Hydrogen Bonding Versus π-Stacking in Charge-Transfer Co-crystals

Abstract

We report a comparative study of two structurally similar donor-acceptor complexes with (1a) and without (2a) H-bonding using X-ray crystallography, spectroscopic analysis, and density functional theory calculations. H-Bonding enhances the donor-acceptor interactions, as manifested in a narrower band gap and shorter π-stacking distance in 1a versus 2a, despite 2 being a stronger donor than 1. Thin-film transistors of 1a showed ambipolar charge transport with "double dip"characteristics, whereas no transistor behavior was observed for 2a.

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Hydrogen Bonding Versus π‑Stacking in Charge-Transfer Co-crystals
Nathan Yee, Afshin Dadvand, Ehsan Hamzehpoor, Hatem M. Titi, and Dmitrii F. Perepichka*
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sıSupporting Information
ABSTRACT: We report a comparative study of two structurally similar donor−
acceptor complexes with (1a) and without (2a) H-bonding using X-ray
crystallography, spectroscopic analysis, and density functional theory calcu-
lations. H-Bonding enhances the donor−acceptor interactions, as manifested in
a narrower band gap and shorter π-stacking distance in 1a versus 2a, despite 2
being a stronger donor than 1. Thin-film transistors of 1a showed ambipolar
charge transport with “double dip”characteristics, whereas no transistor
behavior was observed for 2a.
Charge transfer (CT) complexes of π-electron donors (D)
and acceptors (A) have been of special interest as
electronic materials since the discovery of metallic conductivity
in the tetrathiafulvalene:tetracyanoquinodimethane complex
1
(TTF:TCNQ) in the 1970s. A number of attractive properties
such as superconductivity and ferromagnetism have been
observed in CT materials.
2−4
The emergence of organic
semiconducting devices such as organic light-emitting diodes
(OLEDs), organic photovoltaics (OPVs), and organic field-
effect transistors (OFETs) and the need of bicomponent
materials to optimize the efficiency of these devices has
motivated a renewal of interest in CT complexes.
5−13
Interaction between the highest occupied molecular orbital
(HOMO) of the donor and the lowest unoccupied molecular
orbital (LUMO) of the acceptor can result in charge transfer,
the degree of which (ρ) is governed by the difference between
the HOMO of the donor and LUMO of the acceptor
(ΔEoffset). Complexes with electrochemical ΔEoffset >∼0.4 eV
display a low ρ(essentially neutral in the ground state), while
significant CT in the ground state occurs for complexes with
lower electrochemical offset (e.g., TTF:TCNQ ΔEoffset ≈0.2
eV, ρ= 0.6), and such complexes often display metallic
conductivity, when crystallizedwithsegregatedD/A
stacks.
14,15
On the one hand, the self-assembly of CT materials with
low/moderate ρrelies on strategies such as shape comple-
mentarity (e.g., fullerene/annulene systems)
16,17
and specific
supramolecular interactions such as H-bonding and halogen
bonding.
18−20
On the other hand, the self-assembly of strong
CT complexes (high ρ) is primarily guided by CT interactions,
which lead to an electrostatic attraction of ionized D·+and A·−
components.
14,21
H-Bonding has also been used in the
assembly of strong D/A pairs.
22−25
For example, a single-
point H-bonding was used to control the assembly of a TTF-
imidazole donor with various acceptors, where the H-bonding
polarization strengthens the CT interactions, leading to a near-
IR (NIR) absorption and, often, metallic conductivity in
complexes with a large ΔEoffset (up to 0.7 eV).
22,23
Recently,
our lab reported that complementary two-
26,27
and three-
point
28−30
H-bonding can be used for a reliable coassembly of
various π-electron donors and acceptors with a relatively high
ΔEoffset (0.8−2.5 eV). In the case of unidirectional (two-point)
H-bonding we observed a strong enhancement of the D−A
interactions due to H-bonding.
