Charge and Spin Transport in an Organic Molecular Square

Article abstract of DOI:10.1002/anie.201504576

Understanding electronic communication among multiple chromophoric and redox units requires construction of well-defined molecular architectures. Herein, we report the modular synthesis of a shape-persistent chiral organic square composed of four naphthal

Full text of DOI:10.1002/anie.201504576

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201504576
German Edition: DOI: 10.1002/ange.201504576
Organic Semiconductors
Charge and Spin Transport in an Organic Molecular Square
Yilei Wu, Siva Krishna Mohan Nalluri, Ryan M. Young, Matthew D. Krzyaniak,
Eric A. Margulies, J. Fraser Stoddart,* and Michael R. Wasielewski*
Abstract: Understanding electronic communication among
multiple chromophoric and redox units requires construction
of well-defined molecular architectures. Herein, we report the
modular synthesis of a shape-persistent chiral organic square
composed of four naphthalene-1,8:4,5-bis(dicarboximide)
(NDI) sides and four trans-1,2-cyclohexanediamine corners.
Single crystal X-ray diffraction reveals some distortion of the
cyclohexane chair conformation in the solid state. Analysis of
the packing of the molecular squares reveals the formation of
highly ordered, one-dimensional tubular superstructures, held
enantiomerically pure cyclic oligomers of NDI—namely, (À)-
and (+)-3NDI (trimer)[10a] and (À)- and (+)-2NDI
(dimer)[10b]—and investigated their electronic and magnetic
properties. The p systems of these units in the macrocycles
interact strongly. Their one-electron chemical reduction
results in stable radical anions in which the electron is
shared fully between the NDI units. In the quest to advance
our understanding of how molecular geometry affects
through-space orbital overlap and electron sharing, we have
designed and developed a modular synthesis (Scheme 1) of
a pair of chiral shape-persistent[11] macrocyclic diimide
squares[12]—namely, (À) and (+)-4NDI. In the present
work, we have investigated the photophysical and magnetic
properties of 4NDI and compared them with those of the
smaller macrocycles—namely, 2NDI and 3NDI—as well as
with a monomeric reference compound Ref-NDI. 4NDI has
has been characterized by 1) high-resolution mass spectrom-
etry in the gas phase, 2) 1D and 2D NMR spectroscopies, as
well as 3) optical spectroscopies in the solution phase, and
4) single-crystal X-ray diffraction in the solid state. Further-
more, we show by means of electronic spectroscopy and cyclic
voltammetry how the excitonic and electronic interactions
between neighboring chromophores are strongly dependent
on their relative orientations and center-to-center distances.
We have employed ultrafast transient absorption spectrosco-
py to reveal how the excited-state dynamics in these non-
conjugated multichromophoric architectures also depends on
conformation. Finally, we established, by means of electron
paramagnetic resonance (EPR) and electron-nuclear double
resonance (ENDOR) spectroscopies, that electron sharing
occurs between all four NDI units in monoreduced [4NDI]CÀ,
a remarkable result given the small interchromophoric over-
lap of the p orbitals enforced by the square geometry.
À
=
together by means of multiple [C H···O C] hydrogen-bond-
ing interactions. Steady-state and time-resolved electronic
spectroscopies show strong excited-state interactions in both
the singlet and triplet manifolds. Electron paramagnetic
resonance (EPR) and electron-nuclear double resonance
(ENDOR) spectroscopies on the monoreduced state reveal
electron sharing between all four NDI subunits comprising the
molecular square.
C
onstruction of ordered organic molecular architectures
capable of delocalization or fast hopping of charge among
neighboring units is a prerequisite for organic electronics and
photovoltaics.[1] Extensive experimental effort has been made
by us[2] and others[3] to develop multichromophoric systems
capable of significant charge delocalization. While hole
hopping between up to seven porphyrins has been observed[3a]
in multiporphyrinic assemblies by using electron paramag-
netic resonance (EPR) spectroscopy, the direct observation of
electron sharing over more than three aromatic units is
unprecedented.
Naphthalene-1,8:4,5-bis(dicarboximide)
(NDI)
(Scheme 1) and its derivatives[4] have attracted considerable
attention as n-type electroactive materials in organic field-
effect transistors[5] (OFETs) and photovoltaics[6] (OPVs).
