Rubrenes: Planar and twisted

Article abstract of DOI:10.1002/chem.200800802

Surprisingly, despite its very high mobility in a single crystal, rubrene shows very low mobility in vacuum-sublimed or solution-processed organic thin-film transistors. We synthesized several rubrene analogues with electron-withdrawing and electron-donating substituents and found that most of the substituted rubrenes are not planar in the solid state. Moreover, we conclude (based on experimental and calculated data) that even parent rubrene is not planar in solution and in thin films. This discovery explains why high mobility is reported in rubrene single crystals, but rubreneshows very low field-effect mobility in thin films. The substituted rubrenes obtained in this work have significantly better solubility than parent rubrene and some even form films and not crystals after evaporation of the solvent. Thus, substituted rubrenes are promising materials for organic light-emitting diode (OLED) applications.

Full text of DOI:10.1002/chem.200800802

FULL PAPER
DOI: 10.1002/chem.200800802
Rubrenes: Planar and Twisted
Abhimanyu S. Paraskar,[a] A. Ravikumar Reddy,[a] Asit Patra,[a] Yair H. Wijsboom,[a]
Ori Gidron,[a] Linda J. W. Shimon,[b] Gregory Leitus,[b] and Michael Bendikov*[a]
Abstract: Surprisingly, despite its very
high mobility in a single crystal, ru-
brene shows very low mobility in
vacuum-sublimed or solution-processed
organic thin-film transistors. We syn-
thesized several rubrene analogues
with electron-withdrawing and elec-
tron-donating substituents and found
that most of the substituted rubrenes
are not planar in the solid state. More-
over, we conclude (based on experi-
mental and calculated data) that even
parent rubrene is not planar in solution
and in thin films. This discovery ex-
plains why high mobility is reported in
rubrene single crystals, but rubrene
shows very low field-effect mobility in
thin films. The substituted rubrenes ob-
tained in this work have significantly
better solubility than parent rubrene
and some even form films and not crys-
tals after evaporation of the solvent.
Thus, substituted rubrenes are promis-
ing materials for organic light-emitting
diode (OLED) applications.
Keywords: field-effect transistors ·
materials science · organic electron-
ic materials · rubrenes · substituent
effects
Introduction
40 cm2 VÀ1 sÀ1 was reported for 1.[5,6] Surprisingly, despite its
very high mobility in a single crystal, compound 1 shows
very low mobility in vacuum-sublimed or solution-processed
organic thin-film transistors.[7,8] To overcome this problem,
thin films based on crystalline mixtures of 5,12-diphenylan-
thracene and 1 were prepared, with these reported to yield
a hole mobility of up to 0.7 cm2 VÀ1 sÀ1,[8] and transistor
arrays based on single crystals of 1[9] were also fabricat-
ed.[10,11]
Apparently, good crystal packing is responsible for the ob-
served high mobility,[12] and indeed, examination of the crys-
tal structure of 1[13] shows that it has p–p stacking in one di-
rection and a herringbone motif in another direction. The
molecular structure of 1 consists of an absolutely planar tet-
racene backbone in the solid state,[13] although very similar
molecules were found to be twisted.[14,15] In the X-ray struc-
ture of rubrene, significant steric repulsion between the
phenyl rings shifts them above and below the tetracene
plane forming a dihedral angle of 258. The tetracene core is
still practically planar, thus steric repulsion between the
phenyl rings is balanced by the rigidity of the tetracene
core. Even large twisting of the acene backbone does not
change the HOMO–LUMO gap of acenes significantly.[16]
On the other hand, the energy required for twisting acenes
by 10–208 is only a few kcalmolÀ1, however, it increases to
40 kcalmolÀ1 for twisting more than 808.[16,17] Recently, the
Rubrene (5,6,11,12-tetraphenyltetracene; 1) has been known
since the beginning of the last century.[1] The electrolumines-
cence and chemiluminescence of 1 were well studied in the
1960s and it is a classic example of a material with excellent
electrochemiluminescent properties.[2] Nowadays, compound
1 is used as a dopant and a photosensitizer in organic light-
emitting diodes (OLEDs). Recently, compound 1 was inves-
tigated as an active semiconductor in organic field-effect
transistors (OFETs).[3] An exceptionally high field-effect
mobility of up to 15 cm2 VÀ1 sÀ1 was measured for single crys-
tals of 1,[4] and later, a contact-free intrinsic mobility of
[a] Dr. A. S. Paraskar,# Dr. A. R. Reddy,# Dr. A. Patra,# Y. H. Wijsboom,
O. Gidron, Dr. M. Bendikov
Department of Organic Chemistry
Weizmann Institute of Science
Rehovot 76100 (Israel)
Fax : (+972)8934-4142
E-mail: michael.bendikov@weizmann.ac.il
[b] Dr. L. J. W. Shimon, Dr. G. Leitus
Chemical Research Support Unit
Weizmann Institute of Science
Rehovot 76100 (Israel)
[#] These authors contributed equally to the work.
supramolecular self-assembly of 1 on an AuACTHNURTGNEU(GN 111) surface
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200800802.
was studied by STM and other methods.[18]
Chem. Eur. J. 2008, 14, 10639 – 10647
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
10639

Despite great interest in 1 in materials science, very few
substituted rubrenes are known[19] and the electronic and
structural properties of substituted rubrenes have never
been studied. The parent rubrene (1) is also only modestly
soluble in organic solvents. Our original motivation was to
design rubrene derivatives suitable for thin film transistors
by improving charge mobility and solubility by appropriate
chemical substitution. To this end, we synthesized several
rubrene analogues 2–7 (Scheme 1) with electron-withdraw-
Scheme 1. Structure of rubrenes 1–7 synthesized in this work.
ing and electron-donating substituents and found that most
of the substituted rubrenes are not planar in the solid state.
Moreover, we conclude that even 1 is not planar in solution
nor, most probably, in thin films. This discovery explains
why high mobility is reported in single crystals of 1, but 1
does not show any field-effect mobility in conventional thin
films. The substituted rubrenes obtained in this work have
significantly better solubility than 1 and some even form
films and not crystals after evaporation of the solvent. Thus,
substituted rubrenes are promising materials for OLED
Scheme 2. Synthesis of rubrenes 1–7. See text for a description of the
conversion of 11–13 into 14–20.
To prepare monomethoxydifluoro-substituted rubrene 6,
methoxy-substituted isobenzofuran 9 was prepared in two
steps from commercially available 3-phenylpthalide by addi-
tion of 4-methoxyphenyllithium and subsequent dehydration
with acetic anhydride. A cycloaddition reaction between 9
and 1,4-naphthaquinone gave quinone 12. Quinone 12 was
treated with 1-bromo-4-fluorobenzene in the presence of
nBuLi to provide 19, which was treated with HI in diethyl
ether to give the rubrene 6. Introduction of methoxy group
in the tetracene core of rubrene was achieved by treating
1,3-diphenyisobenzofuran (8) with 10 (R2 = OMe) to give
13. Quinone 13 was treated with phenyllithium to provide
20, which was subjected to aromatization to give rubrene 7
(Scheme 2). Rubrenes 1–7 were purified by sublimation
before characterization (in addition to column chromatogra-
phy as described in the Experimental Section).
