Highly emissive diborylphemlene-containing bis(phenylethynyl)benzenes: Structure-photophysical property correlations and fluoride ion sensing

Article abstract of DOI:10.1002/chem.200900864

A series of 2,5-bis(dimesitylboryl)-1,4-bis(arylethynyl)benzenes 1-6 that contain various p-substituents on the terminal benzene rings, including NPh ₂ (1), OMe (2), Me (3), H (4), CF₃ (5), and CN (6) groups, were synthesized, and th

Full text of DOI:10.1002/chem.200900864

FULL PAPER
DOI: 10.1002/chem.200900864
Highly Emissive Diborylphenylene-Containing Bis(phenylethynyl)benzenes:
Structure–Photophysical Property Correlations and Fluoride Ion Sensing
Cui-Hua Zhao, Eri Sakuda, Atsushi Wakamiya, and Shigehiro Yamaguchi*[a]
+
Abstract: A series of 2,5-bis(dimesityl-
boryl)-1,4-bis(arylethynyl)benzenes 1–6
that contain various p-substituents on
the terminal benzene rings, including
NPh2 (1), OMe (2), Me (3), H (4), CF3
(5), and CN (6) groups, were synthe-
sized, and the effects of the p-substitu-
ents on the absorption and fluores-
cence properties were investigated
both in solution and in the solid state.
Linear relationships were obtained not
between the sp constants and the
fluorescence quantum yields in the
solid state. An important finding ex-
tracted from these results is that the
suppressed fluorescence quenching in
the solid state is a common feature for
the present laterally boryl-substituted
p-conjugated skeletons. Hence, the di-
borylphenylene can serve as a useful
core unit to develop highly emissive or-
ganic solids. In fact, most of the deriva-
tives showed more intense emission in
the solid state than in solution. In addi-
tion to these studies, the titration ex-
periment of
1 by the addition of
nBu4NF was conducted, which showed
the stepwise bindings of two fluoride
ions with high association constants as
well as a drastic change in the fluores-
cence spectra, while constantly main-
taining high quantum yields (0.61–
0.76), irrespective of the binding
modes. This result also demonstrated
the potential utility of the present mol-
ecules as an efficient fluorescent fluo-
ride ion sensor.
+
only between the Hammett sp con-
Keywords: boron · fluorescence ·
fluoride ion sensor · structure–prop-
erty relationships
effects
stants of the p-substituents and the ab-
sorption and fluorescence maxima,
quantum yields, and excited-state dy-
namics parameters in solution, but also
·
substituent
Introduction
the main chain or bear the boryl groups at the terminal posi-
tions of the p-conjugated framework. In contrast, we and
other research groups recently reported the introduction of
the boryl groups at the lateral positions of the p-conjugated
framework as a new design of the boron-based materi-
als.[16–18] In particular, we reported that the incorporation of
the boryl groups on the electron-donating p-conjugated
skeleton, such as bis[(p-Ph2N-phenyl)ethynyl]benzene (1), is
an efficient way to create emissive organic solids.[16a] This
design has a certain generality. Thus, we also demonstrated
that boryl-substituted oligothiophenes showed intense solid-
state emissions, the color of which could be easily tuned in a
full-color region from blue to red by appropriate structural
modification.[16c] Similar boryl-substituted polythiophenes
were also independently synthesized by Jꢀkle and co-work-
ers.[17]
Tri-coordinate boron-containing p-conjugated systems have
attracted considerable attention due to their intriguing elec-
tronic and photophysical properties, which arise from the
pp–p* conjugation between the vacant p orbital on the
boron atom and the p* orbital of the p-conjugated frame-
work.[1] The compounds classified into this category have
been utilized in a wide range of fields, including nonlinear
optics,[2,3] two-photon absorption and emission materials,[4,5]
organic light-emitting diodes (OLEDs),[6–9] and chemical
sensors for fluoride ions.[10–15] Most of the reported organo-
boron materials typically either contain the boron atoms in
The design and synthesis of highly emissive organic solids
is a fundamental subject for various photonic and optoelec-
tronic applications. Although a number of emissive fluoro-
phores with a fluorescence quantum yield of near unity in
solution exist, the examples of highly emissive organic solids
are still limited, because of the severe fluorescence quench-
ing as a result of certain intermolecular interactions.[19]
[a] Dr. C.-H. Zhao, Dr. E. Sakuda, Dr. A. Wakamiya,
Prof. Dr. S. Yamaguchi
Department of Chemistry, Graduate School of Science
Nagoya University, Furo, Chikusa, Nagoya 464-8602 (Japan)
Fax : (+81)52-789-5947
E-mail: yamaguchi.shigehiro@b.mbox.nagoya-u.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200900864.