NDIs can also be used 1) as versatile units for molecular
recognition,[7] 2) as electron acceptors for photoinduced
charge- and energy-transfer studies,[8] and 3) as platforms
for organocatalysis.[9] Recently, we have reported two pairs of
4NDI was prepared in an overall yield of 8% by stepwise
condensations, starting from naphthalenetetracarboxylic dia-
nhydride and (R,R)-trans-1,2-cyclohexanediamine in DMF at
1308C (Scheme 1). This stepwise route[13] facilitates purifica-
tion and leads to an improved yield of 4NDI compared to
a one-pot approach (ca. 1% yield; see the Supporting
Information for further details). The NDI square 4NDI was
identified 1) by atmospheric-pressure photoionization cou-
pled to high-resolution mass spectrometry (APPI-HRMS;
Figure S1 in the Supporting Information) and 2) by NMR
spectroscopy. The 1H NMR spectrum (Figure S2) exhibits the
characteristic resonances from the protons of the NDI unit in
the region, d = 8–9 ppm, and the aliphatic protons of the
linker cyclohexane chains at higher field, d = 1.5–6.5 ppm.
The observation of only a single set of resonances in the
13C NMR spectrum (Figure S3) of 4NDI is indicative of high
symmetry (point group D4) and restricted conformational
[*] Y. Wu, Dr. S. K. M. Nalluri, Prof. R. M. Young, Dr. M. D. Krzyaniak,
E. A. Margulies, Prof. J. F. Stoddart, Prof. M. R. Wasielewski
Argonne-Northwestern Solar Energy Research (ANSER) Center and
Center for the Chemistry of Integrated Systems (CCIS)
Department of Chemistry, Northwestern University
2145 Sheridan Road, Evanston, IL 60208-3113 (USA)
E-mail: stoddart@northwestern.edu
m-wasielewski@northwestern.edu
Supporting information for this article (including experimental
1
details on synthesis, mass spectrometry, H and 13C NMR, steady-
state and time-resolved electronic spectroscopy, electrochemical
measurements, EPR and ENDOR spectroscopy) is available on the
WWW under http://dx.doi.org/10.1002/anie.201504576.
1
mobility. A comparison of the H (Figure 1) and 13C (Fig-
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linked together by four
(S,S)-trans-1,2-cyclohexane-
diamine corners, resulting in
a rigid square-shaped mac-
rocycle with a face-to-face
distance of 11 Š and an
easily accessible interior
cavity.[16] A careful analysis
of the X-ray diffraction data
of all three NDI macrocy-
cles reveals the ring pucker-
ing of the cyclohexane
bridging units from their
ideal chair conformations,
a situation which is sup-
ported by NMR spectrosco-
py. The values of selected
bond length, bond angles,
and torsional angles show
(Figure S7) an increase of
the ]N1-C1-C2 (and ]C1-
C2-N2) bond angles from
1128 in a strain-free cyclo-
hexane[17] with vicinal dii-
mide substituents in the
equatorial positions up to
1168 in 2NDI and 4NDI,
and a decrease of ]N1-C1-
Scheme 1. Stepwise preparation of (À)-4NDI from (R,R)-trans-1,2-cyclohexanediamine and naphthalenetetra-
carboxylic dianhydride. The enantiomer (+)-2NDI was obtained in a similar fashion from (S,S)-trans-1,2-
cyclohexanediamine.
ure S4) NMR spectra of this series of macrocyclic diimides
shows a downfield shift of the two diastereotopic proton
resonances (from 8.26 and 8.23 ppm for 2NDI, to 8.49 and
8.47 ppm for 3NDI, and 8.60 and 8.57 ppm for 4NDI), which
are resolved into separate doublets (3JAB = 7.5 for 2NDI and
3JAB = 7.7 for 3NDI and 4NDI) because of the restricted
C2-N2 gauche torsional angles from an unstrained value of
548[17] down to 488 in 3NDI, 458 in 4NDI, and 378 in 2NDI. The
average torsional angle magnitudes within the cyclohexane
rings are 598 in 2NDI, 588 in 3NDI, and 578 in 4NDI. These
values are significantly higher than that (54.98) reported[16] for
a cyclohexane ring in an unstrained chair conformation,
indicating significant ring puckering. The strain energy
enforced by the macrocyclization is also localized on the
NDI units, forcing their aromatic planes to be bent with
a displacement of 0.4 Š in 2NDI and 0.1 Š in both 3NDI and
4NDI.[18] The relative total strain energy can be estimated by
comparing the computed (B3LYP/6-31G**) ground-state
energies (Figure S8), which show that 3NDI is the least
strained compound among the three NDI polygons. In the
single-crystal superstructure of 4NDI, two crystallographi-
cally distinct NDI squares are observed (Figure 2b) to be held
À
rotation around the C N bonds and the rigidity of cyclohex-
ane ring (Table 1).[14]
In order to elucidate further structural details (i.e.,
constitution, molecular shape, bond lengths, bond angles),
we performed an X-ray diffraction analysis[15] (Figure 2) on
single crystals, obtained by slow vapor diffusion of n-hexane
into a 1.0mm solution of 4NDI in CHCl3 at room temperature.
