Axially and Helically Chiral Cationic Radical Bicarbazoles: SOMO-HOMO Level Inversion and Chirality Impact on the Stability of Mono- And Diradical Cations

Article abstract of DOI:10.1021/jacs.0c08948

We report persistent chiral organic mono- and diradical cations based on bicarbazole molecular design with an unprecedented stability dependence on the type of chirality, namely, axial versus helical. An unusual chemical stability was observed for sterically unprotected axial bicarbazole radical in comparison with monocarbazole and helical bicarbazole ones. Such results were experimentally and theoretically investigated, revealing an inversion in energy of the singly occupied molecular orbital (SOMO) and the highest (doubly) occupied molecular orbital (HOMO) in both axial and helical bicarbazole monoradicals along with a subtle difference of electronic coupling between the two carbazole units, which is modulated by their relative dihedral angle and related to the type of chirality. Such findings allowed us to explore in depth the SOMO-HOMO inversion (SHI) in chiral radical molecular systems and provide new insights regarding its impact on the stability of organic radicals. Finally, these specific electronic properties allowed us to prepare a persistent, intrinsically chiral, diradical which notably displayed near-infrared electronic circular dichroism responses up to 1100 nm and almost degenerate singlet-triplet ground states with weak antiferromagnetic interactions evaluated by magnetometry experiments.

Full text of DOI:10.1021/jacs.0c08948

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Article
Axially and Helically Chiral Cationic Radical Bicarbazoles: SOMO−
HOMO Level Inversion and Chirality Impact on the Stability of
Mono- and Diradical Cations
Sitthichok Kasemthaveechok, Laura Abella, Marion Jean, Marie Cordier, Thierry Roisnel,
Nicolas Vanthuyne, Thierry Guizouarn, Olivier Cador, Jochen Autschbach,* Jeanne Crassous,
and Ludovic Favereau*
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ABSTRACT: We report persistent chiral organic mono- and diradical cations
based on bicarbazole molecular design with an unprecedented stability
dependence on the type of chirality, namely, axial versus helical. An unusual
chemical stability was observed for sterically unprotected axial bicarbazole
radical in comparison with monocarbazole and helical bicarbazole ones. Such
results were experimentally and theoretically investigated, revealing an
inversion in energy of the singly occupied molecular orbital (SOMO) and
the highest (doubly) occupied molecular orbital (HOMO) in both axial and
helical bicarbazole monoradicals along with a subtle difference of electronic
coupling between the two carbazole units, which is modulated by their relative
dihedral angle and related to the type of chirality. Such findings allowed us to
explore in depth the SOMO−HOMO inversion (SHI) in chiral radical
molecular systems and provide new insights regarding its impact on the
stability of organic radicals. Finally, these specific electronic properties allowed
us to prepare a persistent, intrinsically chiral, diradical which notably displayed near-infrared electronic circular dichroism responses
up to 1100 nm and almost degenerate singlet−triplet ground states with weak antiferromagnetic interactions evaluated by
magnetometry experiments.
(CP) light,20−30 radical CP luminescence (CPL),31,32 and
INTRODUCTION
■
possibly enhanced spin-filtering properties.33 However, design-
Stable organic molecules with open-shell electronic structures
have attracted significant interest in chemistry and biochem-
ing organic chiral open-shell chromophores remains a
formidable challenge, owing to the high chemical reactivity
istry due to their specific reactivity and their use as agents for
of radicals and poor configurational stability.34 As a result,
in vivo imaging.1−3 This class of compounds has also received
known examples remain rare and are mainly limited to
increasing attention in materials science because of the
monoradicals with very few examples of stable diradi-
possibility to combine electron and spin conduction in
molecular electronics such as organic light-emitting diodes
(OLEDs), organic field-effect transistors (OFETs), and
cals.24,25,27,28 Thus far, the chemical and configurational
stability issues have been overcome using mainly two
strategies: shielding the unpaired electrons with bulky
organic magnets.4−17 In this context, organic diradicals are of
substituents or enhancing their delocalization over the
particular interest since they can adopt two different spin
states, namely, singlet and triplet, related to the interaction
between the unpaired electrons within the molecular back-
molecular backbone.8,9,27,35−38 While such approaches can be
efficient, they often require additional synthetic steps and may
preclude significant intermolecular interactions for charge and
bone.4,18 Organic π-conjugated radicals also allow us to gain
spin transport applications.17
fundamental insights on the nature of chemical bonds and their
delocalization in addition to their other features such as narrow
Received: August 22, 2020
HOMO−LUMO energy gaps, low-lying doubly excited states,
and redox amphoterism.5,10,11,19 It has recently emerged that
combining these electronic properties with a chiral organic π-
conjugated system gives rise to molecular architectures with
unique magnetic and chiroptoelectronic features, such as
specific absorption of near-infrared (NIR) circularly polarized
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Figure 1. (a) Examples of reported stable monoradicals with a SOMO−HOMO inversion (SHI) where the molecular fragment providing the
HOMO level above the SOMO is highlighted in red, and schematic illustration of the triplet ground-state diradical obtained from oxidation of a
radical with a SHI.42 (b) Novel axially and helically mono- and diradicals cationic carbazole derivatives. The unoccupied counterpart of the SOMO
in the SHI systems is calculated to be above the near-degenerate energies of the spin orbitals constituting the HOMO level, i.e., there is no electron
hole among the occupied levels.
