From Monodisperse Thienyl- and Furylborane Oligomers to Polymers: Modulating the Optical Properties through the Hetarene Ratio

Article abstract of DOI:10.1002/chem.201706124

The application of our newly developed B–C coupling method by catalytic Si/B exchange is demonstrated for the synthesis of a series of triarylboranes (1), monodisperse thienyl- and furylborane dimers (2) and trimers (9), extended oligomers (3) and polymers (3′), as well as mixed (oligo)thienyl-/furylboranes. The structures of 1 aa^Tip, 1 bb^Tip, and 2 bbb^Mes*, determined by X-ray crystallography, reveal largely coplanar hetarene rings and BR₃ environments, which are most pronounced in the furylborane species. Photophysical investigations, supported by TD-DFT calculations, revealed pronounced π-electron delocalization over the hetarene backbones including the boron centers. With an extended series of derivatives of varying chain lengths available, we were able to determine the effective conjugation lengths (ECL) of poly(thienylborane)s and poly(furylborane)s, which have been reached with the highest-molecular-weight derivatives of our study. Through variation of the furan-to-thiophene ratio, the photophysical properties of these materials are effectively modulated. Significantly, higher furan contents lead to considerably increased fluorescence intensities. Compounds 1 aa^Tip, 1 bb^Tip, and 3 a^Tip showed the ability to bind fluoride anions. The binding process is signaled by a distinct change in their optical absorption characteristics, thus rendering these materials attractive targets for sensory applications.

Full text of DOI:10.1002/chem.201706124

A Journal of
Accepted Article
Title: From Monodisperse Thienyl- and Furylborane Oligomers to
Polymers - Modulating the Optical Properties Through the
Hetarene Ratio
Authors: Artur Lik, Sangeth Jenthra, Lars Fritze, Lars Müller, Khai-Nghi
Truong, and Holger Helten
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10.1002/chem.201706124
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From Monodisperse Thienyl- and Furylborane Oligomers to
Polymers – Modulating the Optical Properties Through the
Hetarene Ratio
Artur Lik, Sangeth Jenthra, Lars Fritze, Lars Müller, Khai-Nghi Truong, and Holger Helten*
Dedicated to Prof. Dr. Alexander C. Filippou on the occasion of his 60th birthday
Abstract: The application of our newly developed B–C coupling meth-
od by catalytic Si/B exchange is demonstrated for the synthesis of a
series of triarylboranes (1), monodisperse thienyl- and furylborane di-
mers (2) and trimers (9), extended oligomers (3) and polymers (3′), as
for organic (opto)electronics, anion or amine sensing, and bio-im-
aging. The combination of inherently electron-deficient trivalent
borane moieties with electron-rich organic π systems results in the
formation of donor–acceptor arrays. Thiophene, a ubiquitous
building block in organic electronic materials in general,[19] has
proved particularly useful for this purpose.[10,11,13-15] For instance,
Jäkle and Marder and co-workers presented dithienylboranes
1aaMes* and 1aaFMes and bisboranes 2aaaMes* and 2aaaFMes, which
feature extended conjugated π systems involving largely coplanar
thiophene rings and the boron centers (Figure 1).[14] The bulky
aromatic side groups, 2,4,6-tri-tert-butylphenyl (supermesityl,
Mes*; R = tBu) and 2,4,6-tris(trifluoromethyl)phenyl (fluoromesityl,
FMes; R = CF3), adopt a nearly perpendicular arrangement to this
π plane and provide kinetic stabilization, thus making these spe-
cies chemically very robust. Compounds 1aaFMes and 2aaaFMes
and also the B-mesityl substituted derivative 1aaMes (R = Me) by
Miyasaka, Kobayashi, and Kawashima[11i] are able to bind and
detect fluoride anions via an optical response. Recently, Jäkle et
al. reported the synthesis of poly(thienylborane) 3aMes* and relat-
ed polymers comprising oligothiophene building blocks by Sn/B
exchange or Stille cross-coupling polycondensation, respective-
ly.[15]
well as mixed (oligo)thienyl-/furylboranes. The structures of 1aaTip
,
1bbTip, and 2bbbMes*, determined by X-ray crystallography, reveal
largely coplanar hetarene rings and BR3 environments, being most
pronounced in the furylborane species. Photophysical investigations,
supported by TD-DFT calculations, revealed pronounced π-electron
delocalization over the hetarene backbones including the boron cen-
ters. Having extended series of derivatives of varying chain lengths
available, allowed us to determine the effective conjugation lengths
(ECL) of poly(thienylborane)s and poly(furylborane)s, which have
been reached with the highest-molecular-weight derivatives of our
study. Through variation of the furan-to-thiophene ratio, the photo-
physical properties of these materials are effectively modulated. Sig-
nificantly, higher furan contents lead to considerably increased fluo-
rescence intensities. Compounds 1aaTip, 1bbTip, and 3aTip showed the
ability to bind fluoride anions. The binding process is signaled by a
distinct change in their optical absorption characteristics, thus render-
ing these materials attractive targets for sensory applications.
R
R
R
tBu
Introduction
R
R
R
R
In the last few decades, π-conjugated materials have been the
subject of tremendous research activity,[1] driven by applications
in electronic and optoelectronic devices such as (polymer-based)
organic light-emitting diodes (OLEDs/PLEDs),[2] field-effect tran-
sistors (OFETs),[3] and organic photovoltaic cells (OPVs),[4,3b] as
well as biomedical[5] and sensory applications.[6] In recent years,
organic–inorganic hybrid materials generated by doping of π sys-
tems with inorganic main group elements have emerged as versa-
tile alternatives with special features.[7] Conjugated organobo-
ranes[7-11] and organoborane polymers[7,8,10,12-18] have attracted
particular attention. The incorporation of the vacant pπ orbital of
boron into an extended π system (p–π conjugation) causes intrigu-
ing optical and electronic properties, thus enabling applications
tBu
tBu
R
R
S
B
B
B
S
B
S
S
S
S
n
1aaMes*: R = tBu
1aaFMes: R = CF3
2aaaMes*: R = tBu
2aaaFMes: R = CF3
3aMes*
1aaMes
: R = Me
Figure 1. Dithienylboranes 1aaMes*, 1aaFMes, 1aaMes, thienylborane dimers
2aaaMes*, 2aaaFMes, and poly(thienylborane) 3aMes*
.
