Formation of azaborines by photoelimination of b,n-heterocyclic compounds

Article abstract of DOI:10.1002/anie.201300873

Highly fluorescent π-conjugated polycyclic azaborines can be prepared from B,N-heterocyclic compounds with a BR₂-CH₂ unit through the elimination of an R-H molecule (see scheme). These clean photoelimination reactions occur both in solution and in polymers doped with the precursors. Copyright

Full text of DOI:10.1002/anie.201300873

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Angewandte
Communications
DOI: 10.1002/anie.201300873
Photochemistry
Formation of Azaborines by Photoelimination of B,N-Heterocyclic
Compounds**
Jia-Sheng Lu, Soo-Byung Ko, Nicholas R. Walters, Youngjin Kang, Francoise Sauriol, and
Suning Wang*
Organoboron compounds have broad applications in organic
synthesis,[1] catalysis,[2] optoelectronic devices,[3] sensors,[3,4]
and probes.[5] Among p-conjugated organoboron compounds,
ꢀ
azaborine molecules or B N-containing aromatic molecules
have attracted much research interest and efforts.[6] The
ꢀ
replacement of a C C unit in an aromatic molecule with an
ꢀ
isoelectronic B N unit has been shown to impart interesting
electronic, photophysical, luminescent, and chemical proper-
ꢀ
ties, which are often very distinct from those of the C C-
containing aromatic analogues, to the molecule.[6] This
ꢀ
observation has led to many potential applications of B N-
containing aromatic compounds, such as hydrogen storage
materials,[7] optoelectronic materials,[8] sensors,[9] and bioac-
tive molecules.[10] Nonetheless, examples of polycyclic p-
conjugated azaborine molecules and derivatives remain rare
ꢀ
compared to the vast number of C C-containing aromatic
compounds and heterocyclic aromatic compounds. The syn-
theses of azaborine compounds are in general very challeng-
ing and often involve multi-step reactions or the use of
transition-metal catalysts.[6] Thus, the development of effi-
ꢀ
cient and simple synthetic methods for B N-containing
aromatic compounds is highly important to advance the
chemistry and applications of this important class of com-
pounds.
Scheme 1. Formation of azaborines through photoelimination of B,N
heterocycles.
We herein disclose the discovery of a new and facile
ꢀ
synthetic method for B N-containing aromatic compounds,
namely, the photoelimination method (Scheme 1). Although
examples of organoboron compounds that display unusual
photochemical reactivity have been reported previously,[11]
the photoelimination reaction presented here is highly
elimination reaction also occurs readily in a polymer sub-
strate. The details are presented herein.
Compound BN-1 was synthesized in good yield by the
reaction of nBuLi with 2-(2-methylphenyl)pyridine at ꢀ788C,
followed by the addition of BMes2F. The dimethyl analogue
BN-2 and the benzothiazolyl B,N-heterocyclic compound
BN-3 were obtained in good yields using the same procedure.
For BN-4, BMes2F was replaced by BMe2Br in the synthesis.
The presence of tetramethylethylenediamine (TMEDA) in
the lithiation step is necessary to ensure that the lithiation
takes place at the methyl site and not at the H atom in ortho
position of the phenyl ring, which can lead to the formation of
the undesired phenyl–pyridyl or phenyl–benzothiazolyl che-
late products that are analogues of B(ppy)Mes2 (ppy = phe-
nylpyridine) and derivatives.[11b] The yield of BN-4 is poor,
which may be caused by the reaction of BMe2Br with
TMEDA.
The B,N-heterocyclic compounds are generally colorless,
except BN-3, which is light yellow both in solution and in the
solid state. They are stable in air and upon heating and do not
show any appreciable change when heated to 1108C in
toluene. They were fully characterized by 1H, 13C, and
11B NMR spectroscopy, and HRMS/elemental analysis. The
crystal structures of BN-1, BN-3, and BN-4 were determined
ꢀ
unusual and unprecedented because of the breaking of a C
H bond and of a B C bond. Furthermore, this photoelimi-
nation process is a generic reaction for the B,N-heterocyclic
compounds shown in Scheme 1, in which the R group can be
either an alkyl or an aryl group and the N heterocycle can be
either a pyridyl or a benzothiazolyl. Furthermore, this photo-
ꢀ
[*] J. S. Lu, Dr. S. B. Ko, N. R. Walters, F. Sauriol, Prof. Dr. S. Wang
Department of Chemistry, Queen’s University
Kingston, Ontario, K7L 3N6 (Canada)
E-mail: wangs@chem.queensu.ca
Prof. Dr. Y. Kang
Division of Science Education, Kangwon National University
Chuncheon 200-701 (Republic of Korea)
[**] We thank the Natural Sciences and Engineering Research Council
for financial support.
