X-Shaped Oligomeric Pyromellitimide Polyradicals

Article abstract of DOI:10.1021/jacs.7b12124

The synthesis of stable organic polyradicals is important for the development of magnetic materials. Herein we report the synthesis, isolation, and characterization of a series of X-shaped pyromellitimide (PI) oligomers (X_n-R, n = 2-4, R = Hex or Ph) linked together by single C-C bonds between their benzenoid cores. We hypothesize that these oligomers might form high-spin states in their reduced forms because of the nearly orthogonal conformations adopted by their PI units. ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopies confirmed the isolation of the dimeric, trimeric, and tetrameric homologues. X-ray crystallography shows that X₂-Ph crystallizes into a densely packed superstructure, despite the criss-crossed conformations adopted by the molecules. Electrochemical experiments, carried out on the oligomers X_n-Hex, reveal that the reductions of the PI units occur at multiple distinct potentials, highlighting the weak electronic coupling between the adjacent redox centers. Finally, the chemically generated radical anion and polyanion states, X_n-Hex^?- and X_n-Hex^n(?-), respectively, were probed extensively by UV-vis-NIR absorption, EPR, and electron nuclear double resonance (ENDOR) spectroscopies. The ENDOR spectra of the radical monoanions X_n-Hex^?- reveal that the unpaired electron is largely localized on a single PI unit. Further reductions of X_n-Hex^?- yield EPR signals (in frozen solutions) that can be assigned to spin-spin interactions in X₂-Hex^2(?-), X₃-Hex^3(?-), and X₄-Hex^4(?-). Taken together, these findings demonstrate that directly linking the benzene rings of PIs with a single C-C bond is a viable method for generating stabilized high-spin organic anionic polyradicals.

Full text of DOI:10.1021/jacs.7b12124

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Article
X-Shaped Oligomeric Pyromellitimide Polyradicals
Yilei Wu, Ji-Min Han, Michael Hong, Matthew D. Krzyaniak, Anthea K Blackburn,
Isurika R. Fernando, Dennis D. Cao, Michael R. Wasielewski, and J. Fraser Stoddart
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b12124 • Publication Date (Web): 07 Dec 2017
Downloaded from http://pubs.acs.org on December 7, 2017
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X-Shaped Oligomeric Pyromellitimide Polyradicals
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Yilei Wu, JiꢀMin Han, Michael Hong, Matthew D. Krzyaniak,
Anthea K. Blackburn,
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Isurika R. Fernando, Dennis D. Cao, * Michael R. Wasielewski, * J. Fraser Stoddart *
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Department of Chemistry, ArgonneꢀNorthwestern Solar Energy Research (ANSER) Center,
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and Institute for Sustainability and Energy at Northwestern,
Northwestern University, Evanston IL 60208ꢀ3113
ABSTRACT: The synthesis of stable organic polyradicals is important for the development of
magnetic materials. Herein, we report the synthesis, isolation, and characterization of a series of
Xꢀshaped pyromellitimide (PI) oligomers (X -R, n = 2–4, R = Hex or Ph) linked together by
n
single C–C bonds between their benzenoid cores. We hypothesize that these oligomers might
form highꢀspin states in their reduced forms because of the nearly orthogonal conformations
1
13
adopted by their PI units. H and C nuclear magnetic resonance (NMR) spectroscopies
confirmed the isolation of the dimeric, trimeric, and tetrameric homologues. XꢀRay
crystallography shows that X -Ph crystallizes into a densely packed superstructure, despite the
2
crissꢀcrossed conformations adopted by the molecules. Electrochemical experiments, carried out
on the oligomers X -Hex, reveal that the reductions of the PI units occur at multiple distinct
n
potentials, highlighting the weak electronic coupling between the adjacent redox centers. Finally,
•–
n(•–)
the chemically generated radical anion and polyanion states, X -Hex and X -Hex
,
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respectively, were probed extensively by UVꢀVisꢀNIR absorption, EPR and electron nuclear
double resonance (ENDOR) spectroscopies. The ENDOR spectra of the radical monoanions Xn-
•
–
Hex reveal that the unpaired electron is largely localized on a single PI unit. Further reductions
•–
of X -Hex yield EPR signals (in frozen solutions) that can be assigned to spinꢀspin interactions
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(•–)
3(•–)
4(•–)
in X -Hex , X -Hex , and X -Hex . Taken together, these findings demonstrate that
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directly linking the benzene rings of PIs with a single CꢀC bond is a viable method for generating
stabilized highꢀspin organic anionic polyradicals.
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■
INTRODUCTION
The search for organic molecules having large numbers of magnetically ordered unpaired spins
leading to molecular magnets has a long history focused on polyradicals, polycarbenes, and
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organometallic complexes.
that has received much less attention
chemically stable materials for applications, ranging from car paints to highꢀtemperature
Polyradicals of aromatic diimides represent a class of molecules
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despite having been utilized to create thermally and
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adhesives and insulators, organicꢀelectrode batteries,
ionꢀtransport channels,
These applications are enabled by the two
reversible oneꢀelectron reduction processes that aromatic diimides typically undergo to form,
organic
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photovoltaics,
and other organic electronics.
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first of all, a radical anion and then subsequently a spinꢀpaired dianion. Each of these redox
states has a characteristic optical signature along with unique electrochemical and photophysical
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properties that can be leveraged in applications.
Investigations into this class of compounds
have been bolstered by the fact that a myriad of Nꢀfunctionalized aromatic imide derivatives can
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be produced by reacting the corresponding anhydrides or carboxylic acids with almost any
primary amine. In the larger aromatic diimides, core functionalization has proven to be a viable
strategy for controlling molecular packing, introducing solubilizing groups, and tuning orbital
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energy levels.
Directly coupled naphthalene diimide oligomers have been shown by
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Polander et al. to be amenable to incorporation into organic fieldꢀeffect transistors. These coreꢀ
functionalization strategies have not been applied widely to the smallest of the diimides, namely
pyromellitimide (PI), despite the low molecular weights of PIs, which are attractive in organic
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electronics applications where efficient chargeꢀtoꢀmass ratios are crucial for device utility.
