In-Fjord Substitution in Expanded Helicenes: Effects of the Insert on the Inversion Barrier and Helical Pitch

Article abstract of DOI:10.1002/chem.202102585

A series of expanded helicenes of different sizes and shapes incorporating phenyl- and biphenyl-substituents at the deepest part of their fjord have been synthesized via sequential Au-catalyzed hydroarylation of appropriately designed diynes, and their racemization barriers have been calculated employing electronic structure methods. These show that the overall profile of the inversions (energies, number of transition states and intermediates, and their relative position) is intensively affected by the interplay of steric and attractive London dispersion interactions. Hence, in-fjord substitution constitutes an additional tool to handle the mechanical properties in helicenes of uncommonly large diameter. The photochemical characterization of the newly prepared helical structures is also reported.

Full text of DOI:10.1002/chem.202102585

Accepted Article
Accepted Article
Title: In-Fjord Substitution in Expanded Helicenes: Effects of the Insert
in the Inversion Barrier and Helical Pitch
Authors: Manuel Alcarazo, Samuel Suárez-Pantiga, Pablo Redero,
Xaiza Aniban, Martin Simon, Christopher Golz, and Ricardo
A. Mata
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
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To be cited as: Chem. Eur. J. 10.1002/chem.202102585
Link to VoR: https://doi.org/10.1002/chem.202102585
01/2020

10.1002/chem.202102585
Chemistry - A European Journal
RESEARCH ARTICLE
In-Fjord Substitution in Expanded Helicenes: Effects of the Insert
in the Inversion Barrier and Helical Pitch
Samuel Suárez-Pantiga,[a] Pablo Redero,[a] Xaiza Aniban,[b] Martin Simon,[a] Christopher Golz,[a]
Ricardo A. Mata,[b],* and Manuel Alcarazo[a],*
[a]
Dr. S. Suárez-Pantiga, P. Redero, M. Simon, Dr. C. Golz, Prof. Dr. M. Alcarazo
Institut für Organische und Biomolekulare Chemie, Georg-August-Universität Göttingen, Tammannstraße 2, 37077 Göttingen, Germany
E-mail: malcara@gwdg.de
[b]
X. Aniban, Prof. Dr. R. A. Mata
Institut für Physikalische Chemie, Georg-August-Universität Göttingen, Tammannstraße 6, 37077 Göttingen, Germany
E-mail: rmata@gwdg.de
Supporting information for this article is given via a link at the end of the document.
Abstract: A series of expanded helicenes of different sizes and
shapes incorporating phenyl- and biphenyl-substituents at the
deepest part of their fjord have been synthesized via sequential Au-
catalyzed hydroarylation of appropriately designed diynes, and their
racemization barriers have been calculated employing electronic
structure methods. These show that the overall profile of the
inversions (energies, number of transition states and intermediates,
and their relative position) is intensively affected by the interplay of
steric and attractive London dispersion interactions. Hence, in-fjord
substitution constitutes an additional tool to handle the mechanical
properties in helicenes of uncommonly large diameter. The
photochemical characterization of the newly prepared helical
structures is also reported.
A well-known strategy to freeze the racemization in helicenes
consists of the incorporation of substituents at one or both termini
of their fjord region.[12] Alternatively, higher racemization barriers
are achieved by the embedment of the helicene moiety into more
extended π-systems (extended helicenes),[13] or into polyhelical
scaffolds (multi-pole helicenes);[9,14] these structures require the
simultaneous deformation of several helicene units to achieve
helical inversion. We hypothesized however, that an additional
manifold to control inversion barriers might be operative in the
case of extended helicenes. Each linear fusion incorporated to the
helicene framework entails positioning a C-H moiety at their inner
fjord region. Installation of suitable substituents at one or some of
these in-fjord positions should increase the helical pitch of the
resulting helicene and expectedly also the strain during the
enantiomer interconversion process. As a result, augmented
configurational stability is expected (Figure 1).