The chiral (++)-4NDI is made (Figure 2a) from four NDI faces
Table 1: 1H and 13C chemical shifts obtained from the 1H and 13C NMR
spectra recorded in CDCl3 at 298 K, for the cyclohexane ring protons and
carbon atoms in 2NDI, 3NDI and 4NDI in comparison with the
resonances for the analogous protons and carbons in 2NIA.
À
=
together by means of multiple intermolecular [C H···O C]
hydrogen bonds between the carbonyl oxygen atoms and the
NDI protons (3.0 Š < dC···O C = 3.5 Š, 2.1 Š < dCÀ
<
=
C
=
H···O
2.7 Š, 1388 < ]CÀH···O < 1528) interactions. These supramolec-
ular dimers, which are composed of two 4NDI molecules with
a twist angle of around 458, stack along the c-axis (Figure 2b
and Figure S6) to form infinite coaxial channels filled with
disordered solvent molecules (Figure 2c). Bundles of these
sheath-like organic nanotubes are then tightly packed to form
well-ordered arrays that constitute the single crystal (Fig-
ure S6b).
d [ppm]
HC HD HE HF HG C1/ C3/ C5/ Distortion[a,b]
C2
C4
C6
2NIA 6.32 2.70 2.02 2.00 1.70 54.0 29.2 25.5
0[c]
2NDI 5.59 2.69 2.32 2.04 1.72 57.1 30.8 26.1 13.1
3NDI 6.23 2.48 1.99 1.94 1.67 54.0 30.1 25.9
4NDI 6.40 2.34 1.88 1.85 1.63 54.9 30.6 25.7
1.0
3.1
[a] The distortion on the cyclohexane chair is expressed as sum of the
square of the deviation of the chemical shifts with respect to those for
2NIA. [b] To a first approximation the distortion can be correlated with
the relative strain energy computed by DFTmethods (RB3LYP/6-31G**).
See Figure S24. [c] By definition.
The optical properties were investigated by UV/Vis
absorption spectroscopy (Figure 3a). These spectra show an
!
!
intensity reversal for the 0 0 and 0 1 vibronic bands of the
!
S1 S0 electronic transition (with maxima at 360 and 380 nm,
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respectively) in nNDI, where n = 2
and 3, with respect to those of
monomeric Ref-NDI, as a conse-
quence of interchromophoric exci-
ton coupling between neigh! boring
!
1
NDI units.[19] The A0 0/A0
ratio
for these vibronic bands increases as
the distance between the electronic
transition moments, which lie paral-
lel to the long axis of each NDI unit,
becomes larger, in good agreement
with the excitonic model developed
by Kasha.[19] The chiroptical activi-
ties in nNDI are indicated by the
strong signal in the electronic circu-
lar dichroism (CD) spectra (Fig-
ure 3b), which display several sets
of degenerate exciton-type Cotton
effects. The sign and intensity of
these CD bands are correlated with
the distance between the NDI chro-
mophores and the dihedral angle
between their electronic transition
dipoles.[17] In particular, the negative
Cotton effect at around 380 nm of
(À)-nNDI, which is obtained from
(R,R)-trans-1,2-cyclohexanedia-
mine, reflects the negative gauche
torsional angle of the N1-C1-C2-N2
bond system of the cyclohexane
skeleton. The unsymmetrical shape
of the CD band at 380 nm can be
explained by the effect of cancella-
tion by the closely lying CD band at
360 nm.