In contrast to thiophene, the use of furan as a component for π-
conjugated materials is significantly less well explored. The gen-
eral value of furan-based materials for organic electronics has
been recognized only quite recently, although such compounds
show several favorable features and they are in some respect
complementary to their well-established thiophene conge-
ners.[20,21] Significantly, different from most other components of
organic electronic materials, furan rings are biodegradable and
can be obtained from entirely renewable resources. The combina-
tion of boron with furan moieties, however, has been largely unex-
plored so far.[11t,22]
[*]
A. Lik, S. K. Jenthra, L. Fritze, L. Müller, K.-N. Truong, Dr. H. Helten
Institute of Inorganic Chemistry, RWTH Aachen University
Landoltweg 1, 52056 Aachen (Germany)
E-mail: holger.helten@ac.rwth-aachen.de
http://www.ac.rwth-aachen.de/extern/helten/index.html
Supporting information for this article is given via a link at the end of
the document.
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10.1002/chem.201706124
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Recently, we communicated the development of a highly effi-
cient, environmentally benign method for the formation of B–C
bonds using a novel catalytic Si/B exchange condensation ap-
proach and demonstrated its application to the synthesis of aryl-
borane molecules, oligomers, and polymers, including the first
oligo- and poly(furylborane)s.[23,24] Herein, we report full details of
these studies and an extension thereof to longer monodisperse
oligomers, i. e., dimers and trimers, as well as mixed thienyl-/furyl-
borane oligomers prepared by modular synthesis. This allows us
to determine the effective conjugation lengths of these classes of
compounds, and we demonstrate that their optoelectronic proper-
ties are effectively modulated through the heterocycle ratio. We
also investigate the effect of the aromatic side group on the photo-
physical properties of the new materials and their ability to bind
and detect F−.
appropriate aryllithium reagent to furnish the kinetically stabilized
triarylboranes 1. In the cases of 6aa and 6bb, TipLi (Tip = 2,4,6-
triisopropylphenyl, tripyl) was used, which afforded the air- and
moisture-stable products 1aaTip and 1bbTip. As our further inves-
tigations revealed that longer furylborane oligomers were water-
sensitive despite having the sterically demanding Tip group at-
tached to the boron center (see below), we additionally deriva-
tized 6bb with the even bulkier Mes*Li to obtain 1bbMes*. The
mixed-substituted species 6ab was derivatized with Mes*Li as
well to give the stabilized triarylborane 1abMes*. Only the diphenyl
derivative 1ccMes* was found to be air- and moisture-sensitive in
spite of the Mes* group at boron; it decomposed gradually within
24 h under air (Figure S34, SI). However, we succeeded in iso-
lating 1ccMes* in pure form by re-crystallization.
All other triarylboranes 1 were purified by column chromatogra-
phy on silica and isolated in moderate to good yields (35–69%) as
colorless crystalline solids. Their constitution was unambiguously
ascertained by 1H, 11B, and 13C NMR spectroscopy, mass spec-
trometry, and single-crystal X-ray diffraction. In the case of
1abMes*, however, the quality of the X-ray data obtained was too
poor to allow the discussion of structural parameters. The 11B
NMR resonance of compound 1abMes* appears at a chemical shift
of 50.2 ppm, which is intermediate between that of 1aaTip (56.1
ppm) and 1bbMes* (47.7 ppm) or 1bbTip (48.6 ppm), respectively.
Compared to the resonance of 1ccMes* (67.6 ppm), these signals
are at significantly higher field, indicating more effective π-electron
donation from the heteroaromatic rings to the vacant pπ orbital on
boron. The 1H NMR spectrum of 1bbMes* showed broad reso-
nances for the protons at the furan rings (see Supporting Informa-
tion, Figure S26), and the spectrum of 1abMes* showed broadened
resonances for the furyl and the thienyl protons (Figure S23, SI).
This was not observed for 1bbTip (Figure S29, SI). Presumably,
this is a result of the hindered rotation of the rings in the super-
mesityl derivatives 1bbMes* and 1abMes* in solutionat ambient tem-
Results and Discussion
Synthesis of triarylboranes. As outlined in our preliminary com-
munication,[23] in the first step of our investigations we explored
the reaction between 2-(dibromoboryl)thiophene (4a) and 2-(tri-
methylsilyl)thiophene (5a) to give bromo-bis(thien-2-yl)borane
6aa (Scheme 1). Some condensation was observed by 1H NMR
spectroscopy in a 1 M CH2Cl2 solution at ambient temperature al-
ready without an initiator added. However, the reaction did not go
to completion within 12 d. Addition of catalytic amounts (5 mol%)
of Me3SiOTf yielded significant rate acceleration with 95 % con-
version after 5 d. The stronger electrophilic species Me3SiNTf2,
finally, proved to be a highly efficient catalyst: complete and selec-
tive conversion to 6aa was observed with 5 mol% of Me3SiNTf2 at
ambient temperature within 72 min. Analogous conditions were
adopted to generate 6bb and the new, mixed-substituted deriva-
tive 6ab. Both reactions proceeded quantitatively (by 1H NMR
spectroscopy) within less than 2 h as well. The formation of the
phenyl derivative 6cc was considerably slower, and it required in-
creased substrate concentration (4 M) and higher catalyst loading
(25 mol%). Nevertheless, this reaction was fully selective, too,
and afforded 95 % conversion within 3 d at room temperature.
perature, in contrast to 1bbTip
.
Synthesis of monodisperse oligomers. In order to systematic-
ally investigate the effect of chain elongation on the photophysical
properties of linear extended thienyl- and furylboranes and also
mixed species, we prepared a series of monodisperse oligomers
(Scheme 2). The bisboranes 2, which can be regarded as hetaryl-
borane “dimers”, were synthesized by the reaction of a 2,5-bis(tri-
methylsilyl)hetarene 7a or 7b with two equivalents of either 4a or
4b in CH2Cl2 at room temperature in the presence of 5 mol% of
Me3SiNTf2, and subsequent post-modification with TipLi or Mes*Li
Ar3Li
toluene
cat.
Ar3
Br
CH2Cl2, r.t.
1B
Ar Br2 + Ar2SiMe3
B
B
Ar1
Ar2
Ar1
1aaTip
Ar2
– LiBr
– Me3SiBr
4a-c
5a-c
6aa
6bb
6ab
6cc
:
Ar3 = Tip
Ar3 = Tip
: Ar3 = Mes*
: Ar3 = Mes*
: Ar3 = Mes*
1bbTip
:
in toluene, respectively. The products 2aaaTip, 2bbbMes*, 2abaMes*
,
1bbMes*
1abMes*
1ccMes*
and 2babMes* were isolated by column chromatography on silica,
and their identity was unambiguously ascertained by multinuclear
NMR spectroscopy and mass spectrometry. The structure of
2bbbMes* in the solid-state was additionally determined by a
single-crystal X-ray diffraction study.
Ar1, Ar2
=
(a),
b
( ),
c
( )
S
O
cat. = Me3SiNTf2 (or Me3SiOTf)
Scheme 1. Synthesis of triarylboranes 1 via catalytic B–C coupling.
