Crystal structures of anti- and syn-9,10-di(1′-naphthyl)anthracene and isomerization in solid state

Article abstract of DOI:10.1016/j.tet.2010.02.072

9,10-Di-(1′-naphthyl)anthracene is often used as electroluminescence materials in organic light-emitting diodes. Because of the hindered rotation about the σ-bond between naphthyl and anthracene chromophore, two possible stereoisomers can be isolated. HPLC, ¹H NMR, and ¹³C NMR spectra gave two different sets of peaks and the X-ray single crystal analysis confirmed the structures of the two isomers, anti and syn. syn was more soluble than anti in THF as well as toluene and the thermal properties of the two were quite different. Differential scanning calorimetry study and HPLC analysis showed that the isomerization between anti and syn in the solid state took place at >370 °C.

Full text of DOI:10.1016/j.tet.2010.02.072

Tetrahedron 66 (2010) 3360–3364
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Crystal structures of anti- and syn-9,10-di(10-naphthyl)anthracene and
isomerization in solid state
Cho Hee Lee a, Mi Jong Kim a, Sang-Pil Han a, Yoo-Soon Lee a, Sung Kwon Kang a, Jae Hee Song b,
,
Jong Tae Je c, Hyoung-Yun Oh d, Yeong-Joon Kim a
*
a Department of Chemistry, Chungnam National University, Daejeon 305-764, Republic of Korea
b Department of Chemistry, Sunchon National University, Sunchon 540-747, Republic of Korea
c SFC Co., Ltd, Ochang TechnoVillage 641-5, Gak-ri, Cheongwon, Chungbuk 363-883, Republic of Korea
d LG Display R&D Center, 1007 Deongeun-ri, Wollong-myeon, Paju-si Gyeonggi-do 413-811, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
9,10-Di-(10-naphthyl)anthracene is often used as electroluminescence materials in organic light-emitting
Article history:
Received 13 November 2009
Received in revised form
21 February 2010
Accepted 22 February 2010
Available online 1 March 2010
diodes. Because of the hindered rotation about the
s-bond between naphthyl and anthracene chro-
mophore, two possible stereoisomers can be isolated. HPLC, 1H NMR, and 13C NMR spectra gave two
different sets of peaks and the X-ray single crystal analysis confirmed the structures of the two isomers,
anti and syn. syn was more soluble than anti in THF as well as toluene and the thermal properties of the
two were quite different. Differential scanning calorimetry study and HPLC analysis showed that the
isomerization between anti and syn in the solid state took place at >370 ꢀC.
Keywords:
Ó 2010 Elsevier Ltd. All rights reserved.
Isomerization
Aryl–aryl rotation
X-ray structure
Anthracene
1. Introduction
9,10-Diarylanthracenes, when the substituent groups are
bulky, basically have two stereoisomers due to the hindered
Conjugated aromatic compounds have attracted great interest
from not only scientific but also industrial community because of
their potential applications in the field of organic electronics such
as organic thin film transistors (OTFTs) and organic light-emitting
diodes (OLEDs). Anthracene derivatives have been the important
ones among many of the leading materials due to their fluorescence
characteristics from the anthracene chromophore and many
studies have found applications as important class of highly
efficient, stable, blue-light emitting materials as well as new
class of host luminescent materials.1–12 However, there is a sig-
nificant electronic interaction between the chromophores in
solid phase, which results in an undesired broadening of the
emission spectrum and low fluorescence yield. To overcome this
weakness, bulky substituents at 9 and 10 positions of anthra-
cene are typically employed and the several compounds have
been developed. 9,10-Di(10-naphthyl)anthracene is a good ex-
ample, where the rigid naphthyl substituents effectively isolate
the anthracene chromophore, generating blue EL.
rotation about the s-bond between aryl and anthracene moiety.
Several calculations on these types of molecules have been
reported and the barrier of rotation is known to be 35–40 kcal/mol.
