Access to Functionalized Pyrenes, Peropyrenes, Terropyrenes, and Quarterropyrenes via Reductive Aromatization

Article abstract of DOI:10.1002/anie.202100686

Herein we report a versatile concept for the synthesis of fourfold functionalized, soluble pyrenes, peropyrenes, terropyrenes, and quarterropyrenes. They were obtained by a modular stepwise approach towards the rylene scaffold via Suzuki–Miyaura cross coupling, oxidative cyclodehydrogenation in the presence of caesium hydroxide under air, and finally zinc-mediated reductive silylation. The silylated reaction products were characterized by X-ray crystallography. The first example of a synthesized and crystallized quarterropyrene is presented and its oxidation reaction investigated. The functionalized ropyrenes were systematically characterized by means of UV/Vis–NIR and photoluminescence spectroscopy showing a bathochromic shift of 80 nm per naphthalene unit and a nearly linear increase of the extinction coefficients. Cyclic voltammograms and DFT calculations identify them as electron-rich dyes and show a narrowing of the electrochemically determined HOMO–LUMO gap and lower oxidation potentials for the higher homologues.

Full text of DOI:10.1002/anie.202100686

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How to cite:
International Edition: doi.org/10.1002/anie.202100686
German Edition: doi.org/10.1002/ange.202100686
Organic Dyes
Access to Functionalized Pyrenes, Peropyrenes, Terropyrenes, and
Quarterropyrenes via Reductive Aromatization
Simon Werner, Tobias Vollgraff, and Jçrg Sundermeyer*
In memory of Professor Siegfried Hꢀnig
Abstract: Herein we report a versatile concept for the synthesis
of fourfold functionalized, soluble pyrenes, peropyrenes,
terropyrenes, and quarterropyrenes. They were obtained by
a modular stepwise approach towards the rylene scaffold via
Suzuki–Miyaura cross coupling, oxidative cyclodehydrogena-
tion in the presence of caesium hydroxide under air, and finally
zinc-mediated reductive silylation. The silylated reaction
products were characterized by X-ray crystallography. The
first example of a synthesized and crystallized quarterropyrene
is presented and its oxidation reaction investigated. The
functionalized ropyrenes were systematically characterized by
means of UV/Vis–NIR and photoluminescence spectroscopy
showing a bathochromic shift of 80 nm per naphthalene unit
and a nearly linear increase of the extinction coefficients. Cyclic
voltammograms and DFT calculations identify them as
electron-rich dyes and show a narrowing of the electrochemi-
cally determined HOMO–LUMO gap and lower oxidation
potentials for the higher homologues.
Figure 1. Chemical structures of a) the substance class of rylene
diimides and b) pyrene and its higher homologues.
rylene diimides were synthesized by the group of Mꢀllen
using a modular approach of cross couplings starting from
halogenated perylene or naphthalene monoimides and their
corresponding boronic acid esters followed by oxidative
[
10–15]
cyclizations to form the rylenes.
with up to eight naphthalene units were formed, and also
Thereby, rylene diimides
[10–17]
terrylene and quarterrylene were accessed.
Another model system for molecular rylenes is peropyr-
ene (Figure 1b, n = 2). Peropyrene is a potential candidate
for singlet fission materials and effective synthesis strat-
egies for substituted peropyrenes were developed during the
past years. Terropyrenes, for example, the unsubsti-
tuted stem system or bent varieties for cyclophane syn-
theses, are rare in literature. All published strategies
required multi-step organic syntheses, especially for accessing
[18]
[
19]
P
olyaromatic hydrocarbons (PAHs) offer a huge variety of
applications related to their unique (opto-)electronic proper-
ties, for example in materials for organic electronics and
[
17,20–22]
[
1]
[23]
photovoltaics. Studies elucidating structure–property rela-
tions gave rise to the synthesis of nanographenes with defined
structures and tailored properties. Especially the edge
structure and width determine their electronical properties
[24]
[2]
[21,22,25,26]
the higher homologues, terropyrenes.