ACHTUNGTRENNUNGapplications.
Results and Discussion
Synthesis of rubrenes 1–7: Rubrenes 1 and 2 were synthe-
sized according to a literature procedure.[19a] Rubrenes 3–7
were prepared from the corresponding 6,11-diaryltetracene-
5,12-diones (11–13) by using aryllithiums or Grignard re-
agents followed by HI-mediated aromatization of the corre-
sponding diols (16–20; Scheme 2).
Nucleophilic addition of 11 and 4-fluorophenylmagnesium
bromide, prepared from 1-bromo-4-fluorobenzene and
iPrMgCl·LiCl,[20] was carried out in THF under an inert at-
mosphere to give 16. HI-promoted aromatization of 16 pro-
vided rubrene 3 as a red solid. Quinone 11 was treated with
4-bromobenzaldehyde diethyl acetal in the presence of BuLi
in THF to obtain 17 followed by aromatization with HI in
diethyl ether at reflux to give rubrene 4 as a red solid. Simi-
larly, cyano-substituted rubrene 5 was prepared in a good
yield (Scheme 2).
X-ray crystal structures: Single crystals of rubrenes 2, 3, 4,
and 6 were grown from a mixture of dichloromethane and
ethanol. Unexpectedly, the X-ray structure of 2 shows that
the molecule adopts a strongly twisted conformation with
approximately D2 symmetry (Figures 1 and 2).[21] The end-
to-end twist of the tetracene core is 44.08. The twist is un-
evenly distributed with the four tetracene rings contributing
4.8, 15.6, 16.2, and 7.48 to the twist. X-ray structures of all
twisted rubrenes examined in this paper form centrosym-
10640
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 10639 – 10647

Rubrenes
FULL PAPER
metric crystals in which the
same number of right- and
left-twisted molecules (both
enantiomers) are present. On
the other hand, the X-ray
structure of difluoro-substitut-
ed rubrene 3 adopts a planar
conformation very similar to
that of 1. The fact that some
acene-type molecules adopt
different
conformations
(planar and twisted) depending
on the substituents has been
previously observed for substi-
tuted naphthalenes and ben-
zannulated anthracenes.[22] In-
terestingly, crystals of 1 and 3
are isostructural with almost
the same unit cell parameters
Figure 1. Crystal structures of 2 (top, showing the twist around the tetracene core) and 3 (bottom, showing the
planarity of the tetracene core).
Figure 2. Packing arrangements of a) 1, b) 3, c) 2, d) 4, and e) 6. Only tetracene cores are shown for clarity.
Chem. Eur. J. 2008, 14, 10639 – 10647
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10641

M. Bendikov et al.
except for the a lattice dimension, which is longer in 3. The
X-ray structures of 6 (with both difluoro and methoxy
groups), for which the end-to-end twist of the tetracene core
is 35.28, and of diformyl 4, for which the end-to-end twist of
the tetracene core is 37.78, are similar to that of 2. However,
crystal structures of 2, 3, 4, and 6 have different packing ar-
rangements (Figure 2). Twisting of the tetracene core results
in worse crystal packing relative to the rubrenes with a
planar tetracene core (as indicated by a lower packing
index[23]) and consequently leads to weaker intermolecular
interactions. As a result, twisted rubrenes are not good can-
didates for applications in OFET. Based on solid-state X-ray
structures of 1 and substituted rubrenes, we have shown that
the solid-state structures, and consequently, the properties
of rubrenes can be modified by substitution.
UV/Vis absorption and emission spectroscopy: The absorp-
tion and emission data for all substituted rubrenes are sum-
marized in Table 1. The UV/Vis absorption (lmax,abs =520–
529 nm) and emission (lmax,ems =553–568 nm) spectra are
very similar for all substituted rubrenes (Figure 3). In solu-
tion, the effect of the phenyl ring substituents on the elec-
tronic properties of rubrene is small. Substituted rubrenes
are strongly fluorescent, some with a quantum yield of close
to 100%. The quantum yields for rubrene analogues with
methoxy-substituted phenyl rings (2 and 6) are relatively
Table 1. Optical properties of substituted rubrenes 1–7.
lmax,abs
elmax
[mÀ1 cmÀ1
lmax,ems
F
HOMO–LUMO gap [eV]
[a]
[b]
[nm]
G
]
[nm]
1
2
3
4
5
6
7
528
528
526
529
528
526
520
5666
557
562
554
568
565
558
553
0.96
0.23
0.82
0.51
0.96
0.25
1
2.29
2.27
2.29
2.25
2.27
2.29
2.31
2.35
2.35
2.36
2.34
2.35
2.36
2.38
Computational results: The planar (C2h symmetry) structure
of rubrene lies 3.8 kcalmolÀ1 higher in energy than the mini-
mum structure (D2 symmetry) for which the tetracene core
is highly twisted by 428 (at the B3LYP/6-31G(d) level of
theory).[23,24,25] The planar D2h structure of rubrene is a
second-order saddle point lying 6.5 kcalmolÀ1 above the
minimum-energy structure. Interestingly, neither donor nor
acceptor substituents on the phenyl rings change the twisting
or planarization energies. Thus, in compounds 2 and 5, the
twisting angles are 43.2 and 42.38, respectively, with the
planar structures (Cs symmetry) lying 4.0 and 3.6 kcalmolÀ1
higher in energy, respectively. So, twisting of the central tet-
racene core in rubrene does not result from an electronic
effect of the phenyl ring substituents.
6125
7500
7102
8100
5392
[a] HOMO–LUMO gap calculated from the intersection point of the ab-
sorbance and florescence spectra. [b] HOMO–LUMO gap calculated
from the lmax of the absorbance spectra.
In solution, even 1 has a twisted structure based on a
comparison of the measured and calculated (GIAO-B3LYP/
6-311GACHTUNGTRENNUNG
(2df,p)//B3LYP/6-31G(d)) 13C NMR spectroscopy
values, with the latter corrected for the chemical shifts of
9,10-diphenyl anthracene.[23] The average difference between
the measured NMR chemical shifts of the carbon atoms of
the tetracene core and the shifts calculated for a twisted
conformation (by using the minimum-energy structure) is
0.46 ppm, whereas this difference is significantly larger
(1.85 ppm) if the measured shifts are compared with shifts
calculated for planar rubrene.[23] The difference in the calcu-
lated chemical shifts of the central quaternary carbon atoms
of the tetracene core (carbon atoms 5a and 11a, Scheme 1)
for the planar and twisted structure is especially pro-
nounced, while all other carbon atoms are less sensitive to
twisting. Calculated values for carbon atoms 5a and 11a in a
twisted conformation are just 0.3 ppm off the measured
values, whereas the difference between the experimental
and calculated values is 4.0 ppm for a planar conforma-
tion.[23,26] We can conclude that the rubrene molecules are
intrinsically nonplanar and steric repulsions are responsible
for twisting in rubrenes, whereas crystal-packing forces are
responsible for the planar tetracene core observed in the
crystals of 1 and 3. Based on these data, a nonplanar geome-
try is expected for rubrene and its derivatives in convention-
al thin films.