Figure 1. A comparison of the 1H NMR spectra (500 MHz, CDCl3, 298 K) of (À)-2NIA, (À)-2NDI,
(À)-3NDI, and (À)-4NDI.
The excited state dynamics of this series of cyclic diimide
molecules in CH2Cl2 were followed using femtosecond Vis/
NIR transient absorption spectroscopy (fsTA). The cyclic
tetramer 4NDI and trimer 3NDI show (Figure 3c) similar
behavior to that of the monomeric Ref-NDI. FsTA spectra,
following excitation with a 350 nm laser pulse, reveal
1
formation of *NDI in all three molecules within the instru-
ment response time (< 200 fs). Global target analysis of the
three-dimensional DA versus time and wavelength dataset
reveals (Figures S9–S12) species-associated differential spec-
1
tra that indicate the formation of short-lived *NDI states
(1.2 Æ 0.1, 2.5 Æ 0.1 and 2.0 Æ 0.1 ps for Ref-NDI, 3NDI, and
4NDI, respectively), which absorb at 606, 730, and 1135 nm,
followed by relaxation and fast intersystem crossing to long-
3
lived *NDI states, which absorb at 460 and 488 nm.[20] The
triplet state decays were followed by nanosecond transient
absorption (nsTA, Figures S16–S18). Remarkably, we
observed a time-dependent reversal of the intensities of the
two peaks of the triplet transient absorption spectrum on the
nanosecond timescale for 3NDI (12.6 Æ 0.6 ns) and 4NDI
(19.4 Æ 0.3 ns), an observation which is not present in the case
of the monomeric Ref-NDI. These spectral changes could be
indicative of the relaxation of a delocalized triplet exciton to
a more localized monomer-like triplet excited state, perhaps
Figure 2. a) Top view of a tubular representation of the crystal struc-
ture of molecular square (+)-4NDI, showing the nanometer-sized
cavity. The hydrogen atoms are omitted for the sake of clarity.
b) Expanded view of the cyclohexane skeleton. c) Side-on view showing
the supramolecular dimer of two crystallographically distinct (+)-4NDI
molecules held together by means of multiple intermolecular CH···O
hydrogen bonds. Bond angles are shown in red and torsional angles in
green. d) A blend of tubular and space-filling representations of the
solid-state superstructures of (+)-4NDI showing the tubular alignment
along the c-axis.
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Figure 3. a) UV/Vis absorption and b) CD spectra of (À)-2NDI (red traces), (À)-3NDI (green traces), and (À)-4NDI (blue traces) in CH2Cl2
compared with the absorption and CD spectra of the Ref-NDI (black traces). (T=298 K and the concentration was adjusted in order to have the
absorbance equal 1.0 at 360 nm.) c) Combined visible and near-infrared femtosecond transient absorption spectra of Ref-NDI, (À)-2NDI, (À)-
3NDI, and (À)-4NDI in CH2Cl2 at 298 K following excitation with a 350 nm laser pulse (0.6 mJ/pulse). d) Cyclic voltammograms (0.5 mm in
CH2Cl2, 100 mm TBAPF6, 50 mVsÀ1, 298 K) of Ref-NDI (black trace), (À)-2NDI (red trace), (À)-3NDI (green trace) and (À)-4NDI (blue trace).
The (+)-nNDI analogues shows identical results except for CD spectra, which display similar Cotton effects, but with inverted signs.
