Bowl-Shaped Pentagon- and Heptagon-Embedded Nanographene Containing a Central Pyrrolo[3,2-b]pyrrole Core

Article abstract of DOI:10.1002/anie.202104092

A bowl-shaped nitrogen-doped nanographene composed of a pyrrolo[3,2-b]pyrrole core substituted with six arene rings circularly bonded with one another has been prepared via a concise synthetic strategy encompassing the multicomponent tetraarylpyrrolopyrrole (TAPP) synthesis, the Scholl reaction, and intramolecular direct arylation. This synthesis represents the first case of programmed sequential intramolecular direct arylation reactions utilizing the different reactivity of C–Br and C–Cl bonds. The target compound contains two central pentagons confined between two adjacent heptagons—the inverse Stone–Thrower–Wales topology. The presence of both five- and seven-membered rings in the final structure is responsible for interesting properties such as a perpendicularly aligned dipole moment, absorption and fluorescence in the orange-red region, weak emission originating from the charge-transfer character of a low-energy absorption band, and a high lying HOMO. In the solid state slipped convex-to-convex π–π stacking dominates.

Full text of DOI:10.1002/anie.202104092

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Buckybowls
Hot Paper
Bowl-Shaped Pentagon- and Heptagon-Embedded Nanographene
Containing a Central Pyrrolo[3,2-b]pyrrole Core
´
Maciej Krzeszewski, Łukasz Dobrzycki, Andrzej L. Sobolewski,* Michał K. Cyranski,* and
Daniel T. Gryko*
Dedicated to Professor Janusz Jurczak on the occasion of his 80th birthday
and to Professor Kenichiro Itami on the occasion of his 50th birthday
Abstract: A bowl-shaped nitrogen-doped nanographene com-
posed of a pyrrolo[3,2-b]pyrrole core substituted with six arene
rings circularly bonded with one another has been prepared via
a concise synthetic strategy encompassing the multicomponent
tetraarylpyrrolopyrrole (TAPP) synthesis, the Scholl reaction,
and intramolecular direct arylation. This synthesis represents
the first case of programmed sequential intramolecular direct
arylation reactions utilizing the different reactivity of C–Br and
C–Cl bonds. The target compound contains two central
pentagons confined between two adjacent heptagons—the
inverse Stone–Thrower–Wales topology. The presence of both
five- and seven-membered rings in the final structure is
responsible for interesting properties such as a perpendicularly
aligned dipole moment, absorption and fluorescence in the
orange-red region, weak emission originating from the charge-
transfer character of a low-energy absorption band, and a high
lying HOMO. In the solid state slipped convex-to-convex p–p
stacking dominates.
carbons.[15–19] On top of the pursuit of well-defined carbon-
rich materials,[20–23] multifaceted research on bowl-shaped
nanographenes contributes to fundamental science, address-
ing questions regarding the nature of aromaticity,[24–26] strain
energies,[27,28] and metal complexation.[29] Inducing a curvature
in p-extended systems, usually requires harsh conditions
which rarely tolerate electron-rich heterocycles. Incorpora-
tion of heteroatoms into polycyclic frameworks, however, is
an elegant method of tuning the intrinsic physicochemical
properties of polycyclic aromatic hydrocarbons (PAHs).[30–32]
Furthermore, azabuckybowls have been envisaged as precur-
sors for the bottom-up syntheses of azafullerenes[33] and
nitrogen-doped CNTs.[34] Historically, the first example of an
azabuckybowl was triazasumanene reported by Higashibaya-
shi and Sakurai in 2012.[35] Although elusive for a long time,
recently the groups of Shinokubo and Nozaki independently
reported synthetic pathways towards azacorannulene[36,37]
(Figure 1a) and subsequently developed its chemistry and
studied its properties.[38–43] Nonetheless, examples of other
nitrogen-embedded, bowl-shaped compounds remain rather
scarce.[44–47] Herein, we present the synthesis of a bowl-shaped
polyarene based on the most electron-rich small aromatic
molecule—pyrrolo[3,2-b]pyrrole.[48] Unlike classical bucky-
bowls consisting of five- and six-membered rings satisfying
the isolated pentagon rule (IPR),[49] our azabuckybowl
