Boron–Nitrogen-Doped Nanographenes: A Synthetic Tale from Borazine Precursors

Article abstract of DOI:10.1002/chem.201905794

In this work, a comprehensive account of the authors’ synthetic efforts to prepare borazino-doped hexabenzocoronenes by using the Friedel–Crafts-type electrophilic aromatic substitution is reported. Hexafluoro-functionalized aryl borazines, bearing an ortho fluoride leaving group on each of the N- and B-aryl rings, was shown to lead to cascade-type electrophilic aromatic substitution events in the stepwise C?C bond formation, giving higher yields of borazinocoronenes than those obtained with borazine precursors bearing fluoride leaving groups at the ortho positions of the B-aryl substituents. By using this pathway, an unprecedented boroxadizine-doped PAH featuring a gulf-type periphery could be isolated, and its structure proven by single-crystal X-ray diffraction analysis. Mechanistic studies on the stepwise Friedel–Crafts-type cyclization suggest that the mechanism of the planarization reaction proceeds through extension of the π system. To appraise the doping effect of the boroxadizine unit on the optoelectronic properties of topology-equivalent molecular graphenes, the all-carbon and pyrylium PAH analogues, all featuring a gulf-type periphery, were also prepared. As already shown for the borazino-doped hexabenzocoronene, the replacement of the central benzene ring by its B₃N₂O congener widens the HOMO–LUMO gap and dramatically enhances the fluorescence quantum yield.

Full text of DOI:10.1002/chem.201905794

A Journal of
Accepted Article
Title: Boron-nitrogen-doped nanographenes: a synthetic tale from
borazine precursors.
Authors: Jacopo Dosso, Tommaso Battisti, Benjamin D Ward, Nicola
Demitri, Colan Hughes, Andrew P. Williams, Kenneth D.M
Harris, and Davide Bonifazi
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To be cited as: Chem. Eur. J. 10.1002/chem.201905794
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10.1002/chem.201905794
Chemistry - A European Journal
- Chemistry - A European Journal -
- Full Paper –
Boron-nitrogen-doped nanographenes: a synthetic tale from borazine
precursors.
Jacopo Dosso, Tommaso Battisti, Benjamin D. Ward, Nicola Demitri,
Colan E. Hughes,P. Andrew Williams, Kenneth D. M. Harris, and Davide
Bonifazi*
[a] J. Dosso, T. Battisti, Dr. B. D. Ward, Dr. C. E. Hughes, Dr. P.
A. Williams, Prof. Dr. K. D. M. Harris, Prof. Dr. D. Bonifazi;
School of Chemistry, Cardiff University, Park Place, Cardiff
CF10 3AT (UK). E-mail: bonifazid@cardiff.ac.uk.
[b] Dr. N. Demitri; Elettra - Sincrotrone Trieste, S.S. 14 Km 163.5
in Area Science Park, 34149 Basovizza - Trieste, Italy.
[ ] D.B. gratefully acknowledges Cardiff University and the EU
through the MSCA-RISE funding scheme (project INFUSION and
PHOTOTRAIN) for the financial support. TB thanks the project
ITN-ETN PHOTOTRAIN project for his PhD fellowship. The authors
also acknowledge the use of the Advanced Computing (ARCCA) at
Cardiff University, and associated support services.
Supporting information for this article is available on the
https://onlinelibrary.wiley.com or from the author. Crystallographic
data for the structures reported in this paper have been deposited
at the Cambridge Crystallographic Data Centre as supplementary
publication no. 1963744, 1963743, 1963745 and 1973297 for compounds
18, 7, 11 and 1 respectively.
Keywords: polycyclic aromatic hydrocarbons, heteroatom doping, BN-
doping, borazine, coronenes, gulf-type peripheries,
optoelectronics, powder X-ray diffraction.
1
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Abstract. In this paper we report a comprehensive account of our
synthetic efforts to prepare borazino-doped hexabenzocoronenes using
the Friedel-Crafts-type electrophilic aromatic substitution. We
showed that hexafluoro-functionalized aryl-borazines, bearing an
ortho fluoride leaving group on each N- and B-aryl rings, could lead
to cascade-type electrophilic aromatic substitution events in the
stepwise
C-C
bond
formation
giving
higher
yields
of
borazinocoronenes than those obtained with borazine precursors
bearing fluoride leaving groups at the ortho positions of the B-aryl
substituents. It is with this pathway that an unprecedented
boroxadizine-doped PAH featuring
a
gulf-type periphery could be
isolated, and its structure proved through single crystal X-ray
diffraction analysis. Mechanistic studies on the stepwise Friedel-
Crafts-type
cyclisation
suggest
that
the
mechanism
of
the
planarization reaction proceeds through extension of the π system.
To appraise the doping effect of the boroxadizine unit on the
optoelectronic
properties
of
topology-equivalent
molecular
graphenes, the all-carbon and pyrylium PAH analogues, all featuring
a gulf-type periphery, were also prepared. As already shown for the
borazino-doped hexabenzocoronene, the replacement of the central
benzene ring by its B3N2O congener widens the HOMO-LUMO gap and
dramatically enhances the fluorescence quantum yield.
Figure to table of contents
2
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Introduction.
