α-Oligofurans

Full text of DOI:10.1021/ja9093346

Published on Web 01/29/2010
r-Oligofurans
Ori Gidron,† Yael Diskin-Posner,‡ and Michael Bendikov*,†
Department of Organic Chemistry and Chemical Research Support Unit, Weizmann Institute of Science,
RehoVot 76100, Israel
Received November 3, 2009; E-mail: michael.bendikov@weizmann.ac.il
Scheme 1. Synthetic Route to Long R-Oligofurans
Organic molecules with long π conjugation have received much
attention because of recent progress in solar cells, organic light-
emitting diodes (OLEDs), organic field-effect transistors (OFETs),
batteries, and sensors.1 Stability, good solid-state packing, pro-
cessability, rigidity/planarity, high fluorescence, and a HOMO-
LUMO gap in the semiconductor region (which also leads to
absorption/emission in the visible range) are among the main
requirements for useful organic electronic materials. Among the
workhorses in this field are long R-oligothiophenes [nT’s, in
particular R-sexithiophene (6T)]2 and oligoacenes (in particular,
pentacene),3 which have been extensively studied because of their
superior electronic properties, including relatively high field-effect
mobility. There is an ongoing search for new compounds possessing
most of the advantages of oligo- and polythiophenes but also having
enhanced fluorescence, solubility, and rigidity. For example,
polyselenophenes were recently introduced as an alternative to
polythiophenes that shows better planarity and good electrochromic
properties.4 However, long R-oligofurans (nF’s), which are close
oligothiophene analogues, are not known, and even short R-oligo-
furans have never been structurally characterized. It has been
suggested that R-oligofurans comprising more than four rings (i.e.,
with more rings than 4F) are unstable5 since they are very electron-
rich, and the longest reported substituted R-oligofuran consists of
five rings.6,7 R-Oligofurans include only first-row elements. Usually,
organic electronic materials are not biodegradable, and extensive
usage of these compounds may generate a significant amount of
hazardous waste. Furan-only materials are biodegradable, and in
addition, furans (in contrast to other types of conjugated materials)
can be obtained from entirely renewable resources.8
when concentrated from solution. To obtain 9F, the preferred route
is coupling of 5 with 6, since the coupling of 4 with 2 results in a
complex reaction mixture (for the synthesis of compounds 5 and
1
6, see the SI). 5F-9F were characterized by a combination of H
and 13C NMR spectroscopy, high-resolution mass spectrometry
(HRMS), thermogravimetric analysis (TGA), differential scanning
calorimetry (DSC), UV-vis spectroscopy, fluorescence, and cyclic
voltammetry (CV). In addition, compound 6F was characterized
by single-crystal X-ray crystallography.
The oligofurans were found to be relatively stable. For example,
6F is stable in air up to 250 °C on the basis of the onset of the
TGA curve (see the SI). Long R-oligofurans are stable in powdered
form and even as a dioxane solution when exposed to air (including
moist air) at room temperature for up to at least a few weeks.
However, as expected, R-oligofurans are unstable with respect to
a combination of light and oxygen, since 2,5-disubstituted furans,
especially those with electron-donating groups, are known to react
with singlet oxygen to form unsaturated esters via ring opening of
furan endoperoxide.13 Other light-induced decomposition pathways
are also possible.
Good solubility is required for processing of organic electronic
materials from solution. We found that the solubility of oligofurans
is significantly better than that of the corresponding oligothiophenes
(the solubility of 6F is 0.7 mg/mL, compared with only <0.05 mg/
mL for 6T), which allows crystallization from heptane.14 Longer
oligothiophenes are completely insoluble, while 5F-8F are reason-
ably soluble; 13C NMR spectra of 5F-6F can be measured in
solution, and 9F can be extracted using a Soxhlet extraction (see
the SI). The single-crystal X-ray structure of 6F15 shows the
oligomer to be completely planar with herringbone packing, as
observed for 6T16 (Figure 1). The average inter-ring C-C bond
lengths in 6F (1.428 Å) are significantly shorter than in 6T (1.442
Å). This may indicate a less aromatic and more quinoid structure
for 6F. The exo-bond C-C-C angle is ∼5° larger in 6F than in
6T (C8-C9-C10 ) 133.4 and 129.0°, respectively; see the SI).
