Asymmetrical fluorene[2,3-b]benzo[d]thiophene derivatives: Synthesis, solid-state structures, and application in solution-processable organic light-emitting diodes

Article abstract of DOI:10.1002/chem.200900860

A series of novel asymmetrical fused compounds containing the backbone of fluorene[2,3-b]benzo[d]-thiophene (FBT) were effectively synthesized and fully characterized. Single-crystal X-ray studies demonstrated that the length of the substituent side chain

Full text of DOI:10.1002/chem.200900860

FULL PAPER
DOI: 10.1002/chem.200900860
Asymmetrical FluoreneACTHNUTRGNE[NUG 2,3-b]benzo[d]thiophene Derivatives:
Synthesis, Solid-State Structures, and Application in Solution-Processable
Organic Light-Emitting Diodes
Chunyan Du,[a, b] Shanghui Ye,[a, b] Jianming Chen,[a, b] Yunlong Guo,[a, b] Yunqi Liu,*[a]
Kun Lu,[a, b] Ying Liu,[a, b] Ting Qi,[a] Xike Gao,[a] Zhigang Shuai,[a] and Gui Yu[a]
Abstract: A series of novel asymmetri-
cal fused compounds containing the
backbone of fluoreneACTHNUGRTNEUNG[2,3-b]benzo[d]-
the FBTs have large band gaps, low-
lying HOMO energy levels, and there-
fore good stability toward oxidation.
Moreover, the substituents strongly in-
fluence the fluorescence properties of
the resulting FBT derivatives. The di-n-
hexyl compound exhibits intense fluo-
rescence in solution with the highest
quantum yield of up to 91%. Solution-
processed green phosphorescent organ-
ic light-emitting diodes with the di-n-
butyl derivative as the host material ex-
thiophene (FBT) were effectively syn-
thesized and fully characterized.
Single-crystal X-ray studies demon-
strated that the length of the substitu-
ent side chains greatly affects the solid-
state packing of the obtained fused
compounds. DFT, photophysical, and
electrochemical studies all showed that
Keywords: fused-ring systems
organic light-emitting diodes
·
·
hibited
a
maximum brightness of
14185 cdmÀ2 and a luminescence effi-
ciency of 12 cdAÀ1.
solid-state structures · substituent
effects · sulfur heterocycles
Introduction
be tailored by changing the degree of conjugation through
different backbone units or incorporating heteroatoms, such
as sulfur, selenium, phosphorus, nitrogen, and oxygen, as
bridging atoms.[9–11] Furthermore, these modifications can
also significantly influence their organization in the solid
state.[12] The solid-state morphology of organic semiconduc-
tors plays an important role in the characteristics of organic
electronic devices. Both exciton migration and carrier mobi-
lity strongly depend on the solid-state stacking of the conju-
gated materials. In general, great intermolecular overlap,
thus strong electronic interactions, between the adjacent
molecules leads to high charge mobility. As we know, the in-
corporation of heteroatoms would lead to a variety of intra-
and intermolecular interactions, such as van der Waals inter-
actions, p–p stacking, and heteroatom interactions, thus af-
fecting not only the electronic structures but also the solid-
state structures.[13,14] However, there are still no definite
guidelines for molecular design to combine high perfor-
mance with easy preparation. It is still a major challenge to
understand the relationship between molecular structure
and properties.
Conjugated materials, which are one of the most investigat-
ed classes of advanced materials, have attracted much atten-
tion in recent years because of their potential applications in
electronics, photonics, and optoelectronics.[1–4] In contrast to
conjugated polymers, conjugated oligomers are character-
ized because of their well-defined structures and superior
chemical purity.[5,6] Among these, ladder-type fused mole-
cules have been the focus of research interest because of
their enhanced degree of p-conjugation, highly efficient
photoluminescence, and remarkable charge mobility.[7,8] The
physical properties of the ladder-type fused molecules can
[a] C. Du, S. Ye, J. Chen, Y. Guo, Prof. Dr. Y. Liu, K. Lu, Y. Liu,
Dr. T. Qi, Dr. X. Gao, Prof. Dr. Z. Shuai, Prof. Dr. G. Yu
Beijing National Laboratory for Molecular Sciences
Organic Solids Laboratory, Institute of Chemistry
Chinese Academy of Sciences, Beijing 100190 (China)
Fax : (+86)10-62559373
E-mail: liuyq@iccas.ac.cn
[b] C. Du, S. Ye, J. Chen, Y. Guo, K. Lu, Y. Liu
Graduate School of the Chinese Academy of Sciences
Beijing 100190 (China)
Electrophosphorescent organic light-emitting diodes
(OLEDs) with the potential to offer 100% internal efficien-
cies because they can use both the singlet and triplet exci-
tons for emission have been rapidly developed in recent
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200900860.