26,27
However, not having X-ray
crystallographic data for corresponding D/A cocrystals without
H-bonding, the understanding of the effect of H-bond
polarization on the CT interactions relied on the density
functional theory (DFT) predictions and solution studies.
Here we report two D/A cocrystals (1a and 2a) consisting
of a diindolopyrrole (DIP) π-donor and 2,7-dinitrophenan-
thraquinone acceptor. The two N−Hbondsin1are
complementary with the two COonaenabling two-point
H-bonding. However, no H-bonding is possible for 2, since the
N−H protons are replaced with ethyl groups. The ΔEoffset
between the individual components 1or 2and a(0.5−0.7 eV)
can be classified as “intermediate”(vide infra) and are expected
to lead to partial ρ, lying between strong CT complexes such as
TTF-TCNQ and the weaker H-bonded complexes that we
previously studied.
26,27
We investigated the role of H-bonding
on the electronic structure using X-ray crystallography, DFT
calculations, vis−NIR absorption, and electrochemical meas-
urements, photoelectron yield spectroscopy, and electrical
characterization in OFET devices.
Cyclic voltammograms (CV, Figure S1) of DIPs 1and 2
exhibit reversible oxidations at 0.00 and −0.19 V versus
ferrocene (Fc), respectively. The lower oxidation potential of 2
compared to 1results from the electron-donating ethyl
Received: March 20, 2021
Revised: April 6, 2021
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substituents. Quinone aexhibits a reversible reduction at
−0.70 V versus Fc. From the CV measurements, HOMO
energies of −4.80 eV (1) and −4.61 eV (2) and a LUMO
energy of −4.10 eV (a) were estimated, corresponding to
ΔEoffset of 0.70 eV (1a) and 0.51 eV (2a). Such small
HOMO−LUMO differences are expected to lead to CT
interactions and the formation of charge-transfer complexes
regardless of H-bonding. This is in contrast to our previously
studied systems in which the D/A interactions are weak
(ΔEoffset > 0.8 eV), and no coassembly takes place without H-
bonding.
26,27
Co-crystals were obtained by a slow evaporation from
dimethylformamide (DMF) (1a) or a 1:1 mixture of dioxane/
chlorobenzene (2a). The X-ray crystallographic analysis of
both complexes shows mixed π-stacks of alternating D and A
components (Figure 1). 1a also displays two-point H-bonding
between the D and A molecules of adjacent stacks (N−H···
OC contacts ∼2.2 Å), while such interactions are not
possible in 2a. The π−πdistances in 1a (3.3−3.4 Å) are
slightly shorter than in 2a (3.4−3.5 Å), which may be due to
stronger D/A interactions in the former
22,23,26,27
(vide infra),
although we also cannot exclude the possible steric effects from
the ethyl groups in the latter.
We investigated the electronic properties and donor−
acceptor interactions in both cocrystals by DFT using
crystallographic geometries optimized only for the position
of hydrogen atoms (which are not reliably determined by an X-
ray analysis).
31
Interestingly, the calculations show that, unlike
in similar H-bonded complexes of a weaker donor and
acceptor,
26,27
the LUMO of 1a is not fully localized on the
acceptor component, resulting in a substantial HOMO/
LUMO orbital overlap despite the absence of any π−π
contacts between the components (Figure S4). The HOMO/
LUMO overlap integrals (SHL) determined by wave function
analysis
32
indicate that the D/A interactions in a H-bonded
(edge-to-edge, denoted --) complex (SHL = 0.232) are nearly as
strong as in a π-stacked (face-to-face, denoted =) complex (SHL
= 0.288). However, for 2a, predictably D/A interactions are far
weaker for a (--) complex (SHL = 0.022) compared to a (=)
complex (SHL = 0.175).
Figure 2a shows the perturbation of the orbital energies due
to H-bonding and π-stacking for 1a. The calculated ΔEoffset
between the individual 1and ais 0.65 eV, but the HOMO−
LUMO gap (HLG) of the complex (1a) in the gas phase is
only 0.52 eV. This is a result of H-bonding polarization, which
destabilizes the HOMO of the donor and stabilizes the LUMO
of the acceptor.
