Study on facile synthesis, crystal structure, and solid-state fluorescence of dicyclohexane-annelated anthracene

Article abstract of DOI:10.1246/bcsj.81.754

Dicyclohexane-annelated anthracene was synthesized by a new efficient method, which employed the Diels-Alder reaction between in situ-generated 2,3-dehydronaphthalene and dicyclohexane-fused furan. The molecule adopted anti-parallel face-to-face slipped π-stacking in its crystal structure, which induced an excimer-like emission in the solid state.

Full text of DOI:10.1246/bcsj.81.754

754 Short Articles
Bull. Chem. Soc. Jpn. Vol. 81, No. 6, 754–756 (2008)
planned to synthesize some 1,4-disubstituted anthracene deriv-
atives. 1,2,3,4,5,6,7,8-Octahydrodibenz[a,c]anthracene (1a) as
well as 1,4-dialkylanthracene were selected as target mole-
cules. The former can be ascribed to an extended 1,4-dialkyl-
anthracene, which has two cyclohexane rings annelated to an-
thracene. Anthracene 1a was first prepared by Lawrence et al.
in 1935 by a classical anthracene synthesis method in five
steps that involves a Friedel–Crafts reaction between phthalic
anhydride and octahydrophenanthrene.10 However, detailed
description of the experimental procedure was not provided.
In addition, although octahydrophenanthrene is commercially
available, it is expensive, which reflects the difficulty in its
preparation. To prepare the tetraalkylanthracenes mentioned
above, we developed a new, efficient, and easily achievable
synthetic approach, which involved the Diels–Alder reaction
of dibenzyne and 2,5-disubstituted furans, and subsequent de-
oxygenation. The success led to the preparation of 1a using the
Diels–Alder reaction of a type of aryne, namely 2,3-dehydro-
naphthalene, with dicyclohexane-annelated furan 3a. In this
paper, we report on the facile synthesis, crystal structure,
and solid-state fluorescence of 1a. In addition, for comparison,
the results of 1,4-dipropylanthracene (1b) are also presented.
The cyclohexane-fused furan 3a11 was obtained as shown
in eq 1.
Study on Facile Synthesis,
Crystal Structure, and Solid-State
Fluorescence of Dicyclohexane-
Annelated Anthracene
ꢀ1
Chitoshi Kitamura, Chika Matsumoto,1
Nobuhiro Kawatsuki,1 Akio Yoneda,1
Kohei Asada,2 Takashi Kobayashi,2
and Hiroyoshi Naito2
1Department of Materials Science and Chemistry,
Graduate School of Engineering, University of Hyogo,
2167 Shosha, Himeji 671-2280
2Department of Physics and Electronics,
Graduate School of Engineering,
Osaka Prefecture University,
1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531
Received October 23, 2007; E-mail: kitamura@eng.u-hyogo.
ac.jp
O
1) NiO2, MeCN
O
60 °C, 22 h, 81%
Dicyclohexane-annelated anthracene was synthesized
by a new efficient method, which employed the Diels–Alder
reaction between in situ-generated 2,3-dehydronaphthalene
and dicyclohexane-fused furan. The molecule adopted anti-
parallel face-to-face slipped ꢀ-stacking in its crystal struc-
ture, which induced an excimer-like emission in the solid
state.
2) p-TsOH, benzene
reflux, 16 h, 44%
3a
2
ð1Þ
Thus, the homocoupling of cyclohexanone 2 with NiO2,
followed by Paal–Knorr furan synthesis using p-toluene-
sulfonic acid, resulted in a moderate yield of 3a. Next, anthra-
cene 1a was synthesized as shown in eq 2.
R1
O
R1
n-BuLi
toluene
The design and synthesis of organic materials that can fluo-
resce in the solid state received considerable attention because
of their promising applications, such as light-emitting diodes
and solar cells.1 Solid-state packing effects are now being rec-
ognized as critical factors for understanding fluorescence prop-
erties such as spectral shape and quantum efficiency.2–6 How-
ever, molecular design for controlling solid-state fluorescence
is not completely understood;5 therefore, this topic will be dis-
cussed in the following sections. Anthracene is a representa-
tive member of fluorophores. For example, solid-state fluores-
cence studies concerning chain-bridged anthracene deriva-
tives,7 organic salts of anthracene-2,6-disulfonic acid with
amines,4 and supramolecular complexes of anthracene–resor-
cinol hosts with guest molecules8 have been reported. Recent-
ly, we prepared a high-symmetry structure of 1,4,5,8-tetraal-
kylanthracenes,9 where alkyl substituents varied from methyl
to hexyl. In addition, we elucidated the relationship between
the fluorescence properties in the solid state and the corre-
sponding packing patterns. Then, we demonstrated that the al-
kyl chains can act as spacers and modulate the intermolecular
distance and position between anthracene cores. To further in-
vestigate the substituent effect on the solid-state fluorescence,
we studied the low-symmetry structure of anthracenes. We
Br
Br
R2
R2
R2
R2
O
+
−30 °C
then, rt, 1 h
R1
R1
4
3a: R1,R2 = −(CH2)4−
3b: R1 = n-Pr, R2 = H
5a, 74%
5b, 65%
ð2Þ
R1
R2
R2
TiCl4, Zn, THF
40 °C, 16 h
R1
1a, 66%
1b, 68%
Treatment of 2,3-dibromonaphthalene (4)12 with excess n-
BuLi and 3a at ꢁ30 ꢂC in toluene, where 2,3-dehydronaphtha-
lene was generated in situ and then trapped with 3a, resulted in
a 74% yield of adduct 5a. For the transformation into 1a, the
widely used two-step sequence of hydrogenation and acid
treatment could not be applied because hydrogenation of 5a
was not possible. Instead, one-step deoxygenation with low-
valent titanium12 was employed for the synthesis of 1a, which
was obtained in 66% yield. The yields of the Diels–Alder re-
action and the deoxygenation employed for the preparation
of 1b were 65 and 68%, respectively. This two-step anthracene
Published on the web June 11, 2008; doi:10.1246/bcsj.81.754