Figure 3. Absorption and emission spectra of rubrenes 1–7.
10642
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 10639 – 10647

Rubrenes
FULL PAPER
low, in the range of 23 to 25%. On the other hand, rubrene
with methoxy-substitution on the tetracene core (7) gives a
quantum yield of 100%. For rubrenes with substituents such
as F and CN (3 and 5), the quantum yield is 82–96%. The
extinction coefficients of all rubrene analogues are in the
range of 5400 to 8100mÀ1 cmÀ1, which is similar to the extinc-
tion coefficient of the parent rubrene.[27] The optical
HOMO–LUMO gaps calculated from the lmax of the absorb-
ance spectra are in the range of 2.34 to 2.38 eV, whereas
those calculated from the intersection point of the absorb-
ance and fluorescence spectra for all rubrenes are in the
range of 2.25 to 2.31 eV.
does not change under repeated cycling. As expected, the
presence of methoxy substituents on the tetracene core re-
1
sults in the E = ox of 7 (0.58 V) being smaller than that of 1
2
1
(0.74 V), whereas the E = red (À1.87 V) of 7 is larger than that
2
of 1 (À1.58 V). Methoxy substitution on the phenyl rings (2)
has little effect on the half-wave potentials (Table 2). The
differences between the oxidation and reduction potentials
(the electrochemically measured HOMO–LUMO gap) are
similar for all of the rubrenes and are in the range of 2.20 to
2.46 V (Table 2).
Interestingly, the electronic properties of rubrenes, such
as the HOMO–LUMO gap and the absorption and emission
spectra, are insensitive to twisting of the tetracene core.[16]
Calculated HOMO–LUMO gaps (B3LYP/6-31G(d)) are
2.51 and 2.60 eV, respectively, for C2h symmetry (planar tet-
racene core) and twisted D2 symmetry (lowest energy struc-
ture) geometries of 1. The calculated HOMO–LUMO gaps
of substituted rubrenes are similar to rubrene 1, for exam-
ple, for the twisted (C1 symmetry) and planar (Cs symmetry)
dimethoxy-substituted rubrene 2, HOMO–LUMO gaps are
2.50 and 2.60 eV, respectively.
Conclusion
We have shown that the planar structure of 1 found in
single crystals is due to crystal-packing forces and most sub-
stituted rubrenes are not planar in single crystals. Rubrenes
(including parent rubrene 1) also adopt a twisted structure
in solution. We have found that the balance between the
planar and nonplanar structures of 1 and its derivatives is
very delicate and may change as a result of different crystal-
packing forces.[28] We have clearly shown that the structure
of acene molecules differs between solution and the solid
state, and that their planarity is strongly affected by crystal-
packing forces. Significant modification of the solid-state
structure and properties of rubrenes has been achieved by
substitution of peripheral phenyl rings. The fact that 1 shows
a high field-effect mobility as single crystals, but has yet to
be made into a working thin-film device might be related to
the change in its planarity. The high efficiency of rubrene in
LED devices might also be associated with its twisted struc-
ture.[29] Due to low field-effect mobility in thin films, ru-
brenes may not be good candidates for use in thin film
FETs;[30] however, they are excellent candidates for applica-
tions in LEDs. We expect that the substituted rubrenes ob-
tained in this study will have a small tendency to crystallize
and will prove to be good candidates for applications in
LEDs.
Electrochemistry: The electrochemical properties of substi-
tuted rubrenes were investigated by cyclic voltammetry
(CV; see Table 2, Figure 4, and the Supporting Information).
Both the oxidation and reduction processes are reversible
for all rubrenes (expect for the reduction of 4), demonstrat-
ing excellent electrochemical stability, and the CV trace
Table 2. Electrochemical properties of substituted rubrenes.
1
1
1
1
2
E = ox [V]
E = red [V]
E = oxÀE = red [eV]
2
2
2
1
2
3
4
6
7
0.75
0.76
0.80
0.88
0.76
0.58
À1.56
À1.59
À1.62
À1.32
À1.62
À1.88
2.31
2.35
2.42
2.20
2.38
2.46
Experimental Section
General: THF and Et2O were distilled from sodium and benzophenone.
Hydroiodic acid was purchased from Aldrich as a 57% aqueous solution
stabilized with phosphorous acid. 1H and 13C NMR spectra were mea-
sured at 250 and 400 MHz, respectively, with a Brucker 250 or a Bruck-
er 400 spectrometer. 1H NMR chemical shifts are reported in parts per
million (ppm) relative to tetramethylsilane (0.0 ppm) or chloroform
(7.26 ppm). Data are reported as follows: chemical shift, multiplicity (s=
singlet, d=doublet, t=triplet, q=quartet, m=multiplet), coupling con-
stants (Hertz), and integration. In 13C NMR spectra sometimes some sig-
nals are missing due to poor signal to noise ratio because of poor solubili-
ty. Flash chromatography was performed by using silica gel 60 (230–400
mesh). Analytical thin-layer chromatography (TLC) was performed by
using 0.25 mm Merck precoated silica gel plates (60-F254). All air- and/
or moisture-sensitive reactions were conducted in flame- and/or oven-
dried glassware under a dry argon atmosphere with standard precautions
taken to exclude moisture. All mass spectrometer measurements were
performed with a Micromass platform LCZ 4000 instrument in ESI
mode. Elemental analysis was carried out with a FlashEA 1112 Thermo
Figure 4. CV traces of 1–3 on a platinum electrode versus Ag/AgCl in
1,2-dichloroethane/tetrabutylammonium perchlorate (0.1m).
Chem. Eur. J. 2008, 14, 10639 – 10647
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10643

M. Bendikov et al.
Finnigan CHN elemental analyzer. Crystals were grown by dissolving ru-
brenes in CH2Cl2 and then adding EtOH until it formed 20% of the mix-
ture. Crystals were grown in a special desiccator under a constant contin-
uous flow of nitrogen.
(m, 4H); 13C NMR (62.5 MHz, CDCl3): d=184.2, 144.0, 140.3, 135.6,
134.9, 133.6, 128.9, 128.8, 128.6, 128.4, 127.1, 126.9 ppm.
