Influence of Extended Conjugation on Photophysical/Electronic Properties and Photoelimination of BN-Heterocycles

Article abstract of DOI:10.1021/acs.organomet.7b00290

A 1,1-hydroboration reaction was used successfully to create brominated BN-heterocyclic compounds, which are amenable to Stille coupling reactions for the construction of new BN-heterocyclic compounds, including a new polymeric BN-heterocycle that has an extended π-conjugated backbone. The conjugated backbone of the new BN-heterocycles was found to have a great influence on the photophysical and electronic properties of this class of compounds. In addition, the photoelimination reactivity of the new BN-heterocycles was also found to be greatly dependent on the extent of the conjugated backbone. Several new 1,2,4-triazole-fused boranaphthalenes have been obtained successfully via photoelimination.

Full text of DOI:10.1021/acs.organomet.7b00290

Article
pubs.acs.org/Organometallics
Influence of Extended Conjugation on Photophysical/Electronic
Properties and Photoelimination of BN-Heterocycles
Yong-gang Shi,† Xiang Wang,‡ Nan Wang,† Tai Peng,*,† and Suning Wang*,†,‡
†Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of Chemistry, Beijing Institute of
Technology, Beijing 100081, People’s Republic of China
‡Department of Chemistry, Queen’s University, Kingston, Ontario K7L 3N6, Canada
S
* Supporting Information
ABSTRACT: A 1,1-hydroboration reaction was used successfully to create
brominated BN-heterocyclic compounds, which are amenable to Stille
coupling reactions for the construction of new BN-heterocyclic compounds,
including a new polymeric BN-heterocycle that has an extended π-
conjugated backbone. The conjugated backbone of the new BN-heterocycles
was found to have a great influence on the photophysical and electronic
properties of this class of compounds. In addition, the photoelimination
reactivity of the new BN-heterocycles was also found to be greatly
dependent on the extent of the conjugated backbone. Several new 1,2,4-triazole-fused boranaphthalenes have been obtained
successfully via photoelimination.
optical energy gap are necessary. However, the effect of
extended π-conjugation on the photophysical/electronic
properties and the PE reactivity of B,N-heterocycles with a
core structure such as 1a has not been established. Therefore,
we set out to develop synthetic methods for extended π-
conjugated molecules containing a PE active B,N-heterocyclic
unit.
We have found that postmodification of molecules that have
a benzyl-pyridyl backbone such as A and its halogen-substituted
derivatives for C−C coupling to extend the backbone
conjugation is difficult. On the other hand, we have discovered
that bromo-substituted B,N-heterocycle 1a is amenable for post
C−C cross-coupling reactions. Therefore, in this study, we
selected 1a as the core structure and progressively increased
extended conjugation of the backbone via compounds 2b−5b
shown in Scheme 1. Investigation of these new molecules
revealed the distinct effect of extended conjugation on the
electronic properties and PE reactivity of the B,N-heterocycles
and luminescence of the corresponding B,N-arenes. The details
are presented herein.
INTRODUCTION
■
BN-embedded aromatic compounds (BN-arenes) are an
attractive class of materials that have attracted much recent
research attention,1,2 mainly because of their unique electronic/
photophysical properties and versatile applications in organic
optoelectronic materials/devices.3,4 Various synthetic methods
have been developed for achieving a variety of B,N-arenes.3−6
In particular, B,N-heterocycles such as A in Scheme 1 have
been found to be convenient precursors for the generation of
B,N-arenes B via photoelimination (PE) reactions.7 One
advantage of the PE reaction is that the precursor B,N-
heterocycle may be readily prepared either by lithiation of the
appropriate substrate, followed by the addition of BR2X, or by
the catalyst-free 1,1-hydroboration of pyrido[1,2-a]isoindole
and derivatives.7b 1,1-Hydroboration has also been realized for
analogues of pyrido[1,2-a]isoindole, leading to new B,N-
heterocycles such as 1a, which also undergoes a PE reaction,
forming B,N-arenes such as 1c.7d Pammer and co-workers have
shown that 1,2-hydroboration of 2-(2-vinylphneyl)pyridine and
derivatives is also an effective strategy for the synthesis of B,N-
heterocycles such as D, which contains an asymmetric carbon
atom, although the photochemistry of D has not been
established.8 These new synthetic methods make it possible
to access a large variety of B,N-heterocyclic compounds and
carry out a comprehensive study of the PE phenomenon. Our
recent successful demonstration of exciton-driven elimination
(EDE) of B,N-heterocycles for in situ generation of B,N-arenes
in OLEDs9 opens up the possibility of using B,N-heterocyclic
precursor compounds for the in situ generation of functional
B,N-arenes in optoelectronic devices such as organic field effect
transistors (OFETs) and organic solar cells. For these
applications, extended π-conjugated molecules or polymers
for easy processing and achieving the desired energy levels/
RESULTS AND DISCUSSION
■
Syntheses and Structures of BN-Heterocycles 2b−5b.
1-Methyl-2-phenyl-1H-[1,2,4]triazolo[5,1-a]isoindole (the het-
erocycle L in Scheme 1) can be obtained readily and undergoes
facile 1,1-hydroboration as pyrido[2,1-a]isoindole does, provid-
ing easy access to B,N-heterocycles.7d The precursor B,N-
Special Issue: Tailoring the Optoelectronic Properties of Organo-
metallic Compounds with Main Group Elements
Received: April 15, 2017
© XXXX American Chemical Society
A
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1a (72%). Nonetheless, this is the first successful demon-
stration of 1,1-hydroboration involving brominated substrates.
