A Boat-Shaped Tetracationic Macrocycle with a Semiconducting Organic Framework

Article abstract of DOI:10.1002/anie.201702019

We report the synthesis of a tetracationic macrocycle which contains two N,N′-bis(methylene)naphthalenediimide units inserted in between the pyridinium rings of the bipyridinium units in cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺ or “blue box”) an

Full text of DOI:10.1002/anie.201702019

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201702019
German Edition: DOI: 10.1002/ange.201702019
Organic Semiconductors
A Boat-Shaped Tetracationic Macrocycle with a Semiconducting
Organic Framework
Minh T. Nguyen, Matthew D. Krzyaniak, Magdalena Owczarek, Daniel P. Ferris,
Michael R. Wasielewski, and J. Fraser Stoddart*
Abstract: We report the synthesis of a tetracationic macrocycle
which contains two N,N’-bis(methylene)naphthalenediimide
units inserted in between the pyridinium rings of the bipyr-
idinium units in cyclobis(paraquat-p-phenylene) (CBPQT4+
or “blue box”) and describe the investigation of its potential
use in materials for organic electronics. The incorporation of
the two naphthalenediimide (NDI) units into the constitution
of CBPQT4+, not only changes the supramolecular properties
of the tetracation in the solid state, but also has a profound
influence on the electrochemical and electronic behavior of the
resulting tetracationic macrocycle. In particular, the solid-state
(super)structure, investigated by single-crystal X-ray diffrac-
tion, reveals the formation of a three-dimensional (3D)
supramolecular framework with ca. 2.8 nm diameter one-
dimensional (1D) hexagonal channels. Electrochemical studies
on solid-state thin films of the macrocycle show that they
exhibit semiconducting properties with a redox-conductivity of
up to 7.6 ꢀ 10À4 SmÀ1. Moreover, EPR and ENDOR spectros-
copies show that charge is equally shared between the NDIs
within the one-electron reduced state of the NDI-based macro-
cycle on the time scale of these techniques.
workers[1b] as “Texas-sized or burnt orange molecular boxes”
(Figure 1)—have also been investigated in the context of
anion recognition[10] and as supramolecular organic frame-
works.[11]
Figure 1. Structural formulas of CBPQT4+ or “blue box”, the Texas-
sized or “burnt orange box”, and NDITM4+ reported in this study.
Despite their long history in supramolecular chemistry,
organic polycationic macrocycles have not been investigated
extensively as materials with potential use in organic elec-
tronics, such as organic field-effect transistors (OFETs) and/
or photovoltaics. One of the obstacles, for cationic cyclo-
phanes in general, is the Coulombic repulsion between the
positively charged units in these macrocycles. The intrinsic
repulsion prevents intermolecular stacking and results in
materials devoid of the facile conductivity characteristics of
organic semiconductors.[12]
O
rganic polycationic cyclophanes, especially those capable
of hosting electron-rich guests by dint of donor–acceptor
interactions, have been employed[1] widely over the past few
decades in a number of different contexts. The scientific
community find these cyclophanes attractive, not only
because of the role they play as electron-deficient receptors
in molecular recognition processes,[2] but also on account of
their usefulness as building blocks for the construction of
more complex systems, i.e., mechanically interlocked mole-
cules (MIMs) such as catenanes,[3] rotaxanes,[4] and catenane-
or rotaxane-containing supramolecular architectures.[5]
Among these organic polycationic cyclophanes, pyridinium-
and imidazolium-based systems are the two most commonly
studied classes. The former compounds have been explored
widely for applications in molecular recognition[6] and molec-
ular machines, including switches,[7] attenuators,[8] and
memory devices.[9] The later compounds—imidazolium-
based cyclophanes, recently reported by Sessler and co-
Recent studies in our laboratory have revealed,[13] how-
ever, that the reduction of a pyridinium-based tetracationic
cyclophane—cyclo-bis(paraquat-p-phenylene)[14] (CBPQT4+
or “blue box”)—to its bis-radical dicationic state
[CBPQT2(C+)] facilitates the formation of an inclusion complex
+
with the methyl viologen (MVC ) radical cation. This complex,
which exhibits intermolecular stacking in the solid state
through radical-radical interactions,[13] has been considered as
a candidate for use in OFETs.[15] Although the reduced solid-
state superstructure is stable under ambient conditions for
weeks, oxidation back to the tetracationic state would,
however, induce molecular repulsion and device degradation,
limiting its long-term operation in air under ambient con-
ditions. We anticipated that incorporating air-stable building
blocks, which exhibit intermolecular p-stacking, into tetra-
cationic cyclophanes could increase the viability of these
systems as long-lasting molecules for electronic devices. In
addition, the electron-deficient characteristics associated with
tetracationic cyclophanes might also be beneficial when
[*] Dr. M. T. Nguyen, Dr. M. D. Krzyaniak, Dr. M. Owczarek,
Dr. D. P. Ferris, Prof. M. R. Wasielewski, Prof. J. F. Stoddart
Department of Chemistry, Northwestern University
2145 Sheridan Road, Evanston, IL 60208 (USA)
E-mail: Stoddart@northwestern.edu
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201702019.
