Two-dimensional bricklayer arrangements of tolans using halogen bonding interactions

Article abstract of DOI:10.1039/c5cc02225h

Diphenylacetylene (tolan) derivatives with self-complementary aryl halides and halogen bond-accepting nitriles form 2D bricklayer packing motifs when halogen bonding occurs. When halogen bonding is absent, as occurred with fluorinated aryl bromides, the molecules adopt other packing motifs. These results suggest halogen bonding is potentially useful for producing rarely observed 2D bricklayer motifs in organic semiconductors.

Full text of DOI:10.1039/c5cc02225h

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Two-dimensional bricklayer arrangements of
tolans using halogen bonding interactions†
Cite this:DOI: 10.1039/c5cc02225h
Fanny Frausto, Zachary C. Smith, Terry E. Haas and Samuel W. Thomas III*
Received 16th March 2015,
Accepted 22nd April 2015
DOI: 10.1039/c5cc02225h
www.rsc.org/chemcomm
Diphenylacetylene (tolan) derivatives with self-complementary aryl examples that yield intermolecular overlap of p-orbitals of
halides and halogen bond-accepting nitriles form 2D bricklayer packing highly conjugated small molecules.14–18
motifs when halogen bonding occurs. When halogen bonding is
In comparison to hydrogen bonding, halogen bonding (XB) is a
absent, as occurred with fluorinated aryl bromides, the molecules less often encountered, but increasingly popular, non-covalent
adopt other packing motifs. These results suggest halogen bonding interaction.19–22 Halogen bonds are interactions between areas of
is potentially useful for producing rarely observed 2D bricklayer negative potential, such as nitrogen-based lone pairs of electrons,
motifs in organic semiconductors.
and areas of positive potential on large halogens, most often Br
and I, located directly opposite of the carbon–halogen bond (the
The intermolecular arrangements of molecules with highly s-hole); the resulting n–s* interactions are more highly directional
delocalized, p-conjugated structures are critical to their solid-state than hydrogen bonds.23,24 XB has been used in a number of
properties and performance as semiconductors. Intermolecular functional materials, including cocrystals and gels.25,26 In the class
electronic coupling between molecules in the solid state is key, for of highly delocalized organic molecules, reports include self-
example, to charge-carrier mobility in field-effect transistors as thin complementary diarylethylenes,27–29 diarylacetylenes,30 phenoxy-
films or single crystals.1,2 Edge–face interactions, such as C–H/p boron subphthalocyanines,31 azobenzenes,32 and co-crystals
interactions, often lead to herringbone-type arrangements, which of stilbenes and diiodinated aromatics,33–35 as well as a number of
in many cases precludes effective electronic coupling between the studies on halogen bonding in TTF derivatives.36 Recently, multi-
p-systems of individual molecules. Overcoming the tendency for point halogen bonds were discovered in crystals of dihalodiimides,37
candidate organic semiconductors to adopt herringbone packing and weak halogen bonding was discovered in halogenated
motifs is therefore an important goal for organic electronics.3 N-heterocyclic acenes.38 Herein we describe the dependence of
There are few generally applicable strategies, however, for the packing motif on chemical structure of simple models for highly
rational design of molecules to accomplish this goal.
delocalized rigid rods—diphenylacetylene derivatives—with self-
Trialkylsilylethynyl-substituted pentacenes, heterocyclic analogs complementary halogen bonding substituents.
thereof, and structurally related compounds show predictable trends
Two observations provided initial inspiration for this work:
of herringbone, 1-D slipped-stack, and 2-D bricklayer packing that (i) the crystal structures of p-halobenzonitriles, which comprise
correlate with the size of the trialkylsilyl groups.4–8 A number of close packed, infinite one dimensional chains of molecules, with
studies have demonstrated that organic semiconductors with 2-D Nꢀ ꢀ ꢀX halogen bonds along the axes of the one dimensional
bricklayer packing generally yield superior charge carrier mobilities chains,39–41 (ii) C–Br and C–I bonds are present in many synthetic
to those with herringbone or 1-D slipped-stack packing.9–13 An intermediates of conjugated materials because of the prevalence
alternative approach to overcoming edge–face interactions of cross-coupling reactions in their preparation. We therefore
would be to design molecules with edge–edge interactions that designed a series of diphenylacetylenes (tolans), with the hypo-
could compete with edge–face interactions. Hydrogen bonding thesis that halogen bonding between the 4 and 40 positions would
is one class of edge–edge interactions of which there are several yield close-packed, one-dimensional chains. Chart 1 shows the
chemical structures of diphenylacetylenes 1–6, all of which con-
tain benzonitrile rings. Five of the structures (1–5) have potential
halogen bond donating moieties, from the strongly electropositive
tetrafluoroiodide 1 to the weakly electropositive non-fluorinated
Department of Chemistry, Tufts University, Medford, MA 02155, USA.