The 11B{1H} resonances of 2aaaTip (58 ppm) and 2bbbMes* (49
ppm) appeared in the same range as those of their respective
monoborane congeners, 1aaTip and 1bbMes*, however, the signals
were significantly broadened (FWHM = 2980 for 2aaaTip and 2120
Hz for 2bbbMes*) as an effect of chain elongation. Although the
mixed derivatives 2abaMes* and 2babMes* both contain two boron
After the formation of the respective bromodiarylborane 6 was
complete (or almost complete, in the case of 6cc), the reaction
mixture was evaporated to dryness, the crude product was re-dis-
solved in toluene and treated with a solution or suspension of the
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atoms in distinct chemical environment, they each show only one
signal in their 11B{1H} NMR spectra (δB = 51 ppm for 2abaMes* and
δB = 49 ppm for 2babMes*), most likely a result of the broadened
signal patterns. Furthermore, an overall increase in the ratio of
side of the molecule.[14] The largely coplanar orientation of the het-
aryl rings with respect to the BC3 moiety, and to lesser extent also
that of the phenyl rings in 1ccMes*, indicate potential π conjugation
between these groups over the boron center. In 1aaTip, both thio-
phene rings are disordered by rotation about the B–C bonds,
while in the furyl derivatives 1bbTip, 1bbMes*, and 2bbbMes* the het-
erocycles consistently adopt all-anti conformation in the solid-
state.
furan to thiophene rings leads to a slight highfield shift of the 11
B
signal (from 2abaMes* to 2babMes* by ca. 2 ppm).
1) Me3SiNTf2 cat.
CH2Cl2, r.t.
2) 2 ArLi,
2
4a
B
Br2
X
Ar
Ar
Y
: X = S
B
B
+
toluene
4b: X = O
Y
X
X
Me3Si
SiMe3
2aaaTip: X = Y = S, Ar = Tip
2bbbMes*: X = Y = O, Ar = Mes*
2abaMes*: X = S, Y = O, Ar = Mes*
2babMes*: X = O, Y = S, Ar = Mes*
7a
: Y = S
7b
: Y = O
1) Me3SiNTf2 cat.
CH2Cl2, r.t.
2
Ar
BBr2
X
2) 2 ArLi,
toluene
B
Ar
Y
4a: X = S
4b: X = O
+
X
X
B
B
X
Y
B
SiMe3
Ar
X
Ar
9aaaaTip: X = Y = S, Ar = Tip
Me3Si
8aaTip: X = Y = S, Ar = Tip
8abMes*: X = S, Y = O, Ar = Mes*
9aabaMes*: X = S, Y = O, Ar = Mes*
9bbabMes*: X = O, Y = S, Ar = Mes*
Scheme 2. Synthesis of monodisperse hetarylborane oligomers.
The synthesis of the “trimers” 9 required prior preparation of di-
silylated compounds 8. This was done via dilithiation of the appro-
priate triarylborane 1 and subsequent reaction with Me3SiCl (2
equiv.). Compounds 8 were then treated with two equivalents of
4 at ambient temperature in a catalytic reaction using Me3SiNTf2
(Scheme 2). Post-modification with TipLi or Mes*Li afforded 9.
The derivatives 9aaaaTip and 9aabaMes* were isolated by column
chromatography, and their constitution was unambiguously as-
certained by NMR spectroscopy and mass spectrometry. The
11B{1H} resonance of 9aaaaTip (57 ppm) was in the expected
range and further broadened (FWHM = 3050 Hz) compared to
those of the “dimers” and “monomers”; for 9aabaMes* the reso-
nance was not detected. The formation of 9bbabMes* was evi-
denced by mass spectrometry (m/z = 1051.1; see SI, Figure
S127), however, the product suffered from decomposition during
the chromatographic work-up (see SI, Figures 62 – 64).[25]
Figure 2. Molecular structures of 1aaTip, 1bbTip, and 2bbbMes* in the solid state
(H atoms omitted for clarity).
Table 1. Selected structural data for triarylboranes 1 and dimer 2bbbMes*
.
Compd.
Interplanar
angle BC3–
Tip/Mes* [°]
Interplanar
angle
(het)aryl–
(het)aryl [°]
B–C1 bond
length [Å]
B–C5 bond
length [Å]
1aaTip
1bbTip
1bbMes*
86.0
85.6
30.2[c]
5.1
1.532(3)
1.529(6)
1.541(3)
1.539(6)
Structural studies. The solid-state structures of 1aaTip, 1bbTip
,
[a,b]
83.0,
87.0
7.7,
4.6
1.527(6),
1.542(5)[d]
1.529(6),
1.547(5)[e]
1bbMes*, 1ccMes*, and the furylborane dimer 2bbbMes* have been
determined by single-crystal X-ray diffraction (Figure 2 and Table
1; the molecular structures of 1bbMes* and 1ccMes have been dis-
played in our previous communication[23]). In each of these com-
pounds the boron centers are trigonal-planar coordinated (sums
of the bond angles around boron, Σ(BC3), ca. 360° each). The
Tip/Mes* substituents are nearly perpendicular to the respective
BC3 plane, thus ruling out any possible π interaction of the vacant
p orbital on boron with this aromatic system. Interestingly, the two
Mes* substituents of 2bbbMes* point into opposite directions,
which is different from the thienylborane dimers 2aaaMes* and
2aaaFMes (with FMes = 2,4,6-(CF3)3C6H2) recently described by
Jäkle and Marder wherein the aryl side groups are on the same
[a]
1ccMes*
83.4
43.8
1.579(3)
1.581(3)
2bbbMes*
84.4,
88.9
6.0,
5.7
1.534(6),
1.531(6)[f]
1.546(6),
1.545(5)[g]
[a] Taken from ref. [23]. [b] The asymmetric unit of 1bbMes* contains two
independent molecules. [c] Mean value from the four different relative
conformations of the disordered rings. [d] B2–C27. [e] B2–C31. [f] B2–C9. [g]
B2–C8.