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Recently, we reported the synthesis of the parent 3,6ꢀdihaloꢀPIs, which exhibit significant twoꢀ
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point halogen bonding interactions in the solid state.
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We envisioned that these 3,6ꢀdihaloꢀPIs
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could be readily transformed into coreꢀfunctionalized PI compounds. Indeed, 3,6ꢀdibromoꢀPI
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derivatives were utilized by Guo et al. as the starting point for the synthesis of a series of
donorꢀacceptor poly(phenyleneethynylene)s. In these mixtures, which showed broad absorption
profiles, the PI units were spaced apart by bisethynylaryl donors to create a phenylethynylene
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backbone. Independently, Rhee et al. have reported that dehalogenative coupling of 3,6ꢀ
dibromoꢀPIs produces a solutionꢀprocessible polydisperse mixture that could be cast into thin
films.
We hypothesized that directly coreꢀcoupled oligoꢀPI compounds would display attractive selfꢀ
assembly, electrochemical, photophysical and magnetic properties. Therefore, we have sought to
obtain and evaluate these PI oligomers. Herein, we describe the synthesis and isolation of a
series of torsionally frustrated PI oligomers X -R—where n is the number of PI units (n = 2–4)
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and R is the functional group attached to the nitrogen atoms—in which the PI units are coupled
directly together by C–C bonds between their benzenoid rings. Additionally, we report (i) the Xꢀ
ray crystal structure of X -Ph, (ii) the electrochemical behavior of the oligomers X -Hex, (iii)
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the UVꢀVisꢀNIR absorption, EPR and electron nuclear double resonance (ENDOR) spectra of
the chemically generated radical monoanionic and dianionic states of X -Hex, and (iv) present
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evidence for the formation of spin multiplets (S > ½) upon multiply reducing X -Hex.
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■
RESULTS AND DISCUSSION
Synthesis. 3ꢀBromoꢀPIs 1-R were synthesized following a similar protocol to the one that was
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used to produce the dihaloꢀ3,6ꢀPIs in our previous report. Briefly, commercially available 3ꢀ
bromoꢀ1,2,4,5ꢀtetramethylbenzene was oxidized with KMnO to yield the tetracarboxylic acid,
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which was dehydrated by Ac O and reacted with hexylamine or aniline to yield 1-Hex and 1-Ph,
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respectively (Scheme S1). We hypothesized that by subjecting 1 to a homocoupling reaction
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Scheme 1. Synthesis of X -R compounds. (a) Ullman coupling of the monobromide 1-R
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alone yields X -R whereas (b) exposing a mixture of monoꢀ and dibromides 1-R and 2-R to
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the same conditions yields oligomers X -R (n = 2–4). R = C H or Ph. Reaction conditions:
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(i) Activated Cu Bronze, NMP, 150 °C for 3.5 h.
exhibit weak to moderate electronic coupling and stabilization over multiple imide sites.
Inspection of molecular models of X -Hex revealed that it is impossible for the aryl–aryl bond to
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experience free rotation as a consequence of significant steric and electronic repulsions between
the imide carbonyl oxygen atoms. Initial attempts to conduct the Ullman coupling of 1-Hex
utilizing anhydrous DMF as solvent produced in 81% yield a bright orange sideꢀproduct, which
was identified to be an undesired dimethylamination byproduct. No oligomeric pyromellitimides
were isolated. Through the course of reaction optimization, it was found that NMP was a suitable
replacement solvent for DMF. MALDI mass spectrometry of the crude reaction mixture after
Ullman homocoupling of 1-Hex in NMP did not display any bromine isotope patterns,
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suggesting completion of the reaction and consumption of the starting material. Purification by
preparative TLC yielded X -Hex, in addition to the reduction byproduct N,N′ꢀ
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dihexylpyromellitimide (PI-Hex), in 30 and 37% yields, respectively. Subsequently, we sought
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to synthesize the higher order oligomers X -R. We envisioned that by conducting the Ullman
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coupling on a mixture of monoꢀ and dibromoꢀPIs, the distribution of products could be
controlled statistically. MALDI mass spectrometry performed (Figure S1) on the crude mixture,
obtained after reacting 1-Hex with 2-Hex in a 2:1 ratio under Ullman coupling conditions,
revealed the presence of oligomers up to nine PI units in length. After purification by either
preparative GPC or TLC, we were able to isolate the monomeric, dimeric, trimeric, and
tetrameric products, which we designate as having the constitutions PI-Hex, X -Hex, X -Hex,
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and X -Hex, respectively.
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NMR Spectroscopy. These constitutional designations were confirmed by NMR spectroscopy.
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In the H NMR spectra (Figure 1a) of these compounds, the ratio of the integrals of the terminal
PI core proton resonances to those of the NCH protons alpha to the imide nitrogen atoms
correspond to the assigned number of PI units. In X -Hex and X -Hex, two different triplet
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resonances are observed around 3.5 ppm in the spectrum on account of the differences in
chemical environments between the “outer” and “inner” PI units leading to constitutional
heterotopicity between the Nꢀmethylene protons. A downfield shift of the terminal PI core proton
resonances is observed with increasing oligomer lengths. This observation can be rationalized by
the inductive electronꢀwithdrawing effects of the imide groups on the additional linked PI units,
thus further confirming the direct coupling of these units. Unsurprisingly, the magnitude of this
signal shift is found to be much smaller between X -Hex and X -Hex, than it is between PI-Hex
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and X -Hex, because the additional PI units in the larger oligomers are further away from the
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terminal aromatic protons. The carbonyl region of the C NMR spectra (Figure 1b) clearly
reveals the increasing number of heterotopic imide carbon atoms with increasing oligomer
lengths.
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Figure 1. Selected portions of the H and C NMR spectra of X -Hex, X -Hex, X -Hex, and
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PI-Hex, from top to bottom. The top half of the structural formulas, which are related by
rotation about a C axis (gray dashed line), are not depicted for the sake of clarity. (a) The
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ratio of integrations of the core (blue) and alphaꢀcarbon (green and red) proton resonances in
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the H NMR spectra correspond to constitutionally heterotopic “inner” and “outer” PI units.
(b) The expected number of resonances are also observed for the carbonyl carbon atoms in
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the C NMR spectra.