The doping of polycyclic aromatic hydrocarbons (PAHs) is currently
emerging as one of the most promising approaches to tailor the
optoelectronic
properties
of
organic
semiconductors.[1–4]
The
substitution of C=C bonds with isoelectronic and isostructural
dipolar boron-nitrogen (BN)[5–7] and boron-oxygen (BO)[8–11] couples is
an efficient method to gain control on the HOMO-LUMO band gap
without significant structural modifications of the molecular
scaffold.[12,13] The use of polar BN and BO bonds typically widen the
HOMO-LUMO gap[14,15] and imparts dipole-driven self-assembly behaviors
in the solid state[3,16] and on surfaces.[17–19] Noticeable examples
include
BN-doped
pyrenes,[16]
phenanthrenes,[20]
anthracenes[21]
,
perylenes[22,23] and nanographenes.[9,24–26] In these PAHs, the doping
units are isolated BN couples selectively placed either at the
peripheries or in the core of aromatic precursors bearing amino
groups at specific positions. Another strategy consists in the use
of preorganized doping units, in which the BN and BO couples are
clustered in six-membered rings and used to replace benzenoid rings.
Examples include the long-known inorganic B3NnOO-based six-membered
rings displayed in Figure 1: borazine (B3N3),[31] boroxadizine
(B3N2O),[28–30] bordioxazine (B3NO2)[28,29] and boroxine (B3O3).[27]
Figure 1. Inorganic B3NnOO-based six-membered rings that can be used for
doping graphenoid structures.
3
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Amongst all inorganic rings, only the B3N3, B3N2O and B3NO2 units can
be fully integrated into fully planarized nanographene systems. One
can hardly fail to notice that the B3N3 ring can replace any
benzenoic units in a molecular graphene scaffold, whereas the B3N2O
and B3NO2 rings can only be placed at the peripheries. For instance,
a
bordioxazine-fused PAH displaying catalytic activity in the
dehydrative amidation of carboxylic acids and amines has been very
recently reported by Shibasaki and Kumagai.[32] Borazine derivatives
have been mainly used as precursors to build BN-doped graphitic-like
carbon materials. Important examples include graphenes[33–35] and
polymers.[36–40] On the other hand, the use of the boroxadizine ring to
dope PAHs remains elusive to the best of our knowledge.
Certainly, the most intriguing example in the field is that of
hexabenzoborazinocoronene
(1),
the
B3N3-doped
congener
of
hexabenzocoronene (HBC), that can be considered as the smallest
borazino-doped nanographene structure (Scheme 1). Bettinger and co-
workers firstly reported the thermolytic synthesis of 1, in which
the central benzene ring has been replaced by
a
borazine core
(Scheme 1).[34] In a parallel avenue, our group contributed to the
field with the engineering of borazine-doped polyphenylenes[37,40] and
reported the rational synthesis of
a
soluble derivative of 1,
Friedel-Crafts-type
molecule through six intramolecular
2,[33]
reactions[33,41] using a hexafluoro-functionalized borazine precursor
(Schemes 1-2). In our synthetic approach, the fluoride leaving
groups are placed at the ortho positions of the B-aryl substituents
in
the
borazine
precursors
and
the
electrophilic
aromatic
substitution reaction led to molecule 2 in 5% yield along with a
non-cyclic helicoidal side-product 7 (Scheme 2). Capitalizing on the
Friedel-Crafts-type electrophilic aromatic substitution,[41] herein we
report a comprehensive account of our synthetic efforts to prepare
borazino-doped hexabenzocoronenes. Aiming at enhancing the yield of
soluble borazinocoronene 2, we reasoned that the hexafluoro-
functionalized isomer of molecule 5, bearing an ortho fluoride
leaving group on each N- and B-aryl rings, could lead to cascade-
type electrophilic aromatic substitution events in the stepwise C-C
bond formation. It is during these synthetic investigations that we
could isolate the first boroxadizine-doped PAH featuring a gulf-type
4
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periphery and prove its structure through single crystal X-ray
diffraction analysis. Propelled by these findings, we decided to
investigate further the doping effect of the boroxadizine unit and
prepared all-carbon and pyrylium[42–45] PAH analogues, all featuring a
gulf-type[46] periphery. As observed for the case of molecule 2, the
replacement of the central benzene ring by its B3N2O congener widens
the HOMO-LUMO gap and it enhances the fluorescence quantum yield.
Xylyl moieties were introduced on all PAHs to favor solubility.
Scheme 1. Synthetic planarization protocols for preparing borazino-doped
hexabenzocoronenes 1 and 2.
Results and discussion.
Synthesis and structure determination of BN-doped nanographenes. In
our early report,[33] we prepared B3N3-doped hexabenzocoronene
2
capitalizing on the Friedel-Crafts reaction using difluoroarenes
(route A, Scheme 2).[41] In this strategy, hexafluoro borazine 5 was
obtained after reaction of 4-xylyl aniline 3 with BCl3 followed by
addition of 2,6 difluorophenyl lithium 4 (Scheme 2).[33] Subsequent
planarization with [iPr3Si-CB11H6Cl6] and Me2SiMes2 at 110 °C in PhCl
gave molecule 2 (5% yield) and partially fused BN-derivative 7 as
the major product (17% yield). Further purification of the reaction
mixture allowed us to isolate partially fused derivative A1. Also,
5
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mass
analysis
investigations
showed
that
molecular
ions
corresponding to A2 were present in the reaction crude (see also
below for the hypothesized stepwise cyclization mechanism).
