Blue Fluorescence from BF2 Complexes of N,O-Benzamide Ligands: Synthesis, Structure, and Photophysical Properties

Article abstract of DOI:10.1021/acs.inorgchem.7b02013

Small molecules having intense luminescence properties are required to promote biological and organic material applications. We prepared five types of benzamides having pyridine, pyridazine, pyrazine, and pyrimidine rings and successfully converted them i

Full text of DOI:10.1021/acs.inorgchem.7b02013

Article
pubs.acs.org/IC
Blue Fluorescence from BF Complexes of N,O-Benzamide Ligands:
2
Synthesis, Structure, and Photophysical Properties
,
†
†,⊥
‡
‡
§
Minoru Yamaji,* Shin-ichiro Kato, † Kazuhiro Tomonari, Michitaka Mamiya, Kenta Goto,
∥
§
Hideki Okamoto, Yosuke Nakamura, and Fumito Tani
†
Division of Molecular Science, Graduate School of Science and Engineering, Gunma University, Kiryu, Gunma 376-8515, Japan
Education Program of Materials and Bioscience, Graduate School of Science and Engineering, Gunma University, Kiryu, Gunma
‡
3
76-8515, Japan
§
Institute for Materials Chemistry and Engineering, Kyushu University, Fukuoka 819-0395, Japan
Division of Earth, Life, and Molecular Sciences, Graduate School of Natural Sciences and Technology, Okayama University,
∥
Okayama 700-8530, Japan
*
S Supporting Information
ABSTRACT: Small molecules having intense luminescence properties are
required to promote biological and organic material applications. We prepared
five types of benzamides having pyridine, pyridazine, pyrazine, and pyrimidine
rings and successfully converted them into three types of the difluoroboronated
complexes, Py@BAs, as novel blue fluorophores. Py@BA having a pyridine
moiety (2-Py@BA) showed no fluorescence in solution, whereas Py@BAs of
pyridazine and pyrazine moieties (2,3-Py@BA and 2,5-Py@BA, respectively)
emitted blue fluorescence with quantum yields of ca. 0.1. Transient absorption
measurements using laser flash photolysis of the Py@BAs revealed the triplet
formation of 2,3- and 2,5-Py@BAs, while little transient signal was observed for
2
-Py@BA. Therefore, the deactivation processes from the lowest excited singlet
state of fluorescent 2,3- and 2,5-Py@BAs consist of fluorescence and intersystem
crossing to the triplet state while that of the nonfluorescent Py@BA is governed
almost entirely by internal conversion to the ground state. Conversely, in the solid state, 2-Py@BA emitted intense fluorescence
with a fluorescence quantum yield as high as 0.66, whereas 2,3- and 2,5-Py@BAs showed fluorescence with quantum yields of ca.
0
.2. The crystal structure of 2-Py@BA took a herringbone packing motif, whereas those for 2,3- and 2,5-Py@BAs were two-
dimensional sheetlike. On the basis of the difference in crystal structures, the emission mechanism in the solid state was
discussed.
INTRODUCTION
with fluorescence quantum yields of ca. 0.8 depending on the
substituted positions. We also determined fluorescence
■
11
Fluorescent probes are widely used in the field of biological and
organic material science. Much attention has been paid to
1
quantum yields in the solid state of these BF DKs ranging
2
from 0.05 to 0.25, which were smaller than those in solution.
difluoroboronated complexes ligated with dipyrromethane and
dibenzoylmethane skeletons as small and intensely fluorescent
systems. One of the former is known as a derivative of 4,4-
difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY), having high
fluorescence quantum yields, high extinction coefficients, sharp
absorption and fluorescence emission spectra, and high light
Ikeda and co-workers reported unique “excited multimer”
12
formation of BF complexes of dibenzoylmethane derivatives.