26,27
Subsequent π-stacking of the complex
leads to a strong CT, which, in contrast, stabilizes the HOMO
and destabilizes the LUMO and thus increases the HLG to
1.12 eV. For comparison, the π-stacked complex 2a shows a
larger HLG of 1.24 eV, despite the smaller calculated ΔEoffset
between 2and a(0.58 eV). Unfortunately, it was not possible
to directly measure the orbital perturbation by H-bonding in
solution by CV measurements
26,33
due to the extremely low
solubility of 1a in noncompeting solvents. The photoelectron
yield spectroscopy in air (PESA) of cocrystals showed that the
ionization potentials (IP) of 1a and 2a are slightly (∼0.1 eV)
higher compared to those of pure 1and 2(Figure 2c).
We also calculated the one-dimensional (1D) band structure
(Figure 2b) on infinite π-stacks of H-bonded pairs of 1a
(1a(||)) and compared it to a single stack of 1a (1a(|)), which
exhibits the same π-interactions but no H-bonding. The H-
bonded stack 1a(||) exhibited a smaller band gap (1.46 eV) and
higher band dispersion energies along the direction of π-
stacking than 1a(|) (band gap 1.80 eV), suggesting that H-
bonding strengthens the electronic interaction in the crystals.
The relatively low dispersion energies for both HOMO (∼60
meV) and LUMO (∼130 meV) bands were expected based on
Figure 1. Crystal structures of complexes with π−πdistances
displayed. Phenyl groups have been omitted for clarity. Note that
1a shows two types of H-bonded π-stacks with interplanar angle, 6.6°
and 33.6°, between the H-bonded molecules.
Figure 2. (a) Calculated [B3LYP/6-31G(d)] orbital energies of the individual 1,2, and a(red lines), the H-bonded complex 1a (black lines), and
π-stacked clusters (green lines) observed in cocrystals 1a and 2a (full DFT optimization was done for individual molecules and the H-bonded
complex, while only the position of the hydrogen atoms was optimized in crystallographic structure of the clusters). (b) Band structure calculations
for infinite stacks of complexes: (left) 1a(||) denotes two stacks that are H-bonded to one another; (right) 1a(|) denotes a single π-stack (no H-
bonding). (c) PESA measurements for 1,2,1a, and 2a.
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Cryst. Growth Des. XXXX, XXX, XXX−XXX
B

their mixed stacking arrangements. For reference, for the
metallic TTF-TCNQ, which exhibits segregated stacks, the
dispersions were calculated to be 0.65 eV for the HOMO and
0.7 eV for the LUMO, in agreement with experimental
estimates (∼0.5 eV).
34
The optical absorption properties of the complexes were
studied via diffuse reflectance UV/vis/NIR spectroscopy
(Figure 3a). Both complexes exhibited low-energy CT bands
with a red edge at 1500 nm (1a) and 1200 nm (2a). We
applied the Kubelka−Munk transform to the diffuse reflectance
spectra to extract the band gaps, which were determined to be
0.86 eV for 1a and 1.07 eV for 2a, in agreement with values
obtained by DFT (Figure S3). Thus, 1a exhibits a band gap
that is 0.21 eV narrower than that of 2a despite 2being a
stronger (by ∼0.2 eV) donor than 1.
The infrared spectra of cocrystals of 1a exhibited a
significant 70 cm−1red-shift in the N−H stretching vibrations
versus that of 1indicating strong H-bonding interactions
(Figure 3b). Notably, the CO vibration stretch of 1a is also
red-shifted by 40 cm−1versus that of a, which is indicative of a
reduction in bond order and has been used to estimate the
charge transfer degree ρin CT complexes.
