Functionalization of boron-doped tri(9,10-anthrylene)s

Article abstract of DOI:10.1016/j.tet.2013.06.036

9,10-Dianthryl-9,10-dihydro-9,10-diboraanthracene derivatives bearing two peripheral chloro (3) or bromo (5) substituents have been synthesized. X-ray crystallography reveals a mutually perpendicular orientation of the individual planes within each of the molecules. The two bromo substituents in 5 can be replaced by NMe₂ (6) or thiophen-2-yl (7) groups using Me ₃Sn-NMe₂ or nBu₃Sn-C₄H₃S and Pd-mediated C-N or C-C coupling protocols. Compounds 3, 5, and 7 show TICT photoluminescence and a pronounced positive solvatochromism in agreement with a charge-separated excited state.

Full text of DOI:10.1016/j.tet.2013.06.036

Tetrahedron 69 (2013) 7073e7081
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Functionalization of boron-doped tri(9,10-anthrylene)s
Claas Hoffend, Kai Schickedanz, Michael Bolte, Hans-Wolfram Lerner,
*
Matthias Wagner
€
€
Institut fur Anorganische Chemie, J.W. Goethe-Universitat Frankfurt, Max-von-Laue-Strasse 7, D-60438 Frankfurt (Main), Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 May 2013
Received in revised form 5 June 2013
Accepted 11 June 2013
Available online 18 June 2013
9,10-Dianthryl-9,10-dihydro-9,10-diboraanthracene derivatives bearing two peripheral chloro (3) or
bromo (5) substituents have been synthesized. X-ray crystallography reveals a mutually perpendicular
orientation of the individual planes within each of the molecules. The two bromo substituents in 5 can be
replaced by NMe2 (6) or thiophen-2-yl (7) groups using Me3SneNMe2 or nBu3SneC4H3S and Pd-
mediated CeN or CeC coupling protocols. Compounds 3, 5, and 7 show TICT photoluminescence and
a pronounced positive solvatochromism in agreement with a charge-separated excited state.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Organoboranes
Charge transfer
Electrochemistry
Halogenation reactions
Fluorescence
Stille coupling reaction
1. Introduction
molecular design for efficient electron storage3 (ii) After the olig-
omers have been charged with one electron per anthryl fragment,
Functional organic materials are becoming increasingly impor-
tant for applications in optoelectronic devices due to their low
price, light weight, high mechanical flexibility, and pronounced
the resulting oligoanions tend to adopt high-spin states,4,5 a key
precondition for the occurrence of organic ferromagnetism6 (iii)
Oligo(9,10-anthrylene)s (n¼2, 3, 4) have shown to undergo pho-
toinduced intramolecular charge separation in polar solvents.7 The
elucidation of the underlying photodynamic mechanisms is rele-
vant for an understanding of primary processes of photosynthesis8
and for the design of artificial light-harvesting systems.
tunability.1 Most of these materials contain extended
p-electron
systems that are composed of alkenylene, alkynylene, and/or
(hetero)arylene fragments. The overall electronic properties are
largely determined by the resulting conjugation lengths: Extensive
conjugative interaction leads to small HOMOeLUMO gaps and,
upon charge injection, to full charge delocalization. A prominent
example is polyacetylene, which can be converted to the metallic
state upon oxidative doping.2 Disruption of conjugation, on the
other hand, provides an array of electronically decoupled subunits
in which the kinetics of electron transfer between multiple po-
tential minima decides on the charge distribution. Electronic
structures of this kind are realized in compounds like oligo(9,10-
As exemplified by 9,90-bianthryl (A), ultraviolet excitation of the
compound in a polar environment leads to an equilibrium between
the nonpolar, locally excited species ALE and the charge-transfer
species ACT (Fig. 1). Dual fluorescence is observed when the ALE
/
ACT mixture relaxes to the electronic ground state.9,10 Given that the
two anthryl halves of A are chemically equivalent (i.e., there is no
natural electron donor or acceptor fragment in the molecule), the
excited-state charge separation is essentially solvent-induced and
consequently the CT spectrum is sensitive to the solvent configu-
ration about the fluorophore.
anthrylene)s, in which the individual
p systems adopt mutually
perpendicular conformations due to steric congestion.3,4
Oligo(9,10-anthrylene)s have application potential in a variety of
areas: (i) each anthryl fragment can act as two-electron acceptor
and the electrostatic interaction between the individual redox
centers is small. Oligo(9,10-anthrylene)s thus possess a promising
Attempts have been made to increase the charge-transfer rates
through the combination of polycyclic p-conjugated moieties with
more complementary electron donor/acceptor properties. Exam-
ples are 9-(90-anthryl)carbazole (B)10 and 9,10-dianthryl-9,10-
dihydro-9,10-diboraanthracenes C (Fig. 1).11
In the case of C-type compounds, the 9,10-dihydro-9,10-
diboraanthracene (DBA) cores are isoelectronic with the ex-
tremely reactive anthracene dication.12 As a result, highly efficient
* Corresponding author. E-mail address: Matthias.Wagner@chemie.uni-
frankfurt.de (M. Wagner).
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.06.036

7074
C. Hoffend et al. / Tetrahedron 69 (2013) 7073e7081
that the resulting 9,10-dianthryl-DBAs are formed as pairs of
atropisomers.11 We are therefore focusing solely on 2,7-di-tert-
butylanthracene in the present study, which will lead to com-
pounds C with either four inward-pointing (located at C-2 and C-7
in Fig. 1) or four outward-pointing tBu groups (located at C-3 and
C-6 in Fig. 1). The selection of the preferential substitution pattern
will depend on the specific case: (i) inward-pointing tBu groups
provide additional kinetic shielding to the boron atoms and
therefore help to render the molecules stable toward air and
moisture. Moreover, they stay out of the way of the substituents R,
which are therefore less restricted in size. (ii) Outward-pointing tBu
groups offer the possibility to synthesize boron-doped graphene
flakes through dehydrogenative CeC coupling reactions between
the DBA core and the two anthryl fragments.13,14
Our initial attempts at the synthesis of soluble 9,10-dianthryl-
DBAs bearing bromine atoms at the peripheral positions of their
anthryl substituents relied on 9,10-dibromo-2,7-di-tert-butylan-
thracene (9,10-Br2-2,7-DTBA) as starting material. The compound is
readily available from 2,7-di-tert-butylanthracene (2,7-DTBA)11 and
2 equiv of N-bromosuccinimide in the presence of a catalytic
amount of FeCl3 (cf. Supplementary data for full details). The in-
troduction of the anthryl substituent into the DBA core requires
Li/Br exchange on 9,10-Br2-2,7-DTBA and a subsequent nucleophilic
substitution reaction with 9,10-dibromo-DBA (2; Scheme 1). We
hoped to be able to discriminate between the two bromine atoms
by using 1 equiv of the sterically demanding tert-butyllithium in
the Li/Br exchange reaction. However, after quenching of the
anthryllithium regent with H2O no preparatively useful selectivity
between 9- and 10-lithiation was observed. We therefore aban-
doned 9,10-Br2-2,7-DTBA and turned our attention to 10-bromo-9-
chloro-2,7-di-tert-butylanthracene (1; Scheme 1).
