Molecular Coplanarity and Self-Assembly Promoted by Intramolecular Hydrogen Bonds

Article abstract of DOI:10.1021/acs.orglett.6b03225

Active conformational control is realized in a conjugated system using intramolecular hydrogen bonds to achieve tailored molecular, supramolecular, and solid-state properties. The hydrogen bonding functionalities are fused to the backbone and precisely pr

Full text of DOI:10.1021/acs.orglett.6b03225

Letter
pubs.acs.org/OrgLett
Molecular Coplanarity and Self-Assembly Promoted by
Intramolecular Hydrogen Bonds
Congzhi Zhu, Anthony U. Mu, Yen-Hao Lin, Zi-Hao Guo, Tianyu Yuan, Steven E. Wheeler,
and Lei Fang*
Department of Chemistry, Texas A&M University, 3255 TAMU, College Station, Texas 77843, United States
S
* Supporting Information
ABSTRACT: Active conformational control is realized in a conjugated system
using intramolecular hydrogen bonds to achieve tailored molecular, supra-
molecular, and solid-state properties. The hydrogen bonding functionalities are
fused to the backbone and precisely preorganized to enforce a fully coplanar
conformation of the π-system, leading to short π−π stacking distances,
controllable molecular self-assembly, and solid-state growth of one-dimensional
nano-/microfibers. This investigation demonstrates the efficiency and significance
of an intramolecular noncovalent approach in promoting conformational control
and self-assembly of organic molecules.
olecular conformation represents one of the most
Mimportant structural features for organic molecules. In
a π-conjugated compound, the conformation not only shapes
the properties of individual molecules but also plays an essential
role in determining intermolecular interactions and self-
assembly behaviors. Generally, a coplanar backbone of a
conjugated molecule facilitates extended π-delocalization,1
consequently inducing enhanced intramolecular charge transfer
and a narrowed band gap.2 Moreover, coplanarity of conjugated
skeletons promotes strong intermolecular π−π interactions and
electronic coupling,3 leading to self-assembly in solution,4
formation of well-defined nano/microstructures,5 and con-
densed packing modes in the solid state.6 Therefore, pursuing
coplanar conformations becomes an important strategy in the
development of functional π-conjugated organic materials.
The prevailing strategy to achieve such a goal is the
construction of fused-ring constitutions by forming additional
strands of covalent bonds so that a coplanar conformation is
enforced. On one hand, this method is efficient in affording
excellent optoelectronic properties of individual molecules7 and
endorses strong intermolecular interactions, which can be
further translated into the formation of nano-/microscale
assemblies in the solid state.5b,8 On the other hand, the
covalently fused constitution often leads to synthetic and
processing challenges and does not allow active control over
the torsional conformation of these conjugated molecules.
Alternatively, the employment of intramolecular noncovalent
bonds, such as hydrogen bonds,2a,3,4c,9 B−N Lewis acid−base
pairing,2b,4c,10 metal coordination,2c and van der Waals forces,11
could serve as a promising approach to induce the desired
conformation in a π-conjugated molecule while enabling
dynamic conformational control. Unlike covalent bonds, the
thermodynamically driven formation of noncovalent bonds is
reversible,2a,9b,10a enabling on-demand control of molecular
conformation and therefore active manipulation over molec-
ular/supermolecular properties.9b
Among various types of noncovalent forces, hydrogen bonds
stand out as ideal candidates because of their tunable strength
and directionality,12 allowing for precise control of molecular
conformation. The introduction of conjugated coplanarity by
intramolecular hydrogen bonds, however, is still a challenging
task. For example, in a 2-phenylpyridine-based system (Figure
1a), the coplanarity is distorted by 2,2′-H,H repulsion even in
the presence of an intramolecular hydrogen bond.13 Such steric
repulsion can be avoided in a 1,2-diphenylethyne derived model
compound, in which linear intramolecular hydrogen bonds are
installed in parallel (Figure 1b).2a,9c,d,14 Modern theoretical
Figure 1. (a) An intramolecular hydrogen bonded 2-phenylpyridine
derivative with a dihedral angle measured from the single crystal
structure.13 (b) A p-phenylene ethynylene derivative with a parallel
linear hydrogen bond.16 (c) The structural formula of 1 and the
potential energy surface scan [B97D/6-31G(d,p)] of 1 by changing
the dihedral angle between the hydrogen bond donating units and
accepting units. (d) The structural formula and DFT optimized
conformation [B3LYP/6-311G(d,p)] of 2.