features two abutting pentagons between two adjacent
Introduction
Aesthetically pleasing, bowl-shaped molecules, referred
to as buckybowls and geodesic polyarenes, are of great
theoretical and practical interest among the scientific com-
munity.[1–4] Owing to their close relationship to fullerenes[5]
and carbon nanotubes (CNTs)[6] which revolutionized mate-
rials science, investigations into the chemistry of these
contorted polyaromatics flourish.[7] The smallest curved
subunits of C60, corannulene,[8–12] and sumanene,[13,14] have
been exhaustively investigated and serve as scaffolds for the
syntheses of diverse, topologically unique molecular nano-
[*] Dr. M. Krzeszewski, Prof. D. T. Gryko
Institute of Organic Chemistry, Polish Academy of Sciences
Kasprzaka 44–52, 01-224 Warsaw (Poland)
E-mail: dtgryko@icho.edu.pl
´
Dr. Ł. Dobrzycki, Prof. M. K. Cyranski
Faculty of Chemistry, University of Warsaw
Pasteura 1, 02-093 Warsaw (Poland)
E-mail: mkc@chem.uw.edu.pl
Prof. A. L. Sobolewski
Institute of Physics Polish Academy of Sciences
Al. Lotnikꢀw 32/46, 02-668 Warsaw (Poland)
E-mail: sobola@ifpan.edu.pl
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.202104092.
Figure 1. Representative examples of bowl-shaped, nitrogen-embedded
polyaromatics.
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heptagons identified as the inverse Stone-Thrower-Wales
C-X activation in the intramolecular direct arylation. Sec-
defect in graphene. The term “buckybowl” is, however, used
throughout this manuscript with respect to the title compound
to underline its dome-shaped geometry.
ondly, due to its smaller van der Waals radius, steric
congestion around the central core is minimized, enabling
crucial, double seven-membered ring formation via the Scholl
reaction in the preceding step.[57,58] Retrosynthetic analysis
leads to the TAPP substituted with biphenyls at the pyrrolic
nitrogen atoms and aryls, attached to the central ring, bearing
simultaneously bromine and chlorine atoms at the two ortho
positions. Finally, to ensure good solubility of the final
compound and each of its parent precursors in common
organic solvents, we decided to introduce six tert-butyl groups
at the periphery.
Accordingly, adequate starting materials that is, 2-bromo-
4-(tert-butyl)-6-chlorobenzaldehyde (1) and 3’,5-di-tert-butyl-
[1,1’-biphenyl]-2-amine (2) were prepared (see ESI for de-
tails). The one-pot multicomponent condensation, catalyzed
by iron(III) perchlorate,[59,60] between 1, 2 and butane-2,3-
dione (3) gave TAPP 4 in 25% yield (Scheme 1). Subjecting 4
to the first intramolecular direct arylation with Pd(OAc)2,
Cs2CO3 and PPh3 in toluene at 1208C for 5 h led smoothly to 5
in 76% yield. The order of steps here is critical to the success
of this strategy as we have proven that if the intramolecular
Scholl reaction is attempted first, a 1,2-aryl shift occurs
leading to a rearranged product.[55] Importantly, the formation
Results and Discussion
Design and Synthesis
Recently, we reported the on-surface synthesis of an
azabuckybowl possessing a pyrrolo[3,2-b]pyrrole core (Fig-
ure 1b).[50] The target bowl-shaped molecule was obtained
from a precursor possessing two pre-formed heptagons in its
structure. On-surface cyclodehydrogenation led to the effi-
cient closure of the two remaining hexagons. This on-surface
process proved the only viable method as all attempted in-
solution oxidation methods failed. The final compound was
characterized with ultra-high-resolution scanning tunneling
microscopy (UHR-STM) supported by density functional
theory (DFT) calculations, followed by the computational
evaluation of its aromaticity.[51] Undoubtedly, rapidly devel-
oping surface-assisted techniques have proved to be extreme-
ly powerful tools for the construction of a large variety of
nanostructures with atomic precision,[52] however, from the
organic chemistꢀs perspective, it is still flawed due to its
miniature scale. An inability to scrutinize the fundamental
physicochemical properties of the obtained nitrogen-embed-
ded buckybowl through the NMR, UV-vis and fluorescence
spectroscopies, prompted us to design an alternative synthetic
route.
