Boron-doped tri(9,10-anthrylene)s: Synthesis, structural characterization, and optoelectronic properties

Article abstract of DOI:10.1002/chem.201202280

The reaction of 9,10-dibromo-9,10-dihydro-9,10-diboraanthracene (9,10-dibromo-DBA, 3) with two equivalents of 9-lithio-2,6- or 9-lithio-2,7-di-tert-butylanthracene gave the corresponding 9,10-dianthryl-DBAs featuring two (4) or four (5) inward-pointing tert-butyl groups. Compound 4 exists as two atropisomers, 4 and 4′, due to hindered rotation about the exocyclic Bi-C bonds. X-ray crystallography of 5 suggests that the overall interactions between facing tert-butyl groups are attractive rather than repulsive. Even in solution, 4/4' and 5 are stable toward air and moisture for several hours. Treatment of 3 with 10-lithio-9-R-2,7-di-tert-butylanthracenes carrying phenyl (R=Ph), dimesitylboryl (R=Mes₂B), or N,N-di(p-tolyl)amino (R=Tol₂N) groups gave the corresponding 9,10-dianthryl-DBA derivatives 9-11 in moderate to good yields. In these molecules, all four solubilizing tert-butyl groups are outward pointing. The solid-state structures of 4, 5, 9, and 10 reveal twisted conformations about the exocyclic Bi-C bonds with dihedral angles of 70-90. A significant electron-withdrawing character was proven for the Mes₂B moiety, but no appreciable +M effect was evident for Tol₂N. Compounds 5, 9, and 11 show two reversible DBA-centered reduction waves in the cyclic voltammogram. In the case of 10, a third reversible redox transition can be assigned to the Mes₂B-anthryl substituents. The UV/Vis absorption spectrum of 5 is characterized by a very broad band at λ_max=510 nm, attributable to a twisted intramolecular charge-transfer interaction from the anthryl donors to the DBA acceptor. The corresponding emission band shows pronounced positive solvatochromism (λ_em=567 nm, C ₆H₁₂; 680 nm, CH₂Cl₂) in line with a highly polar excited state. The charge-transfer bands of 10 and 11, as well as the emission bands of 9 and 10, are redshifted relative to those of 5. The Tol₂N derivative 11 is essentially nonfluorescent in solution, but emits bright wine-red light in the solid state. Twist and radiate: The air- and moisture-stable 9,10-dianthryl-9,10-dihydro-9,10-diboraanthracenes (see figure) are reversible redox systems that show photoluminescence due to twisted intramolecular charge-transfer interactions, both in solution and in the solid state. Copyright

Full text of DOI:10.1002/chem.201202280

DOI: 10.1002/chem.201202280
Boron-Doped Tri(9,10-anthrylene)s: Synthesis, Structural Characterization,
and Optoelectronic Properties
Claas Hoffend, Frauke Schçdel, Michael Bolte, Hans-Wolfram Lerner, and
Matthias Wagner*[a]
Abstract: The reaction of 9,10-dibro-
mo-9,10-dihydro-9,10-diboraanthracene
(9,10-dibromo-DBA, 3) with two
equivalents of 9-lithio-2,6- or 9-lithio-
2,7-di-tert-butylanthracene gave the
no (R=Tol2N) groups gave the corre-
sponding 9,10-dianthryl-DBA deriva-
tives 9–11 in moderate to good yields.
In these molecules, all four solubilizing
tert-butyl groups are outward pointing.
The solid-state structures of 4, 5, 9, and
10 reveal twisted conformations about
the case of 10, a third reversible redox
transition can be assigned to the
Mes2B–anthryl substituents. The UV/
Vis absorption spectrum of 5 is charac-
terized by a very broad band at lmax
=
corresponding
9,10-dianthryl-DBAs
510 nm, attributable to a twisted intra-
molecular charge-transfer interaction
from the anthryl donors to the DBA
acceptor. The corresponding emission
band shows pronounced positive solva-
featuring two (4) or four (5) inward-
pointing tert-butyl groups. Compound 4
exists as two atropisomers, 4 and 4’,
due to hindered rotation about the
ꢀ
the exocyclic B C bonds with dihedral
angles of 70–908. A significant elec-
tron-withdrawing character was proven
for the Mes2B moiety, but no apprecia-
ble +M effect was evident for Tol2N.
Compounds 5, 9, and 11 show two re-
ꢀ
exocyclic B C bonds. X-ray crystallog-
tochromism
(lem =567 nm,
C6H12;
raphy of 5 suggests that the overall in-
teractions between facing tert-butyl
groups are attractive rather than repul-
sive. Even in solution, 4/4’ and 5 are
stable toward air and moisture for sev-
eral hours. Treatment of 3 with 10-
lithio-9-R-2,7-di-tert-butylanthracenes
carrying phenyl (R=Ph), dimesitylbor-
680 nm, CH2Cl2) in line with a highly
polar excited state. The charge-transfer
bands of 10 and 11, as well as the emis-
sion bands of 9 and 10, are redshifted
relative to those of 5. The Tol2N deriv-
ative 11 is essentially nonfluorescent in
solution, but emits bright wine-red
light in the solid state.
versible
DBA-centered
reduction
waves in the cyclic voltammogram. In
Keywords: boranes · charge trans-
fer · conjugation · electrochemistry ·
luminescence
yl (R=Mes2B), or N,N-diACTHUNTGRNEUNG(p-tolyl)ami-
Introduction
Boron-containing conjugated p-electron systems are enjoy-
ing increasing attention because of their outstanding opto-
ACHTUNGTRENNUNG
electronic properties,[1–6] which find applications, for exam-
ple, in new conducting and semiconducting polymers for
plastic electronics[7] or in anion sensor technology.[8]
In recent years, 9,10-dihydro-9,10-diboraanthracene
(DBA) derivatives A (Figure 1) were recognized for the fol-
lowing reasons as versatile building blocks of boron-doped
organic materials. 1) Their rigid framework guarantees opti-
mal p overlap between the boron atoms and the 1,2-phenyl-
ene bridges. Appropriately designed A-type compounds are
therefore highly luminescent and/or behave as reversible
Figure 1. Symmetrically (R1 =R2 =H, halogen, OR, NR2, alkyl, aryl,
vinyl) and unsymmetrically (R1 =Mes, R2 =H, Br, OR, vinyl; Mes=mesi-
tyl) substituted 9,10-dihydro-9,10-diboraanthracenes A; stabilization of A
by structural constraint as exemplified in B; protection of the vacant
boron-centered p orbitals in C by mesityl groups; 9,10-dianthryl-9,10-di-
hydro-9,10-diboraanthracenes D and their structural relationship with C.
[a] Dipl.-Chem. C. Hoffend, F. Schçdel, Dr. M. Bolte, Dr. H.-W. Lerner,
Prof. Dr. M. Wagner
Institut fꢀr Anorganische und Analytische Chemie
Goethe-Universitꢁt Frankfurt
Max-von-Laue-Strasse 7, 60438 Frankfurt am Main (Germany)
Fax : (+49)69-798-29260
E-mail: Matthias.Wagner@chemie.uni-frankfurt.de
two-step redox systems.[6,9–13] 2) Its cyclic structure renders
the DBA skeleton more stable than comparable open-chain
derivatives.[13,14] 3) The proximity of the two boron atoms
leads to binding cooperativity (e.g., the ditopic Lewis base
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201202280.
15394
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Chem. Eur. J. 2012, 18, 15394 – 15405