Received: October 27, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b03225
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Letter
calculations, however, suggested that the linear hydrogen bond
was not as favorable as a bent hydrogen bond in terms of the
strength.15 Moreover, most hydrogen bond donating and
accepting moieties were connected to conjugated backbones
through rotatable single bonds, which could lead to increased
flexibility and hence a greater entropy penalty when adopting
the coplanar conformation. In addition, although some
promising molecular properties have been reported,2a,9b the
expected self-assembly and favorable solid-state packing have
not been clearly demonstrated yet. Herein, we report the
achievement of unambiguous molecular coplanarity, control-
lable self-assembly, and solid-state fiber formations of a large
model π-system 1 (Figure 1c), in which well-defined intra-
molecular hydrogen bonds play a crucial role in governing these
characters.
Scheme 1. Synthesis of the Coplanar Model Compound 1
and Its Non-coplanar Counterpart 2
In compound 1, 1,4-bis(phenylethynyl)benzene was selected
as the backbone to avoid the aforementioned 2,2′-H,H steric
repulsion.2a,9c,d,14 The hydrogen bonding moieties were
incorporated into the backbone through a fused-ring
constitution. This strategy decreases the entropy penalty
when the molecule adopts the desired coplanar conformation
and, therefore, enhances the strength of the intramolecular
hydrogen bonds and the molecular rigidity. Meanwhile, these
preorganized hydrogen bonds adopted a bent structure (∠N−
H···O ≈ 165° measured from optimized geometry) and a
favorable binding orientation at oxygen.15,17 Using the reported
bond lengths from crystal structures of related building
blocks,18 the distance between hydrogen bonding nitrogen
and oxygen was estimated to be ∼2.84 Å (Figure 1c), falling
into the range where hydrogen bonds can be formed
effectively.19 In addition, condensed solid-state packing,3,6a
aggregation in solution, and self-assembly into well-defined
nano/microstructures5 were expected once the strong hydro-
gen bonds and high molecular rigidity were achieved.
To validate this molecular design, density functional theory
(DFT) was employed to examine the torsional energy
landscape of 1. By fixing the anthraquinone units into the
same plane, the torsional potential energy was computed by
changing the dihedral angle between the hydrogen bond donor
and the anthraquinone plane. The lowest energy was found in
the coplanar conformer. The 13.9 kcal/mol potential well
suggested that this coplanar conformation was strongly
stabilized by the hydrogen bonds (Figure 1c). In parallel, a
control molecule (2) was designed by inhibiting the hydrogen
bonds by methylation. Geometry optimization of this molecule
revealed a twisted backbone due to the steric interaction
between methly groups and the anthraquinone units (Figure
1d).
temperature, in contrast to that in its precursor 6 (7.72 ppm),
corroborating the formation of hydrogen bonds in solution at
room temperature (Figure S9). Because of the preorganized
geometry and rigid nature of the hydrogen bonding moiety, the
hydrogen bonds in 1 were expected to be strong and robust in
solution and in the solid state. Although variable-temperature
1H NMR experiments revealed a slight dissociation of these
hydrogen bonds at high temperatures, the chemical shift of the
N−H proton remained in the downfield region at 9.99 ppm at
65 °C in CDCl3 and 10.27 ppm at 80 °C in d6-benzene,
respectively (Figure S10). This result indicated that intra-
molecular hydrogen bonds maintained control over the
coplanar molecular conformation even in these boiling
solutions. The Fourier transform infrared spectra (Figure
S11) showed significantly weakened N−H stretching in 1
(3325 cm−1) compared to a non-hydrogen bonded amide N−
H stretching (3450−3460 cm−1),17 confirming these robust
hydrogen bonds in the solid state.