6-(4-Methoxyphenyl)-11-phenyltetracene-5,12-dione (12): Solid 9 (1.05 g,
3.5 mmol) was slowly added to 1,4-naphthaquinone (0.55 g, 3.5 mmol)
dissolved in CH2Cl2 (70 mL), and the solution was stirred at room tem-
perature for 12 h. The mixture was cooled to À788C followed by the
Absorption and emission spectroscopy: UV/Vis spectra were obtained on
a Jasco 570 spectrophotometer. Fluorescence spectra were recorded on a
Varian Cary Eclipse fluorimeter in a 1 cm quartz cuvette with CH2Cl2 as
the solvent. All measurements were made under a nitrogen atmosphere
and the spectrometer was purged with nitrogen for 5 min before each
measurement. The quantum efficiency was calculated by using rhodamine
B in EtOH (Aldrich) as the reference.
dropwise addition of BBr3 (4.0 mL of
a 1m solution in CH2Cl2,
4.0 mmol). After 1 h at À788C, the dark reaction mixture was slowly
warmed to room temperature, continuously stirred for another 6 h, then
heated at reflux for 4 h, and subsequently cooled to room temperature.
The solution was then poured into water and the aqueous portion was ex-
tracted with CH2Cl2. The combined organic extracts were washed with
brine, dried (MgSO4), and concentrated in vacuo to give a yellow solid,
which was purified by column chromatography (hexane/EtOAc 3:1) to
give 12 as bright yellow crystalline needles (1.33 g, 86%). 1H NMR
(CDCl3, 250 MHz): d=8.11–8.06 (m, 2H), 7.68–7.48 (m, 8H), 7.33–7.29
(m, 2H), 7.25–7.22 (m, 3H), 7.15–7.11 (m, 2H), 3.96 ppm (s, 3H).
CV measurements: CV measurements were performed with a three-elec-
trode cell in
a solution of 0.1m tetrabutylammonium perchlorate
(Bu4NClO4) in 1,2-dichloroethane at a scan rate of 100 mVsÀ1. All the
measurements were performed under an argon atmosphere and before
each measurement argon was purged through the solution for 20 min to
deoxygenate the system. A Pt wire was used as the counter electrode, Pt
was used as the working electrode, and Ag/AgCl was used as the refer-
ence electrode. Its potential was corrected to the saturated calomel elec-
trode (SCE) by measuring the ferrocene/ferrocenium couple in this
system (0.40 V versus SCE).
2,3-Dimethoxy-6,11-diphenyltetracene-5,12-dione (13): Compound
8
(0.361 g, 1.34 mmol) was added slowly as solid to 10 (0.291 g,
a
1.34 mmol) dissolved in CH2Cl2 (15 mL) and the solution was stirred at
room temperature for 12 h. Additional CH2Cl2 (3 mL) was added, and
the mixture was cooled to À788C followed by dropwise addition of BBr3
(1.5 mL of a 1m solution in CH2Cl2, 1.5 mmol). After 1 h at À788C, the
dark reaction mixture was slowly warmed to room temperature, continu-
ously stirred for another 6 h, then heated at reflux for 4 h, and subse-
quently cooled to room temperature. The solution was then poured into
water and the aqueous portion was extracted with CH2Cl2. The combined
organic extracts were washed with brine, dried (MgSO4), and concentrat-
ed in vacuo to give a yellow solid, which was purified by column chroma-
tography (CH2Cl2) to give 13 as bright yellow crystalline needles (0.470 g,
82%). 1H NMR (CDCl3, 250 MHz): d=7.62–7.45 (m, 12H), 7.32 (d, J=
2.5 Hz, 2H), 7.29 (d, J=2.5 Hz, 2H), 3.92 ppm (s, 6H); 13C NMR
(100 MHz, CDCl3): d=183.1, 153.7, 143.9, 140.7, 135.5, 133.0, 130.6,
129.8, 129.6, 128.7, 128.5, 128.3, 127.0, 108.1, 56.5 ppm; MS: m/z: 471
[M]+, 493 [M+Na]+, 510 [M+K]+, 964.30 [2M+Na]+.
DFT calculations: All calculations were performed by using DFT with
the B3LYP hybrid functional[31] and the 6-31G(d) basis set. The Gaussi-
an 03 program was used for all computations.[32] Frequency calculations
were performed at the same level for all stationary points to define them
as minima or saddle points. We note that, in contrast to many ab initio
methods (such as MP2, CISD, CCSD), the B3LYP level always yields
real frequencies for benzene regardless of the basis set used.[33] The
13C NMR chemical shieldings were calculated by using the gauge-includ-
ed atomic orbitals (GIAO) method[34] coupled with B3LYP and the 6-
311GACHTUNGTRENNUNG(2df,p) basis set using the B3LYP/6-31G(d) optimized geometries
(these calculations are denoted as GIAO-B3LYP/6-311GACTHNUTRGNE(NUG 2df,p)//B3LYP/
6-31G(d)). The 13C NMR chemical shifts were referenced to TMS, which
was calculated at the same level of theory (s=182.75 ppm).
1-(4-Methoxyphenyl)-3-phenylisobenzofuran (9):[35] A solution containing
1-bromo-4-methoxybenzene (4.9 g, 26.3 mmol) was dissolved in THF
(75 mL) and cooled to À788C, and nBuLi (16.5 mL of a 1.6m solution in
hexanes, 26.3 mmol) was added dropwise under an argon atmosphere
while maintaining the temperature at below À608C. The solution was
5,12-Dihydro-5,12-bis(4-methoxyphenyl)-6,11-diphenyltetracene-5,12-diol
(15):[19a] nBuLi (6 mL of a 1.6m solution in hexanes, 9.6 mmol) was added
dropwise to a stirred solution of 4-bromoanisole (1.25 mL, 10 mmol) in
THF (10 mL) at À788C. The mixture was allowed to stir at this tempera-
ture for 15 min before being transferred by cannula to a stirred solution
of quinone 11 (0.41 g, 1 mmol) in THF (10 mL) at À788C. The resulting
dark reaction mixture was then allowed to warm to room temperature
slowly over a 12 h period and then quenched by pouring into a saturated
aqueous solution of NH4Cl. The aqueous portion was subsequently ex-
tracted with Et2O, and the combined organic extracts were dried
(MgSO4) and then concentrated in vacuo to give 15 (0.5 g, 80%).
1H NMR (250 MHz, CDCl3): d=7.71 (dd, J=3.0, 5.3 Hz, 2H), 7.56 (q,
J=3.2 Hz, 2H), 7.48 (d, J=8.7 Hz, 4H), 7.32–7.28 (m, 2H), 7.25–7.19 (m,
10H), 6.97–6.94 (m, 2H), 6.58 (d, J=8.7 Hz, 4H), 3.74 ppm (s, 6H);
13C NMR (62.5 MHz, CDCl3): d=158.8, 150.2, 144.8, 137.0, 132.1, 131.9,
131.1, 130.0, 129.9, 127.8, 127.4, 127.0, 126.3, 126.0, 125.8, 122.5, 113.0,
55.2 ppm.
stirred for 2 h followed by the dropwise addition of a solution of ACTHNUTRGNEU3GN -phe-
nylphthalide (5.0 g, 23.7 mmol) in THF (35 mL) over a 45 min period.
