π-Expanded borazine: An ambipolar conjugated B-π-N macrocycle

Article abstract of DOI:10.1002/anie.201203788

Staying at a B 'N' B: A highly symmetric cyclic structure in which N donor and B acceptor sites are alternating (see picture) is found for the first ambipolar π-conjugated B-N macrocycle. The donor-π-acceptor arrangement results in mutual interactions between B and N and a pronounced solvatochromic effect on the emission. The strong luminescence in solution also lends itself to use in anion recognition. Copyright

Full text of DOI:10.1002/anie.201203788

Angewandte
Chemie
DOI: 10.1002/anie.201203788
Boron Macrocycles
p-Expanded Borazine: An Ambipolar Conjugated B–p–N
Macrocycle**
Pangkuan Chen, Roger A. Lalancette, and Frieder Jꢀkle*
In reference to its isoelectronic and isostructural relationship
with benzene, borazine is commonly referred to as “inorganic
benzene”.[1] This concept has generated renewed interest in
=
recent years, as the judicious replacement of C C double
bonds with isoelectronic and isosteric B–N fragments has
resulted in a plethora of interesting molecules for applications
ranging from hydrogen-storage materials to analogues of
aromatic natural products, and new optical and electronic
materials.[2–4] For instance, introduction of a B–N fragment in
benzene results in polarization of the molecule, which in turn
leads to unusual reactivity, including nucleophilic substitution
and hydrogenation under mild conditions.[3,5] B–N function-
alization of extended organic p-systems alters the electronic
structure, resulting for example in low-lying LUMO levels
and corresponding bathochromic shifts in the emission
spectra.[4,6] In the previous examples, B and N are directly
connected. An alternative design has borane acceptor (A)
moieties separated from amine donors (D) by an organic p-
conjugated linker. This D–p–A approach has proven success-
ful for the development of nonlinear optical materials,
ambipolar charge carriers in organic light emitting devices
(OLEDs), and fluorescent anion sensors.[7,8]
Conjugated macrocycles, on the other hand, are an
attractive class of materials for optoelectronic applications
as they comprise discrete, monodisperse structures, represen-
tative of an infinite polymer chain without any end groups.[9]
Another interesting feature is their ability to self-assemble
into tubular supramolecular structures and to form well-
defined and highly symmetric arrays upon deposition on
surfaces.[10] Numerous conjugated organic cyclic compounds
have been explored. More recently, heteroatom-containing
systems have attracted interest because the added function-
ality can offer unique properties and possibly open the door
to new applications.[11,12] As an example, Tanakaꢀs cyclic
hexaanilines A (p = phenylene) provide a platform for studies
on the aromaticity and molecular magnetism that results from
spin delocalization in the radical cation and dication.[11] We
have introduced an electron-deficient charge-reverse ana-
logue[13] to A, the conjugated macrocyclic organoborane B
with fluorene as the p-system.[14] Herein, we report the first
ambipolar macrocycle, which contains nitrogen as donor and
boron as acceptor sites, bridged by p-conjugated phenylene
groups. This new type of macrocycle C may be viewed as a p-
expanded borazine; however, introduction of the phenylene
bridges results in remarkably different properties in compar-
ison to borazine, including strong blue fluorescence, solvato-
chromic emission, and redox processes that reflect the
ambipolar structure of this unique D–p–A type macrocycle.
Initially, the linear oligomer 2-Si2 was prepared in 66%
overall yield by Sn/B exchange of the stannyl group in 1-SiSn
with the boryl group in 1-SiB, followed by treatment with
triisopropylphenyl copper (TipCu) for steric protection of the
boron center (Scheme 1). The selectivity of the Sn/B
exchange relies on the much higher reactivity[15] of the Sn
ꢀ
ꢀ
C in comparison to the Si C bond in 1-SiSn. Formation of the
ambipolar macrocycle 3 was accomplished by reaction of 2-Si2
with 2 equivalents BBr3, followed by cyclization under
pseudo-high dilution conditions upon simultaneous addition
of stoichiometric amounts of the resulting borylated species
and 1-Sn2 (1:1) to a large quantity of toluene. Treatment of the
initially generated B-Br functionalized macrocycle with
2 equivalents of TipCu in refluxing toluene for 2 days gave
the desired product 3. GPC analysis indicated that the crude
sample after standard workup consists of the targeted macro-
cycle as the major product in addition to a small amount of
larger cyclic species and/or higher linear polymers.[16] Purifi-
cation by preparative size exclusion column chromatography
on Bio-beads with THF as the eluent gave analytically pure 3
as a pale yellow powdery solid in 38% overall yield over three
steps.
