Synthesis and fluorescence properties of pyrimidine mono- and bisboron complexes

Article abstract of DOI:10.1021/jo400879g

Novel fluorescent mono- and bisboron complexes based on pyrimidine bearing β-iminoenolate ligands were synthesized, and their fluorescence properties were investigated. The nonsubstituted and trifluoromethyl-substituted monoboron complexes showed higher f

Full text of DOI:10.1021/jo400879g

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Article
Synthesis and Fluorescence Properties of
Pyrimidine Mono- and Bisboron Complexes
Yasuhiro Kubota, Yousuke Ozaki, Kazumasa Funabiki, and Masaki Matsui
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/jo400879g • Publication Date (Web): 24 Jun 2013
Downloaded from http://pubs.acs.org on June 30, 2013
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The Journal of Organic Chemistry
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Synthesis and Fluorescence Properties of Pyrimidine
Mono- and Bisboron Complexes
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Yasuhiro Kubota,* Yousuke Ozaki, Kazumasa Funabiki, and Masaki Matsui*
Department of Materials Science and Technology, Faculty of Engineering, Gifu University, 1ꢀ1
Yanagido, Gifu 501ꢀ1193, Japan
Eꢀmail : kubota@gifuꢀu.ac.jp, matsuim@gifuꢀu.ac.jp
Abstract
R = H
R = NMe2
R = CF3
Novel fluorescent monoꢀ and bisboron complexes based on pyrimidine bearing βꢀketoiminate
ligands were synthesized and their fluorescence properties were investigated. The nonsubstituted and
trifluoromethylꢀsubstituted monoboron complexes showed higher fluorescence quantum yield in solid
state than in solution. The dimethylamino derivative of the monoboron complex exhibited positive
solvatochromism in the fluorescence spectra. The bisboron complex showed significantly higher molar
absorption coefficient and redꢀshifted maximum absorption and maximum fluorescence wavelengths
than the corresponding monoboron complex.
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Introduction
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Boron complexation is a simple yet effective strategy to express or enhance fluorescence. For
example, although dipyrromethene,1 pyridomethene,2 diketone,3 iminoketone,4 and azo dyes5 generally
show no fluorescence, their boron complexes are known to be fluorescent. Among the boron complexes,
especially boron dipyrromethene (BODIPY) dyes, show excellent fluorescence in solution and are used
as molecular probes,6 in photodynamic therapy,7 as laser dyes8 and solar cells.9 However, most
BODIPY dyes show small Stokes shift and lose their fluorescence in solid state.10 Therefore, the
modification of BODIPY core11 and the development of new boron complexes12 are being actively
investigated.
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On the other hand, in recent years, much attention has been paid to multinuclear boron
complexes.13 Bisboron complexes with interesting properties such as longꢀwavelength absorption and
fluorescence,14 fluorescent near infrared (NIR)ꢀlabeling,15 in vivo imaging,16 twoꢀphoton absorption,17
nonlinear optical properties,18 and solidꢀstate fluorescence19 have been reported. Recently, we have
reported that monoboron complexes based on pyrazine or thiazole structures bearing a βꢀketoiminate
ligand show interesting fluorescence properties such as a large Stokes shift, solidꢀstate fluorescence and
aggregationꢀinduced emission enhancement (AIEE) effect.4 In the course of our studies on the
development of fluorescent boron complexes bearing
βꢀketoiminate ligands, we are interested in the
synthesis of the boron complex based on pyrimidine structure. In this paper, we describe the synthesis
and fluorescence properties of novel monoꢀ and bisboron complexes based on pyrimidines bearing
ketoiminate ligands.
βꢀ
Results and Discussion
Synthesis
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The reaction of 4,6ꢀdichloropyrimidine with acetophenone in the presence of sodium hydride
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3
gave bidentate ligand 1, which consists of a tautomeric mixture of iminoketone 1a and iminoenol 1b in
4
5
6
1
CDCl3 (Scheme 1). The iminoketone and iminoenol structures were confirmed by H NMR spectra of
7
8
the methylene signal at
δ 4.47 (s, 2H) of 1a and olefinic signal at δ 6.01 (s, 1H) and hydroxy signal at δ
9
14.6 (brs, 1H) of 1b. The ratio of 1a and 1b was 1 : 15 in CDCl3. The tautomeric mixture of 1 was
treated with boron trifluoride diethyl ether complex in the presence of triethylamine to give monoboron
complex 4. Solution of 4 and acetophenone in tetrahydrofuran was refluxed, quenched with water,
extracted with dichloromethane, and concentrated. The residue was reacted with boron trifluoride
diethyl ether complex in the presence of triethylamine to yield bisboron complex 7.
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Similar reactions of 4,6ꢀdichloropyrimidine with 4'ꢀ(trifluoromethyl)acetophenone and 4'ꢀ
dimethylaminoacetophenone gave the corresponding monodentate ligands 2 and 3 (Scheme 1). The
ligands 2 and 3 were allowed to react with boron trifluoride diethyl ether complex in the presence of
triethylamine to give trifluoromethylꢀ and dimethylaminoꢀsubstituted BF2 complexes 5 and 6,
respectively.
Scheme 1. Synthesis of pyrimidine boron complexes.