ꢀ
of two new C C bonds around the core proceeds through
selective double C-Br activation leaving the two C-Cl bonds
intact. Then, dye 5 was subjected to the double Scholl reaction
mediated by iron(III) chloride. As we previously reported,
X-ray crystallographic analysis of the precursorꢀs deriva-
tive (Figure 2a) revealed a large distance, reaching 3.28 ꢁ,
between the two carbon atoms involved in the cyclodehy-
drogenation step. This, plausibly accounts for the impediment
of the final oxidative aromatic coupling.[53–55] Therefore, we
designed an alternative precursor, bearing additional chlorine
atoms attached to the essential carbon atoms located inside
the helicene-like fjord regions (Figure 2b). We envisioned
that introduction of halogen atoms in ortho positions with
respect to the central ring, would enable double six-mem-
bered ring closure via palladium-mediated intramolecular
direct arylation.[56] Chlorine was chosen over bromine or
iodine atoms for two reasons. Firstly, the lower activity of Cl
vs. Br/I in the cross-coupling reactions allows for sequential
Scheme 1. Synthetic route to azabuckybowl 7. Reagents and condi-
tions: a) Fe(ClO4)3·xH2O (6 mol%), AcOH/toluene, 908C, 3 h, 25%;
b) Pd(OAc)2 (20 mol%), Cs2CO3 (2.4 equiv), PPh3 (48 mol%), toluene,
1208C, 5 h, 76%; c) FeCl3 (6.0 equiv), DCE/MeNO2, 808C, 16 h, 29%;
d) Pd(OAc)2 (2.0 equiv), HPt-Bu2MeBF4 (6.0 equiv), DBU/DMA, 1808C,
24 h, 83%. DCE: 1,2-dichloroethane.
Figure 2. Design considerations. a) Crystal structure (CCDC
1589418)[50] of a derivative of the precursor used in the on-surface
synthesis of an azabuckybowl. b) The computed model of its analogue
containing two additional chlorine atoms. t-Butyl groups and hydrogen
atoms omitted for clarity.
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FeCl3 proved to be the most efficient oxidizing agent
regarding oxidative coupling of TAPPs, whereas PIFA and
DDQ/TfOH system have had detrimental impact on the
pyrrolo[3,2-b]pyrrole core.[55] Furthermore, in order to close
two seven-membered rings, elevated temperature and longer
reaction time are crucial.[50] Thus, reaction was conducted in
a mixture of dichloroethane/nitromethane at 808C for 16 h.
The transformation from 5 to 6 turned out to be very sensitive
to changing conditions such as excess of oxidant and scale (see
ESI). The major by-products, according to the ESI-MS,
formed with the use of large excess (8–12 equiv) of FeCl3 are
over-chlorinated compounds (see ESI, Figures S30–S31).
After scrupulous investigation it was found that 6 equivalents
of FeCl3 are optimal for this particular process. The desired
compound 6 containing two newly-formed heptagons was
isolated in 29% yield. Although the reaction yield is
significantly lower compared to the analogical Scholl reaction
with an unsubstituted precursor (29% vs. 60%),[50] the
additional steric hindrance around the core imposed by the
chlorine atoms, was not deleterious for the reaction entirely.
