Isostructural Charge-Transfer Cocrystals Based on Triptycene End-Capped Quinoxalinophenanthrophenazine

Article abstract of DOI:10.1021/acs.cgd.0c01619

Charge transfer (CT) crystals of an electron donor and an acceptor have electronic properties that are different from the two pure components. Despite the great potential of such CT crystals have for organic electronics, the control of the spatial arrangement of donor and acceptor molecules in the cocrystals plays a crucial role for the charge transfer itself or, for example, for charge transport. In most cases, by changing e.g. the acceptor components, various cocrystals are accessible but in almost all cases with different packing motifs and thus crystallographic parameters. This makes a direct and systematic comparison of different donor-acceptor pairs difficult or even impossible. On the basis of a triptycene end-capped dimethoxy-quinoxalinophenanthrophenazine (QPP-OMe) as a donor with six small electron-deficient molecules as acceptors, an isostructural packing could be realized, where the QPP-OMe donor is forming voids with nearly the same orientation and size to enclathrate the acceptor molecules, as has been revealed by single-crystal X-ray diffraction. The CT complexes were studied spectroscopically, supported by density functional theory calculations.

Full text of DOI:10.1021/acs.cgd.0c01619

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Article
Isostructural Charge-Transfer Cocrystals Based on Triptycene End-
Capped Quinoxalinophenanthrophenazine
Lucas Ueberricke, Farhad Ghalami, Wen-Shan Zhang, Vaishnavi Rao, Frank Rominger,
̈
Rasmus R. Schroder, Marcus Elstner, and Michael Mastalerz*
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ABSTRACT: Charge transfer (CT) crystals of an electron donor
and an acceptor have electronic properties that are different from
the two pure components. Despite the great potential of such CT
crystals have for organic electronics, the control of the spatial
arrangement of donor and acceptor molecules in the cocrystals
plays a crucial role for the charge transfer itself or, for example, for
charge transport. In most cases, by changing e.g. the acceptor
components, various cocrystals are accessible but in almost all
cases with different packing motifs and thus crystallographic parameters. This makes a direct and systematic comparison of different
donor−acceptor pairs difficult or even impossible. On the basis of a triptycene end-capped dimethoxy-quinoxalinophenan-
throphenazine (QPP-OMe) as a donor with six small electron-deficient molecules as acceptors, an isostructural packing could be
realized, where the QPP-OMe donor is forming voids with nearly the same orientation and size to enclathrate the acceptor
molecules, as has been revealed by single-crystal X-ray diffraction. The CT complexes were studied spectroscopically, supported by
density functional theory calculations.
the whole range from insulators,34 over semiconduc-
INTRODUCTION
■
tors,25,31,33,35−52 conductors,53,54 and in rare cases even
The formation of organic cocrystals, which are composed of
two or more different molecules in a defined stoichiometry, is
an attractive route to create new materials with physical
superconductors,55−57 making CT crystals promising candi-
dates for applications as ferroelectrica58,59 for data storage or
ambipolar organic field-effect transistors.60−64
properties that can be entirely different from the individual
Despite the great potential of cocrystalline materials, their
components.1−6 For example, the solubility of pharmaceuticals
formation can be challenging, as cocrystallization requires
can be modified without changing the physiological effect.7 In
certain criteria, such as fitting geometrical demands65 or
the field of organic electronics, cocrystallization was used to
complementary supramolecular interactions, like hydro-
modify the π-stacking of acenes and phenazines by
gen2,8,10,66−70 or halogen bonding,67,71−76 or π stack-
encapsulation,8 hydrogen bonding,9−11 and host−guest recog-
ing.19,42,77−81 Co-crystallization can alter the packing in an
nition.12
unpredictable way, and very often polymorphism additionally
In this respect, charge-transfer (CT) crystals of an electron-
hampers the rational design of such materials.1,45,54,82−87 This
rich donor (D) molecule and an electron-deficient acceptor
gives rise to the demand for the synthesis and investigation of
(A) molecule have gained special attention as a method to
new cocrystals in order to gain a deeper insight into their
tune optoelectronic properties, depending on the interplay
formation and to elucidate their structure−property relation-
between the electronic structure of the individual components,
ship.
stoichiometry, their relative spatial arrangement, and their
During our ongoing research on triptycene end-capped
supramolecular interactions in the solid state.13−21 Here,
quinoxalinophenanthrophenazines (QPPs) as crystal engineer-
electron density is transferred from the highest occupied
ing synthons to promote and control the formation of dimers
molecular orbital (HOMO) of the donor to the lowest
with tight face-to-face π stacking of the QPP planes,88−92 we
unoccupied molecular orbital (LUMO) of the acceptor,
resulting in partially charged14,22 or, in the case of full electron
transfer, even ionic species.14,22 Typically, two different
Received: November 30, 2020
Revised: January 4, 2021
Published: January 13, 2021
stacking modes, namely, segregated or mixed stacks of donor
and acceptor molecules, are observed.13−19 The charge-transfer
interaction has a significant impact on the electronic structure,
which allows tuning of the electronic gap (“band gap
engineering”).23−33 For instance, the conductivity can cover
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Figure 1. (a) Molecular structure of QPP-OMe. (b) Crystal structure of QPP-OMe (solvent molecules omitted for clarity).91 The blue ellipsoid
indicates the void filled by disordered solvate (dπ,D‑D = 3.43 Å, dx = 4.11 Å, dx′ = 13.19 Å, dD‑D = 6.95 Å). (c) Acceptor molecules used in this study
for cocrystallization.