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years.[15] The way to realize efficient electrophosphorescent
OLEDs is to choose suitable host materials that allow for
exothermic energy transfer between a host excited state and
a lower-energy guest unoccupied orbital, which in turn re-
sults in an energetically favorable excited-state transition
between molecules.[15] The reported host materials are
mostly bulky and sterically hindered molecules, and no
fused planar molecules have been reported as host materials
in phosphorescent OLEDs.
Fluorene-based materials have been widely studied for
their unique optoelectronic properties.[16–18] At present, we
find that the studies have mainly focused on polyfluorene[19]
and oligomers.[20] Until now, there have only been a few
linear fused-ring compounds with the introduction of the
fluorene substructure,[21] which may be due to the lack of ef-
ficient synthetic strategies. The introduction of fluorene
would have a great influence on the solid-state stacking and
electronic structure of the compounds, as the C9 position of
the fluorene unit could be readily functionalized with a vari-
ety of groups. However, how the substituent side chains
affect the solid-state structure and the physicochemical
properties are still ambiguous.[22] To the best of our knowl-
edge, the introduction of the fluorene substructure in an
asymmetrical linear fused-ring molecule has not been re-
ported yet. We recently reported the synthesis and charac-
terization of an asymmetrical heteroacene anthraACTHNUTRGENUG[N 2,3-b]ben-
Scheme 1. Synthesis of 4a–d.
zo[d]thiophene (ABT).[23] Expanding on our previous work,
we have focused on the synthesis and structure–property re-
lationship of asymmetrically substituted fluoreneACTHNUTRGENUG[N 2,3-b]ben-
zo[d]thiophene (FBT) compounds. Herein, we have de-
signed and synthesized a series of asymmetrically substituted
compounds 4a–d (see Scheme 1), in which a fluorene unit is
The Suzuki–Miyaura cross-coupling reaction between 9,9-di-
methyl-2-fluoreneboronic acid and 2-bromo(methylsulfinyl-
benzene) produces 3b in 80% yield. Methyl-substituted
FBT 4b was obtained by treating 3b with trifluoromethane-
sulfonic acid for 12 h and subsequent demethylation in
water/pyridine. Compounds 4a, 4c, and 4d were synthesized
by using a slightly different route because the corresponding
fluoreneboronic acids were not commercially available.
Compounds 1a, 1c, and 1d were coupled with (2-methylsul-
fanyl)benzeneboronic acid by using the Suzuki–Miyaura
cross-coupling reaction to afford 2-(2-methylsulfanylphen-
yl)-9,9-dialkylfluorene derivatives 2a, 2c, and 2d. The oxi-
dation of 2a, 2c, and 2d with hydrogen peroxide gave rise
to the key 2-(2-methylsulfinylphenyl)-9,9-dialkylfluorene
precursors 3a, 3c, and 3d. The final compounds 4a, 4c, and
4d were obtained by treating 3a, 3c, and 3d in a similar
procedure to 3b. As expected, the solubility of the FBTs
4a–d increases as a function of the length of the alkyl
chains. Furthermore, all these compounds exhibit good solu-
bility in common solvents, such as petroleum ether, ethyl
acetate, and dichloromethane, thus enabling ready purifica-
tion by column chromatography and device fabrication with
solution processing.
fused with benzothiophene, that is, fluoreneACTHNUGRTENUNG[2,3-b]benzo[d]-
thiophene, motivated by the aforementioned characteristics.
Accordingly, a detailed investigation of the photophysical
properties and single-crystal structural studies of substituted
FBT is presented to fully explore the influence of different
substituents on the inherent electronic properties and solid-
state packing arrangement. Furthermore, we prepared a
series of solution-processable phosphorescent OLEDs con-
taining FBTs as the host materials for the dopant [IrACHTNUTRGNEUNG(ppy)3]
(ppy=phenylpyridine) to evaluate the application of the
FBTs in OLEDs. To the best of our knowledge, this type of
fused compound bearing the thiophene ring used as the host
material of phosphorescent OLEDs has been rarely de-
scribed. Herein, we report a flexible molecular design that
satisfies the demand for phosphorescent devices.[24]
Results and Discussion
Synthesis: The synthesis of the FBTs was conducted with
the acid-induced condensation of aromatic methyl sulfoxide
groups as a key reaction, which has been used extensively
for the preparation of high-molecular-weight polymers and
thiophene-based heteroacenes.[25,26] First, the substrates for
the condensation were synthesized as shown in Scheme 1.