FULL PAPER
pyridazine binds more strongly than two equivalents of pyri-
dine).[15,16] Given this background, a variety of symmetrically
(R1 =R2 =H,[9] halogen,[13,17–20] OR,[13] NR2,[19] alkyl,[21,22]
aryl,[12,14,23] vinyl;[9] Figure 1) and unsymmetrically (R1 =Mes,
R2 =H, Br, OR, vinyl; Mes=mesityl)[13] substituted DBAs
have been synthesized and thoroughly characterized.
pounds can be regarded as boron-doped analogues of
tri(9,10-anthrylene)s and a comparison of the optoelectronic
properties of the two classes of compounds will therefore be
revealing.
However, a serious problem often encountered with com-
Results and Discussion
1,2
ꢀ
pounds A lies in the sensitivity of their exocyclic B R
bonds toward air and moisture. Kinetic stabilization of DBA
derivatives has so far been achieved either by structural con-
straint or by the introduction of steric bulk. In the first case,
the tetracoordinate intermediate preceding hydrolytic de-
composition is destabilized by embedding the boron atoms
into a rigidly fixed planar framework (see B; Figure 1).[14] In
the second case, mesityl substituents (R1 =R2 =Mes) are in-
troduced into the molecule so that the vacant boron-cen-
tered p orbitals are protected from the top and bottom by
methyl groups (see C; Figure 1).[12] Solutions of 9,10-dimesi-
tyl-9,10-dihydro-9,10-diboraanthracene (C) are stable for ex-
tended periods of time even in nondried solvents and under
an ambient atmosphere (NMR spectroscopic control). Un-
fortunately, the compound offers only a limited degree of
tunability.
In the search for alternative substituents, which combine a
level of steric protection similar to that of the mesityl group
with a larger number of set-screws for manipulating the op-
toelectronic properties of the corresponding DBA deriva-
tive, 9-anthryl moieties appear particularly attractive. 1) H-
C1 and H-C8 in D (Figure 1) serve a similar purpose to the
ortho-methyl groups in C. 2) 9-Anthryl substituents contain
an extended p-electron system; many of them are therefore
both luminescent and redox active. 3) The 10-position of the
9-anthryl group is well-suited for further functionalization
(R3), either with the aim of influencing its electronic proper-
In a first series of exploratory experiments, we treated 9,10-
dibromo-DBA (A; R1 =R2 =Br)[13] with two equivalents of
9-lithioanthracene[36] in diethyl ether and obtained a brick-
red microcrystalline solid, which was insoluble in all
common solvents. We therefore decided to introduce solubi-
lizing side chains into the 9-anthryl substituents and selected
the readily available 2,6-di-tert-butyl derivative for further
investigations.
Synthesis of soluble 9,10-dianthryl-9,10-dihydro-9,10-dibora-
AHCTUNGTREGaNNUN nthracenes: Friedel–Crafts alkylation of anthracene with
tBuCl/AlCl3 in CHCl3 provided a 3:2 mixture of the litera-
ture-known isomers 2,6- and 2,7-di-tert-butylanthracene.[37]
The products were conveniently separated by selective ex-
traction of the 2,7-isomer into acetone (see the Supporting
Information). Treatment of 2,6-di-tert-butylanthracene with
N-bromosuccinimide/FeCl3 in CHCl3 gave 1 (Scheme 1) in
excellent yield.[33] Lithium–bromine exchange on compound
1 with nBuLi, followed by the addition of 9,10-dibromo-
DBA (3; Scheme 1) led to the formation of the 9,10-di-
AHCTUNGTREGUNaNN nthryl-DBAs 4 and 4’ (Scheme 1). The deep-red com-
pounds 4 and 4’ are soluble in C6H6, toluene, THF, CHCl3,
and CH2Cl2, can be purified by flash column chromatogra-
phy on silica gel under an ambient atmosphere, and are
stable in nondried C6D6 for several hours (NMR spectro-
scopic control; first signs of decomposition are detectable
after approximately 6 h).
ꢀ
ꢀ
ties or to prepare oligomers [ {B
(m-C6H4)2B} (9,10-anthra-
ꢀ
cenediyl) ]n. 4) For steric reasons, the DBA and 9-anthryl
planes in D should be orthogonal to each other. Pronounced
charge-transfer interactions can be anticipated between both
fragments if the HOMO levels of the 9-anthryl moieties and
the LUMO level of the DBA core are appropriately adjust-
ed.
Compounds 4 and 4’ are rotamers, which arise from im-
ꢀ
peded rotation about the exocyclic B C bonds. The
11B{1H} NMR resonances of 4 and 4’ are broadened beyond
1
detection. In the H NMR spectrum, 4 and 4’ give rise to
two sets of signals with overall integral ratios of 4:5. Even
though we succeeded in growing single crystals of 4 (see
below), separation of the two rotamers on a preparative
scale was not possible. Thus, we cannot unambiguously
assign a specific signal set to either 4 or 4’. Similar rotamers
have previously been reported for a 4-type tri(9,10-anthry-
lene), in which the DBA core is replaced by 2,6-di-tert-butyl-
Any system in which two chromophores are linked by a
single bond possesses, in the perpendicular conformation,
several twisted intramolecular charge-transfer (TICT) states,
which may give rise to unique optoelectronic properties.[24–26]
Because the radiative back transition from the TICT state
to the ground state is overlap forbidden, many TICT states
are nonluminescent and therefore lead to intramolecular flu-
orescence quenching. However, by coupling to suitable vi-
brations this transition can gain intensity from more allowed
states,[24] which explains the appreciable TICT fluorescence
quantum yields of various compounds like, for example,
oligo(9,10-anthrylene)s.[27–35]
AHCTUNGTREGaNNUN nthracene-9,10-diyl and for which the sterically less con-
gested atropisomer corresponding to 4’ is preferentially
formed and has been structurally characterized.[33]
The X-ray crystal structure analysis of 4 fully confirms its
proposed molecular structure (Figure 2; Table 1). In the
solid state, the molecule possesses Cs symmetry. The length
ꢀ
of the exocyclic bond B1 C1 (1.577(7) ꢃ) is essentially the
Herein, we report on the synthesis and structural charac-
terization of compounds D carrying electronically innocent
(i.e., Ph), potentially p-accepting (i.e., Mes2B), and p-donat-
ing (i.e., Tol2N; Tol=p-tolyl) substituents R3. These com-
same as in 9,10-dimesityl-DBA (1.589(2) ꢃ[12]). The 9-anthr-
yl substituent in 4 and the plane spanned by C31, B1, and
C34 are orthogonal (dihedral angle=88.9(1)8) and the two
symmetry-related 9-anthryl moieties are nearly coplanar to
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15395

M. Wagner et al.
3.6–3.9 ꢃ). Despite this steric
interaction, the two tert-butyl
groups do not seem to have a
pronounced tendency to avoid
each other because the DBA
core in 4 is only slightly distort-
ed from planarity (dihedral
angle between the two ortho-
phenylene rings=169.2(3)8).
Given the structural features
of 4, the question arises of how
far the limits of steric demand
around the DBA center can be
pushed and whether the mole-
cule is able to accommodate
four inward-pointing tert-butyl
substituents. To synthesize the
corresponding
molecule
5
(Scheme 1), 2,7-di-tert-butylan-
thracene was first brominated
selectively at its 9-position by
applying the same conditions as
in the case of the 2,6-derivative
(see the Supporting Informa-
tion). The resulting compound
2 was lithiated and the lithio
anthracene species was reacted
with 3 to obtain 5 in 86% yield.
Scheme 1. Synthesis of compounds 4/4’ and 5. Reagents and conditions: i) nBuLi (3 equiv), Et2O, ꢀ788C!
room temperature, 1 h; ii) Et2O/toluene, ꢀ788C!room temperature, 12 h.
Table 1. Crystallographic data for 4 and 5.
Compared to 4 and 4’, 5 shows a similarly good solubility
and even enhanced stability toward air and moisture.
The H NMR spectrum of 5 is characterized by one signal
4
5
formula
Mr
C56H58B2
752.64
C56H58B2 ꢄC6H6
830.75
1
at d=1.24 ppm (integrating 36H) for the four tBu groups,
two multiplets at d=6.72 (4H) and 7.62 ppm (4H) for the
DBA core, and four signals for the 9-anthryl moieties. The
13C{1H} NMR spectrum is also in full agreement with the
proposed highly symmetric molecular structure. In the
11B{1H} NMR spectrum, 5 gives rise to an extremely broad
signal at d=73 ppm.
X-ray crystallography of 5 (Figure 2; Table 1) reveals a Ci-
symmetric framework with a planar DBA core. In contrast
to 4, the plane of the anthryl group in 5 is inclined by an
angle of 72.4(2)8 relative to the DBA plane. However, the
two anthryl substituents remain coplanar, with the shortest
C···C (H···H) distances between facing substituents being
3.876(7) ꢃ (2.7–3.0 ꢃ). We also note that the increased
steric interaction in 5 does not lead to an elongation of its
color, shape
T [K]
radiation, l [ꢃ]
crystal system
space group
a [ꢀ]
b [ꢀ]
c [ꢀ]
a [8]
b [8]
red, plate
173(2)
MoKa, 0.71073
orthorhombic
Cmc21
29.405(3)
9.1756(10)
16.6838(16)
90
red, block
173(2)
MoKa, 0.71073
orthorhombic
Pccn
22.9053(16)
24.3415(13)
9.2511(7)
90
90
90
90
90
g [8]
V [ꢃ3]
Z
4501.4(8)
4
1.111
1616
0.062
5157.9(6)
4
1.070
1784
0.059
1calcd [gcmꢀ3
]
F
A
]
crystal size [mm3]
reflns collected
0.24ꢄ0.24ꢄ0.13
10352
0.25ꢄ0.24ꢄ0.19
32634
ꢀ
exocyclic B1 C1 bonds (1.576(5) ꢃ) as compared to 4
independent reflns (Rint
)
2231 (0.1186)
2231/1/262
0.959
0.0603, 0.1027
0.0930, 0.1118
0.164, ꢀ0.171
4553 (0.0813)
4553/9/276
1.018
0.0956, 0.2004
0.1296, 0.2190
0.319, ꢀ0.260
data/restraints/parameters
(1.577(7) ꢃ). This observation, together with the fact that
the facing tert-butyl groups could easily avoid each other if
the two anthryl planes were inclined in opposite directions,
suggests that the tBu···tBu interaction in 5 is not entirely re-
pulsive but, on the contrary, that dispersion forces are caus-
ing a stabilizing attraction. In this context, we refer to hexa-
kis(3,5-di-tert-butylphenyl)ethane, in which the tert-butyl
groups stabilize the compound relative to its dissociation
product radicals by as much as 40 kcalmolꢀ1 through attrac-
tive dispersion interactions.[38] Here, the shortest tBu···tBu
GOF on F2
R1, wR2 [I>2s(I)]
R1, wR2 (all data)
largest diff. peak and hole [eꢃꢀ3
]
each other (dihedral angle=2.3(1)8). This conformation re-
sults in a close approach of the two tert-butyl substituents at
C23 and C23A (shortest distance between two carbon
atoms: C10···C10A=4.455(11) ꢃ; shortest H···H contacts:
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Chem. Eur. J. 2012, 18, 15394 – 15405

