to be alligator clips
for the synthesis of molecular devices.6 They can also
afford interesting supramolecular architectures with fasci-
nating properties by intermolecular interactions and coordi-
nation with metal ions.7 In this context, three BDF deriva-
tives 4-6 with pyridine termini and different spacers have
been synthesized via the corresponding Suzuki, Heck,
and Sonogashira coupling reactions of the key precursor
2a, which can be obtained after hexylation of 1a. Moreover,
their dimethylated compounds 7-9 have been prepared
as viologen analogues. Viologens have been widely used in
the fundamental study of electrochemical, photoelectro-
We envisioned the same methodology as for 1a to be
applicable to the synthesis of 1b (Scheme 2). Thus, 2,5-bis-
(trimethylsilylethynyl)-1,4-benzoquinone was used, which can
be prepared in excellent yield from 1,4-diiodo-2,5-dimethoxy-
benzene through a Sonogashira coupling and subsequent
oxidation reaction. Under the reaction conditions of the final
step, desilylation was concomitantly induced to give 1b di-
rectly, but only in low yield. Seemingly, the trimethylsilylethy-
nyl groups are unstable and interfere with the reaction process.
Next, Pd-catalyzed Sonogashira coupling reactions of 1a
with 4-ethynylpyridine and of 1b with 4-iodopyridine have
been tested. However, due to the poor solublility of these
BDF compounds, the desired coupling products could not be
isolated. In order to circumvent this problem, hexyl groups
have been introduced to afford 2a in satisfactory yield
(Scheme 1). The synthesis of 2b consists of Pd-catalyzed
Sonogashira coupling and subsequent deprotection of the
alkynes. Compound 2a was readily converted into BDF
derivatives 4-6 with pyridine termini via the Suzuki cou-
pling with 4-pyridylboronic acid, the Sonogashira coupling
with 4-ethynylpyridine, or the Heck coupling with 4-vinyl-
pyridine (Scheme 3). Additionally, an alternative approach
to 5 involving the Sonogashira coupling reaction of 2b with
4-iodopyridine has been investigated, showing that with this
protocol 5 can only be isolated in 29% yield (see the
Supporting Information). The lower yield indicates that
the self-coupling of alkynes takes place under the tested
reaction conditions. Notably, as illustrated in Scheme 3,
quaternization of the pyridine termini of 4-6 was performed
by treatment with an excess of methyl iodide in CH3CN
under microwave irradiation to afford the corresponding
conjugation-extended viologen derivatives 7-9 in good
yields. All new compounds were unambiguously character-
ized by spectroscopic analyses and also by single-crystal
X-ray analyses in the case of 1b and 4-6.
(5) (a) Dalton, G. T.; Cifuentes, M. P.; Watson, L. A.; Petrie, S.; Stranger,
R.; Samoc, M.; Humphrey, M. G. Inorg. Chem. 2009, 48, 6534. (b) Tang, Y.;
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Angew. Chem., Int. Ed. Engl. 1994, 33, 385.
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Leutwyler, S.; Decurtins, S. J. Org. Chem. 2008, 73, 3596. (b) Yi, C.; Blum, C.;
Liu, S.-X.; Frei, G.; Neels, A.; Stoeckli-Evans, H.; Leutwyler, S.; Decurtins, S.
Tetrahedron 2008, 64, 9437. (c) Yi, C.; Blum, C.; Liu, S.-X.; Ran, Y.-F.; Frei,
G.; Neels, A.; Stoeckli-Evans, H.; Calzaferri, G.; Leutwyler, S.; Decurtins, S.
Cryst. Growth Des. 2008, 8, 3004.
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(b) Barltrop, A. J.; Jackson, C. A. J. Chem. Soc., Perkin Trans. 2 1984, 367.
(c) Monk, P. M. S. The viologens: Physicochemical Properties, Synthesis and
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SCHEME 1. Synthetic Routes to 2a and 2b
SCHEME 2. Synthetic Route to 1b
or the b-axis (6). Distinct molecular directions are twisted for
adjacent molecules within the stacks by angles of ∼70° (5)
or ∼90° (6). In the case of 5, the stacking alignment is mainly
caused by π π interactions; the interplanar separations
3 3 3
between the least-squares planes of the molecular cores
˚
amounts to 3.24(1) A. In contrast, the molecular arrangement
for 6 is mainly driven by van der Waals interactions between
the aliphatic chains. Consequently, the interplanar distances
˚
are >5 A. The crystal structures and crystal packings of 1b
and 4 are shown in Figures S2-S3 and S4-S5 (Supporting
Information), respectively. In contrast to the nearly planar
structures of 5 and 6, the compound 4 adopts a nonplanar
conformation with a dihedral angle of 53.1(1)° between the
pyridine rings and the BDF unit due to ortho-repulsive
interactions between cyano groups and pyridine rings.
Electrochemistry. The electrochemical properties of 4-9
in CH2Cl2 were investigated by cyclic voltammetry (CV).
Their electrochemical data are collected in Table 1, and
Figure 3 exhibits as a typical example the CV curves for
compound 6 and its viologen derivative 9. All compounds
undergo two well-separated and reversible single-electron
oxidation processes to the BDF centered radical cation and
dication states. Compared to molecules 4 and 6, compound 5
exhibits high oxidation potentials, very probably because of
the strong electron-withdrawing effect of the 4-pyridylethy-
nyl groups. Note that, upon protonation or methylation,
the oxidation potentials are significantly positive-shifted,
which can be attributed to the significant electron-withdrawing
effect of the pyridinium groups. In addition, the methylated
BDF derivatives 7 and 9 show a reversible two-electron
reduction peak suggesting that they behave as the π-extended
viologens. Obviously, their cation radical states are thermo-
dynamically unstable because of their nonplanar structures,
which is in good agreement with previously published
results.14 Compound 9 shows a less negative reduction poten-
tial (E1/2 = -0.67 V) than 7 (E1/2 = -0.89 V) due to the
Structural Studies. Compound 5 crystallizes as solvate-free
orange rods in the triclinic space group P1, while compound 6
crystallizes as solvate-free orange needles in the monoclinic
space group P21/c. In both cases, two crystallographically
independent molecules (A, B) are found in the unit cell showing
only small differences in their structural parameters; the crystallo-
graphic inversion center is located at the C6 ring. The BDF core
is nearly planar; the rms deviations from least-squares planes
˚
˚
through the heterocyclic skeletons are 0.01 A (A) and 0.22 A (B)
˚
of compound 5 and 0.02 A (A and B) for compound 6,
respectively. The dihedral angles formed between the pyridine
rings and the BDF unit are 18.7(4)° and 21.5(5)° for 5 and
14.9(4)° and 16.0(4)° for 6. There are no exceptional geometrical
features, and all bond lengths and angles are within the expected
range; they compare well with reported structures of BDF
derivatives.12,13 As an example, the molecular structure of 5 is
shown in Figure 2 (Figure S6, Supporting Information for 6).
In the crystal lattices (Figures S1 and S7, Supporting
Information), the molecules are stacked along the a-axis (5)
(12) Shukla, R.; Wadumethrige, S. H.; Lindeman, S. V.; Rathore, R. Org.
Lett. 2008, 10, 3587.
(13) Choi, H. D.; Seo, P. J.; Son, B. W.; Lee, U. Acta Crystallogr. 2007,
E63, o2920.
(14) Takahashi, K.; Nihira, T.; Akiyama, K.; Ikegami, Y.; Fukuyo, E.
J. Chem. Soc., Chem. Commun. 1992, 620.
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SCHEME 3. Synthetic Routes to Compounds 4-9
TABLE 1. Redox Potentials (V versus Ag/AgCl), Onset Oxidation and
Reduction Potentials, Ionization Potentials (IP), Electron Affinities (EA),
and Electrochemical Band Gaps Egel of Compounds 4-9
el
IPc
EAd Eg
ox1
ox2
E1/2
red
E1/2
ox1
Eonset
red
Eonset
compd E1/2
(eV) (eV) (eV)
4
5
6
7
8
9
0.82
0.90
0.83
0.97
0.99
0.93
1.15
1.24
1.15
0.72
0.77
0.70
0.80
-5.08
-5.13
-5.06
1.19a -0.89
-0.71 -5.16 -3.65 1.51
-0.63 -5.23 -3.73 1.50
-0.53 -5.12 -3.83 1.29
1.36 -0.72b 0.87
1.25 -0.67
0.76
aQuasi-reversible peak. birreversible peak. cEstimated from the onset
oxidation potentials using empirical equations: IP = -(Eonsetox1 -0.44
þ 4.8) eV. dEstimated from the onset reduction potentials using empiri-
cal equations: EA = -(Eonsetred -0.44 þ 4.8) eV.
and electron affinities (EA, LUMO levels) of 3.65-3.83 eV
were calculated according to the following equation:
EHOMO/ELUMO=[-(Eonset (vs Ag/AgCl) - E1/2 (ferrocene) þ
4.8)] eV, where 4.8 eV is the energy level of ferrocene below the
vacuum level.15 Again, it is worthwhile to note that electroche-
mical band gaps (Eg =IP - EA) of 7-9 are apparently varied
when different spacers are incorporated into the molecular
structures.
Photophysics. The electronic absorption spectra of
4-6, recorded in CH2Cl2, are shown in Figure 4. For the
FIGURE 2. ORTEP view of molecule
A (30% probability
ellipsoids) of compound 5. Hydrogen atoms are omitted for clarity.
extended π-conjugation. Compound 8, however, only under-
goes one irreversible reduction process, indicative of an EC
mechanism, i.e., an electrochemical step followed by a fast
chemicalreactionstep. Theproduct of thisreductive processis
not clear at this point. It can therefore be deduced that the
nature of the spacers plays an important role in the electro-
chemical properties.
el
The moderate onset oxidation potentials and onset reduc-
tion potentials of 7-9 were observed between 0.76 and
0.87 V and -(0.53-0.71) V, respectively, from which the
ionization potentials (IP, HOMO levels) of 5.12-5.23 eV
(15) Chen, C. P.; Chan, S. H.; Chao, T. C.; Ting, C.; Ko, B. T. J. Am.