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intergrated into organic semiconductors since they would
result in a material with high electron affinity, thereby
increasing the air-stability of the materials during cycling
operations.[16]
detected (Figure S2) in the gas phase at m/z = 697.113 and
1
416.442, respectively. Both the H and 13C NMR spectra of
NDITM·4PF6 support the existence of a single and highly
symmetrical compound in which the assignments of all the
resonances were confirmed (Figure S3 to S7) by two-dimen-
sional 1H-1H and 1H-13C correlation spectroscopy experi-
ments.
Herein, we report the design, synthesis, and character-
ization of
a
novel organic tetracationic macrocycle
(NDITM4+) that has naphthalenediimide units (NDIs)—
components used in the preparation of many organic semi-
conductors[17]—incorporated into the constitution of
CBPQT4+. The macrocycle was designed so that both the
organic-electronic properties inherent in the NDI units and
the electron-deficient nature of a tetracationic cyclophane
could be utilized simultaneously to produce an air-stable
system for potential use as an organic semiconductor. The
incorporation of two NDIs into the “blue box” archetype has
resulted in a novel tetracationic macrocycle NDITM4+ which
exhibits a unique kind of self-assembly to afford a supra-
molecular organic framework that displays semiconducting
properties with high electron affinity in the solid state.
The solid-state (super)structure (Figure 2a and S8) of
NDITM4+, which was determined unambiguously by X-ray
diffraction analysis of a single crystal obtained by the slow
evaporation of NDITM·4PF6 solution in MeCN during 3 days,
reveals a tetracation with a boat-like conformation. Two
crystallographically equivalent NDI units are positioned in
the hull of the boat conformation, separated by 4.6 ꢀ (defined
by the distance between the two closest hydrogen atoms), and
have a dihedral angle of 1358 relative to each other (Fig-
ure S11). Unlike the solid-state structure (Figure S13) of the
precursor DB2+ and other PMNDI-containing molecules,[18]
all pyridinium units in NDITM4+ are oriented (Figure S12)
toward the same side relative to the NDI unit to which they
are connected, supporting a dihedral angle of 688. This
conformation is not only commensurate with the formation of
the macrocycle, but it also facilitates the face-to-face inter-
molecular p–p stacking for NDI units (see below). The solid-
state superstructure of NDITM4+ reveals that it crystallizes in
the hexagonal space group P63/m and forms (Figure 2a)
a porous network with one-dimensional (1D) channels of ca.
28 ꢀ in diameter. Since solvent molecules and some of the
The synthesis of NDITM4+ was carried out using a well-
established synthetic approach (Scheme 1) for the prepara-
tion of cationic macrocycles. The starting material in the
synthesis—bis(4-pyridylmethyl)naphthalenediimide
(PMNDI)—was prepared from an imide condensation
between naphthalene-1,4:5,8-tetracarboxy dianhydride and
4-(aminomethyl)pyridine in DMF at 1308C overnight. Reac-
tion of PMNDI with 1,4-bis(bromomethyl)benzene, followed
by anion exchange from bromide to hexafluorophosphate
ions in aqueous MeOH, yielded the dibromide DB·2PF6. The
final cyclization to produce NDITM4+ involves the reaction of
DB·2PF6 with 1 equiv of PMNDI in the presence of tetra-
butylammonium iodide (TBAI) as a catalyst. The tetracat-
ionic macrocycle was precipitated from an aqueous solution
of NH4PF6, collected, and purified by column chromatogra-
phy (SiO2: 0.5% NH4PF6 in Me2CO) to yield NDITM·4PF6.