Key of the
[
3]
(
band structure). The family of poly-perinaphthalenes
synthesis route reported by Chalifoux and co-workers are p-
(
rylenes, see Figure 1b), also known as 5-armchair graphene
extentions of aromatic systems using two- or tetrafold
[
4]
[21,22,27]
nanoribbons (5-AGNRs), is a broadly studied class of PAHs.
Compared to other GNRs their simple structure, broad
spectral light absorbance and singlet fission properties, as
well as length-dependent band gap sizes were motivation for
intensive research efforts. With regard to molecular and
soluble rylene compounds beyond the archetypical family of
perylene diimides (PDIs, see Figure 1a), higher homologue
alkynylated precursors in acid-mediated
or InCl3-
[25]
catalyzed benzannulations.
Inspired by the modular construction principle of higher
[5]
[6]
[
7]
[11,13,15]
rylene diimide dyes
and the novel reductive function-
[
8]
[28]
alization approaches for naphthalene diimide (NTCDI)
[29,30]
and perylene diimide (PTCDI)
in our group and in the
[
9]
group of Miyake, we intended to reductively access higher
homologue ropyrenes. The precursors 2, 3, 11, and 12 of the
final peropyrenes, terropyrenes, and quarterropyrenes are
synthesized in analogy to the modular synthesis of soluble,
liquid crystalline dihydroxy-ropyrene-quinones reported by
[
*] S. Werner, T. Vollgraff, Prof. Dr. J. Sundermeyer
Fachbereich Chemie and Material Science Center (WZMW)
Philipps-Universitꢀt Marburg
[31]
Hans Meerwein Strasse 4, 35032 Marburg (Germany)
E-mail: JSU@staff.uni-marburg.de
Buffet and Bock (Scheme 1).
Peropyrenequinone 2 was
directly reduced and silylated, reacting with Zn as reducing
agent in the presence of trimethylsilyl chloride to the orange
air sensitive peropyrene silylether 4. Zinc turned out to be the
best reducing agent. It is readily available, non-toxic, and
processable even under non-inert conditions. For reductive
silylation of the smaller dihydroxy pyrenequinone 2 we could
introduce four triisopropylsilyl groups, leading to highly
soluble pyrene-tetrasilyl ether 5 in rather poor yield. Next,
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202100686.
ꢁ
2021 The Authors. Angewandte Chemie International Edition
published by Wiley-VCH GmbH. This is an open access article under
the terms of the Creative Commons Attribution License, which
permits use, distribution and reproduction in any medium, provided
the original work is properly cited.
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1
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Scheme 1. a) Previously reported synthesis of dihydroxy-peropyrenequinone (2); b) our approaches for the synthesis of peropyrene-silylether 4 and
pyrene-silylether 5 by reductive functionalization; c) the higher homologue terropyrene-silylethers 13 and 14 and quarterropyrene-silylether 15 by
modular approaches.
we turned our attention to the application of a similar
reduction protocol to obtain the higher homologues of 2,
terropyrenequinone 11 and quarterropyrenequinone 12.
triisopropylsilyl chloride with imidazole, a reductive silylation
of 11 and even 12 yielded the air stable terropyrene-
triisopropyl silylether 14 and royal blue colored quarter-
ropyrene-triisopropylether 15 with superior solubility, high
purity, however low yields. Both 14 and 15 could be isolated
by column chromatography with dichloromethane. We fur-
ther synthesized a congener of 15 with n-butyl groups in both
terminal positions and TMS groups (S4) in order to improve
the solubility, but the solubilizing effect of non-bulky butyl
groups was not significantly more effective. Unfortunately, all
attempts to synthesize a higher homologue penterropyrene
were not successful so far, since the corresponding penterro-
pyrene-quinone was too insoluble for further transformations.
