Electron Delocalization in Perylene Diimide Helicenes

Article abstract of DOI:10.1002/anie.201607878

We report two new helicenes derived from the double fusion of an acene with two perylene diimide (PDI) subunits. These PDI-helicene homologs exhibit very different structural and electronic properties, despite differing by only a single ring in the link between the PDI units. The shorter inter-PDI link brings the two PDI subunits closer together, and this results in the collision of their respective π-electron clouds. This collision facilitates intramolecular through-space electronic delocalization when the PDI-helicene is reduced.

Full text of DOI:10.1002/anie.201607878

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Communications
Chemie
International Edition: DOI: 10.1002/anie.201607878
German Edition: DOI: 10.1002/ange.201607878
Helicenes
Electron Delocalization in Perylene Diimide Helicenes
Nathaniel J. Schuster, Daniel W. Paley, Steffen Jockusch, Fay Ng, Michael L. Steigerwald,* and
Colin Nuckolls*
Abstract: We report two new helicenes derived from the
double fusion of an acene with two perylene diimide (PDI)
subunits. These PDI-helicene homologs exhibit very different
structural and electronic properties, despite differing by only
a single ring in the link between the PDI units. The shorter
inter-PDI link brings the two PDI subunits closer together, and
this results in the collision of their respective p-electron clouds.
This collision facilitates intramolecular through-space elec-
tronic delocalization when the PDI-helicene is reduced.
H
ere we describe new perylene-diimide-based molecules
that demonstrate how narrowing the distance between redox-
active subunits can reinforce the delocalization of electrons
added to a fully conjugated helical framework. Perylene-
3
,4,9,10-tetracarboxylic diimide (PDI) provides a wellspring
of intensely absorbing, robust dye materials with substantial
[
1]
electron affinities. Readily prepared and easily derivatized,
[
2–4]
these materials excel as n-type semiconductors
in organic
and organic photovoltaics
Some of the most efficient non-fullerene-based
[
5,6]
field effect transistors
[7,8]
(
OPVs).
[
9–11]
OPVs incorporate oligomeric PDI-nanoribbons.
In these
nanoribbons, a succession of steric interactions contort the p-
surface to form a twistacene (Figure 1a)—a polyaromatic
helix in which the stereogenic axis transects the core of the
[
12]
molecule.
By coiling PDI-based nanoribbons about an
alternative axis (Figure 1b), new helical scaffolds—PDI-
helicenes—can be realized.
A helicene consists of angularly fused aromatic subunits
arranged consecutively in a helix. Its p-surface coils around
a nonintersecting stereogenic axis to give a strained cylin-
drical core. This strain often encumbers the synthesis of
[13,14]
helicenes,
wealth of fascinating chiral and chiroptical properties.
but they remain compelling targets due to their
[15]
Figure 1. The helical contortion of PDI-nanoribbons to make a) twist-
acenes or b) helicenes. c) Acene fusion with the bay of the PDI yields
PDI-helicenes (n=0 and 1). d) Fusion with two PDI moieties forms
Additionally, p-to-p interactions between redox-active qui-
nones within helicene radical anions have been shown to
(M)- and (P)-NPDH and (M)- and (P)-APDH.
[16,17]
enhance electronic delocalization
and to promote the
assembly of chiral supramolecular aggregates with pro-
[
18,19]
nounced nonlinear optical properties.
More recently,
integrating redox-active PDI into helicenes, we intended to
combine the desirable properties of both types of molecular
structures into a single nanoribbon.
This manuscript details the synthesis and characterization
of two new helicenes that result from the double fusion along
one of the zigzag edges of an acene with the bay of two PDI
monomers (Figure 1c, n = 0 and 1). Fusion with naphthalene
or anthracene forms naphthyl- or anthracenyl-linked PDI-
dimer helicene (NPDH and APDH). We separated the left-
installing vinyl ruthenium and alkynyl iron complexes on the
termini of [6]helicene has been shown to amplify the
chiroptical properties of the helicene core, as well as to
[20À22]
enable redox-triggered chiroptical switching.