Chem. Soc. 2008, 130, 12828.
(16) Brannon, J. H.; Magde, D. J. Phys. Chem. 1978, 82, 705.
(17) Hamai, S.; Hirayama, F. J. Phys. Chem. 1983, 87, 83.
J. Org. Chem. Vol. 75, No. 10, 2010 3353

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shifts (>5000 cm-1) of the lowest energy absorptions of
the corresponding methylated derivatives 7-9 (Table 2,
Figure 5, and Figure S9 (Supporting Information)). In both
cases, for protonated and methylated compounds, there is no
emission observed. Taking 6 as an example, the acidic
titration experiments on fluorescence intensities were per-
formed (Figure S10, Supporting Information). Upon addi-
tion of 3 equiv of Hþ, the emission is completely quenched.
Upon further deprotonation, however, there is a restitu-
tion of emission. Thus, the BDF derivatives 4-6 can act as
pH-dependent fluorescence switches. Notably, the differ-
ences between the band gap values directly measured by
CV (Egel between 1.29 and 1.51 eV) and the optical band gap
opt
values obtained from UV-vis spectra (Eg between 1.57
and 1.82 eV) lie within an acceptable range.
FIGURE 3. Cyclic voltammograms of 6 (black line) and 9 (green
line) in CH2Cl2 (2 ꢀ 10-4 M); supporting electrolyte 0.1 M
Computational Studies. To rationalize the electronic abs-
orption spectra of 4-9, a computational study was per-
formed, employing density functional theory (DFT) as well
as time-dependent DFT (TD-DFT) calculations with the
B3LYP functional and the valence triple-ζ plus polarization
(TZVP) basis set. All calculations were carried out with the
TURBOMOLE V6.0 program package.18 The ground-state
optimized structures of 5-6 and 8-9, but with NH2 instead
of N(hexyl)2, were constrained to C2h symmetry. In contrast,
the pyridine rings in the calculated minimum-energy geome-
tries of compounds 4 and 7 are twisted by approximately 50°
with respect to the BDF core. The calculated geometries of
compounds 4-6 are in good agreement with X-ray struc-
tures (see the Supporting Information).
The vertical TD-DFT calculations predict the S0fS1
electronic excitation to be an intense in-plane π-π* transi-
tion at 27920 cm-1 (358 nm) for 4, at 22640 cm-1 (442 nm)
for 5, and at 21750 cm-1 (460 nm) for 6, with oscillator
strengths fcalc = 0.18 for 4, 0.44 for 5, and 0.53 for 6 (see stick
spectra in Figure 4), all in fairly good agreement with the
experimental data; the lowest energy absorption bands
show fexp = 0.20 for 4, 0.24 for 5, and 0.22 for 6. This
electronic transition, polarized approximately along the long
molecular axis (horizontal in Figure 6), is dominated (>90%
for 4-6) by the one-electron HOMOfLUMO contribution
(Figure 6). This low energy electronic excitation mainly
corresponds to π-electron flow from the BDF core including
the substituents on the furan rings toward the pyridine
groups and their spacers. This result clearly demonstrates
that due to the extended π-conjugation, the HOMO-
LUMO gap decreases substantially, so that the S0fS1
absorption for 5 and 6 is fully shifted into the visible part
of the spectrum.
(Bu4N)PF6; scan rate 100 mV s-1
.
3
molecules 5 and 6, the most important feature is the strong
(ε ≈ 2 ꢀ 104 M-1 cm-1) and broad absorption band between
18000 cm-1 (555 nm) and 25000 cm-1 (400 nm), which peaks
at around 20800 cm-1 (480 nm), well into the visible part of
the optical spectrum, as evidenced by the intense orange-red
color of both compounds. A series of further intense absorp-
tion bands is observed within the UV region of the spectrum
at energies higher than 25000 cm-1 (400 nm). These optical
absorption spectra of 5 and 6 are in stark contrast with
those of other reported BDF derivatives bearing different
substitution patterns, which therefore exhibit no electronic
absorptions at energies lower than 25000 cm-1 (400 nm).3a,12
As discussed below, this remarkable difference originates
mainly from the nature of the specifically extended
π-conjugated backbone of compounds 5 and 6. In contrast,
but as expected from the yellow color of compound 4, its
corresponding lowest energy absorption band is substantially
blue-shifted and peaks around 25500 cm-1 (392 nm). This
observation can be rationalized by the nonplanarity of mole-
cule 4, as demonstrated by its single-crystal structure. All these
results clearly reflect the effect of the different spacers between
the BDF core and the pyridine groups.
Furthermore, the chromophoric molecules 4-6 show
strong fluorescence emission in CH2Cl2 solution at 20500
cm-1 (487 nm), 18800 cm-1 (532 nm), and 17800 cm-1
(562 nm) with Stokes shifts of νST = 4950, 2000, and
3000 cm-1 for 4-6, respectively (Figure 4 and Table 2). The
corresponding luminescence quantum yields ΦF are quite high
with values >0.75. The fluorescence excitation spectra com-
pare well with the corresponding absorption profiles. All three
compounds exhibit also a fluorescence emission in the solid
state (Figure S8, Supporting Information). The solid-state
emission is red-shifted with respect to the solution emission
by 450, 2300, and 2000 cm-1 for 4-6, respectively. Table 2 also
includes some optical data for the strongly emitting compounds
2b and 3.