X-Ray Crystallography. In order to gain further insight into the conformations adopted by these
oligomeric pyromellitimides, we turned our attention to obtaining solidꢀstate structures. Initial
attempts to crystallize X -Hex were unsuccessful, no doubt as a consequence of the relatively
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flexible alkyl chains. Hence, we synthesized X -Ph starting from 1-Ph, following the same
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protocols (Scheme 1a) as described for X -Hex. Purification of the longer Nꢀphenylated
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derivatives X -Ph, obtained following the protocol outlined in Scheme 1b, was challenging
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because of their relatively lesser solubilities in chlorinated solvents, compared with those in the
X -Hex series. Milligram quantities of X -Ph, contaminated by a phthalate impurity which most
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likely arises from decarboxylative side reactions, were isolated after repeated preparative TLC.
Nonetheless, we proceeded to attempt the crystallization of both X -Ph and X -Ph.
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Figure 2. Tubular representation (a/b) of the solidꢀstate structure of X -Ph. (a) A sideꢀon
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view down the N–N axis of one of the PI units reveals a slight tilt with respect to the adjacent
one. (b) A view down the C–C bond between the PI units reveals their near orthogonal
arrangement with respect to each other. (c) A tubular and spaceꢀfilling representation of the
solidꢀstate superstructure reveals that despite the geometrical complexity of X -Ph, the
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Single crystals of X -Ph suitable for Xꢀray diffraction, were obtained by vapor diffusion between
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a solution of the compound in 1:1 CH Cl/MeOH and a MeOH reservoir. The solidꢀstate structure
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reveals that X -Ph crystallizes in the tetragonal I4 /a space group (No. 88). The asymmetric unit
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consists of one complete PI unit which constitutes half of an X -Ph molecule. The C–C bond
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formed during the homocoupling reaction has a bond length of 1.48 Å, a value which does not
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differ significantly from the aryl–aryl bond length (1.51 Å) in biphenyl in its solid state. In each
X -Ph molecule, the PI units project away from the axis containing the aryl–aryl bond, such that
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the two PI units are tilted 97° apart (Figure 2a). The PI units are oriented in a nearꢀorthogonal
and crissꢀcrossed manner with respect to each other with a torsional angle of approximately 89°
(Figure 2b). The solventꢀfree superstructure is characterized by the fact that, despite their crissꢀ
cross shape, the molecules of X -Ph are able to pack in a surprisingly efficient manner into a unit
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cell that has only 21.9% void volume in a way that maximizes πꢀπ stacking interactions between
the PI units. Throughout the superstructure, pairs of X -Ph molecules are packed together
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intimately with their vertices pointed directly at each other, while the remaining aromatic PI
surfaces are satisfied by faceꢀtoꢀface πꢀπ stacking with additional X -Ph molecules, as evidenced
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by the distance of 3.26 Å between the PI benzenoid planes (Figure 2c). While small, needleꢀlike
single crystals of X -Ph were obtained by vapor diffusion of hexanes into a CHCl solution of
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the compound, we were unable, however, to detect any significant diffraction on the part of these
crystals.
Electrochemistry. The solidꢀstate structure of X -Ph confirms that the imide groups are located
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in close proximity to each other, suggesting that they may experience favorable spinꢀspin
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interactions once the compounds have been transformed into their reduced states.
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oligomer containing n PI groups is capable of accepting a total of 2n electrons. The redox
behavior of X -Hex was probed (Figure 3) by electrochemical experiments (0.2 mM, 0.1 M
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TBAPF , 50 mV s ) in THF and referenced internally to the Fc/Fc couple. The accessible halfꢀ
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2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
PI
–
1
Figure 3. Cyclic voltammograms (0.2 mM, THF, 0.1 M TBAPF , 50 mV s ) of PI-Hex, X -
6
2
Hex, X -Hex and X -Hex, from top to bottom. Ferrocene was used as an internal standard
3
4
+
and the labeled peak voltages were calibrated against the Fc/Fc couple.
wave reduction potentials are listed in Table 1. In the cyclic voltammograms (CV) obtained from
a solution of X -Hex in THF, four distinct processes were observed. These processes present
2
themselves as two pairs of oneꢀelectron reductions, with the first and second oneꢀelectron
1
2
•–
reduction waves (E1/2 = –1.35 V, E1/2 = –1.59 V) corresponding to the formation of X -Hex
2
2
(•–)
and X -Hex , respectively. The observed splitting is attributed to the Coulombic interactions
2
that come into play when a second electron is introduced into the molecules. The initial reduction
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1
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1
1
1
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2
2
2
2
2
2
2
2
3
3
3
3
3
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3
3
3
3
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1
occurs at a slightly more positive potential compared to that (E1/2 = –1.38 V) of PI-Hex, an
observation which suggests that there is no significant conjugative interactions between the
3
4
individual PI units. The second pair of oneꢀelectron reduction waves (E1/2 = –2.13 V, E1/2 = –
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
Table 1. Electrochemical Data for PI-Hex and X
n
-Ra
1
2
3
4
5
6
Compound
PI-Hex
E
½
/ V
E
½
/ V
E
½
/ V
E
½
/ V
E
½
/ V
E
½
/ V
–1.38
–1.35
–1.36
–1.38b
–2.05
–1.59
–1.44
–1.60
X
2
X
3
X
4
-Hex
-Hex
-Hex
–2.13
–1.77
–1.87
–2.30
–2.00
–2.17
–2.17
–2.24
–2.29b
a
+ b
Halfwave potential values are given vs Fc/Fc . Multielectron process.
3
–
4–
2
.30 V), which are associated with the formation of X -Hex and X -Hex , respectively, are
2 2
2
shifted toward more negative potentials than the second reduction (E1/2 = –2.05 V) of PI-Hex.