Scheme 2. Routes A and B for preparing BN-doped nanographenes.
Figure 2. a) Front view of 7 with BN distances. b), c) front view and side
view with axial enantiomers (M and P highlighted) of 7. Atom colors: pink
B, blue N, gray C; space group: P 21/n.
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The structure of partially planarized derivative
7
was further
confirmed by single-crystal X-ray analysis (Figure 2), which
displayed an helicene-like arrangement of the aromatic rings, with a
37° tilt around a central cove-type periphery. The BN bond lengths
for 7 are: 1.414(4)-1.422(3) Å for the BNH couple and 1.448(3)-
1.454(4)
Å
for the NBN fragment, slightly different from those
reported for 2 (1.433(3)-1.442(2) Å).
Aiming at improving the reaction yield, we redesigned the borazine
precursor and prepared a BN-derivative bearing one fluorine atom on
the ortho-position of each ring (route B, scheme 2). In this
synthetic route, aniline 8 is reacted with BCl3 in refluxing toluene,
followed by the addition of 2-fluorophenyl lithium
9
to give
hexafluoro borazine derivative 10 in a 36% yield. Subsequent ring-
closure reaction in the presence of Me2SiMes2 and [iPr3Si-CB11Cl6H6] at
110°C in degassed PhCl afforded borazino-coronene 2 in 15% yield.
While purifying the reaction mixture, an additional product was
isolated in a 4% yield. To our surprise the product proved to be
B3N2O-doped PAH 11 featuring a gulf-type periphery. To the best of
our knowledge, this molecule represents the first example of a PAH
embedding a B3N2O cycle. To validate its chemical structure, crystals
suitable for X-ray diffraction were grown, and the resulting
structure is displayed in Figure 3. Focusing on the bond lengths in
the B3N2O ring, the BN distances varies between 1.437(4) Å and
1.441(4) Å, similarly to the values reported for molecule 2.[33] The
BO bond lengths are 1.386(4) and 1.375(4), very close to the
distances measured for boric acid and phenyl boroxine.[47]
Figure 3. Front view of molecule 11 with BN and BO bond lengths. Atom
colors: pink B, blue N, red O, gray C; Space group: P 21/c.
To probe the versatility of route B, we undertook the synthesis of
unsubstituted borazinocoronene 1, and prepared hexafluoro borazine
7
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12. As for intermediate 10, each B- and N-aryl rings in precursor 12
bear an ortho-fluoride atom (Scheme 2). Friedel-Crafts reaction led
to 1 as an insoluble material, whose structure was confirmed by
mass-spectrometry in line with the seminal report by Bettinger and
co-workers.[34] Given the insolubility of molecule 1, we have carried
out structure determination from powder X-ray diffraction data.
Crystal Structure Determination of
1
from Powder XRD Data.
A
previous powder XRD study has been reported[34] for 1, involving
profile-fitting (using the Le Bail technique) of the powder XRD
pattern recorded for a two-phase sample comprising 1 together with
another crystalline phase arising from one of the reagents involved
in the synthesis. The Le Bail fitting showed that the unit cell of 1
closely matches the unit cell in the reported crystal structure[48,49]
of hexa-peri-hexabenzocoronene (which contains
a
central C6 ring
instead of a B3N3 ring). However, the previous powder XRD study[34]
only considered the preliminary profile-fitting stage of the
structure determination process, and did not proceed to carry out
structure solution and refinement. The powder XRD pattern of
1
prepared in the present work closely matches the one reported
previously,[34] but in our case it is clear from the powder XRD
pattern that our sample is a pure phase of 1 (as discussed below).
This powder XRD pattern was used for structure determination of 1.
Profile fitting and unit cell refinement from the powder XRD data
were carried out using the Le Bail technique[50] implemented in the
program GSAS,[51] starting from a unit cell (a = 12.929 Å, b = 5.119
Å, c = 17.9983 Å,
β = 108.984°) and space group (P21/n) that were
derived from the reported[49] unit cell of the carbon analogue of 1 by
transformation to the conventional setting (which has the shortest
lengths of the a- and c-axes and the value of
good-quality Le Bail fit was achieved (Figure 4a; Rwp = 0.57%, Rp
0.44%), with the following refined unit cell parameters:
12.921(5) Å, b = 5.0996(7) Å, c = 18.002(5) Å,
β closest to 90°). A
=
=
a
β
= 109.151(16)°. The
fact that this unit cell accounts for all peaks observed in the
experimental powder XRD data confirms the assignment of the sample
as a pure crystalline phase of 1. From the unit cell volume and
density considerations, it is deduced that there are two molecules
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of 1 in the unit cell, and therefore the asymmetric unit for space
group P21/n comprises one half molecule of 1.