Thus, by
[
*] N. J. Schuster, Dr. D. W. Paley, Dr. S. Jockusch, Dr. F. Ng,
Dr. M. L. Steigerwald, Prof. C. Nuckolls
Chemistry Department, Columbia University
New York, NY 10027 (USA)
(
(
M) and right-handed (P) helicenes of both compounds
Figure 1d) and discovered that NPDH resists racemization
E-mail: mls2064@columbia.edu
cn37@columbia.edu
at very high temperatures (> 2508C). The enantiomers of
APDH interconvert at room temperature. Intermolecular p-
to-p stacking mediates the self-assembly of single-handed
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201607878.
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supramolecular helixes of APDH in the solid state. The large
gap separating the PDI subunits of APDH minimizes intra-
molecular p-to-p contact. Conversely, the [6]helicene core of
NPDH positions its PDI subunits within 3.2 ꢀ of one another.
This proximity facilitates the intramolecular collision of their
p-electron clouds, which enhances the delocalization of
electrons added to NPDH.
Scheme 1 illustrates the simple synthetic route to prepare
NPDH and APDH in near-quantitative yields. Our strategy
stemmed largely from two previous findings: 1) bromination
can occur exclusively at the bay position of N,N’-dialkylated
enantiopure [6]helicene racemizes completely in solution in
[29]
about 13 minutes at 2208C,
extended double [6]helicene isomerizes in solution at
and a recently reported p-
[
30]
2308C. Its ease of synthesis and enantiostability at high
temperatures make NPDH an excellent candidate for various
applications in chiral nanoscience, a conclusion reinforced by
the wealth and intensity of the circular dichroism (CD)
transitions of its enantiomers (Figure 2). The particularly
pronounced Cotton effect at 523 nm underscores the syner-
gistic combination of the inherent chirality of [6]helicene with
the strong absorptivity of the PDI chromophores.
Scheme 1. Two-step synthesis of PDI-helicenes NPDH and APDH from
brominated PDI 2 [R=CH(C H ) ].
Figure 2. CD spectra of (M)- and (P)-NPDH in THF (10 mm, 1 cm
pathlength).
5
11 2
[
23]
PDI;
and 2) acenes fuse preferentially at their peri-
We further investigated the molecular structures of
NPDH and APDH using density functional theory (DFT)
and single-crystal X-ray diffraction (SCXRD). Solutions of
racemic APDH readily yielded crystals of sufficient quality
for SCXRD. The solid-state structure (Figure 3a) closely
resembles the DFT-optimized, gas-phase model of APDH
(Figure S2). DFT shows the planes within the two perylene
cores of each PDI bisect at 458, whereas in the solid state the
same angle measures 498. Looking down the stereogenic axis
of APDH reveals that four pairs of p-bonded atoms eclipse
one another (Figure S3a). The splaying of the p-surface in the
solid state separates the nearest of these eclipsing pairs by
5.7 ꢀ (5.8 ꢀ in the gas-phase structure). This distance exceeds
the range of substantive p-to-p interaction. The crystal
structure of APDH reveals extensive intermolecular p-to-p
interactions between the helicenes, with successive PDI pairs
slip-stacking cofacially within 3.4 ꢀ of each other (Figure 3b).
These interactions guide the formation of supramolecular
helixes that complete one revolution every third helicene
(Figures S3b and S3c). The enantiomers of APDH impart
their chirality to the helixes: (P)-helicenes form (P)-helixes
while (M)-helicenes form (M)-helixes within a racemic crys-
tal.
positions during intramolecular oxidative photocycliza-
[
24]
[25–27]
tions, particularly with PDI and its precursors.