2.1. Dichlorinated 9,10-dianthryl-DBA with outward-pointing
tert-butyl groups
Treatment of 2,7-DTBA11 with 1 equiv of N-chlorosuccinimide/
HCl results in the selective monochlorination at the 9-position of
the anthracene scaffold (9-Cl-2,7-DTBA). 9-Cl-2,7-DTBA was sub-
sequently transformed into 1 using N-bromosuccinimide/FeCl3 (cf.
Supplementary data for more details). Li/Br exchange with 1 equiv
of n-butyllithium, followed by the addition of 9,10-dibromo-DBA
(2) gave the dichlorinated 9,10-dianthryl-DBA 3 in 57% yield
(Scheme 1).
Fig. 1. Equilibrium between the locally excited (LE) and the charge-transfer (CT) state
in electronically excited 9,90-bianthryls A, 9-(90-anthryl)carbazole B, and substituted
9,10-dianthryl-9,10-dihydro-9,10-diboraanthracenes C.
donoreacceptoredonor triads are formed, the fluorescence spectra
of which show no indication of emission from locally excited
anthryl states but exclusively the radiative back transition from the
charge-transfer state to the ground state.11 Substituents R in the 10-
position of the anthryl fragments provide excellent set-screws for
a fine-tuning of the emission wavelength of C, but also a means to
incorporate C into more extended aggregates.
In the past, we have already introduced phenyl rings as well as
dimesitylboryl and N,N-di(p-tolyl)amino groups as substituents R.
All of them were attached to the anthracene scaffold prior to the
final assembly of the triad.11 The purpose of this paper is to describe
facile routes to halogeno-substituted derivatives C (R¼Cl, Br),
which allow for an a posteriori modification of the molecule. We
will show that Pd-catalyzed Stille reactions are well-suited for the
replacement of R¼Br by NMe2 or C4H3S (thiophen-2-yl) and we will
describe key optoelectronic properties of the compounds obtained.
The 1H NMR spectrum (CDCl3) of 3 shows one singlet at
1.50 ppm (integrating 36H) for the four magnetically equivalent
tBu groups, two multiplets for the DBA core (7.29 ppm (4H),
7.42 ppm (4H)), and three signals for the anthryl substituents
(3ꢀ4H). The 13C{1H} NMR spectrum is also in full agreement with
the proposed highly symmetric molecular structure; a resonance at
128.5 ppm can be assigned to the Cl-bonded carbon atom. Com-
pound 3 does not exhibit a 11B{1H} NMR resonance, likely because
the signal is broadened beyond detection.
2.2. Difunctionalized 9,10-dianthryl-DBAs with inward-
pointing tert-butyl groups
A seemingly straightforward strategy for the preparation of
dihalogenated 9,10-dianthryl-DBAs with inward-pointing tert-bu-
tyl groups would be to invert the reaction sequence leading to 1, i.e.,
to take 9-bromo-2,7-di-tert-butylanthracene (9-Br-2,7-DTBA)11
and treat it with N-chlorosuccinimide/HCl. In our hands, this ap-
proach led to product mixtures and partial Br/Cl exchange and was
therefore not pursued further. Instead, we conducted an a posteri-
ori functionalization of the 9,10-dianthryl-DBA 411 with 2 equiv of
N-bromosuccinimide/FeCl3 to obtain the dibrominated species 5 in
90% yield (Scheme 1). An advantage of compound 5 over 3 lies in
2. Results and discussion
9,10-Dianthryl-DBAs (C) have to be equipped with alkyl side-
chains in order to obtain soluble compounds. In the past, we se-
lected tert-butyl (tBu) groups for this purpose, because 2,6- and 2,7-
di-tert-butylanthracene are easily available and help to accomplish
a compromise between good solubility and high crystallinity.11
2,6-Di-tert-butylanthracene, however, bears the disadvantage

C. Hoffend et al. / Tetrahedron 69 (2013) 7073e7081
7075
Scheme 1. Synthesis of compounds 3, 5, 6, and 7. Reagents and conditions: (i) nBuLi (2.2 equiv), Et2O, ꢂ78 ꢁC/rt, 1 h. (ii) Et2Oetoluene, ꢂ78 ꢁC/rt, 12 h. (iii) NBS (2 equiv), FeCl3,
CHCl3, rt, 48 h. (iv) 6: Me3SneNMe2 (2.2 equiv), (tBu3P)2Pd, toluene, 80 ꢁC, 24 h; 7: nBu3SneC4H3S (5.5 equiv), (tBu3P)2Pd, toluene, 80 ꢁC, 48 h. NBS¼N-bromosuccinimide.
the fact that arylbromides tend to undergo Pd-catalyzed coupling
reactions more readily than arylchlorides. Given that 5 is an aryl-
borane and that Suzuki-coupling protocols are based on the
cleavage of BeC bonds, we decided against the Suzuki reaction and
relied on the Stille coupling for further derivatization. The bromine
atoms in 5 can efficiently be replaced by NMe2 groups or thiophen-
2-yl rings upon the Pd-mediated reaction with Me3SneNMe2 or
nBu3SneC4H3S, respectively (cf. 6, 7; Scheme 1).
The NMR spectra (C6D6) of 5 reveal similar general features as
those of the non-brominated compound 4.11 Noteworthy differ-
ences are (i) the absence of the H-10 signal in the proton spectrum
of 5 and (ii) a downfield shift of the H-4,5 resonances from 8.10 ppm
(4) to 8.87 ppm (5).
are detected at 126.9 (C-
a), 127.6 (C-b), 129.7 (C-g), and 140.6 ppm
(C- ).
d
2.3. X-ray crystal structure analyses of 3, 5, 6, and 7
Selected crystallographic data for 3, 5, 6, and 7 are summarized
in Tables 1 and 2.
The solid-state structure of 3 reveals two chloro substituents at
C(2) and C(2A) as well as four outward-pointing tBu groups (Fig. 2).