2
Their fluorescence quantum yields varied from ca. 0.2 to 0.7
depending on the molecular aggregation state in the single
crystals. Research and development for highly efficient
fluorophores are required for their application to organic
solid-state electronic devices, such as organic light-emitting
devices (OLEDs). However, there are very few applications of
2
−4
and chemical stability. Similarly, aromatic difluoroboronated
β-diketones (BF DKs) emit intense fluorescence not only in
2
5
−7
solution but also in the solid state. It is reported that varying
the arene systems using biphenyl, naphthyl, and anthryl
moieties as the chromophores in BF DKs is effective for
13,14
BF DK molecules to OLEDs,
although tremendous efforts
2
have been devoted to investigating BF DKs in the solid state,
2
2
8
−10
such as single crystals and polymer dispersion films. To design
modulating the photophysical properties.
Fluorescence
new BF complexes with efficient emission properties in the
quantum yields of these BF DKs in solution vary from 1 ×
2
2
−
3
solid state, especially blue emission, we need to create boron
10
to unity depending on the chromophores and the
1
0
substituents. In our previous research, we prepared BF DKs
2
having phenacene moieties such as phenanthrene and chrysene
Received: August 5, 2017
and showed that they emitted intense fluorescence in solution
©
XXXX American Chemical Society
A
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Inorganic Chemistry
Article
complexes different from BODIPYs and BF DKs by developing
new ligands.
Currently, an efficient and durable solid-state blue emitter is
required. We have been motivated to create a blue emitter with
compounds and the analytical data are given in the Supporting
Information.
2
Absorption, Emission, and Transient Absorption Measure-
ments. Absorption spectra were recorded on a JASCO V-550
spectrophotometer. Emission spectra in solution were recorded on a
Hitachi F-4010 fluorescence spectrophotometer. The fluorescence
quantum yields and fluorescence spectra in the solid state were
measured with a Hamamatsu Photonics absolute PL quantum yield
boron complexes different from BODIPYs and BF DKs. In this
2
context, we started to design ligand systems chelating a boron
atom with nitrogen and oxygen atoms as a hybrid feature
between BODIPY and BF DK which have ligation systems with
measurement system (C9920-02). Fluorescence lifetimes (τ
determined with a Hamamatsu Photonics fluorimeter system
Quantaurus Tau) on the basis of the time-correlated single-photon
) were
f
2
two nitrogen atoms and two oxygen atoms, respectively.
Hachiya et al. prepared difluoro(amidopyrazinato-O,N)boron
derivatives by mimicking the molecular structures of cypridina
(
counting method. Solution samples in a quartz cell with a 1 cm optical
path length were prepared in the dark, and purged with Ar when
necessary. The third harmonic (355 nm) from a Nd:YAG laser system
1
5
oxyluciferin. In the present research, we were successful in
preparing ligands of benzamides having pyridine, pyridazine,
pyrazine, and pyrimidine rings and attempted to synthesize
difluoroboron complexes with these ligands (see Chart 1).
(LT-2137 from Lotis-TII) was used for laser flash photolysis as the
excitation source. The details of the detection system for the time
16
profiles of the transient absorption have been reported elsewhere.
Transient absorption spectra were obtained using a Unisoku USP-
T1000-MLT system, which provides a transient absorption spectrum
with one laser pulse. The obtained transient spectral data were
analyzed by using the least-squares best-fit method.
Chart 1. Molecular Structures and Abbreviations for the
Ligands and Difluoroboronated Complexes (Py@BA) Used
in This Study
Theoretical Calculations. The calculation was performed at the
17
DFT level with the use of the Gaussian 09 software package. The
fully optimized geometries of the Py@BAs were calculated by using
18−22
the 6-31G(d) basis set with the B3LYP method under vacuum.
Atom coordinates for the optimized geometries are deposited in the
Supporting Information. Time-dependent DFT (TD-DFT) calcula-
2
3−25
26,27
tions
were carried out at the TD B3LYP/6-31+G(d) level
by
using the optimized ground-state geometries.
RESULTS AND DISCUSSION
■
Absorption and Emission Measurements. Figure 1
shows absorption and emission spectra of the Py@BAs
prepared in the present work.
