Synthesis, electronic properties, and x-ray structural characterization of tetrarylbenzo[1,2-b:4,5-′]difuran cation radicals

Article abstract of DOI:10.1021/ol801356e

(Chemical Equation Presented) Electroactive tetraarylbenzo[1,2-b:4,5- ′]difuran (BDF) and model diarylbenzofuran derivatives are synthesized and their structures are established by X-ray crystallography. Isolation and X-ray crystallographic characterizati

Full text of DOI:10.1021/ol801356e

ORGANIC
LETTERS
2008
Vol. 10, No. 16
3587-3590
Synthesis, Electronic Properties, and
X-ray Structural Characterization of
Tetrarylbenzo[1,2-b:4,5-b′]difuran Cation
Radicals
Ruchi Shukla, Shriya H. Wadumethrige, Sergey V. Lindeman, and
Rajendra Rathore*
Department of Chemistry, Marquette UniVersity, P.O. Box 1881, Milwaukee, Wisconsin 53201
rajendra.rathore@marquette.edu
Received June 16, 2008
ABSTRACT
Electroactive tetraarylbenzo[1,2-b:4,5-b′]difuran (BDF) and model diarylbenzofuran derivatives are synthesized and their structures are
established by X-ray crystallography. Isolation and X-ray crystallographic characterization of the robust cation-radical salts of BDF
derivatives confirm that a single charge in the BDFs is stabilized largely by the benzodifuran and coplanar r-aryl groups lying on the
longitudinal axis. These findings suggest that the linear arrays of BDFs may allow the construction of molecular wires suitable for
long-range electron transport.
The design and syntheses of new electroactive chromophores
have attracted considerable attention over the past decade
owing to their potential for the practical applications in the
emerging areas of molecular electronics and nanotechnol-
ogy.1 Tetraarylbenzo[1,2-b:4,5-b′]difurans (simply referred
to hereafter as BDF), a largely unexplored2,3 class of
electroactive chromophores, possess close structural similari-
ties to the electroactive moieties utilized for the construction
of, extensively explored, polyphenylenevinylene (PPV) and
its alkoxy-substituted derivatives (i.e., Figure 1).4
Figure 1. Structural similarities between polyphenylenevinylenes
and tetraarylbenzo[1,2-b:4,5-b′]difurans.
Although BDF derivatives have been known for over a
century2 and can be easily prepared in excellent yields via
Lewis acid-catalyzed condensations of benzoin or substituted
benzoins with p-hydroquinone, their potential as electroactive
materials has received little attention.3
Our continuing interest in the design and syntheses of
chromophores that form stable organic cation radicals led
us to explore the potential of BDF derivatives as hole
carriers. As such, the cation radicals, or hole carriers, are of
fundamental importance to organic material science since
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Chem. Soc. 2007, 129, 11902. (b) Destrade, C.; Tinh, N. H.; Gasparoux,
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.
10.1021/ol801356e CCC: $40.75
Published on Web 07/11/2008
2008 American Chemical Society