Theoretical calculations for 4a–d: To obtain further insight
into the molecular structure and electron distribution and to
estimate the position of the frontier orbitals of 4a–d, DFT
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FluoreneACHTUNGTRENNUNG[2,3-b]benzothiophene Derivatives as Phosphorescent Devices
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À
calculations were performed by using Gaussian 03 at the
B3LYP/6–31G(d) level. For 4b–d, the optimized structures
are similar to those obtained by X-ray crystallographic stud-
ies. The HOMOs and LUMOs for each molecule are spread
out over the entire FBT backbone, without localized frontier
orbitals. However, with the increase in the length of the
alkyl chains at the C9 position of fluorene, the electron den-
sity located on the sulfur atoms increased greatly from 4a to
There are three types of short C H···p contacts between ad-
jacent molecules within each layer of 4b (Figure 2).
Single crystals of di-n-butyl-substituted 4c, obtained from
the slow evaporation of hexane, crystallizes in the triclinic
4d (Figure 1). The energy values of the HOMO (EHOMO
)
Figure 2. a) Molecular structure of 4b in the solid state. b) Stacking dia-
À
gram of 4b that shows C H···p hydrogen bonds in the solid state and all
the values are given in ꢃ. c) Layer structure packing of 4b viewed along
the a axis.
¯
system space group P1. Compound 4c has two independent
Figure 1. HOMO and LUMO for 4a–d according to DFT calculations at
the B3LYP/6–31G(d) level.
molecules in one unit cell and each molecule possesses an
asymmetrical geometry with the two n-butyl substituents ar-
ranged vertically around the backbone at one side (see
Figure 3 and Figure S2 in the Supporting Information). In-
terestingly, the two n-butyl substituents in each molecule
were arranged in a different manner with the four carbon
atoms of one n-butyl substituent adopting a mean ꢁantiꢂ ge-
ometry and the other a ꢁsynꢂ geometry (Figure 3). This ar-
rangement may be because of the steric hindrance of the
two molecules in one unit cell. From the stacking structure
shown in Figure 3b, we can see that the two independent
molecules construct two types of stacking columns arranged
and LUMO (ELUMO)orbitals of 4a–d were calculated by
DFT studies to be
À5.55 eV; ELUMO =À1.24, À1.26, À1.25, and À1.25 eV; and
Eg =4.34, 4.31, 4.31, and 4.30 eV for 4a–d, respectively.
E
HOMO =À5.58, À5.57, À5.56, and
Solid-state structures of 4b–d: Recent studies have shown
that the solid-state morphology of conjugated materials
plays an important role in the performance characteristics of
electronic devices;[1a,2b] thus, X-ray diffraction studies were
performed on crystals of 4b–d to determine their solid-state
order and the effect of the side-chain substitution on the
solid-state structures. Compound 4b, obtained as colorless
plate crystals upon evaporation from CH2Cl2, crystallizes in
the orthorhombic system space group Pca2(1).[27] The struc-
ture of 4b reveals a nearly planar molecular conformation
with the two methyl substituents arranged vertically around
the backbone. The compound has p-stacked packing in two
dimensions and forms a layer-by-layer solid structure along
the c axis (see Figure S1 in the Supporting Information).
À
in a face-to-face fashion. Two types of short C H···p con-
tacts (i.e., 2.85 and 2.83 ꢃ, respectively; Figure 3a and 3b)
exist between the two independent molecules. The two adja-
cent molecules in a stacking column are almost parallel, and
there are intermolecular C···C interactions (3.34 and 3.35 ꢃ,
respectively; Figure 3c and 3d) that exist between adjacent
molecules in each column. Besides, the third type of short
À
C H···p contact (2.80 ꢃ; Figure 3) was observed between
two face-to-face molecules in the two columns. As for the
À
pendant n-butyl chains, short C H···C contacts exist be-
tween the anti n-butyl chain of one molecule and the syn
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Y. Liu et al.
Figure 3. a) Molecular structure of 4c in the solid state. b) Stacking dia-
À
gram of 4c that shows p–p interactions and C H···p hydrogen bonds in
the solid state and all the values are given in ꢃ. c) Crystal packing of 4c
viewed along the a axis.
Figure 4. a) Solid-state structure of 4d, top view and side view.
À
b) Stacking diagram of 4d that shows p–p interactions and C H···p hy-
drogen bonds. c) Crystal packing of 4d viewed along the a axis.
chain of another (Figure 3), which may explain why the two
n-butyl chains adopt different arrangements.
588C respectively) with lengthening alkyl chain probably be-
cause longer alkyl substitution largely increases the degree
of freedom of the FBTs (see Figure S4 in the Supporting In-
formation). However, the thermal decomposition tempera-
ture of 4a–d (Td ꢀ3008C) increased on the order 4b<4c<
4a<4d, as shown by thermogravimetric analysis (TGA; see
Figure S4 in the Supporting Information), thus indicating
that 4d, with the longest side chain, has the best thermal sta-
bility.