Accord-
ingly, we prepared NPDH and APDH in two steps from
brominated PDI 2. First, the Suzuki–Miyaura cross-cou-
[28]
pling of 2 with either 2,7-diborylated naphthalene or 2,7-
diborylated anthracene afforded the acene-bridged PDI
dimers 3a or 3b, respectively. Subsequent oxidative ultra-
violet photocyclizations of these intermediates provided
NPDH and APDH as racemates in quantitative yields. As
such, trituration and filtration were sufficient to obtain
analytically pure NPDH and APDH as orange and red
solids, respectively. There was no indication, in either case, of
cyclization at the alternative ortho-position of the acenes.
Due to their contorted conformations and four branched alkyl
tails, both PDI-helicenes dissolved readily in a variety of
organic solvents, including dichloromethane (DCM), benzene
(
PhH), and tetrahydrofuran (THF). They were practically
insoluble in methanol, ethanol, and hexanes.
We readily separated the (M)- and (P)-PDI-helicenes of
NPDH and APDH using chiral HPLC (see Figure S1 in the
Supporting Information) and examined their resistance to
thermally induced racemization. Analysis of the previously
resolved samples of APDH by chiral HPLC revealed that its
enantiomers interconvert at room temperature in solution.
Conversely, the steric congestion intrinsic to the [6]helicene
core of NPDH significantly impedes inversion. Remarkably,
a single enantiomer of NPDH heated at 2508C in diphenyl
ether for one hour showed no isomerization. For comparison,
We have been unable to grow crystals of NPDH of
sufficient quality for SCXRD. Nevertheless, the rigidity of
NPDH suggests the DFT model (Figure 3c) adequately
represents its molecular structure. Similar to APDH, the
PDI planes in NPDH intersect at a 488 angle, although this
distortion does not open a significant gap between the PDI
2
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Figure 4. UV/Vis absorbance spectra of PDI 1, NPDH, and APDH in
DCM (10 mm, 1 cm pathlength).
NPDH resemble the vibronic progression of PDI 1, whereas
the lowest-energy absorption of APDH (lmax = 561 nm)
manifests as a broad band. DFT calculations indicate that
the HOMO of APDH (Figure S5) localizes on the electron-
rich anthracenyl link. The LUMOs of both PDI-helicenes
(Figures S5 and S6) distribute across the electron-poor PDI
subunits. Thus, the broad band in APDH corresponds to
anthracene-to-PDI intramolecular charge transfer (ICT).
Excitation of APDH at 561 nm generates emission peaks
that shift bathochromically with increasing solvent polarity,
ranging from 575 nm in toluene to 623 nm in N,N-dimethy-
lacetamide. This positive solvatochromism supports our
Figure 3. a) Structure of (M)-APDH from SCXRD. b) Left-handed
supramolecular helix assembled in the solid state. Free solvent, the
CH(C H ) tails, and hydrogen atoms have been removed to provide
5
11 2
a better view of the p-surface. Thermal ellipsoids are set at 20%
probability. c) DFT-optimized model of (M)-NPDH (B3LYP/6-31G**).
Its methyl groups, which substitute for the CH(C H ) tails to simplify
5
11 2
assignment of the least energetic absorption of APDH as
the calculation, and hydrogen atoms have been removed in the image.
[37]
ICT.
For NPDH, the HOMO delocalizes across the
naphthyl link and PDI subunits, but the HOMO-2 lies
primarily on the naphthyl link (Figure S6). Thus, acene-to-
PDI ICT bands in NPDH originate from the HOMO-2,
although they are obscured by intense electronic transitions in
this region of the spectrum.
subunits. Looking down the stereogenic axis of NPDH shows
that ten pairs of p-bonded atoms eclipse one another (Fig-
ure S4). This model predicts the distance between the nearest
of these eclipsing pairs in NPDH to be 3.2 ꢀ, well within
twice the van der Waals radius of the carbon atom (3.4 ꢀ). As
such, we expected intramolecular p-to-p collisions to assist in
delocalizing electrons subsequently added to NPDH.