It is noteworthy that the most pronounced coplanarity of the
rings is observed in the furyl derivatives 1bbTip, 1bbMes*, and
2bbbMes*. The interplanar angles are 5 – 8° only. The mean twist
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angle between the thiophene (Thi) planes in 1aaTip is 30.2°, which
is somewhat larger compared to those in Mes*B(Thi)2 (19.0°),[14]
FMesB(Thi)2 (16.3°),[14] and MesB(Thi)2 (8.4 and 4.2°).[11i] The ob-
servation that the furylboranes show greater planarity than their
thienylborane congeners parallels the trend found for oligofurans
vs. analogous thiophene oligomers.[21] The torsion angle between
the phenyl groups in 1ccMes is 43.8°. The lengths of the B–C
bonds to the thiophene or furan substituents, respectively, in
1aaTip, 1bbTip, 1bbMes*, and 2bbbMes* are roughly in the same
range, and also in the same range with those of MesB(Thi)2 (av-
eraged: 1.548 Å)[11i] and FMesB(Thi)2 (1.541(7) and 1.526(6) Å),[14]
but significantly shorter than in Mes*B(Thi)2 (1.589(6) and
1.568(4) Å).[14]
Tip/Mes*-containing disilyl compounds 8aaTip and 8bbMes* as the
monomers in combination with BBr3 in analogous catalytic reac-
tions. In the case of 8bbMes*, we furthermore changed the solvent
to the more polar 1,2-dichlorobenzene (o-DCB). Apparently due
to the improved solubility of the macromolecular intermediates
12aTip and 12bMes*, the products 3a′Tip and 3b′Mes* were obtained
with significantly increased molecular weights, corresponding to
number average degrees of polymerization of 10 and 9, respec-
tively.
Table 2. GPC data[a] for 3aTip, 3a′Tip, 3bMes*, and 3b′Mes*
.
Compd.
3aTip
Mn
MW
PDI
1.1
1.3
1.5
1.6
DPn
4
1550
3270
1880
3260
1760
4100
2890
5220
Polycondensation reactions. As communicated recently,[23] we
performed polycondensation reactions leading to oligo- and poly-
(thienyl/furylborane)s via two different routes (Scheme 3).[23]
Route A: Reactions of 7a and 7b with one equivalent of BBr3 at
−78 °C selectively yielded the AB monomers 10a,b, which contain
both functional groups required for polycondensation (SiMe3 and
BBr2) in one molecule. Addition of 5 mol% of Me3SiNTf2 to CH2Cl2
solutions of 10a,b, initiated polycondensation thereof to give
11a,b. These intermediates were subsequently derivatized with
TipLi (11a) or Mes*Li (11b).[26] After aqueous work-up and purifi-
cation by precipitation with ethanol this afforded the air- and
moisture-stable products 3aTip and 3bMes*. Gel permeation chro-
matography (GPC), however, suggested that they were of rela-
tively low molecular weight with narrow polydispersity (Table 2).
We assume that this is a result from the observed precipitation of
the intermediates 11a,b at some point during the polycondensa-
tion process.
3a′Tip
10
5
3bMes*
3b′Mes*
9
[a] In THF, vs polystyrene standards.
Photophysical studies. All monomers 1, dimers 2, trimers 9,
oligomers 3, and polymers 3′ prepared were studied by UV–vis
absorption and fluorescence emission spectroscopy in THF
(Table 3) which, building on related previous studies,[14,15,23] leads
to a detailed picture of the photophysical properties of such spe-
cies. In the UV–vis spectra, the dihetarylborane monomers show-
ed each an intense band at λabs,max = 315 – 325 nm and a further
band at higher energy (λabs,max = 260 – 280 nm), which we assign
both to π–π* transition processes based on insights gained from
our time-dependent density functional theory (TD-DFT) calcula-
tions (vide infra). Comparison of the data for 1bbTip and 1bbMes*
reveals a significant effect of changing the stabilizing aryl substitu-
ent from Tip to Mes*. This causes a decrease in the wavelength
of the lowest-energy absorption maximum by 9 nm (315 vs. 324
nm). On the other hand, the emissivity is significantly enhanced
through the substituent change. Whereas 1bbTip was found to be
basically non-emissive in THF, compound 1bbMes* showed rela-
tively strong blue fluorescence with a quantum yield of 24 %. Pre-
sumably, this is associated with the increased rigidity of the mole-
cules of 1bbMes* compared to 1bbTip due to the hindered rotation
of the furan rings in the former, as evidenced by 1H NMR spectros-
copy (vide supra), thus favoring radiative decay processes.
Route A
B
Br3
n-pentane, –78 °C
Me3Si
Me3Si
B
Br2
SiMe3
X
X
– Me3SiBr
10a,b
7a,b
Me3SiNTf2 cat.
CH2Cl2, r.t.
X
S a O b
( ), (
=
)
– Me3SiBr
ArLi, toluene
– LiBr
1/n
B
1/n
B
X
n
X
n
Ar
Br
11a,b
3aTip: Ar = Tip
3bMes*: Ar = Mes*
Route B
1) BBr3
Ar
Ar
2) Me3SiNTf2 cat.
Br
B
B
X
B
X
CH2Cl2 or o-DCB, r.t.
SiMe3
1/n
X
X
– 2n Me3SiBr
ArLi, toluene
In the case of the couple 1aaTip / 1aaMes* [14]
on the other hand,
,
Me3Si
n
12aTip: Ar = Tip
12bMes*: Ar = Mes*
8aaTip: Ar = Tip
8bbMes*: Ar = Mes*
variation of the bulky aryl substituent has negligible influence on
the absorption and emission characteristics of the compounds.
The maximum of the lowest-energy absorption band for both de-
rivatives appears at virtually identical wavelength. Notable fluo-
rescence was observed for neither of them. While 1aaMes* was
described as non-emissive in THF,[14] compound 1aaTip showed
weak fluorescence with a quantum yield of 3 %.
– LiBr
2/n
3a'Tip: Ar = Tip
B
X
n
Ar
3b'Mes*: Ar = Mes*
Scheme 3. Polycondensation reactions.
Route B: As we found that the Tip and Mes* side groups impart
pronounced solubility, in the next step, we decided to employ the
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Table 3. Photophysical data[a] for triarylboranes 1, dimers 2, trimers 9,
oligomers 3, and polymers 3′.
Compd.
λ
λ
Φ
F
εmax
abs,max
em,max
[103 L mol−1
[nm]
[nm]
[%]
cm−1
]
[b]
1aaTip
325
325
324
315
320
271
370
360
360
361
359
387
381
409
412
407
411
242
410
–
3
[c]
[d]
1aaMes*
1bbTip
–
409
257
200
194
–
–
[b]
[b]
1bbMes*
1abMes*
1ccMes*
2aaaTip
400
366
–
24
10
–
Figure 3. UV–vis absorption spectra of triarylboranes 1 (high-energy bands of
1aaTip, 1abMes*, and 1bbMes* omitted for clarity).
Also within the series of dimeric compounds, the thienylborane
239
–
–
derivative 2aaaTip absorbs at the longest wavelength (λabs,max
=
370 nm). The band of its Mes* congener 2aaaMes* is slightly blue-
shifted (λabs,max = 360 nm),[14] which correlates with the trend ob-
served for the couple 1bbTip / 1bbMes*. 2aaaMes*, the mixed dimers
2abaMes* and 2babMes*, and the pure furylborane derivative
2bbbMes* have very similar absorption characteristics with maxima
at around 360 nm. Only the latter shows some fluorescence in
solution.