Recently, theoretical and experimental studies have reported
that the stability of radicals can be considerably enhanced if
they are in an uncommon electronic configuration, such that
the energy of the singly occupied molecular orbital (SOMO) is
below the highest doubly occupied molecular orbital
(HOMO) level (Figure 1). Systems with such a SOMO−
HOMO inversion (SHI), which has sometimes (albeit
somewhat imprecisely39) been construed as a non-aufbau
electronic configuration,21,27,40−42 have been notably successful
as achiral luminescent radical emitters in OLED devices.43
While this approach has appeared rather interesting to increase
the stability of already persistent radicals such as nitroxide and
perchlorotriphenylmethyl (PTM, Figure 1), the impact of SHI
on known unstable radicals remains unexplored. Regarding
chiral radicals, a SHI has been mentioned only for one helical
derivative under its racemic form.21 In addition, compounds
with such a peculiar electronic configuration are often viewed
as promising intermediates to form triplet ground-state
diradicals with nondegenerate SOMOs upon oxidation,
resulting from the removal of one electron of the HOMO
level.21,42,44 Accordingly, a deep understanding of the
electronic and steric factors that govern monoradical stability
displaying a SHI is urgently needed, as it may help to develop
open-shell compounds with higher stability and bring about
new synthetic strategies to design innovative chiral organic
high-spin materials.
Herein, we report a simple molecular design of persistent
chiral organic cationic radicals based on C2-symmetric
bicarbazole derivatives. Investigations of their photophysical,
chiroptical, and electrochemical properties revealed a high
chemical stability for unprotected carbazole axial radical 1•+ in
comparison to unstable helical 2•+ (Figure 1b). Interestingly,
both compounds exhibit SHI, indicating that such an
electronic configuration is not a sufficient prerequisite to
provide radical stability. Our results rather suggest that in
addition to the SHI, the electronic coupling between the
radical center and the electron-rich unit on which the HOMO
is centered plays a crucial role in the radical reactivity. In 1•+
and 2•+, this aspect depends on the dihedral angle between the
carbazole moieties, which is governed by the axial and helical
chirality of the bicarbazole systems.45 Such findings bring new
insights regarding the stability of organic radicals and allowed
us to further design and isolate chemically and configuration-
ally stable chiral diradical 32•2+, which displayed promising
near-infrared chiroptical properties and nearly degenerate
singlet−triplet ground states.
RESULTS AND DISCUSSION
■
Synthesis and Structural Characterization. We first
prepared bicarbazole derivatives 1 and 2. The synthesis was
inspired from that of BICOL, a ligand previously used in
asymmetric catalysis.46,47 Briefly, carbazole CBzOMe was first
hydrolyzed and then engaged in an oxidative homocoupling
reaction using vanadium complex as catalyst to afford 4,4′-
bicarbazole-3,3′-diol, BICOL, derivative (Scheme 1). The
latter was then alkylated at both the nitrogen and the oxygen
positions to give rac-1, which was resolved to its corresponding
axially chiral enantiomers (+)-/(−)-1 using chiral HPLC (see
the Electronic Supporting Information, ESI for details). Helical
(+)-/(−)-2 were successively obtained from (+)- and (−)-1
through hydrolysis and subsequent bridging of the hydroxy
groups using diiodomethane in 90% yield. Further function-
alization of monocarbazole CBzOMe afforded N-methylated
1′, which was then converted to sterically hindered 3′ with the
tert-butyl group at the 6 position (Scheme 1). Steric protection
was also applied to (+)-/(−)-1 and gave (+)-/(−)-3 in 90%
yield. Under similar conditions, helical bicarbazole 2 did not
afford the expected bis-tert-butyl derivative 4 but rather a
complex mixture of oligomers with multiple tert-butyl groups.
As a result, compound 4 was obtained from 3 using a similar
synthetic strategy to that for 2 in an overall 25% yield.
X-ray structure analyses provide further structural insights
for rac-1 and rac-2, which crystallized in the P-1 and C2/c
space groups, respectively. Dihedral angles of 87.1° and 52.4°
B
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population of 4% at 298 K). Therefore, due to crystal packing,
3 does not crystallize in its lowest energy conformer. The
calculated properties of 3 further support the higher abundance
of the conformer with a dihedral angle of 65° in solution by
comparison with the experimental chiroptical properties (vide
infra). All of these observations indicate that the electronic
coupling between the two carbazole units of these chiral
compounds should differ significantly depending on the
dihedral angle, which may impact their corresponding
photophysical, chiroptical, and electronic properties.