The overall splitting of the reductions into four distinct processes suggests that, in the
symmetrically identical PI redox centers, the imide units in X -Hex are located in close enough
2
proximity to each other to impose repulsive Coulombic interactions which shift subsequent
reductions toward more negative potentials. Cyclic voltammetry also confirmed that X -Hex
3
6
–
undergoes six distinct reversible oneꢀelectron reductions to the hexaꢀanion state X -Hex . For
3
X -Hex, 8ꢀelectron reversible reductions are detected, which proceed from the neutral state to the
4
octaꢀanion state. Notably, the first two electron reduction waves in X -Hex are overlapping,
4
indicating that these two electrons communicate with each other very weakly, most likely
because they are injected into the two terminal PI units to avoid unfavorable Coulombic
repulsions. The subsequent two singleꢀelectron processes are most likely associated with the
sequential reductions of the two “inner” PI units of X -Hex. All of the initial reduction potentials
4
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are essentially independent of the oligomer length, thus excluding any significant conjugation
effect. In order to obtain further information on the electronic states for the reduced species of
X -Hex, we measured the optical absorption changes of X -Hex during the course of reduction
n
n
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
from neutral to anionic states by performing chemical reduction titration experiments.
•–
UV-Vis-NIR Absorption Spectroscopy. While the monoreduced radical anion X -Hex could
2
be generated by chemical reduction with 1 equiv of cobaltocene (CoCp , E = –1.33 V vs Fc)
2
1/2
(Figure S2 and S3), it was necessary to employ the more strongly reducing
2(•–)
decamethylcobaltocene (CoCp* , E = –1.91 V) to obtain X -Hex . Accordingly, a solution
2
1/2
2
of CoCp* was added into a 0.4 mM solution of X -Hex in DMF in order to obtain the singly and
2
2
doubly reduced states of the compound and their UVꢀVisꢀNIR spectra (Figure 4) were recorded.
The wavelengths and molar absorptivities of major bands are summarized in Table 2. The UVꢀ
•–
VisꢀNIR spectrum of the radical anion X -Hex reveals two absorption bands, centered at 651
2
3
9,59ꢀ60
and 716 nm, corresponding
closely to the wavelengths of the absorption bands for PI-
•–
Hex . In addition to these two bands, however, the spectrum also displays a new band at 790 nm
with a molar absorptivity similar to the one present at 716 nm. The new band might be assigned
to the intervalence charge transfer band (IVCT, Figure 4), characteristic of mixedꢀvalence
2
(•–)
compounds. Addition of a second equivalent of CoCp* to the solution yielded X -Hex
2
2
quantitatively, as indicated by the disappearance of the IVCT band. The absorption spectrum of
•–
•–
the doubly reduced state, which resembles PI-Hex rather than X -Hex , has an intense band
2
centered at 730 nm. Unsurprisingly, addition of excess equivalents of CoCp* to the solution did
2
not produce any significant changes in the UVꢀVisꢀNIR spectra, confirming that the further
reduced X -Hex species could not be generated because the potentials required to access them
2
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
•–
n(•–)
Figure 4. (a) UVꢀVisꢀNIR absorption spectra (DMF, 296 K) of PI-Hex and X -Hex
(n
2
n
=
2–4), which were generated by chemical reduction of X -Hex with n equiv of CoCp* ,
n
respectively.
•–
are more negative than the reduction potential of CoCp* . The monoreduced X -Hex anion
2
3
displays a similar IVCT band at ca. 790 nm, corresponding to the optically induced CT between
the terminal PI and the central one. Notably, an additional lower energy and weaker transition is
observed at around 955 nm (IVCT′, Figure 4), which is assigned to the CT interaction between
the two terminal PIs. This assignment is further corroborated by the disappearance of the 955 nm
band upon addition of the second equivalent of CoCp* to reduce both terminal PIs. The spectral
2
2
(•–)
3(•–)
•–
2(•–)
features of X -Hex
and X -Hex
resemble closely those of X -Hex and X -Hex
,
3
3
2
2
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respectively. Similar trends in spectral changes are observed upon sequential reduction of X4-
4
(•–)
2(•–)
3(•–)
Hex, where X -Hex
is relatively similar to X -Hex
and X -Hex
.
4
2
3
Table 2. UVꢀVis Absorption Properties of PI-Hex and X -Hex as well as their Reduced States.
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
–1
Compound
PI-Hex
λ
max / nm
320
ε / 10 M cm–1
2.6
7.1
PI-Hex•–
650
7
16
30.9
4.2
X
2
-Hex
327
650
X
2
-Hex•–
5.1
7
16
10.4
8.6
790
730
X
2
-Hex2(•–)
29.3
Computational Results. DFT (B3LYP/6ꢀ31G*) calculations for PI-Me, X -Me, X -Me and X -
2
3
4
Me (in which hexyl groups are simplified to ꢀCH ) provided insight into their conformations and
3
energetics. In the oligomers, the PI units are severely twisted relative to one another, which is
consistent with the singleꢀcrystal structure; φ, the torsional angle between the planes containing
each PI units, varies from 62º to 90º depending on the redox and electron spin states (Figures S4
and S5, Tables S1 and S2).
Frontier molecular orbitals for PI-Me, X -Me, X -Me and X -Me were also computed (DFT,
2
3
4
B3LYP/6ꢀ31G*). The LUMO and LUMO+1 of the X -Me are bonding and antibonding
2
combinations, respectively, of two PI LUMOs (Figure S4). The degrees of electronic
communication in the PI oligomers can be quantified by considering the energy splitting between
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2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
61
the LUMO and LUMO+1 orbitals of the neutral states. The computed splitting values, which
corresponds to twice the electronic coupling (V) according to energyꢀsplittingꢀinꢀdimer (ESD)
6
2
–1
method combined with Koopmans’ Theory (KT), was 476, 532, 387 cm (gas phase,
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
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7
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9
0
1
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8
9
0
1
2
3
4
5
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7
8
9
0
B3LYP/61G*) for X -Me, X -Me and X -Me, respectively. Thus, the corresponding computed
2
3
4
electronic coupling between the PI units is significantly smaller than the reorganization energy
–
1
associated with the rylene diimide anion formation (around 0.28 eV or 2300 cm in gas phase).
Therefore, these compounds are expected to localize the charge on a single PI unit (RobinꢀDay
63
class II mixed valence rather than allowing full electron delocalization. The small computed
electronic coupling in the oligomeric systems, which agrees well with experimental
electrochemical results showing very little anodic shift of their first reduction potential compared
to that of monomeric reference compound, can be rationalized by the nearly orthogonal
conformation between the PI units and the presence of a nodal plane in the CꢀC bond linking
them. In order to further corroborate our interpretation on the electronic distribution in these
compounds we performed EPR/ENDOR measurements.