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Crystal structures
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7
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The structures of 4–7 were confirmed by Xꢀray crystallographic analysis. The ORTEP drawings
of 4–7 are shown in Figures 1–4, respectively. The crystal structure of 7 shows C2 symmetry along the
C4–C5 axis. The C1–O1 bond lengths of 4 (1.32 Å) and 7 (1.33 Å) are between typical C–O (ca. 1.43
Å) and C=O (ca. 1.23 Å) bond lengths (Figures S1 and S2). The C2–C3 (4: 1.41 Å, 7: 1.42 Å) bonds are
longer than C1–C2 bonds (4: 1.35 Å, 7: 1.36 Å). The B1–O1 bond lengths of 4 (1.44 Å) and 7 (1.44 Å)
are considerably smaller than the B1–N1 bond lengths of 4 (1.59 Å) and 7 (1.59 Å). These results
suggest that the sixꢀmembered rings of 4 and 7 have delocalized iminoenolate structures.
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The boron atoms of 4 and 7 have a tetrahedral geometry. Although the B1 atom of 4 lies in the
same plane as the pyrimidine ring, the B1* and B1 atoms of 7 are located above and below the
corresponding pyrimidine ring (Figures S1 and S2). As a result, while 4 is almost planar, 7 is slightly
bent. Additionally, the F2 and F1 atoms of 4 are evenly situated above and below the pyrimidine ring.
On the other hand, two fluorine atoms of 7 (F1* and F1) are oriented perpendicularly above and below
the pyrimidine ring and the others (F2* and F2) are arranged horizontally to the pyrimidine ring.
Furthermore, the central pyrimidine ring of 7 is more distorted than that of 4. In the pyrimidine ring of 4,
dihedral angle between the C12–C4–C3 plane and the N2–C5–N1 plane is 0.5° (Figure S1). Meanwhile,
in the case of 7, the corresponding dihedral angle is 2.2° (Figure S2). The adjacent bidentate
coordination of two boron atoms through the pyrimidine moiety causes a larger distortion in the
structure of 7 than in that of 4. This may be due to the relief of the steric repulsion between neighboring
boron moieties.
The torsion angles of C2–C1–C6–C7 of 4–7 are 5.9°, 0.8°, 6.7° and 10.2°, respectively (Figure
S3). This indicates that the terminal aryl groups allow the extension of πꢀconjugation. The C1–C6 bond
length of 4–7 is as follows: 1.477 Å for 4, 1.481 Å for 5, 1.461 Å for 6 and 1.479 Å for 7 (Figure S3).
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The C1–C6 bond length of 6 is shorter than those of 4, 5 and 7, suggesting that contribution of the
resonance structure shown in Figure S4 is important in the ground state of 6.
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Cl1
C4
N1
C10
C11
C7
C2
C12
N2
C3
C1
O1
C9
C8
C6
B1
C5
F2
F1
Figure 1. ORTEP view of 4 (CCDC 931994). Hydrogen atoms have been omitted for clarity.
C10
C10*
C7*
C11
C6
C9
C9*
C8*
C11*
C2*
F1*
C4
C2
C3*
C3
C1*
C1
C6*
O1*
C8
C7
B1
O1
N1*
N1
B1*
C5
F2
F2*
F1
Figure 2. ORTEP view of 7 (CCDC 931992). Hydrogen atoms have been omitted for clarity.
Cl1
C4
C3
F5
C11 C10
C9
C13
C2
C1
C12
N2
F4
C6
C7
N1
C5
C8
O1
F1
F3
F2
Figure 3. ORTEP view of 5 (CCDC 931993). Hydrogen atoms have been omitted for clarity.
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Cl1
C4
C14
N3
C11 C10
C9
C12
N2
C2
C3
C1
C6
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N1
F2
C5
O1
F1
C7
C8
C13
Figure 4. ORTEP view of 6 (CCDC 931995). Hydrogen atoms have been omitted for clarity.
Optical properties
The UV–vis absorption and normalized fluorescence spectra of 4–7 in dichloromethane are
shown in Figures 5 and 6, respectively. The maximum absorption wavelength (λmax) of monoboron
complex 4 (375 nm) was slightly redꢀshifted compared to that of ligand 1 (346 nm) (Table 1). Although
ligand 1 did not show fluorescence, complex 4 exhibited weak blue fluorescence at 429 nm. The
fluorescence quantum yields (
Bisboron complex 7 showed a sharp absorption peak at 475 nm along with a vibrational peak at 449 nm.
The λmax of 7 was 100 nm redꢀshifted compared to that of 4. Molar absorption coefficient ( ) of 7
Φf) of 1 and 4 in dichloromethane were 0.00 and 0.02, respectively.
ε
(107,300) was significantly higher than that of 4 (42,600), which may be because of the extension of πꢀ
conjugation by annulation and the two terminal phenyl groups. Bisboron complex 7 exhibited relatively
strong yellowꢀgreen fluorescence (Φf = 0.55) and the maximum fluorescence wavelength (Fmax) was
490 nm. The Stokes shift of 7 (645 cmꢀ1) was very small, similar to other boron complexes such as
BODIPY dyes1 (400–600 cmꢀ1, in most cases) and pyridomethene–BF2 complexes2b (250–400 cmꢀ1).
This suggests that 7 has rigid molecular structure with little difference between the ground and excitedꢀ
state structures.20 The
λ
max of 5 (372 nm) was slightly blueꢀshifted and ε was lower (35,800) compared
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to that of 4 (375 nm and 42,600), respectively. On the other hand, 6 showed relatively large
bathochromic shift of max (459 nm) and increase in (70,800).