Finally, the precisely designed precursor 6 was employed for
the second intramolecular direct arylation reaction under the
more stringent conditions reported by Nozaki and co-work-
ers.[41] The reaction was carried out with Pd(OAc)2 and HPt-
Bu2MeBF4 in a DBU/DMA mixture in a pressure tube at
1808C for 24 h. To our delight, the target azabuckybowl 7 was
formed in 83% yield. Its structure was corroborated through
nuclear magnetic resonance (NMR) spectroscopy, high-reso-
lution mass spectrometry (HRMS) and single crystal X-ray
diffraction. The 1H NMR spectrum of azananographene 7
measured in [D6]benzene at 258C revealed the presence of six
singlets (two of them overlapped) manifesting in C2-symme-
try (see ESI). All of the signals are strongly downfield shifted
(d = 8.35–8.87 ppm), comparable to the chemical shift of the
bay-region hydrogen atoms of phenanthrene indicating full
conjugation of the molecule.
atoms (C20, C25, C37, C42) reached 2.05 ꢁ (Figure 3c),
which is slightly deeper than that of recently reported
azacorannulenes (1.70 ꢁ[36] and 1.73 ꢁ[37]) and significantly
deeper than the calculated bowl-depth of a “naked” azabuck-
ybowl deposited on an Au(111) surface (1.20 ꢁ).[50] The apex
angle of the cone was experimentally determined to be 132.88.
The bond lengths measured for 7 ranged between 1.36–1.40 ꢁ
(aromatic bond character) for the central pyrrolo[3,2-b]pyr-
role ring, 1.39–1.44 ꢁ (aromatic bond character) for the
peripheral benzene rings and 1.49–1.55 ꢁ (single bond
character) for the bridging bonds (Figure S2). These results
suggest that, similarly to azacorannulene, the major contri-
bution of the bond alternation pattern in 7, comprises the
pyrrolo[3,2-b]pyrrole core surrounded by six Clar-type ben-
zene rings. The curvature of 7 was determined by the p-orbital
axis vector (POAV) angles.[61] The largest (POAV) angles of 7
were found to be 9.08 and 9.18 at the central C4 and C5
positions, respectively (Figure S3). These values are slightly
higher than the POAV angle of corannulene (8.78)[61] and
relatively close to the POAV angle reported for azacorannu-
lene (9.28).[36] Unlike in solution, where a flexible, uncon-
strained molecule of 7 can equalize its substituents around the
rim through
a bowl-to-bowl inversion, the solid-state
“freezes” a single conformation occupying a unique environ-
ment through the crystal packing forces. The presence of two
heptagons in the molecular structure of 7 imposes a negative
Gaussian curvature within its surface. This entails helicene-
like distortion of two sites of the bowl, thus inducing an axial
chirality resulting in C2 point group for the optimized single
molecule. However, the energy barrier for the bowl-to-bowl
inversion is very small (vide infra) thus the solution has to
contain both conformers/enantiomers. Indeed, in the centro-
symmetric space group of 7 enantiomer pair (P,P)-7 and
(M,M)-7 are present in the unit cell (Figure 3d). However,
due to interatomic interactions in the crystal lattice the
molecular symmetry of 7 is C1 but still is quite close to C2
point group. The molecules of this enantiomeric pair arranged
themselves in a one-dimensional packing structure with
a slipped convex-to-convex p-p stacking, where the closest
distance between two enantiomeric molecules reaches 3.27 ꢁ
(Figure 3e–g). Each stacking chain is separated from another
by a channel-gap filled with solvent molecules between two
concave faces of neighboring enantiomers. Most likely con-
vex-to-concave stacking is restricted due to the presence of six
bulky t-butyl groups.