EPR/ENDOR Spectroscopy. EPR and ENDOR spectroscopies were performed on the
chemically generated monoꢀ and diꢀreduced states of X -Hex and compared with the EPR
2
•–
spectrum of PI-Hex with particular attention being paid to understanding spin localization or
delocalization with respect to each PI unit in the monoꢀanion and the presence of higher spin
•–
multiplicities in the more highly reduced states. The solution state cwꢀEPR spectra of PI-Hex ,
•–
•–
•–
X -Hex , X -Hex , and X -Hex , respectively, are shown in Figure 5a. Spectral simulation of
2
3
4
•–
PI-Hex , overlaid in red, provides the hyperfine coupling values presented in Table 3, which
6
4
•–
agree well with those reported in the literature. Although the spectrum of X -Hex shows the
2
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
•
–
•–
Figure 5. (a) Solution (0.4 mM, DMF, 250 K) Xꢀband EPR spectra of PI-Hex , X -Hex ,
2
•–
•–
X -Hex , and X -Hex . (b) Solution (0.4 mM, DMF, 250 K) cwꢀENDOR spectra of PI-
Hex , X -Hex , and X -Hex . The corresponding spectral simulation are overlaid in red.
3
4
•–
•–
•–
2
3
•–
same spectral width as PI-Hex , the lines are considerably broadened, an observation which
makes the simulation of both the nitrogen and hydrogen hyperfine couplings difficult. The cwꢀ
•–
•–
•–
EPR spectra of X -Hex , and X -Hex also show the same spectral width as PI-Hex but are
3
4
•–
•–
broadened further as compared to X -Hex . This line broadening observed in X -Hex , X -
2
2
3
•–
•–
Hex , and X -Hex is likely caused by the slowing down of tumbling rate associated with
4
6
5ꢀ66
increasing molecular size.
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1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Table 3. EPR/ENDOR Spectroscopic Data for the Radical Monoanions of PI-Hex and X -Hex
2
Compound
PI-Hex•–
a
N
/ MHz
a
Hcore / MHz
a
H
(CH ) / MHz
2
3.35 (x2)
ꢀ
1.80 (x2)
1.82
0.73 (x4)
0.70
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
X
-Hex•–
2
The number of equivalent nuclei sharing the hfcc values are given in the parentheses.
In order to extract the hyperfine couplings, continuous wave ENDOR spectroscopy was
•–
•–
•–
performed on PI-Hex , X -Hex , and X -Hex , and electron spin echo envelope modulation
2
3
67
•–
•–
(
ESEEM) spectroscopy was performed on both PI-Hex and X -Hex . The ENDOR spectrum
2
•–
of PI-Hex shows a pair of proton resonances that were simulated with isotropic hyperfine
couplings of 1.82 and 0.70 MHz (Figure 5b). These values were used in the simulation of the cwꢀ
•–
•–
EPR spectrum. The ENDOR spectra of X -Hex , and X -Hex both show splitting patterns
2
3
•–
similar to those found in PI-Hex , an observation which suggests that the unpaired electron spin
density is localized on a single PI unit in the dimer. If the electron was delocalized or hopping
rapidly among multiple PI units, the proton hyperfine coupling would be expected to decrease
6
8ꢀ69
linearly with the number of units following the McConnell relationship.
The cwꢀENDOR
•–
spectrum of X -Hex was barely observed above the noise level and is shown in Figure S6. As
4
•–
additional verification of spin localization, 3ꢀpulse ESEEM was performed on both X -Hex and
2
•–
PI-Hex , Figure S7. The two spectra show no differences, a result which confirms that both the
number of equivalent imide nitrogens and the magnitude of their hyperfine coupling are the
same.
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Figure 6 shows the Wꢀband (94 GHz) EPR spectra recorded in frozen solution at 120 K of the
•–
•–
2(•–)
radical monoꢀanions X -Hex and PI-Hex , the radical dianion X -Hex , the radical trianion
2
2
3
(•–)
4(•–)
X -Hex
3
and the radical tetraanion X -Hex , with their corresponding simulation overlaid in
4
•–
•–
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
red. Simulation of X -Hex and PI-Hex provides the same gꢀtensor, g = [2.00485, 2.00485,
2
•–
•–
2
.00213], which is further evidence that suggests X -Hex and PI-Hex are magnetically
2
•–
2(•–)
equivalent. Additional reduction of X -Hex to X -Hex , leads to the appearance of an EPR
2
2
signal characteristic of a triplet spin state, the spectrum also shows a monoreduced impurity in
•–
Figure 6. Wꢀband EPR spectra of frozen solutions (0.4 mM, DMF, 120 K) of PI-Hex , X -
2
•–
2(•–)
3(•–)
4(•–)
Hex , X -Hex , X -Hex
and X -Hex
.
2
3
4
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4
4
4
4
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5
6
the central spectral region. Simulation of the two interacting PI units was achieved by rotating
the gꢀtensor of one PI unit by 80° about the yꢀaxis relative to the other and using a dipoleꢀdipole
coupling of 143 MHz collinear with the yꢀaxes of the gꢀtensors. The observed dipoleꢀdipole
2
(•–)
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
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5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
coupling allows an estimation of the mean distance between the interacting spins in X -Hex
2
70
by the pointꢀdipole approach, which is calculated to be 7.1 Å. The structural details obtained
2
(•–)
from fitting the Wꢀband EPR spectrum of X -Hex
agree well with the solidꢀstate structure
2
(
Figure 2b) of X -Ph and shows that the each electron is essentially localized within one PI unit.
2
D12
D12
D12
D13
D13
D14
4
(•–)
Figure 7. Schematic diagram for possible dipoleꢀdipole couplings in X -Hex
.