Figure 2. Single-crystal X-ray structures and optical micrographs (transmission mode) of QPP-OMe cocrystals. (a−c) QPP-OMe/DNT (dπ,A
=
3.30 Å, dπ,B = 3.34 Å, dD‑D = 6.65 Å, dO···H = 2.53 Å). (d−f) QPP-OMe/TCNQ (dπ,A = 3.28 Å, dπ,B = 3.37 Å, dN···H = 2.63 Å, dD‑D = 6.65 Å). (g−i)
(QPP-OMe)2(F4TCNQ) (dπ,A = 3.32 Å, dπ,B = 3.33 Å, dD‑D = 6.56 Å). (j−l) (QPP-OMe)2(PQDC) (dπ,A = 3.37 Å, dπ,B = 3.39 Å, dD‑D = 6.76 Å).
(m, n) QPP-OMe/FTCNQ (dD‑D = 6.65 Å). (o, p) (QPP-OMe)2(F2TCNQ) (dD‑D = 6.64 Å). Solvent molecules are omitted for clarity. π-stacking
is indicated by arrows. Disordered acceptor molecules in (g) and (i) are represented by a red ellipsoid.
found within the aforementioned series of more than 20 single-
crystal X-ray structures that the dimethoxy-substituted
triptycene end-capped QPP-OMe is an exception and did
not show continuous π-stacking in the solid state but a rather
different pattern, in which two adjacent QPP dimers were
separated by 6.95 Å, creating a “void” filled with disordered
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Figure 3. Isostructural packing of cocrystals of QPP-OMe with DNT (a), TCNQ (b), FxTCNQ (x = 1, 2, 4) (c−e), and PQDC (f). Disordered
acceptor molecules in (c, d) are represented by a red ellipsoid.
Table 1. Crystallographic Data of QPP-OMe and its Cocrystals with DNT, TCNQ, FxTCNQ (x = 1, 2, 4), and PQDC
QPP-OMe/
DNT
QPP-OMe/
TCNQ
QPP-OMe/
b
(QPP-OMe)2
b
(QPP-OMe)2
(F4TCNQ)
(QPP-
OMe)2(PQDC)
a
cmpd
space group
QPP-OMe
FTCNQ
(F2TCNQ)
all P1 (triclinic)
enclathrated solvent
molecules
x CHCl3
1 CHCl3
none
x CHCl3
x CHCl3
3 CHCl3
2 CHCl3
1 MeOH
15.58
19.52
20.04
108.1
104.4
107.9
5098.8
70 657
14 656
0.084
0.225
1.03
a [Å]
b [Å]
c [Å]
α [deg]
β [deg]
γ [deg]
cell volume V [Å3]
reflections collected
independent reflections
R1 (I > 2σ(I))
wR(F2) (I > 2σ(I))
GOF
11.56
14.29
16.01
88.6
89.4
71.7
2509.4
44 904
8897
0.096
0.262
0.95
11.79
13.98
15.77
85.7
88.9
73.9
2490.8
22 970
5322
0.113
0.281
1.03
11.69
14.7
15.58
84.8
84.4
69.5
2493.9
14 341
3070
0.186
0.404
1.49
11.63
14.26
15.93
86.4
88.4
73.5
2527.2
14 505
5236
0.132
0.339
1.31
11.71
14.01
16.03
86.1
89.5
74.7
2531.7
22 301
8828
0.113
0.301
2.03
15.61
19.5
20.03
107.4
105.3
109.6
4997.3
68 698
19 729
0.066
0.157
1.03
habitus
yellow
dark yellow
green bars
greenish bars
dark green needles brownish green
plates
dark yellow
needles
3.46
4.37
6.76
13.09
3.37−3.39
−131.2, −133.5
−469.4
needles
flakes
c
dπ,D‑D [Å]
3.43
4.11
6.95
13.19
3.40
4.32
6.65
12.48
3.30−3.34
−84.4, −92.7
−550.6
2
3.42
4.31
6.65
12.63
3.28−3.37
−87.3, −89.0
−488.6
2
3.44
4.46
6.65
12.82
3.49
4.38
6.64
12.96
3.41
4.44
6.56
12.75
d
dx [Å]
dD‑D [Å]
d
dx′ [Å]
e
dπ,D‑A [Å]
3.32−3.33
−105.5, −107.9
−1020.7
4
f
ED‑A [kJ/mol]
f
Etot [kJ/mol]
−250.7
2
1
g
Z
4
3
h
A.U.
2
2
3
a
b
c
Taken from ref 91. Not all data could be obtained due to heavy disorder of the acceptor molecule. π−π distances between two adjacent donors
d
(see Figure 1). Displacement of two donors along the long molecular axis (see Figure 1 and the Supporting Information for the determination
e
f
procedure). π−π distances between donor and acceptor molecules. Intermolecular potentials between π-stacked D/A dimers and total packing
energy calculated by the UNI force field method implemented in Mercury.99,100 Total number of molecules in the unit cell including refined
g
h
solvate molecules. Number of molecules in the asymmetric unit, solvent molecules not counted.
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solvent molecules (Figure 1).91 This was attributed to the
electron-enriched nature of QPP-OMe in contrast to the other
QPPs within this series, resulting in an increased electronic
repulsion.91
increases from dx = 4.11 Å in the parent structure to dx =
4.31−4.46 Å upon cocrystallization. The interplanar distance
between two QPP molecules is dD‑D = 6.95 Å for the pristine
QPP-OMe structure. Upon intercalation this distance gets
reduced to dD‑D = 6.64−6.65 Å with DNT, TCNQ, FTCNQ,
and F2TCNQ, while with F4TCNQ an even smaller distance is
found (dD‑D = 6.56) Å. A larger distance was measured in the
cocrystal with PQDC (dD‑D = 6.76 Å). At the same time the
displacement along the long axis decreases from dx′ = 13.19 Å
in the parent structure to dx′ = 12.48−13.09 Å in the cocrystals;
that is, the void (depicted in blue in Figure 1) gets smaller in
both dimensions. This contraction indicates an attractive
interaction with the acceptor molecules, which stabilizes the
cocrystal structures and is in agreement with intermolecular
potentials calculated by the UNI force field method
implemented in the program Mercury.99,100 Between adjacent
donor−acceptor pairs potential energies between ED‑A = −84.4
kJ/mol for QPP-OMe/DNT and ED‑A = −133.5 kJ/mol for
QPP-OMe/PQDC were calculated. As a result of this
additional stabilization the total packing energies of the
cocrystals with DNT, PQDC, TCNQ, and FxTCNQ (x = 1,
2) are between Etot = −469.4 and −550.6 kJ/mol. By far the
highest packing energy (Etot = −1020.7 kJ/mol) has (QPP-
OMe)2 (F4TCNQ).