At higher energy, the next calculated electronic transition,
S0fS2, at 28220 cm-1 (354 nm) and 26500 cm-1 (377 nm) for
5 and 6, respectively, corresponds also fairly well with the
experimental data (see stick spectra in Figure 4). It is
dominated (>89% for 5 and 6) by the one-electron
HOMO-1fLUMO contribution; hence, it is as well an in-
plane π-π* transition in its electronic nature. Again, there
occurs a substantial charge flow within the BDF skeleton
and the pyridine groups through the links.
It is important to point out that the pyridine-substituted
BDF derivatives 4-6 can be protonated and deprotonated
under acidic and basic conditions, respectively. Upon pro-
tonation, the lowest energy absorptions are bathochromi-
cally shifted (Figure 5 for 6 and Figure S9 (Supporting
Information) for 4 and 5) due to the strong electron-
withdrawing effect of the pyridinium groups. The same
statement also holds true for the significant bathochromic
€
(18) (a) Ahlrich, R.; Bar, M.; Haser, M.; Horn, H.; Kolmel, C. Chem.
Phys. Lett. 1989, 162, 165. (b) Bauernschmitt, R.; Ahlrich, R. Chem. Phys.
€
€
€
Lett. 1996, 256, 454. (c) Bauernschmitt, R.; Haser, M.; Treutler, O.; Ahlrich,
R. Chem. Phys. Lett. 1997, 264, 573.
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JOCArticle
FIGURE 4. Electronic absorption (black line, 2 ꢀ 10-5 M) and fluorescence emission (red line, 2 ꢀ 10-6 M) spectra of 4 (a), 5 (b), and 6 (c) in
deaerated CH2Cl2 solution at room temperature, together with the calculated S0fSn electronic transitions.
TABLE 2. Lowest Energy Absorption and Emission Maxima Amax and Emax, Respectively, Extinction Coefficient ε, Excitation Wavelength λex, Stokes
Shift νST, Luminescence Quantum Yield ΦF, Optical Band Gap Egopt, and Calculated HOMO-LUMO Gaps Egcalc of Compounds 2b and 3-9 in CH2Cl2
compd
Amax (cm-1
)
ε (M-1 cm-1
)
λ
ex (cm-1
)
Emax (cm-1
)
ν
ST (cm-1
)
ΦF
Egopt (eV)
Egcalc (eV)
2b
3
4
5
6
7
8
9
23640
23150
25500
20800
20800
17400
15300
15200
25000
20000
16000
21000
18000
17000
14000
15000
22990
22990
27030
23260
23260
21640
21280
20550
18800
17800
2000
1870
4950
2000
3000
0.98a
2.80c
2.74c
2.80c
2.44c
2.35c
1.82d
1.63d
1.57d
c
1a
0.88b
1a
0.75a
3.92
3.14
2.98
2.13
1.94
1.94
aFluorescein in 0.1 M NaOH (ΦF = 0.95)16 as a standard. bDiphenylanthracene in EtOH (ΦF = 0.9)17 as a standard. Determined from the
intersection of the absorption and emission spectra in solution. dDetermined from the onset of the lowest energy electronic absorptions in the
corresponding UV-vis spectra in solution.
FIGURE 6. Molecular orbitals of 4 (left), 5 (middle), and 6 (right)
that are involved in the S0fS1 (HOMO to LUMO) and S0fS2
(HOMO-1 to LUMO) transitions.
HOMO-LUMO gaps correspond also fairly well with the
optically determined ones (Egopt).
FIGURE 5. Electronic absorption spectra of 6 (black line), its
protonated form (red), and its methylated form 9 (green) at room
temperature.
Conclusions
We developed an efficient synthetic route to the key
precursors 2a and 2b, which, for instance, can readily under-
go Suzuki, Heck, and Sonogashira coupling reactions to
afford a manifold of extended π-conjugated BDF derivatives
with different functional groups and spacers in good yields.
The analogous calculations for the methylated com-
pounds 7-9 (see Figures S11 and S12, Supporting
Information) give similar results as discussed above
for molecules 4-6. As shown in Table 2, the calculated
J. Org. Chem. Vol. 75, No. 10, 2010 3355

Yi et al.
JOCArticle
Herein, the synthetic protocols are demonstrated with the
pyridyl end-capped molecules 4-6. Importantly, for these
systems, a strong electronic absorption profile extends far
into the visible spectral region. These are also highly lumi-
nescent materials that undergo two reversible oxidation
processes. The effect of different spacers on the photophysi-
cal and electrochemical properties has been demonstrated.