Scheme 1. Schematic Synthetic Route for 1, 1′, 2, 3, 3′, and
a
4
Computational Details. Kohn−Sham density functional
theory (DFT) as implemented in the Gaussian (G16) package
was used for all computations,51 employing the PBE0
functional52 and the def2-SV(P) basis.53,54 For neutral and
oxidized species, solvent effects were considered by means of
the polarizable continuum model (PCM) for dichloromethane
and acetonitrile,55 respectively, to match the experimental
conditions. “D3” dispersion corrections were included in the
calculations.56 Excited state energies, transition moments,
excited state structures, and their vibrational normal modes
were obtained from time-dependent DFT (TD-DFT) response
theory. For the absorption and electronic circular dichroism
(ECD) spectra we calculated the 200 lowest energy vertical
spin-allowed electronic excitations. The transitions were
subsequently Gaussian broadened with σ = 0.20 eV to simulate
the spectral envelopes. For overviews of the theoretical
approach to model natural optical activity by quantum
chemical calculations, in particular, via TD-DFT, see, for
example, available reviews.57,58 Different functionals and
solvent effects were tested for these compounds; see the
Supporting Information for details.
a
Reaction conditions: (i) Me2SO4, 22 M NaOH, acetone, reflux, 77%;
(ii) BBr3, CH2Cl2, rt, >95%; (iii) VO(acac)2, MeCN, O2, rt, 50%; (iv)
t-BuCl, ZnCl2, MeNO2, rt, >90%; (v) CH2I2, K2CO3, acetone, Ar,
reflux, >90%. ORTEP drawing of 1, 2, 3, and 4 with the
corresponding dihedral angle between the carbazole units. Hydrogen
atoms are omitted for clarity.
between the carbazole units were measured for 1 and 2,
respectively, which for the latter appeared to be reminiscent of
that found for carbo[6]helicene.48−50 This structural difference
was confirmed by density functional theory (DFT) calculations
(Figure S49) and also probed by the 1H NMR spectra. Protons
H5 (Scheme 1, doublet signal in Figures S33 and S37) are
more shielded for 1 than for 2 (6.53 and 6.95 ppm,
respectively) owing to an unequal current ring effect coming
from either the opposite aromatic phenyl ring in 1 or the N-
heterocycle one in 2. rac-3 and rac-4 crystallized in the P21/n
space group and displayed dihedral angles between 98° and
102° for rac-3 (two molecules within the same unit cell) and of
55.5° for rac-4. However, two conformers were found for 3 in
the calculations with dihedral angles of 65° and 106.6°, the
latter being higher in energy by 1.8 kcal·mol−1 (Boltzmann
Photophysical Properties of 1, 2, and 1′. Carbazole 1′
and bicarbazoles 1 and 2 display similar UV−vis spectra with
two maxima of absorption at 270 and 300 nm (ε ≈ 3.7 × 104
and 3.0 × 104 M−1 cm−1), respectively, and a vibronic band
between 325 and 390 nm (ε ≈ 1.0 × 104 M−1 cm−1, Figure 2).
Whereas 1 and 2 show the expected higher absorption
Figure 2. (a) UV−vis absorption spectra of 1 (black), 2 (red), and 1′ (green). (b) Normalized fluorescence spectra of 1 (black), 2 (red), and 1′
(green). (c) Calculated (Calc.) absorption spectra for 1 (black) and 2 (red) with selected transitions and oscillator strengths indicated by “sticks”
bars. (d) Details for selected transitions and occupied (occ)−unoccupied (unocc) MO pair contributions (greater than 10%) to the transition
density for (+)-1 and (+)-2. H and L indicate the HOMO and LUMO, respectively, for which isosurfaces ( 0.035 au) are shown in (e).
C
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intensity, about twice that for 1′, the UV−vis spectrum of 2
exhibits red-shifted absorption bands at 330 and 370 nm,
indicating a stronger electronic interaction between the
bridged carbazole systems, as expected from the smaller
dihedral angle. For both bicarbazole compounds, the lowest
energy excitation with a sizable oscillator strength (f)
corresponds to the HOMO−LUMO transitions at 327 and
355 nm for 1 and 2 respectively. In each case, the HOMO and
LUMO are delocalized over the entire molecule and the
LUMO exhibits nodes at the nitrogens (Figure 2). Both
compounds have quasi-degenerate HOMO and HOMO−1
levels separated by 0.112 and 0.149 eV for 1 and 2,
respectively. Visual inspection of the MO isosurface plots
reveals that the HOMO−1 and HOMO are essentially in-
phase and out-of-phase (+/−) linear combinations of
individual carbazole fragment frontier orbitals (FFOs). The
different energetic splitting of the HOMO−1 and HOMO
indicates that the FFO interactions are more pronounced in
compound 2, which goes along with the significant red shift
and slightly larger intensities for the calculated transitions at
285−290 and 327−355 nm in comparison to 1 (Figure 2 and
Tables S7 and S8). The chiroptical properties of 1 and 2 have
also been studied, both experimentally and theoretically, as
detailed in the Supporting Information (Figures S5 and S54).