GPC analysis of purified 3 gave a single, monodisperse
band corresponding to a molecular weight of Mn = 1483 Da
(polydispersity index (PDI) = Mw/Mn = 1.01), which is close to
the theoretical value of 1540 Da. Successful synthesis of the
macrocyclic species was further confirmed by high-resolution
MALDI-MS, which showed a single signal at m/z 1540.0705
that can be assigned to the molecular ion peak (calcd
1540.0706).[16] Consistent with the highly symmetric cyclic
structure, only one set of sharp signals was observed in the 1H
[*] P. Chen, Prof. Dr. R. A. Lalancette, Prof. Dr. F. Jꢀkle
Department of Chemistry, Rutgers University, Newark
73 Warren Street, Newark, NJ 07102 (USA)
E-mail: fjaekle@rutgers.edu
Homepage: http://andromeda.rutgers.edu/~fjaekle
[**] This work is supported by the National Science Foundation under
Grants No. CHE-0809642 and CHE-1112195.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201203788.
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endocyclic B–C (1.546(6), 1.555(7) ꢁ) and N–C
(1.416(6), 1.432(6) ꢁ) distances are similar to the
exocyclic ones of 1.573(6) and 1.419(6) ꢁ, respec-
tively, and the endocyclic C-B-C (119.8(4)8) and C-
N-C (121.3(3)8) angles are close to 1208, indicating
that the ring system is not significantly strained.
However, as evident from the side view in Fig-
ure 1b, rather than adopting perfect D3 symmetry,
the B3N3 core is distorted towards a chair-like
conformation, and all the exocyclic substituents
point into one direction relative to the mean plane
described by the B3N3 core. Six of the dichloro-
ethane solvent molecules are located in layers that
alternate with layers consisting of the main
molecules 3 (Figure S7, Supporting Information).
The remaining two solvent molecules are highly
disordered and are positioned in channels that
propagate along the crystallographic c axis. The
channels are smaller than the cavity of the
individual macrocycles, because the aryl substitu-
ents and solvents in layers above and below each
macrocycle reach into the channels (Figure 1c).
Scheme 1. Synthesis of the donor–p–acceptor macrocycle 3.
Compound 3 forms colorless crystals that are
and 13C NMR spectra, and a broad 11B NMR resonance at d =
72 ppm is indicative of tricoordinate B centers.[16]
blue-emissive when exposed to UV light. In solution, the
absorption spectra of 3 show two distinct bands around 420
and 390 nm, independent of the solvent (Figure S8, Support-
ing Information). These bands are attributed to intramolec-
ular charge transfer (ICT) from triarylamine donor sites (nN/
p-centered orbitals) to triarylborane acceptor sites (nB/p*-
centered orbitals). Photoexcitation gives rise to an intense
emission (CH2Cl2: lem = 460 nm, F = 0.76), which experiences
a pronounced red-shift with increasing solvent polarity (Fig-
ure 1d and Figure S8, Supporting Information). This solvato-
chromic effect in the emission, but not the absorption spectra,
suggests a more polarized excited state upon ICT, a phenom-
enon that is consistent with the
Colorless hexagon-shaped single crystals that show blue
fluorescence were obtained by slow vapor diffusion of
dichloroethane into a solution of 3 in toluene (Figure 1).
ꢀ
Compound 3 crystallizes in the trigonal space group R3 with
eight molecules of dichloroethane per macrocycle for a total
of six macrocycles and 48 solvent molecules per unit cell.[17]
Due to the large size and exceedingly fast solvent evaporation
from the crystals, and despite the use of dichloroethane in
place of dichloromethane, the uncertainties are relatively
large and only allow for a qualitative discussion. The
formation of
a
B/N D–p–A
system.[7]
DFT and TDDFT calculations
(B3LYP/6-31G*) were carried out
on a simplified derivative (3th, R =
Ph and phenylene p-system in C) in
D3 symmetry (minimization in C3
symmetry gave slightly higher
energy) and the results are sum-
marized along with experimental
data in Table 1.[18] The HOMO of
3th is degenerate (e) with contribu-
tions of the nitrogen atoms and the
bridging phenylene rings, while the
degenerate LUMOs (e) are mostly
localized on the boron p orbitals
with smaller contributions of the
exocyclic phenyl rings (Figure 2).