We envisioned that, when crystallized in the presence of
electron-poor molecules, small enough to intercalate into these
voids, various donor−acceptor cocrystals should be possible
without altering the overall structure. This is to the best of our
knowledge still a difficult task and is very rarely reported for
other systems.8,42 In this context, CT crystals of meso-diphenyl
tetrathia[22]annulene[2,1,2,1] (DPTTA) as a donor and
Fxtetracyanoquinodimethane (x = 0, 1, 2, 4) or other
TCNQ-based acceptors are an exception with six structures
being either isomorphous or having an isostructural pack-
ing.31,36,37,93 With BTBT and BSBS (benzothienobenzothio-
phene and its selenium congener) an isostructural and
isomorphous cocrystal series with FxTCNQs (x = 0, 1, 2)
was reported as well.40
Here, we present our triptycene end-capping concept as a
crystallographic synthon to cocrystallize CT complexes, which
self-assemble with nearly the same crystal packing independent
of the chemical nature of the acceptor molecule, allowing a
deeper understanding of structure−property relationships.
QPP-OMe/DNT crystallized as yellow flake-like crystals in
a 1:1 stoichiometry with one molecule DNT, one QPP-OMe,
and one CHCl3 molecule in the asymmetric unit. Two adjacent
DNT molecules are sandwiched between two QPP planes via
π-stacking (dπ = 3.30−3.34 Å) in the void, where formerly
solvate was enclathrated in the pristine QPP structure (Figure
2a−c). The two DNT molecules interact with each other by
twofold weak-to-moderate hydrogen bonding between the
RESULTS AND DISCUSSION
■
For the formation of CT crystals, six small electron-deficient
compounds were chosen: (i) 2,4-dinitro toluene (DNT), a
common target analyte for the sensing of explosives,94 (ii)
phenanthroquinoxaline dicarbonitrile (PQDC), which has a
slightly larger aromatic system with pronounced π-stacking
tendency,95 and (iii) TCNQ and its fluorinated derivatives
FxTCNQ (x = 1, 2, 4), which are, due to their extremely low
LUMO levels, benchmark electron acceptors and are beneficial
for increased charge transfer.13,53,54,86,87,96,97 The synthesis and
crystal structures of QPP-OMe91 and PQDC95,98 have already
been described before. The cocrystals were all grown by vapor
diffusion of methanol into solutions of equimolar mixtures of
QPP-OMe and the various acceptors in CHCl3. Light to deep
green crystals were obtained with TCNQ, FTCNQ, and
F2TCNQ, whereas cocrystals with DNT, F4TCNQ, and
PQDC were yellow to brown (Figure 2).
oxygen of the nitro groups and the aromatic protons (dO···H
=
2.53 Å, θCH···O = 162.4°).101
QPP-OMe/TCNQ crystallized as green bars in a 1:1
stoichiometry with one molecule of TCNQ and one of QPP-
OMe in the asymmetric unit. No solvate molecules were
found. Similar to the structure with DNT, two TCNQ
molecules, which interact via weak hydrogen bonding67
(dN···H = 2.63 Å, θCH···N = 124.8°), are intercalated with dπ =
3.28−3.37 Å between the QPP planes (Figure 2d−f).
(QPP-OMe)2(F4TCNQ) crystallized as brownish-green
platelets in a 2:1 stoichiometry with three CHCl3 molecules
in the asymmetric unit. Here, F4TCNQ intercalates with dπ =
3.32−3.33 Å into the QPP stacks (Figure 2g−i). (QPP-
OMe)2(PQDC) crystallized also in a 2:1 stoichiometry with
two CHCl3 molecules and one molecule MeOH in the
asymmetric unit as dark yellow needles. As all other acceptors,
PQDC is enclathrated between the QPP planes. The π-
stacking distance between PQDC and the two adjacent QPP-
OMe molecules is dπ = 3.37−3.39 Å (Figure 2j−l). It is
noteworthy that, in the latter two CT crystals, these 2:1
stoichiometries were obtained, although all solutions contained
equimolar amounts of QPP-OMe and acceptor. This suggests
a clear preference for the 2:1 stoichiometry in cocrystals with
F4TCNQ and PQDC. Whereas in the latter case, the larger
size of PQDC prevents the intercalation of two molecules
between the same QPP planes, this is not necessarily the case
for F4TCNQ. Despite its similar size as TCNQ, only one
molecule is stacked between two QPP planes. Most likely the
additional hydrogen bonding between adjacent TCNQ
molecules, which is not possible in F4TCNQ, helps to stabilize
the structure. Table 1 summarizes all crystallographic details.