E-mail: sam.thomas@tufts.edu; Tel: +1-617-627-3771
† Electronic supplementary information (ESI) available: Complete synthetic and
bromide 5; control molecule 6 has no halogens. Several studies
have established hierarchies of halogen bond donors represented
crystallographic details. CCDC 1054201–1054206. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c5cc02225h
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of the molecules. Molecules 1, 2, and 5 arrange in crystal
structures similar to analogous p-halobenzonitriles:40 (i) there
is clear Nꢀ ꢀ ꢀX halogen bonding between the Lewis basic nitrile
and the Lewis acidic s-hole of iodine or bromine as character-
ized by linear (179–1801) Nꢀ ꢀ ꢀX–C angles and Nꢀ ꢀ ꢀX distances
approximately 10–16% less than the sum of their van der Waals
radii (Table 1); (ii) the halogen bonds between the termini of
molecules yield infinite one-dimensional chains of molecules
in the direction of their long axes; and (iii) the linear chains
pack into two-dimensional arrays. The conjugated backbones
of 1, 2 and 5 are somewhat twisted, with 19–221 torsional angles
between the two rings in both structures. Unlike p-iodobenzo-
nitrile, there is no ‘‘roll’’—displacement between chains in these
arrays along their short axes—in the structures of 2 or 5. In addition,
the lengths of the tolans enable each molecule to participate in
cofacial interactions with four other molecules, two from each
adjacent one-dimensional chain, with minimal inter-chain CꢀꢀꢀC
distances of 3.43 Å (2) and 3.34 Å (5) between rings rotated towards
each other. With the exception of compound 2, which shows several
extra peaks in its powder diffractogram, there is good agreement
between the X-ray powder diffractograms calculated from the single
crystal structures of 1, 2, and 5 and those that were measured
Chart 1 Diphenylacetylene derivatives with self-complementary halogen
bonding moieties (1–5) and a control molecule lacking any halogens (6).
Table 1 Relevant crystallographic and theoretical parameters of 1–5a
XB?
dNꢀ ꢀ ꢀX (Å)
dNꢀ ꢀ ꢀX/SrvdW
Vmax (I, Br)
Vmax (ArX face)
1
2
3
4
5
Yes
Yes
No
No
Yes
2.97
3.16
—
—
3.07
0.84
0.90
—
—
0.90
100
58.5
ꢁ11.1
62.5
30.4
ꢁ8.6
60.6
83.5
62.7
43.3
a
All potentials given in kJ molꢁ1
.
in our molecules as the following: 1 4 3 4 2 4 5.42,43 We experimentally (ESI†).
prepared all compounds with Sonogashira reactions between
The resultant structures are two-dimensional bricklayer
either (i) 4-ethynylbenzonitrile and appropriate aromatic halides arrangements of molecules, which are uncommon for conjugated
(for 1–5) or (ii) 4-iodobenzonitrile and phenylacetylene (6). molecules, but are reported to be advantageous for charge trans-
A stoichiometric excess of symmetric dihalide prevented a port in organic field effect transistors in some circumstances.
significant quantity of the three-ring phenylene–ethynylene Additional non-covalent interactions of the halogen atoms exist
byproduct from forming in the syntheses of 1–4. The non- between 2D brickwall arrays, in these cases between their
fluorinated tolans (2,44 5,45 and 646) are known; compounds 1, negatively charged areas (C–XꢀꢀꢀH angles = 841–871) and the edges
3, and 4 have not been reported.
of aromatic rings (dI–H = 3.07 Å in 2, dBr–H = 2.92 Å in 5).
We were successful in growing crystals suitable for X-ray Compound 1, an analog of 2 that has four fluorine atoms on
crystallography of all compounds listed in Chart 1. Table 1 the iodoarene instead of four hydrogen atoms, shows a similar
summarizes some of the key structural parameters obtained from packing motif to 2 and 5: 1-D chains along the long molecular axis
these crystal structures. Fig. 1 shows the X-ray crystal structure that also contain the halogen bond axis, packed into 2-D brick-
of non-fluorinated compounds that have self-complementary layers. In comparison to 2, a smaller NꢀꢀꢀI distance (2.955 Å) along
benzonitrile and iodine (2) or bromine (5) on opposite ends the chains is consistent with a stronger halogen bond of the
Fig. 1 X-ray crystal structures of compounds 2 and 5, with thermal ellipsoids shown at 50% probability. Left: the 2-D brickwork structure of each in the
crystallographic bc plane. Right: viewed along the crystallographic c axis, with hydrogen atoms omitted.