Di-n-hexyl-substituted 4d, obtained from methanol/di-
chloromethane at À208C, crystallizes in the monoclinic
system space group P21/n. Compound 4d has a single mole-
cule in one unit cell and possesses a different packing struc-
ture from 4b and 4c, although these compounds have the
same backbone structure (see Figure 4 and Figure S3 in the
Supporting Information). The central FBT core was flat and
the two n-hexyl substituents were arranged almost vertically
around the plane of the backbone, with the carbon atoms in
both substituents in a mean anti geometry. We observed that
Optical properties: The photophysical properties of FBTs
4a–d are shown in Figure 5 and Table 1 (also see Figure S5
in the Supporting Information). For the series of FBTs, 4a
without substituents showed a much weaker absorption in-
tensity at long wavelengths than 4b–d. With the introduc-
tion of substituent chains, the absorption maxima of 4a–d
exhibited a slight red shift from 341 to 346 nm. However,
for the substitued FBT derivatives 4b–d, the variations in
the absorption spectrum in a dilute solution are much less
pronounced, thus indicating that the length of the alkyl
chains has no significant effect on the absorption spectrum
of the FBTs. In comparison to those spectra recorded in so-
lution, red shifts of dl=12, 6, and 9 nm in the absorption
maxima for pristine films of 4a, 4b and 4c, and 4d, respec-
tively, were observed, thus suggesting that moderate inter-
molecular interactions were present in the solid-state.
À
short C C contacts (i.e., 3.37 ꢃ) exist between neighboring
À
conjugated planes and that short C H···p intermolecular in-
teractions exist between the pendant n-hexyl chain and the
conjugated plane (2.37 ꢃ), which result in the formation of
stacking columns in an edge-to-face fashion (see the packing
structure shown in Figure 4).
These very different solid-state structures of the three rel-
atives of the same FBT family nicely illustrate how the
effect of the substituents can efficiently be utilized to tune
the organization in the solid-state of organic semiconduc-
tors. Even the simplest variations, such as lengthening the
side chain from one to four–six carbon atoms, have tremen-
dous impact on the packing motif of the compound. Further
research would yield complementary and useful information
in the study of structure–property relationships in organic
semiconductors.
The FBTs all exhibited intensive fluorescence both in so-
lution and in the solid state. For 4a, the solid-state fluores-
cence emission wavelength maximum is observed at lmax
=
Thermal properties: Differential scanning calorimetry
(DSC) studies of 4a–d revealed that these compounds
showed decreased melting points (i.e., 240, 183, 120, and
398 nm and exhibits a red shift of about 34 nm from solution
(i.e., 364 nm), which is indicative of strong intermolecular
interactions, likely through p stacking. Compounds 4b–d ex-
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FluoreneACHTUNGTRENNUNG[2,3-b]benzothiophene Derivatives as Phosphorescent Devices
FULL PAPER
to that of the others, which may be attributed to the fact
that 4d has the longest alkyl chains, which are arranged ver-
tically around the backbone plane and preclude p stacking
in the solid state.
Fluorescence quantum yields were determined by using
standard procedures with naphthalene (ff =0.23Æ0.02)[28] as
a reference. Fluorescence quantum yields in dichlorome-
thane were 0.28, 0.50, 0.78, and 0.91 for 4a–d, respectively.
The fluorescence quantum yields increase with lengthening
of the alkyl substituents, and 4d, with n-hexyl substituents,
appears to be significantly more highly fluorescent with the
highest quantum yield of 0.91. It is worth noting that the
Stokes shift of the fluorescence band of 4a in the solid state
was higher than that of 4b–d (i.e., 45 versus 21, 30, and
30 nm, respectively), thus indicating there are different
structural variations between the excited and ground states.
Relative to indenofluorene (IF),[29] the absorption spec-
trum wavelength maximum of 4a showed a red shift of dl=
7 nm from lmax =333 of IF[29] to 341 nm of 4a in solution,
and the optical gap estimated from the absorption edges of
the solution spectra for 4a is 3.51 eV, which is smaller than
that of IF (i.e., 3.71 eV). The structural difference between
4a and IF is that the indene unit in IF was changed to the
benzothiophene unit in 4a, thus resulting in spectral change
and demonstrating that the introduction of the p-electron-
rich thiophene moiety impacts greatly on the optical proper-
ties.
Cyclic voltammometry: The electrochemical stabilities of
FBTs were investigated by cyclic voltammetry (CV), and
the electrochemical characteristics are listed in Table 2. For
4a, a quasi-reversible oxidation wave was observed and the
oxidation peak maximum was 1.54 eV. However, 4b–d
Figure 5. a) Absorption and b) emission spectra of 4a–d in CH2Cl2.
Table 1. Photophysical data of 4a–d.
Table 2. Cyclic voltammogram data[a] and optical band gaps of 4a–d and
IF for comparison.