Reverse-phase analytical HPLC of this compound using
a H2O/MeCN (0.1% TFA) gradient elution revealed a pres-
ence of pure NDITM4+ eluting with a 32 min retention time
(Figure S1, Supporting Information). High resolution electro-
spray ionization-mass spectrometric (ESI-MS) analysis indi-
cated the existence of the NDITM·4PF6 with the loss of
À
PF6 counterions are disordered and exhibit partial occu-
pancy in these relatively large pores, they have been
eliminated during the refinement using an implemented
program in the crystallographic software.[19] Each hexagonal
channel is comprised of two alternating layers each containing
three NDITM4+ macrocycles that are positioned such that
every macrocycle establishes eight sets of short contacts with
+
=
four of its neighbors through [N ···O C] (mean dN···O
=
À
3.14 ꢀ) and [C H···O] (mean dC···O = 3.14 ꢀ) interactions
(Figure S12). Using these weak interactions, two repeated sets
of three NDITM4+ macrocycles are stacked in a coaxial
manner with a perpendicular rotation angle of 608 forming an
infinite hexagonal channel (Figure 2b). These 1D channels
2–4 PF6 counterions, i.e., [M-2PF6]2+ and [M-3PF6]3+ were
are held together by p–p interactions (dNDI-NDI
=
plane-plane
À
3.35 ꢀ, Figure 2c) between the NDI units. Consequently,
a three-dimensional (3D) superstructural framework with
infinite 1D channels (pore diameter ca. 20 ꢀ) along the c-axis
is formed. The incorporation of NDI units plays a critical role
in the formation of this supramolecular organic framework.
These units are involved, not only in intermolecular p–p
stacking, but also in short contacts between the carbonyl
groups and the bis(pyridinium)phenylene fragments.
The intermolecular p–p interactions of NDI units along
the c-axis in the solid-state superstructure of NDITM4+,
combined with charge hopping between the two NDI units
within the same NDITM4+ macrocycle (see below), implies
that electron transport could be occurring along this 1D
direction as NDI p–p stacking and charge hopping are
known[17] to cause increased charge mobility in n-type organic
semiconductors. In order to investigate the electronic proper-
ties induced by the NDI units, we have examined the
Scheme 1. Synthesis of NDITM4+. Reagent and conditions: (i) 1) 1,4-
Bis(bromomethyl)benzene/DMF/1108C/5 h, followed by 2) NH4PF6/
H2O. (ii) 1) PMNDI/ TBAI/DMF/1208C/24 h, followed by 2) NH4PF6/
H2O.
2
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Figure 2. a) Graphical representations of the single-crystal X-ray (super)structure of NDITM4+ showing the assembly into a 3D framework with
relatively large 1D channels. The SEM insert shows the extended hexagonal shape of one microscopic crystal. b) Plan and side-on views of
a representative 1D channel that has two consecutive layers (blue alternating with yellow and back) containing three NDITM4+ macrocyles as
a cyclic array in each layer. c) Side-on (left) and plan (right) views of the NDI stacks which propagate the extension of the 1D hexagonal channels
to afford a 3D framework. Counterions and hydrogen atoms have been omitted for the sake of clarity.
electrochemical properties of NDITM4+ at the molecular
level by carrying out cyclic voltammetry (CV) on this
tetracationic macrocycle. Two NDI reference compounds
(Figure 3)—one neutral (NRef) and the other dicationic
(CRef2+)—were also investigated in order to explore the
effects of both cyclization and the presence of the positive
charges on the redox properties of NDI units (see the SI). The
CV of NRef displays two reversible redox waves for NDI at
À593 and À1050 mV vs. Ag/AgCl, corresponding to the
formation of NDICÀ radical anions and NDI2À dianions,
respectively. On account of the effect of the positive charges
on the pyridinium units, the first reduction peak of CRef2+ is
shifted ca. 158 mV to a more positive potential (À435 mV),
while the second reduction (À830 mV) is quasi-reversible and
can be assigned to the reduction of the pyridinium units. See
the SI for the details of the assignments. On the other hand,
the CVof NDITM4+ exhibits three reversible reduction peaks
at À350, À820, and À1056 mV which can be assigned,
respectively, to 1) the formation of NDI radical anions,
2) the reduction of the pyridinium rings, and 3) the reduction
of the NDI radical anions to dianions. The significant anodic
shift of the first reduction peak in the CV of NDITM4+,
compared to that of either NRef (DE = 240 mV) or CRef2+
Figure 3. Cyclic voltammograms of NRef (a), CRef2+ (b), and
NDITM4+ (c) recorded at a 100 mVsÀ1 scan rate using a Pt working
electrode in 0.1m TBAPF6/MeCN electrolyte.
