White light emission from a single component system: Remarkable concentration effects on the fluorescence of 1,3-diaroylmethanatoboron difluoride

Article abstract of DOI:10.1016/j.tetlet.2012.05.122

Concentration effects on the fluorescence (FL) emission of 1,3-dibenzoylmethanatoboron difluoride (1aBF₂) and its diisopropyl derivative (1bBF₂) in KBr and CH₂Cl₂ were investigated. Powder samples of 1aBF₂ and 1bBF₂ in KBr exhibit yellow and white FL emissions, respectively, whose intensities and wavelengths are not significantly affected by concentration. In contrast, remarkable concentration effects on FL properties of these compounds in CH ₂Cl₂ solutions were observed. Increases in the concentrations of 1aBF₂ and 1bBF₂ from 1 × 10 ^-7 to ca. 2 × 10^-1 M lead to dramatic changes in the FL colors from blue (398 and 411 nm, respectively) to yellow (548 and 558 nm) via white. Careful analysis of the FL spectra, involving lifetime determinations and wave deconvolutions, reveals that emissions from 1BF₂ involve two FL domains, corresponding to an excited monomer and an excimer, and that concentration increases promote a continuous change from the former to the latter major FL domain. Thus, white FL of 1aBF₂ and 1bBF₂ is achieved by modulation of the dual FL of the excited monomer (blue) and excimer (yellow). These findings indicate that 1,3-diaroylmethanatoboron difluoride (1BF₂) represents a new white emitting material that has advantageous features which arise from the fact that it is an easily prepared, low molecular weight, single component system not containing a heavy metal atom.

Full text of DOI:10.1016/j.tetlet.2012.05.122

Tetrahedron Letters 53 (2012) 4138–4141
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
White light emission from a single component system: remarkable
concentration effects on the fluorescence of 1,3-diaroylmethanatoboron
difluoride
Atsushi Sakai a, Mirai Tanaka a, Eisuke Ohta a,b, Yuichi Yoshimoto a, Kazuhiko Mizuno a,b, Hiroshi Ikeda a,b,
⇑
a Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
b The Research Institute for Molecular Electronic Devices (RIMED), Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Concentration effects on the fluorescence (FL) emission of 1,3-dibenzoylmethanatoboron difluoride
(1aBF2) and its diisopropyl derivative (1bBF2) in KBr and CH2Cl2 were investigated. Powder samples of
1aBF2 and 1bBF2 in KBr exhibit yellow and white FL emissions, respectively, whose intensities and wave-
lengths are not significantly affected by concentration. In contrast, remarkable concentration effects on FL
properties of these compounds in CH2Cl2 solutions were observed. Increases in the concentrations of
1aBF2 and 1bBF2 from 1 ꢀ 10ꢁ7 to ca. 2 ꢀ 10ꢁ1 M lead to dramatic changes in the FL colors from blue
(398 and 411 nm, respectively) to yellow (548 and 558 nm) via white. Careful analysis of the FL spectra,
involving lifetime determinations and wave deconvolutions, reveals that emissions from 1BF2 involve
two FL domains, corresponding to an excited monomer and an excimer, and that concentration increases
promote a continuous change from the former to the latter major FL domain. Thus, white FL of 1aBF2 and
1bBF2 is achieved by modulation of the dual FL of the excited monomer (blue) and excimer (yellow).
These findings indicate that 1,3-diaroylmethanatoboron difluoride (1BF2) represents a new white emit-
ting material that has advantageous features which arise from the fact that it is an easily prepared, low
molecular weight, single component system not containing a heavy metal atom.
Received 11 May 2012
Revised 22 May 2012
Accepted 25 May 2012
Available online 30 May 2012
Keywords:
White emission
Fluorescence
Organoboron complex
Intermolecular interaction
Excimer
Ó 2012 Elsevier Ltd. All rights reserved.
Introduction
of their small molecular size and lack of heavy metal atoms.8–13
The parent 1aBF2 was synthesized for the first time by Morgan
White fluorescent organic materials have attracted great atten-
tion in connection with their application to white organic
light-emitting diodes (WOLEDs).1 To obtain white emission, with
the Commission Internationale de l’Eclairage (CIE) coordinate (x, y) =
(0.33, 0.33), mixing three primary colors (blue, green, and red) or
two complementary colors (such as blue and yellow) is necessary.