Chem. Eur. J. 2009, 15, 10603 – 10612
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10603

Results and Discussion
Synthesis: In the previous work, the synthetic route to the
boryl-substituted bis(arylethynyl)benzene 1 was established
based on the Pd0/CuI-catalyzed Sonogashira reaction with
1,4-diethynyl-2,5-bis(dimesitylboryl)benzene (9) as a key
precursor.[16a] This facile synthetic route allowed us to pre-
pare a series of the derivatives 2–6 containing various termi-
nal substituents by using appropriate p-substituted phenyl
iodides, as shown in Scheme 1. The coupling reaction of
Therefore, the rational design of such emissive organic
solids is still a challenging issue.[20] In this regard, the eluci-
dation of the structure–property relationship of the known
highly emissive systems should provide an important basis
for further rational designs. On the basis of this motivation,
we herein investigate the structure–property correlations in
a series of bis[(p-substituted phenyl)ethynyl]diborylbenzenes
1–6, including the already reported p-Ph2N-substituted de-
rivative 1.
Scheme 1. Synthesis of bis[(p-substituted phenyl)ethynyl]diborylbenzenes
1–6.
In the previous study on 1, we learned some important
issues about the effect of the lateral dimesitylboryl groups,
through comparisons of 1 with other analogous compounds
that contain bulky triisopropylsilyl groups (7) or electron-
withdrawing cyano groups (8), instead of the boryl groups.
Among these compounds, only 1 can maintain an intense
emission in the solid state. These facts demonstrated that
there are two crucial requirements for achieving an intense
solid-state emission. One is steric bulkiness that can prevent
an intermolecular interaction. The other is a p-electron-ac-
cepting character that leads to an intramolecular charge-
transfer (CT) transition with a large Stokes shift, which
should effectively suppress self-quenching in the condensed
phase. Notably, the lateral dimesitylboryl groups in 1 can
satisfy these two requirements.
To elucidate the more detailed structure–property correla-
tion, we herein focused our attention on the substituent ef-
fects of the terminal phenyl rings. We prepared a series of
bis(arylethynyl)benzene derivatives 1–6 that contain elec-
tron-donating Ph2N (1) and MeO (2) groups, electronically
indifferent Me (3) and H (4) groups, and electron-withdraw-
ing CF3 (5) and CN (6) groups as the p-substituents and
comprehensively studied their photophysical properties, in-
cluding the excited-state dynamics and solid-state fluores-
cence. We also studied their application to a fluorescent che-
mosensor for fluoride ions, to demonstrate the utility of
these highly emissive fluorescent molecular systems.
phenyl iodides that contain electron-withdrawing p-substitu-
ents, such as CF3 and CN proceeded in higher yields than
with those that contain electron-donating groups, such as
OMe and NPh2, probably because the electron-donating
groups on the aryl halides generally have a detrimental
effect on the coupling reaction.[16b,21] All the obtained boryl-
substituted p-conjugated compounds are stable in air and
water and can be purified by silica-gel column chromatogra-
phy.
X-ray crystal structure analysis: Among the newly prepared
bis(arylethynyl)benzenes, the structures of nonsubstituted 4
and cyano-substituted 6 were determined by X-ray crystal-
lography. Their ORTEP drawings are shown in Figure 1.