The solid-state structure of 1 was elucidated unambiguously
using single crystal X-ray diffraction. The distances between the
hydrogen-bond-donating nitrogen atom and the hydrogen-
bond-accepting oxygen atom were 2.819(7) Å and 2.840(7) Å
on each side respectively (Figure 2a), close to the predicted
values, falling into a strong-hydrogen-bond-forming distance.19
The bond lengths of the inner CO were longer than those of
the outer CO, in agreement with the decreased carbonyl
stretching frequency after accepting hydrogen bonds (Figure
S11). An almost coplanar backbone of 1 was observed (dihedral
angles less than 4° between the planes of the hydrogen bond
donor and acceptors). The ethynylene linker was slightly bent
The synthesis toward 1 started with compound 3 prepared
according to a literature report (Scheme 1).20 The linear n-octyl
chains were installed to increase the solubility of 1 without
perturbing the potentially strong π−π stacking between the
coplanar backbones.21 After reducing the nitro groups in 3 into
amine functionalities, the intermediate was treated directly with
molten urea, affording the hydrogen bond donating inter-
mediate (4). After bromination, Sonogashira coupling of 5 gave
intermediate 6. The final product 1 was obtained after
deprotection of 6 followed by another Sonogashira coupling
with 1-iodo-anthraquinone. Compound 2 was prepared from 6
as well. After methylation, the intermediate 7 was converted
into 2 using a similar tandem procedure.
Figure 2. (a) Front view and (b) side view of single crystal structure of
1. The thermal ellipsoids were scaled to the 50% probability level. (c)
A side view of the packing mode of 1 in single crystal. The octyl chains
and C2H2Cl4 solvent molecules are omitted for clarity.
For compound 1, a remarkable downfield shift of the N−H
proton signal (10.15 ppm) was observed in CDCl3 at room
B
DOI: 10.1021/acs.orglett.6b03225
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in the crystal structure due to the formation of hydrogen bonds.
The extended coplanar backbone allows for the condensed
packing with a π−π stacking distance of 3.30 Å (Figure 2c),
which is even shorter than the interlayer distance of graphite
(3.35 Å).22
In comparison to the control compound 2, the hydrogen
bond formation and the conformational coplanarity of 1
impacted its electronic structure by facilitating a more coherent
electronic conjugation of the backbone.2a,b,9c Despite the very
similar constitutional structures of 1 and 2, the absorption
onset of 1 was 20 nm red-shifted compared to that of 2 (Figure
S13). Furthermore, cyclic and differential pulse voltammetry
(Figures S15, S16) revealed a lowered LUMO level (−3.44 eV)
and a smaller band gap (1.98 eV) of 1 compared to that of 2
(2.04 eV).
solvent, such as DMSO, to a solution of nonpolar compounds
is expected to increase the aggregation because of the enhanced
hydrophobic interaction in between the solute molecules.23 For
compound 1, however, it was expected that DMSO should
decrease the association constant because of the disruption of
intramolecular hydrogen bonds. Indeed, by adding up to 14%
(volume fraction) of DMSO to a CDCl3 solution of 1, the
aggregation enthalpy became less negative and the association
constant decreased (Figure 3c). The hydrophobic aggregation
was not observed until the fraction of d6-DMSO was increased
over 17%, where the hydrophilic nature of DMSO started to
predominate. This observation confirmed that the intra-
molecular hydrogen bond was playing a crucial role to affect
the aggregation behavior of 1 by keeping the coplanar
conformation and locking the torsional motion. Such control-
lable aggregation behavior of 1 demonstrated the advantage of
the intrinsic dynamic nature of noncovalent bonds when used
for conformational control.
Figure 3. (a) The aromatic region of 1H NMR spectra of 1 in CDCl3
in different concentrations. (b) Temperature- and concentration-
dependent H NMR chemical shifts of proton f of 1, fitted with the
Figure 4. (a) SEM images of nano-/microfibers of 1 grown from slow
evaporation of CH2Cl2 solutions. (b) TEM images of these fibers with
the SAED pattern.
1
isodesmic model; the inset is a van’t Hoff plot of aggregation constants
at different temperatures. (c) Aggregation enthalpy of 1 in CDCl3/d6-
DMSO mixed solvents while increasing the volume fraction of d6-
DMSO.