(DE = 85 mV), reveals the influence of both the positive
charge and the macrocyclic ring formation on the electron
affinity of the system. The lowest unoccupied molecular
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orbital (LUMO) energy of NDITM4+ was estimated to be
from a MeCN solution onto a Pt electrode, was recorded
(Figure S16) in a CH2Cl2 electrolyte at scan rates in the 25–
400 mVsÀ1 range. The linear relationship between the peak
current of the oxidation/reduction and the scan rates reveals
that the solid-state thin films of NDITM·4PF6 are highly
electroactive and porous to the charge-compensating ions
under these conditions,[23] a feature which is commonly
observed in organic semiconductive thin films. This observed
electroactive characteristic led us to investigate the electronic
properties by determining the electrical conductivity of the
NDITM·4PF6 thin film using a Pt interdigitated electrode
array (IDA, Figure 5a and b). Films of varying thickness were
À4.20 eV using the equation[20]
E
LUMO = À4.8À(Ered, onsetÀ
Eferrocene) eV. This low-lying LUMO level indicates that
NDITM4+ is a strong electron-accepting macrocycle and
promising for use as an n-type organic semiconductor.
In order to probe the extent of possible electron hopping
within NDITM4+ as arising from it being a macrocycle,
electron paramagnetic resonance (EPR) spectroscopy was
performed on the NDITMC(3+) and CRefC(+) reduced states
which are generated (Figure S15) by reacting the cationic
compounds with Zn dust in MeCN. The CW-EPR spectrum
(Figure 4a) of CRefC(+) and corresponding simulation agree
well with previously reported[21] EPR spectrum and hyperfine
coupling values for a NDI radical anion, where the unpaired
electron spin density is localized to a single NDI unit. The
EPR spectrum (Figure 4b) of the NDITMC(3+) radicals and
simulation, on the other hand, show a broadening and loss of
hyperfine features, a change consistent with the unpaired spin
density shared across two NDI units. An electron-nuclear
double resonance (ENDOR) experiment was performed in
order to confirm the proton hyperfine coupling values used in
the simulation of the CW-EPR spectrum of NDITMC(3+). The
1H CW-ENDOR spectrum (Figure 4c) of NDITMC(3+) and
simulation reveal an electron-to-proton hyperfine coupling
constant of 2.65 MHz. This value is about one half of the
hyperfine coupling constant found[22] for radicals localized on
a single NDI unit, indicating that the unpaired electron hops
between the two NDI units at a rate exceeding the ENDOR
experiment timescale[22] (ca. 107 sÀ1).
Figure 5. a) Photographs of a Pt-interdigitated electrode array (IDA)
used in the in situ conductivity measurements. b) Schematic illustra-
tion of the conductivity measurement setup using a 10 mm Pt IDA.
c) Solid-state CV and in situ conductivity of NDITM·4PF6 thin film.
prepared by spin coating NDITM·4PF6 from a MeCN solu-
tion. The conductivity of the film was calculated using a well-
established relationship between the observed drain current
and the bulk material conductivity.[24] See Equation (1) in the
SI. The solid-state CV and the potential-dependent conduc-
tivity profile of a 500 nm thick NDITM·4PF6 film were
recorded (Figure 5c) in CH2Cl2. The first conductivity peak
appears at À426 mV vs. Ag/AgCl which is approximately the
same potential as the first reduction peak observed in the
solid-state CV. The significant increase in conductivity can be
attributed to the generation of mobile radicals on the NDI
units upon reduction. The second, more negative and larger
magnitude, peak in the conductivity profile appears at ca.
À1121 mV and has a global maximum conductivity of 7.6 ꢁ
10À4 SmÀ1. This maximum conductivity was observed for
NDITM·4PF6 in a reduced state in which the macrocycle has
the highest possible number of radicals per macrocycle
(Figure S18), consistent with an increase in the conductivity
of an organic semiconductor upon chemical doping[25] to
generate free charge carriers. Although these measurements
show conductivity values of one-to-three order(s) of magni-
tude lower than those for conducting polymers and/or some
NDI-based materials,[24g,26] the fact that the solid-state super-
structure is composed of a large-pore framework, while the
Figure 4. CW-EPR Spectra of the CRefC(+) (a) and NDITMC(3+) (b)
radicals, generated by reducing the cationic compounds with Zn dust
in MeCN, recorded at 298 K. The corresponding simulations are
overlaid in black. The hyperfine coupling constants (in MHz) used for
the simulations are provided. c) 1H CW-ENDOR spectrum of
NDITMC(3+) radicals at 246 K.