Solution
Film[e]
llum
[a]
[b]
[c]
[b]
[c]
Compound
labs
llum
Eg
ff[d]
labs
Eg
Compound
4a
4b
4c
4d
IF[g]
[nm]
[nm]
[eV]
[nm]
[nm]
[eV]
Eoxpeak [eV]
1.54
1.59, 1.75[f] 1.57, 1.76[f] 1.54, 1.73[f] 1.25
4a
4b
4c
4d
341
344
346
346
351, 364
357
358
3.51
3.48
3.47
3.47
0.28
0.50
0.78
0.91
353
350
352
355
398
371
382
365
2.83
3.06
3.03
2.65
EHOMO [eV][b] À5.58 À5.57
À5.56
À5.83
À1.25
À2.36
3.47
À5.55
À5.83
À1.25
À2.36
3.47
À5.43
À5.65
À1.13
À1.94
3.71
EHOMO [eV][c]
ELUMO [eV][b]
ELUMO [eV][d]
Eg[e] [eV]
À5.84 À5.84
À1.24 À1.26
À2.33 À2.36
359
3.51
3.48
[a] Absorption maxima measured in a dilute solution of CH2Cl2. [b] Ex-
cited at the absorption wavelength maximum. [c] Estimated from the
onset of absorption (Eg =1240 eVlonsetÀ1). [d] Measured in solution with
CH2Cl2 with naphthalene[28] as a standard at room temperature. [e] Films
were drop-cast using a solution of CH2Cl2 on a quartz substrate.
[a] Performed in
a .
solution of Bu4NPF6 in CH3CN; n=100 mVsÀ1
[b] Calculated through DFT studies. [c] Obtained according to UV/Vis
spectra by using the empirical equation: EHOMO =À(4.4+Eoxonset). [d] Cal-
culated from the Eg and EHOMO values. [e] Estimated from the onset of
absorption in CH2Cl2 (Eg =1240 eVlonsetÀ1). [f] Second oxidation.
[g] From ref. [29].
hibit similar fluorescence spectra in solution with the wave-
length maxima at lmax =357, 358, and 359 nm, respectively.
But the thin-film fluorescence emissions of 4b–d exhibit dif-
ferent red shifts with the wavelength maximum at lmax =71,
382, and 365 nm, respectively. As for 4d, the fluorescence
emission in the thin film shows only a small red shift relative
showed two quasi-reversible oxidation waves (see Figure S6
in the Supporting Information) and the first oxidation peaks
maxima of 4a–d were +1.59, +1.57, and +1.54 V versus Ag/
AgCl, respectively. The HOMO levels (EHOMO) of 4a–d were
estimated from the first oxidation onsets to be À5.84, À5.84,
À5.83, and À5.83 eV, respectively, which are much lower
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Y. Liu et al.
than that of indenofluorene (À5.65 eV)[29a] and other report-
ed fused compounds that incorporate a sp3 carbon atom as a
carbon bridge[30] and are indicative that FBTs are more
stable under ambient conditions. Based on the HOMO
levels and the optical-band gaps evaluated from the onset
wavelength of the UV/Vis absorption spectra, LUMO
energy levels of the FBTs were calculated to be ELUMO
=
À2.33 and À2.36 eV for 4a and 4b–d, respectively. As com-
pared with IF, changing indene with the benzothiophene
unit in FBTs has a large influence on the HOMO and
LUMO energy levels because of the heteroatom effective-
ness[13,14] of the thiophene unit. However, the length of the
alkyl chains has little effect on the energy levels, thus leav-
ing the HOMO and LUMO energy levels almost unchanged
from 4b to 4d.
Device fabrication of OLEDs: To explore the application of
the FBTs in OLEDs, we investigated the use of FBTs as the
host material and [IrACHTNUGTRNEUNG(ppy)3] as the emitter for green phos-
phorescent devices, with the device structure of indium tin
oxide (ITO)/polyethylene dioxythiophene–polystyrene sulfo-
nate (PEDOT–PSS; 30 nm)/4a–d: (w, 10%), [IrACTHUNRGTNEUNG(ppy)3]
(45 nm)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline
(BCP; 40 nm)/Ca–Ag (100 nm). The conducting polymer
PEDOT–PSS was used as the hole injection layer. The emit-
ting layer consisted of 4a–d doped with 10 wt% of green
phosphor [Ir
In the electroluminescence and film spectra (lmax =514 and
510 nm) of 10% [Ir(ppy)3] in 4a–d, only [Ir(ppy)3] emissions
ACHTUNGTRENNUNG(ppy)3]. BCP was used as a hole-blocking layer.