Dihedral angles C(11)C(1)C(21)//C(31)B(1)C(32A) of 70.3(3)ꢁ un-
equivocally prove the lack of p-conjugation between the associated
electron clouds. Only negligible differences are evident between
the exocyclic and endocyclic BeC bond lengths and none of the
bond angles about the trigonal-planar boron centers deviates by
more than 1ꢁ from the ideal angle of 120ꢁ.
Upon going from 5 to 6, NMe2 signals appear at 3.38 ppm and
45.1 ppm in the 1H and 13C{1H} NMR spectra, respectively. The
successful introduction of the amino group is further indicated by
the absence of a C(aryl)eBr resonance at 123.0 ppm (5) and the
presence of a characteristic C(aryl)eN resonance at 145.8 ppm (6).
13C{1H} NMR spectroscopy provides a useful diagnostic tool to map
The dibrominated derivative 5 crystallizes from toluene with
two crystallographically independent molecules in the asymmetric
unit (5A, 5B). Since most structural parameters of 5A and 5B are
closely similar, only the molecular structure of 5A is shown in Fig. 2.
5A possesses an inversion center in the solid state, which renders
the two anthryl substituents coplanar; the same is true for 5B. The
dihedral angles C(11)C(1)C(21)//C(31)B(1)C(32A) between the
anthryl fragments and the DBA cores differ between 5A (85.4(2)ꢁ)
and 5B (70.2(3)ꢁ). Moreover, the anthryl scaffolds in 5A are signifi-
cantly twisted with dihedral angles C(12)C(13)C(14)//C(22)C(23)
C(24) of 16.1(7)ꢁ, whereas 5B contains essentially planar anthryl
moieties (C(12)C(13)C(14)//C(22)C(23)C(24)¼0.6(8)ꢁ). The space-
the
p
-electron density distribution in acenes15 and thereby to
evaluate the donor effect of the NMe2 groups on the anthryl frag-
ments. Compared to 4, the carbon atoms in the positions ortho and
para to the NMe2 substituents experience upfield shifts of only
1.5 ppm (C-4a, 10a) and 1.9 ppm (C-9), which suggests that the þM
effect of the amino groups in 6 is rather small.
The 1H and 13C{1H} NMR spectra of the 9,10-dianthryl-DBA core
of 7 do not deviate significantly from those of 4. An H-10 signal is,
however, absent in the proton spectrum of 7; three doublets of
doublets appear instead, which are assignable to the thiophen-2-yl
rings (7.13, 7.29, 7.36 ppm). The corresponding 13C NMR resonances
filling models of 5A/5B clearly reveal that the vacant boron
bitals are efficiently shielded not only by the hydrogen atoms at
C(12) and C(22) but also by the tBu groups. We find B(1)eC(1) bond
p or-

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C. Hoffend et al. / Tetrahedron 69 (2013) 7073e7081
Table 1
Crystallographic data for 3 and 5
3
5
Formula
Mr
C56H56B2Cl2ꢀ4 CHCl3
1299.00
C56H56B2Br2ꢀ1.5 C7H8
1048.65
Color, shape
Red, block
Red, plate
T [K]
Radiation,
173(2)
173(2)
MoKa, 0.71073
ꢀ
l
[A]
MoK
a
, 0.71073
Crystal system
Space group
Monoclinic
P21/c
14.7190(7)
9.0634(7)
23.9955(12)
90
91.331(4)
90
3200.2(3)
2
Triclinic
Pꢂ1
ꢀ
a [A]
9.9122(10)
15.0820(13)
19.324(2)
79.074(8)
84.207(8)
77.152(8)
2760.3(5)
2
ꢀ
b [A]
ꢀ
c [A]
a
b
g
[ꢁ]
[ꢁ]
[ꢁ]
3
ꢀ
V [A ]
Z
Dcalcd [g cmꢂ3
F(000)
]
1.348
1336
0.639
1.262
1094
1.509
m
[mmꢂ1
]
Crystal size [mm3]
Rflns collected
0.42ꢀ0.38ꢀ0.22
0.49ꢀ0.28ꢀ0.07
38,217
35,615
Independent rflns (Rint
)
5647 (0.0895)
5647/0/343
1.077
10,373 (0.1020)
10,373/19/637
1.013
Data/restraints/parameters
GOF on F2
R1, wR2 [I>2
R1, wR2 (all data)
s
(I)]
0.0858, 0.2316
0.1071, 0.2461
1.255, ꢂ1.018
0.0604, 0.1090
0.1050, 0.1219
0.625, ꢂ0.353
Largest diff peak and hole
ꢀꢂ3
[e A
]
Table 2
Crystallographic data for 6 and 7
6
7
Formula
Mr
C60H68B2N2
838.78
C64H62B2S2ꢀ2.5 C6H6
1112.14
Color, shape
T [K]
Gray, plate
173(2)
Red, block
173(2)
MoKa, 0.71073
ꢀ
Radiation,
l
[A]
MoK
a, 0.71073
Crystal system
Space group
Monoclinic
P21/c
9.1712(12)
25.295(2)
10.6954(13)
90
94.101(10)
90
2474.8(5)
2
Triclinic
Pꢂ1
ꢀ
a [A]
15.2919(9)
15.3394(9)
16.6380(10)
72.752(4)
85.225(5)
60.642(4)
3239.5(4)
2
ꢀ
b [A]
ꢀ
c [A]
Fig. 2. Molecular structures of 3 (top) and 5A (bottom) in the solid state; displacement
a
b
g
[ꢁ]
[ꢁ]
[ꢁ]
ellipsoids at the 50% probability level, H atoms omitted for clarity. Selected bond
ꢀ
lengths (A), bond angles (deg), and dihedral angle (deg): 3: Cl(1)eC(2) 1.747(4), B(1)e
C(1) 1.575(6), B(1)eC(31) 1.566(6), B(1)eC(32A) 1.555(6), C(31)eC(32) 1.431(5); C(1)e
B(1)eC(31) 119.0(3), C(1)eB(1)eC(32A) 121.0(3), C(31)eB(1)eC(32A) 120.0(3); C(11)
C(1)C(21)//C(31)B(1)C(32A) 70.3(3). Symmetry transformation used to generate
equivalent atoms: A: ꢂxþ1, ꢂy, ꢂzþ1. 5A: Br(1)eC(2) 1.909(3), B(1)eC(1) 1.594(5),
B(1)eC(31) 1.556(6), B(1)eC(32A) 1.551(6), C(31)eC(32) 1.423(5); C(1)eB(1)eC(31)
121.1(3), C(1)eB(1)eC(32A) 119.3(3), C(31)eB(1)eC(32A) 119.6(3); C(11)C(1)C(21)//
C(31)B(1)C(32A) 85.4(2). Symmetry transformation used to generate equivalent atoms:
A: ꢂxþ1, ꢂyþ1, ꢂzþ1.
