Stringing the Perylene Diimide Bow

Article abstract of DOI:10.1002/anie.202004989

This study explores a new mode of contortion in perylene diimides where the molecule is bent, like a bow, along its long axis. These bowed PDIs were synthesized through a facile fourfold Suzuki macrocyclization with aromatic linkers and a tetraborylated perylene diimide that introduces strain and results in a bowed structure. By altering the strings of the bow, the degree of bending can be controlled from flat to highly bent. Through spectroscopy and quantum chemical calculations, it is demonstrated that the energy of the lowest unoccupied orbital can be controlled by the degree of bending in the structures and that the energy of the highest occupied orbital can be controlled to a large extent by the constitution of the aromatic linkers. The important finding is that the bowing results not only in red-shifted absorptions but also more facile reductions.

Full text of DOI:10.1002/anie.202004989

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Accepted Article
Title: Stringing the PDI Bow
Authors: Taifeng Liu, Jingjing Yang, Florian Geyer, Felisa Conrad-
Burton, Raul Hernandez-Sanchez, Hexing Li, Xiaoyang Zhu,
Shengxiong Xiao, Colin Nuckolls, and Michael Steigerwald
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202004989
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10.1002/anie.202004989
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Stringing the PDI Bow
Taifeng Liu,[a] Jingjing Yang,[b] Florian Geyer,[b] Felisa S. Conrad-Burton,[b] Raúl Hernández Sánchez,[b]
Hexing Li,[a] Xiaoyang Zhu,[b] Shengxiong Xiao,*[a] Colin P. Nuckolls,*[b] and Michael L. Steigerwald*[b]
Abstract: This study explores a new mode of contortion in perylene
diimides where the molecule is bent, like a bow along its long axis.
We synthesize these bowed PDIs through a facile 4-fold Suzuki
macrocyclization with aromatic linkers and a tetraborylated perylene
diimide that introduces strain and results in a bowed structure. By
altering the strings of the bow, the degree of bending can be
controlled from flat to highly bent. Through spectroscopy and
quantum chemical calculations, we demonstrate that the energy of
the lowest unoccupied orbital can be controlled by the degree of
bending in the structures and the energy of the highest occupied
orbital can be controlled to a large extent by the constitution of the
aromatic linkers. The important finding is that the bowing results not
only in red-shifted absorptions but also more facile reductions.
1d). The important finding is that the bowing results not only in
red-shifted absorptions but also more facile reductions. This
mode of molecular contortion provides a previously unexplored
tuning knob for the singlet and triplet energies of PDI[8] and its
related polycyclic aromatic hydrocarbons[8-9] as demonstrated by
our earlier work[8] that compound 1d showed an increase in the
singlet fission rate by 2 orders of magnitude.
In this study, we unveil a new type of contortion in perylene-
3,4,9,10-tetracarboxylic diimide (PDI, Figure 1a) that bends it
like a bow along its long axis and in the process, tunes and
decreases the excited state energy as the degree of bowing
increases. PDI and its derivatives have been extensively
investigated as electronic and optoelectronic materials in organic
field effect transistors (OFETs),[1] organic photovoltaics (OPVs),[2]
organic redox flow batteries,[3] organic photodetectors,[4] and in
many other applications.[5] PDI plays a prominent role in such
important processes as charge transport, energy transport,
singlet fission, and photoinduced electron transfer at the core of
these organic materials.[1d, 6] For many applications of PDI, it is
beneficial to contort the core of the PDI away from planarity.[1f, 1i,
Scheme 1. Synthesis of 1a-1d.
2, 4]
For example, a non-planar, contorted core drastically
improves solar cell device performance and provides a rich
design motif for non-fullerene electron acceptors in
optoelectronic devices.[2]
What we describe here is an understanding of how a new
type of contortion tunes the energy of PDI’s electronic transitions.