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more electropositive tetrafluoroiodide, while a smaller minimal of 4 showed no significant short contacts. Instead, each nitrile of
C–C distance between chains in each bricklayer stack (3.37 Å) 4 participates in hydrogen bonds with two C–H bonds, which
reflects strong cofacial interactions between arenes and fluorinated together with cofacial interactions between the fluorinated and
arenes. In addition, the crystal structure of 5 shows evidence of non-fluorinated rings yielded a 1-D slipped-stack arrangement.
some disorder with respect to the direction of packing of chains
To help us understand structure–property relationships, we
(parallel or antiparallel), as inversion twinning occurs in its struc- calculated the molecular electrostatic potential surfaces of all
ture. Although the carbon–nitrogen bond length of the nitrile of 2 is molecules, using crystallographically-determined geometries, at the
significantly shorter than expected (1.026 Å), we note no other DFT level (Fig. 3). In all cases, the potentials on the end of the nitrile
crystallographic evidence for disorder in this structure.40 Not groups are strongly negative. The trends in maximum positive
surprisingly, molecule 6 does not crystallize into a 2D bricklayer electrostatic potential at the iodine or bromine (see Table 1)
packing motif; edge–face interactions dominate its herringbone for 1–5 matches the trend reported for similarly substituted
packing. In addition, the experimentally determined powder X-ray compounds:42,43 (i) fluorinated arenes have more positive s-holes
diffractogram of 6 shows excellent agreement with that calculated on I or Br than those without fluorine atoms, (ii) iodides have
from the single-crystal structure.
more positive s-holes than analogously substituted bromides,
Single crystal structures 3 or 4 do not show Nꢀ ꢀ ꢀBr halogen and (iii) tetrafluorinated aryl bromides have more electropositive
bonding; each of these molecules have multiple fluorine atoms on s-holes than non-fluorinated aryl iodides. Although fluorination
the bromoaromatic rings, which we prepared with the hypothesis does indeed increase the positive potential of the s-holes of 1, 3,
that the more positive s-hole on Br would lead to stronger observed and 4, it also increases the positive potential at other locations
halogen bonding than in 5, as we observed in the comparison of 1 on the molecular surface, particularly the face of the haloarene,
and 2. In contrast, the bromine atoms in the crystal structure of 3 but also on the hydrogen atoms on the edges of both rings.
do not interact with the nitrile group, which instead participates in These other areas of positive potential are not sufficiently
s–p* interactions with electropositive carbon atoms on fluorinated charged to compete with Nꢀ ꢀ ꢀI halogen bonding, as evidenced
rings of nearby molecules; these edge–face interactions preclude by the crystal structure of 1; they do disrupt, however, the Nꢀ ꢀ ꢀBr
the formation of 2D bricklayers (Fig. 2). Electrostatically favorable halogen bonding present in non-fluorinated bromide 5, yielding
contacts of the bromine atoms of 3 also involve the tetrafluorinated instead edge–face (3) and other edge–edge (4) interactions of the
ring, with bromine interacting with (i) electronegative fluorine nitriles in spite of the deeper s-holes (by 20–30 kJ molꢁ1) on Br
atoms (d = 3.09, 3.14 Å, y = 1431, 1451)47 and (ii) electropositive of single crystals of these compounds. Therefore, considering
carbon atoms (d = 3.46–3.50 Å, y = 821–1051). Powder X-ray
diffraction analysis of 3 shows clear polymorphism, as the experi-
mental powder diffractogram is different from that calculated from
single crystal structure of 3 (ESI†).
We interpret these different diffractograms to be indicative
of polymorphism of 3, highlighting a delicate balance between
different non-covalent interactions. Although an effort to reduce
the electropositive nature of the face of the tetrafluorinated ring
of 3 by preparing difluorinated 4 did result in the absence of
significant edge–face interactions and the long axes of all the
molecules pointing along the same direction, the bromine atoms
Fig. 2 Packing of 3 in a single crystal, viewed along the crystallographic a
axis, with hydrogen atoms omitted for clarity and with thermal ellipsoids
shown at 50% probability. Edge–face interactions between the nitrogen of
the nitrile group and the electron-poor face of the fluorinated arene
contribute to herringbone-like packing.
Fig. 3 DFT calculated electrostatic potential maps of 1–5, using crystallo-
graphically determined geometries and the B3LYP/6-311+G(d,p) basis set. Iodine
was treated with the B3LYP/DGDZVP basis set. Scale bar is in kJ molꢁ1
.
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