Dibenzothiophene derivatives: From herringbone to lamellar packing motif

Article abstract of DOI:10.1021/cg100863q

It is generally believed that π-π stacking would be much more efficient than herringbone stacking for the transporting of charge carriers. The electron-withdrawing group sulphone unit was introduced into dibenzothiophene (DBT) derivatives, and lamellar st

Full text of DOI:10.1021/cg100863q

DOI: 10.1021/cg100863q
Dibenzothiophene Derivatives: From Herringbone to Lamellar
Packing Motif
2010, Vol. 10
4155–4160
Chengliang Wang,†,§ Huanli Dong,† Hongxiang Li,‡ Huaping Zhao,† Qing Meng,† and
Wenping Hu*,†
†Beijing National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids, Institute of
Chemistry, CAS, Beijing 100190, China, ‡Laboratory of Material Science, Shanghai Institute of
Organic Chemistry, CAS, Shanghai 200032, China, and §Graduate University of Chinese Academy of
Sciences, Beijing 100039, China
Received June 29, 2010; Revised Manuscript Received July 25, 2010
ABSTRACT: It is generally believed that π-π stacking would be much more efficient than herringbone stacking for the
transporting of charge carriers. The electron-withdrawing group sulphone unit was introduced into dibenzothiophene (DBT)
derivatives, and lamellar structures were observed in the single crystals of the products along with strong, long-range π-π
intermolecular interactions. As a contrast, the reduced materials adopted herringbone packing. We contributed this change of
packing motif to the polarity of the sulphone unit. These results are meaningful to the molecular design to obtain π-π stacking.
Crystal engineering is to design and synthesize functional
molecular solid-state structures, or arrangements, through
tuning the intermolecular interactions.1 The arrangement,
namely the packing motif, plays a very important role in the
organic electronic device performance.2-4 For instance, the
efficient charge transport of the organic field-effect transistors
(OFETs) was strongly dependent on the molecular packing
structures. Research has shown that there are four main pos-
sible packing motifs in organic solid states, named (1) typical
herringbone packing without π-π overlap between neighbor
molecules (e.g., pentacene5); (2) nonclassical herringbone
packing with π-π overlap between neighbor molecules,
which is also called slipped π-stacking in some citations6
(e.g., rubrene7); (3) lamellar packing, one-dimensional (1-D)
π-stacking (we take 1,2,3,4-tetrafluoroanthracene8 as an ex-
ample to demonstrate this packing motif); and (4) lamellar
packing, two-dimensional (2-D) π-stacking (e.g., TIPS-PEN9).
These kinds of packing motifs and the illustrations were
presented in Figure 1. Although pentacene, which is a bench-
mark of the organic semiconductors and adopts herringbone
(arrangements) on the properties. And their chemical struc-
tures, packing motifs (arrangements), and properties are
described in Table 1.
Lots of research has been accomplished (mainly by An-
thony et al.) to prevent the C-H π intermolecular interac-
3 3 3
tions in order toobtainπ-π stacking (orlamellar structure) in
the crystals through substituting at the peri-positions of
acenes.4,15,16 Rubrene and TIPS-PEN were the most repre-
sentative molecules, which adopted slipped π-π stacking and
lamellar structure, respectively, due to this strategy. Besides
this strategy, introducing hydrogen bonds,17,18 halogen-
halogen19 or chalcogen-chalcogen20,21 interactions, or incre-
asing the C/H ratio21 of the molecules was also used to obtain
lamellar structures.
Moreover, introducing polarity, usually as electron-with-
drawing groups, into the molecular structure also could result
in lamellar structure. The design conception is quite concise
and explicit. As we all know, p-type organic semiconductors
usually possess a large π-conjugated system, and this electron-
rich system would lead to a herringbone structure for the
electrostatic repulsion. If polarity is introduced into the che-
mical structure, the dipole-dipole attraction would counter-
balance the electrostatic repulsion and this balance might
result in lamellar structure. On the other hand, polar groups
might also introduce hydrogen bonds and coordinate bonds
into molecular arrangements,1 which might dramatically affect
the packing motif along with the charge transport properties.
However, there are rare citations in the literature16,22 that
discuss this kind of strategy to obtain π-π stacking in crystal
structures. Using linear acenes end-functionalized with a 1,4-
quinone moiety, Nuckolls’s group23 obtained a lamellar
structurewith π-π interactions. They ascribedtheπ-stacking
to the dipolar structure with one end having electron-rich
groups and the other end electron-withdrawing groups.
Similar results (Figures 1C and 2A)8,24 were also obtained
(in work directed by Swager and Gavezzotti et al.) from the
acenes end-functionalized with a tetrafluoro-moiety. π-π
stacking was also observed in the crystals of some derivatives
end-capped with electron-withdrawing groups at both ends
(e.g., trifluoromethylphenyl14 derivatives). In our previous
stacking in crystals with C-H π intermolecular interac-
3 3 3
tions instead of π-π interactions, showed high FET perfor-
mance with mobility up to 5 cm2/(V s) obtained from its
polycrystalline thin films,10 it is generally believed that the
π-π stacking would be the most efficient way to transport
charge carriers. And almost all of the highest mobilities of
FETs reported so far were using semiconductors which ado-
pted the π-π stacking motif as active layers, such as rubrene
(the highest performance for p-type single crystal transistors),
TiOPc (the highest performance for p-type thin film transis-
tors), compound 1 (the highest performance for n-type single
crystal transistors), compound 2 (the highest performance for
n-type thin film transistors), and compound 3 (the highest
performance for n-type thin film transistors based on oligo-
mers). Therein, rubrene adopts slipped π-π stacking and the
other four compounds show 2-D π-π stacking. These results
give a very clear conclusionof the importance of π-π stacking
*To whom correspondence should be addressed. E-mail: huwp@iccas.ac.cn.
Fax: 86 10 62527295. Telephone: 86 10 82615030.
r
2010 American Chemical Society
Published on Web 08/09/2010
pubs.acs.org/crystal