they constitute the smallest unit that stabilizes a cationic
charge as well as an unpaired electron.5
Scheme 1. Preparation of Various BDFs and Model Compounds
Accordingly, in this report we describe the preparation
of various tetraarylbenzodifurans (BDFs) as well as
diarylbenzofurans (as model compounds), and show that
BDFs are highly luminescent materials that undergo
reversible electrochemical oxidations and form stable
cation-radical salts that can be isolated in crystalline form.
The structures of various neutral BDFs and their cation
radicals are determined by X-ray crystallography and
further corroborated by DFT calculations. The structural
studies of BDF cation radicals allow us to delineate that
a single charge in BDFs is stabilized largely by benzodi-
furan and the R-aryl groups lying on the longitudinal axis
(shown in red in Figure 1) while the ꢀ-aryl groups on the
vertical axis (shown in black in Figure 1) contribute little
to the stabilization of the cationic charge. The details of
these findings are described herein.
Thus, tetraphenylbenzodifuran (BDF1) was obtained by
simply heating an intimate mixture of benzoin, p-hydro-
quinone, and zinc chloride in 2:1:2.5 molar ratio for 5-15
min. The resulting mixture was cooled to 22 °C and was
triturated with dichloromethane and water. The dichlo-
romethane layer was separated and washed with a 10%
aqueous sodium hydroxide solution and evaporated to
afford a solid residue that was recrystallized from a
mixture of dichloromethane and acetonitrile to afford
BDF1 in 70% isolated yield. A similar reaction with
commercially available anisoin afforded BDF2 in excellent
yield. It was noted that both BDF1 and BDF2 have limited
solubility in dichloromethane, chloroform, benzene, or
toluene. For example, BDF1 has a solubility of 30 mg/10
mL of CH2Cl2 and BDF2 has a solubility of 40 mg/10
mL in CH2Cl2. Hence, a readily soluble benzodifuran
derivative BDF3 was also prepared by using 4,4′-
dihexyloxybenzoin (Scheme 1), which in turn was ob-
tained from anisoin by using standard procedures.6 The
model diarylbenzofurans (M1-3) were obtained by a
reaction of various benzoin derivatives with 3,4-dimeth-
ylphenol in excellent yields (Scheme 1).
chromic shift going from the tetraphenyl derivative (i.e.,
BDF1) to the corresponding alkoxy-substituted derivatives
(i.e., BDF2-3) (see Figure 2, left). In contrast, the model
Figure 2. Comparison of the emission and excitation spectra of
BDF1-3 (left) and M1-3 (right) in dichloromethane at 22 °C.
The molecular structures of BDF1-3 and M1-3 were
benzofuran derivatives (M1-3) showed only broad emission/
excitation bands (see Figure 2, right).
1
established with the aid of H/13C NMR spectroscopy and
mass spectrometry, and further confirmed by X-ray crystal-
lography (see the Supporting Information for the full
experimental details).
The electron-donor strengths of various BDF derivatives
and the initial indication of the stability of their cation
radicals were evaluated by cyclic voltammetry.
With the various BDF derivatives at hand, we next
examined their emission and excitation spectra in dichlo-
romethane at 22 °C. The highly luminescent BDFs showed
structured emission/excitation bands with a modest batho-
Thus, each benzodifuran and benzofuran derivative was
subjected to electrochemical oxidation at a platinum
electrode as a 2-3 mM solution in dichloromethane
containing 0.1 M n-Bu4NPF6 as the supporting electrolyte.
Figure 3 compiles the cyclic voltammograms of BDF1-3
(left) and benzofuran derivatives M1-3 (right). The
benzodifuran derivatives, BDF1-3, showed two oxidation
waves corresponding to the formation of monocation
radicals and dications, respectively. The phenyl-substituted
BDF1 showed that its first oxidation wave corresponding
to the formation of the cation radical is completely
reversible (Eox1 ) 1.17 V vs SCE, see Figure 3, red curve)
while the second oxidation to the dication occurs irrevers-
(4) (a) Davis, W. B.; Svec, W. A.; Ratner, M. A.; Wasielewski, M. R.
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129, 8070. (b) Kochi, J. K.; Rathore, R.; Magueres, P. L. J. Org. Chem.
2000, 65, 6826. (c) Rathore, S. H.; Abdelwahed, I.; Guzei, A. J. Am. Chem.
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R. J. Org. Chem. 2007, 72, 1765.
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3588
Org. Lett., Vol. 10, No. 16, 2008