Bicarbazoles 1 and 2 display moderately intense fluorescence
around 400 nm with quantum yields of ∼20% (Figure 2). The
enhanced electronic communication between the carbazole
units in 2 remains also present in the excited state as indicated
by the fact that its emission peak maximum is red shifted
relative to the emission peak of 1, similar to how the
absorption of 2 is red shifted. In fact, 1 displays an emission
profile close to that of 1′, evidencing a weak electronic
coupling between the carbazole π-systems also in the excited
state.
Figure 3. (a) Cyclic voltammograms of 1′ (green), 1 (black), 2 (red),
and 3′ (blue) versus saturated calomel electrode (SCE). (b) Synthetic
−
−
route to persistent radical 1•+BF4 (red) and 3′•+BF4 (blue). (c)
−
(Top) UV−vis−NIR absorption spectra of 1 (black), 1•+BF4 (red),
−
and 3′•+BF4 (blue) in dichloromethane at 298 K; (inset) X-band
EPR (ν = 9.4858 GHz) of 1•+BF4−. (Bottom) ECD spectra of (+)-1
(black) and (+)-1•+BF4− (red) with their corresponding enantiomers
marked with a dashed line in dichloromethane at 298 K with an
−
extension of the NIR-ECD spectra of (+)- and (−)-1•+BF4
.
bicarbazole oligomers of different molecular size, which further
confirm the kinetic stability of 1•+BF4 . The fact that the half-
−
life of the radical decreases in the presence of oxygen indicates
that other decomposition pathways may also take place.
The paramagnetic nature of these two monoradical cations
was also assessed by EPR measurements, which display a
similar intense doublet signal of an organic nitrogen-based
radical for both compounds (g = 2.00, Figures 3 and S16). The
Radicals 1•+ and 2•+. Cyclic voltammetry (CV) revealed
two irreversible oxidation processes for 1′ around 0.90 and
1.70 V and two distinct oxidation events for both 1 and 2 in
the range of 0.8−1.4 V (vs SCE, Figure 3). Compound 1
exhibits a first fully reversible one-oxidation process at 0.92 V
and a second less reversible one at 1.11 V with the appearance
of a small reduction peak at 0.70 V in the reverse scan. In stark
contrast, 2 showed two highly irreversible oxidation processes
at +1.14 and +1.40 V, as classically observed for 3- and/or 6-
unprotected carbazole derivatives like 1′, which form unstable
cations under oxidative conditions.59−61 The unusual reversi-
bility of the first oxidation event of 1 as well as the marked
difference of ∼200 mV with that of 2 prompted us to deeper
investigate monoradical cation 1•+. For comparison, 3- and 6-
protected monocarbazole 3′ was selected as a persistent
carbazole radical reference since its CV gives one fully
reversible oxidation process at 0.87 V vs SCE (Figure 3).
Addition of AgBF4 as a chemical oxidant to a solution of 3′
identical optical and magnetic signatures of 3′•+BF4 and
−
1•+BF4 and the lack of intense intervalence absorption
transition for the latter suggest that the unpaired electron is
localized mainly on one carbazole unit in 1•+BF4 and only
−
slightly delocalized over the second one, contrary to class II
and III of organic mixed-valence compounds.63 This situation
is rather surprising since it leads to an unprecedented
persistent carbazole-centered radical without any steric
protection at the reactive 6 position.
According to the photophysical experiments and theoretical
calculations (Figure 2), the two carbazole fragment orbitals in
1 showed weak interaction due to their relative nearly
perpendicular orientation, affording almost degenerate
HOMO and HOMO−1 levels in the neutral state. As a result,
the first two oxidation events occurring for 1 are the sequential
oxidations of each carbazole unit, which partially lift the
pseudodegeneracy of the HOMO in the monoradical cationic
state 1•+. These two anodic signals are followed by a third
irreversible one at +1.63 V, attributed to the second oxidation
of one CBz•+ unit, in accordance with the sequential oxidations
observed for monocarbazole 3′ at +0.85 (3′•+) and +1.53 V
afforded persistent radical 3′•+BF4 , as expected for 3,6-
−
protected carbazole, with a characteristic vibronic visible
signature at 696 and 771 nm (Figure 3).59,60,62 Using the
−
same conditions, 1 was quantitatively converted to 1•+BF4 ,
−
which showed a similar optical signature to 3′•+BF4 with an
additional weak broad IR band (ε ≈ 500 M−1 cm−1 at 1000
nm) and a half-life evaluated to be 3 h in air-saturated CH2Cl2.
Under inert conditions, this half-life extends to 14 h, indicating
(3′2•2+, Figure 3). Following these considerations, 1•+BF4
−
adopts an electronic configuration where its SOMO is lower in
energy than the remaining HOMO of the second nonoxidized
carbazole fragment, partially isolated from the radical center
due to the high dihedral angle between the two π-systems.