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λ
ε
5
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7
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9
Table 1. Optical properties of pyrimidine boron complexes
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In dichloromethane[a]
Solid state
Fmax[g] [nm]
Compd
λ
max [nm] (
ε
)
Fmax[b] [nm]
Φf[b,c]
τs[d] [ns]
kf[e] [109 sꢀ1]
knr[f] [109 sꢀ1]
Φf[c,g]
[h]
375 (42,600)
391 (33,300)
4
411, 429
0.02
525
0.13
[h]
372 (35,800)
388 (24,000)
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7
407, 426
529
0.01
0.78
0.55
488
629
641
0.15
0.20
0.07
459 (70,800)
2.8
1.4
0.28
0.39
0.08
0.32
449 (85,000)
475 (107,300)
490, 517
[a] Measured at a concentration of 1.0 × 10ꢀ5 mol dmꢀ3. [b] The excitation wavelengths (
using an integrating sphere method. [d] Measured using a singleꢀphotonꢀcounting method. [e] Radiative rate constant (kf =
[g] The ex were as follows: 4 (368 nm), 5 (372 nm), 6 (523 nm) and 7 (504 nm). [h] Too short to be measured (τs < 0.1 ns).
λex) were as follows: 4 (377 nm), 5 (377 nm), 6 (452 nm) and 7 (464 nm). [c] Measured
Φ
f/τs). [f] Nonꢀradiative rate constant (knr = (1 ꢀ Φf)/τs).
λ
1
0.5
0
7
6
4
5
300
400
500
600
Wavelength / nm
Figure 5. UV–vis absorption spectra of 4–7 in dichloromethane (1.0 × 10ꢀ5 M).
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6
400
500
600
700
Wavelength / nm
Figure 6. Normalized fluorescence spectra of 4–7 in dichloromethane (1.0 ×10ꢀ5 M).
Theoretical calculation
To understand the absorption properties of 4–7, density functional theory (DFT) calculations
were performed with a Gaussian 09 package.21 The geometries were optimized at DFT/B3LYP level
using a 6ꢀ31G(d,p) basis set. Timeꢀdependent DFT (TDDFT) calculations were also performed using a
B3LYP/6ꢀ311++G(d,p) method. The calculated λmax, main orbital transition, and oscillator strength f are
listed in Table S1. For all complexes, the first absorptions are mostly attributed to HOMO–LUMO
transitions. For each complex, the HOMO and LUMO orbitals are delocalized over the whole molecule
(Figure 7). In particular, in the case of 7, the two terminal phenyl rings significantly contribute to the
delocalization. Therefore, bisboron complex 7 is expected to have redꢀshifted λmax and high ε. The DFT
calculations indicate that dimethylamino derivative 6 shows intramolecular chargeꢀtransfer (ICT)
transition from dimethylamino group to the pyrimidine moiety. The redꢀshifted λmax of 6 are owing to
the ICT transition.
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1
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7
LUMO+1
LUMO
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HOMO
HOMOꢀ1
4
5
6
7
Figure 7. Molecular orbital energy diagram and isodensity surface plots of the HOMO–1, HOMO,
LUMO and LUMO+1 of 4–7.
Discussion of fluorescence quantum yield in solution
Although complexes 4 and 5 hardly showed any fluorescence (Φf ≤ 0.02), complexes 6 (Φf =
0.78) and 7 (Φf = 0.55) exhibited relatively strong fluorescence. Recently, we reported that thiazole
–
monoboron complexes bearing ꢀketoiminate hardly show fluorescence in solution owing to the
β
intramolecular C–Ar rotation, which promotes nonradiative processes.4b In order to consider the effect
of the C–Ar rotation on the Φf value, we investigated the fluorescence properties of 4 in THF–water
mixtures of various rations. The fluorescence color and Fmax were almost unchanged until the addition
of 80% water by volume, because 4 was soluble in the THF–water mixtures. However, when the water
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fraction exceeded 85%, the fluorescence color and Fmax were dramatically changed from blue to yellowꢀ
green and from 430 to 520 nm, respectively (Figures S5, 6). The fluorescence intensity was also
increased with increasing the water fraction. As described below in detail, 4 has higher Φf value and
more redꢀshifted Fmax in solid state than in solution. Thus, these changes are considered to result from
the formation of aggregates in the THF–water mixtures: the formation of nanoparticles was confirmed
by the observation of the Tyndall phenomenon (Figure S7). Since molecular rotations are restricted in
the solid state, the observed increase of fluorescence intensity indicates the intramolecular C–Ar rotation
causes the fluorescence quenching in solution.4b, 22 However, in the light of the fact that the nonradiative
rate constant (knr) of 7 (0.32 × 109 sꢀ1) having two phenyl groups is small compared to those of 4 (5.00 ×
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109 sꢀ1) and 5 (4.57 × 109 sꢀ1), there may be other process in the nonradiative decay of 4 such as a heavyꢀ
atom effect of chlorine (Table S2, S3, and 2).23
The knr value of 6 (0.08 × 109 sꢀ1) was obviously smaller than those of 4 and 5. This suggests that
the higher Φf value of 6 is due to the suppression of nonradiative processes. In the case of 6, the
resonance structure predicts the restriction of the C–Ar rotation (Figure S4). Therefore, the higher Φf
value of 6 may be because of not only the difference of the transition between 4, 5 (π–π* transition) and
6 (ICT transition) but also the restriction of the intramolecular C–Ar rotation.
Solvent effect
The effects of solvent on the absorption and fluorescence properties of 4–6 were investigated.
