Photodissociation of B-N Lewis adducts: A partially fused trinaphthylborane with dual fluorescence

Article abstract of DOI:10.1021/ja506980p

The synthesis of a planarized trinaphthylborane with partially fused structure is presented. This compound shows not only high chemical and thermal stability but also sufficient Lewis acidity to form Lewis adducts with pyridine derivatives in solution. The B-N Lewis adducts exhibit unprecedented photodissociation behavior in the excited state, reminiscent of the photogeneration of carbenium ions from triarylmethane leuco dyes. Consequently, these B-N Lewis adducts exhibit dual fluorescence emission arising from the initial tetracoordinate B-N adducts and the photodissociated tricoordinate boranes.

Full text of DOI:10.1021/ja506980p

Communication
pubs.acs.org/JACS
Photodissociation of B−N Lewis Adducts: A Partially Fused
Trinaphthylborane with Dual Fluorescence
†
†
,†,‡
Kyohei Matsuo, Shohei Saito, and Shigehiro Yamaguchi*
†
‡
Department of Chemistry, Graduate School of Science and Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya
University, Furo, Chikusa, Nagoya 464-8602, Japan
*
S Supporting Information
ABSTRACT: The synthesis of a planarized trinaphthyl-
borane with partially fused structure is presented. This
compound shows not only high chemical and thermal
stability but also sufficient Lewis acidity to form Lewis
adducts with pyridine derivatives in solution. The B−N
Lewis adducts exhibit unprecedented photodissociation
behavior in the excited state, reminiscent of the photo-
generation of carbenium ions from triarylmethane leuco
dyes. Consequently, these B−N Lewis adducts exhibit dual
fluorescence emission arising from the initial tetracoordi-
nate B−N adducts and the photodissociated tricoordinate
boranes.
Figure 1. (a) Photodissociation of a partially fused trinaphthylborane 1-
pyridine complex and (b) the structures of some relevant planarized
triarylboranes.
ncorporation of a tricoordinate, trivalent boron atom, which is
isoelectronic to a carbenium ion, into π-conjugated skeletons
I
dissociation of its Lewis base adducts with various pyridine
derivatives.
enables the construction of charge-neutral electron-deficient
1
molecules. Boron-containing π systems readily react with Lewis
We recently demonstrated that the structural constraint arising
from imposed planarity can be used as an efficient strategy for the
stabilization of Lewis acidic triarylboranes, even in the absence of
kinetic protection of the boron center (Figure 1b). Indeed, fully
planarized triphenylborane 2 showed impressively high stability
toward water, oxygen, silica gel, and amines. We have also
succeeded in the synthesis of some boron-embedded polycyclic
bases to form the corresponding tetracoordinate species. This
fundamental reactivity can be used to induce fascinating
electronic properties and functions. In this context, a primary
strategy is the generation of rigid π-conjugated skeletons with
intense fluorescence by the intramolecular coordination with a2
N-heteroaryl group, such as in BODIPY and related dyes.
Recently, Wang et al. found an intriguing photorearrangement of
intramolecularly N-heteroaryl-coordinated boron compounds, in
which a boracyclopropane is formed, while the B−N bond
7
π-conjugated compounds, such as 3 and a subunit of boron-
8
doped graphene. But in order to develop a more convenient
synthetic pathway to structurally simpler boron-embedded π-
skeletons, we attempted a 3-fold cyclization of tris(8-
bromonaphthyl)borane 4 (Scheme 1). Thus, we treated a
toluene solution of 4 with (Me Si) SiH in the presence of 1,1′-
3
remains intact. Their intensive investigations on the scope of
this reaction demonstrated a potential use for these compounds
in new photochromic systems. In comparison, the reactivity of
intermolecular complexes between boranes and neutral Lewis
bases has not received much attention from a design perspective
regarding functional materials. For this type of complex,
Braunschweig, Engels, et al. recently reported an intriguing
type of photoreactivity: a photomigration of an intermolecula4r
complex between an electron-accepting borole and 2,6-lutidine.
Here, we would like to report yet another, unprecedented type of
photoreactivity for intermolecularly coordinated triarylborane−
pyridine complexes, which photodissociate in the excited state
3
3
azobis(cyclohexanecarbonitrile) (ABCN) as a radical initiator at
8a
1
20 °C, which resulted in the formation of the partially fused
trinaphthylborane 1 with an unexpected connectivity pattern
with respect to the three naphthyl groups as the major product.
Compound 1 was isolated as a red solid in 26% yield. Conversely,
the expected fused product with C symmetry was not observed,
3
and no other byproducts were isolated. Although the detailed
reaction mechanism still remains unclear, 1 is likely generated
9
from successive intramolecular radical cyclizations, followed by
a skeletal rearrangement (Scheme S2).
(
Figure 1a) and exhibit dual fluorescence. While a laser
Despite the partially fused framework, trinaphthylborane 1
showed notably high chemical stability. For example, even after
more than one month in water-containing CH Cl , no
photolysis of borane carbonyl (BH CO) is known as a similar
type of reaction, the present B−N photodissociation is
reminiscent of the photogeneration of carbenium ions from
3
5
2
2
6
triarylmethane leuco dyes. Here, we disclose the synthesis of
Received: July 10, 2014
highly Lewis acidic trinaphthylborane 1 and the photo-
©
XXXX American Chemical Society
A
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Journal of the American Chemical Society
Communication
Scheme 1. Synthesis and Derivatization of Trinaphthylborane
1
3.45 Å. For air-stable triarylboranes, this is an unprecedented
structural feature, and the ability to form columnar stacks would
be beneficial for the construction of supramolecules or discotic
liquid crystals. We evaluated carrier transporting properties as
one of the fundamental properties of the stacked structure. A top-
contact OFET device was fabricated by vacuum vapor deposition
of 1, which showed ambipolar carrier transporting abilities with a
−6
2
−1 −1
hole mobility of μ = 9.3 × 10 cm V
s
and an electron
h
−5
2
−1 −1
mobility of μ = 1.7 × 10 cm V
s
(Figures S6 and S7).
e
Although the performance is poor, this is the first OFET of an air-
stable triarylborane.
Another notable structural feature of 5 is the B−C bond
lengths, which amount to 1.549(10), 1.567(10), and 1.603(10)
Å. These bonds are longer than those in structurally constrained
7a
triphenylborane 2 (1.51−1.54 Å) and comparable to typical B−
10
C bonds in unconstrained triarylboranes (1.57−1.59 Å). This
degradation was observed for 1, as evident from UV−vis
absorption measurements (Figure S10). This result is contrasted
by the fact that the parent tri(1-naphthyl)borane rapidly (t1/2 < 3
h) decomposes in CH Cl solution in the air (Figure S11). But
what is more important from a practical perspective is that the
boron moiety in 1 remained intact during further functionaliza-
tion reactions. For example, the electrophilic bromination of 1
with N-bromosuccinimide successfully afforded brominated 5 in
feature should be relevant to its Lewis acidity.
Upon addition of an excess of pyridine, the broad 11B NMR
signal of 1 at δ 46.8 ppm in chlorobenzene-d completely
5
2
2
disappeared in favor of an emerging sharp signal at δ −3.2 ppm,
which was assigned to the 1·pyridine adduct. The single crystals
of the adduct suitable for X-ray diffraction analysis were obtained
from vapor diffusion of heptane into a toluene/pyridine solution
of 1 (Figure 2b). The asymmetric unit of these crystals contains
two crystallographically independent molecules, both of which
adopt a shallow bowl-shaped structure. The sum of the three C−
B−C bond angles is 340.5° and 342.2°. The most notable feature
in these molecules is the long B−N distance of 1.6927(17) and
8
0% yield. A subsequent Suzuki−Miyaura cross-coupling
between 5 and phenylboronic acid under basic aqueous
conditions produced phenylated derivative 6 in 59% yield. The
observed chemical tolerance is probably due to the chelating
effect of the surrounding polycyclic framework and suggests the
possibility to straightforwardly obtain a variety of derivatives of
this trinaphthylborane core skeleton via well-established
synthetic techniques. Furthermore, the high thermal stability of
1
.6939(16) Å. These values are significantly higher than those in
a sterically hindered dibenzoborole·pyridine adduct [1.638(4)
Å], a pentaphenylborole·2,6-lutidine adduct [1.6567(3) Å],
and a frustrated Lewis pair between B(C F ) and 2,6-lutidine
11
4
6
5 3
1
was demonstrated by thermogravimetric analysis, which
12
[1.661(2) Å], all of which dissociate in solution at room
resulted in the determination of the decomposition temperature
temperature. This comparison suggests a rather weak B−N
coordination in 1.
Despite the weak B−N coordination, the Lewis acidity of 1 is
still higher than those of the previously reported structurally
constrained triarylboranes. This fact was demonstrated by a
titration experiment of 1 with pyridine monitored by UV−vis
absorption spectroscopy (Figure 3a). A toluene solution of 1
for a 5% weight loss (T ) under an N atmosphere at 384 °C.
d5
2
Accordingly, it was possible to vacuum sublimate 1 at 250 °C/
−5
10
Torr.
The bond connectivity in the fused trinaphthylborane skeleton
was unequivocally confirmed by X-ray crystallographic analysis
of 5 (Figure 2a). Unfortunately, single crystals of the parent 1
Figure 2. (a) Crystal structure (left) and the columnar packing (right) of
5. A disordered Br atom is omitted for clarity (for the detail, see
Supporting Information). (b) Crystal structure of 1·pyridine. Thermal
ellipsoids are drawn at the 50% probability.
−5
Figure 3. Titration of 1 in toluene ([1] = 3.4 × 10 M) with pyridine,
0
measured by (a) UV−vis absorption and (b) fluorescence (λ = 390
ex
nm) spectroscopy.
could not be obtained. The crystallographic analysis of 5
confirmed that one of the three naphthyl groups is connected to
the boron center in its 2-position. The three naphthyl groups are
showed its longest absorption maximum at 546 nm and another
intense absorption at 429 nm. Upon addition of pyridine, these
absorption bands gradually decreased, and new bands were
observed at 346 and 374 nm, with a broad tail up to 490 nm.
Isosbestic points were found at 390 and 309 nm. The significant
hypsochromic shifts of the absorption maxima are due to the
disruption of the π-conjugation through the p orbital at boron by
the coordination of pyridine. Notably, the absorption bands of 1
2
2
connected to each other via three C(sp )−C(sp ) bonds.
Consequently, the compound forms a nine ring-fused C B
30
skeleton with a benzofluoranthene substructure. The central
boron atom adopts a trigonal planar geometry. Overall, the fused
trinaphthylborane skeleton adopts an almost planar geometry.
The crystal packing of 5 is best described by a columnar π-
stacked structure, where the average distance between planes is
B
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Journal of the American Chemical Society
Communication
5
(
[1] = 3.4 × 10− M) beyond 500 nm completely disappeared
reforms the Lewis adduct, due to the presence of an excess of
pyridine. Similar fluorescence properties have been reported for
9-phenylxanthen-9-ol, which undergoes photodehydroxylation
under concomitant emission of dual fluorescence from the
excited alcohol and the photodissociated carbenium ion. Thus,
it is the isoelectronic relationship between the carbenium ion and
the tricoordinate borane, which determines the similar behavior
in the excited state.
To gain a deeper understanding of the excited-state behavior
of 1·pyridine, temperature- and solvent-dependent fluorescence
spectra were recorded under the conditions that uncoordinated 1
was not observed in the absorption spectra. As the temperature
decreased from 292 to 193 K, the shorter-wavelength emission
band increased, while the one at longer wavelength decreased.
The photodissociation is hence suppressed at low temperature,
0
−2
after the addition of pyridine up to [pyridine] = 4.2 × 10 M.
This is, in contrast to the fully constrained 2 and 3,
of a quantitative formation of the 1·pyridine adduct under the
applied conditions. The binding constant of 1 toward pyridine
toluene/25 °C) was established to be 5.1 × 10 M (Figure
S12), which is much higher than that of 3 (0.35 M , THF).
The higher Lewis acidity of 1 relative to 3 should be attributed to
the structural difference arising from the smaller constraint in 1,
rather than to the difference in electron-accepting properties.
Cyclic voltammetry measurements in THF showed a lower first
0
7a,8a
indicative
13
3
−1
(
−1
8a
+
reduction potential for 1 (−1.48 V vs Fc/Fc ; Figure S18)
8a
compared to 3 (−1.37 V).
The titration experiments of toluene solutions of 1 with
pyridine were also monitored by fluorescence spectroscopy,
which allowed us to observe an unexpected phenomenon (Figure
indicative of the presence of an activation barrier in S for the
1
photodissociation process (Figure S16). On the other hand, with
increasing solvent polarity, the relative intensity of the shorter-
wavelength emission band increased, whereas the absorption
spectra showed only a subtle solvent dependence (Figure S17).
These results demonstrate that the solvent polarity significantly
affects the excited state and that the photodissociation becomes
more unfavorable in polar solvents. According to TD-DFT
calculations at the B3LYP/6-31G(d) level, the dipole moment of
3
b). In the absence of pyridine, the solution of 1 showed an
orange emission band at 573 nm with a quantum yield of 0.15
and a lifetime of 10.7 ns. Upon the addition of pyridine, a new
band for 1·pyridine appeared at 500 nm, while the original
emission band gradually decreased. This is consistent with the
corresponding spectral change in the UV−vis absorption.
However, the longer-wavelength emission band essentially
remained present, and only slightly decreased in intensity, even
after an excess of pyridine was added, which resulted in a
complete disappearance of the absorption band associated with
the optimized structure of 1·pyridine in S (7.8 D) is bigger than
1
that of 1 in S (1.5 D; Figures S28 and S30). Therefore, the
1
nondissociated Lewis adduct is stabilized to a greater extent in
1
. The excitation spectra monitored for the shorter and longer-
polar solvents in S , relative to the dissociated borane species,
1
wavelength emission bands were identical to each other as well as
to the absorption spectrum of 1·pyridine. Consequently, both
the fluorescence bands should arise from the photoexcitation of
resulting in a suppression of the dissociation.
We moreover conducted titration experiments of toluene
solutions of 1 with various other Lewis bases (Figure 5). Initially,
1
·pyridine. In other words, the Lewis adduct 1·pyridine exhibits
dual fluorescence.
Therefore, time-resolved fluorescence spectra of a toluene
solution of 1·pyridine were recorded at λ = 377 nm (Figure
ex
S15). Immediately after the excitation, the first emission band
appeared around 500 nm, and subsequently the longer-
wavelength emission started to emerge. The fluorescence
lifetime values for the two emission bands in the region of
470−510 and 560−660 nm were determined to be 2.3 and 11.0
ns, respectively, the latter of which is comparable to that of
uncoordinated 1. These results can be rationalized by
considering the photodissociation of 1·pyridine in the lowest
singlet excited state (S ; Figure 4). While the shorter-wavelength
1
fluorescence arises from the locally excited state of 1·pyridine, the
photodissociation occurring in S generates uncoordinated
1
borane 1, which is responsible for the longer-wavelength
fluorescence emission. In the ground state, 1 immediately
Figure 5. Titrations of toluene solutions of 1 ([1] = 3 × 10− M) with
5
0
DMAP (a,b) and PyF (c,d) monitored by UV−vis absorption (a,c) and
fluorescence spectroscopies (λ = 390 nm) (b,d).
ex
we substituted pyridine with the more Lewis basic N,N-
dimethyl-4-aminopyridine (DMAP). Based on UV−vis spectro-
scopic titrations, the binding constant for 1 toward DMAP was
6
−1
estimated to be 6.6 (±1.3) × 10 M (Figure S13). More
importantly, we were unable to observe any photodissociation in
the fluorescence spectra. Upon addition of DMAP to a toluene
solution of 1, both the absorption and fluorescence spectra were
found to change proportionally to the amount of DMAP added.
Ultimately, only one green emission band was observed at 527
Figure 4. A plausible energy diagram for the photodissociation of 1·
pyridine.
C
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Journal of the American Chemical Society
Communication
nm, which was attributed to 1·DMAP. In contrast, the use of the
weaker Lewis basic 3-fluoropyridine (PyF) resulted in an almost
unchanged fluorescence spectrum of 1, even when an excess of
PyF was added after the absorption band of 1 had completely
changed into a new band of the 1·PyF adduct. The binding
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was partly supported by CREST, JST, and a Grant-in-
Aid for Scientific Research on Innovative Area (Stimuli-
responsive Chemical Species, no. 24109007) from the Ministry
of Education, Culture, Sports, Science, and Technology (MEXT)
of Japan. K.M. thanks the JSPS for a Research Fellowship for
Young Scientists.
2
−1
constant of 1 toward PyF was determined to be 1.6 × 10 M
(
Figure S14). These results demonstrate that the photo-
dissociation behavior of these systems strongly depends on the
Lewis basicity of the additive. The observed dual emission of 1·
pyridine is based on the delicate balance between the Lewis
acidity of 1 and the Lewis basicity of the pyridine derivatives.
A highly relevant question in this context is how the Lewis
basicity of the additive affects the behavior in the excited state. In
order to shed some light on this question, DFT calculations at the
B3LYP/6-31G(d) level were carried out for 1·pyridine, 1·
DMAP, and 1·PyF. The HOMOs of these compounds are
comparable and are mainly localized on the benzofluoranthene
moiety. The LUMOs of these compounds, on the other hand, are
distinctly different. While the LUMO of 1·DMAP is delocalized
over the three naphthalene moieties, both 1·pyridine and 1·PyF
exhibit LUMOs localized on the pyridine moieties (Figure S27).
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ASSOCIATED CONTENT
(
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■
*
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AUTHOR INFORMATION
■
Corresponding Author
yamaguchi@chem.nagoya-u.ac.jp
D
dx.doi.org/10.1021/ja506980p | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX