Semiconducting organic assemblies prepared from tetraphenylethylene tetracarboxylic acid and bis(pyridine)s via charge-assisted hydrogen bonding

Article abstract of DOI:10.1021/ja203001z

Principles of crystal engineering have been applied toward the construction of supramolecular assemblies between an acid-functionalized tetraphenylethylene derivative and three different bis(pyridine)s [4,4′-bis(pyridyl)ethylene, 4,4′-bis(pyridyl)ethane,

Full text of DOI:10.1021/ja203001z

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Semiconducting Organic Assemblies Prepared from
Tetraphenylethylene Tetracarboxylic Acid and Bis(pyridine)s via
Charge-Assisted Hydrogen Bonding
Pradeep P. Kapadia,† Lindsay R. Ditzler,† Jonas Baltrusaitis,†,‡ Dale C. Swenson,†
Alexei V. Tivanski,† and F. Christopher Pigge*,†
†Department of Chemistry and ‡Central Microscopy Research Facility, University of Iowa, Iowa City, Iowa 52242, United States
S Supporting Information
b
in polyacenes (a second family of organic conductors which
ABSTRACT: Principles of crystal engineering have been
applied toward the construction of supramolecular assem-
blies between an acid-functionalized tetraphenylethylene
derivative and three different bis(pyridine)s [4,40-bis(pyridyl)-
ethylene, 4,40-bis(pyridyl)ethane, and 4,40-bipyridine]. Each
assembly was structurally characterized, and charge transfer
interactions within each sample were visually apparent.
Quantum chemical calculations were used to determine
crystal band structure and band gap magnitude, and elec-
trical properties of the materials were measured using
conducting probe atomic force microscopy (CP-AFM).
The crystals displayed charge-carrier capability, and the
magnitude of semiconductivity varied systematically as a
function of conjugation in the bis(pyridine) component.
Crystals incorporating 4,40-bis(pyridyl)ethylene and 4,40-
bipyridine displayed conductivities comparable to those
of established organic semiconductors (μeff = 0.38 and
includes tetracenes, pentacenes, perylenes, etc.) also leads to
greater conductivity.3 Consequently, several studies have de-
scribed efforts aimed at directing (controlling) solid state assembly
processes of TTFs, polyacenes, and TTF-acene hybrids to opti-
mize the electronic properties of bulk material.4
We are interested in utilizing tetraarylethylenes as starting
materials in crystal engineering approaches to a range of func-
tional materials. Tetraarylethylenes comprise a family of organic
compounds that possess interesting opto-electronic molecular
properties, such as low redox potentials and solid state
photoluminescence.5 Substituted tetraarylethylene frameworks
can be conveniently prepared in only a few synthetic operations
so that functional groups important in directing intermolecular
interactions (e.g., H-bond donors or acceptors) can be easily
incorporated. Coupled with the reasonably well-defined shape
and geometry of the tetraphenylethylene core, these compounds
are seemingly attractive solid state supramolecular building
blocks. Toward this end, we have prepared an electron-rich
tetraarylethylene derivative symmetrically functionalized with
four acetic acid groups. We report here the successful crystal-
lization of this tetraarylethylene derivative with several bis-
(pyridine) reagents to afford semiconducting organic materials.
We also demonstrate that the charge transport properties of
these crystals can be modulated as a function of the bis(pyridine)
component.
The compounds used in this study are illustrated in Figure 1.
The tetracid 1 was easily prepared from the corresponding
tetraphenol6 via alkylation with ethyl bromoacetate followed
by ester saponification. We initially envisioned that the carboxylic
acid groups in 1 would each serve as H-bond donors along
vectors approximating the sp2 geometry of the central alkene
carbons (despite the flexibility of the OCH2 linkers connecting
the acid groups to the tetraphenylethylene core). We reasoned
that combining the tetratopic H-bond donor 1 with linear
ditopic H-bond acceptors, such as bis(pyridine)s BPE, BPEt,
or Bpy, may afford 2D sheet structures with alternating rows
of tetraphenylethylene and bis(pyridine) components. Stack-
ing of these 2D sheets might then result in potentially
electroactive crystalline architectures featuring segregated
columns of electron-rich and -deficient components (i.e., 1 and
bis(pyridine), respectively).
1.7 ꢀ 10ꢁ2 cm2/V s, respectively).
3
chieving the ability to control the form of supramolecular
A
constructs resulting from the assembly of organic molecules
in the solid state is a central aim of crystal engineering. It is
anticipated that precise control over solid state assembly pro-
cesses will facilitate the synthesis of complex functional materials
imbued with desirable optical, electronic, magnetic, and/or
physical properties starting from carefully chosen yet relatively
simple molecular precursors.1 In turn, organic materials as-
sembled in this manner may exhibit distinct advantages over
their inorganic counterparts in terms of inter alia performance,
miniaturization, and mechanical flexibility.