The λmax, ε, and Fmax values of 4 and 5 were slightly affected by the type of solvent (Figures S8–S11,
Tables S2 and S3). Moreover, the absorption spectra of 6 were hardly affected by the solvent (Figure 8).
However, 6 showed pronounced positive solvatochromism in the fluorescence spectra (Figures 9 and
Table 2). As the solvent polarity increased, the fluorescence spectra got redꢀshifted and broadened. To
estimate the difference in dipole moments between the excited state and ground state (ꢀꢁ), Lippertꢀ
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Mataga plot of 6 was created (Figures 10 and 11).24 Solvent polarity parameters (ꢀf) are obtained from
1
2
3
equation shown in Figure 10.25 The radius of Onsager cavity was calculated by B3LYP/6ꢀ31G(d,p)
4
5
6
method (a = 4.98 Å). The ꢀ
ꢁ value of 6 was estimated to be 11.4 D from the slope of LippertꢀMataga
7
8
9
plot (10,630 cmꢀ1). The dipole moment in the ground state (
ꢁg) was calculated to be 9.17 D by using
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B3LYP/6ꢀ31G(d,p) method. Therefore, the dipole moment of 6 in the excited state (
ꢁ
e) was estimated to
be ca. 20.6 D. The high ꢁe value suggests the formation of ICTꢀexcited state with higher dipolar
moment than the ground state.26 Because the ICTꢀexcited state is stabilized in more polar solvents, Fmax
redꢀshifted with increasing solvent polarity.
Table 2. Absorption and fluorescence properties of 6 in various solvents
Solvent[a]
ꢀf[b]
λ
max [nm] (
ε
)
Fmax[c] [nm]
Stokes shift
[cmꢀ1]
Φf[c,d]
τs[e] [ns]
kf[f] [109 sꢀ1]
knr[g] [109 sꢀ1]
Hexane
Toluene
0.00
0.02
0.15
0.21
0.22
0.31
444 (73,900)
453 (68,900)
460 (67,300)
451 (64,800)
459 (70,800)
452 (63,800)
456
494
512
532
529
565
593
0.82
0.84
0.82
0.60
0.78
0.03
1.9
2.3
2.5
2.6
2.8
0.3
0.44
0.37
0.32
0.23
0.28
0.11
0.10
0.07
0.07
0.15
0.08
3.46
1832
2208
3376
2882
4425
Chloroform
THF
Dichloromethane
Acetonitrile
[a] Measured at a concentration of 1.0 × 10ꢀ5 mol dmꢀ3. [b] Solvent polarity parameter ([(
follows: hexane (445 nm), toluene (452 nm), chloroform (458 nm), THF (451 nm), dichloromethane (452 nm) and acetonitrile (451 nm). [d] Measured using an integrating sphere
method. [e] Measured using a singleꢀphotonꢀcounting method. [f] Radiative rate constant (kf = f/τs). [g] Nonꢀradiative rate constant (knr = (1 ꢀ f)/τs).
ε ꢀ1 )/(2ε λex) were as
+ 1)] ꢀ [(n2 ꢀ1 )/(2n2 + 1)]) (ref. 25). [c] The excitation wavelengths (
Φ
Φ
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0.8
0.6
0.4
0.2
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Hexane
Toluene
Chloroform
THF
Dichloromethane
Acetonitrile
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300
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Wavelength / nm
Figure 8. UVꢀvis absorption spectra of 6 in various solvents (1.0 × 10ꢀ5 M).
1
0.8
0.6
0.4
0.2
0
Hexane
Toluene
Chloroform
THF
Dichloromethane
Acetonitrile
400
500
600
700
800
Wavelength / nm
Figure 9. Normalized fluorescence spectra of 6 in various solvents (1.0 ×10ꢀ5 M).
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Figure 10. LippertꢀMataga equation.
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5000
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1000
0
Acetonitrile
THF
Dichloromethane
Toluene
Hexane
ν
Chloroform
ꢀ
ν
= 10630ꢀf + 932
r² = 0.914
0
0.1
0.2
0.3
0.4
ꢀf
Figure 11. LippertꢀMataga plot of 6.
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Solid-state fluorescence
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4
5
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7
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9
Boron complexes 4ꢀ7 showed solidꢀstate fluorescence (Table 1). The fluorescence spectra of 4ꢀ7
are shown in Figure 12. The Φf values were as follows: 0.13 (4), 0.15 (5), 0.20 (6), and 0.07 (7).
Interestingly, monoboron complexes 4 and 5 showed higher Φf values in solid state than in solution (Φf
≤ 0.05); in other words, 4 and 5 have aggregationꢀinduced enhanced emission (AIEE) character. This is
probably because of the restriction of intramolecular C–Ar rotation in the solid state.4b, 22
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To consider the differences in
Φf, we investigated the crystal packing of 4ꢀ7. In the crystal
packing of 7, the molecules depicted in blue and red (Figure 13) formed independent stacking columns
7A and 7B by intermolecular π–π interactions (C–C: 3.40–3.67 Å). Furthermore, π–π interactions (C–C:
3.56–3.69 Å) were observed between the columns 7A and 7B. As a result, bisboron complex 7 formed
consecutive intermolecular π–π interactions throughout the molecule. In many cases, strong and
continuous intermolecular interactions between the neighboring fluorophores cause fluorescence
quenching in the solid state.27 Therefore, because of the consecutive π–π interactions, 7 is considered to
have relatively low
Φf (0.07).