Electrochemical reductions of NPDH and APDH pro-
vided insight into the extent to which their anions delocalize
electrons. The cyclic voltammetry (CV) reduction waves of
both helicenes (Figure 5a) are reversible. The first reduction
of NPDH and APDH is cathodically shifted by 100 mV
relative to PDI 1, which arises from the LUMO-raising effect
UV/Vis absorbance spectroscopy provided evidence of
enhanced intramolecular through-space excitonic coupling
between the PDI subunits of NPDH relative to those of
APDH (Figure 4). Both dimers retain the characteristic
vibronic progression of the monomer PDI 1 (0-0 transition:
lmax = 525 nm); however, their corresponding progressions
are shifted hypsochromically (0-0 transitions: lmax = 516 nm
and 497 nm for NPDH and APDH, respectively). Intensifi-
cation of the 0-1 transition relative to the 0-0 transition in this
progression indicates the enhancement of through-space
[38]
of the electron-donating acene links. Despite differing by
only a single ring, NPDH and APDH generate very different
voltammograms. Whereas APDH displays two clear reduc-
tion events, NPDH exhibits four reduction events grouped in
two pairs. Spectroelectrochemical measurements (Figures S7
and S8) and, in particular, the four sets of isosbestic points
shared by sequential pairs of oxidation states (e.g., NPDH/
[
31]
À
À
2À
excitonic coupling between the PDI chromophores. Con-
sistent with the relative distances separating their PDI
subunits, NPDH exhibits a larger 0-1:0-0 transition ratio
than APDH. PDI dimers with similarly stacked chromo-
NPDHC , NPDHC /NPDH , etc.) confirm that each event in
the voltammogram of NPDH corresponds to a one-electron
addition, while each event in APDH corresponds to the
addition of two electrons. Therefore, both dimers can accept
four electrons, although the second and fourth reductions of
NPDH must overcome greater Coulombic repulsion than the
corresponding reductions of APDH. This repulsion suggests
NPDH possesses an enhanced ability to delocalize the first
and third added electrons, which cathodically shifts the
second and fourth reductions.
[32–36]
phores also exhibit intensification of the 0:1 transition.
However, conjugation between the chromophores in these
previous dimers is inhibited by the use of linkers orthogonal
to the p-surface of the PDI subunits.
The UV/Vis spectra of the PDI-helicenes differ signifi-
cantly in one respect: the lowest-energy absorptions of
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a contorted naphthyl-linked PDI dimer (CNPD, Figure 5d).
This regioisomer of NPDH also has a twisted naphthyl link
but orients its PDI subunits away from one another; thus, CV
can assess through-bond electronic coupling between the PDI
subunits in the absence of intramolecular p-to-p contact. The
voltammogram of CNPD (Figure 5a) exhibits two broad
reduction events that differ at most by 250 mV, suggestive of
a diminished capacity to delocalize electrons relative to
reduced NPDH. Therefore, intramolecular p-to-p collisions
between the PDI subunits of NPDH reinforce the through-
bond coupling mediated by the contorted naphthyl link to
delocalize electrons in the reduced PDI-helicene.
In summary, we have developed an expeditious, high
yielding synthesis of two acene-linked PDI-dimer helicenes,
NPDH and APDH. Simply by tailoring the length of the
acene link, we have instilled markedly different properties in
these PDI-helicene homologs. NPDH strongly resists ther-
mally induced racemization, whereas the enantiomers of
APDH interconvert in solution at room temperature. Exten-
sive intermolecular p-to-p interactions mediate the formation
of single-handed supramolecular helixes of APDH from
solution into the solid state. The gap separating the PDI
subunits of APDH minimizes significantly the intramolecular
p-to-p contact. Conversely, NPDH positions its PDI subunits
within 3.2 ꢀ of each other, resulting in collisions of their p-
electron clouds. These collisions enhance the delocalization of
electrons added to NPDH. Consequently, through-bond and
through-space electronic delocalization have been married
within a robust PDI-helicene framework. This combination of
advantageous properties charts a clear course towards the
application of NPDH and its derivatives in organic electron-
ics. The delocalization of electrons along a persistent helical
path anticipates the integration of this new molecular
architecture into chiroptical and magnetic materials.