3
ꢀ
V [A ]
Z
Dcalcd [g cmꢂ3
F(000)
]
1.126
904
0.063
1.140
1186
0.126
m
[mmꢂ1
]
Crystal size [mm3]
Rflns collected
0.50ꢀ0.25ꢀ0.15
0.60ꢀ0.55ꢀ0.53
27,406
43,588
Independent rflns (Rint
)
5062 (0.1582)
5062/0/292
1.008
0.0784, 0.1370
0.1561, 0.1642
0.197, ꢂ0.277
12,449 (0.0397)
12,449/18/746
1.034
0.0630, 0.1635
0.0802, 0.1738
0.540, ꢂ0.461
Data/restraints/parameters
Likely for steric reasons, the NMe2 group does not lie in the plane
of its respective anthryl fragment. We observe a concomitant dis-
order of the NMe2 groups indicating that a proportion of 60% adopts
a pyramidalized configuration and only 40% are planar. The dihedral
angle between the planar NMe2 moieties and their adjacent anthryl
groups is around 60ꢁ. The N(1)eC(2) bond length amounts to
GOF on F2
R1, wR2 [I>2
s(I)]
R1, wR2 (all data)
Largest diff peak and hole
ꢀꢂ3
[e A
]
ꢀ
ꢀ
ꢀ
lengths of 1.594(5) A in 5A and 1.587(5) A in 5B, which can be
1.418(4) A and is thus considerably larger than comparable
compared with an exocyclic BeC bond length of 1.577(7) A in 411 or
ꢀ
Me2NeC(phenyl) bond lengths in a variety of N,N-dimethylaniline
ꢀ
16
ꢀ
1.575(6) A in the sterically less congested chloro-derivative 3. We
derivatives with in-plane amino groups (1.375e1.388 A). Our
crystallographic results therefore support the previous in-
terpretation of the 13C{1H} NMR spectrum of 6, namely, that the
therefore conclude that neither the presence of electronegative
substituents at C(2) nor the positions of the tBu substituents (i.e.,
inward- vs outward-pointing) have a systematic influence on the
B(1)eC(1) bonds.
The solid-state structure of 6 (Fig. 3) shows an anthryl twist
(C(12)C(13)C(14)//C(22)C(23)C(24)¼14.9(6)ꢁ) reminiscent of 5A. At
the same time, the anthryl moieties deviate substantially from
a position perpendicular to the DBA core (C(11)C(1)C(21)//C(31)
B(1)C(32A)¼71.9(1)ꢁ), as it has already been observed for 5B.
NMe2 substituents in 6 do not exert their full p-donor capacity.
The crystal lattice of the thienyl derivative 7 also contains two
crystallographically independent molecules in the asymmetric unit
(7A, 7B; cf. Fig. 3 for a plot of 7A). Both molecules are centrosym-
metric; the dihedral angles between the thiophen-2-yl rings and
their respective 9,10-anthrylene moieties amount to 79.3(1)ꢁ (7A)
and 85.8(2)ꢁ (7B). All other key structural parameters of 7A and 7B

C. Hoffend et al. / Tetrahedron 69 (2013) 7073e7081
7077
Fig. 4. Cyclic voltammograms of 6 (solid line) and 7 (dashed line). THF solutions, rt,
[nBu4N][PF6] (0.1 M), scan rate 200 mV sꢂ1, versus FcH/FcHþ
.
chemical reduction of 6 with elemental potassium first led to the
characteristic UV/vis signature of the DBA monoanion (lmax¼539,
750, 835, 945 nm).11,17 Within hours, these bands gradually vanished
and new absorptions developed at lmax¼474 and 665 nm, which are
attributable to the DBA dianion (cf. Supplementary data for plots of
the UV/vis spectra).11,17 A third, irreversible reduction event occurs
in the cyclic voltammogram of 6 at a peak potential of Epc¼ꢂ3.30 V.
Compared to the anthryl reduction of 4 (Epc¼ꢂ3.21 V) there is only
a minor cathodic shift, again indicating that the þM effect of the
NMe2 substituents is small.
In the case of 7, DBA reduction takes place at potential values
E1/2 of ꢂ1.69 and ꢂ2.50 V; these transitions are reversible as long as
Fig. 3. Molecular structure of 6 (top) and 7A (bottom) in the solid state; displace-
the switching potential is not taken below ꢂ2.8
V (cf.
ment ellipsoids at the 50% probability level, H atoms omitted for clarity. Selected
ꢀ
bond lengths (A), bond angles (deg), and dihedral angles (deg): 6: N(1)eC(2)
Supplementary data for more information). Two additional irre-
1.418(4), B(1)eC(1) 1.575(4), B(1)eC(31) 1.561(4), B(1)eC(32A) 1.562(4), C(31)e
C(32) 1.418(4); C(1)eB(1)eC(31) 120.2(3), C(1)eB(1)eC(32A) 120.5(3), C(31)eB(1)e
C(32A) 119.2(2); C(11)C(1)C(21)//C(31)B(1)C(32A) 71.9(1), C(16)C(2)C(26)//C(17)N(1)
C(18) 60.2(4). Symmetry transformation used to generate equivalent atoms: A:
ꢂxþ1, ꢂyþ1, ꢂzþ1. 7A: B(1)eC(1) 1.589(3), B(1)eC(31) 1.556(3), B(1)eC(32A)
1.557(3), C(2)eC(41) 1.480(3), C(31)eC(32) 1.426(3); C(1)eB(1)eC(31) 122.0(2),
C(1)eB(1)eC(32A) 119.4(2), C(31)eB(1)eC(32A) 118.6(2); C(11)C(1)C(21)//C(31)B(1)
C(32A) 85.4(2), C(16)C(2)C(26)//S(42)C(41)C(45) 79.3(1). Symmetry transformation
used to generate equivalent atoms: A: ꢂxþ2, ꢂy, ꢂz.
versible redox waves are visible at Epc¼ꢂ3.10 and ꢂ3.40 V.
2.5. Optical properties of 3, 5, 6, and 7
Table 4 summarizes key UV/vis absorption and emission data of
the 9,10-dianthryl-DBAs under investigation here. Because of the
poor solubility of 3 in C6H12, C6H6, and CH2Cl2, measurements on
this compound were limited to the solvent CHCl3. The UV/vis
spectra of 3 and 5 in CHCl3 are essentially identical, each of them
showing two absorption bands. The respective short-wavelength
band at lmax¼377 nm has a vibrational fine structure and a simi-
lar position and line shape as the 2,7-DTBA absorption.11 It is
are similar to those of the previously discussed molecules and
therefore do not require a detailed discussion.