Contorting the PDI core helically (Figure 1b) decreases the
energy of the HOMO-LUMO transition but to some extent
increases the energy of the LUMO.[7] What we find here is that
bowing the PDI core (Figure 1c) decreases the energy of the
LUMO. The reason for this difference has to do with the overlap
of the nascent π-bond in the LUMO of the PDI, which is
enhanced by bowing the PDI and diminished by twisting (Figure
Figure 1. (a) PDI. (b) helically twisted PDI. (c) bowed PDI. (d) PDI showing the
virtual pi-orbitals in the LUMO between the two naphthalenes, having poorer
overlap in helical configuration and greater overlap in the bowed configuration.
[a]
Dr. T. Liu, Prof. H. Li, Prof. S. Xiao
The Education Ministry Key Lab of Resource Chemistry, Shanghai
Key Laboratory of Rare Earth Functional Materials, Optoelectronic
Nano Materials and Devices Institute, College of Chemistry and
Materials Science, Shanghai Normal University
Shanghai 200234, China
Bent aromatic molecules, such as cycloparaphenylenes
(CPPs),[10] [n]paracyclophanes,[11] [n](2,6)azulenophanes,[12]
[n](2,7)pyrenophanes[13] and [n](2,11)teropyrenophanes,[14] are
synthesized through multi-step cyclization strategies to introduce
the high degree of strain. Here, we bend the PDI core through a
four-fold Suzuki coupling between two aromatic linkers and the
PDI tetraborate[15] (Scheme 1 and ESI). It’s the first time that this
mode of contortion has been applied to a very important class of
optoelectronic materials—the PDI family. The length of the
linkers, the steric repulsion between the central aromatic units
E-mail: xiaosx@shnu.edu.cn
[b]
Dr. J. Yang, Dr. F. Geyer, F. Conrad-Burton, Prof. R. Hernández
Sánchez, Prof. X. Zhu, Prof. C. Nuckolls and Dr. M. L. Steigerwald
Department of Chemistry, Columbia University
New York, NY 10027, USA
Email: cn37@columbia.edu; mls2064@columbia.edu
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.202004989
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and the PDI bay, and the repulsion of the outer aromatic units
with the imide oxygen atoms together bend the PDI into a bow
shape.
and 1d being the most strained (ESI, homodesmotic
calculations). Table 1 tabulates the strain energy from these
calculations and other structural metrics. The structures we
obtained from single crystal X-ray diffraction are essentially the
same values as those obtained from the density functional
theory (DFT) calculations (Figure S6). Table 1 also summarizes
the structural parameters. One measure of the degree of bowing
in these structures is the angle between the two naphthalene
units in the perylene, which increases as the degree of bowing
increases: 34° for 1b, 44˚ for 1c and 45° for 1d. Similarly, the
distance between the two nitrogen atoms of the PDI also
decreases as the degree of bowing increases (Table1).
Figure
2 displays both the chemical structures we
prepared for this study (1a-1d) and the corresponding molecular
structures from single crystal X-ray diffraction (SCXRD). The
important finding is that by altering the strings (the aromatic
linkers), the degree of bending in the bow (the PDI core) was
changed. From the X-ray diffraction analysis, 1a is essentially
flat, having a slight twist to its PDI core. The bowing in 1b-1d
suggests that their strings are under significant tension.
Homodesmotic calculations reveal that indeed the strain
increases through this series with 1a being the least strained
Figure 2. (a) chemical structures of 1a-1d. (b) SCXRD structures of 1a-1d. (ShelXT3 was used for single-crystal analysis.)
Table 1. Structural parameters and calculations of the strain for 1a-1d.
Furthermore, the p-phenyl and the 2,5-thienyl linkers in 1a and
1b are rotating fast on the NMR timescale at all temperatures
tested. It is in good agreement with our DFT calculations on the
energies of the syn/anti isomers of 1a-1d (Table S6). For 1a and
1b, the energy differences are as small as 1.7 kcal/mol, while for
1c and 1d the values are as large as 11.9 kcal/mol. For 1c and
1d, even though the syn/anti isomer conversions were not
detected between 230 K to 400 K (Figure S3 and S4, see ESI
for detailed explanations), the rotation and waggling behaviors of
the 2,5-thienyls in 1c and the p-phenyls in 1d are very different.