In particular, the design of organic semiconductors is an area of
research that may benefit greatly from the development of
successful crystal engineering synthetic strategies. The magnitude
and efficiency of charge transport in solid state materials (crystals,
films, polymers) composed of single-component redox-active
organic compounds or multicomponent combinations of electron
donor/acceptor organic compounds have been shown to critically
depend on the relative orientation of molecular constituents. For
example, numerous theoretical and empirical studies involving the
prototypical organic conductor tetrathiafulvalene (TTF) and TTF
derivatives indicate that charge mobility is enhanced when TTF
components assemble in face-to-face intermolecular orientations.2
Likewise, the presence of extended cofacial areneꢁarene contacts
Received: April 1, 2011
Published: April 27, 2011
r
2011 American Chemical Society
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Figure 1. Crystallization components used in this study.
Figure 3. View of 1 Bpy slightly offset from the b axis illustrating
3
columns of 1 flanked by layers of Bpy. Disordered solvate molecules
omitted for clarity.
molecule of BPE bridges two adjacent molecules of 1 (as we
originally envisaged) to generate 2D layers (Figure 2a). Dis-
ordered solvent molecules (not shown) occupy the voids be-
tween adjacent molecules of 1.
The remaining carboxylic acid residues in 1 BPE are involved
3
in mediating the stacking of 2D layers through H-bonding
interactions with carboxylate groups in adjacent layers. Two
views of the extended packing are shown in Figure 2b,c. As a
consequence of these stacking interactions, individual 2D layers
are aligned 180° in a slightly offset fashion down the c axis to
produce an abab-type pattern. This results in well-defined
segregated columns of 1 and BPE (Figure 2c).
The single crystal structure of 1 BPEt was found to be
3
isostructural with 1 BPE. Charge-assisted H-bonding between
3
formally anionic carboxylate groups on 1 and dicationic bis-
(pyridinium)ethane units results in 2D layers analogous to that
shown in Figure 2a. Layers are then stacked as shown in Figure 2b,
c via additional CO2H ꢁO2C hydrogen bonds (see Supporting
3 3 3
Information (SI)). The only difference between the two structures
is the presence of a π bond linking the two pyridinium groups in
BPE, a structural feature that appears to exert a significant
influence over the conductivity of the material (vide infra).
The structure of 1 Bpy also features bis(pyridine) units
3
bridging molecules of 1 via charge-assisted H-bonding. In this
case, however, the crystal participants are arranged in a step-like
fashion rather than distinct layers. Nonetheless, segregated
columns of 1 are clearly evident (Figure 3). These columns are
separated by layers of Bpy molecules oriented with their long axis
roughly parallel with the direction of tetraarylethylene stacks.
The color of these crystals varied from pink to orange to pale
yellow as the bis(pyridine) component changed from BPE to
Bpy to BPEt. Coloration may indicate varying levels of charge-
transfer interaction in the solid state, presumably facilitated by
the electron-donating ability of 1 and the electron-accepting
abilities of the bis(pyridine)s (which are expected to be enhanced
by their conversion to bis(pyridinium) species in the crystals).
Structural evidence for charge-transfer interactions, however,
was not apparent from X-ray data. For example, elongation of
the central alkene CdC bond in 1 (as might be expected if 1
acquires partial radical cation character) was not observed. This
bond length ranged between 1.358 and 1.363 Å in the crystals
examined, and these values are similar to the bond length reported
for tetraanisylethylene (1.359 Å) and significantly shorter than the
bond length reported for the tetraanisylethylene radical cation
(1.417 Å).7 Consequently, the electronic structure of these crystals
was explored using quantum chemical calculations.
Figure 2. (a) 2D layer formed by charge-assisted H-bonding between
1 and BPE viewed down c (H-bonds shown as black lines). (b) Stacking
of 2D layers (down c) mediated by CO2H ꢁO2C H-bonding.
3 3 3
(c) View of the crystal down b illustrating the segregated columns of
1 and BPE. Disordered solvate molecules omitted for clarity. Crystals of
1 BPEt are isostructural with 1 BPE.
3
3
Combining 1 with 2 molar equiv of BPE, BPEt, or Bpy in a
mixture of acetone and methanol resulted in the deposition of
single crystals upon slow solvent evaporation. Contrary to ex-
pectations, however, crystals of 1:1 stoichiometry were obtained in
each case as determined by single crystal X-ray diffractometry. The
structure of 1 BPE illustrates many features common to all three
3
structures. Molecules of 1 adopt propeller-like conformations
typical of tetraphenylethylenes in the solid state.7 Two carboxylic
acid residues of 1 are engaged in charge-assisted H-bonding with
pyridine nitrogen atoms. From the position of the hydrogen
atoms and the similar CꢁO bond lengths in the relevant
carboxylic acid residues, we conclude that transfer of a proton
to the pyridine has occurred so that the H-bonding interaction
can be described as involving a pyridinium H-bond donor and a
carboxylate H-bond acceptor (N O distance = 2.59 Å). Each
3 3 3
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Figure 4. DOS calculation for 1 BPE. Fermi energy (Ef, dashed line)
3
corresponds to the energy of the HOCO.