A
ACHTUNGTRENNUNG
were observed from the device, thus indicating complete
energy transfer to the phosphorescent dopant from the
FBTs. All the devices exhibited green-light emission with a
peak centered at l=514 nm and Commission International
de LꢂEclairage (CIE) 1931 chromaticity coordinates of
(0.32, 0.61). Figure 6 displays the current density/voltage/lu-
minance and luminance efficiency/luminance characteristics
of the devices with different host materials 4a–d. The best
performance was achieved for the device when using 4c as
the host material with
a
maximum luminance of
14185 cdmÀ2 at 18 V and a luminance efficiency of 12 cdAÀ1
at the luminance of 600 cdmÀ2. The luminance efficiencies
of the devices fabricated with 4b and 4d are 1.2 and
2.8 cdAÀ1, respectively, which are lower than that of 4c but
much higher than that of 4a (0.05 cdAÀ1). Devices based on
4d exhibit a lower performance with a luminance of
2155 cdmÀ2 at 13 V. The poor performance of 4a and 4b
may be attributed to their solid structure because they have
short substituents at the C9 position of fluorene, thereby
forming densely packed solid structures that readily induce
fluorescence quenching. As we know, a suitable host materi-
al in the phosphorescent OLEDs must have limited conjuga-
tion length, and thus rather large energy gaps to prevent re-
verse energy transfer from the guest back to the host and
confine triplet excitons on the guest molecules.[31] For fused
small molecules used as host materials and devices fabricat-
ed with simple spin-coating deposition, our results are com-
parable to the best performance. With further modification
Figure 6. a) Current density/voltage, b) luminance/voltage and c) lumi-
nance efficiency/luminance of the devices with 4a–d.
of the FBTs and optimization of the device structures, these
materials may prove to be excellent host materials for
future OLED display applications.
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FluoreneACHTUNGTRENNUNG[2,3-b]benzothiophene Derivatives as Phosphorescent Devices
FULL PAPER
4c: C27H28S, Mr =384.55, colorless plates, crystal size 0.20ꢄ0.18ꢄ
0.16 mm; triclinic, space group PÀ1; a=8.5753(12), b=13.6692(15), c=
18.691(2) ꢃ; a=90.557(5), b=91.676(7), g=98.547(4)8; V=2165.4(4) ꢃ3,
Z=4, 1calcd =1.180 gcmÀ3; m=0.159 mmÀ1; T=113(2) K; 9268 data (6428,
Conclusions
In conclusion, we have reported the synthesis and properties
of asymmetrical FBT derivatives. To the best of our knowl-
edge, this report is the first of core asymmetrical ladder-type
fused molecules that incorporate fluorene and thiophene
units simultaneously. The FBTs are soluble in common or-
ganic solvents and exhibit different packing with different
lengths of alkyl substituents in the solid state. Fluorescence
spectra show that all the FBTs exhibit intense fluorescence
in solution with quantum yields of up to 0.91 for di-n-hexyl-
substituted 4d. DFT, photophysical, and electrochemical
studies suggest that FBTs have low-lying HOMO levels and
thus higher oxidative stability than most fused-ring com-
pounds. Preliminary results of OLEDs with FBTs indicate
that the FBTs are promising host materials that perform
very efficiently and have low fabrication costs for further
OLED applications. Furthermore, the effect of the substitu-
ents can be efficiently utilized to tune the solid-state struc-
ture and electronic properties of organic semiconductors,
which is a powerful method for obtaining semiconducting
materials with a variety of desirable attributes.
R
int =0.0423, 1.87ꢁqꢁ27.28); GOF=1.073; 509 parameters; R=0.0423.
4d: C31H36S, Mr =440.66, colorless plates, crystal size 0.16ꢄ0.14ꢄ
0.10 mm; monoclinic, space group P21/n; a=12.247(2), b=10.708(2), c=
19.793(4) ꢃ; a=90.00, b=99.63(3), g=90.008; V=2559.1(9) ꢃ3, Z=4,
1calcd =1.144 gcmÀ3; m=0.142 mmÀ1; T=113(2) K; 4502 data (3842, Rint
0.0556, 2.13ꢁqꢁ25.02); GOF=1.099; 291 parameters; R=0.0355.
=
Synthesis of 2-(2-methylsulfanylphenyl)-9,9-di-n-butylfluorene (2c):
K2CO3 solution (2m, 15 mL) was added to a mixture of 1c (1.16 g,
3 mmol) and 2-methylsulfanylbenzeneboronic acid (0.504 g, 3 mmol) in
freshly distilled toluene (20 mL) under argon. [PdACHTNUTRGNE(UNG PPh3)4] (100 mg) was
the added in one portion to the reaction mixture, which was heated to
reflux for 24 h. The mixture was cooled to room temperature and extract-
ed with ethyl acetate. The organic layers were dried, evaporated in
vacuum, the residue was purified by column chromatography on silica gel
with petroleum ether as the eluent to give pure 2c as a white solid (1 g,
83%). MS (EI): m/z (%): 400 [M+]; 1H NMR (400 MHz, CDCl3): d=
0.82–0.85 (m, 10H), 1.22–1.27 (m, 4H), 2.13–2.18 (m, 4H), 2.44 (s, 3H),
7.44–7.53 (m, 8H), 7.64 (s, 1H), 7.85–7.90 ppm (m, 2H); 13C NMR
(400 MHz, CDCl3): d=14.20, 16.32, 23.44, 26.34, 40.48, 55.35, 119.76,
120.08, 123.14, 124.58, 125.13, 125.92, 127.11, 127.38, 128.10, 128.17,
130.35, 137.64, 139.55, 140.77, 141.21, 141.90, 150.69, 151.35 ppm; elemen-
tal analysis calcd (%) for C28H32S: C 83.95, H 8.05; found: C 83.58, H
8.07.