2.4. Electrochemical investigation of 6 and 7
therefore likely attributable to a local
pep electron transition
*
Compounds 6 and 7 were further characterized by cyclic vol-
tammetry (THF, [nBu4N][PF6] (0.1 M), vs ferrocene/ferrocenium
(FcH/FcHþ)). The redox potentials are compiled in Table 3.
within the anthryl systems. A broad, featureless second band is
observed at lmax¼510 nm both in the absorption spectrum of 3 and
of 5. We assign this band to a charge-transfer transition from the
Table 3
Electrochemical data of 6 and 7a
Table 4
E1/2 in [V] versus FcH/FcHþ
UV/vis absorption and emission data of 3, 5, 6, and 7
[molꢂ1 dm3 cmꢂ1])
lmax(em) [nm]
lex [nm])
fF
a
Solvent
CHCl3
lmax(abs) [nm] (
3
Oxidation
Reduction
(
6
7
þ0.23b
ꢂ1.77, ꢂ2.60, ꢂ3.30c
d
ꢂ1.69, ꢂ2.50, ꢂ3.10c, ꢂ3.40c
3
5
377 (17,000), 510 (1400)
655 (520)
0.03
THF, supporting electrolyte: [nBu4N][PF6] (0.1 M), scan rate: 200 mV sꢂ1
.
a
b
c
C6H12
CHCl3
375 (18,900), 548 (2100)
377 (20,800), 510 (2000)
567 (520)
640 (520)
0.03
0.02
Epa value.
Epc value.
6
7
C6H12
C6H6
371 (23,200), 567 (3400)
371 (25,600), 546 (2600)
d
d
d
d
Compound 6 undergoes an irreversible oxidation process at
Epa¼þ0.23 V. A second ill-defined feature appears at slightly more
anodic potential values (Fig. 4). In the cathodic regime, two re-
versible redox waves are observed at E1/2¼ꢂ1.77 and ꢂ2.60 V. An
assignment of these waves as DBA-centered was achieved by com-
parison with the corresponding redox potentials of compound 4
(E1/2¼ꢂ1.73, ꢂ2.57 V)11 and by UV/vis spectroelectrochemistry:
C6H12
C6H6
THF
373 (21,700), 557 (2300)
375 (25,000), 521 (1600)
374 (22,100), 525 (1500)
376 (28,500), 521 (2300)
582 (520)
624 (520)
680 (520)
700 (520)
0.11
0.09
0.04
0.03
CH2Cl2
a
Quantum yields were determined using a calibrated integration sphere; exci-
tation in the long-wavelength absorption band.

7078
C. Hoffend et al. / Tetrahedron 69 (2013) 7073e7081
more electron-rich anthryl donor to the electron-poor DBA accep-
tor. This band is responsible for the red color of the compounds.
The fluorescence spectra of 3 and 5 exhibit broad emissions at
lmax¼655 and 640 nm (CHCl3), respectively, when the molecules
are excited either at lex¼366 nm (anthryl absorption) or
lex¼520 nm (charge-transfer transition).
hydrolysis studies usually provide first indications of de-
composition not earlier than 6 h after the addition of H2O (NMR
spectroscopic control). Fluorescence spectroscopy, however, which
is even more revealing than NMR spectroscopy because the sam-
ples are more dilute by several orders of magnitude, often shows
a weak emission band already soon after sample preparation,
which does not belong to the actual C-type derivative. In the case of
In summary, neither the nature of the halogeno substituent (3:
Cl; 5: Br), nor the position of the tBu groups (3: outward-pointing;
5: inward-pointing) has a noteworthy effect on the absorption or
emission behavior of these 9,10-dianthryl-DBAs.
C
(R¼Ph; all tert-butyl groups outward-pointing), this band
appeared at lem¼440 nm upon irradiation into the anthryl ab-
sorption and gained in intensity in spectra taken after the closed
cuvette had been left standing for extended periods of time outside
the glove box or if air and moisture had been deliberately admitted.
The slow degradation of C (R¼Ph; all tert-butyl groups outward-
pointing) in the presence of air and moisture leads to complex re-
action mixtures rather than to the exclusive formation of the ex-
pected hydrolysis products 9,10-dihydroxy-DBA (cf. 13; Scheme 2)
and 9-phenyl-2,7-di-tert-butylanthracene (cf. 8; Scheme 2). We
In C6H6 solution, compound 6 (Fig. 5) shows two analogously
shaped absorptions as 3 and 5, i.e., a hypsochromic band at
lmax¼371 nm and a bathochromic band at lmax¼546 nm. The latter
is remarkably red-shifted compared to 4 (lmax¼510 nm).11 Neither
in solution nor in the solid state does compound 6 give rise to an
appreciable fluorescence. A possible explanation could be the well-
known quenching effect of amines on anthracenes.18 We therefore
added 2 equiv of ethereal HCl to a solution of 6 in THF-d8 in an
attempt to sequestrate the nitrogen lone pairs. As a result, the NMe2
proton resonances underwent a downfield shift of 0.17 ppm, the
anthryl H-3,6 signals of 0.11 ppm, and the anthryl H-4,5 resonances
of 0.32 ppm. Moreover, a broad hump at 9.88 ppm, integrating
approximately 2H, appeared in the 1H NMR spectrum (cf.
Supplementary data for plots of the spectra before and after the
addition of HCl$Et2O). Nevertheless, the solution remained non-
fluorescent. We therefore assume that the protonation/deproto-
nation equilibrium is not sufficiently shifted to the side of the
ammonium species to suppress the amine quenching effect.
Fig. 5. UV/vis absorption spectra of 6 (solid line) and 7 (dashed line); UV/vis emission
spectrum (dotted line) of 7; C6H6 solutions.
An addition of excess HCl$Et2O is not advisable, because this
leads to extensive protolysis of the molecular framework. In-
terestingly, the closely related 9,10-dianthryl-DBA derivative bear-
ing NTol2 instead of NMe2 substituents is also essentially non-
fluorescent in solution, but emits bright wine-red light in the
solid state (Tol¼p-tolyl).11
The two absorption bands of the thiophen-2-yl derivative 7 in
C6H6 are red-shifted by about 10 nm compared to those of 4. The
fluorescence bands of 7 are strongly influenced by the solvent po-
larity (lem¼582 (C6H12), 624 (C6H6), 680 (THF), and 700 nm
(CH2Cl2)), in line with the postulated charge-separated excited
state (cf. ACT in Fig. 1). The positive solvatochromism of 7 is ac-
companied by a decrease in the quantum yield, i.e., fF¼0.11 (C6H12),
0.09 (C6H6), 0.04 (THF), and 0.03 (CH2Cl2).