Structure solution of
strategy,[52,53] implemented using a genetic algorithm (GA) in the
program In the direct-space structure solution
EAGER.[54–61]
1
was carried out using the direct-space
calculations (see SI for more details), the molecular model was
derived from the molecule in the crystal structure[49] of the carbon
analogue of 1, but with the central C6 ring replaced by a central
B3N3 ring. The asymmetric unit comprised the whole molecule of 1 with
half occupancy of all atoms, and with the centre of the molecule
fixed at a crystallographic inversion centre. Clearly, the inversion
symmetry operation creates disorder in the positions of the B and N
atoms in the central ring, resulting in an occupancy of B0.5N0.5 for
each atomic site in the central ring. The trial structure giving the
best fit to the experimental powder XRD data (i.e., lowest Rwp) in
the structure solution calculations was used as the initial
structural model for Rietveld refinement (see SI for more details),
which was carried out using the program GSAS.[51] The final Rietveld
refinement produced a good-quality fit (Figure 4b; Rwp = 0.60%, Rp
=
0.45%), comparable to the quality of fit obtained in the Le Bail
fitting procedure (Figure 4a), with the following refined unit cell
parameters: a = 12.934(5) Å, b = 5.1022(8) Å, c = 18.009(5) Å,
β =
109.188(17)°, V = 1122.4(7) Å3.
Figure 4. a) Le Bail fitting of the powder XRD pattern of 1, and b) final
Rietveld refinement for 1. Red + marks, experimental data (background
9
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subtracted); green line, calculated data; magenta line, difference plot;
black tick marks, predicted peak positions.
The crystal structure of
1
(Figure 5a,b), is essentially iso-
structural to the crystal structure of the carbon analogue,[49] as
demonstrated by the overlay of the two structures in Figure 5c.
According to the classification of aromatic hydrocarbon crystal
structures,[62] the structure is assigned to the γ packing type. The
molecules are arranged in "slabs" parallel to the ab-plane; all
molecules in a given slab are related by translation and hence have
the same orientation. Adjacent slabs are related by the 21 screw axis
(parallel to the b-axis), and the molecules in adjacent slabs have
different orientations. As discussed above, the crystal structure of
1
is disordered, with each atomic site in the central ring
represented as B0.5N0.5. This representation of the average structure
may be readily interpreted in terms of the molecule of 1 adopting
two orientations with equal probability (the two orientations differ
by 60° rotation about an axis perpendicular to the plane of the
molecule). In the average structure, the positions of the C and H
atoms for the two orientations occupy the same set of sites.
To assess whether there is any significant energy difference between
the
two
molecular
orientations
in
the
disordered
(average)
structure, periodic DFT-D calculations were carried out for two
ordered structures, in each case containing only one of the two
molecular orientations. To generate these ordered structures, the
crystallographic inversion centre was removed to give space group P21
(with one molecule in the asymmetric unit), and the central ring was
an ordered B3N3 ring (i.e., with each atomic site occupied by only B
or only N, and with the B and N atoms alternating around the ring).
The two ordered structures differ by 60° rotation about an axis
perpendicular to the plane of the molecule. Each structure was
subjected to DFT-D geometry optimization (with fixed unit cell). The
resulting geometry optimized structures are very similar (except in
regard to the assignment of B and N atoms to the atomic sites of the
central ring); the largest displacement between atomic sites in the
two structures is 0.111 Å, with an RMSD of 0.072 Å. The energy
difference between the two optimized structures is only 4 × 10–4 kJ
mol–1. This very small energy difference is fully compatible with the
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existence of disorder involving equal populations of the two
molecular orientations in the average crystal structure determined
from powder XRD data.
Figure 5. Crystal structure of 1 viewed a) along the b-axis and b) along
the a-axis. In this disordered structure, the occupancy of each atomic site
in the central ring is B0.5N0.5. The slabs of molecules discussed in the text
are parallel to the ab-plane, which is horizontal in both (a) and (b). c)
Overlay of the crystal structures of 1 (magenta) and the carbon analogue of
1 (cyan). The overlay was constructed by matching the unit cell axes (not
shown) of the two structures.
Computational
investigations
of
the
stepwise
Friedel-Crafts
planarization reaction. To rationalize the synthetic outcomes of the
Friedel-Crafts reaction and set future bases to improve the reaction
protocols for preparing borazine-doped nanographenes, we decided to
study the mechanistic details of the planarization reaction. When
looking at the structures of side-products 7 and 11, one hardly
fails to see that these compounds can only derive from partially
cyclized intermediates, in which a substituent is cleaved possibly
following a hydrolysis. Similarly, the presence of side-products A1
and A2 suggests that hydrolysable intermediates with a lower degree
of annulation are also formed in the reaction mixture. Taken all
together, these observations prompted us to hypothesize that the
ring-closing Friedel-Crafts reactions occur stepwise (i.e., through
six cyclization reactions, labelled here as c1-c6). Considering that
the transition state for each endothermic cyclization resembles the
intermediate more than the precursor,[63] we studied the planarization
reactions through computational simulations of the intermediates
that are possibly formed in each annulation step. The structures of
all postulated intermediates in the Friedel-Crafts reactions were
11
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calculated using density functional theory calculations (with B3LYP
hybrid functional,[64] with a 6-31(d,p) basis set[65] on all centres)
with Gaussian 09.[66] Solvent THF was incorporated using the polarized
continuum model, in which the molecular cavity is defined by the
united atom model that incorporates hydrogen into the parent heavy
atom.[67] The relative energies (∆∆G) are provided in Schemes 3 and 4
and are reported relative to the lowest energy isomer in each
cyclization subset. To ease the nomenclature of the intermediates,
n
n
the structures are labelled as 5
and 10
for synthetic routes A
ci
ci
and B, respectively, with n standing for the intermediate within the
cyclization subset, and ci the cyclization step. For pathway A, it
is clear that only an intermediate can be envisaged for cyclization
reactions c1 and two for c5, whereas up to four isomers could be
formed within cyclization subsets c2-c4 (Scheme 3). Since we used an
isomeric mixture of the starting hexafluoro borazine for pathway B,
one can envisage a complicated pattern of intermediates, including
two for cyclization subsets c1 and c5, eight for c2, fourteen for
c3, and seven for c4 (Scheme 4). The fact that the formation of the
B3N2O-doped product should derive from the hydrolysis of an
intermediate formed within the cyclization subset c4, led us to
restrict our calculations to the intermediates formed in steps c4
and c5. When looking at the relative energies of the intermediates,
it is evident that the lowest energy intermediate for each
a
a
cyclization subset (5ac2, 5a and 5a for pathway A; 10
and 10
for
c3
c4
c4
ci
pathway B) is that displaying the greatest degree of planarity in
the central B3N3 ring. A convenient method to quantify the planarity
of the B3N3 ring is to calculate the RMS deviation of the B atoms
from a plane defined by the three N atoms. RMSD analysis show that
the smallest values (0.083, 0.242 and 0.130) are observed for the
lowest energy intermediates 5a
,
5a
,
and 5a
(Table S2).