The structure of Ph2N-substituted 1 was reported in the pre-
vious paper.[16a] In all these three compounds, the trivalent
boron centers are well protected by the methyl groups at
the o-positions of two mesityl groups, which accounts for
their high stabilities. It is worth noting that the p-substitu-
ents at the terminal phenyl rings of the bis(arylethynyl)ben-
zene skeleton seem to have a certain influence on the copla-
narity of the p-conjugated framework. Thus, while Ph2N-
substituted 1 has a significantly twisted main chain structure,
in which the dihedral angle between the central and termi-
nal benzene rings is 47.58, nonsubstituted 4 has a slightly de-
creased dihedral angle of 37.48, and compound 6, which con-
tains strong electron-withdrawing CN groups, adopts an
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Highly Emissive Diborylphenylene-Containing Bis(phenylethynyl)benzenes
FULL PAPER
Figure 2. Absorption and fluorescence spectra of boryl-substituted bis(ar-
ylethynyl)benezenes 1–6 in benzene.
Table 1. UV/Vis absorption and fluorescence data for boryl-substituted
bis(arylethynyl)benezenes 1–6 in benzene.
Absorption[a]
labs [nm] (log e) lem [nm][b] FF
Fluorescence
Excited-state dynamics
ts [ns] kr [sꢀ1 knr [sꢀ1
[c]
]
]
1
2
3
4
5
6
442 (4.37)
409 (4.05)
559
509
487
480
476
476
0.98 5.4
0.74 7.5
0.36 4.6
0.16 3.2
0.04 1.9
0.04 1.5
1.8ꢂ108
9.8ꢂ107
7.8ꢂ107
5.1ꢂ107
2.1ꢂ107
2.7ꢂ107
3.7ꢂ106
3.4ꢂ107
1.4ꢂ108
2.7ꢂ108
5.1ꢂ108
6.6ꢂ108
Figure 1. ORTEP drawings of a) 4 and b) 6. Thermal ellipsoids are drawn
at the 50% probability level. Hydrogen atoms are omitted for clarity.
401 (4.09)[d]
400 (3.96)[d]
368 (4.27)[d]
377 (4.34)[d]
[a] Only the longest absorption maximum wavelengths are given. [b] Ex-
cited at the longest absorption maximum. [c] Absolute quantum yield de-
termined by a calibrated integrating sphere system. [d] Observed as
shoulder.
almost coplanar main chain structure with a dihedral angle
of only 9.88. Although we cannot rule out the packing effect
on the coplanarity of the p-conjugated skeleton at this stage,
these differences suggest that the p conjugation occurs more
effectively as more electron-withdrawing groups are at-
tached at the terminal positions. On the contrary, as for the
geometry of the boron moieties, the dihedral angles between
the central benzene ring and the boron planes are 44.688 for
6, 36.648 for 4, and 39.378 for 1. Thus, no significant correla-
tion was observed between the electronic nature of the ter-
minal p-substituents and the effectiveness of the pp–p* con-
jugation in the central diborylbenzene moiety.
mainly localized on the diborylphenylene moiety. The latter
band shifts to longer wavelengths as the electron-donating
ability of the p-substituent increases from 6 (377 nm) to 1
(442 nm).
In the fluorescence spectra, the emission properties of the
bis(arylethynyl)benzenes are highly dependent on the elec-
tronic nature of the terminal p-substituents. Similar to the
trend in the longer wavelength band in the absorption spec-
tra, the emission maximum wavelength is red-shifted with
the increase in the electron-donating ability of the terminal
p-substituents, from a greenish/blue fluorescence at 476 nm
for 6 to a yellow emission at 559 nm for 1. In conjunction
with this change, the fluorescence intensity also increases
with the increased electron-donating ability of the terminal
p-substituents.
Photophysical properties: To elucidate the effect of the ter-
minal substituents on the photophysical properties for the
boryl-substituted bis(arylethynyl)benzenes 1–6, we investi-
gated the following three issues: Effects of the p-substitu-
ents on 1) the photophysical properties in solution, 2) the
excited-state character and the delocalization of molecular
orbitals, and 3) the solid-state fluorescence properties.