Figure 5. (a) Representative 3D crystal image for 1 BPE. (b) Repre-
3
Table 1. Physical and Electrical Properties of
sentative IꢁV curves for 1 BPE (red crosses), 1 Bpy (blue circles), and
3
3
1 Bis(pyridine) Crystals
3
1 BPEt (black squares). The 1 BPE curve is scaled downward by a
3
3
factor of 100 to fit on the indicated axes.
Band
gapa
F
σ
μeff
(cm2/V s)c
crystal color (eV) (Ω cm)
(S cmꢁ1
)
performed on individual crystals in an insulating organic solvent
(bicyclohexyl) to prevent water layer contamination. Each
measurement consisted of recording a series of repeated currentꢁ
voltage (IꢁV) curves under 50 nN of force, which provided
sufficient contact between the crystal and the tip without
damaging the crystal. Each crystal was subjected to 15 IꢁV
measurements, and then the crystal was reimaged for comparison
to the original image. Crystals exhibiting significant differences in
images collected before and after conductivity measurements
were not used in data analysis. Representative IꢁV curves are
shown in Figure 5b for 1 BPE, 1 Bpy, and 1 BPEt. Note that
3
3
3
1 BPE pink
1.28 3.6 ( 0.9 0.28 ( 0.06
0.38 ( 0.05
3
1 Bpy orange 1.99 213 ( 70 (4.7 ( 1.3) ꢀ 10ꢁ3 (1.7 ( 0.4) ꢀ 10ꢁ2
3
1 BPEt yellow 2.47 >2.4 ꢀ 106 ncb
ncb
3
a Determined computationally from TDOS calculations. b Not calcu-
lated. c μ = σ0.76; see ref 10.
Experimentally obtained X-ray data were subjected to quan-
tum chemical analysis in the form of atomic position optimiza-
tion followed by density of states (DOS) calculations to identify
the crystalline orbitals involved in charge-transfer processes.8
3
3
3
the IꢁV curve for 1 BPE has been divided by a factor of 100 to
3
be displayed on the same scale as the curves for the other two
samples. From the representative data shown in Figure 5b it is
clear that crystals of 1 BPE are highly conductive, while 1 Bpy is
Atomic projections of densities of states for 1 BPE are shown in
3
Figure 4 as the sum of contributions from (1) all carbon atoms,
(2) CdC bonds, (3) OꢁC bonds, and (4) CO2 functional
groups in 1; (5) nitrogen and (6) carbon atoms in BPE; and
(7) CdC in BPE (bottom to top in Figure 4). The carbocycles in
1 emerged as the major contributors to the highest occupied
crystalline orbital (HOCO) located at ∼ ꢁ4.5 eV. The pyridyl
carbon atoms and the CdC in BPE are calculated to be the
dominant contributors to the lowest unoccupied crystalline
orbital (LUCO) located at ∼ ꢁ3.5 eV in Figure 4. The relatively
3
3
moderately conductive and 1 BPEt shows no measurable cur-
3
rent throughout the voltage bias range of the measurement.
The resistivity (F) of BPE and Bpy crystals was calculated
using the linear region of the IꢁV curves (bias range of (0.15 V).
This bias range was fit to Ohm’s Law to obtain the resistance, and
this value and values for crystal height (measured directly from
AFM images) and probe-sample contact area (calculated to be
1850 ( 50 nm2 using the Hertzian elastic model) were used to
determine resistivity (see SI for details). These data, along with
related conductivity values (σ), are shown in Table 1. The
small band gap found in 1 BPE (1.28 eV) provides a basis for the
3
apparent charge-transfer interactions. Similar computational
treatment of 1 BPEt and 1 Bpy yielded HOCO levels of
3
3
resistivity for 1 BPEt crystals could not be determined due to
comparable energy (ꢁ4.5 eV). The LUCO levels, however, were
shifted to slightly higher energies resulting in increased band gap
values of 2.47 and 1.99 eV, respectively (see Table 1 and SI).
