Radical cations of twisted acenes: Chiroptical properties and spin delocalization

Article abstract of DOI:10.1039/c9cc02735a

We introduce the first series of enantiopure twistacene radical cations, which form reversibly upon chemical or electrochemical oxidation. Their vis-NIR chiroptical properties (Cotton effect and anisotropy factor) increase systematically with the backbone twist. The hyperfine constants observed by EPR demonstrate significant spin delocalization even for large backbone twist angles.

Full text of DOI:10.1039/c9cc02735a

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
first methanofullerene derivatives of Sc
3
N@C2n (2n
=
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DOI: 10.1039/C9CC02735A
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Radical Cations of Twisted Acenes: Chiroptical Properties and Spin
Delocalization
a
b
a
Anjan Bedi, Raanan Carmieli and Ori Gidron *
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
We introduce the first series of enantiopure twistacene radical and P helicities.19 This fluxional behavior greatly complicates
cations, which form reversibly upon chemical or electrochemical efforts to investigate them in enantiopure form. Indeed,
oxidation. Their vis-NIR chiroptical properties (Cotton effect and Rathore’s group investigated the properties of the chiral
anisotropy factor) increase systematically with backbone twist. The anthracene radical cation, but its fast racemization prevented
20
hyperfine constants observed by EPR demonstrate significant spin study of the enantiopure form. To the best of our knowledge,
delocalization even for large backbone twist angles.
the chiroptical properties of twistacene radical cations remain
unexplored.
Chiral molecules capable of forming stable organic radicals are
attracting growing interest for their charge transfer properties,1
as building blocks for enantioselective synthesis, and for their
electro magneto chiral properties. Largely in consequence of
their strong chiroptical properties, considerable research effort
is focused on exploring the potential of helicene and its
derivatives to function as chiroptical redox switches.4-6 While
parent helicenes usually do not form stable cation radicals, they
can be functionalized with redox centers to form chiroptical
We recently introduced a new twistacene family, Ant-Cn, whose
conjugated backbones are helically locked by a diagonal tether
that prevents back and forth flipping around the backbone and,
2, 3
21
consequently, racemization (Fig. 1). By adjusting the length of
the tether, we synthesized a series of stable M and P Ant-Cn
enantiomers. The enantiomerically pure series members exhibit
different twist angles while bearing identical numbers of
backbone substituents, which minimizes substituent effects.
We found the optical and electronic properties of the π‐
conjugated backbone to be strongly and systematically affected
by twist angle, such that increased twisting decreases both the
fluorescence quantum efficiency and optical band gap of Ant-
7
,
8
switches (e.g., transition metals, quinone moieties),
9
phenalenyl to form stable biradicals, and heteroatoms or
nanographenes for stable cations.1
0,
11
By contrast, the
chiroptical properties of the enantiopure radical cations of the
para-fused analogs of helicenes, namely twisted acenes, are
unexplored.
21
Cn. Furthermore, tethering acenes increases their stability and
solubility, both of which are important for the future synthesis
22
of long acenes. The Cotton effect and absorption anisotropy
Acenes and their derivatives are perhaps the most commonly
studied p-type organic semiconductors, as long acenes are
factors (gabs) of Ant-Cn increase systematically with twist angle,
apparently because (unlike the case of helicenes) their magnetic
and electric transition dipole moments lie in parallel.
12, 13
stable in their oxidized state.
While parent acenes are
planar, they are readily twisted out of planarity upon
substitution. Twisted acenes (twistacenes), first introduced by
Cn
Cn
14, 15
Pascal,
parent acenes,
are twisted out of planarity. For example, the rubrene radical
are more stable and soluble compared with their
1
6, 17
to the extent that even their radical cations
FeCl3
1
8
oxidation
‧+
cation is twisted by ~27°. However, most twistacenes undergo
rapid racemization, as they twist back and forth between the M
TBAPC
a.Institute of Chemistry, The Hebrew University of Jerusalem, Edmond J. Safra
Ant-Cn•+
Ant-Cn n = 3 - 6
Campus, Jerusalem 91904, Israel.
Chemical Research Support Unit, Weizmann Institute of Science, Rehovot 76100,
b.
3
Fig. 1 Chemical (FeCl ) and electrochemical oxidation of Ant-Cn (yellow) to the cation
radical, Ant-Cn• (green) in acetonitrile. The colouring of the molecules is as per their
appearance as neutral (left) and cation radical (right) species in acetonitrile solution. The
3
red arches represent the tether, Cn. TBAPC, tetrabutylammonium perchlorate, R = CF .
Israel
†Electronic Supplementary Information (ESI) available: Synthetic details, NMR
spectra, details of calculations, optimized structures, details of EPR measurements,
spectroscopic details, crystal data. CCDC 1901231. See DOI: 10.1039/x0xx00000x
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Journal Name
While charge and spin delocalization are important factors spectra (Fig. S9, see ESI), rendering these materials potential
View Article Online
governing the performance of conjugated systems, the chiroptical switches.4-6
question of how twisting an aromatic system out of planarity
DOI: 10.1039/C9CC02735A
2B2g2B1u
a)
b)
Ant-C6•
Ant-C5•