[c]
[d]
2aaaMes*
2abaMes*
2babMes*
2bbbMes*
9aaaaTip
–
–
427
388
405
358
422
–
–
–
–
385
–
3
–
One conclusion that can be drawn from our studies is that the
furylborane species generally show significantly enhanced fluo-
rescence compared to their thiophene congeners. This parallels
the trend previously observed for purely organic oligofurans vs.
oligothiophenes.[21] Within our series of investigated compounds,
the oligo- and poly(furylborane)s 3bMes* and 3b′Mes* showed the
strongest fluorescence intensities. Presumably, two effects con-
tribute to this phenomenon: the effect of i) the presence of furan
rings in the backbone and ii) the incorporation of Mes* rather than
Tip as the side group.
9aabaMes*
–
–
[b]
3aTip
400[e]
299[e]
147[e]
198[e]
421
455
422
426
1
[b]
3a′Tip
1
[b]
3bMes*
70
71
[b]
3b′Mes*
[a] In THF, except for 1ccMes* (in CH2Cl2). [b] Taken from ref. [23]. [c] Taken
Comparison of the UV–vis spectra for species of different chain
length within each series of comparable compounds reveals con-
siderable bathochromic shifts with increasing number of repeat
units (Figure 4). This provides unequivocal evidence for effective
π conjugation along the thienylborane and furylborane backbones
from ref. [14]. [d] Not reported. [e] Per repeat unit.
Within the series of triarylboranes 1 having the same bulky aryl
substituent (Mes*), the UV–vis spectra reveal a clear trend de-
pending on nature of the conjugated aryl groups (Figure 3; due to
the similarity of the spectra of 1aaTip and 1aaMes* it seems appro-
priate to include the spectrum of 1aaTip in the graphical compari-
son). Here, the wavelength of the absorption maximum increases
in the order: phenyl << furyl < thienyl. The absorption band of
1ccMes* appears at relatively high energy, evidencing rather weak
conjugation between the phenyl groups, as indicated already by
their orientation in the solid-state structure of 1ccMes* (vide supra).
The bands of the hetaryl-substituted compounds are considerably
bathochromic shifted, with the maximum of the mixed-substituted
derivative 1abMes* being right in the middle between those of
1aaTip/1aaMes* and 1bbMes*, though they are in a quite similar
range. This points to comparably strong conjugation between the
thienyl and the furyl groups over the boron center. The fluores-
cence quantum yield of 10 % for the mixed derivative 1abMes* fits
within the trend of increasing emission intensity with increasing
ratio of furan rings.
and also along that of mixed species. With the series 1aaTip
<
2aaaTip < 9aaaaTip < 3aTip < 3a′Tip and 1bbMes* < 2bbbMes* < 3bMes*
< 3b′Mes* at hand, we were able to determine the effective conju-
gation lengths (ECL) of both poly(thienylborane)s and poly(furyl-
borane)s by extrapolation of an exponential fit of the maximum
absorption wavelengths plotted against the number of repeat
units (Figure 4).[27a] This gave an nECL of 9 repeat units for both
classes of compounds. For comparison, this is smaller than the
nECL values of polythiophene (nECL = 17),[27a] and polyfuran (nECL
= 33)[27b] but larger than that of polyfluoreneborane (nECL = 5).[27d]
For an infinite thienylborane polymer chain our study predicts an
absorption wavelength of λ∞ = 414 nm, and for the limiting value
of the infinite poly(furylborane) we obtain λ∞ = 412 nm. Conse-
quently, the effective conjugation lengths for both classes of
polymers have been reached with 3a′Tip and 3b′Mes*, respectively.
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10.1002/chem.201706124
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values of 1aaMes* vs 1bbMes* and 1aaTip vs 1bbTip, respectively,
correlates with the more electron-rich nature of furan vs thiophene.
The redox wave of the mixed-substituted derivative 1abMes* (E1/2
= −2.73 V) appeared halfway between those of 1aaMes* (∆E1/2
150 mV) and 1bbMes* (∆E1/2 = 100 mV).
=
Computational studies. To gain deeper insight into the electron-
ic structures of the species reported herein, we carried out DFT
and TD-DFT calculations using the B3LYP functional in combina-
tion with the def2-SV(P) basis set and including the dispersion
correction DFT-D3(BJ) by Grimme (see the Supporting Informa-
tion, Table S8 and Figures S133 – S139, for complete results).[28-
33] As the bulky aryl substituent on boron (Ar; i.e., the side groups
in the oligomers) we chose mesityl for all compounds for better
comparability and computational convenience.[34] Geometry opti-
mization of 1aaMes, 1bbMes, and 1abMes in the gas phase yielded
structures with largely coplanar hetarene rings and BR3 environ-
ments as observed also by crystallography for the experimental
equivalents in the solid state (vide supra). In accordance with the
solid state structures also is that the coplanarity is significantly
more pronounced in the difurylborane 1bbMes (torsion angle be-
tween the hetaryl rings, 14.4°) than in the dithienylborane 1aaMes
(torsion angle, 1.9°). The phenyl rings in 1ccMes are considerably
more heavily twisted against each other (torsion angle, 47.6°).
The HOMO and the HOMO−1 of the monomeric compounds
1aaMes, 1bbMes, 1abMes, and 1ccMes are localized on the mesityl
ring. However, excitations from these orbitals to the LUMO are
predicted to be of negligible probability. According to our compu-
tations, the strongest low-energy absorption band results from a
transition from the HOMO−2 to the LUMO (Figure 6). Both orbitals
are characterized as π orbitals delocalized over the two (het)aryl
rings,[35] while the LUMO shows significant contribution from the
pπ orbital on boron. Hence, the corresponding excitation process
is characterized as a π–π* transition associated with some degree
of intramolecular charge transfer (ICT) to the boron center.
Figure 4. UV–vis absorption spectra (lowest-energy π–π* absorption bands)
and plots of the respective maximum absorption wavelengths against the num-
ber of repeat units with exponential fits to determine the effective conjugation
lengths (ECL) of poly(thienylborane)s (top) and poly(furylborane)s (bottom).
The UV–vis spectra of the mixed compounds show significant
red-shifts with increasing chain length as well, as shown for the
series 1abMes* < 2abaMes* ≈ 2babMes* < 9aabaMes* (Figure 5). This
demonstrates effective π conjugation in these species between
the different hetarenes and including the boron centers.
Figure 5. UV–vis absorption spectra of mixed thienyl/furylborane species
1abMes*, 2abaMes*, 2babMes*, and 9aabaMes*
.