Photophysical properties in benzene solutions: The UV/Vis
absorption and emission spectra of compounds 1–6 in ben-
zene are shown in Figure 2, the data for which are summar-
ized in Table 1. All these derivatives have an intense absorp-
tion band at ca. 320–340 nm and a weaker band at longer
wavelengths of 368–442 nm. While the former band is as-
signable to the p–p* transition of the bis(arylethynyl)ben-
zene skeleton, the latter band is assigned to the intramolecu-
lar charge-transfer (CT) transition from the HOMO delocal-
ized over the whole p-conjugated framework to the LUMO
We correlated these photophysical data with the Hammett
+
[22]
substituent constants sP
.
We found that not only the ab-
sorption maxima, but also the emission maxima have linear
+
relationships with the sP parameters of the p-substituents.
According to the plots in Figure 3, the relationships shown
in Equations (1) and (2) can be derived, although the corre-
lation value r in the latter case is moderate. These results
may be rationalized by considering that the absorption and
emission maxima of the present systems are highly depen-
+
dent on their HOMO energy levels. As the sP value de-
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S. Yamaguchi et al.
kr ꢂts and kr +knr =tsꢀ1. According to the calculated kr and
knr values, the replacement of the terminal NPh2 groups with
less electron-donating or -withdrawing groups leads to accel-
eration of the nonradiative process and deceleration of the
radiative process at the same time. The linear relationships
were observed between not only kr and sP+, but also knr and
sP+, as shown in Figure 5, from which Equations (4) and (5)
~
*
Figure 3. Correlations between the absorption
( ) and emission ( )
maxima and the Hammett substituent constants (sp+) of the terminal
substituents in the boryl-substituted bis(arylethynyl)benzenes 1–6.
creases, the HOMO energy levels increase, leading to the in-
tramolecular CT transition at the longer wavelength.
nabs ½cmꢀ1ꢁ ¼ 1939spþ þ 25 513 ðr ¼ 0:969Þ
nem ½cmꢀ1ꢁ ¼ 1406spþ þ 20 439 ðr ¼ 0:924Þ
ð1Þ
ð2Þ
~
Figure 5. Correlations between the radiative (kr, ) and nonradiative (knr,
*
) decay rate constants and the Hammett substituent constants (sp+) of
the terminal substituents in the boryl-substituted bis(arylethynyl)ben-
zenes 1–6.
It is worth noting that the fluorescence quantum yields
also have a linear relationship with the Hammett parame-
ters. Figure 4 shows the plot of the quantum yields (FF) of
were obtained, although in the latter case the correlation
value was only moderate. Importantly, the steeper slope in
Equation (5) compared to that in (4) implies that the differ-
ence in knr predominantly contributes to the significant
change in the fluorescence efficiency. These results demon-
strate that the electron-donating terminal groups are essen-
tial for realizing the intense fluorescence in the present
boryl-substituted p-conjugated systems.
kr ½sꢀ1ꢁ ¼ ꢀ0:712 ꢂ 108spþ þ 0:614 ꢂ 108 ðr ¼ 0:970Þ
knr ½sꢀ1ꢁ ¼ 3:10 ꢂ 108spþ þ 3:32 ꢂ 108 ðr ¼ 0:937Þ
ð4Þ
ð5Þ
Figure 4. Correlation between the fluorescence quantum yield (FF) in
benzene and the Hammett constant (sp+) of the terminal substituents in
the boryl-substituted bis(arylethynyl)benzenes 1–6.
Fluorescence solvatochromism and theoretical studies: The
most notable feature of the present boryl-substituted bis(ar-
yl-ethynyl)benzenes is their intramolecular CT character in
the transitions. To gain a deeper insight into their excited
states, we investigated the solvent effects in the absorption
and fluorescence spectra for the representative derivatives,
electron-donating Ph2N-substituted 1, nonsubstituted 4, and
electron-withdrawing CN-substituted 6, the data for which
are summarized in Table 2.