−
a persistent character for 1•+BF4 . Indeed, mass spectroscopy
and UV−vis analysis of the resulting mixture after decom-
position of the radical compound reveal the presence of
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Figure 4. Calculated spin density with an isosurface of 0.002 au; orbital energies (in eV) and isosurfaces ( 0.030) of frontier molecular orbitals
computed for monoradicals 1•+ (left) and 2•+ (right). HOMO−SOMO energy differences are averaged over the highest occupied spin orbital
energies. Oxidation potentials and separation of the two first oxidation potentials in V vs SCE; theoretical (calc.) and experimental (exp.) dihedral
angles between the carbazole fragments.
−
Further insights regarding the SHI were obtained by
2•+BF4 show a similar decomposition pathway, implying
formation of oligomers occurring presumably at the remaining
free 6 and 6′ positions, as occurs for classical nonprotected
carbazole radicals. Since the second carbazole unit is not
attached next to this reactive position of the carbazole radical
theoretical analysis of the electronic configuration of
1•+BF4 . As depicted in Figure 4, the calculated spin densities
−
are represented by spin-polarized Kohn−Sham (KS) systems
where the α-spin HOMO has a matching occupied β-spin
HOMO with both having very similar orbital energies while
the SOMO lies energetically below the HOMO−1. Unlike the
HOMO and HOMO−1 on the neutral systems, the frontier
orbitals of 1•+ are localized on either one of the fragments. The
calculated spin density of 1•+ clearly reflects the excess α-spin
represented by the SOMO and the corresponding β-spin hole
represented by the LUMO. As mentioned already, this SHI has
been claimed as a stabilizing strategy for radicals,21,40−43,64 and
our results suggest that such effects may be also responsible for
unit and is less well oriented for 1•+BF4 than for 2•+BF4
−
−
(related to their dihedral angles, Figure S49), the substituent
steric hindrance seems too far to play a decisive role in the
difference of stability between 1•+BF4 and 2•+BF4 . On the
contrary, the electronic properties between 1 and 2 clearly
differ, as evidenced by the photophysical and chiroptical
characterization, which shows that the electronic interaction
between the two carbazole units is negligible for 1 in
comparison to 2. This is further supported by the separation
of the two first oxidation potentials, ΔEOx, equal to 190 and
260 mV for 1 and 2, respectively (Figures 3 and 4).63,65 In
addition, the second carbazole unit does not help to stabilize
the radical by a resonance effect due to its meta position
regarding the nitrogen atom of the radical unit. On the
contrary, it induces a negative inductive effect which
−
−
the observed stability of 1•+BF4 .
−
−
It is important to note, however, that 2•+BF4 appeared
experimentally unstable, despite an electronic configuration
implying a SHI, and instantaneously affords a complex mixture
of oligomers. This high stability contrast represents therefore a
unique opportunity to gain more insights about the key
parameters that lead to a stable radical with SHI configuration.
We note that different functionals have been tested to ensure
that the SHI for 1•+ and 2•+ does not show up with one
specific functional (see SI). Since the structures of 1 and 2
differ by the dihedral angle between the two carbazole systems,
the difference of reactivity between 1•+BF4− and 2•+BF4− may
be related either to steric arguments or to the electronic
coupling between the radical center and the second carbazole
−
−
destabilizes more radical 2•+BF4 than 1•+BF4 due to the
greater electronic communication between the carbazole
systems in 2•+BF4 . This is experimentally confirmed by the
−
facility of 1 to form the corresponding monoradical 1•+ (EOx
=
0.92 V vs SCE) as opposed to 2•+ (EOx = 1.14 V vs SCE,
Figure 4) and theoretically reproduced by the corresponding
calculated vertical ionization potentials, which indicate a higher
value for 2 (5.42 eV) in comparison to 1 (5.16 eV, Figure 4).
The difference amounts to 25 kJ/mol.
−
fragment. As mentioned above, both radicals 1•+BF4 and
E
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Another notable difference between 1•+ and 2•+ is the orbital
energy gap ΔΕ between the HOMO and the SOMO of 0.8
and 0.4 eV, respectively. Moreover, the energy difference
between the HOMO of 2 and 2•+ is 0.54 eV, much larger than
the difference of 0.18 eV for 1 and 1•+, confirming the stronger
interaction between the radical center and the second
carbazole in 2•+ (Figures 4, S57, and S58). The obtained
results bring further evidence of the crucial role of the
electronic coupling between the radical center and the neutral
electron-rich fragment on stabilizing a molecular open-shell
electronic state with SHI.43
compounds (Figure 5), suggesting a SHI for corresponding
monoradicals 3•+ and 4•+ as in the case of 1•+ and 2•+.