CRYSTAL STRUCTURE ANALYSIS
■
All cocrystals were found in the triclinic space group P1
̅
. To
our delight, as hypothesized a priori in all cases, isostructural
packing was found, in which the acceptor molecule is
sandwiched between π-stacked dimers of QPP-OMe, resulting
in one-dimensional D/An/D (n = 1, 2) stacks, without altering
the spatial orientation and the overall packing of QPP-OMe is
not affected by the intercalation. Its packing motif can
therefore be considered a kind of “host matrix” that
accommodates the acceptor molecules as guests (Figures 2
& 3). The A/D stoichiometry varies between 1:1 and 1:2
depending on the acceptor. Four cocrystals (DNT, TCNQ,
FTCNQ, and F2TCNQ) are isomorphous to the parent QPP-
OMe structure), whereas two cocrystals (PQDC and
F4TCNQ) show larger cell parameters and approximately a
doubled cell volume (for crystallographic details, see Table 1).
The π-stacking distance between two adjacent QPP molecules
remains almost constant in all six cocrystal structures and the
pristine QPP-OMe structure (dπ,D‑D = 3.40−3.49 Å), whereas
the displacement along the molecular long axis slightly
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Light green crystal bars were grown with FTCNQ, and deep
green needles were obtained with F2TCNQ. The acceptor
molecules are enclathrated between the QPP π-stacks. In both
cases they are heavily disordered and could not be crystallo-
graphically resolved (Figure 2m−p). To determine the D/A
pristine F2TCNQ. It is noteworthy that cocrystallization
occurred more readily from 1:1 mixtures, even for the 2:1
cocrystals, while in the 2:1 mixtures the donor and acceptor
had a higher tendency to crystallize separately. Therefore,
DCT measurements were performed for 1:1 mixtures
exclusively for all samples to ensure cocrystallization.
1
ratio, cocrystals were redissolved and analyzed by H NMR
spectroscopy. Whereas FTCNQ is enclathrated in a 1:1 ratio,
cocrystals with F2TCNQ are a 2:1 donor−acceptor complex.
Whereas for the binary mixtures of QPP-OMe with TCNQ
and FTCNQ a weak bathochromic shift toward vCT
cm−1 was observed, for QPP-OMe/F2TCNQ the band shifted
significantly by 8.6 cm−1 toward vCT = 2222.0 cm−1 (Figure
̃
= 2221.8
DEGREE OF CHARGE TRANSFER
̃
■
4a−c; see Figures S3−S5 in the Supporting Information for the
The intense green color of the binary compounds with TCNQ
derivatives (in contrast to the yellow/brownish color of the
pure components) already suggests that the bandgap in these
materials is significantly reduced compared to that of the
isolated compounds, which is typically observed for charge
transfer complexes.23−33 The degree of charge transfer (DCT),
which describes the amount of charge transferred from the
donor to the acceptor, defines the physical properties of the
CT material (e.g., conductivity) and depends, among other
factors, such as the interplay between intra- and intermolecular
interactions, on molecular packing, stoichiometry, electronic
coupling, and on the relative positions of the HOMO of the
donor and the LUMO of the acceptor molecules to each
other.13−19,23−33
The DCT was investigated by infrared (IR) and Raman
spectroscopy. For IR, an amorphous powder, obtained by
evaporating equimolar solutions in CHCl3 of QPP-OMe and
acceptor, was used, whereas Raman measurements were
performed on drop-casted thin films of such mixtures on a
glass substrate. For TCNQ and its fluorinated derivatives
certain vibration modes, such as the CN or CC stretching
vibrations (Raman), are sensitive to the negative charge on the
molecule.13,28,97,102−105 The bathochromic shift can therefore
be used to estimate the DCT from the donor (i.e., QPP-OMe)
to the acceptor molecule via the expression (eq 1)
Figure 4. (a−c) FT-IR spectra (ATR) of TCNQ, FTCNQ, and
F2TCNQ, their Li salts, and their 1:1-mixtures with QPP-OMe. Only
−1
ν12
ν0
̃
̃
i
y
2Δν
̃
j
z
j
j
j
j
z
z
z
z
DCT =
1 −
the region of the CN vibration is shown. (d) Raman spectrum (λex
=
2
ν0
̃
(1)
k
{
785 nm) of drop-casted thin films of F4TCNQ, LiF4TCNQ, and
QPP-OMe/F4TCNQ. Only the region of the CC vibration is
shown. Black: pristine acceptor, red: D/A mixture, green: acceptor
anion. For full spectra see Figures S3−S8 in the Supporting
Information.
with Δν = ν0 − νCT, where ν0, ν1, and νCT are the CN
vibrational bands of the pure acceptor, the acceptor anion, and
the charge transfer compound. For TCNQ and FxTCNQ (x =
1, 2, 4) we prepared the corresponding lithium salts according
to a literature-known procedure106 by reacting the TCNQ
derivative with lithium iodide (see the Supporting Information
for the experimental details) and analyzed the obtained deep
purple salts under the same conditions as described above for
the relevant ν1 frequencies. Their intensities were strongly
enhanced compared to those of the neutral compounds and
showed a splitting into several maxima, which can be attributed
full spectra). In the case of QPP-OMe/F4TCNQ the band
intensity was too weak to be clearly distinguished from the
noise. Thus, we relied on the CC vibration mode in the
corresponding Raman spectrum of a drop-casted thin film
(CHCl3).19,28,30,97,107−109 The measurements were conducted
with spatial resolution via an optical microscope, which
allowed the identification of cocrystalline areas on the thin
to vibrations of different symmetries.103 For Li(TCNQ) v1
2205.0 cm−1 was measured. For Li(FTCNQ) and Li-
(F2TCNQ) we found the most intense vibration at v1
2209.1 and 2213.9 cm−1, respectively. In the case of
Li(F4TCNQ) v1
= 2195.0 cm−1 was measured, which is in
̃ =
film. The C = C vibration was found at v0
pristine F4TCNQ and shifted toward v
CT = 1451.1 cm−1 in the
mixed film (Figure 4d). For LiF4TCNQ v
1 = 1441.0 cm−1 was
measured.