4156 Crystal Growth & Design, Vol. 10, No. 9, 2010
Wang et al.
Figure 1. Possible packing motifs in crystals. (A) herringbone packing (example, pentacene, RefCode PENCEN from CSD); (B) herringbone
packing with π-π overlap between neighbor molecules, slipped π-stacking (example, rubrene, RefCode QQQCIG01 from CSD); (C) lamellar
motif, 1-D π-stacking (example, 1,2,3,4-tetrafluoroanthracene, RefCode MIKGOD from CSD); (D) lamellar motif, 2-D π-stacking (example,
TIPS-PEN, RefCode VOQBIM from CSD). Protons are omitted for clarity.
Table 1. Chemical Structures, Packing Motifs, and Mobilities of Representative Molecules Which Showed the Highest Mobilities in the Related Area

Article
Crystal Growth & Design, Vol. 10, No. 9, 2010 4157
Scheme 2. Synthesis Routes of DBT Derivatives
our previous work.25 3,7-Bis(phenylethynyl)dibenzo[b,d]thio-
phene 5,5-dioxide (DOBEDBT) was synthesized through a
similar route to its reduced counterparts BEDBT by using a
Pd catalyzed modified Sonogashira coupling reaction. All
three compounds were soluble in normal organic solvents
(dichloromethane, tetrahydrofuran, toluene, chlorobenzene,
etc.) and easily formed crystals, and they were all purified
through recrystallization. As for BEDBT, it showed white
rodlike crystals, while, as for DOBEDBT, it displayed pale
yellow grains.
The absorption and fluorescence emission spectra of
DOBEDBT in dilute solution (CH2Cl2) at the concentration
of 10-5 M were shown in Figure 4A. According to the UV-
vis spectra, DOBEDBT showed absorption peaks similar to
those of BEDBT, but broader absorption peaks were ob-
served with maximum absorption about 30 nm red shift for
DOBEDBT. Previous reports26,29,30 have shown that the
sulphone group was not a significant bathochromic group,
so that it was not likely responsible for the red shift. In con-
clusion, the red shift might be attributed to the enhancement
of the conjugated degree for DOBEDBT. The optical energy
gaps determined from the initial absorption were 3.35 and
3.12 eV for BEDBT and DOBEDBT, respectively.
Figure 2. Lamellar motif obtained by introducing polarity into the
chemical structure to balance the electrostatic repulsion: (A) 1,2,3,4-
tetrafluoroanthracene (RefCode MIKGOD from CSD), (B) 3 (the
crystallographic data of 3 can be downloaded free from the Inter-
net as Supporting Information for ref 14 at http://pubs.acs.org).
Protons are omitted for clarity.
Scheme 1. DBT and Its Derivatives: From Herringbone to La-
mellar Packing Motif
To investigate the position change of the frontier orbital of
DOBEDBT compared with BEDBT, density functional the-
ory (DFT) calculations were performed using Gaussion 0331
at the B3LYP/6-31G(d) level. For BEDBT and DOBEDBT,
the largest coefficients in the highest occupied molecular
orbital (HOMO) were mainly delocalized on the hole mole-
cules, as shown in Figure 3A and C. However, the coefficients
in the lowest unoccupied molecular orbital (LUMO) were
most located on the dibenzothiophene core as for DOBEDBT
compared to BEDBT, suggesting the significant role of the
electron-withdrawing group (sulphone unit) for DOBEDBT.
The energy values of the HOMO and LUMO levels of BEDBT
and DOBEDBT were calculated by using DFT (the HOMO
levels were -5.41 and -5.74 eV; the LUMO levels were -1.85
and -2.31 eV, for BEDBT and DOBEDBT, respectively).
The cyclic voltammogram (CV) of DOBEDBT showed
irreversible oxidation peaks in CH2Cl2 solution at the concen-
tration of 10-3 M (Figure 4B) using Bu4NPF6 (0.1 M) as
supporting electrolyte. The electrochemical property of BEDBT
was also described here for comparison. From Figure 4B, the
onset oxidation potentials of DOBEDBT moved 0.33 V by
comparison with BEDBT (the HOMO energy level of DO-
BEDBT was 0.33 eV lower than that of BEDBT, coincident
with the result obtained by DFT calculations). The HOMO
work, we reported some dibenzothiophene (DBT) deriva-