The deep red solution was stirred for another 15 min at À788C. After
that, Ac2O (2.5 mL, 26.5 mmol) was added and the solution was warmed
to room temperature and then heating at reflux for 10 min. Water was
added to the bright yellow solution and the organic layer was separated
and dried over MgSO4 and concentrated. Chromatographic separation
(hexane/EtOAc 9:1) gave 9 (1.05 g, 16%). 1H NMR (250 MHz, CDCl3):
d=7.95–7.77 (m, 6H), 7.45 (t, J=8.0 Hz, 2H), 7.31–7.27 (m, 1H), 7.07–
6.97 (m, 4H), 3.89 ppm (s, 3H); 13C NMR (62.5 MHz, CDCl3): d=159.7,
129.9, 127.5, 127.3, 126.1, 125.53, 125.50, 121.2, 121.0, 115.5, 56.4 ppm;
MS: m/z: 300 [M]+, 301.43 [M+1]+, 323.40 [M+Na]+, 339.38 [M+K]+,
307.42 [M+Li]+, 623.67 [2M+Na]+, 607.70 [2M+Li]+.
6,7-Dimethoxynaphthalene-1,4-dione (10): Compound 10 was prepared
5,12-Bis(4-fluorophenyl)-5,12-dihydro-6,11-diphenyltetracene-5,12-diol
(16): A dry argon flushed 50 mL flask equipped with magnetic stirrer and
a septum was charged with iPrMgCl·LiCl[20] (6 mL of a 1m solution in
THF, 6 mmol). The reaction mixture was cooled to À158C and 4-fluoro-
bromobenzene (1.05 g, 6 mmol) was added in one portion. The tempera-
ture of the reaction mixture was raised to À108C and the Br/Mg ex-
change was completed after 15 min. In another flask, quinone 11 (0.410 g,
1 mmol) was dissolved in THF (10 mL) and the reaction mixture was
cooled at À788C. a solution of Grignard reagent was added dropwise to
this solution through a cannula at À788C. The resulting dark reaction
mixture was allowed to stir at this temperature for 3 h and then allowed
to warm to room temperature slowly over a 12 h period. It was then
quenched by pouring the mixture into a saturated aqueous solution of
NH4Cl. The aqueous portion was subsequently extracted with Et2O, and
the combined organic extracts were dried (MgSO4) and then concentrat-
ed in vacuo. The resulting white solid was washed to obtain pure diol 16
according to the procedure described in the literature.[36]
6,11-Diphenyltetracene-5,12-dione (11):[19a] Compound 8 (5 g, 18.5 mmol)
was added slowly as a solid to 10 (3 g, 18.9 mmol) in CH2Cl2 (50 mL) and
the solution was stirred at room temperature for 12 h. Additional CH2Cl2
(100 mL) was added and the mixture was cooled to À788C followed by
dropwise addition of BBr3 (30 mL of a 1m solution in CH2Cl2, 30 mmol).
After 0.5 h at À788C, the dark reaction mixture was warmed to room
temperature, then heated at reflux for 4 h, and subsequently cooled to
room temperature. The solution was then poured into water and the
aqueous portion was extracted with CH2Cl2. The combined organic ex-
tracts were washed with brine, dried (MgSO4), and concentrated in vacuo
to give a yellow solid, which was purified by column chromatography
(CH2Cl2) to give 11 as a bright yellow crystalline solid (7.0 g, 92%).
1H NMR (CDCl3, 250 MHz): d=8.06 (dd, J=3.5, 6.0 Hz, 2H), 7.64 (dd,
J=3.5, 6.0 Hz, 2H), 7.59–7.54 (m, 8H), 7.51–7.47 (m, 2H), 7.33–7.30 ppm
10644
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 10639 – 10647

Rubrenes
FULL PAPER
1
(0.250 g, 42%). H NMR (250 MHz, CDCl3): d=7.25–7.17 (m, 8H), 7.01–
6.94 (m, 12H), 6.77–6.70 (m, 4H), 6.19 (d, J=7.7 Hz, 2H), 3.91 ppm (s,
2H); 13C NMR (62.5 MHz, CDCl3): d=162.9, 159.0, 145.3, 145.3, 141.0,
139.6, 137.3, 136.4, 134.4, 131.6, 131.2, 128.8, 127.6, 127.3, 127.1, 127.1,
126.9, 126.6, 126.3, 126.2, 114.3, 113.4, 74.8 ppm.
rated, immediately dried over MgSO4, and concentrated. Immediate flash
chromatography (CH2Cl2-hexane) followed by sublimation gave rubrenes
(2–7) as red solids. The yields were determined after flash
AHCTUNGTREGcNNUN hromatography.
5,12-Bis(4-methoxyphenyl)-6,11-diphenyltetracene (2):[19a] Red solid;
yield=62%; 1H NMR (250 MHz, CDCl3): d=7.76 (dd, J=3.2, 7.0 Hz,
2H), 7.64 (dd, J=3.2, 7.0 Hz, 2H), 7.04–6.88 (m, 14H), 6.79 (dd, J=1.8,
8.5 Hz, 4H), 6.53 (dd, J=2.5, 7.0 Hz, 4H), 3.41 ppm (s, 6H); MS: 592
[M]+, 590.99 [MÀ1]+, 615.13 [M+Na]+, 631.27 [M+K]+, 1223.39
[2M+K]+; elemental analysis calcd (%) for C44H32O2: C 89.16, H 5.44;
found: C 89.02, H 4.34.
5,12-Bis(4-(diethoxymethyl)phenyl)-5,12-dihydro-6,11-diphenyltetracene-
5,12-diol (17): Dry THF (10 mL) and nBuLi (3 mL of a 1.6m solution in
hexane, 4.8 mmol) were added sequentially to a 50 mL two-necked flask
equipped with a stirring bar under N2 at À788C. 4-Bromobenzaldehyde
diethyl acetal (0.976 mL, 4.8 mmol) was then added to this solution and
the mixture was stirred at À788C for 3 h. The mixture was slowly
warmed to room temperature and then cooled again to À788C before
the addition of quinone 11 (0.205 g, 0.5 mmol) dissolved in THF (10 mL).