Scheme 2. Photooxidation reactions of 8 and 10 yielding the endoperoxide 9 and the
anthrone 11, respectively; decomposition of 3 in non-dried, non-degassed CDCl3 to
give the anthraquinone 12 and the borinic acid 13. Schematic sketch of the proposed
key intermediate of the degradation reaction of 3. Reagents and conditions: (i) rose
bengal/18-crown-6, CH2Cl2, rt, 24 h. (ii) CDCl3, rt, 3 d.
2.6. Oxidative degradation of 9,10-dianthryl-DBAs
As already noted in a previous publication,11 9,10-dianthryl-
DBAs C (Fig. 1) are pleasingly stable toward moisture. Targeted

C. Hoffend et al. / Tetrahedron 69 (2013) 7073e7081
7079
therefore arrived at the working hypothesis that the initial degra-
4. Experimental section
dation step more likely involves the formation of an endoperoxide
at one of the anthryl moieties than the attack of water on the vacant
boron p orbital. Anthracene endoperoxides are readily obtained
when anthracenes are exposed to light and air: In the first step,
singlet oxygen (1O2) is generated, which then undergoes a cyclo-
addition reaction at the central six-membered ring of the anthra-
cene molecule.19
To test this hypothesis, we irradiated an O2-saturated CH2Cl2
solution of 8 in the presence of air with a fluorescent lamp (9 W, we
also added the photosensitizer rose bengal to boost the formation
of 1O2; Scheme 2). After 24 h, the endoperoxide 9 was isolated in
63% yield after chromatographic workup, thereby indicating that
the substitution pattern present in C (R¼Ph; all tert-butyl groups
outward-pointing) is compatible with the photooxidation reaction
(cf. Supplementary data for more details).
4.1. General materials and methods
Unless otherwise specified, all reactions were carried out under
dry nitrogen or argon using carefully dried and degassed solvents,
flame-dried glassware and Schlenk or glove box techniques. Tolu-
ene, hexane, and C6D6 were dried over Na/benzophenone. CHCl3
and CDCl3 were washed with water (eight times), dried first over
MgSO4, then over CaH2, and freshly distilled prior to use. Solvents
used for UV/vis spectroscopic measurements were purchased as
spectroscopic grade (C6H12, C6H6, THF, CHCl3, CH2Cl2) and degassed
prior to use. Column chromatography was performed using silica
gel 60 (MachereyeNagel). NMR spectra were recorded with Bruker
Avance 300 or Avance 400 spectrometers at rt. Chemical shifts are
referenced to (residual) solvent signals (1H/13C{1H}; C6D6:
d
¼7.16/
We next repeated the experiment using 10-bromo-9-phenyl-
2,7-di-tert-butylanthracene as the starting material (10; Scheme 2).
This time, the endoperoxide was not stable but reacted further to
give 9-hydroxy-9-phenyl-2,7-di-tert-butylanthrone as the final
product (11; cf. Supplementary data for more details including an
X-ray crystal structure analysis of 11).
128.06, CDCl3:
d
¼7.26/77.16) or external BF3$Et2O (11B, 11B{1H}).
Abbreviations: s¼singlet, d¼doublet, dd¼doublet of doublets,
vt¼virtual triplet, m¼multiplet, n.o.¼not observed. Electro-
chemical measurements were performed at rt by using an EG&G
Princeton Applied Research 263A potentiostat with a platinum disc
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
Na/K alloy without added benzophenone at rt and degassed first by
five freezeepumpethaw cycles and then by bubbling helium
through it (40 min). [nBu4N][PF6] (0.1 M) was employed as the
supporting electrolyte. All potential values are referenced
against the ferrocene/ferrocenium couple (E1/2¼0 V). UV/vis ab-
sorption spectra were recorded with a Varian Cary 50 Scan UV/vis
spectrophotometer; for in situ UV/vis monitoring experiments an
immersion probe from Hellma (10.00 mm) was employed. UV/vis
emission spectra were recorded with a Spectrofluorometer FP-
8300 (Jasco). The absolute fluorescence quantum yields (fF) were
determined using a calibrated integrating sphere system (ILF-835
100 mm diameter Integrating Sphere; Jasco), a quantum yield cal-
culation program (FWQE-880; Jasco) and highly diluted samples of
ten different optical densities in each measurement. Mass spectra
were recorded with a Fisons Instruments VG TofSpec mass spec-
trometer. Combustion analyses were performed by the Microana-
lytical Laboratory of the Goethe-University Frankfurt.
The fact that halogenated anthracenes give the corresponding
stable anthrones upon reaction with 1O2, prompted us to finally
select the chlorinated 9,10-dianthryl-DBA 3 for a thorough in-
vestigation of its hydrolysis/photooxidation products. After a solu-
tion of 3 in non-dried, non-degassed CDCl3 had been kept under
ambient light for 3 d, the starting material had completely vanished
and two new compounds had appeared instead in a stoichiometric
ratio of 1:2 (NMR spectroscopic control). The first of these products
was the literature-known borinic acid 13 (Scheme 2),20 while the
NMR signature of the other was identical with that of an authentic
sample of 2,7-di-tert-butylanthracene-9,10-dione (12; Scheme 2;
cf. Supplementary data for more details). The formation of 12 un-
equivocally proves that an oxidation step rather than mere hy-
drolysis is responsible for the degradation of 3. As illustrated in
Scheme 2, we propose that photooxidation of 3 (which also acts
as the photosensitizer) first generates an endoperoxide. As a result,
the anthryl plane is bent away from its original position, thus
leaving room for the nucleophilic attack of a water molecule on the
boron atom with concomitant rupture of the exocyclic BeC bond
and liberation of 12. We wish to emphasize that the photooxidative
decomposition of C-type 9,10-dianthryl-DBAs is therefore not
a specific problem of the boron-containing species, because con-
ventional all-carbon tri(9,10-anthrylene)s do also form endoper-
oxides under comparable conditions.
4.2. Synthesis protocols and characterization data
4.2.1. Synthesis of 3. nBuLi in hexane (1.17 M, 1.2 mL, 1.4 mmol) was
diluted with hexane (3 mL) and added dropwise with stirring
at ꢂ78 ꢁC to a solution of 1 (500 mg, 1.24 mmol) in Et2O (40 mL).
Stirring was continued for 15 min before the reaction mixture was
warmed to rt within 1 h. The resulting turbid solution was cooled
to ꢂ78 ꢁC again, 2 (197 mg, 0.590 mmol) in toluene (25 mL) was
added dropwise with stirring, the reaction mixture was allowed to
warm to rt overnight, and all volatiles were removed under reduced
pressure. The crude product was washed with non-dried C6H6
(3ꢀ5 mL), non-dried MeOH (3ꢀ5 mL), cold non-dried CH2Cl2
(2ꢀ2 mL), and non-dried pentane (3ꢀ5 mL). Yield: 278 mg (57%).