In the case of 4, the molecules depicted in blue and red as well as in orange and gray formed
dimers by π–π interactions (C–C: 3.36–3.70 Å) (Figure 14). Additionally, π–π interactions were
observed for each of the dimers (C–C: 3.52–3.69 Å). Thus, similar to 7, independent stacking columns
4A and 4B were formed by consecutive π–π interactions. However, π–π interactions between the
columns 4A and 4B were not observed. Because the interactions between the stacking columns were
inhibited, monoboron complex 4 could exhibit higher
Φf (0.13) than 7.
As shown in Figure 15, trifluoromethyl derivative 5 also formed dimers by π–π interactions (C–
C: 3.58–3.70 Å). Similar to 4, stacking columns 5A and 5B were formed by π–π interactions between
each of the dimers (C–C: 3.63 Å) in the crystal of 5. However, the interactions between the dimers of 5
(C–C: 3.63 Å) were weaker than those of 4 (C–C: 3.52–3.69 Å). Therefore, 5 is considered to have
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weaker intermolecular interactions than 4, and the slightly higher
weak π–π interactions.
Φf of 5 (0.15) is probably owing to the
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8
9
In the crystal packing of 6, dimers were formed by π–π interactions between the molecules
presented in blue and red (C–C: 3.47–3.68 Å) (Figure 16). However, unlike the other boron complexes,
there were no π–π interactions between the adjacent dimers. Because of the inhibition of interactions
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between the dimers, 6 exhibited relatively strong
Φ
f (0.20) compared to the other complexes.
7
1
0.5
0
6
4
5
400
500
600
700
800
Wavelength / nm
Figure 12. Normalized fluorescence spectra of 4ꢀ7 in the solid state.
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π−π Interactions in stacking column
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4
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7
8
7A
C3-C8, C8*-C3*: 3.40 Å
C3*-C6, C6*-C3: 3.48 Å
C4-C10, C10*-C4: 3.53 Å
C4-C11, C11*-C4, C2-C9, C9*-C2*: 3.54 Å
C3-C9, C9*-C3*: 3.56 Å
C4-C9, C9*-C4: 3.59 Å
C4-C6, C6*-C4: 3.62 Å
C1*-C1: 3.63 Å
C1*-C2, C2*-C1: 3.64 Å
7A
C4-C7, C7*-C4, C4-C8, C8*-C4 , C2*-C6, C6*-C2 : 3.66 Å
C2-C8, C8*-C2*: 3.67 Å
π−π Interactions between stacking columns
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7B
7B
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C9*-C11*: 3.56 Å
C9*-C10*: 3.62 Å
C10*-C10*: 3.69 Å
7B
7A
(b)
7B
7A
Figure 13. (a) Top view of the crystal structure of 7 (Z = 2). (b) Side view of the crystal structure of 7.
(c) List of intermolecular interactions of 7. Greenꢀ and light blueꢀdotted lines show π–π interactions in a
stacking column and between the stacking columns, respectively.
4A
4A
4B
4B
4B
4B
(b)
4A
4A
π−π Interactions in dimer
(c)
π−π Interactions between dimers
C4-C8: 3.36 Å
C9-C12: 3.51 Å
C2-C6: 3.52 Å
C8-C12: 3.58 Å
C4-C7: 3.59 Å
C4-C9: 3.62 Å
C3-C6: 3.70 Å
C5-C11: 3.52 Å
C1-C3: 3.60 Å
C2-C2: 3.66 Å
C1-C2: 3.69 Å
Figure 14. (a) Top view of the crystal structure of 4 (Z = 4). (b) Side view of the crystal structure of 4.
(c) List of intermolecular interactions of 4. Greenꢀ and pinkꢀdotted lines show π–π interactions in a
dimer and between the dimers, respectively.
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1
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4
5
5B
5B
(a)
5B
5A
5B
(b)
5A
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5A
5A
π−π Interactions in dimer
(c)
π−π Interactions between dimers
C3-C11: 3.58 Å
C1-C2: 3.69 Å
C2-C6: 3.70 Å
C5-C8: 3.63 Å
Figure 15. (a) Top view of the crystal structure of 5 (Z = 4). (b) Side view of the crystal structure of 5.
(c) List of intermolecular interactions of 5. Greenꢀ and pinkꢀdotted lines show π–π interactions in a
dimer and between the dimers, respectively.
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(a)
(c)
π−π Interactions in dimer
C3-C11: 3.47 Å
C9-C12: 3.50 Å
C2-C2: 3.52 Å
C1-C2: 3.55 Å
C3-C6: 3.61 Å
C4-C7: 3.61 Å
C2-C6: 3.66 Å
C4-C6: 3.66 Å
C4-C8: 3.68 Å
(b)
No π−π interaction between dimers
Figure 16. (a) Top view of the crystal structure of 6 (Z = 2). (b) Side view of the crystal structure of 6.
(c) List of intermolecular interactions of 6. Greenꢀdotted lines show π–π interactions in a dimer.
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Conclusions
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8
In conclusion, we synthesized monoꢀ and bisboron complexes based on pyrimidine bearing βꢀ
9
ketoiminate ligands and investigated their fluorescence properties both in solution and in solid states.