affects charge delocalization remains open. Having a measure
of spin delocalization vs. twist angle in polyaromatic systems
could serve to indicate the degree of twist that an aromatic ring
can endure before losing delocalization, and so assist in the
design of novel nonplanar aromatic compounds.
[Ox]
•
Ant-C4
Ant-C3
•
[Red]
2B2g2A
u
2B2g2B3g
Here we introduce the first enantiopure series of radical cation
twistacenes. In the vis-NIR spectral region, the series exhibits
strong chiroptical properties (Cotton effect and gabs), which
increase significantly with increasing acene twist. An EPR study
Wavelength (nm)
Wavelength (nm)
reveals the presence of the cation radical, which shows Fig. 2 (a) Circular dichroism (CD) spectra showing the formation of Ant-C4• by stepwise

3
addition of FeCl solution and (b) vis-NIR spectra of the formed radicals at a 1:1 ratio of
increasing charge localization with increasing twist.

Ant-Cn:FeCl in acetonitrile.
3
•
The structures of Ant-Cn
DFT/UB3LYP/6-31G(d) level of theory (Table S2, see ESI). In the
neutral state, twisting in Ant-Cn ranges from 22 to 43 , which
is in line with the range observed in their single crystal
were optimized at the
Since enantiopure Ant-Cn series members are excellent
chiroptical materials, displaying an increase in their Cotton
°
°
2
2
effect with increasing twist, we were interested to examine
whether their radical cations follow a similar trend. The ECD
2
1
•

structures (22
°–38°). The radical cations, Ant-Cn , show a
similar, albeit more narrow, range of twisting angles, from 31
°
•

spectra for Ant-Cn display positive transitions for the M
enantiomer, similar to the π–π* transition in their neutral states
for Ant-C6• to 44
° for Ant-C3 . Thus, tether length induces a

•

systematic twist in the acene core in both the neutral molecule
and its radical cation.
(
7
1
Fig. 3), with the lower energy transitions at 500 nm and 600–
50 nm clearly observed. The vibronic shoulders for the HOMO-
SOMO transitions, with spacing of around 0.17 eV (~1400
cm , corresponding to C=C stretch), indicate that the structure
of the acene remains rigid also in the radical cation state.
Upon addition of FeCl
3
to an acetonitrile solution of Ant-Cn to

produce Ant-Cn• , the pale yellow solution turns bright green

-
1
(Fig. 2a). UV-vis-NIR absorption spectra showed that the π–π*
transition of Ant-Cn at ~400 nm gradually decreases, giving rise
•

to three new transitions for Ant-Cn at ~ 500 nm, 600–750 nm,
and a broad transition at 1000–1800 nm (Fig. 2).
5
0
50
25
0
Ant-C6
Ant-C5
The peak around 500 nm does not show a vibronic pattern, and
25
0
Ant-C6•