Electrochemical studies. The triarylboranes 1 were furthermore
studied by cyclic voltammetry (CV). The CV trace of
dithienylborane 1aaTip (E1/2 = −2.50 V) showed a reversible redox
wave in the same range with that of 1aaMes* (E1/2 = −2.58 V),[14]
while the difurylboranes 1bbTip (E1/2 = −2.76 V) and 1bbMes* (E1/2
= −2.83 V) gave rise to a quasi-reversible reduction process at
slightly more negative potential. The position of the half wave
potentials depends on the nature of both the hetaryl groups and,
to a lesser extent, the sterically demanding aryl substituent. The
compounds with the bulkier Mes* substituent showed an about 70
– 80 mV more negative reduction potential than their Tip conge-
ners. The difference of about 250 – 260 mV between the E1/2
Figure 6. Orbital energy levels and plots of the LUMO and the HOMO−2 (iso-
value: 0.04 au) of 1aaMes, 1bbMes, and 1ccMes (B3LYP-D3(BJ)/def2-SV(P)).
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Compared to the dithienylborane 1aaMes, in the difurylborane
1bbMes the energy levels of all relevant orbitals are increased to
approximately the same degree, which results in similar excitation
energies for the two compounds (Table 4). This correlates with
the more electron-rich nature of furan vs. thiophene. The calculat-
ed excitation energies are in good agreement with the experimen-
tal results, and the slight trend experimentally found for the posi-
tion of the absorption maxima in the series including the mixed
furyl/thienylborane species, is nicely reproduced by the computed
data: 1aaMes (316.2 nm) > 1abMes (309.3 nm) > 1bbMes (305.4 nm).
The diphenylborane 1ccMes has a significantly larger HOMO−2-
to-LUMO energy gap, due to a relatively lower-lying HOMO−2,
which results in a shorter wavelength of the absorption maximum
(286.3 nm) compared to the dihetarylboranes. Our calculations
revealed for all monomeric compounds one or more closely
spaced further π–π* excitations at higher energy (Table 4). These
involve a lower-lying occupied π orbital delocalized over the
(het)aryl rings and the boron center and two π orbitals localized
on either of the two (het)aryl rings, respectively (see SI, Figures
S133 – S136). These excitations are the presumable origin of the
high-energy band in the spectra of the triarylboranes investigated
(vide supra).
and the HOMO−1 in 1bbMes). In the dimer and the trimer, 2bbbMes
and 9bbbbMes, this becomes the HOMO−1 (Figure 7), and in the
tetramer, it represents the HOMO.
Figure 7. LUMO and HOMO−1 (isovalue: 0.04 au) of 9bbbbMes (B3LYP-
D3(BJ)/def2-SV(P)).
Fluoride sensing experiments. In order to explore the ability of
our thienyl- and furylboranes to bind and detect fluoride anions,[6,8]
we performed reactions of the monomers 1aaTip, 1bbTip, and
1bbMes* with tetra-n-butylammonium fluoride (TBAF) in THF. First,
we monitored the reactions by NMR spectroscopy. Whereas the
11B{1H} NMR spectrum of 1bbMes* remained unchanged in the
presence of F−, in the spectra of 1aaTip and 1bbTip, new reso-
nances appeared that are characteristic of four-coordinate boron
species [δB = 2 ppm (1aaTip), SI, Figure S21; δB = 0 ppm (1bbTip),
SI, Figure S31]. Encouraged by these results, we then explored
the possibility to follow the putative fluoride binding processes by
UV–vis spectroscopy (Figure 8a,b). Indeed, upon titration of
1aaTip and 1bbTip in THF with TBAF, the π–π* absorption bands
at λabs,max = 325 and 275 nm (1aaTip) and λabs,max = 324 and 269
nm (1bbTip), respectively, continuously decreased in intensity.
After the addition of about two equivalents of TBAF the low-ener-
gy band was quenched completely. This is consistent with the in-
terpretation that facile binding of fluoride to the boron center oc-
curs; thus, leading to the formation of a borate species (see Figure
8, bottom right). Through this process, π conjugation over the bo-
ron center is effectively disrupted, hence, the corresponding π–π*
absorption bands vanish. The fluoride binding constants were
determined from Benesi-Hildebrand plots as K = 5.0 × 104 M−1 for
1aaTip and K = 3.9 × 104 M−1 for 1bbTip. For the used concentration
(1.5 × 10−5 M in THF) both monomers showed a detection limit at
a F− concentration of about 1 × 10−10 M.
Table 4. Calculated[a] vertical singlet π–π* excitations.
Compd.
1aaMes*
1bbMes*
λ [nm]
Oscillator
strength f
Orbital contributions
|c|2 [%]
316.2
265.7
0.3650
0.0917
HOMO−2 → LUMO
HOMO−5 → LUMO
94.4
86.8
305.4
0.4886
HOMO−2 → LUMO
HOMO−1 → LUMO
HOMO−3 → LUMO
HOMO−4 → LUMO
HOMO−5 → LUMO
75.9
23.0
91.1
89.5
93.5
258.7
233.1
230.3
0.0012
0.0324
0.0138
1abMes*
309.3
261.5
0.4557
0.1202
HOMO−2 → LUMO
HOMO−1 → LUMO
HOMO−4 → LUMO
77.8
20.3
90.6
1ccMes*
286.3
254.1
0.3061
0.0806
HOMO−2 → LUMO
HOMO−5 → LUMO
94.7
91.8
2bbbMes*
373.4
418.8
0.4885
0.6657
HOMO−1 → LUMO
HOMO−1 → LUMO
68.3
89.8
9bbbbMes*
We then aimed at transferring these results to the use of thienyl-
and furylborane polymers 3aTip and 3bMes*. As in the case of the
monomers, the Mes*-substituted derivative 3bMes* showed no
evidence of fluoride binding ability. Upon addition of TBAF to a
solution of 3aTip, on the other hand, the 11B{1H} NMR spectrum
displayed a new resonance 0.2 ppm assigned to a borate boron
center (Figure S73, SI). The π–π* transition band in the UV–vis
spectrum of 3aTip (λabs,max = 409 nm) continuously decreased in
intensity during fluoride titration and was fully quenched when four
equivalents of TBAF had been added (Figure 8c). Concomitantly,
two new bands emerged at λabs,max = 369 and 294 nm.
[a] TD-B3LYP-D3(BJ)/def2-SV(P).