Although the nonsubstituted and trifluoromethylꢀsubstituted monoboron complexes hardly showed any
fluorescence in dichloromethane (Fmax: 425–428 nm, Φf ≤ 0.02), relatively strong fluorescence was
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observed in solid state (Fmax: 488–525 nm,
intramolecular C–Ar rotation that promotes the nonradiative processes. The dimethylaminoꢀsubstituted
monoboron complex exhibited strong and relatively redꢀshifted fluorescence in dichloromethane (Fmax
529 nm, Φf = 0.78). The Fmax of the dimethylamino derivative redꢀshifted with increasing solvent
polarity (Fmax: 456–565 nm). The DFT calculation result and relatively large e value (20.6 D) obtained
Φf: 0.13–0.15), probably because of the restriction of
:
ꢁ
from LippertꢀMataga plot suggest the formation of ICTꢀexcited state. The dimethylamino derivative
showed the highest Φf (0.20) in solid state because of the inhibition of consecutive π–π intermolecular
interactions. The bisboron complex had higher ε (107,300) as well as redꢀshifted λmax (475 nm) and Fmax
(490 nm) compared to the corresponding nonsubstituted monoboron complex, probably because of the
extension of πꢀconjugation by annulation and the two terminal phenyl groups.
Experimental section
General: NMR spectra were recorded on 400 or 600 MHz spectrometers. Chemical shifts are referred to
13
TMS (1H and C) as internal standards and CFCl3 (19F) as an external standard. Infrared (IR) spectra
were recorded in KBr pallets. UV–vis and fluorescence spectra were recorded in a quartz cell (light
path: 10 mm). Absolute fluorescence quantum yields were measured using an integrating sphere.
Fluorescence lifetimes were measured by a singleꢀphotonꢀcounting method. Mass spectra were
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measured in electron impact (EI) mode. Melting points were measured on a Yanagimoto MP–S2 micro–
melting–point apparatus. Analytical Thinꢀlayer chromatography (TLC) was performed on preꢀcoated
plates (Merck, silica gel 60 F254). Silica gel (Wakogel C–200) was used for column chromatography.
Crystallographic Xꢀray data were collected using Mo Kα radiation.
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5
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9
Synthesis of 1: Sodium hydride (60w% in oil, 1.61 g, 40.3 mmol) was added to a THF (150 mL)
solution of 4,6ꢀdichloropyrimidine (2.00 g, 13.4 mmol) and acetophenone (3.22 g, 26.8 mmol) at room
temperature. The solution was refluxed for 1 day. After cooling to room temperature, water was added
to the reaction mixture and extracted with dichloromethane. The extract was dried over Na2SO4 and
concentrated in vacuo. Column chromatography of the residue on silica gel (CH2Cl2, Rf = 0.3), followed
by recrystallization from ethanol to give 1 (875 mg, 28%, 1a : 1b = 1 : 15 in CDCl3) as a yellowꢀwhite
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powder. 1 (mixtuire of 1a and 1b): mp 112.0ꢀ104.0 °C; UVꢀvis (CH2Cl2) λmax 346 nm (log ε = 4.41); IR
(KBr) 3070, 1628, 1566, 1415, 1057, 1096, 864, 756 cmꢀ1; EIMS (m/z) (rel intensity) 234 ([M+2]+; 8),
280 (M+; 23), 105 (100); Anal. calcd for C12H9ClN2O: C, 61.95; H, 3.90; N, 12.04. found: C, 61.98; H,
1
4.04; N, 11.74; 1b: H NMR (400 MHz, CDCl3)
δ 6.01 (s, 1H), 7.03 (s, 1H), 7.43ꢀ7.50 (m, 3H), 7.85
(d, J = 7.4 Hz, 2H), 8.72 (s, 1H), 14.6 (s, 1H); 13C NMR (100 MHz, CDCl3)
δ 92.9, 116.5, 126.1, 128.6,
130.9, 134.6, 155.5, 160.3, 165.3, 169.2.
Synthesis of 2: Sodium hydride (60w% in oil, 1.61 g, 40.3 mmol) was added to a THF (150 mL)
solution of 4,6ꢀdichloropyrimidine (2.00 g, 13.4 mmol) and 4’ꢀ(trifluoromethyl)acetophenone (4.45 g,
23.7 mmol) at room temperature. The solution was refluxed for 1 day. After cooling to room
temperature, water was added to the reaction mixture and extracted with dichloromethane. The extract
was dried over Na2SO4 and concentrated in vacuo. Column chromatography of the residue on silica gel
(CH2Cl2, Rf = 0.7), followed by recrystallization from ethanol gave 2 (852 mg, 21%, 2a : 2b = 1 : 9 in
CDCl3) as a yellowꢀwhite powder. 2 (mixtuire of 2a and 2b): mp 101.0ꢀ103.0 °C; IR (KBr) 3086, 1632,
1574, 1424, 1335, 1115, 1072, 910 cmꢀ1; UVꢀvis (CH2Cl2) λmax 344 nm (log
ε = 4.60); EIMS (m/z) (rel
intensity) 302 ([M+2]+; 23), 300 (M+; 68), 299 (73), 173 (100), 145 (67); Anal. calcd for
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1
C13H8ClF3N2O: C, 51.93; H, 2.68; N, 9.32. found: C, 51.67; H, 2.62; N, 9.31; 2a: H NMR (400 MHz,
1
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3
4
5
6
7
8
9
CDCl3)
1H NMR (400 MHz, CDCl3)
2H), 8.76 (s, 1H), 14.6 (s, 1H); 13C NMR (100 MHz, CDCl3)
125.6, 126.5, 132.4 (q, J = 33.4 Hz), 138.1, 155.7, 160.9, 165.0, 167.2; 19F NMR (376 MHz, CDCl3, ext.
