Oxa[7]superhelicene: A π-Extended Helical Chromophore Based on Hexa-peri-hexabenzocoronenes

Article abstract of DOI:10.1002/anie.201800585

A novel π-extended “superhelicene” based on hexa-peri-hexabenzocoronenes (HBCs) has been synthesized by an efficient four-step synthetic procedure starting from diphenyl ether. Comprehensive structural analysis of the helicene was performed by NMR spectroscopy and mass spectrometry measurements together with X-ray analysis. Physicochemical analysis of the superhelicene and suitable HBC references revealed it had outstanding fluorescent features with quantum yields of over 80 %.

Full text of DOI:10.1002/anie.201800585

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Accepted Article
Title: Oxa-[7]-superhelicene: A π-extended helical chromophore based
on HBCs
Authors: David Reger, Philipp Haines, Frank W. Heinemann, Dirk M.
Guldi, and Norbert Jux
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201800585
Angew. Chem. 10.1002/ange.201800585
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Oxa-[7]-superhelicene: A π-extended helical chromophore based
on HBCs
David Reger,[a],[+] Philipp Haines,[b],[+] Frank W. Heinemann,[c] Dirk M. Guldi,[b] and Norbert Jux*[a]
Abstract: Herein, we report on the synthesis of a novel π-extended
“superhelicene” based on hexa-peri-hexabenzocoronenes (HBC) via
an efficient four-step synthetic procedure starting from diphenylether.
Comprehensive structural analyses were performed by means of
NMR-spectroscopy and mass spectrometry measurements together
with X-ray analysis. Physico-chemical analysis of the superhelicene
and suitable HBC references revealed among others outstanding
fluorescent features with quantum yields of over 80%.
properties. Important is also the solubility of helicenes, which
exceeds that of the planar analogous by far, and, which
constitutes a milestone in the realization of solution-processable
materials.[13]
In the context of converting non-planar PAHs into three
dimensional carbon allotropes, helical building blocks are
considered as key precursors. Promising is in this context the
fact that several π-extended helical structures have recently
been reported.[14]
Herein, we report on the synthesis and the characterization of a
first member of a new class of HBC-based helicenes, namely
“superhelicenes”.[15] The synthesis – scheme 1 – started from
diphenylether 1, which was converted into its 3,6-di-brominated
derivative 2 by aromatic halogenation. A double Sonogashira
coupling with 4-tert-butylphenyl-acetylene followed by a Diels-
Alder reaction with 2.5 eq. of 2,3,4,5-tetrakis(4-(tert-
butyl)phenyl)cyclopenta-2,4-diene-1-one 8 yielded bis-HAB-ether
4. Helical 6 was obtained after reaction with 2,3-dichloro-5,6-
dicyano-1,4-benzoquinone DDQ / triflic acid in CH2Cl2 creating
an overall number of 13 C-C bonds, including the formation of
the furan ring. To the best of our knowledge formation of a furan
ring under such experimental conditions is unprecedented. We
have reason to believe that the closure of the furan ring is the
final step of the cascade, as shorter reaction times like, for
example, 2 h, furnished mixtures of helicene 6 and bis-HBC-
ether 5. Interestingly, changing the reagents to FeCl3 in
nitromethane/CH2Cl2 assisted in avoiding the closure of the furan
ring and in forming bis-HBC-ether 5 selectively. 5 serves as a
perfect reference as it is structurally related to the π-extended
helicene 6 but lacking HBCs in a helical arrangement. Notably,
bis-HBC-ether 5 is light sensitive and transforms into helicene 6
via photocyclization. This phenomenon was observed during
photophysical measurements but was not further studied yet. In
consequence, such measurements with bis-HBC-ether 5 always
show minor amounts of 6. Therefore, two other references, that
is, pentakis-tert-butyl-mono-methoxy-HBC 7 and hexakis-tert-
butyl-HBC 23, were synthesized and characterized – for details
see SI, chapter 2.