̃
= 1452.2 cm−1 in
̃
=
̃
̃
̃
agreement with the value reported in the literature (2194
According to eq 1, a weak charge transfer of 0.02e and 0.05e
was calculated for the QPP-OMe/TCNQ and QPP-OMe/
FTCNQ complexes. As is apparent from the small spectral
shifts, these low values are already near the detection limit for
this method (0.03e) and thus should be taken with care.14,110
Theoretical calculations (see discussion below) were in the
same range. In QPP-OMe/F2TCNQ the DCT strongly
increases to 0.52e. This value is one of the highest reported
cm−1).103 The CN vibrations of pristine TCNQ, FTCNQ, and
F2TCNQ are found at v0
̃
= 2222.1, 2222.5, and 2230.6 cm−1,
respectively. IR spectra of the binary materials were recorded
from 1:1 D/A mixtures in all cases. 2:1 mixtures were
investigated as well; however, no significant differences were
found for the system F4TCNQ/QPP-OMe. In the F2TCNQ/
QPP-OMe mixture, the CN vibration shift was the same as for
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Table 2. Vibrational Bands of Binary Mixtures of QPP-OMe with DNT, TCNQ, FxTCNQ (x = 1, 2, 4), and PQDC, the
a
Corresponding Acceptors in the Neutral and Anionic State and Degrees of Charge Transfer (DCT)
b
b
b
c
d
e
No.
cmpd
mode
v0
̃
[cm−1
]
v1
̃
[cm−1
]
vCT
̃
[cm−1
]
Δv
̃
[cm−1
]
DCTexp
DCTDFT
1
2
3
4
5
6
QPP-OMe/DNT
f
f
f
f
f
f
0.02 (A) - 0.01 (B)
0.04 (A) - 0.05 (B)
f
f
0.12
0.02
QPP-OMe/TCNQ
QPP-OMe/FTCNQ
QPP-OMe/F2TCNQ
QPP-OMe/F4TCNQ
QPP-OMe/PQDC
vCN
vCN
v
v
̃
̃
̃
̃
(IR)
(IR)
CN(IR)
2222.1
2222.5
2230.6
1452.2
f
2205.0
2209.1
2213.9
1441.8
f
2221.8
2221.8
2222.0
1451.1
f
0.3
0.7
8.6
1.1
f
0.02
0.05
0.52
0.10
f
CC(Raman)
f
a
Vibrational bands were obtained either by IR (ATR) of amorphous powders of equimolar D/A mixtures or by Raman spectroscopy (λex = 785
b
nm) of thin films of equimolar mixtures drop-casted onto a glass substrate. Vibration band of the pure acceptor molecule in the neutral state (ν0),
the anionic state (ν1), and in the charge transfer compound (vCT). = ν0 − νCT. Experimentally determined DCT. DCT calculated by Mulliken
population analysis (DFT-B3LYP/6-31G**). A and B refer to the D/A dimers as depicted in Figure 2. Not determined.
c
d
e
̃
̃
̃
f
Figure 5. Normalized absorption spectra of QPP-OMe and QPP-OMe/FxTCNQ (x = 0, 1, 2, 4) complexes in MeOH at saturation measured at rt
(for sample preparation, see the Supporting Information). (a) Whole spectra. (b) CT bands normalized to the most intense peak.
for mixed-stacked CT crystals, and only very few systems with
DCTs around or higher than 0.5e have been reported so far
(0.49−0.68e).13,111,112 Again, one exception is the previously
mentioned DPTTA-F4TCNQ system with an extraordinarily
high DCT of 0.92e.31 In the QPP-OMe/F4TCNQ mixture the
DCT decreases to 0.10e. It is worth mentioning that the
highest DCT is usually observed with the strongest acceptor
F4TCNQ, and this rather unusual behavior within this series
will be discussed below.
DCT was calculated individually for dimers A and B (see
Figure 2). Accordingly, for 2:1 cocrystals (QPP-
OMe)2(F4TCNQ) and (QPP-OMe)2(PQDC) a D/A/D
trimer was used. No calculations were performed on the
cocrystals with FTCNQ and F2TCNQ due to the disorder of
the acceptor. For complexes with TCNQ and F4TCNQ the
calculated values confirm the experimentally determined ones.
A DCT between 0.04e and 0.05e for the D/A dimers A and B
(see Figure 2) was computed for QPP-OMe/TCNQ, which is
in the same range as the DCT obtained by IR. For QPP-OMe/
F4TCNQ a DCT of 0.12e was found, which is close to the
value estimated from the Raman spectrum. Table 2
summarizes the relevant spectroscopic bands and the DCT
values.
In the IR spectrum of the binary mixture of PQDC/QPP-
OMe only a very weak CN vibration band was observed at vCN
̃
= 2231.5 cm−1 (see Figure S3 in the Supporting Information).
This might be explained by the already low intensity of this
band in pristine PQDC at ṽ
CN = 2240 cm−1,95 but it is also in
agreement with the calculated IR spectrum (density functional
theory (DFT)-B3LYP/6-31G**), which predicts a weak
oscillator strength of the CN vibration. For DNT and
PQDC no DCTs could be determined experimentally, as no
spectra of the corresponding anions are available, and
therefore, we had to rely on quantum-chemical calculations
based on the single-crystal structures, using the Mulliken
population analysis (DFT-B3LYP/6-31G**, Table 2).26,113 In
agreement with the experimental observations, only very weak
DCTs of 0.01−0.02e were calculated for QPP-OMe/DNT and
(QPP-OMe)2(PQDC).