The good solubility of our silylated rylenes 4, 13, and 15 in
chlorinated solvents made it possible to obtain X-ray
diffractive single crystals via slow gas phase diffusion of n-
pentane (Figure 2). Trimethylsilyl ethers 4 and 13 crystallize
1
1 and 12 were synthesized in a facile two-step protocol
[
32]
[33]
starting with bis-boronic acid pinacol esters 6
and 7
together with peri-brominated hydroxyphenalenone 8 under
standard Suzuki–Miyaura coupling conditions in a 1:2 ratio.
The resulting ter- and quarternaphthyls 9 and 10 could be
isolated as inseparable isomer mixture (see SI) by a simple
precipitation due to protonation with diluted acetic acid and
subsequent washing with methanol. Non-planar 9 and 10 were
heated in a CsOH melt for three hours to 2808C in a Ni-
crucible on air until the gas evolution ceased. We chose CsOH
as base because of the improved intercalation and interaction
capabilities of caesium cations with intermediately formed
aromatic anions. 11 and 12 were isolated as insoluble dark
blue solids that were directly used for further transformations.
Surprisingly, 11 and 12 reacted under similar conditions as 2
with zinc and trialkylsilyl chlorides in dioxane despite their
insolubility at 1008C. The resulting silylated products were
soluble since their capability of p–p stacking is reduced due to
the sterically demanding silyl groups. Hence, a similar terro-
pyrene-trimethylsilylether 13, as a higher homologue to 4,
could be isolated as purple-red solid. After activating
in the monoclinic space group P2 /c with p–p stacking pairs
1
(dp–p = 3.30 ꢁ (4) and 3.64 ꢁ (13)) of molecules oriented in
a herringbone arrangement for 13. Quarterropyrene 15
crystallizes in the monoclinic spacegroup P2 /n being the
1
first investigated quarteropyrene in the crystalline state to the
best of our knowledge. Its bulkier TIPS groups do not prevent
the face-to-face p–p stacking interactions in the solid state
2
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unit [a bathochromic shift of about 80 nm and an increase of
the extinction coefficient from lmax = 390 nm (e =
ꢀ
1
ꢀ1
2
1
4500 Lmol s )
for
5
to
lmax = 634 nm
(e =
ꢀ
1
ꢀ1
28200 Lmol s ) for 15 (Figure 3a)] with extinction coef-
ficients in slightly smaller magnitudes compared to the related
[
11,13]
rylene diimides.
All four compounds showed nearly no
concentration-dependent absorption maxima (see Figure S2–
S6). TD-DFT (see Figure S13) supports the trend also
regarding the oscillator strengths of the lmax transitions. The
mirrored emission spectra reveal a similar trend with Stokes
shifts of around 15 nm. 4, 13, and 15 show strong vibronic
progressions indicating, together with the narrow Stokes shift,
[34]
small reorganization energies. A broadening of the bands
can be observed for the higher homologues. Measured
fluorescence quantum yields (F , see SI) were only moder-
PL
ately high in case of 4 (0.30), lower for 13 (0.08) and 14 (0.06),
and undetectably low in 4 and 15 with bulky rotating TIPS
groups most likely deactivating the fluorescence through
reorganization energy loss. The predictable evolution of
optical properties can also be rationalized with the Kohn–
Sham molecular orbitals of the HOMO and LUMO for 4, 5,
1
3, and 15, being of similar modular symmetry (Figure 3d).
The frontier molecular orbital energies were experimentally
assigned using cyclic voltammetry (Figure 3c). The electron-
rich dyes 4, 5, 14, and 15 show two pronounced and quasi-
reversible oxidation potential waves, whereas only 14 shows
one and 15 shows two not fully reversible reduction potential
waves within the electrochemical window of dichlorome-
thane. In comparison to reported arylated pero- and terro-
pyrenes, 5 and 14 show lower oxidation potentials and are
[25]
therefore more electron-rich. Additionally, a narrowing of
the electrochemically determined HOMO–LUMO gap with
the growing p-system could be observed. The experimental
HOMO energies were referenced to the vacuum energy level
[
35]
of the ferrocene/ferrocenium redox couple (ꢀ4.8 eV, see
Figure S8–S12) and the LUMO energy was accessed with the
help of the optical HOMO–LUMO gap (determined from the
intersection wavelength of normalized UV/Vis and PL
[
36]
spectra ). Both experiment and theory show an energy gap
(
E ) narrowing from E = 3.02 eV (theory 3.48 eV) for pyrene
g
g
5
to 1.93 eV (theory 1.96 eV) to 15, respectively (see Table 1).