The low hydroboration yield can be attributed to the inductive
electron-withdrawing nature of bromine atom that destabilizes
the key intermediatethe adduct of borane with L-b.7b
Scheme 1. Examples of Hydroboration for B,N-Heterocycle
Synthesis and B,N-Arene Formation via Photoelimination
To extend the π-conjugation of the backbone, we introduced
thienyl units to the central core. First, we attempted to connect
the thienyl units to 2a and 3a via Suzuki coupling methods.
However, this approach failed to produce the desired product
because it led to deborylation of the BMes2 group, attributable
to the basic conditions (Na2CO3 was used as the base) of the
Suzuki coupling method (see the Supporting Information). To
avoid the use of basic conditions, Stille coupling was
investigated as a general method to attach thienyl rings to
compounds 2a and 3a, which turned out to be effective. Using
tris(dibenzylideneacetone)dipalladium (Pd2(dba)3) as the
catalyst and trimethyltin-functionalized thienyl/bis-thienyl
units as the coupling partners, in the presence of P(o-tolyl)3,
thienyl/bis-thienyl units can be attached to 2a and 3a
successfully, producing compounds 2b−4b, respectively, as
shown in Scheme 3. The hexyl group at the ortho position of
Scheme 3. Synthesis of Compounds 2b−5b
heterocycles 2a and 3a for the target compounds examined in
this work were prepared using this approach. The key starting
materials are the monobrominated and dibrominated 1-methyl-
2-phenyl-1H-[1,2,4]triazolo[5,1-a]isoindole (L-b), which can
be prepared by the procedure shown in Scheme 2 via L-I. The
Scheme 2. Synthetic Procedures of L-b and 1a−4a
SnMe3-functionalized thiophene employed in the synthesis of
2b and 3b may be responsible for the relatively low yields of the
products due to steric hindrance in Stille coupling. To reduce
steric hindrance and improve the yield of bis-thienyl-linked
B,N-heterocycle polymers, we prepared (3,3′-dihexyl[2,2′-
bithiophene]-5,5′-diyl)bis(trimethylstannane), according to a
literature procedure,10 and used this tin reagent for the
synthesis of the polymeric compound 5b. Using the same
reaction conditions as employed for 2b−4b synthesis, the
reaction of 3a with the tin reagent in a 1:1 ratio and the end-
capping reagent bromobenzene produced successfully polymer
5b in 42% yield. All four compounds are stable in solution and
in the solid state under ambient conditions.
corresponding precursors (L-a) were prepared in high yields via
the condensation reaction shown in Scheme 2. Methylation of
L-a by dimethyl sulfate, followed by anion exchange with
iodide, produced the iodide salts L-I in good yields. Treatment
of the iodide salts by 3 equiv of KOtBu generated L-b in situ,
which could not be isolated due to their high sensitivity to air.
The addition of 2 equiv of HBMes2 to the solution of L-b led to
the formation of the corresponding B,N-heterocycles. For
comparison, the yields of 1a and its precursor compounds are
also shown in Scheme 2. The one-pot 1,1-hydroboration of L-I
with KOtBu and HBMes2 produced compounds 2a and 3a in
45% and 31% yields, respectively, which are lower than that of
Compounds 2b−5b were fully characterized by NMR,
HRMS, or GPC analyses. All four compounds display a 11B
chemical shift at −1.4, −1.9, 1.1, and 3.1 ppm, respectively,
which is characteristic of four-coordinate boron.7 The polymer
5b has a mean Mn value of 6888, corresponding to about 8
B
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repeating units. The mean PDI value of 5b was determined to
be 1.5 (see the Supporting Information for details). The crystal
structure of 4b was determined by single-crystal X-ray
diffraction analysis and is shown in Figure 1. The B−C bond
Photophysical and Electronic Properties of 2b−5b.
The UV/vis spectra recorded in THF for compounds 2b−5b
along with that of 1a are shown in Figure 2. The impact of
Figure 2. Absorption spectra of 1a and 2b−4b in THF (∼1 × 10−5
M).
extended π-conjugation on the electronic properties of B,N-
heterocycles is evident. From 1a to 5b, the absorption edge is
red-shifted. The absorption bands of 4b and polymer 5b are
bathochromically shifted by 30−50 nm, relative to other
molecules. Although 5b contains about eight repeating units,
the energy of its absorption band/edge is only slightly smaller
than that of 4b, which is clearly caused by the poor conjugation
between the two neighboring thienyl units in 5b.
Compound 4b displays two well-resolved reversible reduc-
tion peaks in its CV diagram, while others show either
irreversible or pseudoreversible peaks in the CV diagrams that
are at potentials much more negative than that of 4b (see the
Supporting Information). To better define the reduction
potential of this group of compounds, differential pulse
voltammetry was performed for all compounds, and the data
are shown in Figure 3 and Table 1. In general, with an increase
Figure 1. (top) Crystal structure of 4b with labeling schemes and 35%
thermal ellipsoids. H atoms are omitted for clarity. Important bond
lengths (Å) and angles (deg): B(1)−N(1) 1.621(4), B(1)−C(1)
1.665(5), B(1)−C(11) 1.652(4), B(1)−C(20) 1.648(5); C(1)−
B(1)−N(1) 99.1(2), C(1)−B(1)−C(11) 118.6(3), C(1)−B(1)−
C(20) 103.0(2), C(11)−B(1)−N(1) 104.9(2), C(20)−B(1)−N(1)
113.1(3). (bottom) DFT optimized structure of the hexyl-substituted
analogue 4b′, showing the twist of the thienyl rings.
lengths in 4b are similar and comparable to those7 of 2-(o-
tolyl)pyridine-BMes2 and precursor compound 1a. However,
the B−N bond in 4b (1.628(2) Å) is shorter than that in 1a
(1.636(2) Å). The bis-thienyl units in 4b are essentially
coplanar, as indicated by the dihedral angles between the S1
and S2 rings (2.4°) and between the S3 and S4 rings (11.6°).