In all these compounds, while the absorption maxima
show only trivial solvent dependence, the fluorescence spec-
tra show substantial bathochromic shifts as the solvent po-
larity increases. These facts demonstrate that these mole-
cules have more polar structures in the excited state relative
to those in the ground state, similar to other boryl-substitut-
ed p-conjugated compounds. To compare the degree of the
polar excited-state structure among these molecules, we em-
ployed the Lippert–Mataga equation [Eq. (6)].
+
1–6 against the Hammett sP constants, which gives Equa-
tion (3). Although electron-withdrawing CN-substituted 6
shows only a weak fluorescence (FF =0.04), the electron-do-
nating Ph2N-substituted 1 exhibits a very intense emission
with a FF of 0.98, even though it has a highly twisted main
chain structure.
FF ¼ ꢀ0:477spþ þ 0:289 ðr ¼ 0:977Þ
ð3Þ
To elucidate the structure–property correlation in more
detail, we also determined the fluorescence lifetime (ts) by a
time-resolved photoluminescence study and calculated the
radiative (kr) and nonradiative (knr) decay rate constants
from the singlet excited state, based on the equations FF =
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Highly Emissive Diborylphenylene-Containing Bis(phenylethynyl)benzenes
Table 2. UV/Vis absorption and fluorescence data of boryl-substituted
FULL PAPER
beyond our expectation. The large quadrupole moment in
the excited state would cause a significant structural relaxa-
tion, which may be one of the factors for the larger knr value
for 6. However, this explanation is not applicable to the ra-
tionalization of the small knr value of Ph2N-substituted 1.
Therefore, the discussion of the difference in the knr values
for these compounds may need a more careful consideration
taking the intersystem crossing process into account.
To elucidate the effect of the terminal p-substituents on
the electronic structures, we conducted single-point calcula-
tions of the three compounds 1, 4, and 6 at the B3LYP/6-
31G(d) level of theory, by using the geometries derived
from their crystal structures. We also performed time-depen-
dent density-functional theory (TD-DFT) calculations of
these three compounds at the B3LYP/6-31G(d) level of
theory to understand their intramolecular CT transitions.
The pictorial drawings of their molecular orbitals are shown
in Figure 7, and the calculated data are summarized in
Table 3.
bis(arylethynyl)benzenes 1, 4, and 6 in various solvents.
[c]
Solvents
labs [nm][a]
lem [nm][b]
Dn [cmꢀ1
]
1
4
6
cyclohexane
benzene
CHCl3
THF
MeOH
cyclohexane
benzene
CHCl3
THF
MeOH
432
442
438
437
536
559
567
601
627
474
480
489
488
496
460
476
484
504
4491
4753
5194
6244
6830
3902
4167
4676
4760
5027
5287
5516
6661
7332
439
400[d]
400[d]
398[d]
396[d]
397[d]
370[d]
377[d]
366[d]
368[d]
cyclohexane
benzene
CHCl3
THF
[a] Only the longest absorption maximum wavelengths are given. [b] Ex-
cited at the longest absorption maximum. [c] Stokes shift. [d] Observed
as shoulder.
2Df
2
ð6Þ
ð7Þ
Dn ¼
Df ¼
ðmeꢀmgÞ þ C
hca3
eꢀ1
ðn2ꢀ1Þ
ꢀ
ð2n2 þ 1Þ
2e þ 1
in which C is a constant, me and mg are the dipole moments
in the excited state and ground state, respectively, and Dn is
the Stokes shift. Df is the solvent polarity and is given by
Equation (7) in which e is the dielectric constant and n is
the optional refractive constant. Although this equation
originally assumes the presence of a molecular dipole, it has
been demonstrated to be applicable to several quadrupole
extended p-conjugated molecules.[23] For the present com-
pounds 1, 4, and 6, we indeed obtained linear relationships
for the plots of Dn as a function of the Df, as shown in
Figure 6. The slopes obtained for compounds 1 and 6 are
7213 and 8974 cmꢀ1, respectively, which are comparable to
each other and much steeper than that of 4 (3327 cmꢀ1).