OPTOELECTRONIC PROPERTIES
■
To get a closer insight into the effect of cocrystallization onto
the optoelectronic properties these were investigated by UV/
vis spectroscopy, recorded in saturated MeOH solution. For
this purpose, QPP-OMe was cocrystallized with the acceptor
compound from 1:1 mixtures in CHCl3, and the solid was
subsequently redissolved in MeOH (for experimental details
see the Supporting Information). Because of the low solubility,
the solutions were almost colorless. An exception from this was
QPP-OMe/F2TCNQ, displaying an intense green color. The
higher solubility of QPP-OMe/F2TCNQ is in agreement with
To take the different stoichiometries into account, for the
1:1 cocrystals QPP-OMe/TCNQ and QPP-OMe/DNT the
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Table 3. Optoelectronic Properties of Donor−Acceptor Compounds of QPP-OMe with DNT, PQDC, TCNQ, and FxTCNQ (x
= 1, 2, 4)
a
b
c
c
c
d
e
f
f
DFT
DFT
DFT
λmax
λonset
[nm]
Eg,opt
E
E
EDg FT
ΔE
tDe ‑A
tehff
teeff
HOMO
LUMO
D‑A
e
No.
cmpd
QPP-OMe
QPP-OMe/DNT
[nm]
[eV]
[eV]
[eV]
[eV]
[eV]
tDh ‑A [meV]
[meV]
[meV] [meV]
1
2
417
417
431
435
2.88
2.86
−5.47
−5.20
−1.95
−2.58
3.52
2.62
2.1
1 (dimer A) 1 (dimer
A)
144
99
1
3
22(dimer
B)
25 (dimer
B)
3
QPP-OMe/TCNQ
837
867
1.43
−5.54
−4.42
1.12
0.4
1 (dimer A) 1 (dimer
A)
0 (dimer B) 0 (dimer
B)
g
g
g
g
g
g
g
g
4
5
6
QPP-OMe/FTCNQ
(QPP-OMe)2(F2TCNQ)
(QPP-OMe)2(F4TCNQ)
842
841
854
873
874
915
1.42
1.42
1.36
n.a.
n.a.
n.a.
n.a.
n.a.
0.2
0.1
−0.2
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
67
n.a.
n.a.
4
g
g
g
g
g
g
n.a.
−5.35
−4.04
1.31
3 (dimer A) 3 (dimer
A)
16 (dimer
B)
19 (dimer
B)
7
(QPP-OMe)2(PQDC)
418
485
2.56
−5.44
−2.28
3.16
2.2
5 (dimer A) 6 (dimer
A)
18
2
50 (dimer
B)
58 (dimer
B)
a
b
c
Absorption maximum at the longest wavelength. Estimated from onset; Eg(opt) = 1242/λonset. Frontier orbital energies and electronic band gap
calculated by DFT-PBE0/6-31G**. For cocrystals supramolecular orbitals were calculated based on the crystal structures. Depending on the
stoichiometry either a D−A dimer or a D/A/D-trimer was used (see Figure 6). In the case of a D/A dimer only the one with the smaller gap was
d
considered (see Figure S18 in the Supporting Information for further details). ΔED‑A = ELUMO (acceptor) − EHOMO (QPP-OMe). See also Figure
e
6. Transfer integrals for hole (h) and electron (e) transport calculated by DFTB (see Figures 2 and S19 for dimer assignment and the Supporting
f
Information for details on the calculation). Effective coupling (superexchange) calculated by DFT-B3LYP/6-31G**. For hole coupling a D-A-D
triad was used, and for electron coupling a A/D/D/A cluster was used (see Figure S20 in the Supporting Information). Only the highest electron
g
transfer integral is shown (see Figure S20 in the Supporting Information for a complete list of clusters and integrals). No calculations could be
performed due to heavily disordered acceptor molecules in the X-ray crystal structures.
the high DCT of 0.52e discussed before, leading to an
increased ionic character and therefore increased polarity. On
the one hand, the QPP-OMe/DNT and QPP-OMe/PQDC
mixtures only gave the superimposed spectra of the single
compounds and no CT bands (see Figure S9 in the Supporting
Information), suggesting that charge transfer is negligible
between QPP-OMe and DNT or PQDC, which is in
agreement with the calculated DCT being close to zero (see
above & Table 2). On the other hand, significant differences
were found in the spectra of the mixtures with TCNQ and
FxTCNQ (Figure 5). Pristine QPP-OMe has absorption
maxima at λmax = 417, 394, 332, and 277 nm, corresponding
to the n−π* and π−π* transitions, respectively.91 An almost
identical pattern in this region is found in the mixtures with
FTCNQ and TCNQ. In the latter case the relative intensity of
the 0−1 n−π* transition at λmax = 394 nm is increased. With
F2TCNQ and F4TCNQ the relative intensity is strongly
enhanced and in the case of F4TCNQ also bathochromically
shifted to λmax = 435 nm. A weak shoulder at ∼480 nm was
observed in the spectra of QPP-OMe/TCNQ and QPP-
OMe/F4TCNQ, which may belong to a photoreaction product
of the TCNQ or F4TCNQ with solvent molecules.114,115
All TCNQ and QPP-OMe/FxTCNQ mixtures showed
similar, strongly red-shifted charge-transfer bands, with a
main peak close to the near-infrared region (λmax = 837−854
nm) and two other peaks of decreasing intensity at λabs = 741−
753 nm and λabs = 677−787 nm. These bands could not be
seen, when measured in CH2Cl2 or CHCl3, suggesting that the
polarized charge transfer state is stabilized in the polar solvent
MeOH.115 The onset wavelengths shift bathochromic from
λonset = 867 nm for QPP-OMe/TCNQ over λonset = 873 and
874 nm for QPP-OMe/FTCNQ and QPP-OMe/F2TCNQ to
λonset = 915 nm for QPP-OMe/F4TCNQ (Table 3). This
trend correlates well with the decrease in LUMO energy in the
order from TCNQ to F4TCNQ. The optical gaps, estimated
from the onset, lie within a narrow range of Eg,opt = 1.43 eV for
QPP-OMe/TCNQ and Eg,opt = 1.36 eV for QPP-OMe/
F4TCNQ.