−
In addition to its chemical stability, 1•+BF4 remains also
configurationally stable, presumably because of the high
enantiomerization barrier of its corresponding neutral form
(ΔG‡ = 173.5 kJ mol−1, 180 °C in 1,2-dichlorobenzene), which
allowed us to record the ECD of the radical enantiomers
(+)-/(−)-1•+BF4 . As depicted in Figure 3, the ECD signals of
−
Figure 5. (a) Cyclic voltammograms of 3 (black) and 4 (red) versus
SCE. (b) UV−vis−NIR absorption spectra of 1•+ (gray), 3•+ (blue),
and 4•+ (green) recorded by spectroelectrochemistry (see ESI for
details).
the neutral form experienced a decrease in intensity and a
global red shift of around 10 nm in the radical state together
with the appearance of new ECD active bands across the
visible and the IR regions. For instance, the intense signals at
302 (Δε = +40 M−1 cm−1) and 360 nm (Δε = −14 M−1 cm−1)
for neutral (+)-1 decreased and broadened to 312 and 370 nm
(Δε = +24 and −4.5 M−1 cm−1, respectively) upon formation
Interestingly, the reversibility of the oxidation processes was
strongly enhanced in comparison to that for 1•+ and 2•+,
highlighting the benefits of the steric protection on the
carbazoles 6 and 6′ positions. Indeed, 3 shows full reversibility
for both the first and the second oxidation signals (EOx = 0.87
and 1.10 V vs SCE) with no degradation after cycling scans. In
contrast, while the first oxidation process of 4 exhibits a fully
reversible response at 1.06 V vs SCE, its second oxidation at
1.37 V shows only a partial reversibility and leads to the
appearance of a weak new reduction peak at 0.85 V in the
reverse scan, suggesting some instability for diradical 42•2+, as
in the case of 1.
of (+)-1•+BF4 , all of these changes being well reproduced by
−
calculated ECD spectra (Figure S60). Importantly, new mirror-
image Cotton effects at 570, 715, and 860 nm (Δε = +0.3,
−0.6, and +1.6 M−1 cm−1, respectively) can be unambiguously
associated with radical absorption transitions, highlighting the
intrinsically chiral nature of this new persistent radical.
Mono- and Diradicals of 3 and 4 and Related
Characterization. The SHI found for 1•+BF4 gives an
−
interesting opportunity to generate the corresponding organic
diradical. As illustrated in Figure 1, oxidation of monoradical
displaying a SHI may result in a diradical with triplet ground-
state multiplicity, in accordance with Hund’s first rule, or an
open-shell singlet with antiferromagnetic coupling of the
The UV−vis−NIR spectra of monoradicals 3•+ and 4•+ were
investigated using spectroelectrochemistry and show similar
photophysical signatures to 1•+ in the UV−vis region but
significant changes in the low-energy part of the spectra
(Figure 4). For both 3•+ and 4•+, the corresponding NIR
absorption bands red shift to 812 and 850 nm, respectively,
and show much less vibronic structure than 1•+. In addition,
the broad NIR band that extends up to 1800 nm for both
radicals appears more intense than that for 1•+ with ε = 760
and 1100 M−1 cm−1 for 3•+ and 4•+, respectively. These
observed optical differences suggest that the radical center is
no longer localized on one carbazole fragment for both 3•+ and
4•+ but interacts with the second donor fragment to some
extent. Moreover, the electronic coupling between the two
carbazoles seems higher for helical radical 4•+ (ΔEOx = 310
mV) than for axial 3•+ (ΔEOx = 230 mV), in line with its
smaller dihedral angle (54° vs 65° for 4•+ and 3•+,
respectively). Despite the presence of the tert-butyl groups,
addition of one equivalent of magic blue to an acetonitrile
unpaired spins (Figure S20). In addition, as 1•+BF4 can be
−
considered as an innovative molecular design of chiral organic
spin-polarized donor,16,66−70 it appears interesting to inves-
tigate the magnetic interaction between the two radical units
upon oxidation of the second carbazole unit. In order to
evaluate the possibility to obtain stable chiral diradicals,
oxidation of 1•+ was investigated using tris(4-bromophenyl)-
aminium hexachloridoantimonate (“magic blue” oxidant, (4-
BrPh)3NSbCl6).71 However, UV−vis titration indicated
instantaneous reactivity of 12•2+2SbF6 , presumably through
−
a polymerization reaction as indicated by formation of a broad
IR band between 800 and 1800 nm (Figure S18) and
anticipated by the nonfully reversible second oxidation process
of 1 for in CV (Figure 3). To circumvent this aspect and
increase radical persistence, we turned our attention to 3 and
4, which are the 6 and 6′ sterically protected analogues of 1
and 2. These new chiral systems show similar chiroptical and
photophysical properties as 1 and 2 but slightly red shifted,
likely owing to the presence of the donor tert-butyl groups
(Figures S5 and S6). Regarding the possible presence of two
conformers of 3 with different dihedral angles, the calculated
ECD spectra of the conformer with the small dihedral angle
(67°) agree well with the experimental ones (Figure S72),
confirming its higher abundance in solution over the
conformer with the large dihedral angle (106°). As for 1 and
2, CVs of 3 and 4 afford two oxidation events for both
solution of 4 did not lead to 4•+SbCl6 but rather polymeric
−
materials as indicated by the obtained broad NIR band
centered at 1440 nm which clearly differs from the optical
signature recorded by spectroelectrochemistry (Figure S10).