δ 4.48 (s, 2H), 7.44 (s, 1H), 7.78 (d, J = 8.6 Hz, 2H), 8.15 (d, J = 8.6 Hz, 2H), 8.95 (s, 1H); 2b:
δ
6.05 (s, 1H), 7.08 (s, 1H), 7.70 (d, J = 8.5 Hz, 2H), 7.94 (d, J = 8.5 Hz,
δ
94.2, 117.1, 123.9 (q, J = 274.1 Hz),
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CFCl3) δ ꢀ62.7 (s, 3F).
Synthesis of 3: Sodium hydride (60w% in oil, 1.61 g, 40.3 mmol) was added to a THF (150 mL)
solution of 4,6ꢀdichloropyrimidine (2.00 g, 13.4 mmol) and 4’ꢀdimethylaminoacetophenone (3.45 g,
21.1 mmol) at room temperature. The solution was refluxed for 2 day. After cooling to room
temperature, water was added to the reaction mixture and extracted with dichloromethane. The extract
was dried over Na2SO4 and concentrated in vacuo. Column chromatography of the residue on silica gel
(CH2Cl2 : AcOEt = 20 :1, Rf = 0.4), followed by recrystallization from ethanol gave 3 (582 mg, 16%, 3a
: 3b = 5 : 1 in CDCl3) as a yellowꢀwhite powder. 3 (mixtuire of 3a and 3b): mp 194.0ꢀ196.0 °C; IR
(KBr) 3059, 1659, 1601, 1570, 1531, 1377, 1331, 1246, 1200, 818 cmꢀ1; UVꢀvis (CH2Cl2) λmax 342 nm
(log
ε = 4.60), 406 nm (log ε
= 3.97); EIMS (m/z) (rel intensity) 277 ([M+2]+; 10), 275 (M+; 30), 148
(100); Anal. calcd for C14H14ClN3O: C, 60.98; H, 5.12; N, 15.24. found: C, 61.04; H, 5.03; N, 15.03;
3a: 1H NMR (400 MHz, CDCl3)
(d, J = 9.2 Hz, 2H), 8.93 (s, 1H); 13C NMR (150 MHz, CDCl3)
153.9, 158.6, 161.3, 166.6, 192.2; 3b: 1H NMR (400 MHz, CDCl3)
J = 8.3 Hz, 2H), 6.91 (s, 1H), 7.76 (d, J = 8.3 Hz, 2H), 8.60 (s, 1H), 14.7 (brs, 1H); 13C NMR (150
δ
3.08 (s, 6H), 4.36 (s, 2H), 6.67 (d, J = 9.2 Hz, 2H), 7.44 (s, 1H), 7.93
δ
40.0, 46.6, 110.8, 122.2, 127.7, 131.1,
δ
3.05 (s, 6H), 5.87 (s, 1H), 6.73 (d,
MHz, CDCl3) δ 40.1, 90.0, 111.4, 115.5, 121.6, 123.8, 152.2, 155.3, 159.5, 165.4, 170.5.
Synthesis of 4: Ligand 1 (500 mg, 2.15 mmol) was dissolved in dry dichloromethane (100 mL).
Triethylamine (0.74 mL, 5.3 mmol) and boron trifluoride diethyl ether complex (0.68 mL, 5.4 mmol)
was added to the solution and stirred at room temperature for 6 h. Then water was added to the solution.
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The solution was extracted with CH2Cl2. The organic layer was dried over Na2SO4. After concentration
of solvent, the residue was purified via chromatography with silica gel (CH2Cl2, Rf = 0.6), followed by
recrystallization from chloroform to afford 4 (537 mg, 89%) as a yellow solid. 4: mp 281.0ꢀ282.0°C; 1H
1
2
3
4
5
6
7
8
NMR (600 MHz, CDCl3)
δ 6.33 (s, 1H), 7.20 (s, 1H), 7.49 (dd, J = 7.6, 7.6 Hz, 2H), 7.57 (t, J = 7.6 Hz,
9
1H), 7.97 (d, J = 7.6 Hz, 2H), 8.84 (s, 1H); 13C NMR (100 MHz, DMSOꢀd6)
δ
92.9, 117.5, 127.4, 129.7,
ꢀ137.9 (q, J = 12.1
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133.1, 133.2, 152.6, 157.2, 162.0, 167.4; F NMR (565 MHz, CDCl3, ext. CFCl3)
δ
Hz, 2F); IR (KBr) 3090, 1597, 1512, 1396, 1138, 1096, 1053, 1033 cmꢀ1; EIMS (m/z) (rel intensity) 282
([M+2]+; 17), 280 (M+; 56), 279 (100), 77 (17); Anal. calcd for C12H8BClF2N2O: C, 51.39; H, 2.88; N,
9.99. found: C, 51.15; H, 2.92; N, 9.97.
Synthesis of 5: Ligand 2 (500 mg, 1.66 mmol) was dissolved in dry dichloromethane (100 mL).
Triethylamine (0.60 mL, 4.3 mmol) and boron trifluoride diethyl ether complex (0.55 mL, 4.4 mmol)
was added to the solution and stirred at room temperature for 6 h. Then water was added to the solution.
