Preparation, crystal structure, and solid-state fluorescence of a CH 2Cl2-solvated crystal of 6,13-bis(t-butylphenyl)-2,3,9,10- tetrapropoxypentaeene

Article abstract of DOI:10.1246/cl.2009.600

Slow evaporation of 6,13-bis(t-butylphenyl)-2,3,9,10-tetrapropoxypentacene in CH₂Cl₂ solution in the dark, yielded CH ₂Cl₂-solvated single crystals easily. The crystals were characterized by X-ray analysis and solid-state fluorescence. Copyright

Full text of DOI:10.1246/cl.2009.600

600
Chemistry Letters Vol.38, No.6 (2009)
Preparation, Crystal Structure, and Solid-state Fluorescence of a CH2Cl2-solvated Crystal
of 6,13-Bis(t-butylphenyl)-2,3,9,10-tetrapropoxypentacene
Chitoshi Kitamura,ꢀ1 Takao Naito,1 Akio Yoneda,1 Takashi Kobayashi,2 Hiroyoshi Naito,2 and Toshiki Komatsu3
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 599-8531
3Goi Research Center, Chisso Petrochemical Corporation, 5-1 Goikaigan, Ichihara 290-8551
(Received March 9, 2009; CL-090242; E-mail: kitamura@eng.u-hyogo.ac.jp)
Slow evaporation of 6,13-bis(t-butylphenyl)-2,3,9,10-tetra-
effloresced within hours of standing in air, turning into powder
subsequently. This observation suggests that the single crystals
we prepared can hold and release CH2Cl2. Using organic sol-
vents other than CH2Cl2 led to powder formation exclusively.
Consequently, we could not produce single crystals consisting
only of 1. Further, 6,13-diphenyl derivatives except 1 contained
t-butyl groups that did not form the solvated crystals.
propoxypentacene in CH2Cl2 solution in the dark, yielded
CH2Cl2-solvated single crystals easily. The crystals were char-
acterized by X-ray analysis and solid-state fluorescence.
Linear oligoacenes have been studied widely as potential or-
ganic semiconducting materials for organic thin film transistors
(OTFTs), organic light-emitting diodes (OLEDs), and photovol-
taic cells.1 In particular, pentacene derivatives have been inten-
sively investigated because they exhibit high hole charge carrier
mobility and a strong red emission.2 The electronic and optical
properties, solubility in organic solvents, and molecular arrange-
ment in the solid state can easily be tuned by structural modifi-
cation of substituents on the backbone. In an effort to tailor these
properties, an increasing number of new pentacenes have been
described.3 Pentacenes that are stable in air and soluble in a
variety of organic solvents have been pursued because of their
processability and low cost involved in the development of
applications. To that end, we prepared a new 6,13-diaryl-
2,3,9,10-tetraalkoxypentacene 1. This compound exhibited a
unique behavior; on slow evaporation only from CH2Cl2 solu-
tion, CH2Cl2-solvated single crystals can easily be formed. We
report here the preparation, crystal structure, and the solid-state
In order to examine the reason for using CH2Cl2 leading to
the formation of single crystals, X-ray structural analysis was
performed at ꢁ100 ꢂC to clarify whether the single crystal crys-
5,6
.
tallized was 1 CH2Cl2. Though there was one disordered set
of propoxy groups, unambiguous structural information was
obtained (Figure 1). Molecule 1 has a crystallographic center
of symmetry, and half of the unit is asymmetric. The pentacene
framework is almost planar and the peripheral benzene rings are
almost perpendicular to the pentacene moiety. The dihedral an-
gle between the planes of benzene and pentacene is 73.15(6)ꢂ.
The carbon atom of the CH2Cl2 solvate is located at a short dis-
˚
tance (3.327 A) above the pentacene plane. The crystal packing
of the pentacene backbone displayed a slipped-parallel arrange-
ment constructed by the stacking of molecular sheets, although
ꢀ-overlapping is absent because of the existence of the solvate.
Within the molecular sheet, a CH2Cl2 molecule is surrounded by
.
fluorescence of 1 CH2Cl2.
In order to obtain 1, we applied a general preparative proto-
col,4 namely addition of 4-(t-butyl)phenyllithium to 2,3,9,10-
tetrapropoxy-6,13-pentacenequinone (2) and subsequent reduc-
tion as shown in Scheme 1.5 We also succeeded in obtaining
the other 6,13-diphenyl derivatives (e.g., phenyl and 2,6-di-
methylphenyl) using this method. Pentacene 1 was a red-violet
solid that showed relatively high air-stability and was soluble
in common organic solvents such as CHCl3 and toluene. The
solutions were unstable in the presence of both light and air.
Slow evaporation of CH2Cl2 solutions at room temperature
in the dark yielded good quality single crystals. These crystals
(a)
(b)
tBu
O
PrO
PrO
OPr
OPr
PrO
PrO
OPr
OPr
i, ii
O
2
1
tBu
.
Figure 1. Crystal structures of 1 CH2Cl2; (a) top view and (b)
view normal to a molecular sheet.
Scheme 1. Reagents and conditions: (i) t-BuC6H4Li, THF,
ꢁ78 ꢂC to rt, 62%; (ii) SnCl2, AcOH–dioxane, 50 ꢂC, 80%.
Copyright Ó 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.6 (2009)
601
state.11 The fluorescence of 1 in powder form can be attributed
to such interacting molecules. The ordered packing structure
1.2
1
1.2
(a)
UV-vis
FL
1
0.8
0.6
0.4
0.2
0
.
and solvated CH2Cl2 molecules in the crystals of 1 CH2Cl2
0.8
0.6
0.4
0.2
0