These results indicate that the quadrupole moments in the
excited state are significant not only in 1 but also in 6.
While it is reasonable that the electron-donating Ph2N-sub-
stituted 1 has such a large quadrupole moment in the excit-
ed state, the steep slope obtained for CN-substituted 6 is
Figure 7. Pictorial drawings of the HOMO and LUMO for a) 1, b) 4, and
c) 6, calculated at the B3LYP/6-31G(d) level of theory by using their
crystal structures.
Table 3. Calculated Kohn–Sham molecular orbital energy levels and the
first excited-state energies for the boryl-substituted bis(arylethynyl)ben-
zenes 1, 4, and 6.
HOMO LUMO HOMO–LUMO Transition energy[a]
f[b]
[eV]
[eV]
gap [eV]
[eV] (l [nm])
1
4
6
ꢀ4.74
ꢀ5.36
ꢀ5.84
ꢀ1.86
ꢀ1.95
ꢀ2.63
2.88
3.41
3.21
2.28 (545)
2.86 (434)
2.81 (442)
0.574
0.0972
0.552
Figure 6. The Lippert–Mataga plots for boryl-substituted bis(arylethynyl)-
~
*
^
benzenes 1 ( ), 4 ( ), and 6 ( ).
[a] The first excited-state transition energy. [b] Oscillator strength.
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S. Yamaguchi et al.
Table 4. Photophysical properties of boryl-substituted bis(arylethynyl)-
The nonsubstituted compound 4 has a HOMO delocalized
over the whole bis(arylethynyl)benzene framework, whereas
its LUMO is mostly localized on the central diborylpheny-
lene moiety. The HOMO and LUMO energy levels are
ꢀ5.36 and ꢀ1.95 eV, respectively, and the calculated first ex-
cited state, mainly consisting of a HOMO!LUMO transi-
tion, has an excitation energy of 2.86 eV (434 nm) with a
small oscillator strength of 0.0972. The incorporation of the
electron-donating Ph2N group onto this skeleton as the p-
substituent (namely, 1) results in a significant increase in the
HOMO energy level by 0.72 eV, whereas the LUMO level
remains almost unchanged. Consequently, the calculated
first excited energy of 1 is significantly lowered to 2.28 eV
(545 nm). Notably, the HOMO of 1 spreads over the entire
p-conjugated skeleton, including the Ph2N moiety. This
change leads to a significant increase in the oscillator
strength of the first excited state (f=0.574). On the other
hand, the introduction of the electron-withdrawing CN
group onto the skeleton of 4 (namely, 6) results in the signif-
icant decreases both in the HOMO and LUMO levels by
0.48 and 0.68 eV, respectively. In conjunction with these
changes, more importantly, the LUMO of 6 spreads over the
entire framework and, consequently, the oscillator strength
of the first excited state again increases to 0.552, although
its excitation energy (2.81 eV, 442 nm) is comparable to that
of 4. Whereas the accuracy of the calculated excitation
energy is not sufficiently high by this level of calculations,
the trend in the order of the oscillator strength is in good
agreement with the order of the experimentally obtained
molecular absorption coefficients for these three molecules.
These calculations suggest that the nature of the transition
significantly depends on the terminal p-substituents. Thus,
while the transition in the Ph2N-substituted 1 has a strong
intramolecular CT character, the transition in 6 seems to
have a mixed character of the intramolecular CT with the
p–p* transition.
benzenes 1–6 in the spin-coated films.[a]
[d]
[f]
labs [nm][b]
lem [nm][c]
FF
t [ns][e]
Dl [cmꢀ1
]
1
2
3
4
5
6
447
412
562
508
486
476
459
462
0.90
0.82
0.62
0.43
0.11
0.09
1.3/6.9 (37/63)[e]
2.3/8.8 (55/45)[e]
2.5/6.5 (57/43)[e]
1.8/5.7 (73/27)[e]
0.69/2.5 (84/16)[e]
0.54/2.0 (88/12)[e]
4580
4590
3700
3990
4950
4670
401[g]
400[g]
374[g]
380[g]
[a] Spin-coated films were prepared from ca. 1.0 mgmLꢀ1 THF solutions.