In order to glean information regarding charge hopping
and molecular interactions of NDITM4+ under conditions that
are more relevant to organic electronics/devices, the electro-
chemical and electronic properties of the macrocycle were
investigated as solid thin films. Thanks to the exclusive
solubility of NDITM·4PF6 in MeCN, thin films prepared by
solution processing from a MeCN solution can be charac-
terized electrochemically in CH2Cl2—a very poor solvent for
NDITM·4PF6—without dissolvation from the surface. A scan-
rate dependence study of a NDITM·4PF6 thin film, drop-cast
4
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conductivity values are still comparable to materials used in
many organic electrodes,[27] suggests a potential use for this
tetracationic macrocycle as an organic semiconductor.
In summary, we have prepared a novel naphthalene-
diimide-based tetracationic macrocycle, which not only
adopts a unique (super)structure, but also displays semi-
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Experimental Section
NDITM·4PF6.
N,N-Bis(4-pyridylmethyl)naphthalenediimide
(PMNDI, 224 mg, 0.5 mmol) was stirred in dry DMF at 1208C
under a N2 atmosphere until all solid had completely dissolved. A
solution of DB·2PF6 (553 mg, 0.5 mmol) and TBAI (37 mg, 20%mol)
in dry DMF (20 mL) was added quickly to the above solution. The
reaction mixture was heated under N2 at 1208C for 1 day and cooled
to room temperature. The yellow precipitate was collected by vacuum
filtration and dissolved in a 1:1 mixture of H2O/MeOH (300 mL). An
excess of NH4PF6 was added to the solution, resulting in the
formation of a white solid which was collected by centrifugation,
washed with H2O (3 ꢁ 20 mL), MeOH (1 ꢁ 20 mL) and dried in vacuo.
The crude product was purified using column chromatography (SiO2:
0.5% NH4PF6 in Me2CO) and the organic solvents were removed by
rotary evaporation. The product was washed with H2O to remove
NH4PF6 and dried in vacuo to yield a white, crystalline solid
ˇ
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1
NDITM·4PF6 (290 mg) in 35% yield. H NMR (600 MHz, CD3CN,
298 K): d = 8.75 (s, 8H), 8.64 (d, J = 6.9 Hz, 8H), 8.00 (d, J = 6.9 Hz,
8H), 7.45 (s, 8H), 5.67 (s, 8H), 5.54 ppm (s, 8H). 13C NMR (125 MHz,
CD3CN, 298 K): d = 164.0, 159.0, 145.2, 135.8, 132.0, 130.8, 127.93,
127.8, 127.8, 64.4, 44.2 ppm. ESI-HRMS calcd for [MÀ2PF6]2+ m/z =
697.143 and [MÀ3PF6]3+ m/z = 416.442, found 697.143 and 416.442,
respectively.
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Acknowledgements
This research is part of the Joint Center of Excellence in
Integrated Nano-Systems (JCIN) at King Abdul-Aziz City for
Science and Technology (KACST) and Northwestern Uni-
versity (NU). The authors would like to thank both KACST
and NU for their continued support of this research. This
work was supported by the U.S. National Science Foundation
under grant no. CHE-1565925 (M.R.W.).
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Conflict of interest
The authors declare no conflict of interest.
Keywords: electron transport · macrocycles ·
molecular electronics · organic frameworks ·
organic semiconductors
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[24] In some respects, the microelectrode devices we have employed
resemble the OFET devices in which the offset potential [VD,
Manuscript received: February 23, 2017
Final Article published: && &&, &&&&
6
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These are not the final page numbers!

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Communications
Organic Semiconductors
Organic electronics: A tetracationic mac-
rocyle containing two naphthalene-
diimide (NDI) units and four pyridinium
rings was synthesized and investigated
for its potential use as an organic semi-
conductor. The incorporation of the NDI
units into the constitution of cyclobis-
(paraquat-p-phenylene) has resulted in
a macrocyle that exhibits a boat-shape
conformation and assembles into an
ultralarge-pore supramolecular frame-
work with semiconducting properties.
M. T. Nguyen, M. D. Krzyaniak,
M. Owczarek, D. P. Ferris,
M. R. Wasielewski,
J. F. Stoddart*
&&&& — &&&&
A Boat-Shaped Tetracationic Macrocycle
with a Semiconducting Organic
Framework
Angew. Chem. Int. Ed. 2017, 56, 1 – 7
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7
These are not the final page numbers!