SYNLETT0936-52141437-2096
© Georg Thieme Verlag Stuttgart · New York
2018, 29, 2529–2534
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Syn lett
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R. Stężycki et al.
Synthesis and Photophysical Properties of Hexaphenylbenzene–
Pyrrolo[3,2-b]pyrroles
Rafał Stężyckia,b
CHO
NH2
t-Bu
t-Bu
t-Bu
t-Bu
David Regerc
t-Bu
t-Bu
t-Bu
t-Bu
Norbert Jux*c
N
2 steps, 15%
+
a
Daniel T. Gryko*
0-02146-28
N
O
a
Institute of Organic Chemistry, Polish Academy of Sciences,
Kasprzaka 44-52, 01-224 Warsaw, Poland
dtgryko@icho.edu.pl
b
Warsaw University of Technology, Faculty of Chemistry,
Noakowskiego 3, 00-664 Warsaw, Poland
c
Department of Chemistry and Pharmacy and Interdisciplinary Center for Molecular Materials, Friedrich-AlexanderUniversität Erlangen-Nürnberg, 91058 Erlangen, Germany
norbert.jux@fau.de
t-Bu
t-Bu
O
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
3 examples
6–80% yield
Published as part of the Cluster Synthesis of Materials
Received: 27.08.2018
Accepted after revision: 29.08.2018
Published online: 26.09.2018
DOI: 10.1055/s-0037-1610286; Art ID: st-2018-v0452-c
Abstract Methods for the synthesis of pyrrolo[3,2-b]pyrroles containing hexaphenylbenzene moieties at the 2- and 5-positions or the 1- and
4-positions have been developed. It was shown that placing a hexaphenylbenzene moiety at the 2- and 5-positions requires a Diels–Alder
reaction between an alkyne-substituted pyrrolopyrrole core and a
2,3,4,5-tetraphenylcyclopenta-2,4-dien-1-one. The resulting dyes show
a strong blue fluorescence that was hypsochromically shifted by chlorination at the 3- and 6-positions. The overall conjugation between the
hexaphenylbenzene moieties and the pyrrolopyrrole core is limited, as
evident from their photophysical properties. The hexaphenylbenzene
moieties attached to the pyrrolo[3,2-b]pyrrole core could not be transformed into hexa-peri-hexabenzocoronenes through intramolecular oxidative aromatic coupling.
Key words chromophores, pyrrolopyrroles, halogenation, multicomponent reaction, hexaphenylbenzene, fluorescence
Propeller-shaped hexaphenylbenzenes1 have been
known for a long time and have been widely utilized in a
variety of applications in materials science, such as molecular wires,2 nonlinear optical materials,3 liquid crystals,4 molecular rotors,5 supramolecular electronic materials,6 and
microporous organic solids.7 These compounds also play a
vital role as precursors to hexabenzocoronenes.8 The classical synthesis of these compounds through the Diels–Alder
reaction and subsequent CO extrusion9 is still commonly
used, although new methods have been recently reported
by Itami10 and Jux11 and their respective co-workers.
Tetraarylpyrrolo[3,2-b]pyrroles (TAPPs) were discovered
in 2013 and have since become a subject of intense investigation as a result of their combination of interesting optical
properties and straightforward synthesis.12–14 Because of
their electron-rich character, pyrrolo[3,2-b]pyrroles have
turned out to be excellent models for studies on symmetry
breaking in the excited state,15 resistive memory devices,16
aggregation-induced emission,17 and photochromic analysis of halocarbons.18 In this context, we hypothesized that
linking hexaphenylbenzene moieties to a pyrrolopyrrole
core at the 1- and 4-positions or at the 2- and 5-positions
might be an excellent strategy for probing the levels of conjugation in such hybrid dyes possessing, effectively, 16 biaryl linkages.
Our initial strategy was to synthesize the corresponding
formylhexaphenylbenzene by following the method described by Jux and co-workers,11 and an aminohexaphenylbenzene by following the classical Diels–Alder route (see
Supplementary Information), for subsequent use directly
under typical conditions for the synthesis of pyrrolo[3,2b]pyrroles. The reaction of the formylhexaphenylbenzene 1
with 3,5-bis(tert-butyl)aniline (2) and biacetyl (3) did not
lead to the formation of the corresponding product
(Scheme 1). All attempts to optimize the reaction conditions such as increasing the reaction time to 24 hours, increasing the reaction temperature to 150 °C, or adding cosolvents failed to alter the reaction output. Both unreacted
aldehyde 1 and the corresponding imine were detected in
the reaction mixture.