Both in solution and in the solid state, 3 shows an intense red
fluorescence when irradiated with UV light (lex¼366 nm). Red
blocks of 3 suitable for an X-ray crystal structure analysis were
grown by gas-phase diffusion of hexane into a concentrated CHCl3
solution.
3. Conclusions
To summarize, we have prepared soluble 9,10-dianthryl-9,10-
dihydro-9,10-diboraanthracene derivatives equipped with two pe-
ripheral chloro (3) or bromo (5) substituents. As expected, the in-
dividual p systems constituting 3 and 5 adoptmutually perpendicular
conformations. Charge-transfer interactions are observed between
the anthryl substituents and the 9,10-dihydro-9,10-diboraanthracene
(DBA) cores, which are more pronounced than in the cases of the
analogous all-carbon tri(9,10-anthrylene)s, because DBA is a strong
electron acceptor. The bromine atoms in 5 can be replaced by other
substituents (e.g., NMe2 (6), thiophen-2-yl (7)) through Pd-mediated
Stille coupling reactions. Compound 6 possesses a very dark color and
is non-emissive in solution or the solid state, whereas the red-purple
compound 7 shows an orange fluorescence (C6H12) and pronounced
positive solvatochromism in solution. The selection of suitable sub-
stituents therefore provides a powerful set-screw for the fine-tuning
1H NMR (400.1 MHz, CDCl3):
d
¼1.50 (s, 36H, C(CH3)3), 7.29 (m,
4H, H-b), 7.42 (m, 4H, H-a), 7.46 (dd, J¼9.0, 1.8 Hz, 4H, H-3,6), 7.64
(d, J¼9.0 Hz, 4H, H-4,5), 8.53 (d, J¼1.8 Hz, 4H, H-1,8); 11B{1H} NMR
(96.3 MHz, CDCl3):
d
¼n. o. ppm; 13C{1H} NMR (100.6 MHz, CDCl3):
of the
p-donor sets and, in turn, the optoelectronic properties of the
d
¼31.2 (C(CH3)3), 35.6 (C(CH3)3), 119.6 (C-1,8), 124.3 (C-3,6), 128.5
donoreacceptoredonor triads.
(C-8a,9a),128.5 (C-9),130.0 (C-4,5),132.5 (4a,10a),134.3 (C-b),140.5

7080
C. Hoffend et al. / Tetrahedron 69 (2013) 7073e7081
(C-a), 140.5
*
(C-10), 146.0
*
(C-c) 149.1 (C-2,7); MS (MALDI): m/z (%):
J¼9.3 Hz, 4H, H-4,5); 11B NMR (96.3 MHz, C6D6):
d
¼n. o.; 13C{1H}
821 (100) [Mþ]; elemental analysis calcd (%) for C56H56B2Cl2
NMR (75.4 MHz, C6D6):
d
¼31.2 (C(CH3)3), 34.8 (C(CH3)3), 125.2 (C-
(821.57): C 81.87, H 6.87; found: C 81.42, H 6.83.
*
This signal was
1,8), 125.4 (C-3,6), 126.9 (C-
10), 129.7 (C- ), 130.5 (C-4a,10a), 134.3 (C-b), 134.9 (C-8a,9a), 140.6
(C-
a), 127.6 (C-b), 127.6 (C-4,5), 128.4 (C-
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.
g
d
),141.0 (C-a),143.9 (C-9),146.4 (C-c),146.8 (C-2,7); MS (MALDI):
m/z (%): 916 (100) [Mþ]; elemental analysis calcd (%) for C64H62B2S2
(916.93): C 83.83, H 6.82, S 6.99; found: C 83.39, H 6.68, S 7.15.
4.2.2. Synthesis of 5. A suspension of N-bromosuccinimide
(236 mg, 1.33 mmol), 4 (500 mg, 0.664 mmol), and FeCl3 (11 mg,
0.068 mmol) in CHCl3 (50 mL) was stirred at rt for 2 d. The
succinimide was removed by filtration, the filtrate was evaporated
to dryness in vacuo, and the resulting solid crude product was
washed with non-dried MeOH (3ꢀ5 mL) to obtain 5 as a deep-red
solid. Yield: 545 mg (90%). Both in solution and in the solid state, 5
shows an intense red fluorescence when irradiated with UV light
4.3. Crystal structure analyses
Compounds 1, 3, 9-Cl-2,7-DTBA, 9,10-Br2-2,7-DTBA, and 12 were
measured on an STOE IPDS-II diffractometer with graphite-
ꢀ
monochromated MoK
a
radiation (
l
¼0.71073 A). Equivalent re-
flections were averaged. An empirical absorption correction with the
program PLATON21 was performed for 1, 3, 9-Cl-2,7-DTBA, and 9,10-
Br2-2,7-DTBA. Compounds 5, 6, 7, and 11 were measured on an
STOE IPDS-II diffractometer using a Genix Microfocus X-ray source
(
lex¼366 nm). Red plates of 5 suitable for X-ray crystallography
were grown by gas-phase diffusion of hexane into a concentrated
toluene solution.
with mirror optics and MoKa radiation. The data were corrected for
1H NMR (400.1 MHz, C6D6):
d
¼1.19 (s, 36H, C(CH3)3), 6.76 (m,
absorptionwiththe frame-scaling procedure contained in the X-AREA
package.22 The structures were solved by direct methods using the
program SHELXS23 and refined with full-matrix least-squares on F2
using the program SHELXL-97.24 All H atoms were geometrically po-
sitioned and refined using a riding model. The methyl groups bonded
to N in compound 6 were allowed to rotate but not to tip.
In compound 5, one of the co-crystallized toluene molecules is
disordered about a center of inversion with equal occupancies;
a second toluene molecule is disordered over two positions with
a site occupation factor of 0.510(9) for the major occupied site.
In compound 6, one methyl group of each NMe2 substituent is
disordered over two positions with a site occupation factor of
0.599(17) for the major occupied site.