The coplanar backbone, strong anisotropic π−π interactions,
negative aggregation enthalpy, and the linear peripheral octyl
chains distinguished 1 as an ideal candidate for the growth of
one-dimensional (1D) organic nano-/microstructures.21b From
a CH2Cl2 solution of 1, 1D fibers with a high aspect ratio (over
500) were obtained feasibly by slow evaporation (Figure 4a)
and vapor diffusion (Figure S18). Under scanning electron
microscopy (SEM), these fibers showed diameters ranging from
0.3 to 8 μm with the lengths reaching several millimeters
(Figure 4a). Under transmission eletron microscopy (TEM), a
series of ring-like diffraction patterns were observed on selected
area electron diffraction (SAED) images (Figure 4b),
suggesting a certain extent of ordered arrangements in these
fibers, consistent with the powder X-ray diffraction measure-
ment (Figure S20).24 In contrast, compound 2 did not exhibit
any self-assembled fibers due to the lack of the intermolecular
aggregation enthalpy as the driving force (Figure S19). Such a
distinctive difference between 1 and 2 indicated the significance
of using intramolecular noncovalent bonds in promoting the
self-assembly of molecules into well-defined solid-state
structures.
In conclusion, the investigation of model compound 1 and its
non-hydrogen bonded analogue 2 clearly demonstrated the
significant impact of preorganized intramolecular hydrogen
bonds on molecular conformation and intermolecular self-
assembly. These hydrogen bonds were designed to force the
molecules to adopt a thermodynamically stable coplanar
conformation. This robust coplanarity induced strong inter-
molecular aggregation and preferential self-assembly into 1D
fibers in the solid state. Overall, this work not only establishes a
fundamental correlation between intramolecular noncovalent
The robust coplanar conformation and tight intermolecular
π−π stacking of 1 were expected to lead to aggregation/self-
assembly of the molecule.23 To study this character, variable-
concentration 1H NMR investigations of 1 and 2 were
performed in CDCl3. At 295 K, as the concentration of 1
increased, the chemical shifts of the aromatic protons were
shifted upfield by ∼20 ppb (Figure 3a), indicating an
aggregation process.21b,23b In contrast, the spectra of 2 did
not show any significant upfield shift even at a much higher
concentration (Figure S17). Such a drastic difference was
attributed to the hydrogen bond induced coplanarity of 1,
which enhanced π−π stacking in solution and promoted the
aggregation, while the twisted backbone and active torsional
motion of 2 prevented efficient aggregation. The self-
association constant of 1 in CDCl3 was calculated to be 83
M−1 at 295 K, assuming an isodesmic aggregation model. Such
a large association constant was comparable with literature
reported rigid macrocyclic molecules.23a To further quantify the
thermodynamic parameters of this aggregation process, self-
association constants were measured at different temperatures
(Figure 3b). The van’t Hoff plot revealed the aggregation
enthalpy change of 1 to be −8.86
0.43 kcal/mol and the
entropy change to be −21.4 1.4 cal/(mol K), demonstrating
a strongly enthalpy-driven process.
To confirm the mechanism of the intramolecular hydrogen
bond-promoted self-assembly, the aggregation properties of 1
were investigated with the addition of DMSO, a hydrogen bond
competing solvent. In general, the addition of a hydrophilic
C
DOI: 10.1021/acs.orglett.6b03225
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.6b03225.
Detailed computation data; experimental procedures;
characterization data and crystallographic data for 1
(PDF)
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: fang@chem.tamu.edu.
ORCID
Steven E. Wheeler: 0000-0001-7824-6906
Lei Fang: 0000-0002-2888-5512
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Texas A&M University (TAMU) for support of this
work. We thank Dr. Nattamai Bhuvanesh for X-ray
crystallography and diffraction support, Dr. Sarbajit Banerjee
for support on electrochemistry. Use of the TAMU Microscopy
and Imaging Center, Mr. Rick Littleton, and use of TAMU
Materials Characterization Facility are acknowledged. We also
thank Dr. Yi Liu at the Molecular Foundry for facility support,
who was supported by the Office of Science, Office of Basic
Energy Sciences, of the U.S. Department of Energy under
Contract No. DE-AC02-05CH11231.
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