On the other hand, the analogous reaction of the aminohexaphenylbenzene 4 with 2-methoxybenzaldehyde (5)
and biacetyl (3) was successful and afforded the corresponding pyrrolo[3,2-b]pyrrole 6 in low yield (Scheme 2).
The low reactivity of substrates bearing a hexaphenylbenzene core in this multicomponent reaction is probably
related to steric effects during the subsequent cyclocondensation steps, rather than to electronic effects, because nei-
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Helen Hoelzelc
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R. Stężycki et al.
t-Bu
t-Bu
t-Bu
CHO
NH2
OMe
CHO +
t-Bu
NH2
t-Bu
t-Bu
+
t-Bu
5
t-Bu
1
2
t-Bu
t-Bu
4
O
O
TsOH, AcOH
toluene
110 °C, 6%
TsOH, AcOH
toluene, 150 °C
O
O
3
3
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
N
OMe
N
MeO
N
N
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
6
t-Bu
Scheme 1 Attempted multicomponent reaction leading to 2,5bis(hexaphenyl)pyrrolo[3,2-b]pyrrole
ther of these substrates possesses any meaningful electrondonating or electron-withdrawing substituent (other than
the reacting groups).
The failure to obtain a pyrrolo[3,2-b]pyrrole containing
hexaphenylbenzene units at the 2- and 5-positions prompted us to investigate an alternative strategy, namely the formation of hexaphenylbenzene moieties on a pyrrolo[3,2b]pyrrole core possessing two phenylethynylphenyl moieties. The reaction of aldehyde 7, constructed through a
Sonogashira coupling, with amine 2 and biacetyl (3) led to
the formation of the corresponding pyrrolopyrrole 8 in 19%
yield (Scheme 3). Reaction of this heterocycle with the corresponding substrate 91b,19 induced a double Diels–Alder reaction followed by extrusion of carbon monoxide, eventually leading to the formation of bis(hexaphenylbenzene)-substituted pyrrolopyrrole 10 in very good yield.20
Scheme 2 Synthesis of 2,5-bis(hexaphenyl)pyrrolo[3,2-b]pyrrole 6
Full conversion was achieved by maintaining the reaction temperature at 250 °C for 42 hours and by using four
equivalents of diene 9. Compound 10 can be regioselectively chlorinated at the 3- and 6-positions, leading to dye 12 in
63% yield (Scheme 4). Interestingly the same molecule
could be obtained by reversing the order of the steps, i.e. by
initially chlorinating pyrrolopyrrole 8 with NCS to give 11,
and subsequent Diels–Alder addition followed by CO extrusion to give dye 12 (Scheme 4). Given the low overall yield
of this latter strategy, the former strategy was deemed to be
more efficient.
Attempts were made to transform dyes 6, 10, and 12
into the corresponding hexa-peri-hexabenzocoronenes.
Various reaction conditions were investigated, such as classical FeCl3/MeNO2/CH2Cl2, as well as PhI(O2CCF3)2/BF3·Et2O,
Fe(OTf)3,21 AlCl3/Cu(OTf)2/CS2, etc. In all cases, full conver-
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CHO
t-Bu
O
t-Bu
NH2
O
N
3
+
t-Bu
t-Bu
t-Bu
N
TsOH, AcOH
toluene
110 °C, 19%
t-Bu
2
t-Bu
t-Bu
t-Bu
8
7
O
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
N
9
t-Bu
t-Bu
N
Ph2O
250 °C
80%
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
10
Scheme 3 Synthesis of 2,5-bis(hexaphenylbenzene)pyrrolo[3,2-b]pyrrole 10
sion was observed after a few minutes, but none of the desired products were detected by using ESI-MS. Moreover,
the complexity of the reaction mixtures, which typically
contained more than ten different products, prevented us
from isolating any product in a pure state. The rationale for
this disappointing result is that the initially formed radicalcation is located on the most electron-rich part of the molecule, i.e. on the pyrrolopyrrole core, thereby obstructing the
typical mechanism for oxidative aromatic coupling.22 This
was confirmed by the fact that the HOMOs in these molecules are mostly located on the pyrrolopyrrole moieties (see
the Supplementary Information, where the detailed tabulated results are presented).
The photophysical properties of dyes 6, 8, and 10–12
were analyzed (Figures 1 and 2 and Table 1). Compound 6
absorbs mainly in the UV region as a result of the presence
of two electron-donating 2-methoxyphenyl substituents at
the 2- and 5-positions. The λmax of 317/352 nm is comparable to that of the analogous 2,5-bis(2-methoxyphenyl)pyrrolo[3,2-b]pyrrole described by us earlier,12b which further
underlines the relatively weak conjugation between the
large substituents on the nitrogen atoms and the heterocyclic core.