The solution was extracted with CH2Cl2. The organic layer was dried over Na2SO4. After concentration
of solvent, the residue was purified via chromatography with silica gel (CH2Cl2, Rf = 0.8), followed by
recrystallization from chloroform to afford 5 (480 mg, 83%) as a yellow solid. 5: mp 209.0ꢀ210.0°C; 1H
NMR (400 MHz, CDCl3)
δ 6.39 (s, 1H), 7.29 (s, 1H), 7.75 (d, J = 8.2 Hz, 2H), 8.07 (d, J = 8.2 Hz, 2H),
13
8.91 (s, 1H); C NMR (100 MHz, CDCl3)
δ
92.5, 116.2, 123.6 (q, J = 276.5 Hz), 125.9, 127.7, 134.1
19
(q, J = 33.4 Hz), 136.3, 151.9, 156.7, 163.4, 168.1; F NMR (376 MHz, CDCl3, ext. CFCl3)
δ ꢀ137.7
(q, J = 11.9 Hz, 2F), ꢀ63.0 (s, 3F); IR (KBr) 3093, 1601, 1508, 1327, 1130, 1069 cmꢀ1; EIMS (m/z) (rel
intensity) 350 ([M+2]+; 18), 348 (M+; 61), 347 (100); Anal. calcd for C13H7BClF5N2O: C, 44.81; H,
2.02; N, 8.04. found: C, 44.76; H, 1.99; N, 7.98.
Synthesis of 6: Ligand 3 (600 mg, 2.18 mmol) was dissolved in dry dichloromethane (100 mL).
Triethylamine (0.90 mL, 6.5 mmol) and boron trifluoride diethyl ether complex (0.83 mL, 6.6 mmol)
was added to the solution and stirred at room temperature for 6 h. Then water was added to the solution.
The solution was extracted with CH2Cl2. The organic layer was washed with water and dried over
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MgSO4. After concentration of solvent, the residue was purified via chromatography with silica gel
(CH2Cl2, Rf = 0.4) to afford 6 (606 mg, 86%) as a red solid. 6: mp 273.0ꢀ275.0°C; 1H NMR (400 MHz,
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CDCl3) δ 3.10 (s, 6H), 6.12 (s, 1H), 6.68 (d, J = 8.9 Hz, 2H), 7.00 (s, 1H), 7.87 (d, J = 8.9 Hz, 2H), 8.65
7
8
9
(s, 1H); 13C NMR (100 MHz, CDCl3)
δ
40.1, 88.4, 111.2, 114.6, 119.3, 127.7, 151.6, 153.5, 156.1,
ꢀ139.2 (q, J = 13.6 Hz, 2F); IR (KBr) 3117,
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160.4, 170.8; F NMR (565 MHz, CDCl3, ext. CFCl3)
δ
1586, 1500, 1431, 1396, 1196 cmꢀ1; EIMS (m/z) (rel intensity) 325 ([M+2]+; 32), 323 (M+; 100), 149
(17); Anal. calcd for C14H13BClF2N3O: C, 51.97; H, 4.05; N, 12.99. found: C, 52.23; H, 4.05; N, 12.72.
Synthesis of 7: Sodium hydride (60w% in oil, 172 mg, 4.29 mmol) was added to a THF (150 mL)
solution of 4 (400 mg, 1.43 mmol) and acetophenone (205 mg, 1.71 mmol) at room temperature. The
solution was refluxed for 2 day. After cooling to room temperature, water was added to the reaction
mixture and extracted with dichloromethane. The extract was dried over Na2SO4 and concentrated in
vacuo. The residue was dissolved in dry dichloromethane (70 mL). Triethylamine (1.0 mL, 7.2 mmol)
and boron trifluoride diethyl ether complex (0.90 ml, 7.2 mmol) was added to the solution and stirred at
room temperature for 6 h. Then water was added to the solution. The solution was extracted with
dichloromethane. The organic layer was dried over Na2SO4. After concentration of solvent, the residue
was purified via chromatography with silica gel (CH2Cl2, Rf = 0.6), followed by recrystallization from
chloroform to afford 7 (94 mg, 16%) as a red solid. 7: mp 235.0ꢀ236.0°C; 1H NMR (600 MHz, CDCl3)
δ
6.43 (s, 2H), 6.79 (s, 1H), 7.51 (dd, J = 8.2, 7.6 Hz, 4H), 7.59 (t, J = 7.6 Hz, 2H), 8.00 (d, J = 8.2 Hz,
4H), 9.04 (s, 1H); 13C NMR (150 MHz, acetoneꢀd6)
155.5, 169.9; 19F NMR (565 MHz, CDCl3, ext. CFCl3)
δ
94.2, 111.5, 128.2, 129.9, 132.6, 133.7, 149.5,
δ
ꢀ140.8 (q, J = 8.6 Hz, 4F); IR (KBr) 3082,
1651, 1555, 1512, 1489, 1397, 1192, 1053 cmꢀ1; EIMS (m/z) (rel intensity) 412 (M+; 100), 411 (92), 105
(36); Anal. calcd for C20H14B2F4N2O2: C, 58.31; H, 3.43; N, 6.80. found: C, 58.29; H, 3.23; N, 6.59.
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The Journal of Organic Chemistry
Supporting Information Available: H and 13C NMR spectra, DFT calculation data, Xꢀray crystal
structures, UVꢀvis absorption and fluorescence spectra and tables of properties are included. This
material is available free of charge via the internet at http://pubs.acs.org.
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Acknowledgements: This work was supported by JSPS KAKENHI (23750228, GrantꢀinꢀAid for
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Young Scientists (B)) and JST AꢀSTEP (AS242Z02734M).
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