Boron-Doped Tri(9,10-anthrylene)s
FULL PAPER
its bromine atom by electron-donating or -withdrawing sub-
stituents. The selection of these substituents had to take into
account the fact that the overall synthetic sequence requires
a second bromination reaction (at the 10-position of the an-
thracene molecule) with which they needed to be compati-
ble. The phenyl, dimesitylboryl (Mes2B), and N,N-di-
ACHUTNRGEN(NUG p-tolyl)ACHTUNGTRENaNUNG mino (Tol2N) groups turned out to be suitable sub-
stituents, which, moreover, cover a broad range of different
electronic properties (see 6–8; Scheme 2). Compounds 6–8
were prepared from
2 through Suzuki coupling with
PhB(OH)2, lithiation/treatment with Mes2BF, and Buch-
wald–Hartwig coupling with Tol2NH, respectively, followed
by bromination with N-bromosuccinimide/FeCl3 (see the
Supporting Information). The same general protocols as in
the cases of 4/4’ and 5 were applied to subsequently prepare
the 9,10-diACTHNUGTRNEUNGanthryl-DBAs 9–11 from 6–8 (Scheme 2). We
wish to emphasize that the synthetic sequence applied nec-
essarily leads to anthryl DBA derivatives in which all four
tert-butyl groups are outward pointing. We also note that 10
can not only be viewed as a 9,10-dianthryl-DBA, but also as
a molecule featuring two 9,10-diborylated anthracene chro-
mophores. Related quadrupolar compounds of the general
formula Mes2B–(p system)–BMes2 have been thoroughly
studied by Marder et al. and reported to possess interesting
linear and nonlinear optical properties.[40]
As already noted for 4/4’, no 11B NMR resonances were
detectable for 9–11. For a discussion of key aspects of the
1H and 13C{1H} NMR spectra of 9–11, it is first helpful to
closely examine the NMR parameters of the anthracene
precursors 9-R-2,7-di-tert-butylanthracene (9-R-2,7-DTBA;
R=Ph, Mes2B, Tol2N). NMR spectroscopy provides essen-
tial information on the electron density distribution within
p-conjugated molecules. Because proton shifts are strongly
influenced by solvent and magnetic anisotropy effects,
13C shifts are generally preferred for the determination of
local p-charge densities.[41] Compared to 9-Ph-2,7-DTBA,
the 13C nuclei 8a, 9a, and 10 in 9-Mes2B-2,7-DTBA, which
are located ortho and para to the boryl group (black circles
in compound 10; Scheme 2), are deshielded by approximate-
ly Dd=3–4 ppm, whereas the meta-carbon resonances (i.e.,
4a, 10a) of both compounds possess essentially the same
shift values (Dd=0.2 ppm). As anticipated, this indicates a
certain degree of p conjugation between the anthracene unit
and the boron atom. In contrast to a priori expectations, the
13C NMR parameters of the atoms 8a/9a and 10 reveal only
negligible differences between 9-Ph-2,7-DTBA and 9-Tol2N-
2,7-DTBA (Dd=0.1 and 0.3 ppm, respectively). Moreover,
the 13C nucleus 9 of 9-Tol2N-2,7-DTBA is far less deshielded
(d=137.5 ppm) than the ipso-13C nuclei of the para-tolyl
rings (d=146.1 ppm); the para-tolyl rings also show strongly
shielded ortho-13C nuclei (d=120.5 ppm). It may therefore
be concluded that the p-donor capacity of the nitrogen atom
is essentially restricted to the para-tolyl groups. The fact
that the Mes2B group exerts a measurable ꢀM effect on the
anthracene p system, whereas the Tol2N substituent does
not appear to have any appreciable +M effect, may be ex-
plained by assuming an (on average) stronger twist about
Figure 2. Molecular structures of 4 (top) and 5 (bottom) in the solid
state; displacement ellipsoids are drawn at the 50% probability level,
H atoms have been omitted for clarity. Selected bond lengths [ꢃ], bond
ꢀ
ꢀ
angles [8], and dihedral angles [8]: 4: B1 C1 1.577(7), B1 C31 1.559(7),
ꢀ
B1 C34 1.562(7); C1-B1-C31 120.2(4), C1-B1-C34 120.9(4), C31-B1-C34
118.8(4); Ar
A
ACHTUNGTRENNUNG(C34) 169.2(3), Ant(C1)//C31B1C34 88.9(1),
ꢀ
ꢀ
ꢀ
Ant(C1)//AntACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
formations used to generate equivalent atoms: A: ꢀx, y, z (4); ꢀx+1,
ꢀy+1, ꢀz+1 (5).
distances amount to about 4.05 ꢃ (C···C) and 2.5 ꢃ
(H···H).[39]
Tuning the electronic properties of 9,10-dianthryl-9,10-dihy-
dro-9,10-diboraanthracenes: Having explored key steric
issues of 9,10-dianthryl-DBAs, we now focus on electronic
substituent effects. Compound 1 was abandoned from fur-
ther studies to circumvent problems of rotamer formation in
the final DBA products (see 4, 4’). Our synthetic efforts
therefore relied on compound 2 and on the replacement of
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15397