accompanied by a geometrical change. The triplet nature of
the latter long-lived excited states is also supported by time-
resolved photoluminescence (TRPL) spectroscopy (Figur-
es S19–S21) and time-resolved EPR (TREPR) spectroscopy
(Figure S22) at 77 K, which reveal phosphorescence and
triplet EPR spectra for all three compounds, similar to those
reported[21] for the NDI triplet state. Conversely, the cofa-
cially p-stacked 2NDI displays ultrafast formation of an
excimer-like state, characterized (Figure 3c) by the appear-
ance[22] of a red-shifted transient absorption in the NIR within
the instrument response time.[23]
The reduction potentials for this series of macrocyclic
NDI molecules are summarized in Table S1 and their cyclic
voltammograms (CVs) are presented in Figure 3d. The CVs
of 2NDI and 3NDI show a clear splitting of the reduction
waves into distinct reversible one-electron processes as
a consequence of strong electronic coupling. On the other
hand, the CVof the tetrameric 4NDI is more complex and less
well-resolved. The differential pulse voltammogram (DPV,
Figure S24) along with the CV shows that the first reduction
at around À0.71 V generates a distinct, singly reduced
[4NDI]CÀ radical anion closely followed by 1) a three-electron
reduction process at around À0.85 V and 2) a broad four-
electron reduction wave at À1.32 V. This observation is
particularly striking as it suggests that, despite the electronic
communication between the NDI units, multielectron reduc-
tion of 4NDI only requires a minimal Coulombic energy
penalty, which makes this nanometer-sized square an appeal-
ing all-organic multielectron acceptor (up to eight electrons)
for potential use in artificial photosynthesis and molecular
supercapacitors. Upon the partial reduction of Ref-NDI,
2NDI, 3NDI, and 4NDI with one equivalent of cobaltocene as
a chemical reductant, the stable monoradical anion, as
monitored by UV/Vis/NIR absorption (Figure S25), was
generated in quantitative yield and subjected to further
EPR spectroscopic studies.
The EPR spectra of [Ref-NDI]CÀ, [2NDI]CÀ, [3NDI]CÀ, and
[4NDI]CÀ and their corresponding simulations are shown in
Figure 4a. If the electron is shared equally within the
macrocycles, it is expected from the McConnell relation-
ship[24] that the isotropic hyperfine couplings should decrease
linearly with the number of NDI subunits present. In order to
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nNDICÀ, is fully shared among all n NDI-based redox sites,
suggesting that the electron migration rate in nNDICÀ is faster
than the timescale (> 107 sÀ1) of the EPR/ENDOR experi-
ments, even when the angle between the p systems
approaches orthogonality and the orbital overlap is minimal,
as in 4NDI. These results also suggest that a fast hopping
mechanism may underlie the efficient electron sharing within
the cyclic NDI oligomers linked by rigid vicinal diamine
linkers. The potential of shape-persistent diimide macrocycles
to carry out long-distance electron transport required for
organic electronics and photovoltaics applications is appar-
ent.
Acknowledgements
We thank Dr. Saman Shafaie for collecting high-resolution
mass spectrometric data, Dr. Amy A. Sarjeant for solving the
single crystal X-ray structures, and Rufei Shi for helpful
discussions. This research was supported by the Chemical
Sciences, Geosciences, and Biosciences Division, Office of
Basic Energy Sciences, DOE, under grant no. DE-FG02-
99ER14999 (M.R.W.) and is also part (Project 32-949) of the
Joint Center of Excellence in Integrated Nano-Systems
(JCIN) at King Abdulaziz City for Science and Technology
(KACST) and Northwestern University (NU). The authors
would like to thank both KACST and NU for their continued
support of this research. R.M.Y. was supported as part of the
ANSER Center, an Energy Frontier Research Center funded
by the U.S. Department of Energy (DOE), Office of Science,
Basic Energy Sciences (BES), under Award #DE-SC0001059
(fsTA experiments). Y.W. thanks the Fulbright Scholars
Program for a Graduate Research Fellowship and also
gratefully acknowledges support of a Ryan Fellowship from
the NU International Institute for Nanotechnology (IIN).
Figure 4. a) CW-EPR spectra and b) 1H ENDOR spectra of [Ref-NDI]CÀ,
[(À)-2NDI]CÀ, [(À)-3NDI]CÀ and [(À)-4NDI]CÀ produced by monoreduc-
tion of neutral states with 1 equiv of cobaltocene. Overlay between the
experimental spectra (black traces, 0.25 mm in CH2Cl2, 298 K) and
their simulated spectra (red traces). Isotropic nitrogen and proton
hyperfine coupling constants are obtained from CW-EPR and 1H
ENDOR, respectively.
measure the isotropic hydrogen hyperfine coupling, aH, more
accurately, continuous-wave ENDOR[25] was also performed
(Figure 4b). The results from fitting the ENDOR spectra
show an n-fold reduction of aH for the [nNDI]CÀ radical anions
relative to those of [Ref-NDI]CÀ, consistent with electron
sharing among n NDI units.[26] The reason for such efficient
electron sharing could be a consequence of the perfect match
of LUMOs associated with the NDI units in the macrocycles
which have much higher symmetry than the p-stacked or
linear arrays.