Synthesis of 2-(2-methylsulfinylphenyl)-9,9-dimethylfluorene (3b):
K2CO3 solution (2m, 15 mL) was added to a mixture of 9,9-dimethyl-2-
fluoreneboronic acid (1.19 g, 5 mmol) and 2-bromo(methylsulfinyl)ben-
zene (1.1 g 5 mmol) in freshly distilled toluene (20 mL) under argon. [Pd-
Experimental Section
Instruments and measurement: 1H NMR (400 MHz) spectra were ob-
tained on a Bruker DMX-400 NMR spectrometer with tetramethylsilane
(TMS) as an internal standard. High-resolution mass spectrometry
(HRMS) and EIMS were carried out on a Micromass GCT-MS spectrom-
eter. Elemental analyses were performed on a Carlo Erba model 1160 el-
emental analyzer. Electronic absorption spectra were measured on a
Jasco V570 UV/Vis spectrophotometer. TGA-differential thermal analy-
sis (DTA) measurements were carried out on a TA SDT 2960 instru-
ments under a dry nitrogen flow with heating from room temperature to
5008C at a heating rate of 108CminÀ1. Cyclic voltammetric measure-
ments were carried out in a conventional three-electrode cell using Pt
button working electrodes 2 mm in diameter, a platinum wire countere-
lectrode, and a Ag/AgCl reference electrode on a computer-controlled
CHI660C instrument at room temperature. X-ray diffraction studies were
carried out in the reflection mode at room temperature on a 2-kW
Rigaku X-ray diffraction system.
ACHTUNGTREN(NUNG PPh3)4] (100 mg) was added in one portion to the reaction mixture,
which was heated to reflux for 24 h. The mixture was cooled to room
temperature and extracted with ethyl acetate. The organic layers were
dried, evaporated in vacuum, and the residue was purified with column
chromatography on silica gel with petroleum ether/ethyl acetate (2:1) as
the eluent to give pure 3b as a pale-white solid (1.3 g, 80%). MS (EI):
1
m/z (%): 332 [M+]; H NMR (400 MHz, CDCl3): d=1.49 (s, 3H), 1.55 (s,
3H), 2.36 (s, 3H), 7.32–7.34 (d, 1H), 7.35–7.38 (t, 2H), 7.40–7.42 (d, 1H),
7.45–7.48 (m, 2H), 7.55–7.59 (t, 1H), 7.61–7.65 (t, 1H), 7.75–7.80 (m,
2H), 8.13–8.16 ppm (d, 1H); 13C NMR (400 MHz, CDCl3): d=27.05,
41.44, 46.97, 120.25, 120.33, 122.70, 123.43, 123.56, 127.16, 127.78, 127.97,
128.60, 130.31, 130.65, 136.64, 138.30, 139.39, 139.84, 143.98, 153.75,
154.24 ppm; elemental analysis calcd (%) for C22H20OS: C 79.48, H 6.06;
found: C 79.23, H 6.12.
Synthesis of 2-(2-methylsulfinylphenyl)-9,9-di-n-butylfluorene (3c): Hy-
drogen peroxide (35%, 0.25 g) dissolved in glacial acetic acid (10 mL)
was added dropwise to 2c (2.5 mmol, 1 g) dissolved in glacial acetic acid
(60 mL). The reaction mixture was allowed to stir at room temperature
for 6 h. The acetic acid was removed by evaporation under vacuum and
the crude product was purified with column chromatography on silica gel
with petroleum ether/ethyl acetate (2:1) as the eluent to afford 3c as a
colorless oil (0.95 g, 92%). MS (EI): m/z (%): 416 [M+]; 1H NMR
(400 MHz, CDCl3): d=0.51–0.63 (m, 8H), 0.76–1.13 (m, 6H), 1.86–1.96
(m, 4H), 2.25 (s, 3H), 7.20–7.27 (m, 4H), 7.30–7.32 (m, 2H), 7.42–7.47 (t,
1H), 7.49–7.53 (t, 3H), 7.62–7.64 (m, 1H), 7.66–7.68 (d, 1H), 8.04–
8.06 ppm (d, 1H); 13C NMR (400 MHz, CDCl3): d=14.29, 21.16, 23.15,
26.30, 41.12, 55.38, 120.15, 120.36, 123.09, 123.70, 123.77, 127.16, 127.83,
128.00, 128.76, 130.57, 130.89, 136.53, 140.13, 140.39, 141.66, 143.74,
150.99, 151.55 ppm; elemental analysis calcd (%) for C28H32OS: C 80.72,
H 7.74; found: C 80.81, H 7.91.