Bull. Chem. Soc. Jpn. Vol. 81, No. 6 (2008)
Ó 2008 The Chemical Society of Japan
755
1.2
1
a
O
b
1b
1a
(a)
0.8
0.6
0.4
0.2
0
c
3.63 Å
80.2°
b
300
350
400
450
500
550
600
(b)
Wavelength / nm
a
O
c
Figure 3. Excitation and fluorescence spectra of 1a (solid
line) and 1b (dotted line) in the solid state.
Table 1. Spectroscopic Data of 1a and 1b in the Solid State
aÞ
Figure 1. Crystal structures of (a) 1a and (b) 1b.
ꢁex
/nm
ꢁem
/nm
Lifetime
/ns
Radiative
lifetime/ns
ꢂbÞ
1a
1b
419
410
474
429
0.92
0.88
27
4.4
29
5.0
(a)
(b)
a) Excitation at 325 nm. b) Absolute quantum yield.
3.67 Å
3.69 Å
cules because 1a has a dipole along the long molecular axis
due to the presence of weak electron-donating alkyl groups.
From B3LYP/6-31G calculations, the dipole moment of 1a
ꢀ
3.89 Å
was calculated to be 1.1 debye. In contrast, the dipole moment
of 1b was 0.1 debye, which indicated that the dipole was
almost nonexistent. The calculation results confirmed that
the alkyl substituents at the 2,3-positions in anthracene and
not those at the 1,4-positions were responsible for the dipole
moment.
3.67 Å
3.63 Å
3.78 Å
Figure 2. Stacking patterns of two vicinal molecules of (a)
1a and (b) 1b.
The excitation and fluorescence spectra of 1a and 1b in
the solid state are shown in Figure 3. 1a illustrates a non-
structured emission band at 474 nm. On the other hand, 1b ex-
hibits structured emission bands at 429, 445 (sh), and 474 (sh)
nm, indicating the similarity to the fluorescence spectra of
1,4,5,8-tetraalkylanthracenes.9 Both fluorescence spectra are
red-shifted with respect to those in dilute solutions, although
the shift of 1a is much larger than that of 1b. A comparison
between the fluorescence and the corresponding excitation
spectra in the solid state reveals that the larger red-shift of
1a mainly results from its larger Stokes shift. In fact, the
Stokes shifts of 1a and 1b are estimated to be 2769 and
1080 cmꢁ1, respectively. Broad spectral shape and large
Stokes shift are characteristic features of excimer emission
due to the face-to-face slipped ꢀ-stacking, which was studied
in great detail in the case of pyrene crystals.14 Thus, the
fluorescence of 1a can be classified as an excimer-like emis-
sion, whereas the fluorescence of 1b is an exciton-like (or
monomer-like) emission. This assignment is also supported
by fluorescence quantum yield and lifetime measurements,
which indicated that the radiative lifetime of 1b is 6 times
greater than that of 1a, as shown in Table 1. In the face-to-face
slipped ꢀ-stacking of 1a, molecules can form an excimer al-
though the distance between the molecules is slightly greater
synthesis is very simple, and would be a versatile tool for
obtaining alkyl-substituted anthracenes.
X-ray analysis of 1a and 1b revealed that a large difference
in packing patterns of molecules exists, as shown in Figures 1
and 2. Thus, 1a showed face-to-face slipped ꢀ-stacking
(Figures 1a and 2a); however, 1b had a herringbone structure
in which the interplanar tilt angle between anthracene rings in
two adjacent columns was 80.2ꢂ (Figure 1b). The anthracene
rings of 1b in one column slipped relative to each other along
the long molecular axis by 1.12 A and along the short molecu-
˚
lar axis by 3.89 A. On the other hand, 1a had an antiparallel
arrangement in the stacked column direction, and the intermo-
lecular distance between the adjacent anthracene planes was
˚
˚
3.63 A (Figure 1a). The antiparallel molecular ꢀ-overlap has
been recently reported in the intramolecular donor–acceptor
system of tetracenes.13 In the case of 1b, neither a ꢀ-overlap
nor an antiparallel arrangement was observed (Figure 2b).
Even in the case of 1,4,5,8-tetraalkylanthracenes, the ꢀ-over-
lap was not observed.9 The shortest distance for overlapping
non-bonded atoms in anthracene for 1a was 3.63–3.67 A,
˚
which is shorter than that for 1b (3.69–3.89 A). The formation
of the antiparallel arrangement for 1a could be mainly ascribed
to weak dipole–dipole interactions between neighboring mole-
˚
˚
than the distance of 3.56 A in pyrene crystals. However, the