4H, H-b), 7.50 (m, 4H, H-a), 7.56 (dd, J¼9.2, 1.8 Hz, 4H, H-3,6), 7.93
(d, J¼1.8 Hz, 4H, H-1,8), 8.87 (d, J¼9.2 Hz, 4H, H-4,5); 11B{1H} NMR
(96.3 MHz, C6D6):
(75.4 MHz, C6D6):
d
¼w75 ppm (h1/2¼3500 Hz); 13C{1H} NMR
d¼31.1 (C(CH3)3), 34.8 (C(CH3)3), 123.0 (C-10),
125.4 (C-1,8), 126.6 (C-3,6), 128.6 (C-4,5), 129.3 (4a,10a), 134.4 (C-b),
135.8 (C-8a,9a), 140.9 (C-a), 142.4 (C-9), 146.1 (C-c), 147.3 (C-2,7);
MS (MALDI): m/z (%): 910 (100) [Mþ]; elemental analysis calcd (%)
for C56H56B2Br2 (910.47): C 73.87, H 6.20; found: C 73.77, H 6.34.
4.2.3. Synthesis of 6. Neat Me3SneNMe2 (50.2 mg, 241
added to a solution of 5 (100 mg, 110 mol) and (tBu3P)2Pd (4.5 mg,
8.8
mol) in toluene (5 mL). The mixture was heated at 80 ꢁC for
mmol) was
m
m
24 h. After cooling to rt, the insolubles were removed by filtration,
washed with non-dried CHCl3 (3ꢀ5 mL), and the filtrate was
evaporated to dryness under reduced pressure. The crude solid
product was washed with non-dried MeOH (4ꢀ3 mL) to obtain 6 as
a dark solid. Yield: 78 mg (85%). Compound 6 fluoresces neither in
the solid state nor in solution when irradiated with UV light
There are two half molecules and 2.5 molecules of benzene in
the asymmetric unit of 7. Both thiophene rings are disordered over
two positions with site occupation factors of 0.504(3) and 0.615(4)
for the major occupied sites. The coordinates and displacement
parameters of the overlying disordered C and S atoms were con-
strained to be the same. Two tert-butyl groups are disordered over
two positions with site occupation factors of 0.673(5) and 0.59(1)
for the major occupied sites. The geometric parameters of one of
the disordered tert-butyl groups were restrained to be equal as
those of a not disordered tert-butyl group; the disordered atoms of
the tert-butyl groups were isotropically refined.
Compound 11 crystallizes together with 0.5 equiv of C6H6. One
tert-butyl group of 11 is disordered over three positions with site
occupation factors of 0.330(4), 0.34(1), and 0.33(1). The disordered
atoms were isotropically refined and their geometric parameters
were restrained to be equal to those of a non-disordered tert-butyl
group. Both hydroxyl hydrogen atoms were freely refined.
CCDC reference numbers: CCDC-914139 (9,10-Br2-2,7-DTBA),
914138 (9-Cl-2,7-DTBA), 914137 (1), 914136 (3), 914134 (5), 914135
(6), 930650 (7), 930651 (11), 930652 (12).
(
lex¼366 nm). Dark gray platelets of 6 suitable for X-ray diffraction
were obtained by slow evaporation (GranopentÒ) of a CHCl3 solu-
tion under inert conditions.
1H NMR (300.0 MHz, C6D6):
d
¼1.28 (s, 36H, C(CH3)3), 3.38 (s,
12H, NMe2), 6.74 (m, 4H, H-b), 7.62 (dd, J¼9.2, 2.0 Hz, 4H, H-3,6),
7.69 (m, 4H, H-a), 8.06 (d, J¼2.0 Hz, 4H, H-1,8), 8.64 (d, J¼9.2 Hz,
4H, H-4,5); 11B{1H} NMR (96.3 MHz, C6D6):
d
¼w75 ppm
(h1/2¼3500 Hz); 13C{1H} NMR (75.4 MHz, C6D6):
d
¼31.3 (C(CH3)3),
34.9 (C(CH3)3), 45.1 (NMe2), 124.3 (C-3,6), 125.7 (C-4,5), 126.1 (C-
1,8), 129.0 (C-4a,10a), 134.0 (C-b), 136.5 (C-8a,9a), 140.1 (C-9), 140.9
(C-a), 145.8 (C-10), 146.5 (C-2,7), 146.6 ppm (C-c); MS (MALDI): m/z
(%): 838 (100) [Mþ]; elemental analysis calcd (%) for C60H68B2N2
(838.78): C 85.91, H 8.17, N 3.34; found: C 86.06, H 8.23, N 3.14.
4.2.4. Synthesis of 7. Neat nBu3SneC4H3S (225 mg, 604
added to a solution of 5 (100 mg, 110 mol) and (tBu3P)2Pd (11 mg,
22
mol) in toluene (5 mL) and the mixture was heated at 80 ꢁC for
mmol) was
m
Acknowledgements
m
48 h. After cooling to rt the solution was evaporated to dryness in
vacuo. The crude oily residue was re-dissolved in non-dried CHCl3
(10 mL) and filtered through a pleated filter paper containing MgSO4
to remove the catalyst. The filtrate was taken to dryness under re-
duced pressure and the crude solid product was washed with non-
dried MeOH until the extract remained colorless. Compound 7 was
obtained as a purple solid and showed a red fluorescence both in
solution and in the solid state (lex¼366 nm). Yield: 75 mg (74%).
M.W. acknowledges financial support by the Beilstein Institute,
Frankfurt/Main, Germany, within the research collaboration
NanoBiC.
Supplementary data
Syntheses and characterization of 9,10-Br2-2,7-DTBA, 9-Cl-2,7-
DTBA, and 1; details of the X-ray crystal structure analyses of
9,10-Br2-2,7-DTBA, 9-Cl-2,7-DTBA, 1, 11, and 12; overlays of the
UV/vis absorption spectra of 6, K[6], and K2[6]; cyclic voltammo-
grams of 6 and 7 with different switching potentials; overlays of the
UV/vis absorption and fluorescence spectra of 3, 5, and 7; 1H NMR
1H NMR (300.0 MHz, C6D6):
4H, H-b), 7.13 (dd, J¼5.1, 3.3 Hz, 2H, H-
H- ), 7.43 (dd, J¼9.3, 1.8 Hz, 4H, H-
), 7.36 (dd, J¼3.3, 1.2 Hz, 2H, H-
3,6), 7.67 (m, 4H, H-a), 8.04 (d, J¼1.8 Hz, 4H, H-1,8), 8.27 (d,
d
¼1.22 (s, 36H, C(CH3)3), 6.78 (m,
), 7.29 (dd, J¼5.1, 1.2 Hz, 2H,
b
a
g

C. Hoffend et al. / Tetrahedron 69 (2013) 7073e7081
7081
14. Dou, C.; Saito, S.; Matsuo, K.; Hisaki, I.; Yamaguchi, S. Angew. Chem., Int. Ed.
2012, 51, 12206e12210.
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oxidation experiments. Supplementary data associated with this
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10.1016/j.tet.2013.06.036.
€
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