[b] Only the longest absorption maximum wavelengths are given.
[c] Fluorescence maximum wavelength, excited at the longest absorption
maximum. [d] Absolute quantum yield determined by a calibrated inte-
grating sphere system. [e] Amplitudes of two lifetimes given in parenthe-
ses. [f] Stokes shift. [g] Observed as a shoulder band.
+
the FF and sp values in the solid state, as shown in
Figure 8, which affords Equation (8). Although a similar
linear relationship between the FF and sp values has been
Figure 8. Correlation between the fluorescence quantum yield (FF) in the
+
spin-coated film and the Hammett sp constant of the terminal substitu-
ents in the boryl-substituted bis(arylethynyl)benzenes 1–6.
Solid-state fluorescence properties: We next investigated the
effects of the p-substituents on the solid-state fluorescence
properties. Thin films of compounds 1–6 were prepared
from their THF solutions with ca. 1 mgmLꢀ1 concentration
on a quartz plate, and their absorption and fluorescence
spectra were directly measured. The absolute fluorescence
quantum yields were determined by a calibrated integrated
sphere system (Hamamatsu C9920). The data are summar-
ized in Table 4.
Both in the absorption and fluorescence spectra, all the
compounds 1–6 maintain almost the same spectra as those
in benzene solutions, in terms of the maximum wavelengths
as well as the full width at the half maximum (FWHM).
These results suggest the formation of neither aggregation
in the ground state nor an excimer in the excited state. The
lateral boryl groups are bulky enough to prevent the inter-
molecular interaction. It is worth noting that, in all the com-
pounds, the fluorescence quantum yields do not significantly
decrease on going from the solution to the solid state. Con-
sequently, a linear relationship is again obtained between
reported for other oligo(phenyleneethynylene) derivatives
in solution,[24] such a relationship in a solid-state emission is
unprecedented to the best of our knowledge. In a compari-
son of Equations (3) and (8), the higher intercept value with
the smaller slope constant in Equation (8) demonstrates a
trend that the value for FF in the solid state becomes higher
than those in solution. This is a notable common feature of
the present laterally boryl-substituted bis(arylethynyl)ben-
zene systems, irrespective of the nature of the p-substituents
at the terminal benzene rings. As far as the molecules have
a sufficiently large Stokes shift (>3700 cmꢀ1), the steric
bulkiness of the central diborylphenylene core seems suffi-
cient to retain the inherent fluorescence properties even in
the solid state, without fluorescence quenching. The in-
creased solid-state FF values observed for compounds 2–6
compared to those in solution may be related to the restrict-
ed motion of the molecules in the solid state.[25]
FF ¼ ꢀ0:422spþ þ 0:409 ðr ¼ 0:977Þ
ð8Þ
10608
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Chem. Eur. J. 2009, 15, 10603 – 10612

Highly Emissive Diborylphenylene-Containing Bis(phenylethynyl)benzenes
FULL PAPER
To accumulate more data on the excited-state behavior in
the solid state, we determined the fluorescence lifetime of
their spin-coated films by a time-resolved photolumines-
cence study. All the compounds 1–6 showed biexponential
decays in the solid state and gave two components, as listed
in Table 4. There is a trend in the amplitudes of the two
components. Thus, as the p-substituent becomes more elec-
tron-donating, the longer-lived excited state becomes domi-
nant. However, this trend seems to not be directly related to
the increment in the FF value in the solid state compared to
that in solution, because all the compounds 1–6 showed
comparable or increased solid-state quantum yields, irre-
spective of the p-substituents. According to the time-re-
solved fluorescence spectra of 4 in the periods of 0–1 and of
3–10 ns right after the excitation, no significant difference in
the spectra shape was observed (see the Supporting Infor-
mation). The characterization of these two components re-
mains unclear at this moment.