Supporting information for this article (including general exper-
imental details, syntheses, and characterization data of all com-
pounds, and computational results) is available on the WWW under
http://dx.doi.org/10.1002/anie.201300873.
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by single-crystal X-ray diffraction analysis and the structures
of BN-1 and BN-3 are shown in Figure 1. The structural data
of BN-4 are provided in the Supporting Information. The
ꢀ
ꢀ
lengths of the B Cmesityl and B N bonds in BN-1 and BN-3 are
similar to those in B(ppy)Mes2.[11b] The B1 C1 bond lengths
ꢀ
(1.645(3) ꢀ, 1.647(4) ꢀ) in both compounds are however
ꢀ
longer than the B CPh bond in B(ppy)Mes2 (1.625 (2) ꢀ) and
related compounds.[11]
Figure 2. UV/Vis (top) and fluorescence (bottom) spectra showing the
conversion of BN-2 and BN-3 in toluene (ꢁ1.0ꢀ10ꢀ5 m) and BN-2 in
a PMMA film (ꢁ10 wt%) to their corresponding azaborines
(lex =300 nm for BN-2 and 350 nm for BN-3). Inset photographs show
the color change (top) of BN-2 and BN-3 before and after irradiation,
and the fluorescence colors (bottom) of BN-2a and BN-3a in solution,
and BN-2a in PMMA film (far right, ꢁ10 wt%) on a glass slide
patterned with a mask (the dark area is BN-2/PMMA film).
1
agree well with DFT-calculated H NMR spectral patterns
(see the Supporting Information). The quantum efficiency for
the conversion of BN-2!BN-2a was determined to be
approximately 4.4%, which is fairly efficient for photochem-
ical reactions. The elimination of methane and the formation
of BN-4a from BN-4 were confirmed by NMR and HRMS
data. However, the reaction of BN-4 is not as clean as that of
BN-1 and BN-2. With prolonged irradiation, the presence of
side products, which were not fully characterized, was evident
in the 1H NMR spectra.
Figure 1. Crystal structures of a) BN-1, b) BN-3, and c) BN-3a, and
d) packing diagram showing intermolecular p stacking in the crystal
lattice of BN-3a. For BN-1 and BN-3, all H atoms except those on C1
are omitted for clarity. Important bond lengths [ꢁ] and angles [8] for
BN-3a: B1–C1 1.475(4), B1–C16 1.578(4), B1–N1 1.485(4), C1–C2
1.390(4), C2–C3 1.435(4), C3–C4 1.395(4), C4–N1 1.372(3); C1-B1-N1
114.4(3), B1-C1-C2 122.5(3), C1-C2-C3 120.2(3), C2-C3-C4 118.6(3),
C3-C4-N1 123.5(3), C4-N1-B1 120.7(2).
The benzothiazole compound BN-3 undergoes a similar
photoelimination as BN-1 upon irradiation at 350 nm, form-
ing compound BN-3a as the major product with the
elimination of one mesityl group. NMR and HRMS data
established that BN-3a is an analogue of BN-1a and BN-2a.
Bright yellow-orange crystals of BN-3a were obtained and
the crystal structure of BN-3a was successfully determined by
X-ray diffraction analysis (Figure 1). The mesityl ring is
approximately perpendicular to the azaborine ring (the
dihedral angle between the two rings is 102.18). The lengths
When excited at 300 nm in dry toluene or benzene under
nitrogen atmosphere, the solutions of BN-1, BN-2, and BN-4
change from colorless to bright yellow. In the UV/Vis spectra,
a distinct new absorption band appears in the 360–520 nm
region and increases in intensity with longer irradiation time,
which accounts for the color change of the solution (see
Figure 2 and the Supporting Information). To elucidate the
structural change, the photoreaction was monitored by 1H and
11B NMR spectroscopy. Upon irradiation, a free mesitylene
molecule was observed and the 1H NMR spectra of BN-1 and
BN-2 underwent a distinct change to those of BN-1a and BN-
2a, respectively (see the Supporting Information.). The most
distinct change in chemical shift is observed for the methylene
protons, the signal of which shifted from 3.02 ppm in BN-
1 and 2.76 ppm in BN-2 to 7.35 ppm in BN-1a and BN-2a,
along with the change in total integration from two protons to
one. The 11B chemical shift also experienced a distinct change
from 1.6 ppm in BN-1 and 1.2 ppm in BN-2, typical for
tetrahedral boron centers, to 35.5 ppm in BN-1a and 35.0 ppm
in BN-2a, similar to some of the previously reported 11B
chemical shifts in 1,2-azaborine compounds.[9b,12] BN-1a and
BN-2a were fully characterized by NMR and HRMS
ꢀ
ꢀ
ꢀ
of the B N, B C, and C C bonds of the azaborine ring in BN-
3a from the X-ray data match very well with those of the
DFT-optimized structure and are similar to those of the
previously reported azaborine compounds.[6d,12a,13] The mole-
cules of BN-3a form one-dimensional and partially p-stacked
columns in the crystal lattice with the shortest atomic
separation distance between the two neighboring aryl rings
being 3.52 ꢀ (Figure 1).
BN-1a, BN-2a, and BN-3a are stable for days in the solid
state and slowly degrade in solution upon exposure to air. In
contrast, BN-4a decomposes rapidly in solution and in the
solid state under air, presumably because of the reaction with
molecular oxygen.[14] The greater chemical stability of BN-1a
to BN-3a is clearly provided by the bulky mesityl group.
Compounds BN-1a, BN-2a, and BN-4a are previously
1
analyses. The H NMR spectral assignments for both com-
pounds were completed with 2D NOESY NMR data, which
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unknown isomers of B,N phenanthrenes. Three other isomers
of B,N phenanthrene and their derivatives (A–C) were
reported previously.[8b,13b,15] The aromaticity of the new B,N
phenanthrene compounds are corroborated by the nucleus-
the diminished intermolecular interactions of BN-2a and BN-
4a as a result of the presence of two extra methyl groups on
the B,N phenanthrene ring. In fact, compound BN-1a shows
concentration-dependent fluorescence in solution (see the
Supporting Information). In addition, the fluorescence spec-
trum of BN-1a in a PMMA film is also significantly red-
shifted and the emission intensity is diminished as the
concentration of BN-1a in the film increases (see the
Supporting Information), while similar concentration-depen-
dent fluorescence in solution and in PMMA films were not
observed for BN-2a, BN-3a, and BN-4a. The relatively low
FFL of BN-3a may be caused by the benzothiazolyl ring.
Because of the contrasting non-emissive and emissive proper-
ties of the precursor compounds and the azaborine com-
pounds, patterned fluorescent films with a high contrast can
be obtained by simply casting the precursor PMMA film on
a glass substrate, then covering the film with a mask and
exposing it to UV light. One example of such patterned films
is shown in Figure 2 (see the Supporting Information for other
examples.)
The absorption and fluorescence spectra of BN-1a, BN-
2a, and BN-4a are strikingly similar, despite their different
substituents, which is an indication that the low-energy
electronic transitions in these molecules are most likely
dictated by the B,N phenanthrene unit. Photoelimination
reactions are known for many organic compounds, such as
azo, azide, and ketone compounds.[17] They are however rare
for organoboron compounds.[11e] To understand the unusual
photoelimination reaction and the photophysical properties
of the new azaborine compounds, time-dependent DFT (TD-
DFT) computational studies were performed for the precur-
sors and the azaborine compounds. In addition, computa-
tional studies were also performed for BN-5a, an analogue of
BN-1a with the mesityl group being replaced by an H atom,
and 9-mesitylphenanthrene for comparison (see the Support-
ing Information).
independent chemical shifts (NICS) obtained from DFT
computational data (see the Supporting Information).
Most remarkable is the observation that the photoelimi-
nation reactions of all four precursors can also occur in the
solid state when a polymer film, such as poly(methyl
methacrylate) (PMMA) or poly(N-vinylcarbazole) (PVK),
are doped with these precursors. The UV/Vis spectra of all
four compounds in PMMA films undergo a similar change as
those recorded in toluene upon irradiation by UV light,
supporting that the same azaborine species are formed. The
UV/Vis spectra for BN-2/PMMA with UV irradiation are
shown in Figure 2, those of BN-1, BN-3, and BN-4 in PMMA
are provided in the Supporting Information.
Unlike the non-emissive precursor compounds, all four
azaborine compounds display bright green or yellow-green
fluorescence in solution and in the solid state (see Figure 2,
Table 1, and the Supporting Information). Thus, the photo-
Table 1: Photophysical properties of BN-1a to BN-4a.
[b]
labs
[nm][a]
lem
[nm] (FFL)
PMMA[c]
labs/lem [nm]
H!L [nm]/f[d]
BN-1a 422, 447, 470 493, 513 (0.27) 447, 470/505 410/0.185
BN-2a 428, 455, 478 500 (1.00)
BN-3a 430, 455, 481 524 (0.16)
BN-4a 425, 449, 474 509 (1.00)
452, 476/510 421/0.199
455, 483/514 425/0.141
454, 479/507 422/0.223
[a] In toluene. [b] In toluene, FFL was determined using Ir(ppy)3
(F=0.92)[19] as the standard under N2 atmosphere. [c] around 10 wt%.
[d] From TD-DFT data.
The computational results show consistently that the
HOMO!LUMO transition has a high oscillator strength and
is responsible for the low-energy absorption band of all B,N
phenanthrene compounds (Table 1). The HOMO and LUMO
levels of BN-1a, BN-2a, BN-4a, and BN-5a are localized on
the B,N phenanthrene ring with no or little contributions from
the substituents (Figure 3). Similarly, for the benzothiazolyl
compound BN-3a, the HOMO and LUMO levels are
elimination reaction can also be followed conveniently by
fluorescence spectroscopy. Compared to the previously
reported B,N phenanthrene isomers A–C[13b,15] and phenan-
threne,[13b,16] the absorption spectra of BN-1a, BN-2a, and
BN-4a are red-shifted by 50–100 nm. The fluorescence
spectra of the new B,N phenanthrene isomers are red-shifted
by approximately 150–160 nm relative to phenanthrene,[13b,16]
170–180 nm to B,N phenanthrene A (R’ = H, R = Ph or
H),[13b] and 40–50 nm to B,N phenanthrene C (R’ = H, R = H,
nBu, Ph, SiMe3).[13b] The relatively low absorption and
emission energies of the new B,N phenanthrene molecules
may be attributed to their greater polarity, compared to
isomers A and C. The fluorescent quantum efficiency FFL was
determined to be 0.27, around 1.00, 0.16, and around 1.00 for
BN-1a, BN-2a, BN-3a, and BN-4a, respectively. Compounds
BN-2a and BN-4a are the brightest emitters among all known
B,N phenanthrene compounds. The much greater FFL of BN-
2a and BN-4a compared to that of BN-1a can be attributed to
ꢀ
localized at the B N-containing arene ring. Thus, the
fluorescence of the new azaborine compounds can be
attributed to a p!p* transition of the p-conjugated azabor-
ine unit. Because of the lack of significant contributions from
the R groups to this transition, the calculated energy of this
transition displays only small variations for BN-1a, BN-2a,
BN-4a, and BN-5a (see Table 1 and the Supporting Informa-
tion), which is in good agreement with the experimental data.
The transition-energy difference of approximately 10 nm
between BN-1a and BN-2a can be attributed to the induction
effect of the methyl substituents. The HOMO!LUMO
transition of 9-mesitylphenanthrene is also localized on the
phenanthrene ring, but at a much higher energy (300 nm) and
with a much lower oscillator strength (0.081) relative to BN-
1a. Compared to 9-mesitylphenanthrene, the LUMO level of
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reaction is therefore thermodynamically favored, and driven
mostly by entropy and the aromatization energy.
In summary, an unprecedented photoelimination reaction
of B,N-heterocyclic compounds has been discovered, which
can be used effectively for the formation of new p-conjugated
ꢀ
polycyclic azaborine compounds by eliminating an R H
ꢀ
molecule from a BR2 CH2 unit. The photoelimination is
applicable to solid substrates, such as PMMA or PVK films
doped with B,N-heterocyclic compounds. This makes it
a simple and convenient method for the in situ generation
of new polycyclic azaborine compounds in polymer substrates
and for creating patterned fluorescent polymer films by light,
which may find applications in the fabrication of optoelec-
tronic devices.
Received: January 31, 2013
Published online: April 5, 2013
Figure 3. Diagrams of the key orbitals involved in the electronic
transitions that are mainly responsible for the low-energy absorption
bands of BN-1 and BN-1a with an isocontour value of 0.04.
Keywords: azaborines · boron · fluorescence · phenanthrenes ·
.
photoelimination
BN-1a is stabilized by 0.52 eV, while the HOMO level is
ꢀ
destabilized by 0.85 eV. Thus, the key impact of B N moiety
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[20] CCDC 919091 (BN-1), 919092 (BN-3), 919093 (BN-4), and
919094 (BN-3a) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
4548
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 4544 –4548