Remarkable differences are found in the intensities of the
CT bands, which correlate with the DCT values determined by
IR/Raman spectroscopy (see Table 2). Weak CT bands are
observed for the combinations with low DCT values (QPP-
OMe/TCNQ and QPP-OMe/FTCNQ). For QPP-OMe/
F2TCNQ, having the highest DCT, the CT band is greatly
enhanced to become the most dominant absorption pattern of
the whole spectrum. The QPP-OMe/F4TCNQ mixture has
the second-most-intense CT band in agreement with the
second-highest DCT.
It must be emphasized that this trend of charge transfers
observed in our FxTCNQ series is contrary to what is
commonly found: charge transfer is usually favored with
decreasing band gaps. Thus, increasing the number of fluorine
atoms should also increase charge transfer. This was
experimentally confirmed for several CT complexes with
TCNQ and its fluorinated derivatives.28,31,86,87 The deviation
of our system from this trend indicates that also other factors,
such as the relative orientation of the donor and acceptor
molecules, have a significant influence on DCTs. The single-
crystal structures partially support this hypothesis: F4TCNQ is
located in the middle of the QPP plane on the pyrene unit,
where the orbital coefficients of the QPP HOMO are small,
and therefore only a weak interaction with the F4TCNQ
LUMO is possible.
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Figure 6. LUMO orbitals of the acceptors PQDC, DNT, TCNQ, and FxTCNQ (x = 1, 2, 4) and HOMO−LUMO diagram (DFT-PBE0/6-
31G**) of QPP-OMe and its cocrystals with DNT, TCNQ, F4TCNQ, and PQDC. For the cocrystals intermolecular hybrid orbitals were
calculated based on the crystal structures (see the Supporting Information for further details).
We were also interested in the solid-state optical properties.
Therefore, thin films were fabricated by drop-casting of
equimolar mixtures of QPP-OMe and acceptor compound in
chloroform, followed by annealing at 60 °C. The films were
largely amorphous, but crystallites were seen as well. Higher
crystallinity was found in the QPP-OMe/F4TCNQ film with
needles greater than 50 μm (see Figure S2 in the Supporting
Information for optical micrographs and absorption spectra of
thin films). For a pure QPP-OMe film three absorption
maxima at λabs = 427, 403, and 340 nm were found, similar to
the ones obtained from solution,91 with a bathochromic shift of
+10 nm of the most red-shifted absorption maximum. The
absorption spectrum of the film made from the mixture with
DNT did not change, except a slight redshift by +8 nm to λabs
= 435 nm. With PQDC the absorption bathochromically
shifted by +14 nm to λabs = 441 nm, accompanied by a peak
broadening. Although the films with TCNQ and its fluorinated
derivatives show charge transfer bands, the intensity is much
lower than that observed in solution, and instead of defined
peaks only a broad shoulder is observed (see Figure S1a the
Supporting Information). The emission spectra of these films
were measured as well. All spectra exhibited a single emission
maximum at λem = 557 nm, which was quenched in the mixed
films to different degrees as compared to the pristine QPP-
OMe film. No correlation to the DCT is observed, and thus
the fluorescence quenching is most likely caused by
aggregation rather than charge transfer (see Figure S1b in
the Supporting Information).
to the frontier molecular orbitals of QPP-OMe and the CT
complexes derived from the X-ray structures. The LUMO
energies of the acceptors are stabilized from ELUMO = −3.3 eV
for PQDC and ELUMO = −3.4 eV for DNT, to ELUMO = −5.1
eV for TCNQ and ELUMO = −5.3 to −5.7 eV for the
fluorinated TCNQ derivatives. This results in increasingly
smaller energy gaps (ΔED‑A = 0.5−0.2 eV) between the QPP-
OMe HOMO and the acceptor LUMO. For F4TCNQ the
LUMO level is even slightly lower than the HOMO of QPP-
OMe (ΔED‑A = −0.2).
For D/A clusters observed in the cocrystals of QPP-OMe
with DNT, TCNQ, F4TCNQ, and PQDC intermolecular
hybrid orbitals (DFT-PBE0/6-31G**) were calculated.24
Depending on the stoichiometry either a D/A dimer for the
1:1 complexes with DNT and TCNQ or a D/A/D trimer for
the 2:1 complexes with F4TCNQ and PQDC was used. In the
case of a D/A-dimer only the one with the smaller gap was
considered (dimer A in Figure 2; see the Supporting
Information for further details). The distribution of the
HOMO orbitals in all structures corresponds to the HOMO
of pure QPP-OMe, whereas the intermolecular LUMO
distribution is identical with that of the corresponding isolated
acceptor. In the QPP-OMe/PQDC trimer an additional
HOMO contribution is found on the PQDC molecule as
well. Minimal LUMO coefficients are located on the QPP-
OMe molecule in the case of the QPP-OMe/DNT dimer and
QPP-OMe/F4TCNQ trimer. The HOMO of the QPP-OMe/
DNT dimer is destabilized by 0.27 eV (EHOMO = −5.20 eV)
compared to that of pure QPP-OMe, whereas the LUMO is
stabilized by 0.63 eV (ELUMO = −2.58 eV). In the QPP-OMe/
PQDC trimer the HOMO energy is almost unaffected (EHOMO
= −5.44 eV), while the LUMO is lowered to ELUMO = −2.28
eV. Consequently, the corresponding electronic band gaps are
DFT CALCULATIONS AND CHARGE TRANSFER
INTEGRALS
■
Figure 6 shows the DFT-calculated LUMO orbitals (B3LYP/
6-311++G**) of the acceptors used in this work in comparison
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Eg = 2.62 and 3.16 eV. In the two CT-complexes with TCNQ
and F4TCNQ the LUMO orbitals are strongly stabilized by
2.47 and 2.09 eV toward ELUMO = −4.42 and −4.04 eV,
respectively. The HOMOs are within a similar range of EHOMO
= −5.54 and −5.35 eV, leading to narrow band gaps of Eg =
1.12 and 1.31 eV.
exception)31 among the highest reported and makes the
material interesting for thermo- or ferroelectrical applica-
tions.13,111,112
The effect of enclathration of further acceptors will be
investigated and reported in due course, as well as organic
electronic device applications.