In fact, the rotational energy barrier for 1,90-naphthylanthracene
is calculated as 38.08 kcal/mol using B3LYP/6-31G
*
//HF/3-21G
method.13
If the physical properties of the two stereoisomers are diff-
erent, it is very important to study on the isolation and charac-
terization of the isomers. In fact, Bard group published a paper
many years ago dealing with the electrochemistry of 9,10-di
(10-naphthyl)anthracene and predicted the possibility of different
electrical properties of the two isomers.14 However, no such
study has been done yet. Very recently, Linker et al. have
published an elegant paper about the synthesis and character-
ization of stereoisomers of 9,10-diarylanthracenes with ortho-
substituted phenyl.15 We believe that 9,10-di(10-naphthyl)
anthracene is a more interesting molecule in industry because of
the actual use of this molecule as OLED material in the current
market. In terms of the process and working condition on OLED,
the thermal stability and isomerization temperature of the two
stereoisomers are also key factors to be considered. Here we
report the crystalline structures from X-ray diffraction results of
* Corresponding author. Tel.: þ82 42 821 5476; fax: þ82 42 821 8896; e-mail
address: y2kim@cnu.ac.kr.
0040-4020/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.02.072

C.H. Lee et al. / Tetrahedron 66 (2010) 3360–3364
3361
the two isomers, anti and syn of 9,10-di(10-naphthyl)anthracene
(Fig. 1) and the solid state isomerization between the two at high
temperature.
Table 1
Crystal data for two structures of 9,10-di(10-naphthyl)anthracene
anti
syn
Chemical formula
Formula weight
Temperature [K]
Wavelength [Å]
Crystal system, space group
a [Å]
C
34H22
C34H22
430.52
295(2)
0.71073
Monoclinic, C2/c
18.1453(10)
16.5882(10)
14.8692(7)
96.094(3)
430.52
174(2)
0.71073 Å
Monoclinic, C2/c
14.671(3)
10.131(2)
21.231(4)
101.27(3)
3094.8(11)
4
b [Å]
c [Å]
b
(ꢀ)
Volume [Å3]
4450.3(4)
8
Z
anti
syn
F(000)
904
1808
Crystal size [mm]
Theta range for data collection
Reflections collected/unique
0.20ꢂ0.18ꢂ0.16
1.96–28.31ꢀ
16,021/3849
[Rint¼0.0385]
3849/0/154
1.052
0.11ꢂ0.06ꢂ0.06
1.67–25.49ꢀ
19,091/4127
[Rint¼0.0816]
4127/0/328
0.937
Figure 1. The structure of two stereoisomers of 9,10-di(10-naphthyl)anthracene.
2. Results and discussion
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I>2
s
(I)]
R1¼0.0494,
wR2¼0.1290
R1¼0.0740,
wR2¼0.1373
0.214 and ꢁ0.223
R1¼0.0653,
wR2¼0.1521
R1¼0.2016,
wR2¼0.2175
0.189 and ꢁ0.194
The preparation of 9,10-di(10-naphthyl)anthracene was
followed the known procedures.16–18 The product was obtained as
yellow powder and the existence of two isomers was confirmed by
HPLC, 1H NMR, and 13C NMR spectra.
R indices (all data)
Largest diff. peak and hole [e Åꢁ3
]
The separation of two isomers was successfully performed after
several times of recrystallization from toluene and xylene. To assign
the structures of isomers, we performed X-ray analysis of the single
crystals. Both anti and syn crystals were grown by slow evaporation
of THF/hexane solution.
Table 2
Selected bond distances (Å) and bond angles (ꢀ) of 9,10-di(10-naphthyl)anthracene
anti
syn
X-ray intensity data were collected on a Bruker SMART APEX-II
C1–C2
1.406(2)
1.406(2)
1.498(2)
1.414(5)
CCD diffractometer using graphite monochromated Mo K
a radia-
C1–C7i
tion (
l¼0.71073 Å). Structure was solved by applying the direct
C1–C8
method using a SHELXS-97 and refined by a full-matrix least-
squares calculation on F2 using SHELXL-97.19 All non-hydrogen
atoms were refined anisotropically. All hydrogen atoms were
placed in ideal positions and were riding on their respective carbon
atoms (Biso¼1.2 Beq). anti Isomer was refined with solvent free.