Moreover, upon methylation or protonation, these mole-
cules become slightly more difficult to be oxidized and show
a considerable bathochromic shift of the longest wave-
length absorption band. Interestingly, they can function as
pH-dependent fluorescence switches. Further studies on
the construction of supramolecular systems by virtue of
hydrogen bonding and metal coordination are currently in
progress.
neutralized with 1 M HCl. The solution was extracted with
CH2Cl2 (4 ꢀ 50 mL). The combined organic phase was evapo-
rated by rotavapor, and the residue was subjected to column
chromatography on silica gel, eluting with a mixture of
hexane and CH2Cl2 (gradient from 0:100 to 100:0) to give a
pale yellow solid (1.1 g, 86%): mp 165-167 °C; 1H NMR (300
MHz, CDCl3) δ 6.90 (s, 2H), 3.82 (s, 6H), 0.26 (s, 18H); 13C
NMR (75.4 MHz, CDCl3) δ 154.2, 116.2, 113.5, 100.8, 100.4,
56.4, -0.01.
2,5-Bis(trimethylsilylethynyl)-1,4-benzoquinone. To a solution
of 1,4-dimethoxy-2,5-bis(trimethylsilylethynyl)benzene (330 mg,
1 mmol) in CH3CN (20 mL) was added (NH4)2Ce(NO3)6 (1.6 g,
3 mmol). Water (2 mL) was added dropwise to completely
dissolve the oxidant. The resulting solution was stirred at rt for
30 min. After evaporation, the residue was dried, redissolved in
CH2Cl2, and extracted with water (3 ꢀ 10 mL). The combined
organic phase was evaporated to give a brown solid. (289 mg,
1
96%): mp 133-135 °C; H NMR (300 MHz, CDCl3) δ 6.87
Experimental Section
(s, 2H), 0.25 (s, 18H); 13C NMR (75.4 MHz, CDCl3) δ 182.4,
137.3, 132.3, 112.4, 96.4, -0.6; HRMS, ESI (positive) calcd for
[C16H20O2Si2Na]þ 323.0894, found 323.0902.
Synthesis. 1,4-Diiodo-2,5-dimethoxybenzene,19 2,5-diiodo-
1,4-benzoquinone,20 and 1,4-dimethoxy-2,5-bis(trimethylsilyle-
thynyl)benzene21 were prepared according to the modified
literature procedures. All chemicals and solvents were pur-
chased from commercial sources and were used without further
purification. All reactions were carried out, unless mentioned,
under normal laboratory conditions in air.
2,6-Diamino-4,8-diethynylbenzo[1,2-b:4,5-b0]difuran-3,7-dicar-
bonitrile (1b). 2,5-Bis(trimethylsilylethynyl)-1,4-benzoquinone
(280 mg, 0.9 mmol) and malononitrile (200 mg, 3 mmol) were
dissolved in THF (50 mL), and then Bu4NF (1 g, 3.2 mmol) was
added. The mixture was stirred at rt for 0.5 h and poured into
water (50 mL). The resulting mixture was extracted with CH2Cl2
(4 ꢀ 50 mL). The combined organic phase was evaporated and
washed with hexane and diethyl ether to give a brown micro-
Microwave reactions were conducted using the Biotage
Initiator Eight EXP microwave apparatus and the correspond-
ing vials. The reaction was performed in glass vials (capacity 5 or
20 mL) sealed with a septum, under magnetic stirring. The
temperature of the reaction mixture was monitored using a
calibrated infrared temperature control mounted under the
reaction vial. The typical experiment was irradiated in a sealed
vial at 120 °C for 30 min.
1
crystalline product (30 mg, 11%): mp 233-235 °C; H NMR
(300 MHz, DMSO-d6) δ 8.42 (s, 4H), 4.84 (s, 2H); 13C NMR
(75.4 MHz, DMSO-d6) δ 166.6, 145.3, 122.3, 114.1, 94.5, 91.2,
73.4, 61.2; HRMS, ESI (positive) calcd for [C16H17N4O2]þ
287.0569, found 287.0560.
2,6-Diamino-4,8-diiodobenzo[1,2-b:4,5-b0]difuran-3,7-dicarbo-
nitrile (1a). To a solution of 1,4-diiodoquinone (2 g, 5.55 mmol)
and malononitrile (990 mg, 15 mmol) in methanol (50 mL) was
added piperidine (4 mL). The mixture was stirredfor 1 h at 60 °C.
A brown precipitate was formed. After centrifugation, the solid
was washed with methanol (2 ꢀ 10 mL) and dried to give a
1,4-Diiodo-2,5-dimethoxybenzene.
A solution of H5IO6
(2.92 g, 12.8 mmol) in CH3OH (20 mL) was stirred for 10 min,
and I2 (6.38 g, 25 mmol) was added. After the solution was
stirred for another 10 min, 1,4-dimethoxybenzene (2.7 g,
20 mmol) was added and the mixture heated at 70 °C for 4 h.
The resulting solution was poured into a solution of Na2S2O5
(5 g, 26 mmol) in water (50 mL). The precipitate was washed
with CH3OH and dissolved in CH2Cl2. After filtration, the
filtrate was collected and evaporated under vacuum to afford
a white crystalline product (7.02 g, 90%).
1
brown powder (1.24 g, 45%): mp 240 °C dec; H NMR (300
MHz, CDCl3) δ 8.36 (s, 4H); 13C NMR (75.4 MHz, CDCl3) δ
166.6, 144.8, 122.8, 114.8, 64.1, 62.7; HRMS (EI) calcd for
[C12H4N4O2I2] 489.8424, found 489.8424.