35
By comparison, a
much smaller (but still substantial) red-shift of the ν(CO) =
17 cm−1versus pure awas observed for 2a suggesting the
lower ρin non-H-bonded complex. Comparing the DFT
calculated ν(CO) for the neutral aand the radical anion
(a.−), the full charge transfer (ρ= 1) should result in an 80
cm−1red shift of the ν(CO), and we estimated ρ= 0.5 for
1a and ρ= 0.2 in 2a (Figure S5).
While such donor−acceptor complexes with strong charge-
transfer interactions often display a pronounced electron
paramagnetic resonance (EPR) signal,
26
which was indeed
originally observed for 1a and 2a, after a careful purification of
the D/A components the freshly prepared complexes were
found to be EPR silent (Figure S2). This should not be
surprising considering the band gap in these CT complexes is
much larger than kT (0.026 eV).
We prepared thin-film OFETs by a vacuum cosublimation of
the H-bonded complex 1a on prepatterned SiO2/Si substrates.
The output and transfer characteristics are displayed in Figure
4and exhibited balanced ambipolar transport although with
relatively low hole and electron mobilities (μh=μe=10
−5
cm2/(V s)). An unusual “double-dip”(two minima in
conductance) was observed in the transfer curves. A similar
behavior has been reported in one of our previously published
H-bonded thin film OFETs.
27
In contrast, 2a did not show any
significant charge transport, which could be due to larger π−π
distances in its stacking arrangement.
In conclusion, we synthesized two structurally similar CT
complexes based on diindolopyrrole donors (1and 2) with a
dinitrophenanthraquinone acceptor (a). Complex 1a is
assembled via two-point DD···AA hydrogen-bonding and π-
stacking interactions, while only the latter is observed in 2a,as
shown by X-ray crystallographic analysis and IR spectroscopy.
Diffuse reflectance measurements reveal the narrower band gap
(by 0.21 eV) and larger degree of charge transfer (ρ= 0.5 vs
0.2) for 1a compared to 2a, despite 2being a stronger electron
donor than 1. DFT calculations show that such a difference
arises from a complex relationship between the HOMO/
LUMO of the individual components, D/A interactions, and
H-bond polarization. 1D band structure calculations show that
H-bonding enhances the electronic interactions in the crystals,
manifested by higher HOMO/LUMO band dispersions. Thin-
film OFETs of the H-bonded complex 1a exhibited ambipolar
charge transport with an unusual double dip characteristic,
while 2a did not exhibit transistor properties.
■ASSOCIATED CONTENT
*
sıSupporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.cgd.1c00309.
Experimental details, cyclic voltammograms, EPR,
Kubelka−Munk plots, additional computational details
(PDF)
Figure 3. (a) Diffuse reflectance spectra of complexes 1a and 2a. (b)
Infrared spectra of individual components 1and awith cocrystals 1a
and 2a.
Figure 4. The (a) output and (b) transfer characteristics of bottom-
contact bottom-gate OFETs based on a thin film of 1a.
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Cryst. Growth Des. XXXX, XXX, XXX−XXX
C

Accession Codes
CCDC 2063558−2063559 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif,orby
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
■AUTHOR INFORMATION
Corresponding Author
Dmitrii F. Perepichka −Department of Chemistry, McGill
University, Montreal QC H3A 0B8, Canada; orcid.org/
0000-0003-2233-416X; Email: dmitrii.perepichka@
mcgill.ca
Authors
Nathan Yee −Department of Chemistry, McGill University,
Montreal QC H3A 0B8, Canada; orcid.org/0000-0003-
1748-6484
Afshin Dadvand −Department of Chemistry, McGill
University, Montreal QC H3A 0B8, Canada
Ehsan Hamzehpoor −Department of Chemistry, McGill
University, Montreal QC H3A 0B8, Canada
Hatem M. Titi −Department of Chemistry, McGill University,
Montreal QC H3A 0B8, Canada; orcid.org/0000-0002-
0654-1292
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.cgd.1c00309
Funding
This work was funded by NSERC Canada. E.H. acknowledges
an MITACS fellowship.
Notes
The authors declare no competing financial interest.
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