The HOMO-2 and LUMO + 2 orbi-
tals are totally symmetric (a2) and
thus feature contributions from all
of the filled N and empty B p orbi-
tals, respectively. Again, p–p deloc-
Figure 1. a) X-ray structure of 3 (solvents and hydrogen atoms omitted). b) Side view without
exocyclic substituents. c) Supramolecular structure of 3 viewed along the crystallographic c axis (only
solvent outside the channels shown; Cl yellow). d) Left: photographs of crystals of 3 without (top)
and with (bottom) UV irradiation at 365 nm; photographs of solutions of 3 in (left to right) toluene,
CH2Cl2, and propylene carbonate, irradiated at 365 nm.
2
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Table 1: Computational and experimental data for 3.
fore assigned to doubly degenerate nNp–nBp* transitions to S2
CV[a]
[eV]
SWV[a]
[eV]
DFT[b]
[eV]
UV/Vis
[nm] ([eV])
TD-DFT[b]
[nm] ([eV])
and S3, for which HOMO-2 and LUMO + 2 contributions are
Parameter
mixed in.[16]
The electrochemical properties of 3 were examined by
cyclic and square wave voltammetry (Figure 3). Three
ELUMO
EHOMO
DEgap/UV
ꢀ2.27
ꢀ5.26
2.99
ꢀ2.30
ꢀ5.30
3.00
ꢀ1.69
ꢀ5.01
3.32
420 (2.96)
432 (2.87)[c]
[a] Reference: ferrocene at 4.80 eV below vacuum, CV=cyclic voltam-
metry, SWV=square-wave voltammetry. [b] Computations performed on
!
3th. [c] Allowed S2 S0 transition.
Figure 3. Cyclic (top) and square-wave (bottom) voltammograms for 3;
oxidation (left) in CH2Cl2 and reduction (right) in THF (0.1m [Bu4N]-
[PF6]) verses Fc0/+ (Fc=[(h-C5H5)2Fe] as an internal reference (indi-
cated with an asterisk).
oxidation and three reduction waves were observed. A
comparison of the electrochemical data with those reported
for the azacyclophane A (phenylene p-system)[11] and bor-
acyclophane B (fluorene p-system)[14] provides insights into
the mutual electronic effects of the adjacent boron and
nitrogen centers in the ambipolar structure of 3 (Table 2).
Relative to the azacyclophane A, 3 is oxidized at more
positive potentials, because the presence of the electron-
deficient neighboring boron atoms decreases the electron
density at nitrogen. Conversely, more negative potentials are
needed to reduce the boron sites in 3 compared with the
boracyclophane B, because of an increase in the electron
density at boron in the presence of the neighboring amine
groups. These experimental findings suggest that the HOMO
level is lowered, while the LUMO is elevated. We also note
that the potentials for reduction of 3 are similar to the last
three reduction steps in B, while the potentials for oxidation
are similar to the last three oxidation steps in A, indicating
that the electronic effect of a reduced borane moiety is
comparable to that of a neutral amine and the effect of an
oxidized amine to that of a neutral borane, respectively.
Importantly, despite the relatively higher LUMO than in A
and the relatively lower HOMO than in B, theoretical
calculations reveal that the HOMO–LUMO gap in the
ambipolar species 3 is the smallest among these macrocycles,
which is consistent with the relatively small optical gap of
2.96 eV determined by UV/Vis spectroscopy.[20]
Figure 2. Kohn–Sham orbital representation for the ground state
frontier orbitals of 3 (B3LYP/6-31G*).
alization into the bridging phenylene rings is more pro-
nounced in the N-centered HOMO-2 than the B-centered
LUMO + 2. This is in contrast to results for the corresponding
hexaaza and hexabora analogues Ath and Bth (R = Ph,
phenylene p-system), for which both the HOMO
and LUMO show delocalization throughout the ring
Table 2: cyclic voltammetry data for 3 and of A and B (vs Fc0/+, v=100 mVsꢀ1).
Species Event E1 E2 E3 E4 E5 E6
system.[16,19] Based on TDDFT calculations, the
!