Polycyclic aromatic hydrocarbons (PAHs) have evolved as
important benchmarks in the study of graphene. Here, their
smaller, modular sizes and their size-tunable features stand
out.[1] Not only the more common, planar PAHs have drawn
significant attention among chemists, physicists, and material
scientists. But, also the design, synthesis, and study of non-
planar PAHs is of great interest. Deviation from planarity offers
novel ways and means in the fields of organic materials and
electronics.[2] Alteration of conjugated π-systems towards the
third dimension is enabled by the introduction of non-six-
membered rings, by the “roll-up” of individual π-systems, or by
the institution of helicity. In the latter approach, π-systems are
placed in a screw-shaped arrangement.[3]
Discovered more than 100 years ago, helical, contorted
aromatics were among the first of these systems to be described
in the literature.[4] Ever since, this family of molecules has always
ignited the curiosity of chemists[5] and has found its way into a
large variety of applications. By virtue of their inherent chirality
they have been applied in enantioselective catalysis; both in the
form of ligands to afford catalytically active metal complexes[6] as
well as in the field of organocatalysis.[7] Notable are also their
roles in light emitting devices,[8] solar cell devices,[9] chiroptical
switches,[10] organic field-effect transistors (OFETs)[11] and non-
linear optics.[12]
At the forefront of contemporary research is the π-extension of
helicenes to afford novel materials with unique and tailorable
[a]
[b]
D. Reger, Prof. Dr. N. Jux
Department Chemie und Pharmazie &
Interdisciplinary Center for Molecular Materials (ICMM)
Friedrich-Alexander-Universität Erlangen-Nürnberg
Nikolaus-Fiebiger-Straße 10, 91058 Erlangen (Germany)
E-mail: norbert.jux@fau.de
P. Haines, Prof. Dr. D. Guldi
Department Chemie und Pharmazie &
Interdisciplinary Center for Molecular Materials (ICMM)
Friedrich-Alexander-Universität Erlangen-Nürnberg
Egerlandstraße 3, 91058 Erlangen (Germany)
Dr. F. W. Heinemann
[c]
[+]
Department Chemie und Pharmazie
Friedrich-Alexander-Universität Erlangen-Nürnberg
Egerlandstraße 1, 91058 Erlangen (Germany)
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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Scheme 1: Synthesis of the oxa-[7]-superhelicene 6: a) Br2 (2 eq.), 1,2-dichloroethane, 1 h at 0 °C, 17 h at r.t., yield 99 %; b) 4-tert-butylphenylacetylene
(2.1 eq.), Pd(PPh3)2Cl2 (3 mol%), CuI (1.5 mol%), THF/diisopropylamine (1:1), N2, 19 h at 100 °C, yield 51 %; c) tetra-(4-tert-butylphenyl)-cyclopentadienone
(2.5 eq.), toluene, N2, 23 h at 220 °C (pressure flask), yield 86 %; d) FeCl3 anhydrous (35 eq.), MeNO2, CH2Cl2, N2, 25 min at 0 °C, 20 h at r.t., yield 60 %; e)
DDQ (15 eq.), triflic acid (30 eq.), CH2Cl2, N2, 25 min at 0 °C, 20 h at r.t., yield 86 %. Side, front and top views of the structure of oxa-[7]-superhelicene 6 as
calculated (DFT-calculation carried out on B3LYP/6-31G* level). Hydrogen atoms omitted for clarity.
absorptions are, due to the larger π-conjugated system, red-
shifted respectively by 14, 3, and 50 nm relative to those of 23, 7,
and 5 – Figure 1. A closer look reveals additional differences:
The α-band absorptions in the long wavelength range are more
intense in superhelicene 6 than in 5 and 7, while the β-band
absorptions in the short wavelength range are less intense. In
principle, p- and α-band absorptions are symmetry-forbidden and,
as such, should be unobservable. On one hand, the p-absorption
bands are enabled by intensity borrowing from the energetically
close lying β-absorption bands. On the other hand, symmetry-
breaking substituents and or structural alterations affect the α-
band absorptions. The functional groups assist in lowering the
symmetry from D6h in 23 to C2v in 7 and 5, and in intensifying the
α-band absorptions. In 6, it is the extended π-conjugation and
the helical structure that both alter the HBC structure. As a
matter of fact, it is hardly surprising that the α-band absorptions
in the helicene 6 are even more intense than in 5 and 7 and
overall red-shifted.[18]
Crystals suitable for x-ray diffraction analysis were grown by
slow diffusion of ethanol into a benzene solution of 6 at room
temperature. Only small, twinned crystals were obtained and the
crystal structure was of moderate quality – for x-ray analysis
details see SI, chapter 11. Fortunately the quality of the
measurement was good enough to present a preliminary
structural analysis of the superhelicene 6. It reveals that 6
crystallizes in a triclinic crystal system with a P-1 space group
and most importantly, the connectivity between the two HBCs is
validated. Also the stacking of the helicenes in the solid state
can be deduced. The M and P isomers pair off and form π-π
stacks between two HBCs with a distance of ~ 3.4 Å. Such a
distance is in very good agreement with those seen for π-π
stacks of hexakis-tert-butyl-HBC 23.[16] Since the crystal
structure was of only moderate quality we also performed DFT
calculations on the structure and the molecular frontier orbitals of
the superhelicene – see scheme 1 and SI, chapter 10.
