Anthradifuran, a furan analogue of pentacene, and its isomers, exhibiting solid-state photoluminescence

Article abstract of DOI:10.1246/cl.2012.957

We have synthesized diphenyl-substituted anthra[2,3-b:6,7- b']difuran, a furan analogue of pentacene, together with its isomers, anthra[2,1-b:6,5-b'] difuran and anthra[1,2-b:5,6-b']difuran. These compounds are stable in the solid state with high decomposition temperatures. They are luminescent in the solid state with a quantum yield of 0.0930.23.

Full text of DOI:10.1246/cl.2012.957

doi:10.1246/cl.2012.957
Published on the web September 1, 2012
957
Anthradifuran, a Furan Analogue of Pentacene,
and Its Isomers, Exhibiting Solid-state Photoluminescence
Hayato Tsuji,*1,2 Kazutaka Shoyama,1 and Eiichi Nakamura*1
1Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033
2Japan Science and Technology Agency, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012
(Received June 29, 2012; CL-120698; E-mail: tsuji@chem.s.u-tokyo.ac.jp, nakamura@chem.s.u-tokyo.ac.jp)
We have synthesized diphenyl-substituted anthra[2,3-b:6,7-
b¤]difuran, a furan analogue of pentacene, together with its
isomers, anthra[2,1-b:6,5-b¤]difuran and anthra[1,2-b:5,6-b¤]di-
furan. These compounds are stable in the solid state with high
decomposition temperatures. They are luminescent in the solid
state with a quantum yield of 0.093-0.23.
Acenes1 play an important role in organic functional
materials.2 A representative compound, pentacene,3 is unstable
in air, and to remedy this problem, heteroacenes,4 such as
thienoacenes, have been synthesized, and their superior stability
and properties in several respects have been reported.5 We
recently demonstrated the utility of fused furans, which have
received much less attention as functional materials than have
thiophenes. Thus, we have reported the unique photophysical
properties of benzodifurans (BDFs) as well as their high carrier
mobilities in the amorphous state, making them suitable for
applications as hole-transporting materials and as host materials
Scheme 1.
in organic light-emitting diodes (OLEDs).6 We have also found
that single crystals of naphthodifurans (NDFs) serve as active
materials in organic field-effect transistors (OFETs) with high
hole mobilities.7 Here, we report the synthesis, structure, and
photophysical properties of linearly fused anthra[2,3-b:6,7-
b¤]difuran (ADF1), in which the anthradifuran (ADF) core has
a structure that is isoelectronic with pentacene. We also
synthesized its zigzag-shaped isomers, anthra[2,1-b:6,5-b¤]di-
furan (ADF2) and anthra[1,2-b:5,6-b¤]difuran (ADF3), having a
structure that is isoelectronic with dibenzo[a,h]anthracene. Their
stability and photoluminescent properties in the solid state are
also described.8
Single-crystal X-ray analysis showed that the ADF core is
very flat and the phenyl substituents are almost coplanar with the
ADF core, as shown in Figure 1 (details are given in SI).10,19
The crystal packing of DPADF1 was in a herringbone arrange-
ment (Figure 1a), similar to that of the polymorphic crystal
structures of pentacene,11 with short intermolecular contacts of
2.79 ¡ for C£H. In contrast, DPADF2 molecules are stacked
along the b axis with an interplanar distance of 3.53 ¡
(Figure 1b). Similarly, DPADF3 molecules stack along the a
axis with interplanar distances of 3.44-3.46 ¡ (Figure 1c). The
different packing of DPADF1 from the other compounds is due
to the CH/³ interaction resulting from the linear molecular
shape of DPADF1.
ADF compounds were synthesized using the recently
developed zinc-mediated double cyclization reaction
(Scheme 1).6a,9 Thus, 3,7-dialkynylanthracene-2,6-diol 1 was
lithiated with butyllithium, transmetallated to zinc, and heated at
120 °C. An internal addition of the zinc phenolate to the alkyne
moiety furnished DPADF1 as an orange solid. Because the
solubility of DPADF1 in organic solvents is very low, the crude
product was sublimed to obtain a pure product in 49% yield. The
product was characterized by NMR, mass spectrometry, and X-
ray analysis, and its purity was confirmed by elemental analysis
(see details in Supporting Information; SI19). The cyclization
reactions of 2 and 3 required a slightly higher reaction
temperature (180 °C) and afforded DPADF2 and DPADF3 as
yellow solids in 41% and 43% yields after sublimation. The
compounds sublime at >380 °C without decomposition (see
SI19). Thin films of these three compounds after exposure for a
long time to air showed no change in their UV-vis spectra,
indicating air stability in the solid state, while DPADF1 was
gradually oxidized in solution under ambient conditions.
Photophysical property measurements of ADFs in the solid
state revealed a difference in absorption between the linear- and
the zigzag-fused ADFs, as well as in their emission properties
(Figure 2 and Table 1). Absorption bands showed a well-defined
vibronic fine structure because of the rigidity of the ADF
framework. Not surprisingly, the linear DPADF1 showed a
much longer absorption maximum (540 nm) than the zigzag
molecules did (451 and 417 nm for DPADF2 and DPADF3,
respectively), as reported for pentacene and dibenzo[a,h]anthra-
cene.12 The ADFs showed photoluminescence in the solid state.
The emission bands are close to being mirror images of the
absorption spectra. DPADF1 showed an orange emission with a
maximum wavelength of 593 nm, while DPADF2 and DPADF3
showed a yellow-green emission at 515 and 489 nm, respec-
tively. The photoluminescence quantum yields (ΦPL) were
0.093, 0.23, and 0.15 for DPADF1, DPADF2, and DPADF3,
respectively. The fact that the ADFs emit in the solid state stands
in sharp contrast to pentacene and the thiophene counterparts,
Chem. Lett. 2012, 41, 957-959
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