S1 S0 transition (HOMO-1 to LUMO + 1/HOMO
1/2
1/2
1/2
1/2
1/2
1/2
to LUMO) is symmetry forbidden because of
cancellation of the transition dipole moments, as is
typically observed for highly symmetric macrocycles,
including A and B.[14,16] While in solution lower
symmetry conformations may be adopted, the struc-
ture of the cycle is expected to be quite rigid. The
A[a]
3
Ox[c]
Ox[d]
Red[e]
ꢀ0.28
ꢀ0.17
+0.20
+0.45
+0.46[g]
ꢀ2.53
ꢀ2.44
+0.72[f]
+0.65[g]
ꢀ2.72
+0.72[f]
+0.94[g]
ꢀ2.84[g]
ꢀ2.70
B[b]
Red[e] ꢀ2.10[b] ꢀ2.10[b]
ꢀ2.27
ꢀ2.57
[a] From Ref. [11]. [b] From Ref. [14]. [c] 0.1m [Bu4N][BF4] in CH2Cl2. [d] 0.1m
[Bu4N][PF6] in CH2Cl2. [e] 0.1m [Bu4N][PF6] in THF. [f] Overlapping. [g] Determined
experimentally observed absorption bands are there- by SWV.
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Figure 4. Titration of 3 with [nBu4N]CN in toluene monitored by a) UV/Vis and b) fluorescence spectroscopy ([3]0 =1.429ꢁ10ꢀ5 m;
[CNꢀ]=1.042ꢁ10ꢀ3 m, lex =419 nm. c) Illustration of electron-donor segments for 3 and the corresponding anion complexes.
The presence of electron-deficient organoborane moieties
also suggests possible use of these molecules in the recog-
nition of anions.[21] The anion binding behavior was evaluated
by titration experiments with the cyanide anion. As shown in
Figure 4, stepwise addition of [nBu4N]CN to a solution of 3 in
toluene resulted in a gradual decrease in the UV absorbance
and emission intensity. Three distinct regimes were observed,
consistent with addition of the anion to the three available
borane moieties, albeit with little or no cooperative
effects[22,23] (lgb11 ꢁ 8.0, lgb12 = 15.7, lgb13 = 23.0). Noteworthy
is that, although the absorption wavelength does not change
dramatically, in the emission spectra a clear bathochromic
tailing is observed, which we attribute to new charge transfer
(CT) pathways upon generation of an electron-rich organo-
borate site (Figure 4c; see also Ref. [8,24]). A comparison to
the quenching behavior of the boracyclophane B provides
further insights. In the case of B, only slightly more than
1 equivalent of quencher is needed to completely turn off the
emission of the host.[14] In contrast, to fully quench the
fluorescence of 3, more than 3 equivalents of CNꢀ are
required, corresponding to full complexation of all three
Lewis acidic B centers. This suggests that emission from CT
states in the partially complexed species [3(CN)]ꢀ and
[3(CN)2]2ꢀ remains strong, whereas a very weakly emissive
low-energy CT state is generated upon anion binding in
[B(CN)]ꢀ. This CT state is believed to serve as an energy trap,
resulting in effective quenching of the emission.[14]
a pronounced solvatochromic effect on the emission. Macro-
cycles, such as 3, combine aspects of electron-rich aza- and
electron-deficient boracyclophanes suggesting possible appli-
cations as ambipolar semiconductor materials. The strong
luminescence in solution also lends itself to use in anion
recognition and our studies indicate that, in the presence of
low levels of cyanide, fluorescence results from emissive
charge-transfer states, which is in stark contrast to the
respective boracyclophane B.
Received: May 16, 2012
Published online: && &&, &&&&
Keywords: ambipolar · anion recognition · luminescence ·
.
macrocycle · organoborane
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!
[20] This trend applies to both the formally forbidden S1 S0 and the
!
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6446; c) G. Zhou, M. Baumgarten, K. Mꢂllen, J. Am. Chem. Soc.
2008, 130, 12477.
Angew. Chem. Int. Ed. 2012, 51, 1 – 6
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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Angewandte
Communications
Communications
Boron Macrocycles
Staying at a B ’N’ B: A highly symmetric
cyclic structure in which N donor and B
acceptor sites are alternating (see pic-
ture) is found for the first ambipolar p-
conjugated B–N macrocycle. The donor–
p–acceptor arrangement results in
mutual interactions between B and N and
a pronounced solvatochromic effect on
the emission. The strong luminescence in
solution also lends itself to use in anion
recognition.
P. Chen, R. A. Lalancette,
F. Jꢁkle*
&&&& — &&&&
p-Expanded Borazine: An Ambipolar
Conjugated B–p–N Macrocycle
6
www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1 – 6
These are not the final page numbers!