For 1c, the fast rotation of the 2,5-thienyls is frozen below 260 K
(Figure S3) and converts to the waggling mode. While in 1d, the
p-phenyls can only waggle at temperature below 340 K and then
start to rotate freely when the temperature is raised above 375 K.
N to N distance
[Å]
Strain energy
[kcal/mol]
Bend angle [°]
crystal
11.36
calc.
11.35
crystal
--
Calc.
--
1a
1b
1c
1d
30
32
33
42
10.90
10.42
10.25
11.04
10.49
10.31
37.57
42.96
42.95
34.09
43.66
44.67
1
Variable temperature (VT) H NMR studies (Figure S1-S4)
reveal these compounds have a much more complex but
interesting behavior in solution. The anti/syn (anti/syn denotes
the two strings are on the opposite/same side of the PDI core)
isomers and their interconversions were detected for 1a and 1b,
but not for 1c and 1d, which are conformationally locked.
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10.1002/anie.202004989
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We first examined the consequences of bowing the PDI on
its optical absorbance. Figure 3a displays the UV-vis spectra for
1a-1d. Bowed PDIs show red-shifted optical absorptions relative
to both flat and helically twisted PDI. For example, a comparison
of slightly bent 1a and the strongly bowed 1d reveals an ~50 nm
red shift. While it is difficult to compare across the set in detail
(for example, the thiophene-containing bridges are chemically
quite different than the phenyl-containing bridges, and charge
transfer between thienyl moiety and the PDI is also a factor in
the UV-vis spectra for 1b and 1c), the more general conclusion
is that a greater degree of bowing leads to greater red-shifted
absorbances. In the simplest approximation, this suggests that
bowing either raises the HOMO energy or lowers the LUMO
energy (or effects some combination of the two). 1a-1d showed
fluorescent maximum at 625, 612, 668 and 617 nm with
quantum yields of 0.6%, 0.9%, 0.8% and 31.6%, respectively
(Figure S5).
to additional influence of a stronger charge transfer than that of
1c (Figure S13-14 and Figure S16). With these data in hand, we
draw the simple conclusion that destabilization of the PDI
nucleus by this longitudinal bowing reduces the LUMO energy.
These results for the longitudinally bowed PDIs stand in
contrast to the behavior of helically twisted PDIs. Destabilizing
the PDI nucleus by helical twisting generally yields more
energetically demanding reductions despite the similar red-
shifted HOMO-to-LUMO transitions.[7] We used DFT calculations
to learn more about these trends. In each of the distortions, the
two naphthalene imide moieties remain largely unaffected,
therefore we dissected PDI into its constituent halves.
Figure 4. The HOMO and LUMO orbitals of: (a) the naphthalene imide and (b)
the PDI.
In Figure 4 we show the HOMO and LUMO of naphthalene
imide and compare these to the HOMO and LUMO of the parent,
planar PDI. The first item of note is that the HOMO of PDI is a
linear combination (appropriately normalized) of the HOMO on
the upper naphthalene imide and the negative of the HOMO on
the lower naphthalene imide. The change of phase between the
two halves (upper and lower) of the orbital indicate that there is
a planar node bisecting this orbital, and this planar node raises
the energy of this orbital. In short, the HOMO of PDI is π-
antibonding with respect to the upper and lower naphthalene
imide fragments.
Figure 3. UV/vis absorption (a) in DCM (1 x 10-5 M) and cyclic voltammetry (b)
of 1a-1d with Bu4NPF6 (0.1 M) as supporting electrolyte, Ag/AgCl as reference
electrode and the Fc/Fc+ redox couple as an internal standard. (The two pink
dashed lines indicate the first redox peaks of 1b).