4
3
(•–)
4(•–)
The Wꢀband EPR spectra of X -Hex
and X -Hex
also show broad features which are
3
4
consistent with ferromagnetic spinꢀspin interactions (Figure 6). Due to the complexity of the
3(•–)
4(•–)
magnetic interactions in X -Hex
and X -Hex
fitting of the Wꢀband spectrum is
3
4
challenging; however, the magnetic interactions can be understood by considering a simple
model and the cwꢀEPR data collected at Xꢀband. The Xꢀband cwꢀEPR spectrum of the radical
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2(•–)
dianion X -Hex
and its simulation are shown in Figure S8. On top of the large monoꢀreduced
3
impurity, the dianion is observed to have a dipoleꢀdipole splitting of 55 MHz, which corresponds
to a distance of 9.8 Å, which is not unreasonable for the distance between the two terminal PI
units. Referring to Figure 7, from this measurement we obtain D , the dipoleꢀdipole coupling
1
1
1
1
1
1
1
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13
2(•–)
between PI 1 and 3, and from the Wꢀband cwꢀEPR of X -Hex
we obtain the nearest neighbor
2
dipoleꢀdipole coupling, D12.
4
(•–)
The Xꢀband cwꢀEPR spectra of X -Hex
shown in Figure 8 was collected as a function of
temperature from 5 to 60 K. The amplitude of the broad features which correspond to the dipoleꢀ
dipole coupling D decrease with decreasing temperature, which occurs concurrently with an
4
12
4
(•–)
Figure 8. Variable temperature Xꢀband EPR spectra of frozen DMF solution of X -Hex
.
4
increase in the amplitude of the narrow, D , features. As the temperature is lowered below about
1
3
n(•–)
20 K, adjacent spins in X -Hex
tend to pair up, turning off the nearest neighbor dipoleꢀdipole
n
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coupling, while the longer range (D ) spinꢀspin interactions increasingly apparent in the overall
1
3
cwꢀEPR spectra.
■ CONCLUSIONS
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In summary, we have reported the synthesis and isolation of three Xꢀshaped oligomeric
pyromellitimides that are readily soluble in organic solvents. The dimeric compound X -Ph was
2
found by Xꢀray crystallography to crystallize in an aesthetically pleasing and highly packed
superstructure that highlights the proximity of the two pyromellitimide units to each other within
the molecule. Cyclic voltammetry reveals that the enforced interaction between imide units leads
to three, five or seven individually accessible redox states. UVꢀVisꢀNIR and EPR/ENDOR
spectroscopic experiments performed on the chemicallyꢀgenerated singlyꢀreduced state of X2-
Hex, shows localization of the unpaired electron on an individual pyromellitimide unit. EPR
spectroscopy also demonstrates antiferromagnetic spinꢀspin interactions for the radical anions
2
(•–)
3(•–)
4(•–)
X -Hex , X -Hex
and X -Hex
at temperatures < ~20K resulting in low spin ground
2
3
4
states, while lowꢀlying thermally accessible highꢀspin (S > ½) states are populated at
temperatures > ~20K. This research shows that decreasing the electronic coupling between the
unpaired spins on adjacent PI molecules by fixing their π systems in a nearly orthogonal
geometry results in the ability to maintain high spin multiplicity spin states in these oligomers,
which may prove useful in developing molecular magnets and quantum information systems.
■
EXPERIMENTAL SECTION
Synthetic Methods. Unless otherwise stated, compounds and solvents were purchased from
commercial vendors and were used as supplied without further purification. Copper bronze was
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activated by a 2% I solution in Me CO immediately prior to use. Nuclear magnetic resonance
2
2
(NMR) spectra were recorded at 298 K on a Bruker Avance III 500 spectrometer, with a working
1
13
frequency of 500 MHz for H and 125 MHz for C. Chemical shifts are listed in ppm on the δ
scale and coupling constants are recorded in Hertz (Hz). Deuterated solvents for NMR
spectroscopic analyses were used as received. Chemical shifts are reported in δ values relative to
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
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3
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3
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0
1
2
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1
2
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9
0
1
2
3
4
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2
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3
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0
the signals corresponding to the residual nonꢀdeuterated solvents (CD COCD : δ 2.05 ppm, δ
C
3
3
H
2
9.84 ppm; CDCl : δ 7.26 ppm, δ 77.23 ppm; CD COCD : δ 2.50 ppm, δ 39.57 ppm). Lowꢀ
3 H C 3 3 H C
and highꢀresolution mass spectra were measured on a Bruker Autoflex III MALDIꢀTOF using
,5ꢀdihydroxybenzoic acid as the matrix.
2
Synthetic Procedures. The syntheses of the X -R compounds described in this research are
n
described in detail below. Detailed procedures for the syntheses of 2-R and 1-R precursor
compounds are provided in the Supporting Information.
X -Hex: A mixture of 1-Hex (100 mg, 216 µmol), activated copper bronze (100 mg), and Nꢀ
2
methylꢀ2ꢀpyrrolidone (5 mL) was heated in an N atmosphere at 150 °C for 3.5 h. After cooling
2
to room temp, the reaction mixture was diluted with EtOAc (30 mL) and washed with brine (3 ×
3
0 mL). The combined organic layers were dried (MgSO ), collected by filtration, and
4
evaporated under reduced pressure. The residue was purified by preparative TLC (2:3
Hexanes/CH Cl (v/v)) to yield X -Hex as a beige solid (25 mg, 30%) as well as the reduction
2
2
2
1
byproduct PI-Hex as a white solid (31 mg, 37%). Spectroscopic characterization of X -Hex: H
2
NMR (CDCl , 500 MHz, 298 K): δ = 8.37 (s, 2H), 3.59 (t, J = 7.3 Hz, 8H), 1.58 (p, J = 7.3 Hz,
3
H
1
3
8
H), 1.30–1.22 (m, 24H), 0.88–0.80 (m, 12H); C NMR (CDCl , 125 MHz, 298 K): δ = 166.5,
3 C
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1
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2
2
2
2
2
2
2
2
2
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3
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3
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3
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1
65.7, 136.8, 135.1, 125.2, 118.7, 38.7, 31.3, 28.3, 26.4, 22.5, 14.0; HRꢀMS (MALDIꢀTOF)
+
+
Calcd for C H N O : m/z = 767.40 ([M + H] ); Found: m/z = 767.35 ([M + H] ).