c2
c3
c4
Interestingly, when looking at the isolated side-products in route A
(Scheme 3), it is clear that products A1, A2, and 7 derive from the
hydrolysis of intermediates 5ac2, 5ac3, and 5ac4, respectively, which
lie on the lowest energy pathway. These observations suggest the
occurrence of a planarity-driven cyclization mechanism. After the
first electrophilic substitution leading to a flat phenanthrene-like
substructure (5 ), the second favored reaction occurs at one of the
c1
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phenanthrene periphery, further extending the conjugation of the π-
a
surface (5 c2). The subsequent cyclization reactions (c3-c5) occur
stepwise progressively extending the outer π-conjugated rim until
the last annulation c6 lead to final hexabenzoborazinocoronene
derivative (Scheme 3).
13
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Scheme 3. Step wise Friedel-Crafts-based planarization mechanism according to route A. Calculated Gibbs free
energies (∆∆G) for the possible intermediates formed at each cyclization step [B3LYP/6-31G(d,p)].
14
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Scheme 4. Friedel-Crafts-based stepwise planarization mechanism according to route B. In this case, the calculated
Gibbs free energies (∆∆G) of the intermediates intervening in the fourth and fifth cyclization steps are considered.
The presence of different isomers in steps 1-3 deterred us from tackling the calculation of all steps [B3LYP/6-
31G(d,p)].
15
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A similar planarity-driven cyclization mechanism to that proposed
for route A, can be envisaged for route B (Scheme 4), in which
the lowest energy intermediates for c4 and c5 are intermediates
10a and 10a . One can hardly fail to notice that the B3N2O-doped
c4
c5
derivative
11
possibly
derives
from
the
hydrolysis
of
intermediate 10a after replacement of a fluorine with a hydrogen
c4
atom. At the present time, further investigations are needed to
unravel the possible source of hydride yielding the fluoride-
hydrogen exchange reaction. Complementary mass-spectrometric
analysis of the reaction mixtures depicted the presence of the
molecular ion associated to one of the intermediates from c5,
further supporting our theoretical investigations and mechanistic
hypothesis.
The greater stability of
intuitive, when considering that a planar system has a greater
degree of delocalization in the -system. However, this
a
planar borazine ring is somewhat
π
electronic explanation can be given increased rigor using Natural
Bonding Orbital (NBO) analyses of the calculated structures. NBO
analysis of the parent borazine 5 indicates two 3-centre, 4-
electron hyperbonds in the B3N3 core, each centered about a NBN
unit and described as a combination of a N-centered p orbital and
a BN
strongly delocalized structure. The NBO analyses of those species
that deviate from planarity indicate substantially less-
π bond as NB:N⟷N:BN (Figure 6, left), thus describing a
delocalized structures. Using 5c as a representative example in
c2
which the boron atoms deviate most from planarity within the
series, the NBO analysis does not find the 3-centre 4-electron
hyperbond (a significant indicator of extensive delocalization),
but second-order perturbation analysis indicates
a
number of
donor-acceptor interactions that involve the N-based occupied p
orbitals and the B-based vacant p orbitals, thereby indicating a
lesser
degree
of
delocalization
(Figure
6,
right).
In
intermediate 5c one of the boron atoms lies on the plane defined
c2
by the three N atoms (Bplane, deviation from plane = 0.001 Å),
16
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whereas the other two lie above and below the plane in equal and
opposite directions (Bout-of-plane, deviation from plane = 0.441 Å);
the NBplane p–p interactions are calculated to have greater
a
strength (84 kcal) than the equivalent NBout-of-plane interactions
(73 and 28 kcal), thus giving a rational explanation for the
calculated stabilities of the two intermediates.
Figure 6. Left: A 3-centre, 4-electron bond described by NBO analysis
of precursor 5; right: NBO-derived donor-acceptor interaction between a
fluorine lone pair and an ipso carbon of the aryl group in 5c
[B3LYP/6-31G(d,p)].
c2
At last, it is noteworthy that the mean deviation of the borazine
ring from planarity does not directly correlate with their
relative free energies; for example, 5c
has the greatest
deviation from planarity (RMSD = 0.294 Å; ∆∆G = 44.6 kJ mol-1)
from the 5ac2–5d series but 5d
has the highest relative free
energy (RMSD = 0.250 Å; ∆∆G = 48.4 kJ mol-1). The relative free
energy of 5c
is lower than expected owing to an extraneous
c2
c2
c2
c2
stabilizing effect relating to the unlinked non-fluorinated aryl
group via two donor-acceptor interactions from the proximal
fluorine lone pairs to a lone valence (vacant p orbital) on the
ipso-carbon of the aryl moiety (Figure 6). These analyses give an
overall picture of the BN delocalization being the dominant
factor in determining stability, but where comparatively minor
stereoelectronic factors can have a significant contribution to
the overall stability of intermediate species in the reaction
process.
17
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Synthesis of pyrylium and all-carbon gulf-type congeners of the
B3N2O-doped PAH. Following the unexpected isolation of B3N2O-doped
PAH 11, we decided to prepare its all-carbon (18) and pyrylium
(26) gulf-type analogues and evaluate the effect of doping of the
central
benzenoid
ring.
Our
syntheses
started
with
the
preparation of 18 (Scheme 5). The synthetic path started with a
trimerization reaction of a 1:2 mixture of acetophenone and 2’-
bromoacetophenone in the presence of a catalytic amount of TfOH
at 130 °C. The reaction produced an inseparable mixture of bromo-
terphenyl derivatives (13), that was directly used in a Suzuki
cross coupling with 14. Subsequently, the isomeric mixture was
demethylated using BBr3 in CH2Cl2, yielding
a
combination of
hydroxyl derivatives that could be separated using silica gel
column chromatography to give targeted molecule 15 in 23% yield
(over 2 steps). Reaction of 15 with (Tf)2O in pyridine afforded
triflate 16, which was cross-coupled with (2,6-dimethylphenyl)-
boronic acid under typical Suzuki conditions to give final
polyphenylene derivative 17. Finally, oxidative ring closure
reaction was achieved using DDQ and TfOH to give planarized
derivative 18 with a 14% yield.
18
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Scheme 5. Synthetic pathway for preparing full-carbon gulf congener 18.
Next, we proceeded with the preparation of the pyrylium gulf-type
analogue, molecule 26 (Scheme 6).[45] Suzuki cross-coupling
reaction between (2,6-dimethylphenyl)-boronic acid and 4-iodo-2-
bromo-toluene gave bromo-aryl derivative 19 in the presence of
Ba(OH)2 as base.[68] Molecule 19 was then converted to boronic
ester 20 (63%) under Miyaura borylation reaction conditions.
Successive Pd-catalyzed cross-coupling reaction led to the
formation of amide 22 in a 73% yield (Scheme 6). Cyclization
reaction triggered by an amide-directed ortho-lithiation and
subsequent addition-elimination reaction on the amide group, gave
phenanthrol 23 in almost quantitative yield. Aldolic condensation
between phenanthrol 23 and benzaldehyde in the presence of HCl
yielded xanthene derivative 24 that, by treatment with PbO2, gave
desired xanthenol 25 with
a
41% yield (Scheme 6). At last,
xanthenol 25 was treated with HBF4·Et2O and the resulting orange
precipitate irradiated at 365 nm in a CH2Cl2/AcOH solution to give
desired pyrylium 26 in high yield (80%).
19
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Scheme 6. Synthetic pathways for preparing gulf pyrylium congener 26.
The structures of all intermediates and targeted compounds were
characterized by spectroscopic and spectrometric techniques.
Suitable crystals for X-ray diffraction analysis for the all-
carbon structure 18 were obtained (Figure 7d). No crystals for
pyrylium 26 could be formed despite numerous attempts. To better
understand the differences in the charge distribution between 11
and 18, ESP surfaces were plotted (Figure 7a-7b). Molecule 18
shows an extended π-surface with low electrostatic potential,
instead the presence of the inorganic ring in congener 11 induces
a relatively high potential region with a strong polarization,
with positively charged B atoms and negatively charged N and O
atoms (Figures 7a-b). Interestingly, the aromatic sextets in 11
depict considerably negative electrostatic potentials compared to
those in 18.
20
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Figure 7. ESP surfaces mapped on the vdW surface up to an electron
density of 0.05 electron/bohr-3 for a) 11 and b) 18. Side (up) and top
(bottom) view of the packing of c) 11 and d) 18. Atom colors: pink B,
blue N, red O, gray C. Space groups: P 21/c and P-1.
The X-ray crystal structures of 11 (Figure 7c) and 18 (Figure 7d)
provide further confirmation of the flat nature of both PAHs
(average RMSD of the atoms in the aromatic scaffold from their
mean plane is 0.13(9) Å for 11 and 0.15(12) Å for 18). The
molecules undergo π···π stacking interactions, with the molecular
units arranged antiparallelly into columns with average distances
of 3.52(20) Å and 3.43(16) Å for 18 and 11, respectively.
NMR properties of the heteroatom-doped nanographenes. To study
the effect of doping units on the ring current properties, we
compared the 1H-NMR spectra of PAHs 11, 18 and 26 (Figure 8b1-b3).
21
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As the diagnostic proton resonance, we selected the chemical
shift ( Ha) of the Ha proton, which is 8.82 ppm for 11, 9.30 ppm
δ
and 9.77 ppm for 18 and 26, respectively. To shed further light
on the effects of different doping units on the magnetic
properties, GIAO-NICS(1) calculations were also performed (Figure
8a). The replacement of the central benzenoid ring with the B3N2O
cluster causes a strong change in the NICS value from -12 ppm
(aromatic) to -0.95 ppm (non-aromatic) in the central region of
the molecule. A similar change in the NICS value has also been
observed for the non-sextet rings of the Clar resonance structure
surrounding the B3N2O system, where a shift from an average of -
2.68 in 18 to -0.79 ppm in 11 was observed.[69] This effect seems
to confirm the enhancement of the localization of the aromatic
sextets induced by the presence of the central B3N2O ring.[12]
Molecule 26 depicts intermediate magnetic properties between
those observed for 11 and 18, with a -6.5 ppm shift for the
central ring (Figure 8a, molecule 26), whereas the other all-
carbon rings feature an overall magnetic anisotropy similar to
that calculated for 18 (Figure 8a). ACID calculations were also
performed for the derivatives (Figure S74), revealing to be in
full agreement with the NICS(1) predictions. The experimental
data for the δHa of molecules 11 and 18 are fully consistent with
both ACID and NICS calculations, while further discussion is
required for the δHa value observed for the pyrylium congener. If
one solely considers the NICS calculations, the δHa value for
molecule 26 should fall in 8.87-9.30 ppm range. However, the
electron deficient character of the carbon atom attached to Ha
proton further deshields the latter, leading to the highest
experimental δHa value in the series.
22
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Figure 8. a) NICS(1)-GIAO surfaces for 11, 18, and 26 calculated from
the B3LYP/6-31+G** optimized geometry in vacuum; b) Detail of the 1H-NMR
spectra (400 MHz, CDCl3) in the aromatic region for 11 (b1), 18 (b2) and
26 (b3).
As previously observed for molecules 2 and HBC,[33] these data
strengthen our hypothesis for which the replacement of an all-
carbon fully benzenoid six-membered ring with an inorganic
congener, e.g. B3N2O and B3N3, reduces the aromatic character of
the π-conjugated scaffold.
23
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Optoelectronic properties of the heteroatom-doped nanographenes.
The steady-state UV-Vis absorption and emission properties of
compounds 7, 11, 18 and 26 in CH2Cl2 are displayed in Figure 9 and
summarized in Table 1. The data for reference compounds HBC and 2
are also reported for comparison purposes.[33] The absorption
envelop for gulf-type molecule 18 appears between 280 and 440 nm,
with a very weak lowest-energy α−band centered at 430 nm (ε <
1000 cm-1M-1).[70,71]
Table 1. Photophysical data in aerated CH2Cl2 and in glassy matrix
for molecules 11, 18, 26, 7, HBC and 2.
Molecule
Absorption
[nm]
ε
[L mol-1cm-1]) a)
Emission
Eopt
λ
λmax,fl
[nm]a)
τfl
[ns]
Φfl
λmax,ph
[nm]e)
τph
[s]e)
∆E
[eV]
(H-L)
[eV]
(
367 (16250)
362 (11860)
348 (13600)
292 (18500)
371
389
409
430
3.36b)
3.37 0.90
-
-
4.23
11
18
430 (800)
408
433
449
458
476
377 (10900)
366 (31300)
332 (70400)
319 (45000)
306 (41500)
540
584
634
3.11c)
0.11d)
<1.0
2.7 3.71
575 (25027)
539 (9635)
480 (4117)
364 (18116)
343 (23378)
2.12b)
26
595
8.1
3.5
0.48
0.83
-
-
4.67
393 (27730)
373 (17800)
356 (15300)
340 (15700)
403
423
443
486
522
559
7
3.12b)
1.45 3.73
446 (1000)
388 (53800)
358 (159100)
HBC
2
485
404
2.76 16.4 0.03
570
492
0.8 3.99
4.0 3.51
375 (24000)
314 (31200)
3.29
8.2
0.43
24
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a) Recorded in an aerated CH2Cl2 solution at r.t. b)Calculated from the lowest energy
intersection between absorption and emission spectra normalized on the lowest
energy peak (Eopt [eV] = 1240.5/
cross[nm]). c)Calculated from the average between the
energies of the highest emission and lowest absorption peak.
d)9,10-
λ
Diphenylanthracene was used as reference standard (0.97 in aerated Cyclohexane).[1]
e)Recorded at 77K in a 1:1 CH2Cl2:CHCl3 glassy matrix.
Upon replacement of the central benzene ring with
congener, a significant blue-shift of the absorption bands is
observed, and the lowest-energy structured electronic ππ
transition appears with a maximum at 367 nm (
16000 cm-1M-1). As
a
B3N2O
ε
≈
expected, the spectral envelops of both gulf-type molecules 11
and 18 are blue shifted when compared to those of reference
benzocoronenes 2 and HBC (Figure 9).
Figure 9. Absorption (
at r.t. and phosphorescence (•••) at 77 K in a 1:1 CH2Cl2:CHCl3 glassy
matrix of 2 (λexc = 315 nm), HBC (λexc = 355 nm), 11 λexc = 265 nm), 7
λexc = 350 nm), 18 (λexc = 265 nm) and 26 (λexc = 267 nm).
—), fluorescence (—) in aerated CH2Cl2 solutions
(
(
Compound 26 shows a broad unstructured absorption band centered
at 575 nm. The substantial redshift of the first electronic
25
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10.1002/chem.201905794
Chemistry - A European Journal
transition of 26 compared to that of the other derivatives is
probably ascribable to the presence of the oxonium specie that,
being electron deficient, could induce a lowering of the HOMO-
LUMO transition energy. Remarkably, the absorption spectrum of
helicene 7 closely resembles that of molecule 2. Regarding the
emission spectra, boroxadiazine derivative 11 displays the most
blue-shifted emission band (λmax = 371 nm, τfl =3.37 ns, Φfl = 0.9),
followed by 7 (λmax = 403 nm,
τfl= 3.5 ns, Φfl = 0.83), 18 (λmax =
408 nm, τfl < 1 ns, Φfl = 0.11) and 26 (λmax = 595 nm, τfl = 8.1 ns,
Φfl = 0.48). As observed for molecule 2, derivatives 11, 7 and 26
display narrow Stokes shifts (27 meV, 70 meV and 69 meV,
respectively), probably due to the rigid molecular scaffold that
prevents most of the geometry-changing vibrational relaxations at
the excited state. The all-carbon gulf-type derivative 18
displays a low emission quantum yield (Φfl = 0.11), which is,
however, three times larger than its parent benzocoronene HBC (Φfl
= 0.03). Interestingly, molecules 11 and 26 show strong emissive
properties, with Φfl values equal to 0.9 and 0.5, respectively. In
particular, B3N2O-doped nanographene 11 shows a two-fold increase
of the Φfl value compared to 2. Notably, no phosphorescence was
detected for molecules 11 and 26 at 77K, while 18 (
λmax= 540 nm,τph
= 2.7s) and helicene 7 ( max= 486 nm,τph = 1.45 s) depict long-
λ
lasting phosphorescence emission signals.
Electrochemical studies were carried out by means of cyclic
voltammetry (CV). Surprisingly, only irreversible or quasi-
reversible oxidative events were detected, with cathodic peaks
ranging between 0.8 and 1.2 V vs Fc+/Fc (1.09 V, 1.01 V, 0.757 V,
and 0.5 V for 11, 18, 7, and 26, respectively) at the scan rate
of 0.2 V s-1 (Figure S71). HOMO-LUMO profiles and energy gap were
calculated in vacuum at the B3LYP/6-31+G** level of theory
(Figure 10 and S73). As expected, the HOMO-LUMO profiles suggest
that the replacement of the central benzene ring with a B3N2O
cluster causes a widening of the bandgap, resulting in a strong
26
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10.1002/chem.201905794
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blue-shift
of
the
absorption
and
emission
properties.
Surprisingly, helicene 7 shows a similar HOMO-LUMO gap to that of
full-carbon gulf-type nanographene 18. Finally, pyrylium 26
presents the narrowest gap in the series, probably due to the
presence of a charge separation state.
Figure 10. Frontier orbital energies estimated from the CV and
photophysical data for 11, 7, 18 and 26. HOMO and LUMO orbitals have
been plotted over optimized structures calculated in vacuum at the
B3LYP/6-31+G** level of theory using Gaussian09/D01.
Conclusions
In conclusion, it was possible to develop a new strategy towards
molecule 2 using a borazine bearing one fluorine atom on the
ortho-position of each ring. This approach leads to higher yields
of 2 and to the unexpected formation of unprecedented B3N2O doped
PAH 11. Mechanistic studies on the stepwise cyclisation for both
pathways led to
a
clearer understanding of the reaction
intermediates. A stepwise mechanism proceeding through extension
of the
π
system is highlighted, providing an effective
27
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10.1002/chem.201905794
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explanation for the presence of byproducts 7, 11, A1 and A2 in
the reaction mixtures. Derivative 11 represents the first example
of a B3N2O doped PAH and to shed light on the properties arising
from this kind of doping, NMR, photophysical and computational
studies were performed on 11 and its congeners 18 and 26. These
studies underlined for the first time the widening of the HOMO-
LUMO gap provided by the substitution of a full-aromatic sextet
of the Clar resonance structure with a B3NnO0 based ring. NICS(1)
studies allowed us to evaluate the effect of doping on
aromaticity of this systems, and along with solution NMR, made
possible to appreciate the decrease in the magnetic anisotropy
arising from borazine based doping of PAHs. Finally, the presence
of the gulf region led to an increase in the quantum yield of
derivatives 11 and 18 compared to parent compounds 2 and HBC.
Given the growing importance of doped PAHs and nanographene
systems in science, this study represents an important step
towards the synthesis of tailored borazino-doped nanographenes.
Moreover, the extensive studies performed on all the obtained
derivatives provide an important guideline for the design of
precisely doped PAHs presenting tailored properties. This result
is, therefore, of fundamental importance in the growing field of
organic optoelectronic devices.
Additional Information
The authors declare no conflict of interest.
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