are considered to weaken the intermolecular interaction and pro-
hibit large nuclear displacement, resulting in a fluorescence
spectrum rather similar to that in solution.
In conclusion, we found that 1 can easily make single crys-
400
500
600
700
800
900
.
tals consisting of 1 CH2Cl2 on slow evaporation from CH2Cl2
solution. Crystal packing of 1 CH2Cl2 was confirmed by X-
Wavelength / nm
1.2
.
(b)
1
UV-vis
FL
800
600
400
200
0
1
.
ray analysis. The fluorescence spectrum of 1 CH2Cl2 was dras-
tically different from that of 1 in powder form.
0.8
0.6
0.4
0.2
0
1·CH2Cl2
1
This work was supported by a Grant-in-Aid (No. 20550128)
from the Ministry of Education, Culture, Sports, Science and
Technology, Japan. We thank the Instrument Center of the Insti-
tute for Molecular Science for the X-ray structural analysis.
400
500
600
700
800
900
Wavelength / nm
Figure 2. (a) UV–vis absorption and fluorescence spectra of 1
in CH2Cl2 and (b) Kubelka–Munk spectrum of 1 in diluted pel-
References and Notes
1
a) M. Bendikov, F. Wudl, D. F. Perepichka, Chem. Rev. 2004, 104,
4891. b) J. E. Anthony, Angew. Chem., Int. Ed. 2008, 47, 452.
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let, and fluorescence spectra of 1 in powder form and 1 CH2Cl2
in crystalline form.
2
two t-butylphenyl and propoxy groups each, and a pentacene
backbone (Figure 1b). The presence of t-butyl groups may be
responsible for the formation of the solvated crystal. It can be
assumed reasonably that the size and configuration of CH2Cl2
matched fortuitously with the vacant position in the molecular
packing. Thus, the solvate molecule plays an indispensable role
as a pillar in stabilizing the crystalline structure.
UV–vis absorption and fluorescence spectra of 1 in a dilute
CH2Cl2 solution were measured (Figure 2a). The former shows a
structured band with peaks at 511, 549, and 594 nm. The latter is
slightly structured with the fluorescence maximum of 610 nm,5
which represents a relatively small Stokes shift of 16 nm. Solid-
state absorption (Kubelka–Munk) and fluorescence spectra of 1
were also measured (Figure 2b).5,7 In the absorption spectrum of
1 in diluted pellet, all the major peaks appear at almost the same
3
For recent examples of substituted pentacene synthesis: a) I.
Kaur, W. Jia, R. P. Kopreski, S. Selvarasah, M. R. Dokmeci, C.
Pramanik, N. E. McGruer, G. P. Miller, J. Am. Chem. Soc. 2008,
130, 16274. b) D. Lehnherr, R. McDonald, R. R. Tykwinski,
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Yoshino, K. Saito, K. Eguchi, M. Tomura, J. Nishida, Y.
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Parkin, J. B. Sherman, R. C. Bakus, II, Chem. Commun. 2007,
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positions of the corresponding peaks of 1 in solution (ꢁmax
¼
.
507, 545, and 589 nm). The fluorescence spectrum of 1 CH2Cl2
in crystalline form is red-shifted by 41 nm with respect to that in
the solution, whereas the fluorescence spectrum of 1 in the pow-
der form shows more drastic change, i.e., the band at 651 nm
4
5
6
.
found in 1 CH2Cl2 disappears and the overall spectrum is fur-
ther red-shifted and broadened. Along with these changes, the
fluorescence quantum yield (ꢂ) decreases in solid state. In fact,
Supporting Information is available electronically on the CSJ-
Journal Web site, http://www.csj.jp/journals/chem-lett/index.html.
Crystallographic details have been deposited with Cambridge Data
Centre as no. CCDC-711919. Copies of the data can be obtained
free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html.
.
ꢂ of 1 in the solution and 1 CH2Cl2 in crystalline form were
0.61 and 0.03, respectively, and ꢂ of 1 in powder form was
too low to be measured (ꢂ < 0:01).5,8 It is reasonable to consid-
er that the lower ꢂ values in the solid state results from concen-
tration quenching,9 efficient energy transfer into quench sites,
and intermolecular interaction. In disordered systems, molecules
form various packing structures without long-range order, and
in many cases they exhibit absorption spectrum similar to that
in the solution.10 On the other hand, solid-state fluorescence
may be dominated by some interacting molecules with lower en-
ergy (e.g. dimers), which are often contained in the disordered
samples. A characteristic feature of solid-state fluorescence is
red-shifted, the spectrum is broadened owing to intermolecular
interaction, and nuclear displacement occurs in the excited
.
It was impossible to measure the absorption spectra of 1 CH2Cl2
in crystalline form technologically.
The lower ꢀ of 1 in powder form is responsible for the poor signal-
to-noise ratio in the fluorescence spectrum in Figure 2b.
Excited States and Photochemistry of Organic Molecules, ed. by
M. Klessinger, J. Michl, VCH publisher, New York, 1995.
7
8
9
10 J. Gierschner, M. Ehni, H.-J. Egelhaaf, B. M. Medina, D.
Beljonne, H. Benmansour, G. C. Bazan, J. Chem. Phys. 2005,
123, 144914.
11 Electronic Process in Organic Crystals and Polymers, ed. by M.
Pope, C. E. Swenberg, Oxford University Press, New York, 1999.
Published on the web (Advance View) May 23, 2009; doi:10.1246/cl.2009.600