Compound 2a. To a solution of 1a (300 mg 0.61 mmol) in
DMF (20 mL) were added 1-bromohexane (2.5 mL, 17.5 mmol)
and K2CO3 (3 g, 21.75 mmol). The resultant mixture was heated
at 90 °C for 1.5 h, poured into water (50 mL), and extracted with
CH2Cl2 for (3 ꢀ 50 mL). The combined organic phase was
concentrated to the volume of 2 mL, precipitated with the
addition of methanol, and centrifuged to give a pale yellow
2,5-Diiodo-1,4-benzoquinone. To a solution of 1,4-diiodo-2,5-
dimethoxybenzene (9.75 g, 25 mmol) in acetonitrile (180 mL)
was added (NH4)2Ce(NO3)6 (42 g, 76 mmol). Then water
(20 mL) was added to completely dissolve the oxidant. The
resulting solution was stirred at rt for 30 min. After evaporation,
the residue was dried, redissolved in dichloromethane, and ex-
tracted with water (3 ꢀ 50 mL). The combined organic phase was
evaporated and dried under high vacuum to give an orange
crystalline product (6.45 g, 70%).
1,4-Dimethoxy-2,5-bis(trimethylsilylethynyl)benzene. A mix-
ture of 1,4-diiodo-2,5-dimethoxybenzene (1.5 g, 3.86 mmol)
and Pd(PPh3)2Cl2 (450 mg, 0.64 mmol) in DMF (10 mL) in a
20 mL vial was bubbled with N2 for 20 min, and then CuI
(350 mg, 1.8 mmol), i-Pr2NH (5 mL), and trimethylsilylacetyl-
ene (1.4 g, 14 mmol) were added. The resulting mixture was
subjected to microwave irradiation by prestirring for 1 min and
reacted at 120 °C for 30 min. The volatile was evaporated under
vacuum, and the residue was dissolved in CH2Cl2 (50 mL) and
1
solid (260 mg, 50%): mp 148-150 °C; H NMR (300 MHz,
CDCl3) δ 3.58 (t, J = 7.2 Hz, 8H), 1.73 (m, 8H), 1.35 (m, 24H),
0.89 (t, J = 7.0 Hz, 12H); 13C NMR (75.4 MHz, CDCl3) δ 163.0,
145.1, 123.5, 116.5, 65.1, 61.3, 50.2, 31.5, 28.4, 26.1, 22.6, 14.0;
HRMS, ESI (positive) calcd for [C36H53N4O2I2]þ 827.2252,
found 827.2258.
Compound 3. A mixture of 2a (826 mg, 1 mmol), Pd(PPh3)2Cl2
(175 mg, 0.25 mmol), CuI (50 mg, 0.26 mmol), and piperidine
(4 mL) in triethylamine (30 mL) was bubbled with N2 for 10 min,
and then trimethylsilylacetylene (600 mg, 6 mmol) was added.
The resulting solution was heated at 110 °C overnight in an inert
atmosphere. The volatile was evaporated by rotavapor, and the
residue was subjected to column chromatography on silica gel,
eluting initially with hexane and then with 1:1 hexane/CH2Cl2
to yield a yellow crystalline product (500 mg, 65%): mp 160-
162 °C; 1H NMR (300 MHz, CDCl3) δ 3.57 (t, J = 7.5 Hz, 8H),
1.71 (m, 8H), 1.33 (m, 24H), 0.88 (t, J = 6.9 Hz, 12H), 0.28
(19) Zhang, J.; Cui, Y.; Wang, M.; Liu, J. Chem. Commun. 2002, 2526.
ꢀ
~
(20) Lopez-Alvarado, P.; Avendano, C.; Menendez, J. C. Synth. Com-
ꢀ
mun. 2002, 32, 3233.
(21) Dirk, S. M.; Price, D. W.; Chanteau, S.; Kosynkin, D. V.; Tour, J. M.
Tetrahedron 2001, 57, 5109.
3356 J. Org. Chem. Vol. 75, No. 10, 2010

Yi et al.
JOCArticle
(s, 18H); 13C NMR (75.4 MHz, CDCl3) δ 163.0, 145.7, 123.2,
115.8, 106.3, 96.1, 93.8, 62.6, 50.1, 31.5, 28.5, 26.2, 22.6, 14.0,
-0.40; HRMS, ESI (positive) calcd for [C46H71N4O2Si2]þ
767.5110, found 767.5132.
1.80 (m, 8H) 1.34 (m, 24H), 0.88 (t, J = 6.8 Hz, 12H); 13C NMR
(75.4 MHz, CDCl3) δ 163.2, 150.4, 144.9, 142.9, 130.6, 123.2,
122.6, 120.8, 118.7, 110.4, 61.3, 50.7, 31.6, 28.7, 26.4, 22.6, 14.0;
HRMS, ESI (positive) calcd for [C50H65N6O2]þ 781.5164, found
781.5187.