(see the Supporting Information section) and are compared in
Figure 4. The absorption spectrum of BDF1•+ was expectedly
Figure 4
.
Spectral changes upon the reduction of 1.3 × 10-5
M
Figure 3. Comparison of the cyclic voltammograms of various
NAP•+ by incremental addition of BDF1 in CH2Cl2 at 22 °C (left)
and a comparison of the absorption spectra of BDF1-3 cation
radicals (middle) and M1-3 cation radicals (right).
benzodifuran (2 mM) and benzofuran (3 mM) derivatives in CH2Cl2
(0.1 M n-Bu4NPF6) at a scan rate of 200 mV s-1. Note that the
first oxidation wave of BDF1 is completley reversible (red curve)
if the scanning is terminated before the start of the second oxidation
event.
similar to that of the alkoxy substituted BDF2•+ [λmax ) 540
(sh), 572, 676, and 1368 (log ꢀ ) 4.37) nm] and BDF3•+ [λmax
) 541 (sh), 576, 705, and 1375 (log ꢀ ) 4.47) nm] as shown
in Figure 4 (middle). A compilation of the absorption spectra
of the cation radicals of benzofurans M1•+ [λmax ) 442, 720
(log ꢀ ) 3.74), and 950 nm], M2•+ [λmax ) 478, 685 (sh), 750
(log ꢀ ) 4.11), and 1050 nm], and M3•+ [λmax ) 480, 695 (sh),
760 (log ꢀ ) 4.17), and 1055 nm] in Figure 4 (right) shows
that they contain similar spectral transitions, as in the spectra
of the cation radicals of BDF derivatives. However, they lack
the intense NIR transition (1180-1400 nm) that is attributed
to a Robin Day III type intervalence transition in BDF cation
radicals.9
ibly (at ∼1.6 V vs SCE). Expectedly, the alkoxy-
substituted BDF2 and -3 showed two reversible oxidation
waves at very similar and relatively lower potentials (i.e.,
BDF2: 0.91 and 1.19; BDF3: 0.89 and 1.15 V vs SCE)
as compared to the BDF1 owing to the presence of
electron-donating alkoxy substituents. The cyclic volta-
mmograms of the corresponding model benzofuran de-
rivatives M1-3 showed at least one reversible oxidation
wave occurring at relatively higher potentials (M1: 1.34
V vs SCE; M2: 1.0 V vs SCE; and M3: 1.04 V vs SCE)
as compared to the corresponding BDF derivatives.
The BDF cation radicals, obtained according to eq 1, are
highly persistent at ambient temperatures and did not show
any decomposition during a 24 h period at 22 °C, as
confirmed by UV-vis spectroscopy. The single crystals of
the BDF1•+ and BDF2•+, suitable for X-ray crystallography,
were obtained by a slow diffusion of toluene into the
The electrochemical reversibility and relatively low oxidation
potentials of benzodifuran and benzofuran derivatives prompted
us to generate their cation radicals using a hydroquinone ether
cation radical (MA•+, Ered ) 1.11 V vs SCE)7 or a naphthalene
cation radical (NAP•+, Ered ) 1.34 V vs SCE)8 as stable
(aromatic) one-electron oxidants.
dichloromethane solutions of BDF1•+SbCl6 and BDF2•+-
-
Thus Figure 4 (left) shows the spectral changes attendant
upon an incremental addition of substoichiometric amounts of
BDF1 to a 1.3 × 10-5 M NAP•+ (λmax ) 672, 616, 503, and
396 nm; ꢀ672 ) 9300 M-1 cm-1)8 in dichloromethane at 22 °C.
Furthermore a plot of formation of the BDF1 cation radical
(i.e., increase in the absorbance at 1100 nm) against the
increments of added neutral BDF1 (see Figure S6 in the
Supporting Information section) established that NAP•+ was
completely consumed after the addition of 1 equiv of BDF1,
and the resulting highly structured absorption spectrum of
BDF1•+ [λmax ) 502, 545, 610 (sh), and 1100 (log ꢀ ) 3.81)
nm] remained unchanged upon further addition of neutral BDF1
(i.e., eq 1).
-
SbCl6 at -10 °C during the course of 2 days (see the
Supporting Information).
The crystallographic analysis of the dark crystals of
BDF1•+SbCl6 and BDF2•+SbCl6 revealed that they pack
by the formation of translational stacks where peripheral
R-aryl groups overlap with the central benzodifuran moieties
of the neighboring molecules at a relatively large dihedral
angle (∼20°), see Figure 5.
-
-
A closer look at the bond length changes in the BDF cation
radicals, together with a comparison of the corresponding
neutral forms, the structures of which were established by
X-ray crystallography, points to the following important
observations: (i) The bond lengths in the central benzodifuran
nuclei are identical in both BDF derivatives (see Table 1).
(ii) In the cation radicals, the “olefinic” bonds (in the furan
rings denoted e in the generic structure in Table 1) undergo
(7) Rathore, R.; Burns, C. L.; Deselnicu, M. I.; Denmark, S. E.; Bui, T.
Org. Synth. 2005, 82, 1. Also see Figure S9 in the SIsection.
(8) Rathore, R.; Burns, C. L.; Deselnicu, M. I. Org. Lett 2001, 3, 2887.
(9) Sun, D.; Lindeman, S. V.; Rathore, R.; Kochi, J. K. J. Chem. Soc.,
Perkin Trans. 2 2001, 158, 5–1594, and references cited therein.
Similarly, the cation radicals of various BDF and benzofuran
derivatives were generated by using NAP•+ as well as MA•+
Org. Lett., Vol. 10, No. 16, 2008
3589