Importantly, 6F is more tightly packed than 6T, since 6F contains
only first-row atoms. The distance between planes in 6F is 2.57 Å,
which is significantly shorter than that in 6T (2.89 Å), although in
both structures there is practically no direct π-π overlap between
adjacent molecules. Also, the molecular density (obtained by
dividing the number of molecules per unit cell by the unit cell
volume) is 17% higher in 6F than in 6T (0.00221 Å-3 for 6F vs
0.00189 Å-3 for 6T; however, the molecular weight and size of
In this paper, we report the synthesis and characterization of a
series of unsubstituted and sufficiently stable R-oligofurans contain-
ing up to nine rings.9-11 The R-oligofurans reported here are the
first oligofuran-based materials to be comprehensively characterized,
including single-crystal X-ray analysis. Importantly, long oligo-
furans are highly fluorescent (relative to oligothiophenes and
alternating furan-thiophene oligomers), electron-rich, and exhibit
tighter herringbone solid-state packing, greater rigidity, and greater
solubility than oligothiophenes.
R-Oligofurans 5F-9F were synthesized using Stille coupling
of shorter oligomers (Scheme 1). Compounds 1 and 2 were obtained
by lithiation of bifuran and terfuran with n-butyllithium followed
by the addition of tributyltin chloride [see the Supporting Informa-
tion (SI)]. The preparation of 3 and 4 was described previously.12
Stille coupling between tributyltin oligofurans (1 and 2) and
dibromooligofurans (3 and 4) afforded analytically pure compounds
5F-8F. The coupling of R-dibromooligofurans with monotin
oligofurans is the preferred synthetic route for 5F-8F, since
monobrominated oligofurans are unstable and tend to decompose
† Department of Organic Chemistry.
‡ Chemical Research Support Unit.
9
2148 J. AM. CHEM. SOC. 2010, 132, 2148–2150
10.1021/ja9093346  2010 American Chemical Society

C O M M U N I C A T I O N S
Figure 1. (left) Herringbone structure and (right) packing of 6F. Color
scheme: gray, C; white, H; red, O. Values in parentheses are for 6T.16
Table 1. Photophysical and Electrochemical Data for Oligofurans
a
a
a
εmax
λabs
(nm)
λflu
(nm)
c d
,
(M-1 cm-1
)
Φfa b
Eox (V)
HOMOe (eV)
,
3F
4F
5F
6F
7F
8F
9F
28700f
28600f
51000
53000
56000
56000
-
331 352, 371f 0.78f (0.07) 0.91 (0.79)
364 391, 413 0.80 (0.18) 0.78 (0.72)
388 421, 449 0.74 (0.36) 0.71 (0.67)
404 442, 472 0.69 (0.41) 0.67 (0.66)
-4.91
-4.73
-4.62
-4.55
-4.51
-4.48
-4.45
417 455, 485 0.67
423 467, 499 0.66
430 473, 507 0.58
0.66 (0.66)
0.67 (0.68)
-
Figure 2. Normalized (a) absorbance and (b) fluorescence spectra of
5F-9F in dioxane.
a Measured in dioxane. b Fluorescence quantum yields for the
corresponding nTs (taken from ref 20) are given in parentheses.
c Oxidation potentials were measured in propylene carbonate (PC) with
0.1 M tetra-n-butylammonium tetrafluoroborate (TBABF4) and a Ag/
AgCl reference electrode (Fc/Fc+
) 0.34 V vs SCE under these
conditions) at a scan rate of 100 mV/s. d Values in parentheses are from
differential pulse voltammetry (DPV) measurements (see the SI).
e Calculated values [B3LYP/6-31G(d)].22 f Data were taken from ref 9
and measured in MeCN.
6T are slightly larger than those of 6F). The herringbone angle of
58° in 6F is midway between the values for pentacene17 (53°) and
6T16 (63°).18 It is interesting that the X-ray structures15 of 3F and
4F exhibit more complicated packing, without herringbone packing,
and are much less ordered than those of 3T and 4T,19 which exhibit
herringbone packing (see the SI).
Figure 3. Calculated [B3LYP/6-31G(d)] relative energies required for spiral
twisting of 6F and 6T as functions of twist angle.
The oligofurans are highly fluorescent, with quantum yields
ranging from 58% for 9F to 74% for 5F (Table 1 and Figure 2).21
The quantum yield slightly decreases with increasing chain length.9
The Stokes shift values are ∼0.25 eV for 3F-9F and are
substantially smaller than those for the corresponding oligoth-
iophenes 3T-6T (∼0.40 eV), possibly indicating that oligofurans
are more rigid. The solid-state fluorescence of 6F shows peaks that
are red-shifted by 0.35 eV relative to their value in solution (see
the SI). In addition to the smaller Stokes shift and closer packing
mentioned above, the rigidity of oligofurans can be seen from the
calculated twisting potentials (Figure 3).22 The energy required to
twist 6F is significantly greater than that for 6T. For example,
twisting 6F to a 36° twist angle requires 12.5 kcal/mol, while simi-
lar twisting in 6T23 requires only 2.3 kcal/mol (we applied the same
methodology for the calculation of the twisting energy as was
described in ref 23). Such large differences in twisting potential
should also allow introduction of different substituents onto the
oligofuran backbone without disturbing its planarity. For compari-
son, even a small increase in twisting potential leads, in some cases,
to significant changes in the structure and electronic properties of
substituted oligoselenophenes relative to oligothiophenes.4d,24 The
rigidity of R-oligofurans also affects their internal reorganization
energy, λ (see the SI). The difference in the twisting potential can
be explained by the less aromatic and more quinoid character of
oligofurans relative to oligothiophenes and is supported by the
Figure 4. (a) CV of 3F-8F in PC with 0.1 M TBABF4 (scan rate 50
mV/s) and (b) repetitive CV scans of 6F in 1,2-dichloroethane with 0.1 M
TBABF4 (scan rate 100 mV/s). Fc/Fc+ ) 0.34 V vs SCE under these
conditions.
difference in the calculated average inter-ring C-C bond lengths.
For example, the average calculated inter-ring C-C bond lengths
in 6F and 6T are 1.432 and 1.443 Å, respectively, which correlates
nicely with the experimental values mentioned above. The smaller
size of the oxygen atom compared with sulfur atom, which leads
to less steric demand in oligofurans than in oligothiophenes, may
also contribute to the significant difference in the rigidity.
CV of 3F-8F shows an irreversible oxidation peak that ranges
from 0.91 V (vs SCE) for 3F and 0.71 V for 5F to 0.67 V for 8F
(Figure 4a and Table 1, calibrated using Fc/Fc+ ) 0.34 V vs
SCE).25 This is in agreement with the calculated difference of only
0.14 eV in the HOMO energies of 5F-8F (Table 1).22 Long
oligofurans are significantly more electron-rich than oligoth-
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C O M M U N I C A T I O N S
of electrochemistry and computational results. This material is available
free of charge via the Internet at http://pubs.acs.org.
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Acknowledgment. We thank Dr. Linda J. W. Shimon and Dr.
Gregory Leitus for X-ray analysis, Elijah Shirman for his help with
fluorescence measurements, Ran Vardimon and Natasha Zamo-
chink for their help with calculations. We thank Profs. Igor F.
Perepichka (University of Central Lancashire) and Milko E. van
der Boom (Weizmann Institute) for useful discussions. We thank
the MINERVA Foundation and the Helen and Martin Kimmel
Center for Molecular Design for financial support. M.B. is the
incumbent of the Recanati career development chair, a member ad
personam of the Lise Meitner-Minerva Center for Computational
Quantum Chemistry, and acknowledges DuPont for a Young
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Supporting Information Available: Experimental procedures;
spectral data; X-ray structural data (CIF) for 3F, 4F, and 6F; and details
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2150 J. AM. CHEM. SOC. VOL. 132, NO. 7, 2010