X-ray Crystallographic Analysis
Single crystals of 7 suitable for X-ray analysis were grown
by recrystallization of 7 from dichloromethane at room
temperature under a N2 atmosphere. The compound crystal-
ꢀ
lizes in the P1 space group with severely disordered solvent
molecules almost continuously filling the space between fully
ordered (including t-butyl moieties) azabuckybowls. The
structure was refined in two manners: including the solvent
molecules modelling, and alternatively using the SQUEEZE
procedure (see ESI for the details). All the further geometry
considerations of the X-ray structure are based on the
refinement including the solvent modelling. The X-ray
crystallographic analysis, shown in Figure 3, unambiguously
confirmed the structure of the final compound comprising
a pyrrolo[3,2-b]pyrrole ring encircled by six 4-(t-butyl)ben-
zene moieties. The analysis revealed full conjugation of the
molecule and its bowl-shaped geometry (Figure 3a,b). The
bowl depth defined as the perpendicular distance between
a centroid of the internal pyrrolo[3,2-b]pyrrole (C4-C5) bond
and the mean plane described by the four furthest rim carbon
Theoretical Investigation of Conformation
Quantum chemical computations were performed for the
azabuckybowl 7 structure with all tert-butyls replaced by
hydrogen atoms (7-H). This only has a marginal influence on
the electronic properties of the systems (Figures 5, S13 and
S14), but significantly speeds up computational explorations.
Two stable conformers of 7-H were determined by MP2
geometry optimization; (i) the global minimum of the
propeller shape (C2 symmetry point group), and (ii) the local
minimum having approximate Cs symmetry which is merely
0.3 kcalmolꢀ1 (100 cmꢀ1) less stable. These minima are
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Figure 3. X-ray crystal structures of azabuckybowl 7 and its calculated racemization pathway. a,b) ORTEP diagrams of the top view (concave face)
and the side view set at 50% probability level with hydrogen atoms and solvent molecules omitted for clarity. c) Stick model diagrams displaying
the bowl depth (top) and the apex angle (bottom). d) Top views (convex faces) of the enantiomer pair of 7 found in the crystal unit cell, (P,P)-
isomer (top) in light gray and (M,M)-isomer (bottom) in orange. e–g) Crystal packing structures viewed along a-, b- and c-axes, respectively, as
stick models with solvent molecules as space-filling models (e,g) and showing enantiomer pair of 7 as space-filling models with solvent
molecules omitted (f). In each case, tert-butyl groups were omitted for clarity. h) Potential-energy landscape of 7-H computed at the MP2/cc-
pVDZ level of theory (zero-point energy corrected).
protected from each other by a barrier of 1.3 kcalmolꢀ1. The
barrier represents a saddle point with single imaginary
vibrational frequency (54 cmꢀ1) related to an out-of-phase
mutual twist of two benzene rings on one side of the 7-H
molecule. A higher barrier of 5.4 kcalmolꢀ1 is related to the
bowl-to-bowl inversion. Interestingly, the transition state (TS)
is not planar, but possesses a rather twisted, S-shaped
conformation with Ci symmetry. Such twisted transition states
have been observed earlier for large bowl-shaped fragments
of fullerenes[62,63] and for hydrazinobuckybowl,[45] with the
bowl-inversion dynamics following double- and triple-well
potential model, classified as type A and type B, respectively.
Intriguingly, the bowl-inversion mechanism of 7-H follows
a quadruple-well potential model, introducing a new type of
these kind of processes. Still, however, the calculated barriers
are markedly lower than those of corannulene (theoretical
value: 10.4 kcalmolꢀ1)[17] and benzoazacorannulene (theoret-
ical value: 15.3–17.0 kcalmolꢀ1)[37] in spite of the deeper bowl
depth.[64,65] The overall potential-energy (PE) landscape of
the racemization process of 7-H is presented in Figure 3h. Its
conformational flexibility has been further corroborated by
variable temperature 1H NMR experiments. In each spec-
trum, measured in [D8]toluene in the temperature ranging
from ꢀ508C to 808C, only time-averaged sets of slightly
broadened signals were present (see ESI, Figures S34,35).