M. Wagner et al.
Table 2). Due to the small crys-
tal size, the data obtained for
10 were poor. We therefore re-
frain from a detailed discussion
of its structural parameters.
Compound 9 crystallizes with
two crystallographically inde-
pendent molecules (9A and 9B)
in the asymmetric unit. Com-
pounds 9A/9B possess a center
of inversion and the tert-butyl
substituents are pointing away
from the DBA core. Neverthe-
ꢀ
less, their exocyclic B1 C1
bond
lengths
(1.589(4)/
1.581(4) ꢃ) are the same as in
the seemingly more crowded
derivatives 4 (1.577(7) ꢃ) and 5
(1.576(5) ꢃ), which lends fur-
ther support to the conclusion
that the inward-pointing tert-
butyl groups do not repel each
other. The DBA moieties in 9A/
9B are planar and form dihedral
angles with the anthryl substitu-
ents of 81.6(2)/88.5(1)8.
Electrochemical measurements
on 9,10-dianthryl-9,10-dihydro-
9,10-diboraanthracenes: Com-
pounds 4/4’, 5, and 9–11 were
investigated by cyclic voltam-
Scheme 2. Synthesis of compounds 9–11. Reagents and conditions: i) 1) nBuLi (3 equiv), Et2O, ꢀ788C!room
temperature, 1 h; 2) 3 (1 equiv), Et2O/toluene, ꢀ788C!room temperature, 12 h. Black circles in 10: 13C nuclei
experiencing deshielding upon going from 9-Ph-2,7-DTBA to 9-Mes2B-2,7-DTBA; white circles in 10:
13C nuclei experiencing deshielding upon going from 9-Mes2B-2,7-DTBA to 10.
Table 2. Crystallographic data for 9 and 10.
ꢀ
ꢀ
the Tol2N C
A
ACHTUNGTRENNUNG(anthryl) bond than the Mes2B C-
9
10
9-Tol2N-2,7-DTBA (9-Mes2B-2,7-DTBA) shows a
dihedral angle of 65.98 (54.48) between anthracene
and the plane spanned by the two remaining ipso-
carbon atoms and the nitrogen (boron) atom (see
the Supporting Information and ref. [40a] for the
related X-ray crystal structure analysis of 9,10-bis-
formula
Mr
color, shape
T [K]
radiation, l [ꢃ]
crystal system
space group
a [ꢀ]
b [ꢀ]
c [ꢀ]
a [8]
b [8]
C68H66B2 ꢄ1.5CH2Cl2
1032.22
red, block
173(2)
MoKa, 0.71073
monoclinic
P21/c
25.5984(13)
10.1849(4)
25.5600(12)
90
118.775(3)
90
5841.0(5)
4
1.174
2188
0.198
0.41ꢄ0.36ꢄ0.33
67536
10758 (0.0810)
10758/0/681
0.981
0.0710, 0.1928
0.0975, 0.2098
0.723, ꢀ0.708
C92H100B4
1248.96
red, plate
173(2)
MoKa, 0.71073
monoclinic
P21/c
17.579(4)
9.1886(15)
24.659(5)
90
94.244(18)
90
3972.2(14)
2
1.044
1344
0.058
0.18ꢄ0.16ꢄ0.02
24853
6978 (0.4522)
6978/18/439
0.836
0.1198, 0.2481
0.3272, 0.3469
0.718, ꢀ0.319
ACHTUNGTRENNUNG
into the DBA scaffold, changes in certain 13C NMR
shift values are apparent in all three cases, which
are qualitatively and quantitatively comparable to
the changes observed upon going from 9-Ph-2,7-
DTBA to 9-Mes2B-2,7-DTBA: the 13C nuclei ortho
to the DBA boron atom experience downfield
shifts of about Dd=3 ppm (i.e., 4a, 10a; white cir-
cles in compound 10; Scheme 2), whereas the reso-
nances of the meta-13C nuclei (i.e., 8a, 9a) remain
unchanged (Ddꢁ0.5 ppm). The three DBA
13C NMR resonances vary by a maximum of Dd=
0.3 ppm between 9, 10, and 11.
g [8]
V [ꢃ3]
Z
1calcd [gcmꢀ3
]
F
A
]
crystal size [mm3]
reflns collected
independent reflns (Rint
)
data/restraints/parameters
GOF on F2
R1, wR2 [I>2s(I)]
R1, wR2 (all data)
Compounds 9 and 10 have been structurally char-
acterized by X-ray crystallography (Figure 3,
largest diff. peak and hole [eꢃꢀ3
]
15398
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Chem. Eur. J. 2012, 18, 15394 – 15405

Boron-Doped Tri(9,10-anthrylene)s
FULL PAPER
substituents and 2) anthracene
and the corresponding 9-R-2,7-
DTBAs (R=H, Ph, Mes2B, and
Tol2N).
Under the measurement con-
ditions applied throughout this
study, compound C gives rise to
two reversible redox waves at
[42]
1
E = =ꢀ1.84 and ꢀ2.73 V; the
2
parent anthracene shows a first
reversible reduction wave at
1
E = =ꢀ2.16 V and an irreversi-
2
ble electron transition at Epc =
ꢀ2.55 V (THF, Na
ACHTUNGRTENN[UNG BPh4], vs.
FcH/FcH+).[32]
The cyclic voltammogram of
5 is characterized by two rever-
1
sible redox waves at E = =ꢀ1.73
2
and ꢀ2.57 V. Moreover, an ad-
ditional ill-defined feature ap-
pears at approximately Epc =
ꢀ3.21 V. Compound 9-H-2,7-
DTBA shows two redox transi-
1
tions at E = =ꢀ2.65 and Epc =
2
ꢀ3.24 V (see the Supporting In-
formation for a CV-plot overlay
of 9-H-2,7-DTBA, C, and 5).
Given these data, the first re-
1
duction of 5 at E = =ꢀ1.73 V
2
clearly occurs at the DBA frag-
ment. An assignment of the
1
second reduction at E = =
2
Figure 3. Molecular structures of 9A (top) and 10 (bottom) in the solid state; displacement ellipsoids are drawn
at the 50% probability level, H atoms have been omitted for clarity. Selected bond lengths [ꢃ], bond
ꢀ2.57 V as either DBA- or
ꢀ
ꢀ
ꢀ
anthryl-centered is less straight-
forward, but was finally ach-
angles [8], and dihedral angles [8]: 9A: B1 C1 1.589(4), B1 C31 1.551(4), B1 C36A 1.570(4); C1-B1-C31
121.5(2), C1-B1-C36A 119.4(2), C31-B1-C36A 119.0(2); Ar(C31)//Ar(C36A) 0.0, Ant(C1)//C31B1C36A
81.6(2), Ant(C1)//Ant(C1A) 0.0, Ant(C2)//Ph(C41) 66.6(1). Ar=phenylene, Ant=anthryl. Symmetry transfor-
mation used to generate equivalent atoms: A: ꢀx+1, ꢀy+1, ꢀz+1.
G
ACHTUNGTRENNUNG
N
ACHTUNGTRENNUNG
ieved by means of UV/Vis spec-
troelectrochemistry. The UV/
Vis spectra of Li[A] and Li2[A]
(R1 =R2 =H) in THF are litera-
metry (CV; THF, [nBu4N]
N
ture-known. Above 400 nm, Li[A] possesses the most in-
tense absorption at lmax =517 nm, together with three broad
humps at lmax =730, 810, and 910 nm; the UV/Vis spectrum
of Li2[A] shows an absorption at lmax =420 nm and a much
weaker, broader band at lmax =500 nm (Figure 5, top).[11]
During exhaustive coulometric one-electron reduction of 9-
The assignment of individual redox waves is based on the
comparison with 1) 9,10-dimesityl-DBA (C; Figure 1) as a
prototypical DBA derivative with redox-innocent boron
H-2,7-DTBA, in situ UV/Vis spec
ACHUTGTNRENUtNG rosAHCTUGNTRENcNGUN opy (THF, [nBu4N]-
G
550, 600, 690, and 740 nm
(Figure 5, middle). For a com-
parative study on 5, we took a
solution in THF, added a small
piece of elemental potassium,
and again performed in situ
UV/Vis spectroscopy. First, the
signature of a DBA monoanion
developed in the spectrum
(lmax =535, 750, 830, 930 nm;
Table 3. Electrochemical and UV/Vis data of 5 and 9–11.
CV[a]
Absorption[c]
Fluorescence[c]
lem [nm] (lex [nm])
E = [V] vs. FcH/FcH+
lmax [nm] (e [molꢀ1dm3 cmꢀ1])
1
2
5
9
10
11
ꢀ1.73, ꢀ2.57, ꢀ3.21[b]
366 (15500), 510 (1050)
374 (28200), 531 (1500)
346 (17400), 442 (18300), 536 (2000)
359 (20300), 429 (14300), 556 (2200)
610 (366)
635 (374)
644 (442)
675 (593)[d]
ꢀ1.68, ꢀ2.46, ꢀ3.32[b]
ꢀ1.70, ꢀ2.37, ꢀ2.64, ꢀ3.11[b]
ꢀ1.69, ꢀ2.42, ꢀ3.27[b], ꢀ3.39[b]
[a] THF, supporting electrolyte: [nBu4N]
peak potential Epc value. [c] C6H6. [d] Solid-state measurement.
[PF6] (0.1m), scan rate: 200 mVsꢀ1, versus FcH/FcH+. [b] Cathodic
ACHTUNGTRENNUNG
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15399