tives25 and their applications in OFETs. In the following
studies, we found that, by introducing sulphone groups, the
packing motifs of two derivatives changed from herringbone
into lamellar structure, as shown in Scheme 1. We attributed
this change to the introduction of polar electron-withdrawing
groups into the molecular structures. The relevant character-
istics were also presented in this communication with the
expectation that they would be helpful to molecular design
through analysis of the structures and properties.
The synthesis routes of the three derivatives of DBT were
outlined in Scheme 2. The synthesis and the physicochemical
properties of dibenzo[b,d]thiophene 5,5-dioxide (DODBT)
could be easily found,26-28 and the synthesis of 3,7-bis(phenyl-
ethynyl)dibenzo[b,d]thiophene (BEDBT) has been reported in

4158 Crystal Growth & Design, Vol. 10, No. 9, 2010
Wang et al.
Figure 3. HOMO orbitals (A and C) and LUMO orbitals (B and D) of BEDBT and DOBEDBT obtained by using DFT calculations: (A and
B) BEDBT; (C and D) DOBEDBT.
Figure 5. (A) Herringbone packing motif without π-π stacking
of DBT (RefCode DBZTHP01 from CSD); (B) lamellar packing
motif of DODBT; (C) hydrogen bonds between neighbor molecules;
(D) overlap between neighbor molecules of DODBT along with
π-stacking (gray, bottom layer; blue, middle layer; green, top layer);
the opposite orientation in sequence could be found. Protons are
omitted for clarity.
Single crystals of DODBT and DOBEDBT suitable for XRD
were grown from chlorobenzene and toluene solution, respec-
Figure 4. (A) UV-vis absorption (solid line) and fluorescence
emission spectra (broken line) of BEDBT and DOBEDBT; (B)
tively. From Figures 5B and 6B, lamellar packing motifs
were observed in the DODBT and DOBEDBT single crystals.
However, as a contrast, the reduced materials DBT and BEDBT
adopted herringbone stacking (as shown in Figures 5A and 6A),
which indicated that the sulphone groups indeed made a great
difference in the molecular arrangements.
As for DODBT, the single crystals belonged to a crystal
system of monoclinic space group C2/c with unit-cell para-
˚
meters of a=10.043(2), b=13.844(3), c=7.0241(14) A and
cyclic voltammograms and (C) orbital levels of DBT (theory
level),32 DODBT (theory level),26 BEDBT,25 and DOBEDBT.
energy level of DOBEDBT determined at the onset oxidation
potentials was -6.20 eV by using ferrocene (Cp2Fe) as inter-
nal standard substance. This high ionization potential (IP,
equal to HOMO) suggested the high stability of DOBEDBT
and could be ascribed to the high IP of DBT and the electron-
withdrawing behavior of sulphone groups. The LUMO en-
ergy level of DOBEDBT was calculated roughly from its
energy gap and HOMO energy level as -3.08 eV, about
0.56 eV lower than that of BEDBT (also consistent with the
DFT calculations), suggesting its strong tendency to lower the
LUMOenergylevel. And the lower LUMO energylevel might
facilitate electron injecting, which was essential for n-channel
FETs. All the HOMO/LUMO energy levels and gaps of
BEDBT and DOBEDBT were summarized in Figure 4C as
well as DBT and DODBT for comparison.
β = 91.34(3)°. Every DODBT molecule interacted with four
neighbor molecules by hydrogen bonds (S-O H) as shown
3 3 3
in Figure 5C and with two molecules by π-π interactions.
Molecules along π-stacking adopted the opposite orientation in
sequence, which suggested that the π-π stacking was induced
by the polar groups. The distance between two π-faces was
˚
about 3.428 A (Figure 5B), and the π-π overlap between
neighbor molecules along π-stacking was almost as large as that
of a benzene ring (Figure 5D). The single crystals of DO-
BEDBT belonged to a crystal system of triclinic space group
P1 with unit-cell parameters of a = 10.031(2), b = 10.146(2),
The arrangements of DODBT33 and DOBEDBT34 were
investigated by their X-ray diffraction (XRD) measurements.