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Optical and Electrochemical Properties
Further p-extension leading to the compound 6 has an
interesting effect on the UV-vis and emission spectra features.
The absorption maximum of 6 is blue-shifted by about 30 nm,
and conversely, the fluorescence maximum is slightly red-
shifted (15 nm) with respect to 5. Plausibly these character-
istics stem from the gross distortion from planarity in 6, thus
breakage of p-conjugation within the molecule caused by the
introduction of two new seven-membered rings embedded in
the framework. It closely resembles the relationship between
the spectral properties of pyrrolo[3,2-b]pyrrole-based double
[6]azahelicenes and their parent precursors.[53] This phenom-
enon translates to a significant divergence between the
UV-vis absorption and fluorescence spectra of dyes 4–7
were measured in dichloromethane and their features were
compared in order to elucidate the influence of the molecular
geometries and degree of conjugation on their electronic
states. These results are summarized in Table 1 and Figure 4.
Parent TAPP 4 displays a main absorption band consisting of
two distinct peaks located in the ultraviolet region of the
spectrum at labs = 298 and 345 nm with molar extinction
coefficients (e) reaching 20000 Mꢀ1 cmꢀ1 and 17000 Mꢀ1 cmꢀ1,
respectively. The first extension of the p-conjugation in 5
compared to its parent counterpart significantly affects its
optical properties. Concurrent bathochromic and hyperchro-
mic shifts of absorption are observed, together with the
appearance of an emission signal. The dye 5 exhibits two main
absorption bands, the first located between 320 and 390 nm
and the second at 400 nm with a shoulder extending to ca.
460 nm. The higher-energy transitions show a peak located at
labs = 362 nm with the e = 39000 Mꢀ1 cmꢀ1. The lower-energy
transitions exhibit a peak at labs = 412 nm with e = 25000 Mꢀ1
followed by two faint humps located at labs = 430 and 455 nm.
Dye 5 emits green light (lem = 526 nm) with a fluorescence
quantum yield (Ffl) = 0.121.
Stokesꢀ shifts of 71 nm (3000 cmꢀ1) and 119 nm (5200 cmꢀ1
)
for 5 and 6, respectively. The particularly large value of the
latter, is attributed to a notable difference in its S1 excited
state and S0 ground state geometries. The formation of two
heptagons, leading to evident contortion of the molecule, has
a detrimental effect on its emission efficiency, with Ffl
dropping to 0.023.
The final p-extension step leading to the azabuckybowl 7
has a prominent impact on the spectroscopic properties. Its
UV-vis spectrum reveals complex features with the first
intense major band located between 280 and 350 nm with
a shoulder extending to ca. 390 nm, showing two distinctive
finger-shaped peaks at labs = 298 and 330 nm with e =
78000 Mꢀ1 cmꢀ1 and 62000 Mꢀ1 cmꢀ1, respectively. The second
band corresponding to the lower-energy transitions, found
Table 1: Spectroscopic and electrochemical properties of the synthesized
dyes 4–7.[a]
between 390 and 470 nm, also shows two peaks at labs
=
399 nm
(e = 16000 Mꢀ1 cmꢀ1)
and
455 nm
(e =
E1/2
EHOMO
[eV]
[b]
Cmpd.
labs
[nm]
e
lem
[nm]
F
11000 Mꢀ1 cmꢀ1). Finally, the third weak band, which corre-
sponds to the longest wavelength absorption at 500 nm tailing
fl
ox1
[V][c]
[ꢁ103 Mꢀ1 cmꢀ1
]
4
5
6
7
345
455
422
530
17
14
25
5
–
526
541
615
–
0.33
0.03
0.07
ꢀ0.09
ꢀ5.13
ꢀ4.83
ꢀ4.87
ꢀ4.71
up to 580 nm with the lowest-energy transition at labs
=
530 nm (e = 5000 Mꢀ1 cmꢀ1). Thus, the absorption maximum
is strongly red-shifted, by 108 nm (4800 cmꢀ1) compared to 6.