Crystallographic data for the structure reported here have been
deposited with the Cambridge Crystallographic Data Center
(deposition nos. CCDC 722475 and CCDC 722476).
C1–C14
C1–C15
C2–C3
C2–C7
C3–C4
C4–C5
C5–C6
C6–C7
1.394(5)
1.498(5)
1.411(5)
1.450(5)
1.361(5)
1.413(5)
1.336(6)
1.440(5)
1.433(2)
1.440(2)
1.348(2)
1.418(2)
1.352(2)
1.429(2)
120.0(1)
119.9(1)
120.1(1)
121.9(1)
117.9(1)
C2–C1–C7i
C7i–C1–C8
C2–C1–C8
C1–C2–C3
C3–C2–C7
C2–C1–C14
C14–C1–C15
C2–C1–C15
The Crystallographic data and refinement parameters for anti-
and syn-isomers are summarized in Table 1. The selected bond
distances and angles are summarized in Table 2. And an ORTEP
view including the atomic numbering scheme is shown in Figure 2.
In anti isomer, there is a crystallographic inversion center located in
the center of anthracene ring. The dihedral angles between an-
thracene and naphthyl planes are 80.0(1)ꢀ and 82.2(2)ꢀ for anti and
syn, respectively. As shown in Figure 3, syn isomer molecules are
held together. The intermolecular distance of 3.670(4) Å between
the two centroids of rings, symmetry code ꢁx, y, ꢁzþ1.5, in the
packing structure indicates that the crystal structure of syn-9,10-di
122.2(4)
119.0(4)
120.7(4)
119.3(3)
120.0(4)
Symmetry code: (i) x, yþ1, z.
intermolecular distances between the C9–C14 ring and H33, H34
atoms are in the range of 2.841(3)–3.397(4) Å). X-ray single crystal
structure of pyridine solvated anti-9,10-di(10-naphthyl)anthracene
has been reported and there seems to be stabilization by weak
(10-naphythyl)anthracene is stabilized by
the two anthracene rings. And there is another weak in-
termolecular C–H/ interaction not shown in Figure 3 (the
p–p interaction between
intermolecular C–H/N and C–H/p
interactions.20
p
Figure 2. ORTEP drawings of two structures of the title compound with atom-numbering scheme; anti (left) and syn (right).

3362
C.H. Lee et al. / Tetrahedron 66 (2010) 3360–3364
ratio was expected from the first method since the diol is known to
be mainly in the anti configuration. This result tells that the steric
effect is not a key issue in the reduction reaction of 9,10-di(10-
naphthyl)-9,10-dihydroanthracene-9.10-diol.
The product right after the synthesis shows 1H NMR spectrum in
Figure 4(c). After successful separation of two isomers, 1H NMR
spectrum of each isomer was much simpler as shown in Figure 4(a)
and (b) and the spectrum of the mixture turns out simply the sum of
Figure 4. 1H NMR spectra of 9,10-di(10-naphthyl)anthracene in chloroform-d
(a) syn (b) anti (c) mixture.
Figure 3. The
p–p interaction (dashed line) between two rings in syn isomer
(symmetry code; (i) ꢁx, y, ꢁzþ1.5).
that of syn and anti. The 1H NMR spectra of syn and anti in toluene-d8
did not change until ca. 100 ꢀC, which means the rotation of
s-bond
between anthracene and naphthalene is very slow at that temper-
ature and isomerization study could not be performed using solu-
tion NMR technique. The 13C NMR spectrum of anti shows 14 peaks
whereas that of syn has much more than 14 peaks with lower
intensities. We interpret this phenomenon for syn as breaking
symmetry due tothe steric congestion. Figure 5 shows the UV and PL
spectra of anti and syn in THF. There was no difference between anti
and syn in solution state even though solid structures were different.
The solubility of the two isomers was quite different. Both syn
and anti were more soluble in toluene than in THF. syn was more
soluble than anti in THF as well as in toluene and the comparison of
the values is shown in Table 3. This makes the large errors in
obtaining the product ratio of anti to syn since the recrystallization
makes higher ratio for anti.