Thus, most of the materials employed for white emission consist
of multiple light-emitting components.2,3 However, some problems
exist with systems of this type, such as the need for a complicated
fabrication process of the emitting device and a change or degrada-
tion of the white color caused by the low stability of fluorescent
compounds. Clearly, use of a single-component, white-emitting
material would be an ideal solution to this problem.4–6 However,
examples of materials of this type are limited to complexes contain-
and Tunstall in 1924.14 A report on FL property in CHCl3 by Karasev
and Korotkikh15 opened up a field of various emission ability of
1aBF2, including FL in the crystalline states16,17 and films,18 FL
induced by two-photon absorption,19 etc. Motivated by these his-
torical works, we have prepared a variety of 4,40-disubstituted
derivatives of 1aBF2 and studied their FL properties in KBr and
CH2Cl2. In the effort described below, we observed that CH2Cl2
solutions of the parent substance, 1aBF2, and its diisopropyl
derivative, 1,3-bis(4-isopropylbenzoyl)methanatoboron difluoride
(1bBF2, Chart 1), display remarkable concentration effects on their
ing heavy metal atoms or organic compounds with highly
p-conju-
gated skeletons.6,7
1,3-Dibenzoylmethanatoboron difluoride (1aBF2, Chart 1) and
its analogues exhibit intense fluorescence (FL) emissions in spite
⇑
Corresponding author. Tel./fax: +81 72 254 9289.
E-mail address: ikeda@chem.osakafu-u.ac.jp (H. Ikeda).
Chart 1. Molecular structures of 1,3-dibenzoylmethanatoboron difluoride (1aBF2)
and its diisopropyl derivative (1bBF2).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.05.122

A. Sakai et al. / Tetrahedron Letters 53 (2012) 4138–4141
4139
Scheme 1. Synthesis of 1aBF2 and 1bBF2.
FL wavelengths. Although no significant difference in FL between
1aBF2 and 1bBF2 exists, the FL colors of these substances undergo
dramatic changes from blue at low concentrations to yellow at
high concentrations, as a consequence of changes from emitting
excited monomers to excimers. Importantly, white emission from
these substances can be achieved by adjusting their concentrations
to bring about a suitable ratio of blue and yellow FL.
Figure 1. Observed FL spectra (solid curves) of 1aBF2 (A) and 1bBF2 (B) in KBr at
294 K with deconvoluted spectra (broken curves) and photographs of measuring
samples (insets). kEX = 365 nm.
Synthesis of 1aBF2 and 1bBF2
The parent substance 1aBF2 and the diisopropyl derivative
1bBF2 were synthesized by treatment of the corresponding
diaroylmethanes 2a and 2b with the boron trifluoride diethyl ether
complex, (Scheme 1).18–20 Preparation of 2b was accomplished by
use of a condensation reaction of methyl 4-isopropylbenzoate with
40-isopropylacetophenone, while 2a is commercially available.
Diffuse reflection and FL properties in KBr
Figure 2. The CIE chromaticity diagrams for FL of 1aBF2 (A) and 1bBF2 (B) in KBr
(squares) and CH2Cl2 (circles) at 294 K. kEX = 365 nm.
The diffuse reflection and FL properties of the parent 1aBF2 and
the diisopropyl derivative 1bBF2 in KBr are summarized in Table
1.21 The diffuse reflection maxima of these substances in KBr pow-
ders (kDR,P) occur at ca. 400 nm, indicating that the isopropyl sub-
stituents do not alter this property.
In contrast, the FL properties of 1aBF2 and 1bBF2 are different.
The sharp FL band of 1aBF2 in KBr powder (kFL,P) appears at around
545 nm while that of 1bBF2 is evidently broadened with some
peaks at 439, 474, and 527 nm (Table 1 and Fig. 1). Moreover,
the FL quantum yield (UFL,P) of 1bBF2 was found to be 0.13, a value
that is much higher than that of 1aBF2 (0.03). Interestingly, the FL
color of 1bBF2 in KBr is almost white, with a CIE coordinate (x,
y) = (0.29, 0.38), while that of 1aBF2 is yellow with a coordinate
(0.35, 0.44) (Fig. 2). Therefore, the incorporation of isopropyl
groups at the 4-position of two phenyl groups in 1aBF2 brings
about a change of FL color of the powder from yellow to white
Wave deconvolutions of the FL spectra using a Gaussian func-
tion were carried out to determine the origin of differences be-
tween the FL properties of 1aBF2 and 1bBF2 in KBr. In both cases,
the original FL spectra were deconvoluted to generate three com-
ponent spectra (Fig. 1), suggesting that FL of 1aBF2 and 1bBF2 in
KBr consists of three components involving multiple FL domains
comprised minimally of excited monomers and excimers. It is
likely that, the observed differences in the FL of 1aBF2 and
1bBF2, caused by the presence of isopropyl groups in the latter
substance, are a consequence of the difference in the degree of
intermolecular interaction existing in the solid state.