The dihedral angles between the S3 ring and the C40 benzene
ring and between the S1 ring and the C5 benzene ring are 17.5
and 42.0°, respectively, while that between the C40 benzene
ring and the triazole ring is 49.3°, indicating good conjugation
between these units. The hexyl-substituted analogue of 4b
(4b′), which could serve as a model compound for the
repeating unit in polymer 5b, was not prepared. Nonetheless, as
shown by the DFT optimized structure of 4b′ in Figure 1, the
thienyl rings in 4b′ are nearly perpendicular to each other due
to the interactions between the hexyl substituents, which
supports the notion that the repeating unit in the polymer 5b is
less conjugated than the backbone of 4b.
Figure 3. Differential pulse voltammetry diagrams of 1a and 2b−5b
recorded in DMF with [NBu4]PF6 as the electrolyte, relative to FC/
FC+.
in the backbone conjugation, the first reduction potential
becomes less negative, consistent with a decrease in the LUMO
energy level. The exception is between 4b and 5b, because 4b
has a more positive reduction potential and a lower LUMO
level in comparison to those of 5b. Again, this can be attributed
to the disruption of conjugation between the two hexyl-
substituted thienyl rings in 5b. The DFT calculated trend of the
LUMO energy level for 1a and 2b−4b agree with the
electrochemical data, as shown in Figure 4.
C
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Table 1. Photophysical and Electrochemical Data
absorption
fluorescence
a
b
d
e
f
e
f
compound
λmax (nm)
Egopt (eV)
λem (nm)
ΦFL
Ered (eV)
HOMO (eV) exptl /DFT
LUMO (eV) exptl /DFT
c
1a
2b
3b
4b
5b
1c
2c
3c
4c
310
310
330
365
352
404
405
410
340
3.35
3.35
3.18
2.82
2.75
2.70
2.70
2.53
2.53
−2.80
−2.67
−2.53
−5.35/−5.10
−5.48/−5.10
−5.45/−5.16
−5.30/−5.12
−5.15/−
−2.00/−1.64
−2.13/−1.66
−2.27/−1.79
−2.50/−2.04
−2.40/−
c
c
c
425
475
478
478
488
505
0.050
0.020
0.13
0.14
0.25
0.17
−2.58, −2.30
c
−2.40
−/−4.70
−/−4.71
−/−4.76
−/4.78
−/−1.33
−/−1.42
−/−1.47
−/−1.90
a
b
c
The first low-energy, resolved absorption band in THF. Egopt = absorption edge. In DMF with [NBu4 ]PF6 as the electrolyte. Potentials vs Fc/Fc+.
d
Determined from the first reduction peak positions in DPV diagrams which agree well with E1/2 from the CV diagrams (see the Supporting
e
f
Information). ELUMO = −(4.8 + Ered); EHOMO = ELUMO − Egopt. DFT calculations were performed using the B3LYP/6-31G(d) level of theory.
and S3) are predominantly charge-transfer transitions from the
Mes groups to the chelate backbone with relatively low
oscillator strengths (∼0.001 to 0.007) for compounds 1a, 2b,
and 3b. For 4b and 4b′, the transitions to S1 and S2 are CT
transitions similar to those of 1a, 2b, and 3b. However, the
transition to the S3 state by 4b and 4b′, which is close in energy
to the S2 state, involves a large contribution from the π-orbital
of the backbone (36% and 46%, respectively) in addition to the
Mes group. Thus, as the backbone conjugation increases, the π
to π* transition localized on the backbone has increased
contributions to the low-lying excited states of the BN-
heterocycles, which may be responsible for the fluorescence of
4b and 5b. The calculated S0 → S1 transition energy for 4b′ is
0.11 eV greater than that of 4b, in agreement with the twisted
structure of 4b′ shown in Figure 1.
Figure 4. HOMO and LUMO energy levels of compounds 1a and
2b−5b: (blue bars) experimentally estimated values; (black bars) DFT
calculated values (not calculated for 5b due to its large size).
Photoreactivity of 2b−5b and Photophysical Proper-
ties of 2c−5c. When they are subjected to UV irradiation at
either 300 or 350 nm in a THF, benzene, or toluene solution,
the monoboron compounds 2b−4b undergo photoelimination,
producing the corresponding triazole-fused boranaphthalenes
2c−4c, respectively, in a manner similar to that for the parent
molecule 1a (Scheme 4). In contrast, compound 5b does not
show any photoreactivity on irradiation under the same
conditions for 22 h. The lack of photoreactivity of 5b may be
BN-heterocycles such as A and 1a are in general not
fluorescent.7 It is therefore not surprising that compounds 2b
and 3b are also not fluorescent. Interestingly, however, 4b and
5b are fluorescent with λem at 425 and 475 nm, respectively,
albeit with a low quantum efficiency in THF (Figure 5). The
polymer 5b is also fluorescent in the solid state with λem ≈ 500
nm and a low quantum efficiency (∼1%). The red shift of the
emission band of 5b in the solid state can be attributed to
intermolecular interactions. TD-DFT data indicate that the
vertical transitions to the first three singlet excited states (S1, S2,
Scheme 4. Photoreactivity of 2b−5b
Figure 5. Fluorescence spectra of 1c−4c and 4b−5b in THF (∼1 ×
10−5 M).
D
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attributed to the extended π-conjugated backbone, which leads
to the backbone-dominated excited states and the lack of
significant contributions from the −CH2−BMes2 portion to the
low-lying excited states accessible at 300 or 350 nm excitation.
1H and 11B NMR spectral tracking at ∼0.01 M concentration
showed that the transformation of 2b−4b to 2c−4c is
quantitative (see the Supporting Information). Compounds
2c−4c display a characteristic 11B chemical shift at 32.8, 33.6,
and 34.3 ppm, respectively, similar to that9 of 1c. Under the
same irradiation conditions, the time needed to achieve full
conversion in the NMR experiments is ∼22 h for 2b to 2c, ∼24
h for 3b to 3c, and ∼72 h for 4b to 4c, respectively. The less
efficient photoelimination of 4b is consistent with its greater
extended π-conjugation of the backbone, relative to those of 2b
and 3b, where electronic transitions localized on the backbone
that do not contribute to the photoreaction pathway can
compete effectively for the excitation energy, hence resulting in
a less efficient photoreaction. The photoelimination process of
2b−4b was also tracked by UV/vis and fluorescence spectros-
copy at ∼1 × 10−5 M concentration. The photophysical data
for 2c−4c are summarized in Table 1. For 2b and 3b, a new
low-energy absorption band at 405 and 410 nm, respectively,
appears as the photoreaction proceeds, which is similar to that
of 1c and is assigned to the BN-arenes 2c and 3c, respectively.
In contrast, on irradiation, the absorption band of 4b at 352 nm
is gradually replaced by an intense high-energy band with well-
resolved vibrational features at 340 nm and a very weak
shoulder peak in the 390−460 nm region (see the Supporting
Information).
there is a much greater contribution from the fused benzene
ring to LUMO in comparison to that in 2c. For 4c, the LUMO
has essentially no contribution from the central core; instead, it
is localized on the phenyl-bis-thienyl unit and the HOMO to
LUMO transition has a low oscillator strength and may be
described as a charge-transfer transition. On the other hand, the
S0 → S2 transition in 4c involves mainly the LUMO+1 orbital
(93%) that has significant central core contributions and the
bis-thienyl units as well with a much higher oscillator strength.
Therefore, the typical π → π* transition of the central
azaborine core is no longer the lowest in energy for 4c, which is
clearly caused by the presence of the conjugated bis-thienyl
units. As a result, the main absorption band of 4c appears at a
higher energy in comparison to those of 2c and 3c.
The new triazole-boranaphthalene compounds are all
brightly fluorescent, as shown in Scheme 4. In the fluorescence
spectra, with irradiation of 2b and 3b, a peak at 478 and 488
nm (Figure 5), respectively, appears, which is responsible for
the characteristic blue-green fluorescent colors of 2c and 3c.
The fluorescent quantum efficiency of 2c (0.14) is similar to
that of 1c (0.13), while that of 3c is much higher (0.25). The
extra thienyl ring in 3c that is bound to the boranaphthalene
ring seems effective in red-shifting the emission energy and
enhancing ΦFL, relative to that of 1c. For 4b, under irradiation,
its fluorescent color changes from blue to green, and a new
fluorescent band at 505 nm, which is red-shifted by more than
80 nm in comparison to that of 4b, appears, which is assigned
to the photoelimination product 4c and likely originates from
the HOMO to LUMO+1 transition shown in Figure 6. The
fluorescence quantum yield of 4c is 0.17, comparable to those
of 1c and 2c.
To understand the different electronic properties of 2c−4c,
TD-DFT computational studies for these three compounds
were performed and the results are summarized in Table 1,
Figure 6, and the Supporting Information. As shown in Figure
6, the S0 → S1 transition for all three compounds involves
HOMO−LUMO orbitals (98%). The HOMO compositions
for all three compounds are similarlocated exclusively on the
central triazole-boranaphthalene unit. However, the LUMO
composition varies considerably. For 2c, the LUMO is
dominated by the phenyl ring and the triazole unit. For 3c,
CONCLUSIONS
■
In summary, we have demonstrated that brominated N-
heterocyclic compounds L-I can be successfully converted to
BN-heterocycles via one-pot 1,1-hydroboration reactions.
Furthermore, we have shown that brominated BN-heterocycles
based on a triazolyl-benzyl chelate core can be further
functionalized on the backbone via Stille coupling to produce
extended π-conjugated BN-heterocycles. All monoboron
species produced in this manner have been found to undergo
quantitative photoelimination, producing the corresponding
triazole-fused boranaphthalenes, which are brightly fluorescent.
The photoelimination reaction efficiency decreases with an
increase in the extended conjugation of the backbone. A
polyboron compound with a conjugated backbone was
obtained successfully, which does not show any photoreactivity
due to the effective competition of electronic transitions
localized on the π-conjugated backbone with the photo-
elimination pathway in the excited state. These findings provide
valuable guides for the future design of photoresponsive boron-
based materials.
EXPERIMENTAL SECTION
■
General Procedure. All solvents were freshly distilled over sodium
metal and stored under nitrogen prior to use. All starting materials
were purchased from Energy Chemical. HBMes2,11 3-hexyl-2-
(trimethyltin)thiophene,12 5-trimethylstannyl-2,2′-bithiophene,13 and
(3,3′-dihexyl[2,2′-bithiophene]-5,5′-diyl)bis[trimethylstannane]10
were synthesized according to procedures reported in the literature. All
reactions were performed under nitrogen and in dry solvents. 1H, 13C,
and 11B NMR spectra were recorded on a 400 or 700 MHz
Figure 6. Diagrams for orbitals involved in S0 → S1 (S0 → S2) vertical
excitation of 2c−4c obtained by TD-DFT calculations (B3LYP/6-
31G(d)).
1
spectrometer. H and 13C chemical shifts are in ppm, relative to the
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signals corresponding to the residual nondeuterated solvents (CDCl3,
576.2245. Data for 3a: white solid (yield: 31%); 1H NMR (400 MHz,
C6D6) δ 7.35 (s, 1H), 7.15−7.14 (m, 1H), 7.10 (dd, J = 7.9, 3.7 Hz,
2H), 7.04 (dd, J = 8.3, 1.7 Hz, 1H), 6.88 (s, 4H), 6.58 (d, J = 8.3 Hz,
1H), 6.48 (t, J = 7.9 Hz, 1H), 3.17 (s, 2H), 2.39 (s, 12H), 2.29 (s, 3H),
2.22 (s, 6H); 13C NMR (176 MHz, C6D6) δ 153.37, 152.18, 151.46,
141.32, 134.38, 133.98, 133.56, 131.06, 130.92, 130.64, 128.63, 127.37,
126.92, 125.31, 122.73, 119.38, 33.85, 25.37, 20.97; 11B NMR (225
MHz, C6D6) δ −1.84; HR-ESIMS (m/z) [M + 1]+ calcd for
C34H35BBr2N3 654.1291, found 654.1341.
1
1H 7.26 ppm, 13C 77.23 ppm; d6-DMSO, H 2.50 ppm, 13C 39.52
ppm; C6D6, 1H 7.16 ppm, 13C 128.06 ppm). 11B chemical shifts are in
ppm, relative to BF3·OEt2 (0.00 ppm). High-resolution electrospray
ionization mass spectra data were obtained via an ESI (Agilent (Q-
TOF 6520)) analyzer. UV/vis spectra were obtained on an Agilent
Cary 300 UV/vis spectrophotometer. Fluorescence spectra were
recorded on a Photon Technologies International Quanta Master
Model C-60 spectrometer. Fluorescent quantum efficiencies were
determined using a Hamamatsu Quantaurus-QY spectrometer
(C11347). Cyclic voltammetry experiments were conducted on an
AUTOLAB-CV-75W analyzer with a scan rate of 100 mV s−1. The
electrochemical cell was a standard three-compartment cell composed
of a Pt working electrode, a Pt auxiliary electrode, and a Pt-wire
reference electrode. All measurements were performed using 0.10 M of
NBu4PF6 in DMF as the supporting electrolyte. The potentials are
reported relative to the ferrocene/ferrocenium couple. The purity of
Synthesis of Compound 2b. In an oven-dried Schlenk flask were
placed 2a (50 mg, 0.087 mmol), 3-hexyl-2-(trimethyltin)thiophene
(32 mg, 0.095 mmol), Pd2(dba)3 (4 mg, 0.0044 mmol), and P(o-
tolyl)3 (5 mg, 0.017 mmol). The flask was evacuated and filled with N2
three times, and then dried toluene (4 mL) was injected into the
mixture. The resulting solution was stirred at 100 °C for 12 h under
N2. After being cooled to room temperature, the solvents were then
removed under reduced pressure. The crude product was purified by
silica gel chromatography (petroleum ether/CH2Cl2, 2/1). 2b was
1
all compounds reported here was established by H NMR and 13C
1
obtained as a white solid in 40% yield (23 mg): H NMR (400 MHz,
NMR spectra.
C6D6) δ 7.44 (s, 1H), 7.35 (d, J = 7.7 Hz, 1H), 7.22 (d, J = 7.8 Hz,
1H), 7.11 (d, J = 7.4 Hz, 1H), 6.93 (dd, J = 6.7, 4.9 Hz, 4H), 6.90 (s,
4H), 6.82 (dd, J = 12.8, 6.6 Hz, 2H), 3.34 (s, 2H), 2.66 (s, 3H), 2.59−
2.51 (m, 2H), 2.48 (s, 12H), 2.24 (s, 6H), 1.50 (dt, J = 15.3, 7.6 Hz,
2H), 1.24−1.13 (m, 6H), 0.84 (t, J = 6.9 Hz, 3H); 13C NMR (176
MHz, C6D6) δ 153.13, 152.09, 150.98, 141.51, 139.83, 136.44, 135.99,
133.20, 132.16, 131.85, 131.63, 130.52, 129.95, 129.68, 129.54, 128.80,
128.52, 128.33, 125.78, 124.83, 124.21, 124.03, 120.74, 34.23, 31.98,
31.24, 29.46, 28.94, 25.45, 22.97, 21.04, 14.30; 11B NMR (225 MHz,
C6D6) δ −1.37; HR-ESIMS (m/z) [M + 1]+ calcd for C44H51BN3S
664.3897, found 664.3929.
DFT Calculation Details. Geometry optimization and TD-DFT
calculations were carried out using the Gaussian 09 software15 at the
High Performance Computing Virtual Laboratory (HPCVL) at
Queen’s University. All computations were performed at the B3LYP
level of theory using the 6-31G(d) basis set14 for all atoms.
Synthesis of Compounds L-I1−L-I3. The precursor compounds
L-1a−L-3a were prepared using a modified procedure reported
previously for L-1a.9 See the Supporting Information for details. The
reaction was performed in air. A mixture of compound L-a (0.2 mmol)
and dimethyl sulfate (0.5 mL) was stirred for 5 min at 155 °C and then
another 30 min at 100 °C. The mixture was poured slowly into ethyl
ether (4 mL). Filtration gave a light gray solid intermediate. The solid
was dissolved in 1.5 mL of water and reprecipitated by adding 0.5 mL
of 30% aqueous potassium iodide and filtered; the residue was then
treated with water (1 mL × 3) and dried to give L-I. Data for L-I1:
Synthesis of Compound 3b. In an oven-dried Schlenk flask were
placed 3a (50 mg, 0.076 mmol), 3-hexyl-2-(trimethyltin)thiophene
(71 mg, 0.21 mmol), Pd2(dba)3 (7 mg, 0.0076 mmol), and P(o-tolyl)3
(9.3 mg, 0.03 mmol). The flask was evacuated and filled with N2 three
times, and then dried toluene (9 mL) was injected into the mixture.
The resulting solution was stirred at 100 °C for 12 h under an N2
atmosphere. After being cooled to room temperature, the solvents
were then removed under reduced pressure. The crude product was
purified by silica gel chromatography (petroleum ether/CH2Cl2, 1/1).
1
light yellow solid (yield: 66%); H NMR (300 MHz, d6-DMSO) δ
8.43 (d, J = 7.7 Hz, 1H), 7.97−7.71 (m, 8H), 5.78 (s, 2H), 4.21 (s,
1
3H). Data for L-I2: light yellow solid (yield: 90%); H NMR (400
MHz, d6-DMSO) δ 8.44 (d, J = 7.7 Hz, 1H), 8.06 (s, 1H), 8.01−7.94
(m, 2H), 7.90 (dd, J = 13.0, 4.9 Hz, 2H), 7.81 (t, J = 7.6 Hz, 1H), 7.69
(t, J = 7.9 Hz, 1H), 5.79 (s, 2H), 4.21 (s, 3H). Data for L-I3: gray solid
1
3b was obtained as a light yellow oil in 25% yield (14 mg): H NMR
1
(400 MHz, C6D6) δ 7.45 (s, 2H), 7.36 (d, J = 7.7 Hz, 1H), 7.23 (d, J =
7.9 Hz, 2H), 7.01−6.92 (m, 4H), 6.90 (d, J = 7.8 Hz, 4H), 6.81 (dd, J
= 5.2, 2.4 Hz, 2H), 3.34 (s, 2H), 2.68 (s, 3H), 2.62−2.52 (m, 4H),
2.53−2.42 (m, 12H), 2.26 (s, 6H), 1.53 (ddd, J = 26.3, 14.7, 7.2 Hz,
4H), 1.35−1.16 (m, 12H), 0.87 (dt, J = 13.8, 4.9 Hz, 6H); 13C NMR
(176 MHz, C6D6) δ 153.09, 151.80, 151.38, 141.42, 140.28, 139.85,
137.24, 136.45, 136.00, 133.18, 131.94, 130.48, 130.20, 129.95, 129.69,
129.54, 128.58, 128.33, 125.79, 125.26, 124.78, 124.50, 119.48, 34.20,
32.17, 31.46, 31.24, 29.47, 28.95, 25.44, 22.97, 21.05, 14.30; 11B NMR
(128 MHz, C6D6) δ −1.91; HR-ESIMS (m/z) [M + 1]+ calcd for
C54H65BN3S2 830.4713, found 830.4726.
Synthesis of Compound 4b. In an oven-dried Schlenk flask were
placed 3a (40 mg, 0.06 mmol), 5-trimethylstannyl-2,2′-bithiophene
(50 mg, 0.15 mmol), Pd2(dba)3 (6 mg, 0.006 mmol), and P(o-tolyl)3
(7.3 mg, 0.024 mmol). The flask was evacuated and filled with N2
three times, and then dried toluene (9 mL) was injected into the
mixture. The resulting solution was stirred at 100 °C for 12 h under an
N2 atmosphere. After being cooled to room temperature, the solvents
were then removed under reduced pressure. The crude product was
purified by silica gel chromatography (petroleum ether/CH2Cl2, 1/1).
4b was obtained as a yellow solid in 71% yield (35 mg): 1H NMR (400
MHz, C6D6) δ 7.58 (d, J = 1.5 Hz, 1H), 7.47 (t, J = 1.6 Hz, 1H), 7.35−
7.31 (m, 1H), 7.25 (dd, J = 8.1, 1.8 Hz, 1H), 7.20−7.17 (m, 1H), 7.08
(dd, J = 3.6, 1.1 Hz, 1H), 7.03 (dd, J = 3.5, 1.1 Hz, 1H), 6.99−6.95 (m,
2H), 6.94 (s, 4H), 6.91−6.80 (m, 4H), 6.74 (td, J = 5.5, 1.1 Hz, 2H),
6.68 (ddd, J = 6.5, 5.1, 3.6 Hz, 2H), 3.42 (s, 2H), 2.67 (s, 3H), 2.55 (s,
12H), 2.24 (s, 6H); 13C NMR (176 MHz, C6D6) δ 153.16, 151.74,
141.98, 141.44, 137.54, 137.30, 135.16, 133.41, 130.67, 130.07, 128.62,
128.35, 126.13, 125.80, 125.68, 125.36, 125.14, 124.97, 124.38, 121.42,
119.64, 34.28, 30.23, 25.57, 21.03; 11B NMR (128 MHz, C6D6) δ
(yield: 83%); H NMR (400 MHz, d6-DMSO) δ 8.39 (d, J = 8.4 Hz,
1H), 8.24 (s, 1H), 8.07−8.02 (m, 2H), 7.99 (d, J = 8.1 Hz, 1H), 7.90
(d, J = 7.9 Hz, 1H), 7.68 (t, J = 7.9 Hz, 1H), 5.78 (s, 2H), 4.20 (s,
3H).
1,1-Hydroboration of L-I1−L-I3 and Syntheses of Com-
pounds 1a−3a. KOtBu (0.84 mmol) and the correponding salt L-I
(0.27 mmol) were reacted in toluene (10 mL) at room temperature
for 10 h. The insoluble solid and excess base KOtBu were removed by
filtration. The filtrate was concentrated by vacuum until half of the
solvent was left. HBMes2 (0.54 mmol) was added to the filtrate at
room temperature and the mixture was stirred overnight, at which
point the volatiles were removed in vacuo and the product was isolated
by column chromatography on silica gel using a solvent mixture of
hexanes and ethyl acetate (10/1) as the eluent. Data for 1a: white solid
(yield: 72%); 1H NMR (400 MHz, CDCl3) δ 7.66 (dd, J = 7.7, 1.5 Hz,
2H), 7.62−7.49 (m, 4H), 7.29 (dt, J = 12.9, 7.3 Hz, 2H), 7.20 (t, J =
7.5 Hz, 1H), 6.62 (s, 4H), 4.02 (s, 3H), 2.96 (s, 2H), 2.18 (s, 6H),
2.04 (s, 12H); 13C NMR (101 MHz, CDCl3) δ 152.58, 150.43, 149.59,
147.67, 140.35, 132.05, 131.08, 130.59, 130.26, 128.56, 128.47, 128.28,
124.27, 123.29, 123.23, 119.53, 34.66, 31.88, 23.92, 19.85; 11B NMR
(128 MHz, CDCl3) δ −1.80; HR-ESIMS (m/z) [M + 1]+ calcd for
C34H37BN3 498.3081, found 498.3054. Data for 2a: white solid (yield:
45%); 1H NMR (400 MHz, d6-DMSO) δ 7.91−7.83 (m, 2H), 7.78 (d,
J = 7.0 Hz, 1H), 7.70 (d, J = 7.7 Hz, 1H), 7.58 (t, J = 7.9 Hz, 1H),
7.28−7.19 (m, 2H), 7.04 (d, J = 7.3 Hz, 1H), 6.42 (s, 4H), 4.03 (s,
3H), 2.79 (s, 2H), 2.05 (s, 6H), 1.89 (s, 12H); 13C NMR (176 MHz,
d6-DMSO) δ 152.28, 151.51, 148.95, 140.76, 134.01, 131.60, 129.94,
129.28, 128.47, 127.83, 127.29, 125.15, 124.45, 122.19, 120.48, 35.39,
24.48, 22.08, 20.36; 11B NMR (225 MHz, d6-DMSO) δ −3.72; HR-
ESIMS (m/z) [M + 1]+ calcd for C34H36BBrN3 576.2186, found
F
DOI: 10.1021/acs.organomet.7b00290
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
−1.08; HR-ESIMS (m/z) [M + 1]+ calcd for C50H45BN3S4 826.2589,
found 826.2686.
X-ray Diffraction Analysis. The crystal data of 4b were collected
on a Bruker D8-Venture diffractometer with Mo target (λ = 0.71073
Å) at 180 K. Data were processed on a PC with the aid of the Bruker
SHELXTL software package16 and corrected for absorption effects. All
non-hydrogen atoms were refined anisotropically. The positions of
hydrogen atoms were calculated and refined isotropically. The detail of
crystal data, collection parameters, and results of analyses are provided
in the Supporting Information. The crystal data of 4b were deposited
with the Cambridge Crystallographic Data Center with the deposition
number CCDC 1544199. These data can be obtained free of charge
from the Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
Synthesis of Compound 5b. In an oven-dried Schlenk flask were
placed 3a (110 mg, 0.16 mmol), (3,3′-dihexyl[2,2′-bithiophene]-5,5′-
diyl)bis[trimethylstannane] (110 mg, 0.16 mmol), Pd2(dba)3 (6 mg,
0.0064 mmol), and P(o-tolyl)3 (10 mg, 0.032 mmol). The flask was
evacuated and filled with N2 three times, and then dried toluene (8
mL) was injected into the mixture. The resulting solution was stirred
at 120 °C for 48 h under N2. The end-capping reaction was carried out
by adding (3,3′-dihexyl[2,2′-bithiophene]-5,5′-diyl)bis-
[trimethylstannane] (9 mg, 0.013 mmol), and the solution was stirred
at 120 °C for 12 h under N2. Finally, bromobenzene (126 mg, 0.8
mmol) was added and the mixture was stirred at 120 °C for 24 h under
N2. After being cooled to room temperature, the resulting organic
phase was extracted with CH2Cl2 (10 mL × 3) and washed with water.
After the solvents were removed, the residue was dispersed in a small
amount of dichloromethane and added dropwise to a stirred
acetonitrile solution and the precipitate was collected. The obtained
yellow solid was next dispersed in methanol and the precipitate was
collected. Then the yellow solid was dispersed in acetonitrile once
again and the precipitate was collected. Finally, the obtained yellow
solid was dispersed in diethyl ether and the precipitate was collected
and dried in vacuo to afford the desired polymer 5b (55 mg, 42%): 1H
NMR (400 MHz, CD2Cl2) δ 8.27−7.08 (m, 22H), 6.58 (br, 4H), 4.05
(br, 3H), 3.00 (br, 2H), 2.60 (br, 8H), 2.14 (br, 6H), 2.02 (br, 12H),
1.60 (br, 8H), 1.28 (br, 24H), 0.87 (br, 12H); 11B NMR (225 MHz,
CD2Cl2) δ 3.14; GPC (polystyrene standard, 25 °C) Mn = 6888, PDI
= 1.5.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00290.
Synthetic details, characterization data, TD-DFT data,
photoelimination experiments, UV/vis spectra, and
crystal data of 4a (PDF)
Cartesian coordinates of the calculated structures (XYZ)
Accession Codes
CCDC 1544199 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: +44 1223 336033.
Formation of 2c−4c via Photoelimination. 1H NMR scale
photoelimination reactions were carried out using C6D6 as the solvent
and quartz J. Young NMR tubes as the reaction vessels under N2 and a
Shanghai Si-Le Photochemical Reactor as the light source. The
azaborine products 2c−4c were characterized by NMR and HRMS
analysis. For NMR, UV/vis, and fluorescence spectroscopic tracking
AUTHOR INFORMATION
Corresponding Authors
*E-mail for T.P.: pengtai@bit.edu.cn.
■
1
data, see the Supporting Information. Data for 2c: H NMR (400
MHz, C6D6) δ 7.97 (s, 1H), 7.87 (d, J = 8.5 Hz, 1H), 7.76 (d, J = 8.8
Hz, 1H), 7.41 (t, J = 8.6 Hz, 2H), 7.31 (t, J = 7.4 Hz, 1H), 7.10 (s,
2H), 7.03−6.98 (m, 2H), 6.93 (d, J = 5.3 Hz, 2H), 6.80 (d, J = 5.1 Hz,
1H), 2.92 (s, 3H), 2.64 (s, 6H), 2.53 (t, J = 7.8 Hz, 2H), 2.36 (s, 3H),
1.52−1.47 (m, 2H), 1.15−1.21 (m, 6H), 0.83 (t, J = 6.8 Hz, 3H); 13C
NMR (176 MHz, C6D6) δ 153.80, 146.92, 144.23, 141.16, 139.73,
138.96, 136.52, 135.98, 131.81, 130.98, 130.70, 129.88, 129.64, 129.41,
129.14, 128.92, 127.66, 126.42, 125.97, 124.83, 123.03, 117.78, 107.71,
34.90, 31.96, 31.22, 29.42, 28.95, 23.76, 22.93, 21.33, 14.28; 11B NMR
(128 MHz, C6D6) δ 32.8; HR-ESIMS (m/z) [M + 1]+ calcd for
*E-mail for S.W.: wangs@chem.queensu.ca.
ORCID
Suning Wang: 0000-0003-0889-251X
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Y.S., T.P., and S.W. thank the National Natural Science
Foundation of China for financial support (21571017). X.W.
and S.W. thank the Natural Sciences and Engineering Research
Council of Canada for financial support (RGPIN 1193993-
2013) and the High-Performance Computing Virtual Labo-
ratory (HPCVL) at Queen’s University for the computing
facility.
1
C35H39BN3S 544.2958, found 544.2941. Data for 3c: H NMR (400
MHz, C6D6) δ 8.13 (d, J = 1.4 Hz, 1H), 7.78 (d, J = 8.9 Hz, 1H),
7.45−7.39 (m, 2H), 7.31 (dd, J = 8.9, 1.7 Hz, 1H), 7.18 (d, J = 1.4 Hz,
1H), 7.11−7.08 (m, 2H), 7.04−7.00 (m, 2H), 6.93 (dd, J = 5.1, 2.4
Hz, 3H), 6.80 (d, J = 5.2 Hz, 1H), 2.93 (s, 3H), 2.87 (d, J = 8.3 Hz,
2H), 2.63 (s, 6H), 2.53 (d, J = 8.0 Hz, 2H), 2.36 (s, 3H), 1.70−1.64
(m, 2H), 1.52−1.47 (m, 2H), 1.19 (ddd, J = 14.5, 7.3, 2.9 Hz, 12H),
0.82 (d, J = 7.0 Hz, 6H); 13C NMR (176 MHz, C6D6) δ 153.84,
146.75, 144.28, 141.12, 139.74, 139.53, 138.96, 138.82, 136.50, 136.01,
134.61, 132.09, 131.85, 130.69, 130.02, 129.89, 129.46, 129.14, 128.92,
127.67, 127.04, 126.33, 124.85, 124.59, 124.31, 123.47, 119.77, 106.57,
34.85, 32.01, 31.51, 31.22, 29.60, 29.43, 28.96 23.74, 22.98, 22.74,
21.55, 14.28; 11B NMR (128 MHz, C6D6) δ 33.6; HR-ESIMS (m/z)
[M + 1]+ calcd for C45H53BN3S2 710.3774, found 710.3790. Data for
4c: 1H NMR (400 MHz, C6D6) δ 8.23 (d, J = 1.7 Hz, 1H), 7.77 (d, J =
9.0 Hz, 1H), 7.56−7.53 (m, 1H), 7.51 (t, J = 1.6 Hz, 1H), 7.11 (s,
2H), 7.08−7.05 (m, 3H), 6.97−6.94 (m, 3H), 6.83 (dd, J = 5.4, 3.8
Hz, 2H), 6.75 (ddd, J = 4.5, 3.3, 1.3 Hz, 3H), 6.69 (dd, J = 5.2, 3.5 Hz,
3H), 2.89 (s, 3H), 2.70 (s, 6H), 2.35 (s, 3H); 13C NMR (176 MHz,
C6D6) δ 155.10, 154.45, 142.40, 142.26, 141.16, 139.81, 138.96,
137.91, 137.58, 135.86, 135.20, 131.29, 129.65, 129.59, 127.00, 126.81,
126.15, 125.14, 124.89, 124.27, 123.24, 116.17, 106.72, 31.70, 23.80,
21.33, 20.25; 11B NMR (128 MHz, C6D6) δ 34.3; HR-ESIMS (m/z)
[M + 1]+ calcd for C41H33BN3S4 706.1650, found 706.1658.
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DOI: 10.1021/acs.organomet.7b00290
Organometallics XXXX, XXX, XXX−XXX