M. Wagner et al.
Figure 5. UV/Vis absorption spectra of Li[A] and Li2[A] (top; R1 =R2 =
H), of the 9-H-2,7-DTBA monoanion (middle), and of K[5] and K2[5]
(bottom) in THF.
effect of the boron atom on the anthryl substituent out-
weighs its ꢀM effect so that the redox potential of the
anthryl fragment in 5 is shifted to more cathodic values than
suggested by the model system 9-H-2,7-DTBA.
Figure 4. Cyclic voltammograms of 5, 9, 10, and 11 as solutions in THF at
The cyclic voltammogram of 9 is similar to that of 5 and
the two most anodic, reversible transitions appear at essen-
tially the same potential values. Thus, neither the relocation
of the tert-butyl groups nor the introduction of the two
phenyl rings has an appreciable influence on the electro-
chemical properties of the molecules (see the Supporting In-
formation for a CV-plot overlay of 9-Ph-2,7-DTBA, C, and
9).
room temperature; supporting electrolyte: [nBu4N]ACTHNUGTRNENG[U PF6] (0.1m); scan
rate: 200 mVsꢀ1, versus FcH/FcH+.
Figure 5, bottom); after some time, these characteristic
bands vanished and absorptions at lmax =474 and 620 nm ap-
peared instead (Figure 5, bottom). From the comparison of
the spectra compiled in Figure 5, we thus tentatively assign
This situation changes when the phenyl rings are replaced
by Mes2B groups. As in the cases of 5 and 9, compound 10
1
the E = =ꢀ2.57 V wave of 5 to a DBA-centered reduction.
2
1
In turn, the electron transition at Epc =ꢀ3.21 V is most
features reversible redox waves at E = =ꢀ1.70 and ꢀ2.64 V;
2
likely associated with the anthryl substituents. One explana-
tion for the surprising selectivity of the second DBA reduc-
tion over a first anthryl reduction might be that the +I
however, a third reversible transition is now visible at an in-
1
termediate electrode potential of E = =ꢀ2.37 V. A fourth
2
wave is found at Epc =ꢀ3.11 V. In comparison, the cyclic
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Chem. Eur. J. 2012, 18, 15394 – 15405

Boron-Doped Tri(9,10-anthrylene)s
FULL PAPER
voltammogram of 9-Mes2B-2,7-DTBA is characterized by
reduction events at Epc =ꢀ2.46 and ꢀ3.00 V (see the Sup-
porting Information for a CV-plot overlay of 9-Mes2B-2,7-
DTBA, C, and 10). Coulometric measurements on 10 pro-
vided clear evidence that the first (DBA-centered) reduction
is a one-electron transition. When the potentiostat was set
to a value between the second and third redox waves, two
more electrons were transferred; coulometry at a slightly
more cathodic potential value than the third redox wave led
to an overall electron count of four. Spectroelectrochemical
measurements carried out at an intermediate electrode po-
tential between those of the second and third redox waves
indicated the formation of both DBA dianions and anthryl
monoanions. We therefore conclude that the second reduc-
tion of the DBA core and the first reduction of the anthryl
substituents cannot be selectively achieved in the case of 10.
A sizeable ꢀM effect of the Mes2B groups on the anthryl
p systems is therefore not only evident from the
13C{1H} NMR spectra (see above), but also from the electro-
chemical studies.
and the second at about lmax =510 nm, which is broad and
featureless and therefore assigned to an anthryl!DBA
charge-transfer transition (Figure 6, Table 3; see the Sup-
The cyclic voltammogram of 11 reveals two reversible
1
waves at E = =ꢀ1.69 and ꢀ2.42 V. Moreover, we observe
2
two features at Epc =ꢀ3.27 and ꢀ3.39 V. Two electron tran-
sitions are visible in the voltammogram of 9-Tol2N-2,7-
1
DTBA with associated redox potentials of E = =ꢀ2.62 and
2
Epc =ꢀ3.47 V, largely identical to those of 9-H-2,7-DTBA.
Thus, in line with 13C{1H} NMR spectroscopy, the Tol2N
group does not seem to exert a notable +M effect on the
anthryl scaffold, because this should result in a cathodic
shift of the reduction potential relative to 9-H-2,7-DTBA.
Although the first electron transition of 11 is again DBA-
centered, the assignment of the second wave is less clear
and might well have contributions from both DBA and
anthryl moieties.
Figure 6. UV/Vis absorption (top) and emission (bottom) spectra of
DBA derivatives in C6H6; 5 (red), 9 (black), 10 (green), 11 (blue).
porting Information for an overlay of the electronic spectra
of 9-H-2,7-DTBA, C, and 5). Thus, upon going from mesityl
to anthryl substituents, the charge-transfer band experiences
a bathochromic shift of more than 100 nm. The fluorescence
spectrum of 9-H-2,7-DTBA reveals a structured emission
band at lem =417 nm (lex =358 nm). Compound C also gives
rise to a structured emission band at 413 nm in cyclohexane
(C6H12)[12] and 460 nm in C6H6 (lex =350 nm), but only one
broad band is visible in THF, at lem =483 nm.[12] The pro-
nounced redshift with increasing solvent polarity suggests
that the emissive state of C is highly polarized and already
has contributions from TICT emission. The associated quan-
tum yields are small (ff =0.02–0.05);[12] however, given the
pronounced vibrational fine structure generally observed for
absorption and emission bands of anthracene derivatives, a
switch from mesityl (see C) to anthryl (see 5 and 9–11)
should have a beneficial effect on the quantum yields ob-
tained (see the Introduction).
Electronic spectra of 9,10-dianthryl-9,10-dihydro-9,10-di-
ACHTUNGTRENNUNGboraACHTUNGTRENNUNGanthracenes: The UV/Vis and fluorescence spectra of
the rotamer mixture 4/4’ are essentially identical to those of
5 and therefore are not considered further. We start our dis-
cussion with a comparison of the electronic spectra of 5 with
those of its constituent chromophores, that is, 9-H-2,7-
DTBA and C (solutions in C6H6). The UV/Vis spectrum of
9-H-2,7-DTBA shows a longest-wavelength absorption band
with vibrational fine structure at lmax =357 nm. The corre-
sponding band of C appears at lmax =349 nm and bears a
shoulder on the bathochromic side. According to time-de-
pendent DFT calculations, the strong absorption can be at-
tributed to p–p* excitations of the DBA core, whereas the
shoulder is due to intramolecular charge transfer from the
electron-rich mesityl groups to the electron-poor DBA
unit.[12] This observation already lends support to our con-
cept to achieve TICT in DBA derivatives by extending the
electron p system of the boron substituents. In line with
that, the UV/Vis spectrum of 5 is characterized by two ab-
sorptions: one at lmax =366 nm, which has a similar line
shape to the 9-H-2,7-DTBA band and thus most likely
arises from electron transitions within the anthryl p system,
In contrast to 9-H-2,7-DTBA and C, the emission band of
5 is bathochromically shifted and appears at lem =610 nm
when the molecule is excited at either lex =366 nm (anthryl
absorption) or lex =510 nm (charge-transfer band; Figure 6,
Table 3). The absorption spectrum of 5 is largely solvent-in-
dependent, but the fluorescence spectrum exhibits pro-
nounced positive solvatochromism, that is, the emission
Chem. Eur. J. 2012, 18, 15394 – 15405
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15401

M. Wagner et al.
duces nonradiative decay pathways operative in solution, a
phenomenon known as aggregation-induced emission.[43]
In summary, we find the following trend in the charge-
transfer bands of 9–11: lmax =531 (9)ꢂ536 (10)<556 nm
(11). Compounds 9 and 10 show essentially identical behav-
ior even though 10 is equipped with p-electron-withdrawing
Mes2B substituents, because the effect of the boryl group is
most pronounced for the anthryl LUMO,[5] whereas the
charge transfer proceeds from the anthryl HOMO into the
DBA LUMO. The presence of p-electron-donating Tol2N
substituents in 11 increases the HOMO levels of the corre-
sponding anthryl moieties,[44] which explains why 11 shows
the most redshifted charge-transfer band. However, the shift
is small compared to 9 and 10, in line with our previous con-
clusions that the nitrogen atom uses most of its p-donor ca-
pacity toward the p-tolyl rings rather than its anthryl sub-
stituent.
With regard to the fluorescence spectra, we note that all
DBA derivatives gave rise to an additional emission at
around lem =440 nm upon irradiation into the respective
anthryl absorption band. When the samples were prepared
in a glovebox with rigorously dried and degassed solvents,
the intensity of this band was very small (approximately 5%
of the intensity of the main band), but it could never be
completely suppressed. The 440 nm emission gained in in-
tensity if the closed cuvette was left standing for extended
periods of time outside the glovebox or if air and moisture
were deliberately admitted. Thus, a certain sensitivity of 5
and 9–11 toward hydrolysis and/or oxidation is apparent
from fluorescence spectroscopy, even though the compounds
can be purified under ambient atmosphere by column chro-
matography (5 and 9) or extraction with nondried MeOH
(10 and 11). As already stated above, targeted hydrolysis
studies, monitored by NMR spectroscopy, gave signs of de-
composition not earlier than 6 h after the addition of
H2O.[45]
In C6H6, 5, 9, and 10 gave quantum yields ff of 0.08, 0.05,
and 0.06, respectively. In contrast, the published quantum
yields of anthracene (ff =0.33), bianthryl (ff =0.42), and
tri(9,10-anthrylene) (ff =0.60) in alkane solvents are consid-
erably higher.[34] We tentatively attribute these differences
to the fact that the DBA core in 5, 9, and 10 is a strong elec-
tron acceptor, thereby favoring TICT processes in compari-
son to the all-carbon systems, which, in turn, result in fluo-
rescence quenching.
Figure 7. Solvatochromism of 5 in solvents of different polarity (C6H12,
C6H6, THF, CH2Cl2); UV/Vis absorption (top) and emission (bottom)
spectra.
band is redshifted in more polar solvents: lem =565 (C6H12),
610 (C6H6), 660 (THF), 680 nm (CH2Cl2; Figure 7). Charac-
teristically, in the least polar solvent, C6H12, both the charge-
transfer band and the fluorescence band of 5 gain a vibra-
tional fine structure.
The UV/Vis and fluorescence spectra of 9 in C6H6 are
similar to those of 5 with all key bands being shifted batho-
chromically by 8–25 nm (Figure 6, Table 3; see the Support-
ing Information for an overlay of the electronic spectra of 9-
Ph-2,7-DTBA, C, and 9).
In the UV/Vis spectra of 10 and 11 (C6H6), two strong ab-
sorptions (instead of one in the cases of 5 and 9) appear at
lmax =346, 442 nm (10) and lmax =359, 429 nm (11). Similar
features are visible in the UV/Vis spectra of 9-Mes2B-2,7-
DTBA and 9-Tol2N-2,7-DTBA, so these two bands are most
likely substituent-centered. In addition, we also observe
charge-transfer bands at lmax =536 and 556 nm for 10 and
11, respectively (Figure 6, Table 3). The Mes2B derivative 10
also shows an emission at lem =644 nm (lex =442 nm),
whereas no appreciable fluorescence of 11 is detectable in
C6H6. In contrast to the solution state, solid 11 shows a
wine-red emission at lem =675 nm (lex =593 nm; see the
Supporting Information for an overlay of the electronic
spectra of 9-Mes2B-2,7-DTBA/9-Tol2N-2,7-DTBA, C, and
10/11). An elucidation of the reasons for the differences in
the solution and solid-state fluorescence of 11 certainly re-
quires more detailed scrutiny. A reasonable working hypoth-
esis would be that restricted rotation in the solid state re-
Conclusion
Five
9,10-dianthryl-9,10-dihydro-9,10-diboraanthracenes
have been synthesized, fully characterized, and found to be
stable toward air and moisture for extended periods of time.
Each of these derivatives carries four tert-butyl groups on its
anthryl substituents. Initially, we assumed that an ideal mo-
lecular design requires all four solubilizing substituents to
be outward pointing. However, it turned out that the molec-
ular framework is compatible even with four inward-point-
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Chem. Eur. J. 2012, 18, 15394 – 15405

Boron-Doped Tri(9,10-anthrylene)s
FULL PAPER
fluorometer FP-8300 (Jasco), respectively. The absolute fluorescence
quantum yields (ff) were determined by using a calibrated integrating
sphere system (ILF-835 100 mm diameter Integrating Sphere; Jasco), a
quantum yield calculation program (FWQE-880; Jasco), and highly dilut-
ed samples of five different optical densities in each measurement. Mass
spectra were recorded with a VG PLATFORM II mass spectrometer or a
Fisons Instruments VG TofSpec mass spectrometer. Combustion analyses
were performed by the Microanalytical Laboratory of the Goethe-Uni-
versity Frankfurt and by the Microanalytical Laboratory Pascher (Rema-
gen, Germany).
ing tert-butyl groups and that their overall interactions seem
to be attractive rather than repulsive. The latter substitution
pattern is therefore particularly interesting, because the tert-
butyl groups serve two purposes: 1) they increase the solu-
bility of the molecule and 2) they provide additional steric
shielding for the boron atoms. We also succeeded in the in-
troduction of phenyl, dimesitylboryl, and N,N-di
ACHTUNGTRENUN(NG p-tolyl)-
ACHTUNGTRENNUNG
handle to influence the optoelectronic properties of the mol-
ecules.
General procedure for the synthesis of DBA derivatives 4/4’, 5, 9, 10, and
11: nBuLi in hexane (3 equiv) was diluted with a threefold volume of
hexane and added dropwise with stirring at ꢀ788C to a solution of the
respective bromoanthracene (2 equiv) in Et2O. Stirring was continued for
15 min before the reaction mixture was warmed to room temperature
within 1 h. The resulting turbid solution was cooled to ꢀ788C again, a
solution of 3 (1 equiv) in toluene was added dropwise with stirring, the
reaction mixture was allowed to warm to room temperature overnight,
and all volatiles were removed under reduced pressure.
The UV/Vis absorption spectra of all five derivatives are
characterized by broad charge-transfer bands in the range
lmax =510–556 nm; the corresponding fluorescence bands
show strong positive solvatochromism. Four of the five 9,10-
dianthryl-DBAs have been investigated by X-ray crystallog-
raphy; all of them reveal twisted conformations about the
ꢀ
exocyclic B C bonds with dihedral angles ranging between
Synthesis of 4/4’: Following the general procedure, 4/4’ were synthesized
from nBuLi (1.4m in hexane, 1.4 mL, 1.9 mmol), 1 (465 mg, 1.26 mmol),
and 3 (200 mg, 0.599 mmol); solvents were Et2O (40 mL) and toluene
(25 mL). The crude product was washed with nondried MeOH (3ꢄ
30 mL) and nondried pentane (3ꢄ30 mL) and further purified by flash
column chromatography (silica gel 60). A mixture of nondried hexane
and CHCl3 (10:1) was employed as the mobile phase until all fractions
showing blue fluorescence had been eluted from the column. Afterwards
the polarity of the mobile phase was increased (hexane/CHCl3 =2:1) to
obtain 4/4’ as a red solid (253 mg, 56%). Both in solution and in the solid
state, 4/4’ show an intense red fluorescence when irradiated with UV
light (lex =366 nm). Single crystals of 4 were grown by gas-phase diffu-
sion of hexane into a concentrated solution of 4 in THF. Rf =0.10 (silica
gel, hexane/CHCl3 =5:1); 1H NMR (400.1 MHz, C6D6): d=1.20 (s, 18H;
708 and 908. We therefore conclude that the electronic spec-
tra of our compounds are dominated by TICT interactions
between the anthryl donors and the DBA acceptor. As a de-
cisive difference, the all-carbon analogues, tri(9,10-anthry-
lene)s, feature three essentially identical subunits, none of
them possessing distinct electron-donor or -acceptor proper-
ties.
The pronouncedly three-dimensional structures, the bulky
tert-butyl groups, and the large Stokes shifts of our 9,10-di-
ACHTUNGTRENNUNGanthryl-DBAs help to suppress p–p interactions and self-
quenching in the solid state. As a result, bright fluorescence
is observed for solid samples and we therefore suggest that
the class of compounds presented herein is particularly
promising for solid-state applications.
C
C
N
ACHTUTNGRENNU(G CH3)3), 1.36 (s, 18H; CACHTUNGTRENN(UGN CH3)3), 1.39 (s, 18H;
3
4
ACHTUGTNREN(UNG H,H)=9.0, JCAHTUNGTREN(NNGU H,H)=2.0 Hz,
G
ACHTUNGTRENNUNG
3J
(H,H)=9.0, 4J
R
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
Experimental Section
A
C
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
Unless otherwise specified, all reactions were carried out under dry nitro-
gen or argon with carefully dried and degassed solvents, flame-dried
glassware, and Schlenk or glovebox techniques. Pentane, hexane, toluene,
C6D6, and Et2O were dried over Na/benzophenone; CHCl3 was dried
over CaH2 and freshly distilled prior to use; CDCl3 was dried over molec-
ular sieves (4 ꢃ). Solvents used for UV/Vis spectroscopic measurements
were purchased as spectroscopic grade, dried over Na/K alloy (cyclohex-
ane, C6H6, THF) or CaH2 (CH2Cl2), and degassed prior to use. Column
chromatography was performed by using silica gel 60 (Macherey–Nagel).
NMR spectra were recorded with Bruker Avance 300 or Avance 400
spectrometers at room temperature. Chemical shifts are referenced to
(residual) solvent signals (1H/13C{1H}; C6D6: d=7.15/128.0 ppm; CDCl3:
d=7.26/77.0 ppm) or external BF3·Et2O (11B, 11B{1H}). Abbreviations: s=
singlet, d=doublet, dd=doublet of doublets, t=triplet, m=multiplet,
br.=broad, app.=apparent, n.r.=multiplet not resolved, n.o.=signal not
observed, i=ipso, o=ortho, m=meta, p=para. Electrochemical meas-
urements were performed at room temperature by using an EG&G
Princeton Applied Research 263A potentiostat with a platinum disk
working electrode (diameter 2.00 mm). The reference electrode was a
silver wire on which AgCl had been deposited by immersing the wire
into HCl/HNO3 (3:1). The solvent (THF) was dried over sodium without
added benzoquinone at room temperature and degassed by five freeze–
125.0, 125.1 (C-5,5’,7,7’), 126.3 (C-9,9’), 2ꢄ129.0 (C-8,8’), 130.0 (C-4’),
130.2 (C-4), 130.7 (C-8a), 130.8 (C-8a’), 2ꢄ132.0 (C-10a,10a’), 133.2 (C-
4a’), 133.4 (C-4a), 134.2 (C-b’), 134.4 (C-b), 134.5 (C-9a’), 134.7 (C-9a),
140.5 (C-a’), 140.9 (C-a), 2ꢄ141.9 (C-10,10’), 146.5 (BC, BC’), 146.7 (C-
6), 146.9 (C-6’), 147.2 (C-2’), 147.3 ppm (C-2); UV/Vis (C6H6): lmax (e)=
367 (20800), 510 nm (1300 molꢀ1 dm3 cmꢀ1); fluorescence (C6H6): lex
=
366 nm, lem =615 nm; MS (MALDI): m/z (%): 753 (100) [M+]; elemen-
tal analysis calcd (%) for C56H58B2 (752.64)ꢄ0.2C4H8O (72.11): C 88.93,
H 7.83; found: C 88.58, H 7.81 (the relative amount of C4H8O present in
1
the sample was confirmed by H NMR spectroscopy).
Synthesis of 5: Following the general procedure, 5 was synthesized from
nBuLi (1.4m in hexane, 1.8 mL, 2.5 mmol), 2 (600 mg, 1.62 mmol), and 3
(270 mg, 0.809 mmol); solvents were Et2O (40 mL) and toluene (25 mL).
The crude product was washed with nondried MeOH (3ꢄ30 mL) and
nondried pentane (3ꢄ30 mL) and further purified by flash column chro-
matography (silica gel 60; hexane/CHCl3 =5:1) to obtain 5 as a red solid
(525 mg, 86%). Both in solution and in the solid state, 5 shows an intense
red fluorescence when irradiated with UV light (lex =366 nm). Single
crystals of 5 were grown by gas-phase diffusion of hexane into a concen-
trated solution of 5 in C6H6. Rf =0.14 (silica gel, hexane/CHCl3 =5:1);
1H NMR (400.1 MHz, C6D6): d=1.24 (s, 36H; C
ACHTUNGTRENNUNG
3
4
b), 7.54 (dd, J
ACHUTGTNREN(NUG H,H)=9.0, JACHTNUGTRENNUNG
pump–thaw cycles. [nBu4N]ACHTNUGTRNEUNG[PF6] (0.1m) was employed as the supporting
3
electrolyte. All potential values are referenced against the FcH/FcH+
a), 8.02 (n.r., 4H; H-1,8), 8.10 (d, JACTHGUNTRNE(NGU H,H)=9.0 Hz, 4H; H-4,5), 8.51 ppm
11
1
1
2
couple (E = =0 V). UV/Vis absorption and emission spectra were record-
(s, 2H; H-10); B NMR (128.4 MHz, C6D6): d=73 ppm (h = =3500 Hz);
2
ed with a Varian Cary 50 Scan UV/Vis spectrophotometer or a Spectro-
13C{1H} NMR (100.6 MHz, C6D6): d=31.2 (C
ACHTUNGTRENNU(G CH3)3), 35.0 (CACHTUNGTRENNUNG(CH3)3),
Chem. Eur. J. 2012, 18, 15394 – 15405
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
15403

M. Wagner et al.
124.7 (C-3,6), 125.1 (C-1,8), 126.1 (C-10), 129.1 (C-4,5), 130.5 (C-4a,10a),
134.1 (C-b), 135.3 (C-8a,9a), 140.9 (C-a), 142.0 (C-9), 146.4 (BC),
146.9 ppm (C-2,7); UV/Vis (C6H6): lmax (e)=366 (15500), 510 nm
(1050 molꢀ1 dm3 cmꢀ1); fluorescence (C6H6): lex =366 nm, lem =610 nm;
MS (MALDI): m/z (%): 753 (100) [M+]; elemental analysis calcd (%)
for C56H58B2 (752.64)ꢄ0.05CHCl3 (119.38): C 88.74, H 7.71; found: C
88.49, H 7.76 (the relative amount of CHCl3 present in the sample was
when irradiated with UV light (lex =366 nm). 1H NMR (400.1 MHz,
C6D6): d=1.23 (s, 36H; CACHTUNRGTNE(UNG CH3)3), 2.08 (s, 12H; p-CH3), 6.91 (m, 12H;
3
4
m-H, H-b), 7.28 (dd, J
3J
(H,H)=8.5 Hz, 8H; o-H), 7.86 (m, 4H; H-a), 8.11 (d, J
4H; H-4,5), 8.69 ppm (d, 4J(H,H)=1.8 Hz, 4H; H-1,8); 11B NMR
(96.3 MHz, C6D6): d=n.o.; 13C{1H} NMR (75.5 MHz, C6D6): d=20.7 (p-
CH3), 31.0 (C(CH3)3), 35.3 (C(CH3)3), 120.2 (C-1,8), 121.3 (o-C), 124.6
ACHUTGTNRENNUG(H,H)=9.0, JCAHTUNGTNER(NUGN H,H)=1.8 Hz, 4H; H-3,6), 7.43 (d,
3
G
ACHTUNGTREN(NUNG H,H)=9.0 Hz,
AHCTUNGTRENNUNG
N
ACHTUNGTRENNUNG
1
(C-3,6), 2ꢄ130.4 (m-C, p-C), 130.8 (C-8a,9a), 131.0 (C-4,5), 134.1 (C-
4a,10a), 134.7 (C-b), 138.6 (C-9), 141.2 (C-a)*, 142.0 (C-10)*, 146.7 (BC),
147.2 (i-C), 148.9 ppm (C-2,7); UV/Vis (C6H6): lmax (e)=359 (20300), 429
confirmed by H NMR spectroscopy).
Synthesis of 9: Following the general procedure, 9 was synthesized from
nBuLi (2.2m in hexane, 1.5 mL, 3.3 mmol), 6 (1000 mg, 2.245 mmol), and
3 (370 mg, 1.11 mmol); solvents were Et2O (40 mL) and toluene (25 mL).
The crude product was washed with nondried MeOH (3ꢄ30 mL) and
nondried pentane (3ꢄ30 mL) and further purified by flash column chro-
matography (silica gel 60). A mixture of nondried hexane and CHCl3
(10:1) was employed as the mobile phase until all fractions showing blue
fluorescence had been eluted from the column. Afterwards the polarity
of the mobile phase was increased (hexane/CHCl3 =2:1) to obtain 9 as a
red solid (857 mg, 85%). Both in solution and in the solid state, 9 shows
an intense red fluorescence when irradiated with UV light (lex =366 nm).
Single crystals of 9 suitable for X-ray diffraction were obtained by gas-
phase diffusion of hexane into a concentrated solution of 9 in CH2Cl2.
Rf =0.42 (silica gel, hexane/CHCl3 =3:1); 1H NMR (300.0 MHz, C6D6):
(14300), 556 nm (2200 molꢀ1 dm3 cmꢀ1); fluorescence (solid state): lex
=
593 nm, lem =675 nm; MS (MALDI): m/z (%): 1143 (100) [M+]; elemen-
tal analysis calcd (%) for C84H84B2N2 (1143.20)ꢄ0.3CH2Cl2 (84.93): C
86.64, H 7.30, N 2.40; found: C 86.38, H 7.32, N 2.23 (the relative
amount of CH2Cl2 present in the sample was confirmed by 1H NMR
spectroscopy). *This signal was broadened beyond detection in the
13C{1H} NMR spectrum; its chemical shift value was determined by using
the cross peak in the HSQC or HMBC spectrum.
Crystal structure analyses: All crystals (except 6, which was measured on
a Siemens CCD three-circle diffractometer) were measured on a STOE
IPDS-II diffractometer with graphite-monochromated MoKa radiation.
An empirical absorption correction with program PLATON[46] was per-
formed for 1, 2, and 6–9. The structures were solved by direct methods[47]
and refined with full-matrix least-squares on F2 with the program
SHELXL97.[48] H atoms were placed on ideal positions and refined with
fixed isotropic displacement parameters by using a riding model.
3
4
d=1.23 (s, 36H; C
(H,H)=1.5 Hz, 4H; H-3,6), 7.38 (t, 3J
(app. t, 3J(H,H)=7.3 Hz, 4H; m-H), 7.79 (m, 4H; H-a), 7.82 (d, 3J-
(H,H)=7.3 Hz, 4H; o-H), 8.10 (d, 3J
(H,H)=9.0 Hz, 4H; H-4,5),
8.15 ppm (d, 4J(H,H)=1.5 Hz, 4H; H-1,8); 11B NMR (96.3 MHz, C6D6):
d=n.o.; 13C{1H} NMR (75.5 MHz, C6D6): d=30.9 (C
(CH3)3), 35.2 (C-
(CH3)3), 122.1 (C-1,8), 124.3 (C-3,6), 128.6 (p-C), 128.8 (m-C), 130.6 (C-
T
ACHTUNGTREN(NUNG H,H)=9.0, J-
A
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
In 2,7-DTBA, a hexane molecule is disordered over two positions with
equal occupancies. The disordered atoms were isotropically refined; the
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
ꢀ
C C bond lengths were restrained to 1.50(1) ꢃ and the C-C-C distances
ACHTUNGTRENNUNG
to 2.50(1) ꢃ.
4,5,8a,9a), 132.0 (o-C), 132.7 (C-4a,10a), 134.6 (C-b), 137.1 (C-9), 140.5
(i-C), 141.1 (C-a), 141.9 (C-10), 146.8 (BC), 147.7 ppm (C-2,7); UV/Vis
(C6H6): lmax (e)=374 (28200), 531 nm (1500 molꢀ1 dm3 cmꢀ1); fluores-
cence (C6H6): lex =374 nm, lem =635 nm; MS (MALDI): m/z (%): 905
(100) [M+]; elemental analysis calcd (%) for C68H66B2 (904.87)ꢄ
0.1CHCl3 (119.38): C 89.21, H 7.27; found: C 89.31, H 7.32 (the relative
The molecule of 1 is located on a crystallographic center of inversion. As
a result, the Br atom and the H atom in the para position are mutually
disordered with equal occupancies.
The absolute structure of 2 has been determined: Flack x parameter
ꢀ0.006(9).
1
amount of CHCl3 present in the sample was confirmed by H NMR spec-
The crystal of 5 was a nonmerohedral twin with three domains (fractional
troscopy).
contributions for the minor domains: 0.167(4) and 0.176(5)). The atoms
Synthesis of 10: Following the general procedure, 10 was synthesized
ꢀ
of the solvent C6H6 were isotropically refined. Equivalent C C bond
lengths and C-C-C bond angles were restrained to be equal.
from nBuLi (2.8m in hexane, 0.44 mL, 1.2 mmol),
7
(510 mg,
0.826 mmol), and 3 (138 mg, 0.414 mmol); solvents were Et2O (40 mL)
and toluene (25 mL). The crude product was washed with nondried
MeOH (3ꢄ30 mL) and nondried pentane (3ꢄ30 mL), redissolved in dry
C6H6, and precipitated into pentane to obtain 10 as a red solid (423 mg,
82%). Both in solution and in the solid state, 10 shows an intense red flu-
orescence when irradiated with UV light (lex =366 nm). Single crystals of
10 were grown by slow evaporation of a concentrated solution of 10 in
THF under inert conditions. 1H NMR (400.1 MHz, C6D6): d=1.16 (s,
In 6, one tert-butyl group is disordered over two positions with a site oc-
cupation factor of 0.643(8) for the major occupied site.
In 9, two tert-butyl groups are disordered over two positions with site oc-
cupation factors of 0.751(6) and 0.640(6) for the major occupied sites.
These disordered atoms were isotropically refined. One CH2Cl2 molecule,
which was anisotropically refined, is disordered about a center of inver-
sion with equal occupancies. Due to the absence of anomalous scatterers
in 9, the absolute structure could not be determined and Friedel pairs
were merged.
36H; CACHTUNGTRENNUNG(CH3)3), 2.08 (s, 12H; o-CH3), 2.18 (s, 12H; p-CH3), 2.49 (s, 12H;
o-CH3), 6.70 (brs, 4H; m-H), 6.93 (br.s, 4H; m-H), 6.96 (m, 4H; H-b),
7.21 (dd, 3J
G
G
The crystal of 10 was very weakly diffracting and the atoms B(2), C(10),
and C(12) were restrained to an isotropic behavior.
3
4
8.08 (d, J
N
H-1,8); 11B NMR (96.3 MHz, C6D6): d=n.o.; 13C{1H} NMR (100.6 MHz,
C6D6): d=21.3 (p-CH3), 24.1, 24.2 (o-CH3), 31.1 (C(CH3)3), 35.3 (C-
ACHTUNGTRENNUNG(CH3)3), 124.0 (C-3,6), 124.8 (C-1,8), 129.5, 129.7 (m-C), 130.9 (C-4,5),
CCDC-887748 (4), CCDC-887749 (5), CCDC-887750 (9), CCDC-887751
(10), CCDC-887752 (1), CCDC-887753 (2), CCDC-887754 (6), CCDC-
887755 (7), CCDC-887756 (8), CCDC-887757 (2,6-DTBA), CCDC-
887758 (2,7-DTBA), CCDC-887759 (9-Ph-2,7-DTBA), CCDC-887760 (9-
Mes2B-2,7-DTBA), and CCDC-887761 (9-Tol2N-2,7-DTBA) contain the
supplementary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
132.7 (C-4a,10a), 134.6 (C-b), 134.7 (C-8a,9a), 139.7 (p-C), 140.6 (o-C),
140.9 (C-a), 141.4 (o-C), 145.5, 145.5 (C-9, C-10), 146.5 (BC), 146.8 (i-C),
147.4 ppm (C-2,7); UV/Vis (C6H6): lmax (e)=346 (17400), 442 (18300),
536 nm (2000 molꢀ1 dm3 cmꢀ1); fluorescence (C6H6): lex =442 nm, lem
=
644 nm; MS (MALDI): m/z (%): 1249 (100) [M+]; elemental analysis
calcd (%) for C92H100B4 (1248.96): C 88.47, H 8.07; found: C 87.75, H
8.10.
Synthesis of 11: Following the general procedure, 11 was synthesized
from nBuLi (1.5m in hexane, 1.8 mL, 2.7 mmol),
8
(1000 mg,
Acknowledgements
1.771 mmol), and 3 (295 mg, 0.884 mmol); solvents were Et2O (50 mL)
and toluene (20 mL). The crude product was washed with nondried
MeOH (3ꢄ30 mL), nondried CH2Cl2 (3ꢄ30 mL), and nondried pentane
(3ꢄ30 mL) to obtain pure 11 as a dark solid (563 mg, 56%). In the solid
state, but not in solution, 11 shows an intense wine-red fluorescence
This work was supported by the Beilstein-Institut, Frankfurt am Main
(Germany), within the research collaboration NanoBiC.
15404
www.chemeurj.org
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 15394 – 15405

Boron-Doped Tri(9,10-anthrylene)s
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Received: June 27, 2012
Published online: October 29, 2012
Chem. Eur. J. 2012, 18, 15394 – 15405
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
15405