This result further indicates that the steric protection at the 6
and 6′ positions of the investigated bicarbazole systems is not a
decisive factor for reaching radical stability. On the contrary,
3•+BF4 was quantitatively obtained and displays a high
−
stability with a measured half-life of 4.1 days in solution
(Figure S28−29, Table S6). For comparison, 3′•+BF4− shows a
half-life of 3.5 days under the same conditions, highlighting a
F
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synergetic effect between the steric protection and the SHI on
the radical stability. Frontier molecular orbitals of 3•+ further
show that the SHI goes along with a (HOMO/SOMO) energy
gap of 0.47 eV, which is in between those for 1•+ and 2•+. This
follows the trend for the calculated dihedral angles between the
π-conjugate systems (77° and 52° for 1•+ and 2•+,
respectively). These results further confirm the importance of
minimizing the electronic interaction between the radical
center and the additional donor unit to increase the stability of
a radical displaying SHI. The high stability found for
between the carbazole π-systems. Importantly, diradical
−
32•2+2SbCl6 showed high chiroptical properties in the NIR
region, as indicated by its absorption anisotropy factors, |gabs|=
1.0 × 10−3 at 920 nm (Figure 6), which is one of the highest
measured for molecular organic diradicals.25,27 The magnetic
properties of 32•2+2SbCl6 were investigated by EPR, which
−
resulted in an intense and broad doublet without any hyperfine
coupling nor Δm = 2 half-field transition, presumably due to a
weak interaction between the two radicals (g = 2.08, Figure
6).72 Given the high stability of 32•2+2SbCl6− (more than 66 h
in the solid state), magnetometry measurements were also
−
3•+SbCl6 prompted us to further explore formation of
−
−
diradical 32•2+2SbCl6 . Addition of two equivalents of magic
performed on powder of 32•2+2SbCl6 at a magnetic field
blue to the solution of 3 results in quantitative formation of
strength of 2 kOe below 20 K and 10 kOe above 20 K to get
further insight about the interaction between radicals within
the chiral environment. χMT is equal 0.73 cm3 K mol−1 at 298
K for 32•2+2SbCl6− (Figure 6), which almost perfectly matches
with the contribution for two uncoupled single electrons with g
= 2.00. This value started to decrease for temperatures below
170 K, indicating an antiferromagnetic coupling between spins.
The χM−1 vs T plot can be fitted with the Curie−Weiss law (χM
= C (T − θ) with C = 0.79 cm3 K mol−1 and θ = −24 K
(Figure 6)). The singlet−triplet gap (ΔEST) estimated from θ
at 67 cm−1 (0.19 kcal mol−1) reveals a weak antiferromagnetic
intramolecular spin-exchange interaction. Interestingly, these
findings confirm the expected formation of a diradical upon
oxidation of the highest doubly occupied molecular orbital of a
−
diradical dication 32•2+2SbCl6 , which represents one of the
rare examples of persistent intrinsically organic chiral diradicals
and thus embedded unique magnetic and chiroptical properties
(Figure 6). 32•2+2SbCl6 displayed a similar UV−vis−NIR
−
−
monoradical with a SHI. For 32•2+2SbCl6 , the two unpaired
electrons are localized on each carbazole fragment with a weak
spatial overlap between the corresponding two SOMOs.42,44
Likewise, the calculations showed that 32•2+ has two unpaired
electrons and a spin-triplet electronic state instead of a singlet
configuration, including two α-spin SOMOs and, lower in
energy, the set of spin-paired orbitals (Figure S64). The
isosurfaces of the two SOMOs (as well as for LUMO and
LUMO+1) are seen to represent in-phase and out-of-phase
linear combinations of frontier fragment orbitals with an
energetic splitting of 0.127 eV whose combined densities are
reflected by the spin density calculations (Figure S65 and ESI
for details). However, spin state energetics are not easy to deal
with in DFT calculations. We found that an open-shell singlet
configuration became effectively isoenergetic with the triplet
when the fractions of the exact exchange in the functional went
to zero, and weak interactions between the electron spins in
adjacent units of 32•2+ in the solid may further complicate the
picture. We thus hypothesize that given the weak electronic
Figure 6. Synthetic pathway to form 3•+SbCl6− and 32•2+2SbCl6−. (a)
−
UV−vis−NIR absorption spectra of 3 (black), 3•+SbCl6 (red), and
−
3
2•2+2SbCl6 (blue) in acetonitrile at 298 K. (b) Absorption
−
dissymmetry factor gabs spectra of (+)-32•2+2SbCl6 (solid line) and
−
(−)-32•2+2SbCl6 (dashed line) in acetonitrile at 298 K. (c) EPR of
3
2•2+2SbCl6− (n = 9.4858 GHz) at 77 K (solid line). (d) χMT vs T of
solid diradical 32•2+2SbCl6−. (e) Calculated spin density for (+)-32•2+
with an isosurface of 0.002 au.