BDF2•+. For a complete listing of the bond lengths of the
peripheral aryl groups in various structures, see Table S1 and
S2 in the Supporting Information.
It is noteworthy that the R-aryl groups, which are more
coplanar with the benzodifuran ring (νR ) 10-34°), undergo a
pronounced quinoidal change owing to the delocalization of
the cationic charge as compared to the less coplanar ꢀ-aryl
groups (νꢀ ) 44-61°) which show almost no change in their
bond lengths (see Table S1 and S2 in the Supporting Informa-
tion).
The experimental observations of the bond length changes
in BDF cation radicals were found to be in reasonable
agreement with the calculated values using DFT calculations
at the B3LYP/6-31G* level (see Table 1).10 Furthermore, it
is noted that the bonds which undergo the most dramatic
lengthening (i.e., bonds a and e) and shortening (i.e., bonds
b and d) in BDF cation radicals are the bonds on which the
HOMO shows the largest bonding and antibonding character,
respectively (i.e., Figure 6).11
Figure 5. The ORTEP diagrams showing the arrangement of
-
-
BDF1•+SbCl6 (right) and BDF2•+SbCl6 (left) in unit cells.
increased delocalization with the central aromatic ring that
leads to their elongation by ∼3 pm and the shortening of
adjacent bonds f and d by ∼2 and ∼3 pm, respectively (see
Table 1). (iii) The central benzene ring acquires a quinoidal
structure, i.e., bonds labeled b shorten by ∼2 pm whereas the
other four bonds (labeled a and c) become elongated by ∼1
and ∼2 pm, respectively. (iv) Interestingly, the structural
changes (described above) in the benzodifuran moiety in
BDF1•+ are roughly 1.5 times more pronounced than those in
the case of BDF2•+ (see Table 1), and such a difference can be
readily attributed to the fact that the unsubstituted phenyl groups
in BDF1•+ are less involved in the stabilization of the positive
charge as compared to the electron-rich p-anisyl groups in
Figure 6. The HOMO’s of BDF1 (right) and BDF2 (left), obtained
by DFT calculations at the B3LYP/6-31G* level.
Table 1. Experimental and Theoretical Bond Lengths of the
(Centrosymmetric) Neutral and Cation Radicals of BDF1 and
BDF2 in Picometers (pm)
In summary, we have demonstrated that various tetraaryl-
benzodifuran (BDF) and benzofuran derivatives can be easily
prepared from readily available starting materials. These highly
luminescent BDFs undergo reversible electrochemical oxidation
and form stable cation-radical salts. The isolation and X-ray
crystal structure determination of the neutral and cation radicals
of BDF derivatives as well as the DFT calculations provide
unequivocal evidence that a single charge (or polaron) in BDFs
is stabilized largely by the benzodifuran and the R-aryl groups.
Efforts are now underway to construct linear arrays of BDF
derivatives to explore their conducting properties for potential
applications in the emerging areas of molecular electronics and
nanotechnology.12
Acknowledgment. We thank the National Science Foun-
dation (CAREER Award) for financial support.
Supporting Information Available: Synthetic details, 1H/
13C NMR data, redox-titration data, and X-ray data of various
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
OL801356E
(10) Calculations were performed with the Spartan 06 software package.
(11) Compare: Banerjee, M.; Lindeman, S. V.; Rathore, R. Chem.
Commun. 2008, 1889.
(12) (a) Weiss, E. A.; Tauber, M. J.; Kelley, R. F.; Ahrens, M. J.; Ratner,
M. A.; Wasielewski, M. R. J. Am. Chem. Soc. 2005, 127, 11842. (b) Dance,
Z. E. X.; Mi, Q.; McCamant, D. W.; Ahrens, M. J.; Ratner, M. A.;
Wasielewski, M. R. J. Phys. Chem. B 2006, 110, 25163, and reference cited
therein.
a Average bond lengths from two independent molecules. b Average
esd for the bond lengths are shown in parentheses.
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