Considering the overall sum of the five torsion angles of the
seven rings forming the helicene we derived values of 86.4 ° (x-
ray structure) and 94.1 ° (DFT-calculation) – for details see SI,
chapter 11 – which are both larger than 78.9 ° seen in the x-ray
structure for oxa-[7]-helicene,[17] prompting to a more distorted
helical system.
The structure is also reflected in the NMR spectra. The signals of
the “inner” tert-butyl groups, which are positioned above and
1
Figure 1: Absorption (left) and normalized fluorescence (right) features of 6,
23, 7 and 5 in THF. The fluorescence spectra are recorded after excitation @
350 nm. The picture on the right of the fluorescence spectra shows a CH2Cl2
solution of 6 under illumination at 366 nm.
below the other HBC are high-field shifted and appear in the H
NMR spectrum at 0.18 ppm. With the help of HSQC, HMBC,
COSY and nuclear Overhauser effect (NOE) measurements it
1
was possible to assign all signals in the H NMR and a majority
of signals in the 13C NMR – for details see SI, chapter 3.
Strong fluorescence evolves for superhelicene 6 in the 490 to
670 nm region, with vibrational fine structure that includes 525,
565, and 620 nm maxima, as well as fluorescence quantum
yields in the range of 80% to 85%. – see SI, chapter 7. Notable
is the small Stokes shift of 10 nm when contrasting the long
wavelength absorption maximum with the short wavelength
fluorescence maximum. The fluorescence features of 7 and 5
are 50 nm blue-shifted relative to those of 6 in sound agreement
with the findings gathered in the absorptions measurements.
They are also vibrationally better resolved as a reflection of
Comparative assays on the basis of photophysical investigations
between hexakis-tert-butyl-HBC 23, pentakis-tert-butyl-mono-
methoxy-HBC 7, bis-HBC-ether 5 and helicene 6 shed light onto
the impact of twisting and extended π-conjugation. The
absorption of, for example, 6 features signatures in the UV and
the visible regions. In accordance with Clar’s rule for PAHs, the
absorption features at 355 / 365 , 393 / 433 nm, and 480 /
515 nm are ascribed to β-, p-, and α-band absorptions,
respectively.[13] At first glance, the β-, p-, and α- band
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structural rigidity. The fluorescence quantum yields of 23 and 7
with 4.1% and 6.9% are more than ten times weaker than those
of 6.[19] In time-resolved fluorescence experiments a decay rate
of 2.3 × 108 s-1 for 6 signifies a significant acceleration relative to
23, 7, and 5 with 0.21 × 108, 0.26 × 108 and 0.28 × 108 s-1,
respectively.[20]
From the absorption and fluorescence measurements we derive
singlet excited state energies for 23, 7, 5, and 6 of 2.8, 2.6, 2.6,
and 2.4 eV, respectively. To investigate the energetics of the
triplet excited state, emission measurements were conducted at
80 K. In contrast to the references, which reveal
phosphorescence features in the 550 to 700 nm range, oxa-[7]-
superhelicene 6 is non-phosphorescent – for details see SI
chapter 4.
The redox features were determined by cyclic voltammetry and
differential pulse voltammetry in CH2Cl2 with Fc/Fc+ as reference
and 0.2 M TBAPF6 as supporting electrolyte. 6 gives rise to two
reductions at -1.5 and -1.8 V as well as three oxidations at +0.9,
+1.2, and +1.5 V – Figure 2. 23 features only one reduction at -
1.7 V and only one oxidation at +1.1 V. 5 exhibits two reductions
at -1.6 and -1.9 V and three oxidations at +1.1, +1.3 and +1.4 V,
while 7 possesses only one reduction at -1.8 V and two
oxidations at +0.9 and +1.1 V – for details see SI, chapter 6.
characteristic maxima at 420, 650, 740, and 860 nm – for more
details see SI chapter 5, figure 21. By means of intersystem
crossing, the singlet excited state transforms into triplet excited
state. Fingerprint absorption of the latter is a 700 nm maximum.
The underlying rates of singlet excited state decay / triplet
excited state formation and triplet excited state decay were
determined for 6 as 2.3 x 108 and 1.3 x 104 s-1, respectively.
In comparison, 23, 7, and 5 show different behaviors. Upon
387 nm excitation of the references, the singlet excited state
forms with maxima at 544 and 1350 nm – for more details see SI
chapter 5, figure 22 - 24. In the following, transformation into the
triplet excited state by intersystem crossing with characteristic
maxima at 500 and 980 nm is observed. The deactivation rates
of the singlet excited states are in the range from 0.21 x 108 to
0.28 x 108 s-1, while those of the triplet excited states range from
0.71 x 104 to 1.5 x 104 s-1.