The resulting dark reaction mixture was allowed to stir at this tempera-
ture for 1 h and then allowed to warm to room temperature slowly over
a 12 h period. It was then quenched by pouring the mixture into a satu-
rated aqueous solution of NH4Cl. The aqueous portion was subsequently
extracted with Et2O, and the combined organic extracts were dried
(MgSO4) and then concentrated in vacuo. The resulting white solid was
purified by column chromatography with CH2Cl2/EtOAc (10:1) as the
eluent to give diol 17 (0.37 g, 98%). 1H NMR (250 MHz, CDCl3): d=
7.60–7.51 (m, 2H), 7.42–7.38 (m, 5H), 7.30–7.28 (m, 2H), 7.17–6.89 (m,
16H), 6.12 (d, J=7.7, Hz, 1H), 5.43 (s, 2H), 3.80 (s, 1H), 3.66–3.50 (m,
8H), 2.04 (s, 1H), 1.29–1.18 (m, 12H); MS: m/z: [M]+ 770, [M+Na]+
794.11; [M+K]+ 810.05.
5,12-Bis(4-fluorophenyl)-6,11-diphenyltetracene (3): Red solid; yield=
96%; 1H NMR (250 MHz, CDCl3): d=7.77–7.71 (m, 2H), 7.59–7.55 (m,
2H), 7.39–7.32 (m, 4H), 7.22 (d, J=2.7 Hz, 2H), 7.19–6.90 (m, 8H),
6.89–6.72 ppm (m, 8H); 13C NMR (62.5 MHz, CDCl3): d=163.2, 159.3,
141.8, 138.4, 133.4, 133.3, 131.9, 130.6, 130.4, 127.3, 126.4, 126.1, 126.0,
124.7, 124.5, 122.4, 114.2, 114.0 ppm; elemental analysis calcd (%) for
C42H26F2: C 88.71, H 4.61; found: C 88.68, H 4.55.
5,12-Bis(4-carboxyaldhydephenyl)-6,11-diphenyltetracene (4): Red solid;
yield=79%;. 1H NMR (250 MHz, CDCl3): d=10.02 (s, 2H), 7.68–7.54
(m, 6H), 7.40–7.33 (m, 3H), 7.30–7.28 (m, 2H), 7.24–7.01 (m, 12H),
6.88–6.83 ppm (m, 3H); 13C NMR (62.5 MHz, CDCl3): d=192.2, 192.0,
148.6, 141.2, 136.8, 136.1, 134.1, 132.7, 132.5, 130.6, 129.6, 128.6, 127.4,
126.5, 126.4, 126.0, 125.5 ppm; MS: m/z: 589 [M]+, 611.83 [M+Na]+,
627.91 [M+K]+; elemental analysis calcd (%) for C44H28O2: C 89.77, H
4.79; found: C 89.58, H 4.65.
5,12-Bis(4-fluorophenyl)-5,12-dihydro-6-(4-methoxyphenyl)-11-phenylte-
5,12-Bis(4-cyanophenyl)-6,11-diphenyltetracene (5): Red solid; 1H NMR
(250 MHz, CDCl3): d=7.41–7.35 (m, 6H), 7.24–7.10 (m, 12H), 7.03–6.97
(m, 4H), 6.90–6.86 ppm (m, 4H); 13C NMR (62.5 MHz, CDCl3): d=
146.7, 141.2, 136.6, 135.5, 132.6, 132.4, 130.9, 130.7, 129.5, 128.4, 128.2,
127.6, 127.2, 126.6, 126.4, 125.8, 125.7, 109.6 ppm.
tracene-5,12-diol (19): nBuLi (4.05 mL of a 1.6m solution in hexane,
6.48 mmol) was added dropwise to
a solution of 1-bromo-4-fluoro-
ACHTUNGTRENNUNG
for 1 h. A solution of quinone 12 (0.300 g, 0.683 mmol) dissolved in THF
(15 mL) was then added to this solution. The resulting dark reaction mix-
ture was stirred at this temperature for 1 h and then allowed to warm to
room temperature over a 12 h period with continuous stirring. The reac-
tion mixture was quenched by pouring into a saturated aqueous solution
of NH4Cl. The aqueous portion was subsequently extracted with Et2O,
and the combined organic extracts were dried (MgSO4) and then concen-
trated in vacuo. Column chromatography with hexane/EtOAc (3:1) as
the eluent gave 19 (0.140 g, 33%). 1H NMR (250 MHz, CDCl3): d=7.70
(dd, J=3.0, 8.5 Hz, 2H), 7.59–7.53 (m, 4H), 7.25–7.21 (m, 4H), 7.04–6.94
(m, 6H), 6.79–6.70 (m, 6H), 6.01 (d, J=8.2 Hz, 1H), 5.91 (dd, J=2.0,
6.5 Hz, 1H), 3.82 (s, 3H), 3.80 (s, 1H), 3.64 ppm (s, 1H); MS: m/z: 632
[M]+, 631.53 [MÀ1]+, 667.43 [MÀ1+Cl]+, 745.23 [MÀ1+TFA]+, 655.61
[M+Na]+.
5,12-Bis(4-fluorophenyl)-6-(4-methoxyphenyl)-11-phenyltetracene
(6):
1
Red crystals; yield=43%; H NMR (250 MHz, CDCl3): d=7.46–7.42 (m,
1H), 7.38–7.31 (m, 3H), 7.18–7.09 (m, 7H), 6.89–6.74 (m, 12H), 6.72–
6.66 (m, 2H), 3.87 ppm (s, 3H); 13C NMR (62.5 MHz, CDCl3): d=158.0,
141.8, 134.2, 133.5, 133.4, 133.3, 133.2, 133.0, 130.3, 127.3, 126.6, 126.5,
126.3, 126.0, 125.0, 114.3, 114.2, 114.0, 113.9, 133.4, 112.9, 55.4 ppm; MS:
m/z: 599 [M]+, 599.65 [M+1]+, 621.62 [M+Na]+, 637.66 [M+K]+,
1220.26 [2M+Na]+.
2,3-Dimethoxy-5,6,11,12-tetraphenyltetracene (7): Red solid; yield=94%;
1H NMR (250 MHz, CDCl3): d=7.37–7.31 (m, 2H), 7.14–7.01 (m, 14H),
6.89–6.84 (m, 8H), 6.52 (s, 2H), 3.59 ppm (s, 6H); 13C NMR (100 MHz,
CDCl3): d=149.6, 142.2, 141.9, 136.3, 134.5, 132.2, 131.8, 129.7, 128.6,
127.8, 127.3, 127.1, 126.5, 125.6, 124.4, 102.8, 55.3 ppm; MS: m/z: 592
[M]+, 593.11 [M+1]+.
5,12-Dihydro-2,3-dimethoxy-5,6,11,12-tetraphenyltetracene-5,12-diol (20):
Dimethoxy quinone 13 (0.470 g, 1.1 mmol) was dissolved in THF (15 mL)
and the reaction mixture was cooled to À788C. PhLi (3 mL of a 1.8m so-
lution in dibutyl ether, 5.4 mmol) was added dropwise to this solution.