The second item to note is that the LUMO of PDI is
similarly a linear combination of the LUMO of the upper
naphthalene imide and that of the lower naphthalene imide,
however in this case the two are combined with the same phase
(sign). This LUMO is π-bonding with respect to the upper and
lower fragments, and this is underscored by the significant
decrease in its orbital energy relative to that of the naphthalene
imide LUMO (-3.5 eV versus -2.4 eV). Our heuristic conclusion
We can use cyclic voltammetry to probe the effect that
bowing has on the reduction potential of PDI and use this to
estimate the LUMO energies of the bowed PDIs. Figure 3b
contains the CVs for 1a-1d. The trend is clear: it is easier to
reduce the PDI to PDI– as the degree of bowing in the molecular
structures increases. It’s worth noting that 1b has a smaller bend
angle than that of 1c, but showed similar reduction potential due
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10.1002/anie.202004989
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COMMUNICATION
is that PDI can be viewed as the fusion of two naphthalene
imides, and σ-bonds accomplish this fusion. In the ground state
the π-interaction between the two halves is repulsive, but in the
HOMO-to-LUMO excited state the repulsive π-interaction
become attractive.
International Joint Laboratory of Resource Chemistry (IJLRC). H.
Li acknowledges financial support from NSFC (21761142011)
and Shanghai Government (18JC1412900).
Keywords: perylene diimides • contorted aromatics • organic
This coupling between the two halves of the molecule in
the PDI system and particularly its effect on the LUMO system
rationalizes why twisting or bending the PDI lowers the energies
of the HOMO-to-LUMO transitions and affects their LUMOs in
very different ways. As PDI twists along its long axis the energy
of its HOMO is affected because this axis is perpendicular to the
central nodal plane of the orbital, however the energy of the
LUMO should increase since the twist is disturbing the smooth
continuity between the two halves of the LUMO. While the
energies of both orbitals may increase, the HOMO is affected
more than is the LUMO. In the case of bowing the situation is
reversed: the energy of the HOMO is raised because orbital
density is forced closer to the central node, while the energy of
the LUMO is lowered because its inter-naphthyl π-bonding is
enhanced. In this case as well, the energies of both orbitals may
be affected with bowing, but that of the HOMO will increase
more than that of the LUMO.
We show here a means to control the degree of bowing in
PDI. We synthesize these bowed PDI structures by a 4-fold
Suzuki macrocyclization. By tuning the groups that bridge the
PDIs we can adjust the degree of bowing and therefore the
energy of the orbitals. We use quantum chemical calculations to
understand the red-shifted absorptions in the bowed structures.
PDI is best viewed as two naphthalene-1,8-imides σ-bonded to
one another. The excited states, and therefore the practical
utility of PDIs, can be rationally manipulated by controlling how
the two naphthalene-1,8-imides interact. The bowed structures
provide better overlap between the nascent pi-bond of the
LUMO and also more facile reductions. These structures also
provide a means to tune the singlet and triplet energy levels,
which will be important in using these molecules as building
blocks for materials exhibiting efficient singlet fission.
electronics • optoelectronic materials
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Experimental Section
For experimental and calculation details, see the Supporting Information.
The crystallographic information files (CIFs) are deposited at the
Cambridge
Crystallographic
Data
Centre
(CCDC,
http://www.ccdc.cam.ac.uk) with CCDC numbers: 1991412 for 1a,
1991413 for 1b, 1991414 for 1c and 2006155 for 1d.
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This work was supported by the U.S. Department of Energy,
Grant DE-SC0014563. F.G. was supported by a Feodor Lynen
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10.1002/anie.202004989
Angewandte Chemie International Edition
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COMMUNICATION
This study explores a new mode of
contortion in perylene diimides, where
it is bent along its long axis like a bow.
By adjusting the tension in the strings
of the bow, the degree of bending can
be controlled from flat to highly
bowed. The important finding is that
the bowing results not only in red-
shifted absorptions but also more
facile reductions.
Taifeng Liu, Jingjing Yang, Florian
Geyer, Felisa S. Conrad-Burton, Raúl
Hernández Sánchez, Hexing Li,
Xiaoyang Zhu, Shengxiong Xiao,* Colin
P. Nuckolls,* and Michael L.
Steigerwald*
Page No. – Page No.
Stringing the PDI Bow
This article is protected by copyright. All rights reserved.