The effect of chain elongation was investigated for furylborane
oligomers,[36] which were calculated up to the tetramer (see SI,
Table S8 and Figures S137 – S139, for complete results). The
bathochromic shifts of the π–π* excitations with increasing num-
ber of repeat units are slightly overestimated by the calculations
(Table 4). However, it is known that B3LYP often tends to yield
somewhat lower excitation energies,[37] and this has a greater ef-
fect on longer-wavelength transitions. Chain elongation causes a
decrease of the gap between the LUMO (π* orbital) and the rele-
vant occupied delocalized π orbital. This leads to a change in the
order of the latter and the Mes-centered orbitals (i.e., the HOMO
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Experimental Section
General procedures. All manipulations were performed under an atmos-
phere of dry argon using standard Schlenk techniques or in an MBraun
glovebox. Solvents (dichloromethane, n-pentane, diethylether, toluene,
and tetrahydrofuran) were dried and degassed by means of a MBraun
SPS-800 solvent purification system. o-Dichlorobenzene was dried over
CaH2 and distilled prior to use. Deuterated solvents for NMR spectroscopy
were dried and degassed at reflux over Na (C6D6) or CaH2 (CDCl3 and
CD2Cl2) and freshly distilled prior to use. Solvents for aqueous work-up
(dichloromethane, ethanol, n-hexane, n-pentane), 2-bromothiophene, tri-
bromoborane, lithium bis(trifluoromethylsulfonyl)imide, bromobenzene,
sulfuric acid, and magnesium turnings were purchased from commercial
sources and used as received. Solutions of n-butyllithium (1.6 M in hexane
and 2.5 M in hexane, respectively) and t-butyllithium (1.7 M in pentane)
were purchased from Sigma Aldrich and used as received as well. Chloro-
trimethylsilane, 2-bromo-1,3,5-triisopropylbenzene, thiophene and furan
were commercially purchased and freshly distilled prior to use. 2-Bromo-
furan,[38] trimethylsilyl bis(trifluoromethylsulfonyl)imide,[39] 2,4,6-triisoprop-
ylphenyllithium,[40] 2-bromo-1,3,5-tri-tert-butylbenzene[41] and 2,4,6-tri-tert-
butylphenyllithium[14] were prepared according to methods described in the
literature. Compounds 5a, 5b, 5c, 4a, 4b, 4c, 7a, and 7b were prepared
according to a procedure previously described by our group.[23] NMR spec-
tra were recorded at 25 °C on a Bruker Avance II-400 spectrometer or on
a Bruker Avance III HD spectrometer operating at 400 MHz. Chemical
shifts (in ppm) were referenced to residual protic impurities in the solvent
(1H) or the deuterio solvent itself (13C) and reported relative to external
SiMe4 (1H, 13C) or BF3·OEt2 (11B) standards. Mass spectra were obtained
with the use of a Finnigan MAT95 spectrometer employing electron ioniza-
tion (EI) using a 70 eV electron impact ionization source or secondary ioni-
zation (SIMS). Elemental analysis was performed with a CHN-O-Rapid
VarioEL by Heraeus. UV–vis spectra were obtained using a Jasco V-630
spectrophotometer. For fluoride titration experiments a solution of tetra-n-
butylammonium fluoride (TBAF) in THF (1 M) was diluted to yield a solution
Figure 8. UV–vis absorption spectra recorded during the titration of 1aaTip (a),
1bbTip (b), and 3aTip (c) with TBAF.
Conclusions
We have presented a systematic study of linear extended thienyl-
and furylboranes as well as derivatives of mixed composition.
Monodisperse monomers, dimers, and trimers have been
prepared by modular synthesis using our newly developed cata-
lytic Si/B exchange condensation method. Polycondensation re-
actions via this approach led to higher oligomers and polymers.
Having extensive series of thienyl- and furylborane oligomers
and polymers of varying chain length at hand, allowed us to deter-
mine the effective conjugation lengths (ECL) of both classes of
compounds, which have been reached with the highest-molecu-
lar-weight derivatives presented in this study. Extended thienyl-
and furylboranes show pronounced π-electron delocalization over
the hetarene backbones including the boron centers. Our photo-
physical investigations further showed that their absorption and
emission characteristics are effectively modulated by the ratio of
furan to thiophene rings incorporated. Significantly, higher furan
ratios lead to considerably increased fluorescence intensities of
the materials. Compounds 1aaTip, 1bbTip, and 3aTip furthermore
showed the ability to bind and detect fluoride anions, accompa-
nied with a distinct change in their optical absorption characteris-
tics. The possibility to easily monitor the binding processes by
UV–vis spectroscopy demonstrates the potential use of our new
materials for sensory applications.
with equivalent concentration with respect to 1aaTip, 1bbTip, and 3aTip
.
Within a glass cuvette different equivalents of TBAF were mixed with a
solution of the corresponding boron-containing compound in THF (c = 1.5
x 10−5 mol/L (1aaTip, 1bbMes*); c = 5.1 x 10−6 (3aTip)) and the reaction was
monitored by determination of the UV–vis absorption spectra. As basic
parameters a response of a moving average of 0.25 s, a bandwidth of 1.5
nm, and a scan speed of 1000 nm min−1 were applied. Fluorescence
spectra were obtained with
a Jasco FP-6500 spectrofluorometer.
Fluorescence quantum yields were determined against commercially
available standards (1abMes* against diphenylanthracene (DPA), 3aTip and
3bMes* against perylene). Melting points (uncorrected) were obtained using
a SMP3 melting point apparatus by Stuart in 0.5 mm (o.d.) glass capillaries,
which were sealed under argon. GPC chromatograms were recorded on
an Agilent 1100 Series, equipped with two SDV linear N columns of 8x300
mm and 8x600 mm measures and 5 mm pore size at 30 °C against a poly-
styrene standard. Detection was carried out via UV signal (λ = 250 nm)
and refractive index (RI), respectively. Cyclic voltammetry experiments
were carried out on a PGSTAT101 analyzer from Metrohm. The three-
electrode system consisted of a Pt disk as working electrode, a Pt wire as
counter electrode and an Ag wire as the reference electrode. The
voltammograms were recorded with ca. 10−3 M solutions in THF containing
Bu4N[PF6] (0.1 M) as the supporting electrolyte. The scans were
referenced after the addition of a small amount of ferrocene as internal
standard. The potentials are reported relative to the ferrocene/ferrocenium
couple.
This study demonstrates the versatile application of our catalytic
B–C bond formation method for the synthesis of conjugated or-
ganoborane oligomers and polymers. It furthermore shows that
furan can keep pace with thiophene groups in their use as building
blocks of π conjugated materials based on triarylboranes. Current-
ly, we are further extending our synthetic approach to access a
broad range of new materials with tailored properties and func-
tions, and we aim to elucidate mechanistic details of their forma-
tion.
General procedure for the synthesis of 1. To a solution of 5 (1 mmol) in
dichloromethane (1 mL) was added TMS-NTf2 (5 mol %) at room tempera-
ture. A solution of 4 (1 mmol) dissolved in dichloromethane (1 mL) was
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10.1002/chem.201706124
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added and the reaction process was monitored using 1H NMR spectros-
copy. The resulting solution was evaporated to dryness to yield crude 6,
which was subsequently re-dissolved in toluene (1.5 mL). To this solution
a solution (TipLi) or a suspension (Mes*Li) of the lithiated arene (1.1 mmol)
in toluene (1.5 mL) was added at −78 °C. The reaction mixture was slowly
warmed to room temperature and stirred over night. The resulting suspen-
sion was diluted with n-pentane and quenched by adding water. The or-
ganic phase was washed with brine, dried over MgSO4, and the solvent
was removed in vacuo. The triarylborane 1ccMes* was purified by two re-
crystallizations from n-hexane at −40 °C. The other products were purified
by column chromatography (silica; n-hexane).
days at room temperature. The resulting suspension was diluted with n-
pentane and quenched by adding water. The organic phase was washed
with brine, dried over MgSO4, and the solvent was removed in vacuo. The
solid product was purified by precipitation with pentane/ethanol.
X-ray crystallographic analysis of 1aaTip, 1bbTip and 2bbbMes*. Suitable
colorless single crystals of 1aaTip, 1bbTip and 2bbbMes* were obtained by
slow evaporation of hexane at ambient temperature. Data were collected
on a Bruker SMART APEX CCD detector on a D8 goniometer equipped
with an Oxford Cryostream 700 temperature controller at 100(2) K using
graphite monochromated Mo-Kα radiation (λ = 0.71073 Å). An absorption
correction was carried out semi-empirically using SADABS[42] (min./max.
transmissions = 0.4052/0.7446 (1aaTip; CCDC-1533828), 0.4324/0.7451
(1bbTip; CCDC-1533828), 0.5925/0.7452 (2bbbMes*; CCDC-1533828)).
General procedure for the synthesis of 2. To a solution of 7 (1 mmol) in
dichloromethane (1 mL) was added TMS-NTf2 (10 mol %) at room temper-
ature. A solution of 4 (1 mmol) dissolved in dichloromethane (1 mL) was
added and stirred over night. The resulting solution was evaporated to dry-
ness and subsequently re-dissolved in toluene (1.5 mL). To this solution a
solution (TipLi) or a suspension (Mes*Li) of the lithiated arene (1.1 mmol)
in toluene (1.5 mL) was added at −78 °C. The reaction mixture was slowly
warmed to room temperature and stirred over night. The resulting suspen-
sion was diluted with n-pentane and quenched by adding water. The or-
ganic phase was washed with brine, dried over MgSO4, and the solvent
was removed in vacuo. The products were purified by column chromatog-
raphy (silica; n-hexane).
The structure was solved with Olex2[43] using Direct Methods (ShelXS[44a]
)
and refined with the ShelXL[44b] refinement package by full-matrix least
squares on F2. All non-hydrogen atoms were refined anisotropically. The
hydrogen atoms were included isotropically and treated as riding. The thio-
phene rings in 1aaTip are splitted. Two molecules of hexane are co-crys-
tallized in the structure of 2bbbMes*
.
Computational methods. DFT calculations were carried out with the
TURBOMOLE V7.0.1 program package.[28] Optimizations were performed
with Becke’s three parameter exchange-correlation hybrid functional
B3LYP[29] in combination with the valence-double-ζ basis set def2-
SV(P).[30] The empirical dispersion correction DFT-D3 by Grimme was
used including the three-body term and with Becke-Johnson (BJ) damp-
ing.[31] The stationary points were characterized as minima by analytical
vibrational frequency calculations.[32] Vertical singlet excitations were cal-
culated by means of time-dependent DFT[33] using the same density func-
tional–basis set combination as specified above.
General procedure for the synthesis of 9. To a solution of 8 (1 mmol) in
dichloromethane (1 mL) was added TMS-NTf2 (10 mol %) at room temper-
ature. A solution of 4 (1 mmol) dissolved in dichloromethane (1 mL) was
added and stirred over night. The resulting solution was evaporated to dry-
ness and was subsequently re-dissolved in toluene (1.5 mL). To this solu-
tion a solution (TipLi) or a suspension (Mes*Li) of the lithiated arene (1.1
mmol) in toluene (1.5 mL) was added at −78 °C. The reaction mixture was
slowly warmed to room temperature and stirred over night. The resulting
suspension was diluted with n-pentane and quenched by adding water.
The organic phase was washed with brine, dried over MgSO4, and the sol-
vent was removed in vacuo. The products were purified by column chro-
matography (silica; n-hexane).
Acknowledgements
Funding by the German Research Foundation (DFG) through the
Emmy Noether Programme and the Research Grant HE 6171/4-
1 and support by the COST action CM1302 (SIPs) is gratefully
acknowledged. We thank Dr. K. Beckerle for GPC measurements,
T. Schindler for assistance with CV measurements, Prof. Dr. U.
Simon for UV–vis access, and Prof. Dr. J. Okuda for generous
support and helpful discussions.
General procedure for the synthesis of 3 (Route A). To a solution of 7
(1 mmol) in pentane (2 mL) was added BBr3 (1 mmol) at −78 °C. The re-
sulting suspension was stirred for 2 h maintaining −78 °C. Then, the mix-
ture was warmed to 0 °C and was stirred until a solution was formed (15
min). All volatiles were removed in vacuo at 0 °C to yield crude 10, which
was subsequently re-dissolved in dichloromethane (2 mL). TMS-NTf2 (5
mol%) was added and the reaction had been stirred for three days at room
temperature. After evaporating to dryness, the resulting solid residue was
dispersed in toluene (3 mL). To this dispersion a solution (TipLi) or a sus-
pension (Mes*Li) of the lithiated arene (1.25 mmol) in toluene (3 mL) was
added at room temperature. The reaction mixture had been stirred for
three days at room temperature. The resulting suspension was diluted with
n-pentane and quenched by adding water. The organic phase was washed
with brine, dried over MgSO4, and the solvent was removed in vacuo. The
solid product was purified by precipitation with pentane/ethanol.
Keywords: boron • conjugated polymers • fluoride sensors •
furan • thiophene
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10.1002/chem.201706124
Chemistry - A European Journal
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A. Lik, S. Jenthra, L. Fritze, L. Müller, K.-
N. Truong, H. Helten*
Page No. – Page No.
From Monodisperse Thienyl- and
Furylborane Oligomers to Polymers –
Modulating the Optical Properties
Through the Hetarene Ratio
Catalytic Si/B exchange polycondensation provided access to a series of thienyl-
and furylboranes, monodisperse oligomers, and polymers. The photophysical
properties of such species are effectively modulated through variation of the ratio of
the heterocycles in the backbone. Careful choice of the constituent rings and the
side groups leads to materials that detect fluoride anions via an optical response.
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