Figure 5. a) Cyclic voltammograms of monomeric PDI 1, APDH,
CNPD, and NPDH in Ar-sparged DCM (1 mm) with 0.1m [Bu N][PF ]
4
6
as the supporting electrolyte. Continuous-wave EPR spectra at 240 K of
monoreduced b) PDI 1 and c) APDH and NPDH in DCM. d) Structure
of CNPD [R=CH(C H ) ].
5
11 2
The electrochemical differences between NPDH and
APDH originate from the intramolecular orientation of
their PDI subunits. Through-bond electronic coupling
between the four redox-active imide groups can occur by
two routes in these dimers: 1) across their perylene cores, an
effective delocalizing conduit as demonstrated previously for
[
39]
monomeric PDI;
and 2) through their contorted acene
links. Dimers with noninteracting PDI subunits would gen-
erate voltammograms comparable to that of PDI 1—two
peaks separated by a potential of 200 mV. Instead, the
reduction events in APDH differ by a potential of 250 mV,
CCDC 1498856 (APDH) contains the crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre.
À
À
À
À
whereas their analogs in NPDH (i.e., 1 to 3 and 2 to 4 )
differ by 350 mV. Accordingly, the PDI subunits couple Acknowledgements
electronically to one another in both dimers, but more
strongly in NPDH. Continuous-wave EPR spectroscopy
We thank Dr. Brandon Fowler for assistance with HPLC, and
reaffirms that the PDI subunits in these helicenes share an
Dr. Raffll Hernµndez Sµnchez for thoughtful discussions. We
thank the Nakanishi lab, particularly Dr. Ana Petrovic,
Manuel Tamargo, and Dr. Nina Berova. We are deeply
indebted to Ryan Hastie for the preparation of the ribbon
graphics. N.S. is supported by the NSF Graduate Research
Fellowship under grant number 11-44155. SCXRD was
performed at the Shared Materials Characterization Labo-
ratory at Columbia University. Use of the SMCL was made
possible by funding from Columbia University. Primary
support for this work was provided by the U.S. Department
of Energy under award number DE-SC0014563.
À
added electron: whereas the spectrum of 1C shows extensive
À
hyperfine splitting (Figure 5b), the spectra of NPDHC and
À
APDHC do not (Figure 5c). This absence of hyperfine
splitting for the PDI-helicene monoanions indicates the
distribution of the radical electron across a p-surface larger
than that of PDI 1. Electron sharing over multiple PDI
[
40,41]
[42]
subunits within nanowires,
ular triangle
DNA hairpins, and a molec-
[
43]
results in a similar loss of spectroscopic
hyperfine structure.
The large gap separating the PDI subunits in APDH
mitigates intramolecular p-to-p contact. Conversely, colli-
sions between the p-clouds of the PDI subunits of NPDH
reinforce through-bond coupling to enhance electronic deloc-
alization in the anions of this helicene. To better examine the
role of these intramolecular p-cloud collisions in facilitating
electronic delocalization in reduced NPDH, we synthesized
Keywords: chirality · helicenes · mixed-valence compounds ·
perylene diimide · p–p interactions
4
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Received: August 12, 2016
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Helicenes
Two helicenes were synthesized by double
fusion of an acene with two perylene
diimide (PDI) subunits. These PDI-heli-
cene homologs show different structural
and electronic properties, despite differ-
ing by only a single ring in the link
N. J. Schuster, D. W. Paley, S. Jockusch,
F. Ng, M. L. Steigerwald,*
C. Nuckolls*
&&&& — &&&&
Electron Delocalization in Perylene
Diimide Helicenes
between the PDI units. The shorter link
brings the two PDI subunits closer
together, and this results in the collision
of their respective p-electron clouds.
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!