Ant-C5•
2
2
might be assigned to the
B
2g

B
1u
transition
LUMO+1). In contrast, the transition at 600–750 nm,
which displays a clear vibronic pattern, can be assigned to the
23
(SOMO
-25
-25
2
2
24-26
B
2g

A
u
transition (HOMO-1

SOMO).
This transition is
expected to be centered around the acene core, and is observed
in parent anthracene at ~720 nm.27 We note that, in a manner
-50
-50
300
3
00
400
500
600
700
800
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
5
0
22
50
25
0
similar to that observed for neutral twistacenes, the extinction
Ant-C4
Ant-C3
-1
-1
coefficient increases with increased twist from 2500 cm M for
2
5
0
Ant-C4•

Ant-C3•
Ant-C6• to 3600 cm M for Ant-C3 . This is probably related

-1 -1
•

to the vibronically allowed transition, whose likelihood
increases as anthracene increasingly deviates from
D
2h
-
25
-25
-50
28
2
symmetry towards C symmetry. This transition is also
bathochromically shifted with increasing twist, as observed for
-50
3
00
400
500
600
700
800
300
400
500
600
700
800
21
the neutral twistacenes.
Wavelength (nm)
Fig. 3 Circular dichroism (CD) spectra of M and P enantiomers (solid and dashed lines,
(blue) in acetonitrile.
Wavelength (nm)
•

The low energy transition that Ant-Cn exhibits at 1000–1800
2
2g2
3g transition, not respectively) of Ant-Cn (red) and Ant-Cn 
nm might be attributed to the
B
B
2
•
4
observed in parent anthracene (forbidden). We found that,
although this transition is similarly not observed for 9,10- Upon increasing the twist angle, there is a significant increase in
diphenyl anthracene, which has D2h symmetry, it can be the Cotton effect (Fig. 4). The molar circular dichroism () for
2
2
-1
-1
observed for 1,5-diphenyl anthracene, which exhibits C2h the B2g

B
1u transition increases three fold from 6.1 M cm
symmetry, as also supported by TD-DFT calculations (see for Ant-C6• (characterized by a 31

°
twist angle) to 19 M cm
-1
-1
•

sections S4 and S5 in the ESI).29 This transition increases in
for Ant-C3
(characterized by a 44
°
twist angle) (Fig. 4, green
2
2
intensity with increasing twist. However, we note that this trace). The  for the B2g
increase is likely to be related not only to the anthracene twist, M cm for somewhat twisted Ant-C6 to 8 M cm for
but also to the orientation of the substituents. Addition of strongly twisted Ant-C3 (Fig. 4, red trace). The Cotton effect
hydrazine results in the full recovery of the neutral Ant-Cn at ~750 nm is relatively strong compared with other all-organic
chiral redox chromophores. The value of gabs (Fig. 4, blue trace)
 A transition also increases from 5
u
-1
-1
•

-1
-1
•

2
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
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Journal Name
increases from 2.2 × 10 for a 31
twist (Fig. S12, see ESI), which is comparable with other C3 to Ant-C6 , the hyperfine coupling constants for open-
cyclophane cation derivatives.30 Overall, the increase in anthracene radical cation do not follow that trend (Fig. S13, see
chiroptical properties does not moderate even for twisting of ESI). Due to the lack of molecular constraint the hyperfine
COMMUNICATION
Although a trend is evident in the hyperfine values from Ant-
-3
-3
°
twist to 3.2 × 10 for a 44
°
View Article Online
DOI: 10.1039/C9CC02735A
•

•

>40°. It is therefore reasonable to assume that applying a larger coupling constants for open-anthracene radical cation
twist to acene radical cations will result in an even stronger represent averaged molecular motions and therefore it do not
•

•

Cotton effect.
follow the trend laid out by Ant-C3 to Ant-C6 . We postulate
that the lack of structural constraints on open-anthracene and
its consequently greater flexibility underlie it not following the
trend of decreasing hyperfine coupling values with increasing
twist angle.
2
1
1
1
1
1
0
8
6
4
2
0
8
6
3
3
3
.4
.2
.0
2B2g® 2B ~ 500 nm
1u
2B2g® 2A ~ 700 nm
u
gabs
a)
b)
2.8
Ant-C3•
3
2
1
0
2.6
2.4
2.2
Cn
Ant-C4•
‧+
32
34
36
38
40
42
44
Twist angle (°)
Ant-C5•