−
signature to that of 3•+SbCl6 , with a two times higher
absorption intensity, as expected from the presence of two
coupling between the carbazole fragments in 32•2+2SbCl6 , the
−
oxidized carbazole units (Figure 6). The UV−vis−NIR
spectrum of 32•2+2SbCl6 shows a 20 nm blue shift of the
−
two SOMOs could be considered as disjointed, which
ultimately favored the singlet ground state, in analogy with
previous 3,3′-dimethylenebiphenyl diradicals molecular design
and using Ovchinnikov parity models.18,73,74
−
signal at 800 nm in comparison to 3•+SbCl6 along with an
increase and red shift of the broad IR band (ε = 2250 M−1
cm−1 at 1400 nm), suggesting some interactions between the
radical centers. The calculated spectrum of 32•2+ reproduced
well the experimental one and allowed us to assign the low-
energy bands between 700 and 1200 nm to transitions among
the set HOMO−1, HOMO and LUMO, LUMO+1 (Figure
S66 and Table S14).
CONCLUSION
■
In conclusion, we described here the first persistent sterically
unprotected axially chiral bicarbazole radical and its diradical
analogue along with their experimental and theoretical optical,
chiroptical, and magnetic properties. We investigated the
structure−properties relationship of organic chiral monorad-
icals based on bicarbazole molecular architectures with either
axial or helical chirality and found that only the former is
chemically and configurationally stable with a half-life of up to
3 h under air conditions. We attributed this unprecedented
result to a complex interplay of SHI and electronic coupling
between the radical center and the remaining neutral carbazole
As for 1•+BF4 , enantiomers of 3•+SbCl6− and 32•2+2SbCl6
−
−
remain also configurationally stable, and the corresponding
mirror-image ECD spectra show a similar profile up to 1100
nm. Both (+)-3•+SbCl6 and (+)-32•2+2SbCl6 displayed
Cotton effects assigned to radical transitions at 660, 820, and
900−950 nm (Δε = +0.7, −2.2, and +2.8 M−1 cm−1,
respectively, Figure S21), which are red shifted in comparison
−
−
to (+)-1•+BF4 , reflecting the higher electronic coupling
−
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Article
fragment, which is directly linked to the carbazoles dihedral
angle and related to axial and helical chirality. Such unique
electronic features provided an opportunity to design and
isolate a persistent, intrinsically chiral, diradical which
displayed near-infrared electronic circular dichroism intensity
up to 1100 nm and almost degenerate singlet−triplet ground
states with weak antiferromagnetic interactions evaluated by
SQUID experiments (ΔEST ≈ 0.2 kcal mol−1). The reported
findings may bring new insights on the key parameters
governing the chemical and configurational stabilities of chiral
organic radicals, which may offer new opportunities to design
innovative organic high-spin materials.
ACKNOWLEDGMENTS
■
̀
́
We acknowledge the Ministere de l’Enseignement Superieur,
de la Recherche et de l’Innovation, the Centre National de la
Recherche Scientifique (CNRS). L.A. and J.A. thank the
National Science Foundation (CHE-1855470) for financial
support and the Center for Computational Research (CCR) at
the University at Buffalo for computational resources.75 The
́
PRISM core facility (Biogenouest, UMS Biosit, Universite de
Rennes 1, Campus de Villejean, 35043 RENNES Cedex,
FRANCE) is acknowledged for NMR characterization and
ECD measurements.
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sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.0c08948.
Crystallographic file for 1 (CIF)
Crystallographic file for 2 (CIF)
Crystallographic file for 3 (CIF)
Crystallographic file for 4 (CIF)
Methods and synthetic procedures, ORTEP diagrams,
table of crystal data, unit cell diagrams, HPLC separation
data, additional photophysiscal and chiroptical charac-
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Jochen Autschbach − Department of Chemistry, University at
Buffalo, State University of New York, Buffalo, New York
14260, United States; orcid.org/0000-0001-9392-877X;
Email: jochena@buffalo.edu
Ludovic Favereau − Université Rennes, Rennes F-35000,
France; orcid.org/0000-0001-7847-2911;
Email: ludovic.favereau@univ-rennes1.fr
Authors
Sitthichok Kasemthaveechok − Université Rennes, Rennes F-
35000, France; orcid.org/0000-0001-5061-5511
Laura Abella − Department of Chemistry, University at
Buffalo, State University of New York, Buffalo, New York
14260, United States; orcid.org/0000-0003-2188-248X
Marion Jean − Aix Marseille University, Marseille 13284,
France; orcid.org/0000-0003-0524-8825
Marie Cordier − Université Rennes, Rennes F-35000, France
Thierry Roisnel − Université Rennes, Rennes F-35000, France
Nicolas Vanthuyne − Aix Marseille University, Marseille
13284, France; orcid.org/0000-0003-2598-7940
Thierry Guizouarn − Université Rennes, Rennes F-35000,
France
Olivier Cador − Université Rennes, Rennes F-35000, France;
orcid.org/0000-0003-2064-6223
Jeanne Crassous − Université Rennes, Rennes F-35000,
France; orcid.org/0000-0002-4037-6067
Complete contact information is available at:
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Notes
The authors declare no competing financial interest.
H
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https://dx.doi.org/10.1021/jacs.0c08948
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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https://dx.doi.org/10.1021/jacs.0c08948
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX