Highly intense fluorescent diarylboron diketonate

Article abstract of DOI:10.1021/jo8017582

(Chemical Equation Presented) Diarylboron diketonates were successfully prepared by the reaction of 1,3-diketone derivatives and arylboron compounds such as triphenylborane [BPh₃] and fluorobis(pentafluorophenyl)borane diethyl etherate [(C₆F₆)₂BF·OEt ₂]. The fluorescent emission of their complexes took place depending on the substituent of the arylboron moiety. In particular, a boron 1,3-bis(4-methoxyphenyl)-1,3-diketonate chelated by a strong electron-withdrawing C₆F₅ group exhibited the most intense fluorescence.

Full text of DOI:10.1021/jo8017582

SCHEME 1. Synthesis of Diarylboron Diketonates
Highly Intense Fluorescent Diarylboron
Diketonate
Atsushi Nagai, Kenta Kokado, Yuuya Nagata,
Manabu Arita, and Yoshiki Chujo*
Department of Polymer Chemistry, Graduate School of
Engineering, Kyoto UniVersity, Katsura, Nishikyo-ku,
Kyoto 615-8510, Japan
chujo@chujo.synchem.kyoto-u.ac.jp
ReceiVed August 8, 2008
chromophore.5 Recently, Fraser et al. have demonstrated that a
hydroxy-functionalized difluoroboron dibenzoylmethane, which
presented high fluorescence quantum yields (ΦF) among re-
ported boron diketonates, was prepared, and was employed as
an initiator in the ring-opening polymerization of lactide to
obtain BF2-dibenzoylmethane end-functionalized polylactide.
This compound also showed high ΦF and room temperature
phosphorescence.6
The presence of B-F bonds in the usual organoboron dyes
results in some instability under irradiation and sensitivity to
polar solvents. To date, to overcome these disadvantages, many
BODIPY derivatives replacing fluoride with aryl, ethynylaryl,
ethynylthienyl, and so on have been reported.7 Such derivatives
shared not only improvement of their stabilities but also large
Stokes’ shifts in their emission spectra, which is of paramount
importance in multicolor labeling, i.e., chromosome mapping.8
In contrast, only a diphenylboron diketonate as an analogous
derivative has been recently reported,9 and its luminescent
property has not yet been investigated. This prompted us to
expand the research of boron 1,3-diketonate dyes by designing
emissive diarylboron complexes. In this paper, we report the
synthesis, spectroscopic properties, and theoretical calculations
of diarylboron diketonates.
Diarylboron diketonates were successfully prepared by the
reaction of 1,3-diketone derivatives and arylboron com-
pounds such as triphenylborane [BPh3] and fluorobis(pen-
tafluorophenyl)borane diethyl etherate [(C6F5)2BF ·OEt2]. The
fluorescent emission of their complexes took place depending
on the substituent of the arylboron moiety. In particular, a
boron 1,3-bis(4-methoxyphenyl)-1,3-diketonate chelated by
a strong electron-withdrawing C6F5 group exhibited the most
intense fluorescence.
Successive reaction between 1,3-diketone derivatives and boron
reagents such as triphenylborane (BPh3) and fluorobis(pentafluo-
rophenyl)borane diethyletherate ((C6F5)2BF·OEt2)10 afforded the
The recent upsurge in the designing studies of fluorescent
organoboron dyes such as 4,4-difluoro-4-bora-3a,4a-diaza-s-
indacenes (BODIPYs) and boron diketonates reflects their
features in the fields of molecular probes,1 photosensitizers,2
and lasers3 because their dyes possess large molar extinction
coefficients, two-photon absorption cross sections, high emission
quantum yields, and sensitivity to the surrounding medium.4
Boron diketonates are generally synthesized by treatment of
bidentate organic π-chromophores with boron reagents such as
trifluoroboron diethyl etherate (BF3 ·OEt2), and are air-stable
due to tetrahedral complexes with one chelating π-conjugated
(5) (a) Maeda, H.; Mihashi, Y.; Haketa, Y. Org. Lett. 2008, 10, 3179. (b)
Ono, K.; Yoshikawa, K.; Tsuji, Y.; Yamaguchi, H.; Uozumi, R.; Tomura, M.;
Taga, K.; Saito, K. Tetrahedron 2007, 63, 9354. (c) Rohde, D.; Yan, C.-J.; Wan,
L.-J. Langmuir 2006, 22, 4750. (d) Mirochnik, A. G.; Fedorenko, E. V.; Kuryavyi,
V. G.; Bukvetskii, B. V.; Karasev, V. E. J. Fluoresc. 2006, 16, 279. (e) Cogne´-
Laage, E.; Allemand, J.-F.; Ruel, O.; Baudin, J.-B.; Croquette, V.; Blanchard-
Desce, M.; Jullien, L. Chem.-Eur. J. 2004, 10, 1445. (f) Go¨rlitz, G.; Hartmann,
H.; Nuber, B.; Wolff, J. J. J. Prakt. Chem. 1999, 341, 167.