Introduction
Classical helicene structures consists of a series of ortho-fused
(hetero)aromatic rings, which on growing adopt a screw-shaped
tridimensional conformation (A, Figure 1).[1] These compounds
have been intensively studied not only for the synthetic challenges
that their preparation holds, but also for the unique properties that
they display in areas as remote as (asymmetric) catalysis,[2]
molecular machines,[3] optoelectronic devices,[4] crystal
engineering[5] or molecular recognition.[6] Recent milestones
achieved in helicene synthesis comprise the thermodynamic
equilibration after cyclization of diastereomeric mixtures to afford
enantiopure [6]- and [7]-helicenes,[7] or the preparation of helical
bilayer nanographenes[8] and multi-pole helicenes.[9]
By relaxing the condition of ortho-fusion between rings, for
example allowing alternating linear and angular ring connections,
expanded helicene scaffolds emerge (B, Figure 1). As a direct
consequence of the more lax ring fusion rules, expanded
helicenes invariably increase their radii and depict more torsional
flexibility than “orthodox” ones. This translates into a slight
reduction of their helical pitches, and much lower configurational
stabilities; indeed, no configurationally stable expanded helicene
that follows the alternating pattern B has been reported to
date.[10,11]
Figure 1. (a) Ring connectivity in helicenes and expanded helicenes; (b)
Positioning in-fjord substituents in expanded helicenes; (c) Known strategies to
fix the conformation in helicenes.
1
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Determined to assess the viability of that working hypothesis, and
as a continuation to our efforts to enable the (asymmetric)
construction of helicenes and related structures,[15] we embarked
on the synthesis of in-fjord-substituted expanded helicenes C
employing phenyl and biphenyl groups as internal substituents.
Given the already proven versatility of Au-catalysis to assemble
complex polyaromatic architectures via alkyne hydroarylation, this
transformation was chosen as the key tool for the final assembly
of the desired structures.[16,17] Thus, we describe herein the
synthesis of three expanded helicenes 1-3 that rigorously follow
an alternation of linear/angular ring fusion pattern, and compound
4, which is a hybrid between classical helicenes (consecutive
angular fusions at both arm termini) and expanded ones (angular-
linear alternation in its central section). All 1-4 derive from π-
extensions of the benzo[a]phenanthro[2,3-o]pentaphene core,
and bear either a phenyl or a biphenyl group at the deepest
position of their internal cavity (Figure 2). The three-dimensional
structures of these compounds have been determined by X-ray
crystallography and their helical inversions processes evaluated
with the help of state-of-the-art quantum mechanical calculations.
Their photophysical characterization is reported as well.
described by Müllen following an otherwise identic route.[18] We
strongly recommend however the use of bistriflates 9a,b due to
their higher solubility in typical organic solvents, which facilitates
their handling, purification and further transformation (Scheme 1).
Scheme 1. Synthesis of the benzo[m]tetraphene core platform. Reagents and
conditions: a) PhCHO (0.5 equiv.), p-TsOH (2 mol%), 110 °C, 32 h., 6a, 84%;
6b, 82%; b) Tf2O (2 equiv.), Et3N (2 equiv.), -78 °C→r.t., 7a, 99%; 7b, 90% ; c)
PbO2 (3.1 equiv.), AcOH, reflux; d) HBF4 (5.0 equiv.), Ac2O, 0 °C, 8a, 53%; 8b,
39% (over two steps); e) BnCO2Na (1 equiv.), 150 °C, Ac2O, 9a, 53%; 9b, 16%.
Initial attempts to couple 9a with an excess amount of boronic acid
10a’ (5.0 equiv.) via Suzuki reaction afforded 11 albeit with quite
modest yield; the protodeborylation of 10a’ is the main process
observed. After extensive experimentation the Negishi reaction
was identified as the most efficient route to couple 9a,b with
internal alkynes 10a-c.[19] Careful optimization of the reaction
conditions (2.5 mol% Pd2(dba)3, 15 mol% S-Phos, 100 °C (µw),
30 min.) allowed the isolation of key precursors 11-14 in 68, 91,
77 and 66 % yields, respectively. The use of microwaves instead
of conventional heating dramatically accelerates this step of the
route and even improves the yields (Scheme 2A).[20] Note that p-
anisyl substituents were strategically located at the alkyne termini
of 11-14 with the aim of suppressing parasitic 5-exo-dig
cyclizations and the subsequent formation of non-benzenoid rings
during the final twofold Au-catalyzed hydroarylation reaction.