Interestingly, the analogous pyrrolopyrrole 10 bearing
hexaphenylbenzene substituents attached at carbon atoms
in the 2- and 5-positions showed a significantly red-shifted
absorption (Figure 1), whereas derivative 12, bearing two
additional chlorine atoms, had a slightly hypsochromically
shifted absorption (λmax = 352 nm). The same trend was observed in comparing the 2,5-bis(phenylethynylphenyl)pyrrolo[3,2-b]pyrroles 8 and 11 (Figure 2).
Table 1 Photophysical Properties of Pyrrolopyrroles 6, 8 and 10–12 in
THF
Dye
Figure 1 Absorption (solid line) and emission (dotted line) of dyes 6
(black), 10 (red), and 12 (blue) in THF
λabs [nm]
λem [nm]
Stokes shift [cm–1] Φfla
6
317/340
415
5 300
0.56
8
404
472
3 600
0.69
10
389
438
2 900
0.69
11
371
434
3 900
0.63
12
352
416
4 200
0.14
a
Reference: quinine sulfate in 0.5 M H2SO4 (Φfl = 0.546).
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t-Bu
t-Bu
Cl
N
NCS, CHCl3
8
rt, 30%
t-Bu
t-Bu
N
Cl
t-Bu
t-Bu
11
9, Ph2O, 250 °C, 39%
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
Cl
N
t-Bu
t-Bu
rt, 63%
N
Cl
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
12
Scheme 4 Synthesis of dye 12 by two different routes
Figure 2 Absorption (solid line) and emission (dotted line) of dyes 8
(black) and 11 (red) in THF
Replacement of the phenylethynylphenyl units in dyes 8
and 11 with hexaphenylbenzene units (10 and 12, respectively) led to hypsochromic shift of both the absorption and
emission (15–34 nm; Figures 1 and 2), which suggests that
the effective conjugation pathway for TAPPs containing less
sterically hindered 4-alkynylphenyl substituents is larger
than for those possessing hexaphenylbenzene moieties. The
photophysical properties of dyes 8 and 11 correspond well
to those of the analogous pyrrolopyrroles reported by us
earlier.23
Pyrrolopyrroles 6 and 12 displayed violet emissions,
whereas dyes 8, 10, and 11 exhibited strong blue emissions.
Interestingly, molecule 6 displayed the largest Stokes shift
in this series (approximately 5300 cm–1), which suggests
there are large differences in the geometries of the ground
and excited states of the molecule. The rest of the studied
dyes showed Stokes shifts of roughly 3000–4000 cm–1. The
fluorescence quantum yields for all the studied dyes were
large (56–69%), except for the dichlorinated TAPP 12 (Table
1).
Basic calculations with Spartan 16 software at the
B3LYP/6-31G* level were performed to obtain further
structural and electronical insights into the novel compounds. For 6, syn- and anti-isomers, with the methoxy
groups on the same or on opposite sides of the pyrrolopyrrole plane, respectively, were calculated. The results for the
LUMO and HOMO energies are listed in Table 2. The trend of
the resulting calculated band gaps was in good agreement
with experimental results seen in UV/Vis measurements. In
the case of dye 12, the HOMO–LUMO gap is larger than that
for the other pyrrolopyrroles, corroborating the experimental results showing a hypsochromically shifted absorption
of 12 compared with that of 10. Generally, new pyrrolopyrroles show consistent LUMO energies but different HOMO
energies.
Table 2 Calculated HOMO and LUMO Energies and Band Gaps of the
Synthesized Pyrrolopyrroles 6, 8, and 10–12
Compound
6 (Syn)
6 (Anti)
HOMO [eV]
–4.4
LUMO [eV]
–0.8
Band gap [eV]
3.6
–4.5
–0.8
3.7
10
–4.3
–0.8
3.5
12
–4.7
–0.8
3.9
8
–4.6
–1.5
3.1
11
–4.8
–1.5
3.3
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10
NCS, CHCl3
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In conclusion, several new TAPPs incorporating hexaphenylbenzene moieties in the 2- and 5-positions and 1and 4-positions have been synthesized. Despite the utility
of the conventional TAPP synthesis for the incorporation of
N-linked hexaphenylbenzene moieties, the multicomponent reaction of aldehydes, primary aromatic amines, and
biacetyl is not an efficient method for the synthesis of pyrrolo[3,2-b]pyrroles bearing hexaphenylbenzene moieties in
the 2- and 5-positions. However, a double Diels–Alder reaction of 2,3,4,5-tetraphenylcyclopenta-2,4-dien-1-one with
alkynyl-functionalized TAPPs, followed by CO extrusion, led
to the formation of the desired hybrid dyes in good yields.
