Enforced face-to-face stacking of organic semiconductor building blocks within hydrogen-bonded molecular cocrystals

Article abstract of DOI:10.1021/ja057939a

We report a method to enforce face-to-face stacking of the aromatic rings of organic semiconductor molecules in the solid state that employs bifunctional hydrogen-bond donors, in the form of semiconductor cocrystal formers, to align semiconductor building blocks. Copyright

Full text of DOI:10.1021/ja057939a

Published on Web 02/11/2006
Enforced Face-to-Face Stacking of Organic Semiconductor Building Blocks
within Hydrogen-Bonded Molecular Cocrystals
Anatoliy N. Sokolov, Tomislav Frisˇcˇic´, and Leonard R. MacGillivray*
Department of Chemistry, UniVersity of Iowa, Iowa City, Iowa 52242-1294
Received November 22, 2005; E-mail: len-macgillivray@uiowa.edu
Enforced face-to-face stacking of aromatics in the solid state is
an important topic in the field of semiconductor materials.1 In this
context, oligothiophenes, anthracenes, and related acenes (e.g.,
pentacene) have emerged as leading candidates for the construction
of organic semiconductor crystalline solids. Many reports have
underscored the importance of maximizing π-orbital overlap to
achieve efficient charge transport properties of such solids.1
Figure 1. X-ray crystal structures of (a) 1 and (b) 2.
Efforts to promote face-to-face stacking of semiconductor
molecules in the solid state have focused on the use of functional
groups expected to direct the assembly process to the prerequisite
packing.2 The intermolecular forces used to promote such arrange-
ments have, thus far, been relatively weak, being based on van der
Waals and/or dipole-dipole interactions. The most successful
implementation of this strategy has, arguably, involved pentacene,
where steric effects of bulky substituents prevented C-H‚‚‚π
forces.3 A much greater π-orbital overlap was achieved, as
compared to pure pentacene. Admittedly, however, the amount of
π-orbital overlap cannot be correlated to the size and position of
the functional groups, owing to a difficulty to control slipping along
the pentacene short axis.3 Indeed, these observations attest to the
sensitivity of crystal packing to changes to molecular structure and
create a challenge to devise methods that employ stronger inter-
molecular forces to direct face-to-face stacking so as to be able to
reliably optimize the performances of such organic semiconductor
solids.
With this in mind, we wish to describe here an approach to
control the organization of the aromatic rings of semiconductor
molecules in the solid state that enforces face-to-face π-stacking
using the strength and directionality of hydrogen bonds. In partic-
ular, our design involves cocrystallizing4 a bifunctional molecule,
hereafter referred to as a semiconductor cocrystal former (SCCF),
with a semiconductor building block (SBB) functionalized with an
appropriate recognition site, or “handle” (Scheme 1).5 The SCCF
π-orbital overlap. Our initial experiments to test this strategy involve
SBBs with a minimal number of thiophene and acene rings since
such semiconductor units generally exhibit less π-orbital overlap
in the solid-state owing to a high number of C-H‚‚‚π forces.7,8
The SCCF used in this study is based on 1,3-dihydroxybenzene,
or resorcinol. We envisioned that a resorcinol could enforce face-
to-face π-stacking of two SBBs based on thiophene and anthracene,
namely 2,5-bis(4-pyridylethynyl)thiophene (1) and 9,10-bis(4-
pyridylethynyl)anthracene (2). The parent SBBs (i.e., thiophene,
anthracene) prefer herringbone packing, as demonstrated by both
experimental and computational studies.8 To provide recognition
sites for the hydrogen bonds, the SBBs were functionalized with
4-pyridylacetylene handles.9,10 The formulations of 1 and 2 were
confirmed by 1H NMR spectroscopy and single-crystal X-ray
diffraction.11
As a control experiment, we examined the crystal structures of
the functionalized SBBs 1 and 2 (i.e., in the absence of a SCCF).11
Whereas the structure of 2 has been recently reported,11 single
crystals of 1 suitable for X-ray diffraction analysis were obtained
by slow evaporation of CH3CN. In the case of 1, the solid11 consists
of dimeric assemblies sustained by C-Hpyridine‚‚‚πpyridine forces such
that the thiophene rings lie approximately antiparallel and offset
(Figure 1). The dimers interact in an offset and parallel manner
via C-Hthiophene‚‚‚N forces to form one-dimensional chains along
the N-N axis. Dimers of adjacent chains lie approximately
perpendicular, being held together by C-Hpyridine‚‚‚N forces. The
pyridyl groups of 1 lie twisted out of the plane of the thiophene
ring by 38.7(3)° and 46.7(2)°. In the case of 2, the pyridyl and
anthracene groups participate in alternating face-to-face π-stacked
Scheme 1. General Scheme of Noncovalent Enforcement of
Face-to-Face Stacking of Semiconductor Building Blocks
arrays. Adjacent arrays are held together via C-Hpyridine‚‚‚πpyridine
,
C-Hpyridine‚‚‚πanthracene, and πpyridine-πanthracene forces. In contrast to
1, a single pyridyl group is twisted out of the plane of the central
anthracene ring, being twisted by 64.5(1)°. As a consequence of
these forces, extended face-to-face stacking of the SBBs is not
observed in 1 and 2, the closest centroid-to-centroid distances being
4.05 and 7.11 Å, respectively.
Having examined the crystal structures of each pure SBB, we
cocrystallized 1 and 2 with a resorcinol. Specifically, cocrystals of
composition 2(5-methylresorcinol)‚2(1) (3) and 2(5-iodoresorci-
nol)‚2(2) (4) were obtained by slow evaporation from CH3CN and
CHCl3/MeOH 5:1 (v:v) solutions, respectively. The formulations
thus employs hydrogen bonds6 to segregate the structure of the SBB
from vexatious effects of crystal packing so as to reliably enforce
of 3 and 4 were confirmed by H NMR spectroscopy and single-
crystal X-ray diffraction.
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C O M M U N I C A T I O N S
face arrangements (Figure 3). The face-to-face geometries can be
ascribed to the larger van der Waals surfaces of the molecular
assemblies, or supermolecules,14 which enable each assembly to
form an extended π-stacked structure.
In this report, we have described a new approach to enforce face-
to-face stacking of the aromatic rings of semiconductor molecules
in the solid state. Specifically, we have employed cocrystals
involving SCCFs to stack SBBs in face-to-face arrangements. We
are currently investigating the ability of this approach to serve as
a synthon14 to control face-to-face stacking of additional SBBs (e.g.,
larger) as well as hybrids. The electronic properties of these, and
related, hydrogen-bonded solids will also be reported.
Figure 2. Hydrogen-bonded molecular assemblies of (a) 3 and (b) 4.
Acknowledgment. We are grateful to the University of Iowa
Mathematical and Physical Sciences Funding program for support
of this work.
1
Supporting Information Available: Synthesis of 1 and 2 and H
NMR and single-crystal data for 1-4. This material is available free
of charge via the Internet at http://pubs.acs.org.
References
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