Figure 2. Solid state structures (left: front view, center: side view, and
right: packing structure) of a) 4; b) 13, and c) 15. Hydrogen atoms are
omitted for clarity and thermal ellipsoids are shown at the 50%
This is indicated by higher HOMO and lower LUMO
energies (from experimentally ꢀ5.01 eV (HOMO) and
[38]
probability level.
ꢀ
2.04 eV (LUMO) of
2.73 eV (LUMO) for 15), with theoretical values supporting
5
to ꢀ4.62 eV (HOMO) and
ꢀ
the observed trends. In comparison with arylated ropyrenes,
[
25]
(dp–p = 3.36 ꢁ) but induce a slight bending of the rylene
silylether substitution leads to higher HOMO levels.
backbone in comparison to the trimethylsilyl-substituted
homologues 4 and 13 and lead to a transversal intermolecular
displacement in order to minimize steric repulsions.
The high HOMO value of 15 and also the existence of
a non-zero baseline in the NIR region of the UV/Vis spectrum
gave rise to the assumption that minor amounts of the
+
The (opto-)electronical properties of the rylenes 4, 5, 13,
oxidation product [15] are present after column chromato-
1
4, and 15 were investigated by UV/Vis–NIR and photo-
graphy with dichloromethane, also observed for other elec-
tron-rich PAHs and resulting from traces of acid in the
presence of air. In order to study the oxidation product of
15, we added the oxidant nitrosyl tetrafluoroborate (NOBF4)
in a UV/Vis–NIR titration experiment. Upon addition of
luminescence spectroscopy as well as cyclovoltammetry. The
optical spectra and cyclovoltammogramms of both quarter-
ropyrene silylethers 13 and 14 were almost identical (see
Figure S7). Typical for rylene dyes,
are dominated by the lowest-energy S1 S -transition. The
[
37]
[11,13]
the absorption spectra
!
excess (10 equiv) of NOBF a gradual color change from blue
0
4
absorption maxima (lmax) and extinction coefficients (e)
increase nearly linearly with each additional naphthalene
to turquoise green and broad NIR absorption bands appear-
ing at 820 and 1050 nm (Figure 4) were observed. Addition of
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ꢀ
5
Figure 3. a) UV/Vis–NIR spectra of 4, 5, 13, and 15 (cꢁ1ꢂ10 m) in CH Cl and photographs of the corresponding solutions (inset).
2
7
2
ꢀ
b) Normalized emission spectra of 4, 5, 13, and 15 in CH Cl (cꢁ1ꢂ10 m, lex. =350 nm). c) Cyclic voltammograms of 4, 5, 14, and 15
2
2
ꢀ1
(
measured in CH Cl , 0.1 m n-Bu NPF , 100 mVs scan rate, glassy carbon working electrode, platinum reference electrode) and corresponding
2 2 4 6
ꢀ
1
differential pulse voltammograms (DPV, dashed lines, 10 mVs scan rate). d) HOMO and LUMO of 5, 4, 13, and 15 calculated by DFT (TIPS
groups simplified by TMS, B3LYP, def-2/TZVPP, isoval. 0.03 a.u.).
Table 1: Summary of experimentally and theoretically determined
HOMO and LUMO energies of 4, 5, 14, and 15.
E
ELUMO,exp.
E
[eV]
ELUMO,theo.
[c]
HOMO,exp.
a]
HOMO,theo.