The electrical properties of these crystalline assemblies were
then measured using conducting probe atomic force microscopy
(CP-AFM). Bulk microcrystalline samples were prepared in good
yield by concentration of equimolar solutions of 1 and BPE,
BPEt, and Bpy. Samples prepared in this way were found to be
homogeneous and possessed forms identical to those found in single
crystals as determined by PXRD. All three microcrystalline samples were
also found to be thermally stable up to ∼175 °C as determined by TGA.
For the CP-AFM studies, each sample was deposited on a
thermally evaporated Au substrate and imaged to determine
the crystal morphology and shape. A typical crystal image for
3
the absence of measurable current within the detection limit of
the instrument (10 pA). Thus, the resistivity value shown in
Table 1 (2.4 ꢀ 106 Ω cm) represents a lower limit.
3
Finally, we attempted to calculate effective charge mobilities (μ)
for the two semiconducting crystalline samples using the space-
charge limited current model.9 This approach was not suited to these
samples, however, as the data in both cases exhibited a nonlinear I vs
V2 relationship. Thus, charge mobilities were estimated using a
simple empirical model advanced by Brown that relates charge
mobility to conductivity (μ = σ0.76) as shown in Table 1.10
The data obtained from CP-AFM conductivity studies clearly
show that 1 BPE and 1 Bpy function as crystalline organic semi-
3
3
conductors. Indeed, the estimated μeff for 1 BPE (0.38 cm2/V s)
3
3
1 BPE is shown in Figure 5a. Crystals prepared from BPEt and
is comparable to charge mobilities determined for single
3
Bpy were similar in appearance. Electrical measurements were
crystals and crystalline films of well-established polyacene and
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thiophene-based organic semiconductors.4,11 Moreover, the re-
sults also demonstrate an ability to tune the semiconducting
properties of these tetraarylethylene crystals as a function of the
bis(pyridine) agent. The most conjugated bis(pyridine) reagent
(BPE) afforded crystals with 1 exhibiting a conductivity approxi-
mately 2 orders of magnitude greater than that for crystals
obtained from 4,40-bipyridine, while crystals derived from 1 and
nonconjugated BPEt were comparatively nonconducting. These
conducting properties correlate nicely with the calculated DOS
which revealed a systematic lowering of the lowest unoccupied
conduction orbital (LUCO) energy in the series 1 BPEt, 1 Bpy,
’ ACKNOWLEDGMENT
We thank the Department of Chemistry, University of Iowa. F.
C.P. thanks the College of Liberal Arts & Sciences and the
Obermann Center for Advanced Studies for sponsorship of a
Career Development Award.
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and 1 BPE (compare Figures 4, S16, and S17). Coupled with the
3
nearly identical HOCO energies across the series, the net effect of
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conductivity exhibited by 1 BPE and the absence of conductivity
3
in 1 BPEt despite their isostructural crystalline networks see-
3
mingly suggests a crucial role for the bis(pyridine) units as
principal charge carriers. In the case of 1 BPE, the combination
3
of protonated BPE molecules arranged in roughly cofacial
orientations and π-conjugation between facially stacked rings
may facilitate charge transport in two dimensions (see Figures 2b,
S1, and S2). In line with this conjecture, the diminished conjuga-
tionin1 Bpy may contribute totheattenuated conductivityinthis
3
sample while the absence of conjugation in BPEt renders this
crystal an insulator.12 This model relegates 1 to the role of a
crystalline scaffold that properly orients and activates (through
protonation/H-bonding) the pyridine components for charge
transport. In this regard, other relatively simple polycarboxylic
acids may fulfill a similar function and studies exploring this
possibility are underway. Alternatively, 1 may also contribute in
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and/or participation in extended H-bonded networks.13 The
synthetic accessibility of additional tetraarylethylene and bis-
(pyridine) derivatives should facilitate formulation of systematic
structureꢁactivity studies designed to shed light on this issue.
In conclusion, we have successfully prepared and character-
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substituted tetraphenylethylene and three bis(pyridine)s. Two of
these supramolecular assemblies were found to exhibit electrical
conductivities comparable to those of established organic semi-
conductors as determined through CP-AFM. Importantly, the
conducting properties of these crystals can be modulated as a
function of a bis(pyridine) partner, opening exciting opportu-
nities for construction of new tunable electroactive organic
materials. This study also illustrates the potential of tetraary-
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(12) We expect the magnitude of conductivity to also depend on the
orientation of the crystal with respect to the AFM surface and tip.
Measurements in this study were performed on randomly oriented
crystals, and so mobility values likely represent an average from multiple
crystal orientations.
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’ ASSOCIATED CONTENT
S
Supporting Information. Synthesis and characterization
b
details of 1 and 1 bis(pyridine) crystals, crystallographic data,
3
additional structural details, description of computational meth-
ods, details of CP-AFM measurements and conductivity calcula-
tions, PXRD, TGA, CIF files. This information is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
chris-pigge@uiowa.edu
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