These diffuse reflection and FL properties, obtained by using
powder samples of 1aBF2 or 1bBF2 (0.2 lmol) grounded with KBr
(500 mg), remain unchanged when different ratios of KBr and the
substances are employed.22
and an improvement of UFL,P
.
Absorption and FL properties in CH2Cl2
The absorption and FL properties of 1aBF2 and 1bBF2 in CH2Cl2
solutions are summarized in Table 1.21 The absorption maxima of
both of these substances in CH2Cl2 solutions (kAB,S) are 380 nm,
indicating that no significant effects of the isopropyl substituents
exist.
Table 1
Diffuse reflection, absorption, and FL properties of 1aBF2 and 1bBF2
Sub.
Diffuse reflection and absorption
KBr CH2Cl2
FL
KBr
CH2Cl2
a
b
a
b
kDR,P (nm)
K–M
kAB,S (nm)
log
e
kFL,P (nm)
kFL,S (nm)
The different FL properties of 1aBF2 and 1bBF2 are also observed
in CH2Cl2. Specifically, at low concentration (e.g., 1 ꢀ 10ꢁ7 M),
CH2Cl2 solutions of both 1aBF2 and 1bBF2 display blue emission
at respective FL maxima (kFL,S) of 398 and 411 nm (Table 1 and
Fig. 3). The 13-nm red shift observed for 1bBF2 versus 1aBF2 is
consistent with the trend seen for FL of members of a series of
1BF2, in which a similar red shift of FL has been observed and ratio-
nalized by alterations of the electron-donating ability of diketonate
1aBF2
1bBF2
400
0.05
365
380
380
4.37
4.43
4.84
545
398
411
398
0.16
439
474
527
a
kDR,P and kFL,P are wavelengths for the diffuse reflection maxima and the FL
maxima, respectively, for the mixtures of 1BF2 (0.2 lmol) and KBr (500 mg) pow-
dered with ballmill at 50 Hz for 15 min.
ligands.9,13 Moreover, the FL quantum yield in CH2Cl2
(UFL,S) of
1bBF2 is nearly unity (0.99) while that of the parent 1aBF2 is only
0.26 (Table 2).
b
kAB,S and kFL,S are wavelengths for the absorption maxima and the FL maxima,
respectively, for 1BF2 in CH2Cl2 when their absorbance values are ca. 0.3 at the
wavelength indicated (ca. 0.5 ꢀ 10ꢁ5 M).

4140
A. Sakai et al. / Tetrahedron Letters 53 (2012) 4138–4141
Figure 3. Concentration effects on the FL spectral profiles of 1aBF2 (A) and 1bBF2
(B) in CH2Cl2 at 294 K. kEX = 380 nm. Photographs of FL of 1aBF2 (C) and 1bBF2 (D) in
CH2Cl2 at 294 K. kEX = 365 nm. The solutions of 1 ꢀ 10ꢁ7
,
1 ꢀ 10ꢁ5
,
1 ꢀ 10ꢁ3
,
1 ꢀ 10ꢁ2, 1 ꢀ 10ꢁ1, and 2 ꢀ 10ꢁ1 M for 1aBF2 or the saturated concentration (ca.
1.5 ꢀ 10ꢁ1 M) for 1bBF2 were serially put on from the left to the right.
Table 2
FL quantum yields and lifetimes of 1aBF2 and 1bBF2 in CH2Cl2
[1BF2] (M)
1aBF2
1bBF2
a
a
a
a
UFL,S s410 (ns) s530 (ns) UFL,S s410 (ns) s530 (ns)
b
0.36
0.28
0.24
1.65
0.46
50.7
6.27
0.47
51.5
2.60
0.44
49.3
1.02
0.32
0.37
0.49
0.31
0.91
48.3
3.11
0.57
57.8
11.5
2.03
52.0
14.2
3.48
23.9
1.95
16.1
1.70
13.2
1.58
1 ꢀ 10ꢁ1
1 ꢀ 10ꢁ2
0.41
0.74
0.80
1.20
0.81
1.86
1 ꢀ 10ꢁ3
1 ꢀ 10ꢁ5
1 ꢀ 10ꢁ7
0.22
0.26
0.26
0.51
0.50
0.50
39.5
0.51
32.5
0.50
0.69
0.44
0.53
0.68
0.99
1.80
1.67
1.56
a
The
s values for the solution at a higher concentration (especially,
[1BF2] > 1 ꢀ 10ꢁ2 M) may involve some systematic errors, because
a probable
intermolecular interaction is not accurately evaluated.