Compound 2b. To a solution of 3 (460 mg, 0.6 mmol) in THF
(15 mL) was added Bu4NF 3H2O (1 g, 3.2 mmol). The mixture
Compound 7. A mixture of compound 4 (30 mg, 0.04 mmol)
and iodomethane (70 mg, 0.5 mmol) in acetonitrile (4 mL) was
treated under microwave irradiation (120 °C, 30 min). The
resulting purple solution was poured into saturated KPF6
aqueous solution (10 mL), and a purple precipitate immediately
formed. The suspension was centrifuged, and then the precipi-
tate was washed with water 3 times and redissolved in aceto-
nitrile. After filtration, the filtrate was evaporated by rotavapor
and dried in vacuum to give a purple solid product (28 mg,
3
was stirred at rt for 30 min and then poured into water (50 mL)
and extracted with CH2Cl2 (3 ꢀ 50 mL). The solvent was
removed under high vacuum to give a pale yellow solid
(240 mg, 64%): mp 123-125 °C; 1H NMR (300 MHz, CDCl3)
δ 3.65 (s, 2H), 3.58 (t, J = 7.5 Hz, 8H), 1.72 (m, 8H), 1.34 (m,
24H), 0.89 (t, J = 6.8 Hz, 12H); 13C NMR (75.4 MHz, CDCl3) δ
163.1, 145.7, 123.5, 115.9, 95.3, 88.1, 73.0, 62.5, 49.9, 31.4, 28.4,
26.1, 22.5, 14.0; HRMS, ESI (positive) calcd for [C40H55N4O2]þ
623.4320, found 623.4334.
1
65%): mp 154-156 °C; H NMR (300 MHz, CD3CN) δ 8.76
Compound 4. A mixture of compound 2a (240 mg, 0.3 mmol),
4-pyridylboronic acid (120 mg, 1 mmol), Pd(PPh3)2Cl2 (40 mg,
0.06 mmol), and Cs2CO3 (200 mg, 0.62 mmol) was added to a
5 mL microwave vial. The reactor was sealed, evacuated, and
purged with N2. Then DMF (2.5 mL) and water (0.5 mL) were
successively added. The reactor was subjected to microwave
irradiation at 120 °C for 30 min. The resulting mixture was
poured into water (10 mL) and extracted with CH2Cl2 (20 mL).
The volatile was evaporated by rotavapor, and the residue was
subjected to column chromatography on neutral alumina, elut-
ing with 100:5 CH2Cl2/triethylamine, to afford a yellow solid
(d, J = 6.4 Hz, 4H), 8.25 (d, J = 6.4 Hz, 4H), 4.37 (s, 6H), 3.54
(t, J = 7.3 Hz, 8H), 1.67 (m, 8H), 1.28 (m, 24H), 0.88 (t, J = 6.2
Hz, 12H); 13C NMR (75.4 MHz, CD3CN) δ 165.0, 148.4, 145.6,
143.7, 130.1, 122.1, 110.6, 61.5, 50.9, 49.2, 32.1, 28.8, 26.6, 23.2,
14.2 ; HRMS, ESI (positive) calcd for [C48H66N6O2]2þ
379.2618, found 379.2627.
Compound 8. A mixture of compound 5 (35 mg, 0.046 mmol)
and iodomethane (30 mg, 0.21 mmol) in acetonitrile (4 mL) was
treated under microwave irradiation (120 °C, 30 min). The
resulting blue solution was poured into saturated KPF6 aqueous
solution (10 mL), and a blue precipitate formed immediately.
The suspension was centrifuged, and then the precipitate was
washed with water for 3 times and redissolved in acetonitrile.
After filtration, the filtrate was evaporated by rotavapor and
dried in vacuum to give a blue crystalline product (36 mg, 73%):
mp 232-234 °C; 1H NMR (300 MHz, CD3CN) δ 8.61 (d, J =
6.6 Hz, 4H), 8.09 (d, J = 6.6 Hz, 4H), 4.28 (s, 6H), 3.71 (t, J =
7.2 Hz, 8H), 1.78 (m, 8H), 1.38 (m, 24H), 0.91 (t, J = 7.0 Hz,
12H); 13C NMR (75.4 MHz, CD3CN) δ 164.5, 146.3, 146.2,
140.0, 129.7, 125.9, 117.0, 96.1, 95.6, 94.0, 62.9, 50.9, 49.2, 32.1,
28.9, 26.7, 23.2, 14.2; HRMS, ESI (positive) calcd for
[C52H66N6O2]2þ 403.2618, found 403.2636.
1
product (180 mg, 85%): mp 92-94 °C; H NMR (300 MHz,
CDCl3) δ 8.77 (d, J = 6.0 Hz, 4H), 7.51 (d, J = 6.0 Hz, 4H), 3.45
(t, J = 7.5 Hz, 8H), 1.64 (m, 8H), 1.26 (m, 24H), 0.88 (t, J = 6.4
Hz, 12H); 13C NMR (75.4 MHz, CDCl3) δ 163.7, 149.5, 142.9,
139.2, 125.1, 120.7, 116.3, 112.1, 61.2, 50.1, 31.5, 28.5, 26.1, 22.5,
13.9; HRMS, ESI (positive) calcd for [C46H61N6O2]þ 729.4851,
found 729.4857.