4
4
55
4
8
X -Hex: A mixture of 1-Hex (90 mg, 0.19 mmol), 2-Hex (50 mg, 92 µmol), activated copper
n
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bronze (150 mg), and Nꢀmethylꢀ2ꢀpyrrolidone (5 mL) was heated in an N atmosphere at 150 °C
2
for 3.5 h. After cooling to room temp, the reaction mixture was diluted with EtOAc (30 mL) and
washed with brine (3 × 30 mL). The combined organic layers were dried (MgSO ), collected by
4
filtration, evaporated under reduced pressure, and the residue triturated with MeOH. The
precipitate was purified by preparative TLC (1:3 hexanes/CH Cl (v/v)) to yield PI-Hex (10.6
2
2
mg, 9.6%), X -Hex (21 mg, 19%), X -Hex (15 mg, 14%) and X -Hex (10 mg, 14%).
2
3
4
1
Spectroscopic characterization of X -Hex: H NMR (CDCl , 500 MHz, 298 K): δ = 8.40 (s,
3
3
H
2
H), 3.66 (t, J = 7.4 Hz, 8H), 3.48 (t, J = 7.3 Hz, 4H), 1.64 (p, J = 7.3 Hz, 8H), 1.54–1.45 (m,
1
3
4H), 1.37–1.19 (m, 36H), 0.86 (t, J = 7.0 Hz, 12H), 0.83 (t, J = 7.0 Hz, 6H); C NMR (CDCl ,
3
1
3
25 MHz, 298 K): δ = 166.6, 166.1, 166.0, 137.1, 135.4, 134.9, 126.1, 125.3, 118.9, 39.0, 31.6,
C
1.5, 28.6, 28.4, 26.7, 26.6, 22.7, 22.6, 14.2, 14.2; LRꢀMS (MALDIꢀTOF) Calcd for
C H N O : m/z = 1148.58 ([M]); Found: m/z = 1148.35 ([M]). Spectroscopic characterization
66 80
6
12
1
of X -Hex: H NMR (CDCl , 500 MHz, 298 K): δ = 8.40 (s, 2H), 3.66 (t, J = 7.3 Hz, 8H), 3.53
4
3
H
(t, J = 7.3 Hz, 8H), 1.68–1.61 (m, 8H), 1.56–1.50 (m, 8H), 1.34–1.16 (m, 48H), 0.86 (t, J = 7.0
1
3
Hz, 12H), 0.88–0.82 (m, 24H); C NMR (CDCl , 125 MHz, 298 K): δ = 166.5, 166.2, 166.1,
3
C
165.9, 137.0, 135.4, 126.0, 38.9, 31.8, 31.6, 31.1, 28.6, 28.4, 26.7, 26.6, 22.9, 22.7, 22.7, 14.4,
+
1
4.2, 14.2; LRꢀMS (MALDIꢀTOF) Calcd for C H N O : m/z = 1531.78 ([M + H] ); Found:
8
8
106
8
16
+
m/z = 1533.80 ([M + H] ).
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X -Ph: A mixture of 1-Ph (100 mg, 224 µmol), activated copper bronze (100 mg), and Nꢀ
2
methylꢀ2ꢀpyrrolidone (5 mL) was heated in an N atmosphere at 150 °C for 3.5 h. After cooling
2
to room temp, the reaction mixture was diluted with EtOAc (30 mL) and washed with brine (3 ×
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
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5
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5
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5
5
5
5
5
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0
1
2
3
4
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7
8
9
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2
3
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7
8
9
0
1
2
3
4
5
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2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
0 mL). The combined organic layers were dried (MgSO ), collected by filtration, evaporated
4
under reduced pressure, and the residue triturated with MeOH to yield X -Ph as a brown solid
2
1
(
29 mg, 35.3%). H NMR (CD COCD , 500 MHz, 298 K): δ = 8.55 (s, 2H), 7.58–7.40 (m,
3
3
H
13
2
0H); C NMR (CD SOCD , 125 MHz, 298 K): δ = 165.2, 164.6, 136.6, 134.8, 131.1, 129.1,
3 3 C
1
28.6, 127.1, 125.7, 118.9; HRꢀMS (MALDIꢀTOF) Calcd for C H N O : m/z = 735.15 ([M +
4
4
23
4
8
+
+
H] ); Found: m/z = 735.12 ([M + H] ).
X -Ph: A mixture of 1-Ph (106 mg, 237 µmol), 2-Ph (75 mg, 0.14 mmol), activated copper
3
bronze (300 mg), and Nꢀmethylꢀ2ꢀpyrrolidone (5 mL) was heated in an N atmosphere at 150 °C
2
for 3.5 h. After cooling to room temp, the reaction mixture was diluted with EtOAc (30 mL) and
washed three times with brine (3 × 30 mL). The combined organic layers were dried (MgSO4),
collected by filtration, evaporated under reduced pressure, and the solids triturated with MeOH.
The residue was purified by preparative TLC (1:3 hexanes/CH Cl (v/v)) to yield X -Ph as a
2
2
3
brown solid (3.8 mg, 35.3%) containing an impurity which could not be separated from the title
compound. The byproduct is believed to be a phthalate produced by undesired decarboxylation
1
during the course of the reaction. H NMR (CD COCD , 500 MHz, 298 K): ppm = 8.58 (s, 2H),
3
3
7
.55–7.33 (m, 30H).
X-Ray Crystallography. XꢀRay crystal data were collected on a Bruker Kappa diffractometer
equipped with a CuK sealedꢀtube source and an APEX II CCD detector. Intensity data were
α
obtained using ω and φ scans spanning at least a hemisphere of reciprocal space for all structures
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(
data were integrated using SAINT). Absorption effects were corrected on the basis of multiple
7
1
equivalent reflections (SADABS). Structures were solved by direct methods(SHELXS) and
7
1
refined by fullꢀmatrix leastꢀsquares against F2 (SHELXL). The structures were solved and
refined using Olex2. The hydrogen atoms were assigned riding isotropic displacement
parameters and constrained to idealized geometries. Crystallographic images were produced
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2
using UCSF Chimera. Atomꢀtoꢀatom distances and torsional angles were measured employing
7
3
Mercury.