Concurrently, the fluorescence maximum is red-shifted to the
orange/red region of the spectrum with lem = 615 nm, exhib-
iting the rather low emission efficiency of Ffl = 0.016. The
Stokesꢀ shift for 7 was determined to be 2600 cmꢀ1 (85 nm).
Positive solvatochromic shift in the emission spectra of certain
TAPP derivatives, attributed to the excited-state symmetry
breaking, has been lately observed and closely studied.[53,66–68]
This prompted us to examine the spectral features depend-
ence on solvent polarity for the synthesized pyrrolo[3,2-
b]pyrroles 4–7. In addition to measurements performed in
dichloromethane, respective emission spectra were collected
for toluene and methyl-tetrahydrofuran. Comparison of
fluorescence maxima indicate that, indeed, dyes 5–7 display
solvatofluorochromism with concomitant decrease of emis-
sion intensity from less polar to more polar solvents (see ESI,
Figures S5–S7). To further elucidate the nature of the
electronic transitions, absorption spectra of 7-H were ob-
tained with the aid of two different theoretical methods. The
spectra are qualitatively similar to each other and both are
composed of several overlapping absorption bands of in-
creasing intensity. The lowest absorption band, assigned to the
S0!S1 transition involves an intramolecular charge-transfer
from the central pyrrolo[3,2-b]pyrrole unit to the side
benzene rings as it is illustrated in Figure S13 and Table S3
of the ESI. The low intensity of this transition is related to the
0.121
0.023
0.016
[a] Measured in dichloromethane. [b] Determined with coumarin 153 in
EtOH as a standard. [c] Cyclic voltammograms were measured in
a deoxygenated solution of 0.1 M tetrabutylammonium hexafluoro-
phosphate in anhydrous dichloromethane, with a glassy-carbon working
electrode, a Ag/AgCl/NaClsat reference electrode, and a platinum-foil
auxiliary electrode (scan rate v=100 mVsꢀ1, Ar, rt). Oxidation potentials
are given relative to Fc/Fc+.
Figure 4. UV-vis absorption spectra (solid lines) and fluorescence
spectra (dashed lines) of compounds 4–7 measured in dichlorome-
thane.
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(1pApA*) which is vertically identified as the S2 state
(Table S3) becomes adiabatically the lowest excited singlet
state of the system. This state is “dark” for absorption from
the ground state, but can be populated via a non-adiabatic
1
transition from the optically “bright” B (pApB*) state. Since
its radiative relaxation to the ground state is forbidden by
symmetry (f = 0), the 1A state can temporarily “trap”
electronic excitation and transmit it to nearby lying triplet
states thus competing with the fluorescence from the 1B state.
The computational studies shed some light on intramolecular
processes responsible for the observed relatively low fluores-
cence yield of investigated compounds. This also indicates the
pathways for chemical modifications in order to increase their
yield of luminescence by substitutions that rise energy of the
“dark” 1A state with respect to the “bright” 1B state.
Finally, the exceptionally electron-rich character of de-
rivatives based on the pyrrolo[3,2-b]pyrrole core and their
potential applicability in the field of organic electronics
encouraged us to investigate the electrochemical properties of
all synthesized dyes 4–7 by performing voltammetric experi-
ments. Cyclic voltammetry measurements revealed that, all of
the obtained pyrrolo[3,2-b]pyrroles are prone to oxidation
(Table 1, Table S2). Except for the parent TAPP 4, all
analyzed compounds underwent two one-electron reversible
oxidation steps (see ESI). In the case of the former the second
oxidation process was irreversible. No reduction waves for 4–
7 were observed in the potential window scanned (ꢁ 2.0 V).