Three of the five ligands were successfully converted to the
corresponding BF complexes. The emission features of the
2
prepared BF complexes (Py@BAs) in solution and the solid
2
state were investigated on the basis of fluorescence and
transient absorption measurements with the aid of theoretical
computations. Crystallographic studies of Py@BAs were also
performed to understand the emission mechanism in the
microcrystalline state.
EXPERIMENTAL SECTION
■
Analytical Instruments. NMR spectra were recorded on NMR
System 400 and 600 MHz spectrometers. High-resolution fast atom
bombardment mass spectra (HR-FAB-MS) were measured with 3-
nitrobenzyl alcohol as a matrix and recorded on a double-focusing
magnetic sector mass spectrometer (JEOL JMS-700). Melting points
were measured with a YANACO micro melting point apparatus.
Materials. Chloroform (spectroscopic grade) from Wako and
ethanol (EtOH, spectroscopic grade) from Kishida were used as
solvents for the spectroscopic measurements without further
purification.
Figure 1. Absorption (black) and fluorescence (blue) spectra in
chloroform at 295 K and phosphorescence spectra (red) in ethanol at
7
7 K obtained for 2-Py@BA (a), 2,3-Py@BA (b), and 2,5-Py@BA (c).
The emission spectra are not corrected.
Py@BAs showed large absorption bands with molar
4
3
absorption coefficients (ε) in the magnitude of 10 dm
−
1
−1
mol cm (see Table 1), which is typical for π,π* transitions.
2
,3-and 2,5-Py@BAs provided weak fluorescence at 295 K,
General Procedures for Preparing the Ligands and Py@BA.
The ligands (benzamide) were prepared by a reaction between the
appropriate amino-substituted pyridine, pyrimidine, or pyrazine and
benzoyl chloride. The prepared ligands were refluxed in benzene for 1
h in the presence of boron trifluoride diethyl etherate. We obtained 2-,
whereas 2-Py@BA showed no fluorescence in solution. The
maximum wavelengths (λ ) of fluorescence are given in Table
flu
1
7
. Phosphorescence from Py@BAs was observable in EtOH at
7 K. On the basis of the quantum yields (Φ ) and lifetimes (τ )
f
f
of fluorescence determined in CHCl , the fluorescence rates
2
,3-, and 2,5-Py@BAs, whereas no difluoroboron complexes of 2,4-L
3
and 2,6-L were formed. The precipitated product was collected and
washed with benzene. Details of the synthetic procedures for all of the
(k
f
), quantum yields (Φnr), and rates (knr) for nonradiative
processes were evaluated by eqs 1−3, respectively.
B
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Table 1. Photophysical Parameters of Py@BA Obtained in Chloroform
3
−1
a
8
−1
b
c
9
−1
ET /kcal mol−1
d
solid
solid
compound
-Py@BA
ε/dm mol cm−1 (λabs/nm) λflu/nm
Φf
τf/ns
k /10 s
Φnr
knr /10 s
λ
/nm Φf
τT/μs
f
flu
2
19500 (335)
15000 (328)
19800 (354)
N/A
392
403
0
N/A
0.62
0.82
N/A
0.81
1.6
1.0
N/A
1.5
64.3
63.6
61.5
415
0.66
0.18
0.16
2.1
5.0
2
2
,3-Py@BA
,5-Py@BA
0.05
0.13
0.95
397
0.87
1.1
426
12.0
a
−1
b
c
−1
d
Determined by the equation k = Φ τ . Determined by the equation Φ = 1 − Φ . Determined by the equation k = Φ τ . Determined from
f
f
f
nr
f
nr
nr f
the origin of the phosphorescence spectrum obtained in ethanol at 77 K.
−
1
k = Φ τ
(1)
(2)
f
f f
Φnr = 1 − Φf
knr = Φ τ −
1
(3)
nr f
The obtained photophysical parameters are given in Table 1.
The k value obtained for 2,5-Py@BA was twice as large as
f
that for 2,3-Py@BA, whereas their ε values are close to each
other. These results indicated that the Strickler−Berg relation-
ship between ε and k values is applicable to the present Py@
f
2
8
BA systems. The absence of fluorescence from 2-Py@BA may
be because of the nonradiative rate, which would consist of
internal conversion from the S to the ground state and
1
intersystem crossing to the triplet (T ) state, substantially larger
1
7
−1
than that for the k value (∼10 s ). The relaxation processes
f
of Py@BAs will be discussed later.
Figure 2 shows fluorescence spectra in the crystalline solid of
Py@BA.
Figure 3. Transient absorption spectra taken at 100 ns upon 355 nm
laser pulsing in the Ar-purged chloroform solution of 2-Py@BA (a),
2
,3-Py@BA (b), and 2,5-Py@BA (c) at 295 K. Insets give time profiles
of the transient signals.
Transient absorption spectra were measured in the 390−720
nm wavelength region, in Ar-purged chloroform. The
intensities of the transient absorption signals decreased with
decay lifetimes (τ ) in the microsecond time region. The τ
T
T
data are given in Table 1. In aerated chloroform, the decay was
accelerated. This fact indicates that the transient signals are due
to triplet−triplet (T-T) absorption. Observation of the triplet
state in solution for 2,3- and 2,5-Py@BAs shows that ISC from
the S to the T state competes with the fluorescence process.
1
1
Figure 2. Fluorescence spectra of 2-Py@BA (a), 2,3-Py@BA (b), and
On the basis of the facts of no fluorescence and little triplet
2
,5-Py@BA (c) in the solid state at 295 K.
formation for 2-Py@BA, the S state deactivates mainly by
1
internal conversion (IC) to the ground state. The IC process of
aromatic molecules in excited states is generally induced by
vibrational and rotational motions. The difference between 2-
and 2,3- or 2,5-Py@BAs is the number of nitrogen atom(s) in
the heterocycle moiety. It is noteworthy that the difference in
the number of the nitrogen atom(s) in the heterocycle moieties
caused a large difference in their relaxation processes, from the
S1 states although the precise mechanism has not been
determined at present.
Samples of 2-Py and 2,3-Py@BAs for emission measurement
were recrystallized from a mixture of chloroform and n-hexane,
whereas that of 2,5-Py@BA was recrystallized from toluene.
solid
The fluorescence maxima (λflu ) in the solid state given in
Table 1 are red-shifted in comparison with those in solution. It
is a typical observation that the wavelength of an emission band
1
2,29
in solution differs from that in the solid state.
The
solid
fluorescence quantum yields (Φ ) in the solid state were
f
solid
f
determined, and the data are also given in Table 1. The Φ
DFT Calculations. To get an insight into the photophysical
values of 2,3- and 2,5-Py@BAs are greater than those (Φ ) in
features of Py@BAs, DFT and TD-DFT calculations were
f
solid
f
26,27
solution. In particular, 2-Py@BA showed a large value of Φ
performed at the B3LYP/6-31+G(d) level.
results are summarized in Table 2.
The calculated
although it displayed no fluorescence in solution.
Transient Absorption Measurements. In order to
Calculated molecular orbitals, the highest occupied molecular
orbitals (HOMO) and lowest unoccupied molecular orbitals
(LUMO), for Py@BAs under vacuum are shown in Figure 4.
In the optimized structures of Py@BAs, the HOMO and
LUMO are located over the ligand moieties. The S → S
investigate the deactivation processes from the S state, laser
1
flash photolysis of Py@BAs was carried out in chloroform.
Transient absorption spectra obtained upon 355 nm laser
pulsing in chloroform solution of Py@BAs are displayed in
Figure 3.