Article
Crystal Growth & Design, Vol. 10, No. 9, 2010 4159
Figure 6. (A) Herringbone packing motif without π-π stacking of BEDBT; (B) lamellar packing of DOBEDBT, (C) interactions between one
DOBEDBT molecule and its neighbor molecules (red dashed line, hydrogen bonds; green dashed line, C-H π interactions; π-π
3 3 3
interactions are not displayed); (D) overlap between neighbor molecules of DOBEDBT along with π-stacking (gray, bottom layer; blue,
middle layer; green, top layer); the opposite orientation in sequence could be found. Protons are omitted for clarity.
˚
c = 11.316(2) A and R = 100.63(3)°, β = 100.92(3)°, γ =
109.63(3)°. And a lamellar packing motif was observed in the
crystals of DOBEDBT, too. But very interestingly, due to the
polarity caused by the introduction of sulphone groups, mole-
cules of DOBEDBT revealed a nonplanar structure. The
dihedral angle between the plane of the twisting phenyl unit
and the plane of the DBT core was about 64.85°. In addition,
every DOBEDBT molecule interacted with two molecules by
to those of BEDBT by using DFT calculations, and the polar
group led to a much lower LUMO energy level, about 0.56 eV
lower than its reduced counterpart, BEDBT, according to the
CV measurements. The sulphone-based derivatives adopted
lamellar structures in their crystals with molecules orientated
oppositely along π-stacking in sequence, while the reduced
materials showed herringbone packing motifs. This result
suggested that the π-π stacking was induced by the polar
groups, and it is meaningful to the molecular design of the
π-stacking motif. The potential application of DOBEDBT in
organic electronics is underway.
hydrogen bonds (S-O H), and the distances between O
3 3 3
˚
atoms and H atoms on the DBT core were about 2.506 A and
those between O and H atoms on the phenyl units were about
˚
2.606 A. Besides the hydrogen-bonded contacts, there were also
C-H π interactions between DOBEDBT and two adjacent
Acknowledgment. The authors would like to thank Ms.
Hua Geng and Prof. Zhigang Shuai for DFT calculations and
further discussions. This work was supported by the National
Natural Science Foundation of China (60771031, 60736004,
20571079, 20721061, and 50725311), the Ministry of Science
and Technology of China (2006CB806200, 2006CB932100),
and the Chinese Academy of Sciences.
3 3 3
molecules and π-π interactions with two other molecules.
Similar to the case of DODBT, molecules along π-stacking
also adopted the opposite orientation in sequence and there
were two kinds of distances between two π-faces, about 3.370
˚
and 3.415 A, respectively. These differences between the crystal
structures of the sulphone-based derivatives DODBT and
DOBEDBT and those of the reduced materials DBT and
BEDBT suggested the important role of the sulphone in the
molecular arrangements, which would be helpful to obtain
π-π stacking.
Supporting Information Available: Description of experimental
procedures and instruments, and crystallographic information files
(CIF) of DODBT and DOBEDBT. This material is available free of
charge via the Internet at http://pubs.acs.org.
In conclusion, the strategies to obtain π-π stacking were
briefly reviewed and two DBT derivatives were obtained with
a lamellar packing motif. This might be attributed to the polar
group (sulphone unit), which balanced the electrostatic repul-
sion. As for DOBEDBT, the largest coefficients in the LUMO
were mostly located on the dibenzothiophene core compared
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˚
˚
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=
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