As an additional difficulty during that last step, each of both
hydroarylation events may occur at two competing sites of the
benzo[m]tetraphene core, the electronically more activated but
sterically hindered positions 1 and 13, or the more accessible
external ones (positions 3 and 11). For that reason, extensive
screening of reaction conditions was necessary, leading to the
identification of catalyst 15 as the optimal one to assemble the
desired helicene skeletons in terms of conversion and
regioselectivity.[21] Under these optimized conditions, compounds
1-4 were isolated in 66%, 72%, 44% and 62% yields, respectively.
(See the Supporting Information for all optimization details and
also for the characterization of undesired regioisomers 16a,b,
obtained during the process).[22]
Figure 2. Target helicoidal structures reported in this work.
Results and Discussion
Synthesis of expanded helicenes. The route developed to
prepare helicenes 1-4 starts from commercially available 2,7-
napthalenediol 5, which was effectively transformed into
xanthenes 6a,b by reaction with half equivalent of the appropriate
aromatic aldehyde under acidic catalysis (Scheme 1).
Subsequent triflation of both alcohols in 6a,b was followed by
oxidation of the polycyclic core with PbO2, and acid-promoted
elimination to afford key pyrilium salts 8a,b in moderate yields
(52% and 34%, respectively; 2 steps). Next, the desired
benzo[m]tetraphene cores were obtained by condensation of 8a,b
with sodium phenylacetate in acetic anhydride followed by in situ
decarboxylation. This sequence affords intermediates 9a,b in
gram scale as white solids, which can be stored for months under
atmospheric conditions without apparent decomposition.
Compounds with structures analogous to that of 9a,b but having
Br-substituents instead of triflates in positions 2 and 12 have been
X-ray crystallography. Single-crystals suitable for X-ray
analyses of 1, 3 and 4 were grown from slow diffusion of hexane
into concentrated solutions of the title compounds in carbon
disulfide, or alternatively by slow evaporation of saturated toluene
solutions (See Figure 3A-C and the Supporting Information).
2
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RESEARCH ARTICLE
While 1 already shows a clear deviation of the planarity due to the
presence of the inner Ph-group, its arms are relatively short and
therefore, the comparison of the structures of 3 and 17[10b] is the
one that best portrays the influence of in-fjord substitution in the
structural deformation, electronic structure, local aromaticity, and
indirectly also on the helical inversion barrier of π-expanded
helicenes (Figure 3). Compound 3 crystallizes as racemate
(space group R-3); the two enantiomers separately stack along
the c-axis, each one rotated 120° respect to its two closest
neighbors, in the way that parallel columns of only P- and M-
enantiomers are formed. Rings D, E and I, J of consecutively
pilled molecules overlap, with a shortest π-stacking distance of
3.5 Å; disordered hexane molecules occupy the channel in
between these columns.
As expected, the helical pitch of 3, measured as the vertical
distance from the centroids of the terminal A and M rings, is
significantly larger than the one of 17 (dA-M = 7.7 Å in 3, versus dA-
M=3.7 Å in 17); but interestingly, the necessary distortion to
accommodate the biphenyl insert does not distribute evenly along
the structure as reflected from the torsion angles along the helical
inner rim (Φ = 4.1°, 4.0°, 14.6°, 21.3°, 1.6°, 4.5° for 3). It
accumulates in the most internal anthracene unit of the fjord cavity,
rings F, G and H, while both helical arms are virtually planar. This
is also evident from the comparison of the root mean square
deviation (RMS) of the carbon atom positions from the mean
plane of the benzene ring to which they belong. The values in %
are represented inside of the rings in Figure 3.
The Y-ring of the biphenyl insert also originates repulsive
interactions with the internal protons of rings E and I. This results
in an opening of the helicene arms in a plane perpendicular to the
helicoidal axis, which increases the helical diameter of 3 (dD-J
=
10.9 Å) when compared with that of the unsubstituted core
structure 17 (dD-J=10.2 Å) (Figure 3B,E). Remarkably, the terminal
edges of the helicene arms in 3 (rings A and M, respectively) are
conveniently stacked on the top and bottom of ring Z of the
biphenyl insert, respectively; being the interplanar distances,
measured from the centroids of the rings, dA-Z= 4.0 Å and dM-Z
=
3.8 Å and dA-M = 7.7 Å. These values are very similar to the pitches
found in high order helicenes such as for example [16]helicene.[24]
Although we were not able to obtain the X-ray structure of 2 our
calculations, which closely reproduce the pitches of 1, 3 and 4,
predict a dA-M = 8.2 Å in that architecture. This counterintuitive
result, which implies a longer helical pitch for 2 than for 3, can only
be explained by presuming the presence of attractive π-π
interactions between the helicene arms and ring Z of the insert in
3 (See the computational section and the Supporting Information).