0
1
transitions of Py@BAs contributed from the HOMO →
C
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Inorganic Chemistry
Article
a
Table 2. Calculated Photophysical Data of Py@BAs under Vacuum
b
c
d
e
compound
HOMO/eV
LUMO/eV
S → S transition
λ /nm
f
coeff
0
1
tr
2
-Py@BA
−6.83 (−6.84)
−7.15 (−7.15)
−7.10 (−7.04)
−2.53 (−2.53)
−2.90 (−2.88)
−3.04 (−3.00)
π,π*
π,π*
π,π*
321 (328)
328 (330)
0.5579 (0.7686)
0.2644 (0.5041)
0.4532 (0.6298)
0.69566 (0.70179)
0.69584 (0.70249)
0.69992 (0.70402)
2
,3-Py@BA
,5-Py@BA
2
339 (350)
b
a
Data in parentheses are calculated results considering the dielectric constant of CHCl . Character of the transitions to the excited singlet states
with the lowest excitation energies. Wavelength estimated from transition energy. Oscillator strength for the S → S transition. For a transition
3
c
d
e
0
1
from HOMO to LUMO.
nitrogen-containing heteroaromatics and the benzene rings
being 3.8, 7.1, and 5.9°, respectively. In the crystals of Py@BAs,
two neighboring molecules form the π-stacked dimer structures
in an anti, head-to-tail manner. The intermolecular distances
between the center of the heteroaromatic moiety of the
molecule and the mean plane of the π skeleton of the facing
molecule in the dimers of 2-Py@BA, 2,3-Py@BA, and 2,5-Py@
BA are 3.40, 3.60, and 3.37 Å, respectively, indicating that the
π−π stacking interactions are efficient.
Figure 6 shows the packing diagrams of the crystal structures
of 2-Py@BA, 2,3-Py@BA, and 2,5-Py@BA.
Figure 4. HOMO and LUMO orbitals of Py@BAs in vacuum.
Any π-stacked dimers of 2-Py@BA, 2,3-Py@BA, and 2,5-
Py@BA interact with other dimers through π−π stacking
interactions to form one-dimensional (1D) column structures
along the a, b, and a axes, respectively. While the 1D columns
in the crystal of 2-Py@BA take a slanted, slipped-stacked form,
those of 2,3-Py@BA and 2,5-Py@BA take a slightly zigzag form.
Noticeably, 2-Py@BA, 2,3-Py@BA, and 2,5-Py@BA differ in
their overall packing motifs despite the similarity of their π-
stacked dimers. In the case of the crystal of 2-Py@BA, the 1D
columns are packed in a herringbone motif as a result of
multipoint CH−O (2.71 Å) and F−π (3.20 and 3.22 Å)
interactions. On the other hand, the columns of 2,3-Py@BA
assemble together by CH−O (2.81 Å) and CH−N (2.43 Å)
interactions to afford two-dimensional (2D) sheetlike struc-
tures. Similar to the crystal structure of 2,3-Py@BA, the
columns of 2,5-Py@BA have short contacts with each other by
CH−N (2.63 Å) and CH−F (2.58 Å) interactions to form 2D
sheetlike structures; toluene molecules are sustained by CH−π
interactions (2.81−3.05 Å). In the crystal structures of 2,3-Py@
BA and 2,5-Py@BA, no noticeable F−π interaction was
observable, implying that the CH−N and/or CH−F
interactions prevail over F−π interactions. Consequently,
these findings show that the difference in the heteroaromatic
moieties, namely, pyridine, pyridazine, and pyrazine rings in 2-
Py@BA, 2,3-Py@BA, and 2,5-Py@BA, respectively, is reflected
LUMO configurations can be assigned as a π,π* transition. The
wavelengths (λ ) estimated from the calculated transition
tr
energies are in the 320−350 nm wavelength region and are
similar to the experimental λ values of the lowest energy
abs
absorption bands in CHCl . The magnitude of the calculated
3
oscillator strengths (f) for the S → S transitions is responsible
0
1
for the allowed π,π* transition. It is known that the Strickler−
Berg equation relates the fluorescence rate k to the oscillator
f
2
8
strength f. In the present study, we definitely observed no
fluorescence from 2-Py@BA in solution, although its calculated
f value was in the same order of magnitude as those of 2,3- and
2
,5-Py@BAs. This consideration supports the proposed
mechanism that the absence of fluorescence from 2-Py@BA
in solution may derive from IC from the S to the ground state.