The resulting dark reaction mixture was stirred at this temperature for
1 h and then allowed to warm to room temperature slowly over a 12 h
period. The reaction was then quenched by pouring the mixture into an
aqueous solution of saturated NH4Cl. The aqueous portion was subse-
quently extracted with Et2O, and the combined organic extracts were
dried (MgSO4) and then concentrated in vacuo. The resulting pale-yellow
solid was purified by column chromatography with CH2Cl2/EtOAc (20:1)
as the eluent to give two isomers of diol 20 (0.218 g, 35%). Isomer A:
1H NMR (250 MHz, CDCl3): d=7.65–7.54 (m, 2H), 7.45–7.33 (m, 2H),
7.19–7.02 (m, 14H), 6.97–6.93 (m, 2H), 6.89–6.83 (m, 2H), 6.55 (s, 2H),
6.19–6.15 (m, 2H), 3.90 (s, 2H), 3.58 ppm (s, 6H). Isomer B: 1H NMR
(250 MHz, CDCl3): d=7.34–7.32 (m, 4H), 7.18–7.03 (m, 14H), 6.96–6.92
(m, 2H), 6.88–6.82 (m, 2H), 6.52 (s, 2H), 6.17–6.14 (m, 2H), 3.61 (s, 2H),
3.57 ppm (s, 6H); MS: m/z: 627 [M], 650.10 [M+Na]+, 1276.96
[2M+Na]+, 625.86 [MÀ1], 1252.80 [2MÀ1].
Acknowledgements
The authors are grateful to Prof. Dmitrii F. Perepichka (McGill Universi-
ty, Canada) and to Prof. Robert Pascal, Jr. (Princeton University) for
useful suggestions, to Alex Banaru for helpful discussions and help with
X-ray structures, to Elijah Shirman for help with florescence measure-
ments and to Natalia Zamoshchik for help with XRD measurements. We
thank the Israel Science 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 the holder of a DuPont Young Professor Award.
General procedure for the synthesis of compounds 2–7: The diol
(0.50 mmol) was dissolved in Et2O (30 mL) in a 100 mL flask under N2
with heating. 57% aqueous HI (6 mL) was then added to this solution at
reflux. The reaction was allowed to proceed for 10 min and cooled to
room temperature and then aqueous saturated sodium metabisulfite
(20 mL) was added to form a red ether layer. The organic layer was sepa-
[1] M. Charles, D. Charles, P. M. Dean, Compt. Rend. 1926, 182, 1440–
1443.
[2] a) E. A. Chandross, J. W. Longworth, R. E. Visco, J. Am. Chem. Soc.
1965, 87, 3259–3260; b) D. M. Hercules, Acc. Chem. Res. 1969, 2,
301–307; c) M. M. Richter, Chem. Rev. 2004, 104, 3003–3036, and
references therein.
Chem. Eur. J. 2008, 14, 10639 – 10647
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10645

M. Bendikov et al.
[3] a) H. E. Katz, Z. Bao, J. Phys. Chem. B 2000, 104, 671–678; b) C. D.
Dimitrakopoulos, P. R. L. Malenfant, Adv. Mater. 2002, 14, 99; c) M.
Bendikov, F. Wudl, D. F. Perepichka, Chem. Rev. 2004, 104, 4891;
d) J. E. Anthony, Angew. Chem. 2008, 120, 460–492; Angew. Chem.
Int. Ed. 2008, 47, 452–483.
[4] V. C. Sundar, J. Zaumseil, V. Podzorov, E. Menard, R. L. Willett, T.
Someya, M. E. Gershenson, J. A. Rogers, Science 2004, 303, 1644.
[5] J. Takeya, M. Yamagishi, Y. Tominari, R. Hirahara, Y. Nakazawa, T.
Nishikawa, T. Kawase, T. Shimoda, S. Ogawa, Appl. Phys. Lett.
2007, 90, 102120.
imaginary frequency and can be formally characterized as a saddle
point. However, more careful examination has shown that this
saddle point connects two minima of C2 symmetry, which are geo-
metrically and energetically undistinguishable from C2h structure.
So, most probably, the imaginary frequency in the C2h structure is a
computational artefact, and such a structure might be a true mini-
mum on the potential-energy surface.
[26] The 13C NMR chemical shift of benzene calculated at the GIAO-
B3LYP/6-311GACTHNUGTRNEUNG(2df,p)//B3LYP/6-31G(d) level is d=133.25 ppm,
whereas the experimental value is d=128.0 ppm. Therefore, such a
calculation systematically overestimates the 13C NMR chemical
shifts of aromatic carbon atoms by 5.25 ppm, since the B3LYP
method apparently does not include sufficient electron correlation
to correctly describe the chemical shifts of aromatic carbon atoms
(see: a) J. Gauss, Chem. Phys. Lett. 1992, 191, 614; b) J. Gauss,
Chem. Phys. 1993, 99, 3629; c) J. J. Buffy, R. West, M. Bendikov, Y.
Apeloig, J. Am. Chem. Soc. 2001, 123, 978–979). If a correction of
only 5.25 ppm is applied then the agreement between the experi-
mental values of the twisted conformation is still significantly better
than for planar rubrene. The average difference between the mea-
sured NMR chemical shifts of the carbon atoms of the tetracene
core and the shifts calculated for a twisted conformation is 1.0 ppm,
and for planar rubrene it is 2.8 ppm. Calculated values for carbon
atoms 5a and 11a in a twisted conformation are 1.2 ppm off the
measured values, whereas the difference between the experimental
and calculated values is 3.1 ppm for a planar conformation.
[6] The stability of 1 in FET devices has also been studied recently, see:
R. Zeis, C. Besnard, T. Siegrist, C. Schlockermann, X. Chi, C. Kloc,
Chem. Mater. 2006, 18, 244–248.
[7] F. Cicoira, J. A. Miwa, D. F. Perepichka, F. Rosei, J. Phys. Chem. A
2007, 111, 12674–12678.
[8] N. Stingelin-Stutzmann, E. Smits, H. Wondergem, C. Tanase, P.
Blom, P. Smith, D. de Leeuw, Nat. Mater. 2005, 4, 601.
[9] a) A. L. Briseno, S. C. B. Mannsfeld, M. M. Ling, S. Liu, R. J. Tseng,
C. Reese, M. E. Roberts, Y. Yang, F. Wudl, Z. Bao, Nature 2006,
444, 913–917; b) A. L. Briseno, R. J. Tseng, M. M. Ling, E. H. L
Falcao, Y. Yang, F. Wudl, Z. Bao, Adv. Mater. 2006, 18, 2320–2324.
[10] Ambipolar behavior for rubrene-based FETs was also demonstrat-
ed, see: T. Takahashi, T. Takenobu, J. Takeya, Y. Iwasa, Appl. Phys.
Lett. 2006, 88, 33505.
[11] See also C. H. Hsu, J. Deng, C. R. Staddon, P. Beton, Appl. Phys.
Lett. 2007, 91, 193505.
[12] D. A. da Silva Filho, E.-G. Kim, J.-L. Brꢁdas, Adv. Mater. 2005, 17,
1072.