756 C. Kitamura et al.
Bull. Chem. Soc. Jpn. Vol. 81, No. 6 (2008)
X-ray Crystallography. Crystallographic data have been de-
posited at Cambridge Crystallographic Data Centre: the deposition
numbers are CCDC-662236 (1a) and 662235 (1b). Copies of this
information can be obtained free of charge via http://www.ccdc.
cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystal-
lographic Data Centre, 12, Union Road, Cambridge, CB2 1EZ,
UK; FAX: +44 1223 336033; e-mail: deposit@ccdc.cam.ac.uk).
formation of an excimer is energetically not possible because
the herringbone structure of 1b does not contain a pair of
molecules that lie exactly face-to-face. Our results suggest
that the shape effect of the side chains on the anthracene
backbone tuned not only the molecular arrangement in the
crystal but also the solid-state fluorescence. The design of
the molecule possessing a dipole along the long molecular
axis is important for achieving an antiparallel ꢀ-stacked
structure.
This work was partly supported by
a Grant-in-Aid
(No. 18750123) from the Ministry of Education, Culture,
Sports, Science and Technology, Japan. We thank the Instru-
ment Center, the Institute for Molecular Science, for assistance
in obtaining X-ray data. We also thank Dr. Yoshiaki Matsuo,
University of Hyogo, for the measurement of diffuse reflec-
tance spectra.
Experimental
Preparation of Dicyclohexane-Annelated Furan 3a. 3a was
prepared by a slightly modified procedure of Hawkins and
Large.11 A mixture of cyclohexanone (8.5 mL, 82.0 mmol) and
freshly prepared NiO2 (35.9 g, 396 mmol) in acetonitrile (60 mL)
was stirred at 60 ꢂC for 22 h. Then, CHCl3 was added, and the
mixture was filtered, washed with brine, and dried over Na2SO4.
After evaporation, the residue was heated at 70 ꢂC under vacuum
to obtain bicyclohexanone (6.46 g, 81%) as a crude yellow oil.
Supporting Information
Materials, fluorescence measurements in the solid state, crystal-
lographic data of 1a and 1b, and UV–vis absorption and fluores-
cence spectra in hexane. This material is available free of charge
on the web at: http://www.csj.jp/journals/bcsj/.
The oil was dissolved in benzene (70 mL), and p-TsOH H2O
ꢃ
(189 mg, 1.0 mmol) was added to the solution. The mixture was
refluxed for 16 h with a Dean–Stark trap. The reaction mixture
was washed with brine and dried over Na2SO4. After evaporation,
column chromatography (SiO2, 1:1 (v/v) CHCl3/hexane) afford-
ed 3a (2.59 g, 44%) as a yellow oil. 1H NMR (CDCl3) ꢃ 1.69–1.73
(m, 4H), 1.79–1.83 (m, 4H), 2.30–2.32 (m, 4H), 2.54–2.57 (m,
4H). 13C NMR (CDCl3) ꢃ 20.60, 23.01, 23.14, 23.20, 116.74,
148.13.
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