OLED fabrication and characterization: All the OLEDs were fabricated
on bare ITO substrates that were cleaned with detergent, deionized
water, acetone, and ethanol. Films of PEDOT–PSS were spin-cast onto
precleaned ITO substrates. A mixture of [IrACTHNUTRGENUG(N ppy)3] and host materials
(10 wt%) was spin-cast from a solution in CHCl3. A Newport 2835-C
multifunction optical meter was used to measure luminance output. Cur-
rent/voltage characteristics were measured with
4140B semiconductor parameter analyzer. CIE coordinates were mea-
sured with a PhotoResearch PR-650 spectrophotometer.
a Hewlett–Packard
Single-crystal X-ray analysis of 4b–d: The measurements of 4b–d were
made on
a diffractometer with MoKa radiation (l=0.71073 ꢃ) at
113(2) K. The structures were solved and defined by direct methods and
SHELXS-97. The hydrogen atoms were located at the calculated posi-
tions. Absorption correction was applied using semiempirical measure-
ments from equivalents.
Synthesis of 9,9-dimethylfluoreneACTHNUTRGENUGN[2,3-b]benzo[d]thiophene (4b): Com-
pound 3b (1 g, 3 mmol) was added to trifluoromethanesulfonic acid
(4.5 mL). The solution was stirred at room temperature for 24 h and then
poured slowly into water/pyridine (90 mL, 8:1). Demethylation was ach-
ieved by heating the mixture to reflux for 30 min. Upon cooling, the mix-
ture was extracted with dichloromethane. The organic extracts were
washed with brine and dried with MgSO4. After removing the solvent by
4b: C21H16S, Mr =300.40, colorless plates, crystal size 0.40ꢄ0.38ꢄ
0.12 mm; orthorhombic, space group Pca2(1); a=13.522(3), b=
6.8691(14), c=17.579(4) ꢃ; a=90.00, b=90.00, g=90.008; V=
1632.8(6) ꢃ3; Z=4, 1calcd =1.222 gcmÀ3
; ; T=113(2) K;
m=0.192 mmÀ1
2812 data (1618, Rint =0.1383, 2.32ꢁqꢁ 25.02); GOF=0.971; 202 param-
eters; R=0.0793.
Chem. Eur. J. 2009, 15, 8275 – 8282
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8281

Y. Liu et al.
evaporation, the residue was purified with column chromatography on
silica gel with petroleum ether as the eluent to afford 4b as a white solid
(0.72 g, 80%). MS (EI): m/z (%): 300 [M+]; 1H NMR (400 MHz,
CDCl3): d=1.59 (s, 6H), 7.35–7.38 (m, 2H), 7.43–7.48 (m, 3H), 7.78–7.80
(d, 1H), 7.83–7.84 (d, 1H), 8.16–8.17 (d, 2H), 7.18–8.20 ppm (d, 1H);
13C NMR (400 MHz, CDCl3): d=27.95, 46.74, 114.13, 115.67, 120.36,
121.52, 122.98, 123.06, 124.48, 126.54, 127.36, 127.91, 135.06, 135.80,
138.76, 138.83, 139.26, 139.97, 151.08, 154.38 ppm; elemental analysis
calcd (%) for C21H16S: C 83.96, H 5.37; found: C 83.90, H 5.46.
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Synthesis of 9,9-di-n-butylfluoreneACTHNUTRGNEUGN[2,3-b]benzo[d]thiophene (4c): Com-
pound 4c was prepared in a procedure similar to 4b, thus affording 4c as
a white solid (0.72 g, 80%). MS (EI): m/z (%): 384 [M+]; 1H NMR
(400 MHz, CDCl3): d=0.63–0.67 (m, 10H), 1.05–1.09 (m, 4H), 2.06–2.09
(m, 4H), 7.35–7.36 (m, 3H), 7.41–7.48 (m, 2H), 7.76–7.78 (d, 1H), 7.83–
7.86 (d, 1H), 8.07 (s, 1H), 8.14 (s, 1H), 8.20–8.22 ppm (d, 1H); 13C NMR
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120.04, 121.59, 123.08, 123.18, 124.44, 126.49, 127.10, 127.72, 134.96,
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ysis calcd (%) for C27H28S: C 84.32, H 7.34; found: C 84.34, H 7.58.
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We acknowledge financial support from the National Natural Science
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Received: April 2, 2009
Published online: July 16, 2009
8282
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ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 8275 – 8282