Keywords: electron transport · hydrogen bonding ·
molecular squares · molecular electronics ·
organic semiconductors
In summary, we have prepared a shape-persistent chiral
molecular square 4NDI using a stepwise approach from
readily available starting materials. We have observed strong
bisignate Cotton effects whose sign supports the absolute
configuration of the compound. The solid-state X-ray dif-
fraction analysis of single crystals corroborates the conforma-
tional information obtained from spectroscopic studies and
reveals a remarkably well-aligned tubular superstructure,
sustained by intermolecular hydrogen bonding interactions.
We envision that the strong induced chiroptical activity
combined with the accessible cavity of 4NDI could be
potentially used for enantioselective recognition, sensing
and catalysis. Moreover, electronic spectroscopies and cyclic
voltammetry of these structurally well-defined nNDI poly-
gons show that the strength of the excitonic-coupling and
electronic communication between the neighboring NDI
units is highly dependent on their separations and relative
orientations, affecting the through-space dipole–dipole inter-
action and interchromophoric orbital overlap. The EPR/
ENDOR investigations indicate that the unpaired electron in
the monoreduced mixed-valence oxidation state, namely
How to cite: Angew. Chem. Int. Ed. 2015, 54, 11971–11977
Angew. Chem. 2015, 127, 12139–12145
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1
(ca. 13-fold) in the dimer. The 2D diffusion ordered H NMR
(DOSY; Figure S5) of nNDI was also performed and shows
a clear decrease in diffusion constants (D = 7.9 m2 sÀ1 for 2NDI,
5.7 m2 sÀ1 for 3NDI and 5.0 m2 sÀ1 for 4NDI) in line with
increasing molecular radii for this series of NDI polygons.
[15] Crystallographic data for C80H56N8O16 (M = 1385.32): ortho-
rhombic, space group P21212 (no. 18), a = 21.680(2), b = 30.545-
(5), c = 15.5369(18) Š, V= 10289(2) Š3, Z = 4, T= 100.0 K, m-
(CuKa) = 0.524 mmÀ1, Dcalc = 0.894 gmÀ1 m3, 17214 reflections
measured (5.69 ꢀ 2q ꢀ 88.98), 7980 unique (Rint = 0.0929, Rsigma
=
0.1715) which were used in all calculations. The final R1 was
0.0552 (I > 2s(I)) and wR2 was 0.1120 (all data). CCDC 1041583
contains the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre.
[16] With a size comparable to three p–p stacking distances, 4NDI
can potentially encapsulate two aromatic guests, such as
benzene, pyrene, or naphthalene derivatives, by means of
aromatic charge-transfer or p-stacking interactions.
´
´
[17] a) J. Gawronski, F. Kazmierczak, K. Gawronska, U. Rychlewska,
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[12] It is worth mentioning that while the metallomacrocyclic
molecular squares have become easily accessible, thanks to the
metal-directed self-assembly approaches, covalent molecular
squares still remain rare on account of the synthetic challenges
they pose. For examples of metallocyclic squares and their
applications, see: a) M. Fujita, J. Yazaki, K. Ogura, J. Am. Chem.
Soc. 1990, 112, 5645 – 5647; b) P. J. Stang, B. Olenyuk, Acc.
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12091; b) J. Gawronski, M. Brzostowska, K. Kacprzak, H.
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Gawronski, M. Brzostowska, K. Gawronska, J. Koput, U.
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[18] The bending of the aromatic systems, such as those found in
cycloparaphenylene, is of considerable interest, not only because
of synthetic challenge and aesthetic beauty, but also because of
the desire to understand how the strain can be tapped to affect
device performance. For recent examples, see: a) R. Jasti, J.
Bhattacharjee, J. B. Neaton, C. R. Bertozzi, J. Am. Chem. Soc.
2008, 130, 17646 – 17647; b) B. L. Merner, L. N. Dawe, G. J.
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Yamago, Y. Watanabe, T. Iwamoto, Angew. Chem. Int. Ed. 2010,
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S. Hiroto, H. Shinokubo, Angew. Chem. Int. Ed. 2013, 52, 5740 –
5743; Angew. Chem. 2013, 125, 5852 – 5855; f) P. J. Evans, E. R.