Compound 5. A mixture of 2a (164 mg, 0.2 mmol), 4-ethynyl-
pyridine (80 mg, 0.77 mmol), Pd(PPh3)2Cl2 (40 mg, 0.06 mmol),
CuI (10 mg 0.05 mmol), and piperidine (2 mL) in triethylamine
(20 mL) was bubbled with N2 for 10 min, and then the resulting
solution was heated at 110 °C overnight in an inert atmosphere.
The volatile was evaporated by rotavapor, and the residue was
subjected to column chromatography on neutral alumina, elut-
ing with a mixture of CH2Cl2 and 1% triethylamine to afford an
orange crystalline product (100 mg, 65%): mp 224-226 °C; 1H
NMR (300 MHz, CDCl3) δ 8.61(d, J = 5.0 Hz, 4H), 7.51 (d, J =
5.4 Hz, 4H), 3.64 (t, J = 7.5 Hz, 8H), 1.78 (m, 8H), 1.36 (m,
24H), 0.88(t, J = 6.8 Hz, 12H); 13C NMR (75.4 MHz, CDCl3) δ
163.1, 149.8, 145.2, 130.9, 125.2, 123.9, 116.2, 97.2, 95.7, 62.6,
50.3, 31.5, 28.5, 26.2, 22.6, 14.0; HRMS, ESI (positive) calcd for
[C50H61N6O2]þ 777.4851, found 777.4872.
An alternative synthetic route to 5: A mixture of 2b (124 mg,
0.2 mmol), 4-iodopyridine (160 mg, 0.8 mmol), Pd(PPh3)2Cl2
(28 mg, 0.04 mmol), and CuI (10 mg, 0.05 mmol) in triethyla-
mine (20 mL) was bubbled with N2 for 10 min, and then the
mixture was heated at 110 °C in an inert atmosphere overnight.
The volatile was evaporated by rotavapor, and the residue was
subjected to column chromatography on neutral alumina, elut-
ing with a mixture of CH2Cl2 and 1% triethylamine to afford an
orange crystalline product (45 mg, 29%).
Compound 6. A mixture of 2a (164 mg, 0.2 mmol), 4-vinylpyr-
idine (90 mg, 0.86 mmol), Pd(PPh3)2Cl2 (40 mg, 0.06 mmol), and
Cs2CO3 (200 mg, 0.62 mmol) in dioxane (10 mL) was bubbled
with N2 for 10 min, and then the resulting solution was heated at
110 °C in an inert atmosphere overnight. The volatile was
evaporated by rotavapor, and the residue was subjected to
column chromatography on silica gel, eluting with CH2Cl2 to
afford a red crystalline product (97 mg, 62%): mp 222-224 °C;
1H NMR (300 Mz, CDCl3) δ 8.59 (d, J = 6.0 Hz, 4H), 8.21 (d,
J = 16.4 Hz, 4H), 7.40-7.35 (m, 6H), 3.63 (t, J = 7.6 Hz, 8H),
Compound 9. A mixture of compound 6 (25 mg, 0.032 mmol)
and iodomethane (70 mg, 0.5 mmol) in acetonitrile (4 mL) was
treated under microwave irradiation (120 °C, 30 min). The
resulting purple solution was poured into saturated KPF6
aqueous solution (10 mL), and a dark-green precipitate formed
immediately. The suspension was centrifuged, and then the
precipitate was washed with water 3 times and redissolved in
acetonitrile. After filtration, the filtrate was evaporated by
rotavapor and dried in vacuum to give a dark-green powder
1
product (26 mg, 73%): mp 195-197 °C; H NMR (300 MHz,
CD3CN) δ 8.50 (m, 6H), 7.94 (d, J = 6.6 Hz, 4H), 7.60 (d, J =
16.4 Hz, 2H), 4.22 (s, 6H), 3.69 (t, J = 7.5 Hz, 8H), 1.80 (m, 8H),
1.32 (m, 24H), 0.87 (t, J = 6.8 Hz, 12H); 13C NMR (75.4 MHz,
CD3CN) δ 164.5, 154.1, 146.1, 144.0, 131.3, 128.5, 124.7, 119.3,
111.3, 61.8, 51.3, 48.4, 32.2, 29.0, 26.8, 23.2, 14.2; HRMS, ESI
(positive) calcd for [C52H70N6 O2]2þ 405.2775, found 405.2788.
Acknowledgment. This work was supported by the Swiss
National Science Foundation (Grant No. 200020-116003
and 200020-130266/1) and EU (FUNMOLS FP7-212942-1).
Supporting Information Available: General experimental
methods and 1H spectra for all new compounds, the solid state
emission spectra of 4-6, additional absorption spectra, compu-
tational studies of 7-9, calculated triplet energies for 6, CIF files
for 1b and 4-6 (CCDC 752868, 773634, 745180, 745181);
molecular structures of 1b, 4, and 6; crystal packing diagrams
of 1b and 4-6. This material is available free of charge via the
Internet at http://pubs.acs.org.
J. Org. Chem. Vol. 75, No. 10, 2010 3357