The assignment of the two HPLC peaks was possible after
obtaining the X-ray diffractiondata of the two isomers. The syn came
first when the separations were performed on a 250 mmꢂ4.6 mm
column packed with 5 mm C18 bonded silica (Waters 600I, mobile
phase was a mixture of H2O/MeCN (5:95, v/v) with a flow rate of
1.5 ml/min.), which means the syn is more polar than anti. The fol-
lowing resultsfor the ratio of syn toanti werefromthe HPLC analysis.
There were many procedures reported for the synthesis of 9,10-
di(10-naphthyl)anthracene. We followed three different literature
conditions and compared the ratio of syn and anti isomers. The first
synthetic method that we used was reduction of 9,10-di(10-naph-
thyl)-9,10-dihydroanthracene-9.10-diol with KI/NaH2PO in acetic
acid.16 The second method was the reaction of 9,10-dibromoan-
thracene with tri(1-naphthyl)indium in the presence of Pd cata-
lyst.17 The third method was the Suzuki–Miyaura reaction of
9,10-dibromoanthracene with 1-naphthaleneboronic acid.18 All
three reactions gave the product in good yields in the similar ratio
of anti/syn as 50:50 (ꢃ5). A large deviation from the 50:50 anti/syn
We have tried to get melting points for the two isomers since
the previous reports gave the only one for the mixture. anti crystal
started to crumple at 369 ꢀC and syn did not melt until 400 ꢀC,
which is the limit of the thermometer. The mixture started to melt
700
a
b
600
500
400
300
200
100
0
1.0
0.5
0.0
-100
200
250
300
350
400
450
500
550
400
450
500
550
Wavelength (nm)
Wavelength(nm)
Figure 5. (a) Absorption (UV–vis) and (b) emission (PL) spectra of anti (d) and syn (.) in THF.

C.H. Lee et al. / Tetrahedron 66 (2010) 3360–3364
3363
Table 3
The DSC in the connection with HPLC experiment clearly shows
that the small DSC signals from 369 ꢀC to 376 ꢀC are due to the
isomerization process from anti to syn. This is a feature of how the
opposite isomer can fit into the crystal lattice.
Solubility of the anti and syn isomers in organic solvents
Solvent
anti (M)
syn (M)
THF
Toluene
9.49ꢂ10ꢁ4
1.04ꢂ10ꢁ3
1.99ꢂ10ꢁ3
2.60ꢂ10ꢁ3
The thermodynamic data for the DSC features of the anti at
368.5 ꢀC and 376 ꢀC show a near cancellation of endothermic and
exothermic. Therefore there is no net heat. We would interpret that
as an isomerization of w30% of the anti at 369 ꢀC, where the lattice
softens enough to permit this. However, this produces syn in an
unfavorable crystal environment, so the process is endothermic.
Relaxation of the crystal at 371 ꢀC is then exothermic, by an equal
amount. Likewise at 375.5 ꢀC another 40% of the anti isomerizes.
In summary, two isomers, anti- and syn-9,10-di-(10-naph-
thyl)anthracene were isolated and characterized with X-ray,
1H NMR, and 13C NMR. This is unprecedented even though 9,10-di-
(10-naphthyl)anthracene is an important compound in organic
electronics industry. DSC study and HPLC analysis provided the
evidence of the isomerization between the syn and anti in solid
state at high temperature. Since syn and anti have different physical
properties in terms of solubility and thermal properties, the
existence of stereoisomers should be an important factor to be
considered in OLED.
from 360 ꢀC as reported in the literature.17 Since the thermal
properties such as melting and sublimation temperature is very
important in OLED, the extensive studies were conducted using
DSC. anti has thermal profiles with three endothermic troughs, two
shallow ones around 369 ꢀC, and 376 ꢀC, and a deep one around
409 ꢀC as shown in Figure 6. The DSC profile for syn shows only one
trough around 409 ꢀC, which is very similar to the last one for that
of anti. The large endotherm at 409 ꢀC is due to melting for both
isomers.