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J. Am. Chem. Soc. 2007, 129, 8942.
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Goeb, S.; Ziessel, R. Org. Lett. 2007, 9, 737. (c) Goze, C.; Ulrich, G.; Ziessel,
R. J. Org. Chem. 2007, 72, 313. (d) Goze, C.; Ulrich, G.; Ziessel, R. Org. Lett.
2006, 8, 4445. (e) Goze, C.; Ulrich, G.; Mallon, L. J.; Allen, B. D.; Harriman,
A.; Ziessel, R. J. Am. Chem. Soc. 2006, 128, 10231. (f) Ulrich, G.; Goze, C.;
Goeb, S.; Retailleau, P.; Ziessel, R. New J. Chem. 2006, 30, 982.
(8) Ju., J.; Ruan, C.; Fuller, C. W.; Glazer, A. N.; Mathies, R. A. Proc. Natl.
Acad. Sci. U.S.A. 1995, 92, 4347.
(9) Bally, I.; Arsene, A.; Bacesou-Roman, M.; Balaban, A. T. Tetrahedron
Lett. 1965, 3929.
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(b) Duchateau, R.; Lancaster, S. J.; Thornton-Pett, M.; Bochamnn, M. Orga-
nometallics 1997, 16, 4995.
(1) Haugland, R. P. The Handbook-A Guide to Fluorescent Probes and
Labeling Technologies, 10th ed.; Spence, M. T. Z., Ed.; Molecular Probes:
Eugene, OR, 2005; Chapter 1, Section 1.4.
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O’Shea, D. F. J. Am. Chem. Soc. 2004, 126, 10619.
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10.1021/jo8017582 CCC: $40.75
Published on Web 10/10/2008
2008 American Chemical Society
J. Org. Chem. 2008, 73, 8605–8607 8605

FIGURE 1. ORTEP drawings of 2a (A) and 2b (B). Thermal ellipsoids
are drawn at the 50% propability level. Crystal packing structures of
2a (C) and 2b (D).
corresponding boron diketonates 1a, 1b, 2a, and 2b in good yields,
respectively (Scheme 1). The tetracoordination states of the boron
atoms of 1a-2b were confirmed by the 11B NMR spectroscopy
in CDCl3. The spectra of 1a (δB ) 8.79 ppm) and 1b (δB ) 8.11
ppm) were downfield-shifted toward those of 2a (δB ) 5.47 ppm)
and 2b (δB ) 5.08 ppm), probably resulting in the substitution for
each 1,3-diketone ligand (discussed later). Further, the basic
structures of the obtained diketonates were also characterized by
1H and 13C NMR, IR and electron ionization mass spectroscopies,
and elemental analysis.
The crystallographically determined molecular structures of
2a and 2b are shown in Figure 1A,B.11 The sp3 orbits hydridized
at boron centers of 2a and 2b appear as a distorted tetrahedron
with bond angles O2-B1-O1 of 107.3° and 108.6°, respec-
tively, slightly narrower than what is observed in difluoroboron
diketonate (111.7°),5b and the angles of C4-B1-C10 were
115.1° and 110.7°, respectively. The average B-O bond length
of 2a (1.52 Å) was slightly longer than that of 2b (1.49 Å),
whose value is comparable to those found in difluoroboron
diketonate analogues (1.48 Å), and the average of bond length
betweenthe boron atom and the aromatic carbon atom (B-C4
and B-C10) of 2a (1.61 Å) was slightly shorter than that of
2b (1.63 Å). These data indicate that the electron density of a
boron atom in 2b is poorer than that of 2a due to the strong
electron-withdrawing C6F5 group, i.e., the chelating ability of
2b is higher than that of 2a. Panels C and D of Figure 1 illustrate
the crystal packing structures of 2a and 2b. The intermolecular
distance of 2a is 3.68 Å, indicating effective π-π stacking
between neighboring phenyl groups. Further, the packing
FIGURE 2. (a) UV-vis and (b) fluorescence spectra of 1a, 1b, 2a,
and 2b in CH2Cl2 (1.0 × 10-5 mol/L).
structure of 2b is interacted between distort boron chelating and
phenyl rings at a length of 3.17 Å (Figure 1C,D). These results
could indicate that the structures of 2a and 2b take the
tetrahedral structures on each boron center.
To examine the optical properties of the obtained boron
diketonates, UV-vis absorption and photoluminescence experi-
ments were carried out in CH2Cl2. The absorption maxima of 2
(2a: λmax ) 405 nm; and 2b: λmax ) 425 nm) are red-shifted as
compared to those of 1 (1a: λmax ) 393 nm; and 1b: λmax
)
385 nm), and the molar absorption coefficients of 2 (2a: ꢀ )
3.5 × 104 M-1 cm-1; and 2b: ꢀ ) 5.5 × 104 M-1 cm-1) are
also higher than those of 1 (1a: ꢀ ) 1.5 × 104 M-1 cm-1; and
1b: ꢀ ) 2.7 × 104 M-1 cm-1) (Figure 2A), corresponding to
πfπ* transition.12 These results suggest that the changes of
these shifted absorptions and coefficients reflect in the electronic
structures of ligand moieties, irrespective of those on boron
atoms. In the fluorescence spectra in CH2Cl2, 1b and 2b showed
emission maxima at 421 (excited at 385 nm) and 441 nm
(excited at 425 nm), respectively (Figure 2A). The absolute
fluorescence quantum yields of 1b and 2b in CH2Cl2 at room
temperature were determined as ΦF ) 0.27 and 0.86, respec-
tively, which are high enough ΦF as compared with those of
any BF2 chelating diketone derivatives reported previously.5,6
In contrast, 1a and 2a showed almost no fluorescent emission
(1a: ΦF e 0.01; and 2a: ΦF ) 0.06). The substituents on the
boron atoms should be responsible for the intensity of fluores-
cence, considering the above results.
(11) (a) The crystal data for 2a are as follows. C29H25BO4, FW ) 448.30,
To investigate the redox properties of the diarylboron
diketonates, cyclic voltammetry (CV) in acetonitrile was
performed (see the Supporting Information, Figure 1S). The
voltammograms showed reversible waves for them. No clear
oxidation waves were observed up to + 2.0 V versus Ag/Ag+.
The reduction potential (E) of 1a (E ) -1.40 V) was lower
than that of 2a (E ) -1.57 V), indicating that the electron
affinity of 1a increases as compared to that of 2a, i.e., the
j
triclinic, P1, a ) 8.36 (3) Å, b ) 13.64 (6) Å, c ) 20.85 (10) Å, R ) 101.61(15)°,
ꢀ ) 95.4(2)°, γ ) 94.3(2)°, V ) 2307 (17) Å3, Z ) 4, Dcalcd ) 1.291 g cm-3
;
the refinement converged to R ) 0.059, Rw ) 0.139, GOF ) 0.851. (b) The
crystal data for 2b are as follows. C29H15BF10O4, FW ) 628.22, monoclinic,
j
P21/c, a ) 19.8126 (18) Å, b ) 10.6735 (10) Å, c ) 25.2963 (17) Å, R ) 90°,
ꢀ ) 106.387(4)°, γ ) 90°, V ) 5132.1 (8) Å3, Z ) 8, Dcalcd ) 1.626 g cm-3
the refinement converged to R ) 0.050, Rw ) 0.075, GOF ) 0.956.
;
(12) (a) Ilge, H.-D.; Birckner, E.; Fassler, D.; Kozmenko, M. V.; Kuz’min,
M. G.; Hartmann, H. J. Photochem. 1986, 32, 177. (b) Morgan, G. T.; Tunstall,
R. B. J. Chem. Soc. 1924, 125, 1963.
8606 J. Org. Chem. Vol. 73, No. 21, 2008

in vacuo. The residual product was purified by silica gel column
chromatography eluted with n-hexane/CHCl3 [2:1(v/v)], followed
by recrystallization from dichloromethane/n-hexane to obtain 1a
(0.34 g, 0.87 mmol) in 87% yield as a yellowish powder. 1H NMR
(CDCl3) δ 6.99 (s, 1H, -CHdC<), 7.20-7.29 (m, 6H, aromatic
protons), 7.52-7.68 (m, 10H, aromatic protons), 8.18 (d, J ) 7.3
Hz, 4H, aromatic protons) ppm. 13C NMR (CDCl3) δ 182.9, 134.3,
133.2, 131.3, 129.0, 128.5, 127.2, 126.5, 94.1 ppm. 11B NMR
(CDCl3) δ 8.79 ppm. IR (NaCl) ν 1540, 1522, 1490, 1355, 1205,
706 cm-1. HRMS (EI) calcd for C27H21O2B (M+) m/z 388.1635,
found m/z 388.1635. Anal. Calcd for C27H21O2B: C, 83.52; H, 5.45.
Found: C, 83.32; H, 5.41.
Synthesis of 1b. (C6H5)2BF·OEt2 (1.71 g, 3.90 mmol) was added
to a solution of 1,3-diphenyl-1,3-propanedione (0.18 g, 0.78 mmol)
in dry dichloromethane (6.0 mL) under nitrogen atmosphere. The
reaction mixture was stirred at room temperature for 3.0 h. After
the insoluble materials were removed by filtration, the solvent was
evaporated in vacuo. The residual product was purified by silica
gel column chromatography eluted with n-hexane/CHCl3 [4:1(v/
v)], followed by recrystallization from dichloromethane/n-hexane
to obtain 1b (0.36 g, 0.63 mmol) in 82% yield as a yellowish
powder.1H NMR (CDCl3) δ 7.16 (s, 1H, -CHdC<), 7.56 (t, J )
7.8 and 15.6 Hz, 3H, aromatic protons), 7.73 (t, J ) 7.3 and 14.6
Hz, 3H, aromatic protons), 8.19 (d, J ) 7.6 Hz, 4H, aromatic
protons) ppm. 13C NMR (CDCl3) δ 180.0, 164.5, 131.3, 130.7,
127.1, 126.3, 125.9, 114.3, 92.1, 55.6 ppm. 11B NMR (CDCl3) δ
5.47 ppm. IR (NaCl) ν 3082, 1539, 1533, 1489, 1475, 1349, 1105,
974, 717 cm-1. HRMS (EI) calcd for C27H11O2F10B (M+) m/z
568.0688, found m/z 568.0692. Anal. Calcd for C27H11O2F10B: C,
57.08; H, 1.95. Found: C, 57.33; H, 2.21.
FIGURE 3. Molecular orbital diagrams for the LUMO and HOMO
of 1a (A) and 1b (B) (B3LYP/6-31G(d, p)//B3LYP/6-31G(d, p)).
electron density around the boron atom of 2a having methoxy
groups is higher than that of 1a. The relationship of 1b (E )
-1.18 V) and 2b (E ) -1.30 V) was the same. From these
results, the 11B NMR chemical shifts of 1a-2b indicate that
substitution of methoxy groups for the 1,3-diketone ligand
strongly influences the chelating moiety as compared to that of
aryl groups.
To provide a more effective understanding for the emission
behavior of 1a-2b, we employed the theoretical calculation
using the density-functional theory (DFT) method at the B3LYP/
6-31G(d p)//B3LYP/6-31G(d,p). Parts A and B of Figure 3
exhibit the lowest unoccupied molecualr orbital (LUMO) and
the highest occupied molecular orbital (HOMO) of 1a and 1b,
respectively. The π orbital of the 1,3-diketone group of 1b lies
on their LUMO and HOMO. As a result, the π f π* excited
state of 1b occupies the lowest excited state, and the transition
between the excited state and ground state is allowed. In
contrast, the HOMO of 1a is not on the 1,3-diketone moieties
but localized on the whole of the phenyl groups. The relatively
high energy level of the occupied orbitals of phenyl groups
prevents the HOMO from locating the π orbital on 1,3-diketone
group of 1a, i.e., the fluorescence quantum yield is extremely
low, analogizing with the emissive driving force of boron-
substituted azobenzenes reported previously by Kawashima et
al.10a The DFTs of 2a and 2b were also identical (see the
Supporting Information, Figure 2S). Therefore, the emission
behavior of diarylboron diketonates should originate from
HOMO inverted by the strong electron-withdrawing C6F5
groups.
In conclusion, we have described the synthesis of highly intense
fluorescent diarylboron diketonates, and illustrated the emission
behavior of diarylboron diketonates by UV-vis and photolumi-
nescence spectroscopies, and theoretical calculation using density-
functional theory (DFT). To the best of our knowledge, this is the
first example of the designs of diarylboron diketonates with
fluorescence. We are currently extending this procedure to the
design and synthesis of higher intense fluorescent diarylboron
diketonates applicable to fluorescent probes of chemical sensors
and organic electroluminescent devices.
Synthesis of 2a. Similarly to the preparation of 1a, 2a was prepared
from 1,3-bis(4-methoxyphenyl)-1,3-propanedione (0.22 g, 0.78 mmol)
and BPh3 (0.95 g, 3.91 mmol) in dry toluene (4.0 mL) in 68% (0.24
g, 0.53 mmol) yield as a yellow crystal. 1H NMR (CDCl3) δ 3.90 (s,
6H, -OCH3 × 2), 6.82 (s, 1H, -CHdC<), 7.00 (d, J ) 9.0 Hz, 4H,
aromatic protons), 7.18-7.28 (m, 6H, aromatic protons), 7.61 (d, J )
6.8 Hz, 4H, aromatic protons), 8.14 (d, J ) 8.8 Hz, 4H, aromatic
protons) ppm. 13C NMR (CDCl3) δ 183.1, 149.4, 147.0, 141.6, 139.2,
138.4, 135.9, 135.5, 132.0, 131.9, 129.3. 129.2, 129.2, 93.8 ppm. 11
B
NMR (CDCl3) δ 8.11 ppm. IR (NaCl) ν 3045, 1605, 1542, 1493, 1242,
1174, 784 cm-1. HRMS (EI) calcd for C29H25O4B (M+) m/z 448.1846,
found m/z 448.1841. Anal. Calcd for C29H25O4B: C, 77.69; H, 5.62.
Found: C, 77.42; H, 5.65.
Synthesis of 2b. Similarly to the preparation of 1b, 2b was
prepared from 1,3-bis(4-methoxyphenyl)-1,3-propandione (0.22 g,
0.78 mmol) and (C6H5)2BF·OEt2 (1.71 g, 3.90 mmol) in dry
dichloromethane (6.0 mL) in 72% (0.35 g, 0.56 mmol) yield as a
yellow crystal. 1H NMR (CDCl3) δ 3.93 (s, 6H, -OCH3 × 2), 6.95
(s, 1H, -CHdC<), 7.04 (d, J ) 9.0 Hz, 4H, aromatic protons),
8.15 (d, J ) 8.8 Hz, 4H, aromatic protons) ppm. 13C NMR (CDCl3)
δ 180.5, 165.5, 149.4, 146.8, 141.6, 139.01, 138.1, 135.8, 131.5,
124.5, 114.6, 92.0, 55.7 ppm. 11B NMR (CDCl3) δ 5.08 ppm. IR
(NaCl) ν 1604, 1558, 1541, 1506, 1495, 1240, 1173, 1097, 978
cm-1. HRMS (EI) calcd for C29H15O4F10B (M+) m/z 628.0904,
found m/z 628.0903. Anal. Calcd for C29H15O4F10B: C, 55.44; H,
2.41. Found: C, 55.65; H, 2.43.
Acknowledgment. We thank Dr. Y, Morisaki and Dr. K,
Tanaka for helpful discussions at Kyoto University.
Supporting Information Available: Text giving typical
experimental procedures, copies of NMR spectra, Figure 1S
showing CVs of 1a-2b, Figure 2S showing the LUMO and
HOMO of 2a and 2b, and crystal structural data for 2a and 2b.
This material is available free of charge via the Internet at
http://pubs.acs.org.
Experimental Section
Synthesis of 1a. BPh3 (1.21 g, 5.00 mmol) was added to a
solution of 1,3-diphenyl-1,3-propanedione (0.22 g, 1.00 mmol) in
dry toluene (4.0 mL) under nitrogen atmosphere. After the reaction
mixture was refluxed at 120 °C for 12 h, the solvent was removed
JO8017582
J. Org. Chem. Vol. 73, No. 21, 2008 8607