The HOMO energy levels (EHOMO) were estimated from the
first oxidation potentials (Table 1). Calculated energy of the
HOMO level of 7-H (Figure 5) was in good agreement with
the experimental data. The half-wave potentials measured for
7 referred to the ferrocene/ferrocenium ion couple were
Figure 5. Selected Kohn–Sham molecular orbitals of the truncated
azabuckybowl 7-H and their energy levels calculated with the aid of
DFT at the B3LYP/cc-pVDZ level of theory. For each MO, the view
corresponds to the convex face of the molecule.
small overlap between molecular orbitals involved in the
transition (Figure 5). Interestingly, the dipole moment is
aligned perpendicularly to the open face of the bowl is 2.55 D
and it decreases in the excited state (see ESI, Table S3).
The computed electronic states diagram of 7-H is
presented in Figure 6. Only the lowest excited (singlet and
triplet) states of the A and B symmetry within the C2
symmetry point group are shown. One can notice upon
inspection of Figure 6 that the singlet state of the A symmetry
E
ox1 = ꢀ0.09 V, and Eox2 =+ 0.30 V (Figure 7). The observed
low value of the first oxidation potential of 7 indicates its
strong propensity to oxidize, comparable to that of hydrazi-
nobuckybowl[45] as well as the highly electron-rich, ladder-
type fluorophores comprising of four consecutively fused
pyrrole rings.[69]
Figure 7. Cyclic voltammogram of azabuckybowl 7.
Figure 6. Schematic potential energy profiles of the lowest excited
states of 7-H constructed on the basis of theoretical results. A and B
relate to the symmetry of molecular orbitals involved in a particular
transition. Vertical arrows represent vertical absorption from the
ground state (black) and fluorescence (blue and red) from the lowest
excited singlet states of a given (B and A) symmetry, respectively, with
values of transition energy (E) and oscillator strength (f) computed at
the ADC(2)/cc-pVDZ theoretical level.
Conclusion
Based on the premise that the different reactivity of
C(sp2)-Br and C(sp2)-Cl bonds could be harnessed in a pro-
grammed way to carry out two sequential direct arylations, we
have demonstrated the successful in-solution synthesis of an
azabuckybowl based on the exceptionally electron-rich
Angew. Chem. Int. Ed. 2021, 60, 2 – 10
ꢀ 2021 Wiley-VCH GmbH
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Acknowledgements
The work was financially supported by the Polish National
Science Centre, Poland (HARMONIA 2018/30/M/ST5/
00460), the Foundation for Polish Science (TEAM
POIR.04.04.00-00-3CF4/16-00 and START scholarship no.
039.2017 to M.K.). M.K. is a recipient of a scholarship
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ꢀ
Keywords: buckybowls · C H activation · dyes/pigments ·
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Manuscript received: March 23, 2021
Accepted manuscript online: April 8, 2021
Version of record online: && &&
,
&&&&
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www.angewandte.org
&&&&
These are not the final page numbers!

Angewandte
Research Articles
Chemie
Research Articles
Buckybowls
Multi-milligram-scale synthesis of the
first example of a bowl-shaped nano-
graphene with a pyrrolo-pyrrole core has
been achieved via sequential, pro-
grammed, intramolecular direct aryla-
tions. This curved molecule possesses an
inverse Stone–Thrower–Wales topology
(a combination of fused pentagons and
heptagons), small bowl-to-bowl inversion
barrier, high-lying HOMO, weak red-
emission at 615 nm, and a small dipole
moment.
M. Krzeszewski, Ł. Dobrzycki,
´
A. L. Sobolewski,* M. K. Cyranski,*
D. T. Gryko*
&&&& — &&&&
Bowl-Shaped Pentagon- and Heptagon-
Embedded Nanographene Containing
a Central Pyrrolo[3,2-b]pyrrole Core
&&&&
www.angewandte.org
ꢀ 2021 Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 2 – 10
These are not the final page numbers!