[c]
[
[b]
[
eV]
[E ] [eV]
g
[E ] [eV]
g
5
4
1
1
ꢀ5.01
ꢀ4.71
ꢀ4.80
ꢀ4.62
ꢀ2.04 [3.02]
ꢀ2.16 [2.55]
ꢀ2.66 [2.14]
ꢀ2.73 [1.89]
ꢀ4.59
ꢀ4.54
ꢀ4.32
ꢀ4.27
ꢀ1.11 [3.48]
ꢀ1.85 [2.69]
ꢀ2.02 [2.20]
ꢀ2.33 [1.94]
4
5
[
a] Electrochemically determined oxidation and reduction potentials
+
referring to Fc/Fc as internal standard (EHOMO(fc)=ꢀ4.8 eV):
EHOMO,exp =ꢀ4.8 eVꢀE
. [b] Determined HOMO and LUMO energies
1/2,ox1
based on the optical HOMO–LUMO energy gap [E ]:
g
ELUMO,exp =EHOMO,exp +Eg,opt. [c] Calculated HOMO and LUMO energies,
level of theory: def2-TZVPP/B3LYP, for carthesian coordinates (XYZ) of
optimized geometries see Table S1–S4.
Figure 4. Changes in the UV/Vis–NIR absorption spectrum of 15
during titration with NOBF in CH Cl /acetonitrile. Inset: Color change
after addition of 10 equiv of NOBF4.
4
2
2
more than 10 equiv NOBF4 had no further effect. Using
H NMR, the formation of a paramagnetic compound could
1
be observed by the absence of distinct signals. TD-DFT
+
calculations (see SI, Figure S15) of [15] describe the novel
Supporting Information available: Additional experimen-
tal details, NMR spectra, CV/DPV measurements, UV/Vis
and TD-DFT spectroscopy, cartesian coordinates of calcu-
lated structures (XYZ), and XRD data. The CIF files of the
presented structures are provided.
band at 1050 nm as SOMO–LUMO transition.
In summary, we report a facile and efficient access to the
higher homologues of peropyrene by a modular and scalable
synthetic bottom-up approach with a reductive aromatization
by Zn/R SiCl as a key step towards functionalized, soluble,
3
and easily isolable electron-rich rylene silylethers. Supported
by comprehensive experimental spectroscopy and spectrom- Acknowledgements
etry, the influence of the enlargement by naphthalene units on
the HOMO–LUMO gap was rationalized by spectroscopy,
Financial support by the LOEWE Program of Excellence of
CV, and DFT calculations. As OSiR groups can be converted
the Federal State of Hesse (LOEWE Focus Group PriOSS
“Principles of On-Surface Synthesis”) is gratefully acknowl-
edged. Open access funding enabled and organized by Projekt
DEAL.
3
[
30]
to OTf functionalities for further transformations, we are
convinced that our new synthesis strategy allows an efficient
general synthetic entry into novel functionalized PAHs for
high-performance organic electronic materials.
4
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2
The authors declare no conflict of interest.
[
Keywords: crystallography · cyclovoltammetry · fluorescence ·
organic dyes · reductive aromatization
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Manuscript received: January 15, 2021
[
[
Revised manuscript received: March 11, 2021
Accepted manuscript online: March 16, 2021
Version of record online: && &&, &&&&
2
Angew. Chem. Int. Ed. 2021, 60, 1 – 6
ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Organic Dyes
An approach for the modular synthesis of
peropyrenes, terropyrenes, and quarter-
ropyrenes via reductive aromatization
and silylation key steps is presented. The
functionalized members of the ropyrene
family are characterized by X-ray crys-
tallography and C NMR spectroscopy.
Optical spectroscopy and cyclic voltam-
metry are discussed in relation to DFT-
calculated frontier molecular orbital
energies.
S. Werner, T. Vollgraff,
J. Sundermeyer*
&&&& — &&&&
Access to Functionalized Pyrenes,
Peropyrenes, Terropyrenes, and
Quarterropyrenes via Reductive
Aromatization
1
3
6
www.angewandte.org
ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 1 – 6
These are not the final page numbers!