b
2 ꢀ 10ꢁ1 M for 1aBF2 and the saturated concentration (ca. 1.5 ꢀ 10ꢁ1 M) for
1bBF2.
Figure 4. Observed FL spectra (black) of 1aBF2 (A) and 1bBF2 (B) with deconvoluted
curves (blue: FL domain attributable to the excited monomer, orange: FL domain
attributable to the excimer).
To gain further insight into the observed substituent effects,
concentration effects of 1aBF2 and 1bBF2 on FL properties, in the
1 ꢀ 10ꢁ7 to 2 ꢀ 10ꢁ1 M region, were explored. As is commonly ob-
served for fluorescent organic compounds, self-quenching causes
the UFL,S of 1bBF2 to significantly decrease (from 0.99 to 0.32) on
increasing concentration (Table 2). Surprisingly, however, the par-
ent 1aBF2 at 2 ꢀ 10ꢁ1 M exhibits a relatively larger UFL,S (0.36) as
compared with that at 1 ꢀ 10ꢁ7 M (0.26). This observation suggests
that radiationless relaxation of the excited monomer or related
species, such as an excimer, of 1aBF2 is effectively retarded at high
concentration.
Importantly, as the concentrations of 1aBF2 and 1bBF2 become
higher, an FL band appears at 548 and 558 nm, respectively, in con-
cert with a drastic FL color change from blue to yellow (Fig. 3).23 It
is significant that at certain concentrations, solutions of the 1BF2

A. Sakai et al. / Tetrahedron Letters 53 (2012) 4138–4141
4141
emit white FL. The CIE chromaticity diagrams for FL of 1aBF2 and
1bBF2 in CH2Cl2 show successive transitions (Fig. 2).24 The CIE
coordinates for 1 ꢀ 10ꢁ1 M solutions are (x, y) = (0.29, 0.35) and
(0.34, 0.40) for 1aBF2 and 1bBF2, respectively, values that are close
to the coordinate for ideal white emission, (x, y) = (0.33, 0.33).
As in the case of KBr powder samples, a study of wave deconvo-
lution analyses is the key to elucidate the effects of substituents on
the FL properties of CH2Cl2 solutions of 1aBF2 and 1bBF2. As dis-
played in Figure 4, the original FL spectra at the lowest concentra-
tions were deconvoluted to generate several spectra with kFL,S at
ca. 400–500 nm (Fig. 4, blue), which can be reasonably assigned
to the excited monomers of 1BF2. In contrast, the original FL spec-
tra at higher concentrations were deconvoluted to generate several
spectra at ca. 400–500 nm and spectra with kFL,S at ca. 550–560 nm
(orange). The latter FL band is usually assigned to the excimer in
the case of 1aBF2.17 Additionally, a stationarity of FL wavelength
against changes in the concentration also implies that the FL
domain is likely associated with the excimers of 1BF2. Thus, the re-
sults of wave deconvolution analyses reveal that the FL of 1BF2 in
CH2Cl2 consists of two types of the FL domains corresponding to
the excited monomers and excimers.
AREA) program by the Ministry of Education, Culture, Sports, Sci-
ence, and Technology (MEXT), Japan. H.I. gratefully acknowledges
financial support in the form of a Grant-in-Aid for Scientific Re-
search on Priority Areas ‘New Frontiers in Photochromism’ (Nos.
20044027 and 21021025 in the Area No. 471) and Innovative Areas
‘p-Space’ (Nos. 21108520 and 23108718 in the Area No. 2007), the
Scientific Research (B) (Nos. 20044027 and 23350023), and the
Challenging Exploratory Research (Nos. 21655016 and 24655037)
from the MEXT of Japan. E.O. also acknowledges financial support
in the form of
a Grant-in-Aid for Young Scientist (B) (No.
24750044). K.M. also acknowledges financial support in the form
of a Grant-in-Aid for the Scientific Research (C) (No. 23550058).