The solid-state structure of 4 is quite informative as well. The
molecule adopts an approximately C2-symmetric conformation,
and its helical pitch measured from the centroids of the rings A,
and K (8.5 Å) is the biggest one in the series due to the angular-
angular junction of terminal rings A-B and J-K, which increases
the unfavorable steric interaction of these moieties with ring Z and
forces ring tilting also at the arms. As a result, the torsion angles
along the helical inner rim (Φ = 20.4°, 20.0°, 20.4°, 25.7°, 21.5°,
17.5°) and the RMS deviation of the carbon atoms from the ideal
benzene plane reveal evenly distributed deformations along with
the complete structure. This distinguishes 4 from 1 (Φ = 1.1°,
14.9°, 25.7°, 6.2°) and 3 (Scheme 3A-C). None of the structures
synthesized is configurationally stable at room temperature.
Scheme 2. (A) Synthesis of helicene precursors. Reagents and conditions: a)
10a (5.0 equiv.), Pd2(dba)3 (2.5 mol%), S-Phos (15 mol %), µw 100 °C, 68%; b)
10b (5.0 equiv.), otherwise as a), 91%; c) 10c (5.0 equiv.), otherwise as a); 12,
77%; 13, 66%; (B) 2-fold Au-catalyzed hydroarylation step, d) 15 (10 mol%),
AgSbF6 (10 mol%), CH2Cl2 or Cl2(CH)2Cl2, 20 °C, 1, 66%; 2, 72% 3, 44%; 4,
62%; C) optimized Au-catalyst; D) Additional regioisomers obtained during the
optimization of the reaction conditions.[23]
3
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RESEARCH ARTICLE
X
X
X
H
G
F
D
E
F
G
9.78
F
5.90
E
3.63
5.56
6.46
7.73
7.18
6.79
6.21
I
E
2.85
C
2.76
G
H
6.67
2.80
D
1.87
dD-J
=
dB-H
=
11.0 Å
3.99
J
D
H
1.50
B
0.70
I
1.02
1.02
10.9 Å
C
Z
5.33
Z
5.57
C
1.52
K
0.42
I
J
4.43
K
2.53
A
0.69
A
3.37
B
5.05
0.44
B
A
0.89
L
M
1.37
0.61
0.92
Z
A
B
θAI = 45.8(1)°
C
Z
D
E
F
X
G
I
H
A
dA-Z = 5.2 Å
B
X
Z
C
D
E
F
G
H
dZ-I = 5.0 Å
I
F
H
G
6.1
6.3
6.2
E
3.2
I
3.6
D
1.0
J
1.7
C
1.2
K
1.5
B
1.4
L
1.4
A
0.7
M
0.7
Figure 3. A)-C) X-ray structures of 1, 3, 4. Only the ipso-carbon from the external p-(MeO)Ph substituents is shown. H atoms are removed for clarity; ellipsoids are
represented at 50% probability for 1 and 4, and 15% for 3. Root mean square deviation (RMS) from mean plane of each benzene ring given in %. D) Calculated
structure of [2]Ph at the PBEh3c level of theory. E) Reported structure of 17.[23]
4
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Electronic structure calculations and helical inversion
mechanism. Having developed an efficient synthesis for 1-4, we
examined their conformational dynamics by Density Functional
Theory (DFT) calculations. In order to obtain specific insight about
the influence of the inner substitution in the inversion barrier
heights, the pathways were calculated not only for compounds 1-
4, but also we included the unsubstituted core architectures. All
structures were optimized at the PBEh3c level of theory,[25] with
energies recomputed with the double hybrid functional
B2PLYP,[26] including D3 dispersion corrections with Becke-
Johnson damping,[27] the def2-TZVP basis set[28] and default fitting
basis.[29] Nudged elastic band calculations (NEB) were carried out
to obtain a conversion profile, with energy estimates for the barrier
provided by the climbing image. In cases where the conversion
mechanism crosses a Cs-symmetric intermediate, the energy
profiles were only computed for one stretch of the pathway and
then symmetrized. The aforementioned calculations were carried
out with the Orca 4.2.1 program package.[30] All substitutions in
the outer rim were removed to minimize computation time. These
structures are represented by the number of the real ones in
parenthesis; the internal substituent is indicated as subindex. The
energies reported are electronic energies unless otherwise noted.