Localization of the HOMO on the pyrrolo[3,2-b]pyrrole core
is responsible for the failure of attempts to transform the
hexaphenylbenzene moieties into hexa-peri-hexabenzocoronenes. The hybrid dyes show blue fluorescence, which,
despite the presence of multiple biaryl linkages, is strong,
Chlorination of these compounds at the 3- and 6-positions
of the electron-rich pyrrolo[3,2-b]pyrrole core changes the
photophysics of this class of compounds, hypsochromically
shifting both the absorption and emission bands.
Funding Information
The authors would like to thank the Foundation for Polish Science
(Grant TEAM/2016-3/22) and Global Research Laboratory Program
(2014K1A1A2064569) through the National Research Foundation
(NRF) funded by the Ministry of Science, ICT & Future Planning (Korea). N.J, H.H., and D. R. gratefully acknowledge support from the
Deutsche Forschungsgemeinschaft via SFB 953 ‘Synthetic Carbon Allotropes’; D.R. acknowledges support from the Graduate School Molecular Science.()
Acknowledgment
We thank Dr. David C. Young for amending the manuscript.
Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/s-0037-1610286. SuportingIformatinSuportingIformatin
References and Notes
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(20) 1,4-Bis(3,5-di-tert-butylphenyl)-2,5-bis{4′′-tert-butyl[2′,3′,5′,6′-tetrakis(4-tert-butylphenyl)]-1,1′:4',1′′-terphenyl4-yl}-1,4-dihydropyrrolo[3,2-b]pyrrole (10)
Pyrrolo[3,2-b]pyrrole 8 (237 mg, 0.250 mmol) and 2,3,4,5tetrakis(4-tert-butylphenyl)cyclopenta-2,4-dien-1-one
(609
mg, 1.00 mmol) were placed in a dry Schlenk flask containing a
magnetic stirring bar. The vessel was evacuated and backfilled
with argon three times, and Ph2O (5 mL) was added. The
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, 2529–2534
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34.7 (3 C), 35.6, 95.4, 119.7 (2 C), 120.4, 123.8 (2 C), 124.1,
125.9, 131.1, 132.0, 132.2, 132.6, 133.1, 135.8, 138.9, 139.1,
139.2, 139.2, 140.0, 140.8, 141.2, 141.4, 141.9, 148.0, 148.2,
148.4, 152.2. HRMS (ESI): m/z [M•+] calcd for C158H182N2:
2107.4303; found: 2107.4348. UV/Vis (THF): λmax (ε) = 389 nm
(69000).
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M.; Gryko, D. T. Org. Biomol. Chem. 2011, 9, 8178.
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Org. Biomol. Chem. 2014, 12, 2874.
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mixture was heated at 250 °C for 42 h then cooled to r.t. EtOH
(20 mL) was added, and the precipitated solid was collected by
filtration and washed with EtOH. The crude product was
refluxed in CH2Cl2 for 10 min, collected by filtration, washed
with MeOH and pentane, and dried under vacuum to give a
white powder, yield: 420 mg (80%); mp ~395 °C (dec.); Rf = 0.37
[silica gel, hexanes–CH2Cl2 (4:1)]. 1H NMR (500 MHz, THF-d8):
δ = 1.07 (s, 36 H), 1.08 (br s, 54 H), 1.33 (s, 36 H), 6.18 (s, 2 H),
6.60 (AA′BB′, J = 8.3 Hz, 8 H), 6.65 (AA′BB′, J = 8.4 Hz, 8 H), 6.68
(AA′BB′, J = 8.3 Hz, 8 H), 6.70 (AA′BB′, J = 8.5 Hz, 4 H), 6.78
(AA′BB′, J = 8.5 Hz, 8 H), 6.80 (AA′BB′, J = 8.5 Hz, 4 H), 6.85
(AA′BB′, J = 8.3 Hz, 8 H), 7.08 (d, J = 1.3 Hz, 4 H), 7.32 (t, J = 1.3
Hz, 2 H). 13C NMR (126 MHz, THF-d8): δ = 31.5, 31.6 (2 C), 31.9,
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, 2529–2534