To evaluate the charge transport properties, direct transfer
integrals t for hole (h) and electron (e) transport between the
QPP-OMe HOMO and the adjacent acceptor LUMO were
calculated using the DFTB method.116−126 In the three
cocrystal structures DNT, PQDC, and F4TCNQ substantial
coupling was found in the B dimers (see Figure 2). They lie
within a range between 16 meV (th) and 19 meV (te) for QPP-
OMe/F4TCNQ and between 50 meV (th) and 58 meV (te) for
QPP-OMe/PQDC. In QPP-OMe/TCNQ direct electronic
coupling is negligible (0−1 meV) for both electron and hole
transport in both dimers. In addition to direct coupling,
electronic coupling in mixed-stacked cocrystals can also occur
via a superexchange mechanism.25,27,41,127,128 In the case of
hole coupling the frontier orbitals of two donor molecules
interact by mixing with the orbitals of the “bridging” acceptor
between them. Electron coupling takes place accordingly
between the two acceptor molecules bridged by a donor
molecule. Interactions arising from superexchange are referred
to as effective transfer integrals teff. For their calculation we
applied an energy-splitting approach (DFT-B3LYP/6-31G**)
based on molecular clustersa D/A/D triad for hole transport
and an A/D/D/A tetrad for electron transport. While the latter
is negligible for all cocrystal structures, the effective hole
transfer integrals rise from tehff = 18 meV (QPP-OMe/PQDC/
QPP-OMe), over tehff = 67 meV (QPP-OMe/F4TCNQ/QPP-
OMe) and tehff = 99 meV (QPP-OMe/TCNQ/QPP-OMe) to
tehff = 144 meV (QPP-OMe/DNT/QPP-OMe). The last three
integrals are thus much larger than the direct coupling between
donor and acceptor, indicating that the superexchange is the
predominant pathway for charge transfer in these structures
and in the same range as reported for other TCNQ-based
cocrystals (e.g., 184 meV for benzidine/TCNQ).25,27,41,127,128
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.cgd.0c01619.
Experimental details on crystal growth and sample
preparation, spectra, crystal data (PDF)
Accession Codes
CCDC 2040320−2040325 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, or by
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
Michael Mastalerz − Organisch-Chemisches Institut and
Centre for Advanced Materials, Ruprecht-Karls-Universität
Heidelberg, 69120 Heidelberg, Germany; orcid.org/0000-
0003-3231-4816; Email: Michael.mastalerz@oci.uni-
heidelberg.de
Authors
Lucas Ueberricke − Organisch-Chemisches Institut and
Centre for Advanced Materials, Ruprecht-Karls-Universität
Heidelberg, 69120 Heidelberg, Germany
Farhad Ghalami − Institut für Physikalische Chemie,
Karlsruher Institute of Technology, 76131 Karlsruhe,
Germany
Wen-Shan Zhang − Centre for Advanced Materials and
Bioquant, Ruprecht-Karls-Universität Heidelberg, 69120
Heidelberg, Germany
Vaishnavi Rao − Centre for Advanced Materials and Institut
für Physikalische Chemie, Ruprecht-Karls-Universität
Heidelberg, 69120 Heidelberg, Germany
CONCLUSIONS
■
On the basis of a triptycene end-capped electron-rich QPP-
OMe, various CT crystals were grown and could be
systematically studied in terms of the effect that the acceptor
compounds have on the overall material, because these were
enclathtrated always in the same crystal voids. To the best of
our knowledge no such investigation was possible for any
similar CT system. In sum, six different acceptors were
successfully enclathrated allowing to fine-tune the degree of
charge transfer as well as the possibility for charge (hole)
transfer by superexchange mechanisms as has been revealed by
DFT calculations. In fact, one CT complex (QPP-OMe/
DNT) with a weak DCT but the right spatial arrangement of
donor and acceptor molecules increases the ability of charge
transfer significantly to teff = 144 meV, which is comparable to
similar CT compounds within the highest values re-
ported.25,27,41,127,128 Within the series of FxTCNQs (n = 0,
1, 2, 4) the CT complexes behave differently than all other
reported series based on these acceptors. Instead of increasing
DCT by increased acceptor strength (F4TCNQ > F3TCNQ,
etc.) in our system the largest DCT of 0.52 e was found for the
F2TCNQ, once more emphasizing how important the
molecular arrangement and control of this is for fine-tuning
electronic properties. The DCT of 0.52 e is (with one
Frank Rominger − Organisch-Chemisches Institut, Ruprecht-
Karls-Universität Heidelberg, 69120 Heidelberg, Germany
̈
Rasmus R. Schroder − Centre for Advanced Materials and
Bioquant, Ruprecht-Karls-Universität Heidelberg, 69120
Heidelberg, Germany
Marcus Elstner − Institut für Physikalische Chemie,
Karlsruher Institute of Technology, 76131 Karlsruhe,
Germany
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.cgd.0c01619
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
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Article
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ACKNOWLEDGMENTS
■
The authors are grateful to “Deutsche Forschungsgemein-
schaft” for supporting this project (within the collaborative
research center SFB1249 “N-heteropolycyclic compounds as
functional materials” (TP-A04, TP-B02, TP-B05, and TP-
C06).
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