The difference between the latter values and Gibbs free energy
barriers are small. In order to better understand some of the
trends observed, we have carried out dispersion interaction
density (DID) analyses of the structures.[31] The energies were
the inversion to take place (ΔE‡ = 82.7 kJ•mol-1). This can be
attributed to two factors: the loss of the stabilizing π-stacking
interaction between the Z-ring of the biphenyl and the arm that
starts the inversion, and the need to bend away the internal
substituent more abruptly than in [2]Ph to reduce steric hindrance.
More striking is that the potential energy diagram is again Cs
symmetric, suggesting that both arms operate independently, one
after the other. In addition, the Cs-symmetric conformer
corresponds to a local energetic maximum, [3]Bi-PhTS2; slight
rotation of the biphenyl Z ring either to the right or the left
reestablish a stabilizing π-staking interaction with one or the other
arm [3]Bi-PhINT1 (Figure 4C).
In order to understand the behavior of the more rigid 4, the
inversion process in structures [4]H and [4]Ph were studied as well
(Figure 4D-E). In [4]H one of the [4]helicene moieties located at
the end of the arms initially inverts via [4]HTS1 to form an
intermediate [4]HINT1, whereby the inverting arm edge ring points
towards the opposing arm. From this point, the Cs symmetric
[4]HTS2 transition state is found, exhibiting a rather similar barrier.
This structure shows the two arms pointing in the same direction.
A completely symmetric process involving the other arm furnishes
ent-[4]H. The barrier to helical inversion for this process was
calculated to be ΔE‡ = 25.5 kJ•mol-1 corresponding to the barrier
in [4]HTS2. Saving the distances in terms of absolute energy
values, the shape of the isomerization profile and the inversion
mechanism in [4]H is very similar to that described for
[8]helicene.[34]
computed at the SCS-LMP2 level of theory with the Molpro2020.1
[32,33]
program package.
Details for the calculation setup, in
Introduction of the internal Ph-substituent in [4]Ph substantially
changes the shape of the isomerization profile. Instead of three
transition states, only two symmetrical relatively high barriers (ΔE‡
= 74.2 kJ•mol-1) are observed. The transition states are similar in
structure to [4]HTS1 (inversion of a terminal [4]helicene moiety),
but the Cs-symmetric structure with the two arms facing each
other is no longer the highest transition state as in [4]H. Instead, it
is a minimum, [4]INT1. This might seem counterintuitive at first,
since the Ph-substituent should always contribute to the steric
strain, imposing more pronounced deformations to the helical
framework; however, a closer look reveals that this is an
incomplete analysis. First, the Ph-substituent adds foremost a
penalty when one arm points towards the center of the helix. If we
compare the barriers between [4]HTS1 and [4]PhTS1 (22.7 vs 74.2
kJ•mol-1), which is a fair comparison given the similarity between
the structures, the difference is 51.5 kJ•mol-1. This is a first
approximation to the added penalty as the result of the
augmented steric clash in [4]Ph. However, in the Cs-symmetric
structure, the Ph-substituent actually acts as a slightly stabilizing
factor. The [4]PhINT1 structure is 18.5 kJ•mol-1 above the global
minimum, while the same structural motif in [4]H, which is actually
a transition state, is 25.5 kJ•mol-1 above the starting configuration.
Attractive London dispersion forces, which are now operative
between the final edges of the arms and the middle phenyl group,
account for this fact. We have plotted in Scheme 5F the dispersion
interaction densities (DIDs) for the interactions between the arms
and the central substituent in [4]Ph and compared them (using the
same scale) to the interactions between the arms in the case of
[4]H . The dispersion interactions (quantified at the SCS-LMP2
level) show a stark contrast. In [4]PhINT1 they amount to 29
kJ•mol-1, with the value going down to just 3 kJ•mol-1 in the case
of [4]HTS2. Hence, we conclude that the London forces derived
from the presence of the Ph-insert effectively compensate the
energetic penalty to be paid in [4]PhINT1 due to the more
particular how the intramolecular analysis was defined, are
provided in Supporting Information.