Probing 6 in the presence of TCNE led to the formation of the
one-electron oxidized form of 6 with transients evolving at 645
and 705 nm – Figure 4 – for more details see SI, chapter 5,
figure 20. As far as the signature of the TCNE radical anion is
concerned, it is discernable in the 400 to 450 nm range in
accordance with literature, where it has been reported to appear
at 430 nm.[21] It is, however, the triplet excited state of 6, which
undergoes charge separation with a bimolecular rate constant of
2.4 x 109 M-1s-1 – for more details see SI, chapter 5, figure 25.
On a timescale of up to 10 µs, the corresponding charge
separated state deactivates via reinstatement of the ground state.
The addition of TCNE to solutions of 23, 7, and 5 led to similar
results. Here, the one-electron oxidized form evolved in the form
of 430, 555, and 860 nm signatures. The corresponding
bimolecular rate constant for charge separation starting from the
triplet state are between 0.9 x 1010 and 1.2 x 1010 M-1s-1 – for
more details see SI, chapter 5, figure 26 - 27.
Figure 2: Differential pulse voltammetry (left) and cyclic voltammetry (right) of
6 in CH2Cl2 with Fc/Fc+ and 0.2 M TBAPF6 as reference and supporting
electrolyte, respectively.
Considering the first reduction and the first oxidation, the HOMO
– LUMO gaps of 23, 7, 5, and 6 are 2.8, 2.7, 2.7, and 2.4 eV,
respectively. All of them are in sound agreement with the singlet
excited state energies – vide supra.
The signatures of
6 upon oxidation were elaborated in
spectroelectrochemical experiments. Upon applying a bias of
+1.0 V versus Ag/Ag+, prominent maxima develop at 600, 650
and 700 nm and are ascribed to the one-electron oxidized form
of 6 – Figure 5 – for more details see SI, chapter 6.
Figure 4: Species associated spectra (left) and time profiles (right) of 6 upon
addition of TCNE in THF received by Target Analysis.
Figure 3: Species associated spectra (left) and time profiles (right) of 6 in THF
received by Target Analysis.
Figure 5: Comparison of TA spectrum of 6 in THF upon addition of TCNE with
0.06 µs time delay with SEC spectrum of 6 in THF upon applying of 1.0 V with
TBAPF6 as electrolyte vs. Ag/Ag+.
Femtosecond transient absorption spectroscopy in combination
with global and target analyses stood at the forefront to shed
light onto the time-dependent excited state features and kinetics.
Figure 3 shows that 387 nm excitation of 6 leads to the
instantaneous formation of its singlet excited state with
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Chem. Eur. J. 2016, 22, 9375-9386; d) L. Shi, Z. Liu, G. Dong, L. Duan,
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9743-9746.
In summary, we synthesized and in depth characterized the first
member of a novel family of π-extended helical chromophores. A
straightforward four-step synthetic protocol was developed
starting from diphenylether. We documented the outstanding
photophysical properties of helical rather than planar HBC
relatives. Currently, we perform complementary investigations
with these novel π-extended helicenes including the tailored
synthesis of even more intriguing members of this molecular
family. Additionally, we are focusing on the enantiomeric
separation and device fabrication such as light emitting devices.
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We thankfully acknowledge the funding by the German
Research Council (DFG) through the Collaborative Research
Center SFB 953 (Synthetic Carbon Allotropes). D.R. thanks the
Graduate School Molecular Science (GSMS) for financial
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[18] Footnote: The auto-oxidation of 5 results in an absorption spectrum,
which also features the characteristics of 6.
[6]
[19] Footnote: As
5 auto-oxidizes under illumination the fluorescence
quantum yields of 5 feature contributions from 6. Uncorrected, the
quantum yield is then 12.1 %. Corrections of the absorption and
fluorescence spectra by subtracting the absorption and fluorescence
features of 6 lead to quantum yields of 6.4 %.
[20] Footnote: In line with the energy gap law of the fluorescence, the red-
shifted fluorescence of superhelicene 6 is linked to shorter lifetimes as
non-radiative deactivation channels start to dominate. This stands,
however, in sharp contrast to the fluorescence quantum yields of
superhelicene , which are higher than those noted for the in the blue
region emitting HBCs 5 and 7. A likely rationale is the lower symmetry
of superhelicene 6 when compared to the parent HBCs.
[7]
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Soc. 1975, 97, 1699-1707.
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COMMUNICATION
The fusion of oxa-[7]-helicene with
HBC subunits in a straight forward
synthesis gave a novel structural
David Reger, Philipp Haines, Frank W.
Heinemann, Dirk M. Guldi, Norbert Jux*
Page No. – Page No.
motif
for
π-extended
helical
chromophores with outstanding
Oxa-[7]-superhelicene: A π-extended
helical chromophore based on HBCs
photophysical
fluorescence
Φf > 80 %).
properties
quantum
(e.g.
yields
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