1
X-ray Crystallography. We investigated the molecular
structures by X-ray diffraction analyses of single crystals of 2-
Py@BA, 2,3-Py@BA, and 2,5-Py@BA for understanding the
emission mechanisms in the microcrystalline state. Crystalline
samples for 2-Py@BA and 2,3-Py@BA were prepared from a
mixture of chloroform and n-hexane, whereas that for 2,5-Py@
BA was from toluene. While the crystals of 2-Py@BA and 2,3-
Py@BA contain no solvent molecule, the crystal of 2,5-Py@BA
contains toluene, which was used in the recrystallization, in a
by their supramolecular arrangement in the solid state.
2
:1 stoichiometry. Figure 5 shows the arrangement of
solid
In the present work, we determined Φ
(=0.66) in the
f
neighboring molecules in the crystals of 2-Py@BA, 2,3-Py@
BA, and 2,5-Py@BA.
solid state for photoluminescence for 2-Py@BA, which was
greater than those for 2,3- and 2,5-Py@BAs (0.16−0.18). By
Molecules of 2-Py@BA, 2,3-Py@BA, and 2,5-Py@BA take a
highly planar form with the dihedral angles between the
solid
f
analogy to Φ , Φ
is expressed as
f
solid
Φf = k fs olid(k
solid
f
solid −1
+ knr
)
(4)
where k fs olid and k ns or lid are respectively rates for fluorescence and
nonradiative processes in the solid state. As the emission
spectra in the solid state and the molecular geometries of the
solid
dimer of Py@BAs are similar to each other, kf in the solid
state may be in the same order of magnitude. The difference in
solid
f
solid
the Φ values may thus be due to that in knr , which closely
correlates to molecular structures in the crystal in terms of
energy dispersion through spin−lattice interaction. The dimers
of 2-Py@BA in the crystal were packed in a herringbone
pattern, whereas those of 2,3- and 2,5-Py@BAs were in one-
Figure 5. Arrangement of neighboring molecules in the crystal packing
for 2-Py@BA (a), 2,3-Py@BA (b), and 2,5-Py@BA (c). The
perspective drawings of 2-Py@BA, 2,3-Py@BA and 2,5-Py@BA are
shown in the Supporting Information.
D
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Inorganic Chemistry
Article
state, while that of 2-Py@BA is governed almost by IC to the
ground state. The difference in the deactivation features
between 2- and 2,3- or 2,5-Py@BAs originated from the
heterocycle moieties of Py@BAs. In the solid state, these three
solid
Py@BAs emitted blue fluorescence. 2-Py@BA showed a Φ
f
solid
value of 0.66, while for 2,3- and 2,5-Py@BAs, the Φ
values
f
were ca. 0.17. Crystallographic studies of Py@BAs showed
similarities and differences in crystal packing among Py@BAs.
π-stacked dimer structures in an anti, head-to-tail manner were
seen for these three Py@BAs. From the viewpoint of crystal
structures, a herringbone motif was seen for 2-Py@BA, whereas
a 1D column structure consisting of the dimer of the molecule
was found for 2,3- and 2,5-Py@BAs. In the present work, we
demonstrated that the lower number of nitrogens in the
heterocyclic moiety for 2-Py@BA in comparison to that for 2,3-
and 2,5-Py@BAs substantially affects the photophysical proper-
ties in solution and the solid state and the crystal structures.
The present results provide a strategy to produce a new blue
emitter by means of N,O-ligation with boron atoms and display
remarkable enhancement of fluorescence efficiency in the solid
state: namely, for 2-Py@BA. Additionally, it is expected that
one can modify the luminescence properties in solution and the
solid state by replacing the phenyl ring in 2-Py@BA with
various aromatic chromophores. Further studies are in progress.
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b02013.
Precise synthesis procedures and analytical data for the
compounds used in this work, decay profiles of
fluorescence, results of DFT calculations including tables
of atom coordinates for the optimized geometries, and
1
13
H and C NMR spectra of prepared compounds (PDF)
Accession Codes
CCDC 1539204−1539205 and 1555564 contain the supple-
mentary 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.
Figure 6. Molecular packing in the crystals for 2-Py@BA (a), 2,3-Py@
BA (b), and 2,5-Py@BA (c). The perspective drawings of 2-Py@BA,
2
,3-Py@BA, and 2,5-Py@BA are shown in the Supporting
Information. Toluene molecules included in the crystal of 2,5-Py@
BA are disordered.
dimensional column structures. The k ns or lid value for a
herringbone structure is smaller than that for two-dimensional
sheetlike structures. It is noteworthy that replacement of
nitrogen atoms in heterocycles of 2,3- and 2,5-Py@BAs with a
C−H moiety affects the fluorescence character of 2-Py@BA in
solution as well as in the solid state, where the large difference
in the crystal structures emerges.
AUTHOR INFORMATION
Corresponding Author
*E-mail for M.Y.: yamaji@gunma-u.ac.jp.
■
ORCID
Minoru Yamaji: 0000-0001-9963-2136
Yosuke Nakamura: 0000-0002-6047-1336
Present Address
CONCLUSION
■
⊥
Department of Materials Science, School of Engineering, The
Five types of benzamides having pyridine, pyridazine, pyrazine,
and pyrimidine rings were prepared, and three types of the
difluoroboronated complexes, 2-, 2,3-, and 2,5-Py@s, were
successfully produced. 2-Py@BA, which has a pyridine moiety,
showed no fluorescence in solution, whereas 2,3- and 2,5-Py@
University of Shiga Prefecture, 2500 Hassaka-cho, Hikone,
Shiga 522-8533, Japan.
Author Contributions
All authors contributed equally.
BAs emitted blue fluorescence with Φ = ca. 0.1. Transient
Funding
f
absorption measurement using laser flash photolysis of the Py@
BAs revealed the triplet formation of 2,3- and 2,5-Py@BAs,
while little transient signal was obtained for 2-Py@BA.
This work was supported by Grants-in-Aid for Scientific
Research (JP26288032 and JP17K05976) from the Japan
Society for the Promotion of Science (JSPS) and was partially
supported by the association for the advancement of science
and technology, Gunma University.
Therefore, the deactivation processes from the S state of 2,3-
1
and 2,5- Py@BAs consist of fluorescence and ISC to the triplet
E
DOI: 10.1021/acs.inorgchem.7b02013
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Notes
[amidopyrazinato-O,N]Boron Derivatives: A New Boron-containing
Fluorophore. Tetrahedron Lett. 2010, 51, 1613−1615.
The authors declare no competing financial interest.
(
16) Yamaji, M.; Aihara, Y.; Itoh, T.; Tobita, S.; Shizuka, H.
Thermochemical Profiles on Hydrogen Atom Transfer from Triplet
Naphthol and Proton-Induced Electron Transfer from Triplet
Methoxynaphthalene to Benzophenone Via Triplet Exciplexes Studied
by Laser Flash Photolysis. J. Phys. Chem. 1994, 98, 7014−7021.
ACKNOWLEDGMENTS
■
This work was supported by the Cooperative Research
Program of the Network Joint Research Center for Materials
and Devices to M.Y. and H.O., respectively. M.Y. acknowledges
the technical staff at Kyushu University for performing the
HRMS spectrometry of the new compounds under the
Cooperative Research Program of the Network Joint Research
Center for Materials and Devices.
(17) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,
B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H.
P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.;
Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin,
K. N.; Staroverov, V. N.; Keith, T.; Kobayashi, R.; Normand, J.;
Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.;
Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.;
Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.;
Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.;
Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador,
P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.;
Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09,
Revision D.01; Gaussian, Inc., Wallingford, CT, 2010.
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DOI: 10.1021/acs.inorgchem.7b02013
Inorg. Chem. XXXX, XXX, XXX−XXX