In the case of unsubstituted 1, i.e. [1]H, no helix is formed, and the
most stable structure adopts a planar conformation, as expected.
Upon introduction of the phenyl substituent, two enantiomeric
conformations of compound [1]Ph become apparent, yet
separated by a single and rather small barrier [1]PhTS1 of only
15.9 kJ•mol-1 in the NEB computed energy profile (free barrier for
activation of 19.1 kJ•mol-1, Figure 4A). More interesting, however,
are the cases of compounds 2-4. Their potential energy
hypersurfaces are more complex, and in all cases the
interconversions take place via stepwise mechanisms (shown in
Figure 4B-E).
The energetic profile for the racemization of [2]Ph is shown in
Scheme 5B. The barrier has been calculated to be ΔE‡ = 59.0
kJ•mol-1, which is very similar to the one reported for the
unsubstituted 17 (ΔG‡ = 54.3). This indicates that the presence of
the phenyl insert in 2 does not significantly modify the height of its
inversion barrier, probably due to the length of the arms. However,
and exceptionally along the series, the potential energy diagram
of [2]Ph is not Cs symmetric, implying that through the minimum
energy pathway both arms move coordinated but at different
paces. Thus, while the left one in Figure 4B nearly achieves the
central anthracene plane, the right one squirms itself to facilitate
this step [2]PhTS1. Once the left arm achieves its final position
[2]PhINT1, the right one proceeds to complete the
enantiomerization, now via a quite low barrier [2]PhTS2, since it is
already appropriately positioned. This avoids the Cs structure
which would be even higher in energy due to the steric clash
between the phenyl substituent and the arms.
The formal exchange of the phenyl insert by a biphenyl one
delivers [3]Bi-Ph. At first glance, our calculations indicate that an
additional penalty of around 20 kJ•mol-1 needs to be satisfied for
5
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pronounced deformation of the polyaromatic skeleton. From this
analysis we concluded that the inversion barrier in [4]Ph is
probably lowered by dispersion as well since this interaction is
also present, at least in part, in [4]PhINT1 between the Ph-insert
and one of the helicene arms.
Figure 4. NEB energy profiles for the enantiomerization mechanisms of: a) [1]Ph, b) [2]Ph, c) [3]Bi-Ph, d) [4]H, e) [4]Ph calculated at the PBEh3c level of theory (single
point calculations at the B2LYP-D3/def2-TZVP level). The independent coordinate λ is the normalized abelian distance between atoms from reactant to transition
state. Only selected transition states and energy-minimum structures are represented together to the energy profiles.; f) DIDs for the interactions between the arms
and the central substituent in [4]Ph calculated at the SCS-LMP2 level.
6
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None of the barriers calculated is sufficiently high to allow the
resolution of the enantiomers of 1-4 at room temperature.
However, these examples do show how upon substitution, the
inversion pathway of the original helicene scaffold can be
Three sets of comparable absorption bands can be identified in
the UV-Vis spectra of 1-4 in the 280-340, 360-400 and 380-420
nm regions. As expected, the spectra for 2-3 are quite similar, but
they appear bathochromically shifted compared with that of 1; this
modified, even by eliminating or creating new barriers to surmount. is a consequence of the increased π-extension in these structures.
In general, the steric impact of the in-fjord substitution increases
racemization barriers, but this effect cannot be simply correlated
with the size of the substituent.
Comparison of the absorption maxima of 2 and 3 with the same
bands reported by 17: 320 nm (ϵ = 1.4 x 105 M-1cm-1), 375 nm (ϵ
= 3.5 x 104 M-1cm-1) and 395 nm (ϵ = 3.8 x 104 M-1cm-1), also
indicates a red shift of the transitions of 2 and 3 by approximately
1220 and 1490 cm-1, respectively.