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.05.
122.
References and notes
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Soc. 2007, 129, 8942–8943.
These conclusions gain support from the results of FL lifetime
(s) measurements. The s values at 410 nm for the FL shorter wave-
length domains of 1aBF2 and 1bBF2 (<1 ꢀ 10ꢁ2 M) are
s410 = ca. 0.5
s values at 530 nm for
and ca. 1.7 ns, respectively. In contrast, the
the longer wavelength FL domains of these substances are
s
530 = ca. 50 ns, much longer than the s410 values. The observed
s
values are typical for emission from excited monomers and exci-
mers of common organic substances.25,26
The results presented above clearly indicate that increasing
concentrations of 1BF2 cause continuous changes of the major FL
domain from the excited monomers to excimers that are accompa-
nied by changes of FL colors from blue to yellow. Mixing blue and
yellow emissions from the excited monomers and excimers,
respectively, in suitable ratios results in white emission.
12. Nagai, A.; Kokado, K.; Nagata, Y.; Arita, M.; Chujo, Y. J. Org. Chem. 2008, 73,
8605–8607.
13. Nagai, A.; Kokado, K.; Nagata, Y.; Chujo, Y. Macromolecules 2008, 41, 8295–
8298.
Conclusion
14. Morgan, G. T.; Tunstall, R. B. J. Chem. Soc., Trans. 1924, 125, 1963–1967.
15. Karasev, V. E.; Korotkikh, O. A. Russ. J. Inorg. Chem. 1986, 31, 493–496.
16. Mirochnik, A. G.; Gukhman, E. V.; Karasev, V. E.; Zhikhareva, P. A. Russ. Chem.
Bull. 2000, 49, 1024–1027.
17. Mirochnik, A. G.; Fedorenko, E. V.; Bukvetskii, B. V.; Karasev, V. E. Russ. Chem.
Bull. 2005, 54, 1060–1062.
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K.; Saito, K. Tetrahedron 2007, 63, 9354–9358.
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Desce, M.; Jullien, L. Chem. Eur. J. 2004, 10, 1445–1455.
20. Franek, W. Monatsh. Chem. 1996, 127, 895–907.
The results of the effort described above show that single com-
ponent, organoboron complexes 1BF2 exhibit white FL not only in
KBr but also in CH2Cl2. The white FL consists of blue and yellow FLs
(complementary colors), corresponding to the FL domains of
excited monomers and excimers, respectively.27 As compared to
conventional systems used for white FL, 1BF2 are single-compo-
nent, low molecular weight materials that are readily synthesized
and do not contain heavy metal atoms.
21. For the detail, see Figure S1 in the Supplementary data.
22. The FL properties of 1BF2 in KBr seem to depend on rather the degree of mixing
or grinding the sample. A study on this phenomenon is now in progress and
results will be published elsewhere.
23. For the detail, see Table S1 in the Supplementary data.
24. Similar concentration effects on FL were also observed in not only less polar
toluene but also polar solvent such as CH3CN. The detail of the solvent effects
on the absorption and FL properties will be given elsewhere.
25. Montalti, M.; Credi, A.; Prodi, L.; Gandolfi, M. T. Handbook of Photochemistry
Third Edition; CRC Press Taylor & Francis Group: New York, 2005. pp 577–579,
Section 10d.
It is interesting that kFL,P of 1bBF2 in KBr experiences a blue
shift, but kFL,S of 1bBF2 in CH2Cl2 undergoes a red shift as compared
to the parent 1aBF2 (Table 1). Although, no clear explanation exists
for these phenomena at this stage, the observation may be a con-
sequence of the existence of another FL domain in the KBr powder,
which probably depends on the molecular arrangement in the solid
state. From this point of view, the relationship between FL proper-
ties and the crystal structures of 1BF2 is an intriguing subject now
being probed in our laboratory.
26. Gould, I. R. In CRC Handbook of Organic Photochemistry Volume II; Scaiano, J. C.,
Ed.; CRC Press: Florida, 1989; pp 197–214. Chapter 6.
27. We observed some excitation spectra of 1BF2 at higher concentration that
might be explainable by assuming an aggregation of 1BF2 in the ground state.
Acknowledgments
However, this hypothesis has not proved yet by absorption spectroscopy or 1
H
This study was supported by the Cooperation for Innovative
Technology and Advanced Research in Evolutional Area (CITY
NMR at this stage. Further study is now in progress and the detail will be given
elsewhere.