958
Figure 1. X-ray crystallographic structures of the DPADF
compounds. Left column: ORTEP drawing (50% probability for
thermal ellipsoids). Right column: packing structures with unit
lattice (viewed from an arbitrary direction).
Table 1. Summary of the properties of the ADF compounds
Compound
-
abs
[nm]a
-
em
[nm] (ΦPL)b
IP [eV]c
DPADF1
DPADF2
DPADF3
540
451
417
593 (0.093)
515 (0.23)
489 (0.15)
5.1
5.5
5.2
aUV-vis absorption maximum wavelength in a thin film.
bEmission maximum wavelength in the solid state. Emission
quantum yields are shown in parentheses. cIonization potential
determined from photoemission yield spectroscopy measure-
ments.
Figure 2. Photophysical properties of ADFs in the solid
state (blue: absorption spectra of thin film, red: photolumines-
cence spectra of powder). (a) DPADF1, (b) DPADF2, and
(c) DPADF3.
This work was supported by a Grant-in-Aid for Scientific
Research on Innovative Areas (for H.T., No. 21108509, “pi-
Space”) from MEXT, Japan. We also thank MEXT (KAKENHI
for E.N., No. 22000008) and the Global COE Program for
Chemistry Innovation. This work was partly supported by
PRESTO, JST.
which show no or little photoluminescence in the solid state.13
The ionization potentials (IPs) of films determined by photo-
emission yield spectroscopy (PYS; Table 1 and SI)14,19 were
5.1, 5.5, and 5.2 eV for DPADF1, DPADF2, and DPADF3,
respectively. These IPs are larger than the value for pentacene
(4.9 eV) and comparable with a thiophene analogue (5.1 eV),15
which are known to serve as good semiconducting materials.
In summary, being chemically and thermally stable, the
linear- and the zigzag-fused ADFs add to the rapidly expanding
repertoire of furan compounds for organic electronic applica-
tions.16-18 Their photoluminescence ability in the solid state
illustrates the difference from the popular thiophene analogues,
which deserves further studies in the area of both chemistry
and materials science, and their application is currently under
investigation.
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10 Single crystals suitable for X-ray analysis were obtained by
physical vapor transport (PVT) for DPADF1 and DPADF3,
and recrystallization from hot chloroform for DPADF2.
Crystal data for DPADF1: C30H18O2, Mr = 410.44, T =
ꢀ
296 K, triclinic, space group P1, a = 5.8523(6) ¡, b =
7.8526(9) ¡, c = 21.788(3) ¡, ¡ = 98.201(7)°, ¢ =
97.343(7)°, £ = 92.859(6)°, V = 980.53(19) ¡3, Z = 2,
R1 = 0.0744 (1436 with I > 2·(I)), wR = 0.2486 (all data),
GOF = 0.987. DPADF2: C30H18O2, Mr = 410.44, T =
296 K, monoclinic, space group P21/n, a = 12.0004(2) ¡,
b = 3.92490(10) ¡, c = 21.0042(4) ¡, ¢ = 99.8721(13)°,
V = 974.66(4) ¡3, Z = 2, R1 = 0.0583 (965 with I >
2·(I)), wR = 0.2079 (all data), GOF = 1.064. DPADF3:
C30H18O2, Mr = 410.44, T = 296 K, triclinic, space group
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1021.88(5) ¡3, Z = 2, R1 = 0.0412 (2924 with I > 2·(I)),
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Chem. Lett. 2012, 41, 957-959
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/