[13] I. Bulgarovskaya, V. Vozzhennikov, S. Aleksandrov, V. Belꢂskii,
Latv. PSR Zinat. Akad. Vestis Kim. Ser. 1983, 4, 53. The same unit
cell parameters were reported by a) D. E. Henn, W. G. Williams,
D. J. Gibbons, J. Appl. Crystallogr. 1971, 4, 256; b) O. D. Jurchescu,
A. Meetsma, T. T. M. Palstra, Acta Crystallogr. Sect. B 2006, 62,
330–334.
[14] R. A. Pascal, Jr., Chem. Rev. 2006, 106, 4809–4819.
[15] On the other hand, the rubrene cation radical has a 278 twist (M.
Kameya, T. Naito, T. Inabe, Bull. Chem. Soc. Jpn. 2000, 73, 61) and
complexation of the four phenyl groups of rubrene with very bulky
RuCp* (Cp=cyclopentadienyl) groups results in a twist of 698 (P. J.
Fagan, M. D. Ward, J. V. Caspar, J. C. Calabrese, P. J. Krusic, J. Am.
Chem. Soc. 1988, 110, 1981–1983).
[27] a) A. P. Darmanyan, Izv. Akad. Nauk SSSR 1982, 8, 1888–1891;
b) J. W. Arbogast, C. S. Foote, J. Am. Chem. Soc. 1991, 113, 8886–
8889.
[28] We note that, in agreement with our results, in recent STM investi-
gations of the self-assembly of 1 on Au
ACHTUNGERTN(NUNG 111), a twisted tetracene
core was assumed for 1.[18]
[29] Usually, good solid-state packing is required for applications in
FETs, whereas weak intramolecular interactions are required for ap-
plications in LEDs. Rubrene (1) was used in LEDs, which suggests
weak intermolecular interactions, on the other hand, crystals of 1
show high field-effect mobility (and consequently strong intermolec-
ular interactions). This apparent discrepancy also supports the con-
clusion that 1 adopts a different structure as a single crystal than as
a thin film.
[30] Recently, high field-effect mobility in rubrene films was obtained by
crystallization of amorphous islands of the rubrene film, see ref.
[11]; this supports our conclusion that crystallization changes the
structure of 1 significantly.
[31] a) R. G. Parr, W. Yang, Density Functional Theory of Atoms and
Molecules, Oxford University Press, New York, 1989; b) W. Koch,
M. C. Holthausen, A Chemistꢀs Guide to Density Functional Theory,
Wiley-VCH, Weinheim, 2000; c) C. Lee, W. Yang, R. G. Parr, Phys.
Rev. B 1988, 37, 785–789; d) A. D. Becke, J. Chem. Phys. 1993, 98,
5648–5652.
[32] Gaussian 03 (Revision C.02), M. J. Frisch, G. W. Trucks, H. B. Schle-
gel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgomer-
y, Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyen-
gar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N.
Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K.
Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian,
J. B. Cross, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann,
O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, P. Y.
Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg,
V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O.
Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Fores-
man, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski,
B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L.
Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Na-
nayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen,
M. W. Wong, C. Gonzalez, J. A. Pople, Gaussian, Inc., Pittsburgh,
PA, 2003.
[16] J. E. Norton, K. N. Houk, J. Am. Chem. Soc. 2005, 127, 4162–4163.
[17] This is contrast to other conjugated systems, such as oligothio-
phenes, in which very little twisting energy is required to achieve a
significant change in HOMO–LUMO gap, see: S. S. Zade, M. Bendi-
kov, Chem. Eur. J. 2007, 13, 3688–3700.
´
[18] a) M.-C. Blꢃm, E. Cavar, M. Pivetta, F. Patthey, F.-D. Schneider,
Angew. Chem. 2005, 117, 5468–5471; Angew. Chem. Int. Ed. 2005,
44, 5334–5337; b) D. Kꢄfer, G. Witte, Phys. Chem. Chem. Phys.
2005, 7, 2850–2853; c) M.-C. Blꢃm, M. Pivetta, F. Patthey, F.-D.
Schneider, Phys. Rev. B 2006, 73, 195409; d) D. Kꢄfer, L. Ruppel, G.
Witte, Ch. Wçll, Phys. Rev. Lett. 2005, 95, 166602.
[19] a) J. A. Dodge, J. D. Bain, A. R. Chamberlin, J. Org. Chem. 1990,
55, 4190, and references therein; b) Y. S. Wu, T. H. Liu, H. H. Chen,
C. H. Chen, Thin Solid Films 2006, 496, 626.
[20] A. Krasovskiy, P. Knochel, Angew. Chem. 2004, 116, 3396–3399;
Angew. Chem. Int. Ed. 2004, 43, 3333–3336.
[21] While this work was completed and prepared for publication, an X-
ray structure of a different twisted rubrene (5,11-bis-4-tert-butyl-
phenyl-6,12-diphenylnaphthacene) was reported, see: S. Haas, A. F.
Stassen, G. Schuck, K. P. Pernstich, D. J. Gundlach, B. Batlogg, U.
Berens, H.-J. Kirner. Phys. Rev. B 2007, 76, 115203.
[22] a) R. P. LꢂEsperance, D. V. Engen, R. Dayal, R. A. Pascal, Jr., J.
Org. Chem. 1991, 56, 688–694; b) D. D. MacNicol, P. R. Mallinson,
C. D. Robertson, J. Chem. Soc. Chem. Commun. 1985, 1649–1651.
[23] See the Supporting Information for details.
[24] A similar structure was also reported in references [7] and [18b].
[25] Interestingly, by using the default conditions in Gaussian 03, the C2h
-
symmetry structure of 1 (as well as of all other rubrenes) has one
10646
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 10639 – 10647

Rubrenes
FULL PAPER
[33] D. Moran, A. C. Simmonett, F. E. Leach III, W. D. Allen, P. v. R.
Schleyer, H. F. Schaefer III, J. Am. Chem. Soc. 2006, 128, 9342.
[34] a) R. Ditchfield, Mol. Phys. 1974, 27, 789; b) K. Wolinski, J. F.
Hinton, P. Pulay, J. Am. Chem. Soc. 1990, 112, 8251; c) J. R. Cheese-
man, G. W. Trucks, T. A. Keith, M. J. Frisch, J. Chem. Phys. 1996,
104, 5497.
[35] Y. Kuninobu, Y. Nishina, C. Nakagawa, K. Takai, J. Am. Chem. Soc.
2006, 128, 12376–12377.
[36] J. Reichwagen, H. Hopf, A. D. Guerzo, C. Belin, H. Bouass-Laurent,
J.-P. Desvenergne, Org. Lett. 2005, 7, 971.
Received: April 27, 2008
Published online: October 17, 2008
Chem. Eur. J. 2008, 14, 10639 – 10647
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10647