Crystallization Procedure and Data. A 0.5ꢀmL solution of X -Ph (1 mg) was dissolved in
2
CHCl /MeOH (1:1 (v/v)) and passed through a 0.45 µ PTFE syringe filter into a crystallization
3
tube. This tube was placed in a 20ꢀmL scintillation vial which contained MeOH (3 mL). After
several weeks, colorless crystals were collected for single crystal Xꢀray crystallography. X -Ph:
2
3
C H N O ; colorless prism, 0.132 × 0.053 × 0.045 mm ; tetragonal, space group I4 /a; a =
44
22
4
8
1
3
15.469(2), b = 15.469(2), c = 34.551(5) Å; α = β = γ = 90°; V = 8268 Å ; Z = 8; ρ
= 1.180 g
calcd
−
3
cm ; 2θ = 50.497°; T = 100 K; 9253 reflections collected, 2142 independent, 253 parameters;
max
−
1
ꢀ = 0.1686 mm ; R = 0.0694 [I > 2σ(I)], wR = 0.1715 (all data); CCDC deposition number
1
2
1
402122.
Electrochemistry. Cyclic voltammetry (CV) experiments were performed on a Gamry
Reference 600 instrument interfaced to a PC using a glassy carbon working electrode (0.071
2
2
cm ). The electrode surface was polished routinely with 0.05 µm alumina/H O slurry on a felt
2
surface immediately before use. The counter electrode was a Pt coil and the reference electrode
was an AgClꢀcoated Ag wire. The concentrations of the samples were 1 mM in 0.1 M electrolyte
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solution (TBAPF in THF or DMF). Electrochemical experiments were conducted on solutions
6
before and after addition of a ferrocene internal standard.
UV-Vis-NIR. The radical anionic and dianionic states of the compounds investigated were
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
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3
3
3
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0
generated by addition of 1 or 2 equiv, respectively, of decamethylcobaltocene (CoCp* ) into
2
DMF solutions under an Ar atmosphere. UVꢀVisꢀNIR spectra were recorded on a Shimadzu UVꢀ
1
800 instrument.
Computational Details. Model structures for the X -R compounds, where R are methyl
n
substituents instead of hexyl groups , were optimized with DFT calculations by using B3LYP
7
4
75
functional, which combines Becke’s three parameter hybridꢀexchange functional with the
7
6
Lee−Yang−Parr correlation functional, and the 6–31G* basis set. Frequency calculations on the
optimized geometries indicated no imaginary frequencies. Note that unrestricted UB3LYP
functional calculation on the anionic states predicts fully delocalized structures for all of
compounds and localization was not achieved by the inclusion of solvation effects with
7
7
polarizable continuum model (PCM) approach. This overꢀdelocalization impeded us to perform
any meaningful TDꢀDFT calculations to support the assignment of UVꢀVisꢀNIR absorption
bands.
EPR Spectroscopy. All EPR sample preparations were performed under an argon atmosphere.
The concentrations of the samples were 0.4 mM in DMF as verified by UVꢀVisꢀNIR absorption
spectra. Samples were loaded into quartz 1.4 mm tubes and sealed with a clear ridged UV
doming epoxy (IllumaBond 60ꢀ7160RCL) and used immediately after preparation. Solution state
continuous wave EPR and ENDOR measurements at Xꢀband (9.5 GHz) were performed with a
Bruker Elexsys E680ꢀX/W EPR spectrometer, equipped with an EN801 ENDOR resonator; solid
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state CWꢀEPR and Pulsed EPR measurements at Xꢀband were performed using a split ring
resonator (ER4118XꢀMS5); and Wꢀband measurements utilized a cylindrical resonator (ENꢀ680ꢀ
1021H). ENDOR experiments were performed using the DICE ENDOR accessory and an ENI
Aꢀ500 RF power amplifier. A 1 kW TWT amplifier (Applied Systems Engineering 117X) was
employed to generate highꢀpower microwave pulses. 3ꢀpulse ESEEM experiments were
collected using the standard π/2 – τ ꢀ π/2 – T ꢀ π/2 pulse sequence, where pulses of turning angle
π/2 were 16 ns in length, τ was fixed at 200 ns, and T was varied in steps of 16 ns. The EPR and
ENDOR spectra were fit in MATLAB® (The MathWorks Inc., Natick MA) using Easyspin
0
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v5.0.16.
■
ASSOCIATED CONTENT
Supporting Information
Detailed synthetic procedures, crystallographic and spectroscopic (NMR, HRMS, UVꢀVisꢀNIR,
and CV) characterization for 1-R, 1-Hex and 1-Ph are included within SI. Additional EPR data
is also presented. This material is available free of charge via the Internet at http://pubs.acs.org.
The crystallographic parameters of single crystal are deposited in the Cambridge
Crystallographic Data Centre (CCDC).
■
AUTHOR INFORMATION
Corresponding Authors
dcao@macalester.edu, mꢀwasielewski@northwestern.edu, stoddart@northwestern.edu
Present Addresses
(D.C.) Chemistry Department, Macalester College, 1600 Grand Ave, St Paul, MN 55105, USA.
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Notes
The authors declare no competing financial interest.
†
Yilei Wu, JiꢀMin Han, and Michael Hong contributed equally to this work.
1
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■
ACKNOWLEDGMENTS
The NMR, MS, and Xꢀray instrumentation in the Integrated Molecular Structure Education and
Research Center (IMSERC) at Northwestern University (NU) were funded by NU and the State
of Illinois. This research is part of the Joint Center of Excellence in Integrated NanoꢀSystems
(JCIN) at King Abdulaziz City for Science and Technology (KACST) and NU. The authors
would like to thank both KACST and NU for their continued support of this research. This
research was supported by the National Science Foundation under grant no. CHEꢀ1565925
(M.R.W.). D.C. was supported by a NSF Graduate Research Fellowship. D.C. and Y.W.
acknowledge support from a Ryan Fellowship awarded by the NU International Institute for
Nanotechnology (IIN). Y.W. thanks the Fulbright Scholars Program for a Graduate Research
Fellowship. A.K.B. acknowledges Fulbright New Zealand for a Fulbright Graduate Award and
the New Zealand Federation of Graduate Women for a Postgraduate Fellowship Award.
■
REFERENCES
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