Fig. 4 Change in the absolute molar circular dichroism () for Ant-Cn• (green and red)
and absorption anisotropic factor (gabs; blue) with twist angle.
H4
H2
H3
Ant-C6•

Charge delocalization in polyaromatic systems is directly related
to their morphology. The deviation from planarity of aromatic
31
3430
3440
3450
30
35
40
45
32, 33
34
systems affects aromaticity,
energy levels, and π-orbital
Magnetic Field (Gauss)
Anthracene twist (degrees)
35, 36
overlap.
The effect of twisting on charge localization is
Fig. 5 (a) EPR signals from Ant-Cn  obtained by electrochemical oxidation of the neutral
parent molecule in acetonitrile solution (coloured lines) compared with the simulated
spectrum (black lines). (b) The change in hyperfine coupling vs. anthracene twist for H –
2
•
important for the design of non-planar polyaromatic
compounds, yet this effect was not studied experimentally in a
systematic manner for lack of a suitable experimental system.
We were therefore interested to observe how charge
localization varies as a function of twist angle in the Ant-Cn•
4
H .
series, using electron paramagnetic resonance (EPR) Conclusions
3
7, 38
spectroscopy.
In summary, we have investigated the first series of twisted
acene radical cations in their enantiopure form. To the best of
our knowledge, this is the first enantiopure radical cation
twistacene system that is configurationally stable at room
temperature. We find a systematic increase in their optical and
chiroptical properties, such that they display a significant
Cotton effect in the NIR spectral region, with increasing twist.
Overall, the findings indicate that twistacenes are potential
components for chiroptical redox switches. The radical cations
display an EPR signal that exhibits only a slight decrease in
In-situ EPR electrochemistry measurements in acetonitrile using
tetrabutylammonium perchlorate as the electrolyte revealed a
signal for all Ant-Cn• (Fig 5a). Simulation of the data revealed
three pairs of weak hyperfine coupling constants assigned to
the three pairs of hydrogens on each terminal rings (Fig. 5a,

39
40
black line). Data analysis suggests that the electron is localized
mostly on the central ring, as is commonly observed for acene
cation radicals. While the changes in the hyperfine values are
subtle, it can be observed clearly that the values decrease with
increasing twist (Fig. 5b). This indicates that delocalization
decreases with increasing twist, most likely because of the
reduced overlap of the π-conjugated system upon twisting. The
hyperfine coupling with increasing twist angle up to a 13° twist
per benzene ring. This may serve as a design principle for future
twisted aromatic systems.
most affected hydrogens are H
2 3
and H , which are more distant
This research was supported by the Israel Science Foundation
from the central ring, while H , located nearer the central ring,
4
(grant No. 1789/16). A.B. is supported by a PBC fellowship. We
is the least affected (Fig. 5b, for calculated spin density see Fig
S1 in the ESI).
It is interesting to note that the hyperfine coupling constants
thank Dr. Benny Bogoslavsky for his help in recording X-ray
structures.
change significantly only for a twisting angle larger than 40° (13°
per benzene ring), whereas the changes are small for smaller
twists. This may indicate that each benzene ring can bear a twist
of up to 13° without significant loss of delocalization. We
Conflicts of interest
There are no conflicts to declare
believe that this finding may serve as a design principle for
future nonplanar aromatic systems.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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29.
Absorption measurments at various concenVtireawtAiortnicsle sOhnolinwe
no concentration dependance DoOf I:t1h0e.10l3o9w/Ce9sCt Ce0n27e3r5gAy
Notes and references
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DOI: 10.1039/C9CC02735A
TOC Entry
A series of enantiopure twistacene radical cations display dependency of the chiroptical properties and
the EPR signal on backbone twist.