2
0
-2
-4
-6
3. Experimental
3.1. General information
All reagents and solvents including bromonaphthalene,
anthraquinone were purchased from commercial sources and used
as received. 9,10-Di(10-naphthyl)anthracene was prepared by
literature procedures.16–18 The isolation of anti and syn isomers was
successfully performed after several times of recrystallization from
toluene and xylene. Both anti and syn single crystals were grown by
slow evaporation of THF/hexane solution. 1H and 13C NMR spectra
were recorded on a JEOL JNM-AL400 spectrometer, operating at
9.39 T. X-ray intensity data were collected on a Bruker SMART
340
360
380
400
420
Temperature (OC)
Figure 6. DSC curves of anti (d) and syn (.) on a heating scan at 10 deg minꢁ1
.
To get an idea why anti has small DSC signals near 370 ꢀC, HPLC
analysis has beenperformed in connectionwith DSC experiment. The
pure antiꢁo1r syn solid was heated using DSC on a scan rate of
10 ꢀC min and the heating was stopped at every 10 ꢀC interval from
340 ꢀC to 440 ꢀC. After cooling the samples, HPLC analysis gave the
anti/syn ratio of each sample and the results were plotted in Figure 7.
APEX-II CCD diffractometer using graphite monochromated Mo K
radiation (
¼0.71073 Å). The ratio of anti/syn was monitored using
LC–UV (Waters 600I). Separations were performed on
m C18 bonded silica.
a
l
a 250 mmꢂ4.6 mm column packed with 5
m
DSC was carried out on a TA instruments DSCQ10 differential
scanning calorimeter with a 10 deg minꢁ1 heating rate under
nitrogen atmosphere. UV and PL spectra were obtained using an
HP8452A and a PERKIN ELMER LIMITED LR64912C (lex¼376 nm),
respectively.
100
80
60
40
20
0
100
80
60
40
20
0
3.1.1. anti-9,10-Di-(10-naphthyl)anthracene. Mp 409 ꢀC (from DSC);
1H NMR (400 MHz, CDCl3)
d
8.09 (d, J¼8.4 Hz, 2H), 8.03 (d, J¼8.0 Hz,
2H), 7.74 (dd, J¼7.2, 8.0 Hz, 2H), 7.66 (d, J¼6.8 Hz, 2H), 7.49 (m, 6H),
7.21(m, 8H); 13C NMR (100 MHz, CDCl3)
136.77, 135.38, 133.73,
d
133.65, 130.68, 129.29, 128.25, 128.16, 127.14, 126.76, 126.31, 126.03,
125.61, 125.24.
3.1.2. syn-9,10-Di-(10-naphthyl)anthracene. Mp 409 ꢀC (from DSC);
340
360
380
400
420
440
460
1H NMR (400 MHz, CDCl3)
d
8.09 (d, J¼8.0 Hz, 2H), 8.04 (d, J¼8.0 Hz,
2H), 7.73 (dd, J¼7.2, 8.0 Hz, 2H), 7.62 (d, J¼7.2 Hz, 2H), 7.50 (m, 6H),
7.30 (m, 4H), 7.21 (m, 4H); 13C NMR (100 MHz, CDCl3)
138.58,
O
Temperature ( C)
d
Figure 7. Mole percent changes of syn (-) and anti (6) at various temperatures.
136.77, 135.41, 133.82, 133.74, 133.62, 130.96, 130.74, 130.68, 129.24,
129.05, 128.92, 128.41, 128.29, 128.16, 127.56, 127.12, 126.69, 126.36,
126.06, 125.67, 125.63, 125.37, 125.26.
Starting with 100% anti, the isomerization occurs at 370 ꢀC and
the percentage of anti dramatically dropped to 20% at 400 ꢀC. After
that the melting process reconstructs the new anti/syn equilibrium
at 50:50. In contrast to this, starting with 100% syn, there was no
isomerization until melt at 409 ꢀC. Again, the anti/syn ratio reached
to 50:50 after melting. This result means the stability of anti and syn
is nearly equal, which is consistent with the calculation results in
gas phase.1
Acknowledgements
This work was supported by the Ministry of Knowledge
Economy, Republic of Korea. C.H.L. is recipient of BK21 fellowship
(2009).

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C.H. Lee et al. / Tetrahedron 66 (2010) 3360–3364
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