Darzi, R. Jasti, Nat. Chem. 2014, 6, 404 – 408. For a review of
cylophane chemistry, see: Modern Cyclophane Chemistry (Eds.:
R. Gleiter, H. Hopf), Wiley-VCH, Weinheim, 2004.
[19] M. Kasha, H. R. Rawles, M. L. El-Bayoumi, Pure Appl. Chem.
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[13] For other examples of preparation of NDI macrocycles using the
protection group strategy, see: a) S. Gabutti, M. Knutzen, M.
Neuburger, G. Schull, R. Berndt, M. Mayor, Chem. Commun.
2008, 2370 – 2372; b) S. Gabutti, S. Schaffner, M. Neuburger, M.
Fischer, G. Schafer, M. Mayor, Org. Biomol. Chem. 2009, 7,
3222 – 3229.
[20] The observed extension of the singlet-excited-state lifetime can
be attributed to the decrease of nonradiative rate constants in
the rigid macrocycles, where some of vibrational modes, needed
for either ISC or internal conversion, are more restricted.
[21] G. P. Wiederrecht, W. A. Svec, M. R. Wasielewski, T. Galili, H.
Levanon, J. Am. Chem. Soc. 2000, 122, 9715 – 9722.
[22] For NIR absorption of aromatic excimers, see, for example: a) R.
Katoh, E. Katoh, N. Nakashima, M. Yuuki, M. Kotani, J. Phys.
Chem. A 1997, 101, 7725 – 7728; b) K. E. Brown, W. A. Salam-
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[23] This excimer-like state decays within 21 ns into a long-lived
triplet state, which has very different TA (Figures S13 – S15) and
EPR spectra (Figure S18) with respect to the monomeric Ref-
NDI because of the strong electronic coupling between the two
NDI subunits. In particular, the TA spectrum of the excited
[14] This decrease of the shielding effect on the protons by adjacent
NDI p systems correlates with the NDI–NDI distance, which
becomes larger as n increases. Moreover, the aliphatic protons of
the cyclohexane rings display the characteristic pattern of
1H NMR resonances of a “locked” chair conformation with
vicinal axial-axial (3Ja–a = 10–12 Hz) and axial-equatorial (3Ja–e
=
3–4 Hz) coupling constants, which confirms the diequatorial
conformation for the imide substituents. Although the spectra
are not resolved well enough to permit an accurate determi-
nation of J values, the pronounced differences in chemical shifts
of the cyclohexane protons suggest a substantial distortion of its
skeleton in the three NDI polygons compared to an open-chain
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Angewandte
Chemie
triplet state of 2NDI is considerably red-shifted (750 nm)
compared to that (488 nm) of the Ref-NDI. The triplet EPR
spectrum of 2NDI is also narrowed. Fitting a simulation to the
signal (Figure S21) gave zero-field splitting parameters of j D j=
607 and E/D = 0.0132 (units of 10À4 cmÀ1). In contrast, zero-field
splitting parameters for typical NDI monomers have been
reported[20] as j D j= 743 and E/D = 0.0162. These results may
indicate that a charge transfer configuration, in which the two
unpaired electrons constituting the triplet state each reside on
adjacent NDI molecules, contributes to the overall triplet
wavefunction. See: M. C. Thurnauer, J. J. Katz, J. R. Norris,
Proc. Natl. Acad. Sci. USA 1975, 72, 3270 – 3274.
[25] ENDOR spectroscopy of the radical anions was performed in
CH2Cl2 solution using the resonance condition nÆ =j nn Æ aH/2 j ,
in which nÆ is the ENDOR transition frequency and nn is the
proton Larmor frequency. For more details, see, for example: H.
Kurreck, B. Kirste, W. Lubitz, Electron Nuclear Double Reso-
nance Spectroscopy of Radicals in Solution, VCH, Weinheim,
1988.
[26] The nitrogen isotropic hyperfine interaction, aN, was obtained
through simulation of the cw-EPR spectra by keeping aH fixed to
the values obtained from ENDOR.
[24] a) H. M. McConnell, J. Chem. Phys. 1956, 24, 632 – 632; b) H. M.
McConnell, J. Chem. Phys. 1956, 24, 764 – 766; c) J. R. Norris,
R. A. Uphaus, H. L. Crespi, J. Katz, Proc. Natl. Acad. Sci. USA
1971, 68, 625 – 628.
Received: May 26, 2015
Published online: August 20, 2015
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