The fluorescence spectra follow a tendency that is consistent with
the absorption spectra. The maxima for 2-4 also appear red
shifted when compared to 1 (λem,max = 457, 479, 472 and 471 nm.
for 1-4, respectively).
UV-Vis absorption and emission. The UV absorption and
emission spectra of 1-4 and 16a,b have been measured in
chloroform at room temperature (absorption) and 10 °C
(emission); those for 1-4 are summarized in Figure 5. Table 2
contains all relevant numerical data.
DFT calculations have also been carried out to obtain the spectra
of compounds 1-3 (see the Supporting Information). We have
made use of the simplified time dependent density functional
theory formalism (sTD-DFT)[35] with the B97X functional[36] and
the def2-TZVP basis set. The functional was chosen for its correct
long-range asymptotic exchange, which is important in such
extended systems. Following the same procedure as in previous
absorption studies for helicenes, the computed absorption
energies were shifted by 1 eV.[37] The trends observed in the
measured spectra are well replicated in the DFT spectra (based
on PBEh3c structures). The red shifts of the transitions of 2 and 3
if compared to 17 can be traced back to their increased helical
pitches. A comparison between computed and experimental
absorption spectra is provided in the Supporting Information.
1
1,2
1
1,2
1
2
3
4
0,8
0,6
0,4
0,2
0
0,8
0,6
0,4
0,2
0
260
360
460
560
660
Wavelength [nm]
Figure 5. (a) Normalized UV-Vis absorption (continuous line) and fluorescence
spectra (dotted line) of 1-4 in HCCl3 at room temperature.
Conclusion
Table 2. Photophysical properties[a]
In summary, we describe herein the design and synthesis of
expanded helicenes containing substituents attached to the inner
part of their cavity. Key for the preparation of these unique
architectures was the identification of a highly reactive Au-catalyst,
15, which allows the key hydroarylation steps to proceed not only
efficiently in terms of yield, but also with high regioselectivity.
Comparison of the enantiomerization energy profile of the
targeted helicenes with these of their unsubstituted parent
structures show significant differences in the relative energy
values and in the general shape of the profile (number of transition
states and intermediates). These changes cannot be simply
correlated with the size of the insert. Hence, our results
demonstrate that deep in-fjord substitution can be used as an
additional tool to handle the mechanical properties of expanded
helicenes. Moreover, they also highlight the impact of dispersion
interactions in the enantiomerization of these architectures, and
how they may counteract the expected tendencies from the
classical concept of steric hindrance.
Compound
Absorption
λmax (nm)
ϵmax
λem (nm)
Φf
(M-1•cm-1)
(nm)[b]
1
300
366
388
3.26 x 104
3.88 x 104
2.51 x 104
457
490
0.29 (366)
0.07 (333)
0.02 (330)
2
3
290
333
388
409
1.62 x 105
1.59 x 105
8.78 x 104
7.69 x 104
479
515
293
336
390
411
1.31 x 105
1.09 x 105
6.15 x 104
5.27 x 104
472
506
4
312
383
406
7.66 x 104
5.49 x 104
3.79 x 104
471
508
0.09 (312)
0.33 (310)
0.12 (321)
16a
16b
310
339
390
3.40 x 105
2.60 x 105
2.70 x 105
494
521
Acknowledgements
292
321
370
394
1.66 x 105
1.68 x 105
1.17 x 105
1.27 x 105
506
533
Financial support from the Deutsche Forschungsgemeinschaft (Al
1348/4-3, INST 186/1237-1, INST 186/1298-1 and GKR 2455) is
gratefully acknowledged.
[a] Measured in HCCl3 at 25 °C; [b] Excitation wavelength.
Keywords: Expanded helicenes • Au catalysis • hydroarylation •
polycyclic aromatic compounds • racemization barriers
7
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10.1002/chem.202102585
Chemistry - A European Journal
RESEARCH ARTICLE
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10.1002/chem.202102585
Chemistry - A European Journal
RESEARCH ARTICLE
Entry for the Table of Contents
Deep in-fjord substitution is presented as a new tool to control the mechanical properties of expanded helicenes. The syntheses of a
series of these compounds containing internal inserts is described and their overall inversion profile is calculated. Interestingly, the
interplay between steric and dispersion interactions play a major role in shape of the inversion profiles (energies, number of transition
states and intermediates).
9
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