Fluorescent sensing of fluoride ions: Because the fluoride
ion is highly relevant to human health and environmental
issues,[26] the selective detection of it has attracted great at-
tention. Fluorescence sensing is one of the most powerful
methods due to its high sensitivity. Tri-coordinate boron
compounds are one of the effective fluoride sensors based
Figure 9. Fluoride ion sensing by Ph2N-substituted bis(arylethynyl)ben-
on the principle that coordination with fluoride ion disrupts
the pp–p* conjugation between the boron center and the at-
tached p-conjugated chromophore and thus leads to a signif-
icant change in the UV/Vis absorption, fluorescence, and
two-photon excited fluorescence.[10–15] Considering the char-
acteristic intramolecular CT emission of the present laterally
boryl-substituted p systems, we envisioned that the coordi-
nation of a fluoride ion to the boron center would interrupt
the strong intramolecular CT transition and activate the
emission based on the p–p* transition of the bis(arylethy-
nyl)benzene framework, leading to a significant change in
the fluorescence spectra. On the basis of this idea, we inves-
tigated the fluoride sensing ability of the most emissive 1 as
a representative compound for the present borylated p sys-
tems.
The titration experiment of 1 with the fluoride ion was
carried out in THF by using nBu4NF (TBAF) as the fluoride
source. The fluorescence spectral change of 1 (1.96ꢂ10ꢀ6 m)
upon the addition of TBAF is shown in Figure 9. A two-step
stepwise sensing of the fluoride ions was observed. Thus, as
the concentration of TBAF increased, the emission band at
601 nm decreased, and a new blue-shifted band appeared at
477 nm with an isosbestic point at 566 nm (Figure 9a) This
change became saturated when the concentration of TBAF
amounted to 5.52ꢂ10ꢀ6 m. This spectral change was associat-
ed with a dramatic emission color change from reddish
orange to greenish blue. The molar ratio analysis showed
that the spectral change in this concentration range was at-
tributed to the formation of a 1:1 complex (see the Support-
ing Information). The binding constant was determined to
be 1.25ꢂ105 mꢀ1 at 208C, which is comparable to those of
other tri-coordinate organoboron compounds.[10–15] Signifi-
zene 1: Fluorescence spectra change of 1 in THF upon addition of TBAF
with the range of a) 0–5.52ꢂ10ꢀ6 m and b) 5.52ꢂ10ꢀ6ꢀ60.0ꢂ10ꢀ6 m (lex
=
390 nm).
cantly, the further addition of a large excess of TBAF
caused the appearance of a more blue-shifted emission band
at 412 nm with a new isosbestic point at 460 nm (Figure 9b).
This spectral change resulted in an emission color change to
bright sky blue. This change can be interpreted as a com-
plexation with a second fluoride ion, and the binding con-
ꢀ1
stant was determined to be 1.79ꢂ104 m
at 208C. Notably,
the complexation with fluoride ions did not cause any de-
crease in the FF value (1: 0.61; 1·F22ꢀ: 0.76). These high fluo-
rescence efficiencies demonstrate the advantages of our
system over the precedent boron-based fluoride ion sensors.
Conclusions
The introduction of the bulky dimesitylboryl groups onto
the lateral positions of the electron-donating p-conjugated
systems is an effective design for highly emissive organic
solids. We demonstrated the utility and generality of this
new design by synthesizing two series of compounds, 2,5-di-
borylphenylene-cored oligo(phenyleneethynylene) systems
and 3-boryl-2,2’-bithiophene systems. In this article, we fo-
cused our attention on the former class of compounds and
investigated the more detailed structure–property relation-
ships. Thus, we synthesized a series of 1,4-bis(arylethynyl)-
2,5-diborylbenzenes 1–6 that contain various p-substituents
on the terminal benzene rings and investigated the substitu-
Chem. Eur. J. 2009, 15, 10603 – 10612
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www.chemeurj.org
10609

S. Yamaguchi et al.
(s, 6H), 2.29 (s, 12H), 2.08 ppm (s, 24H); 13C NMR (CDCl3): d=152.1,
142.5, 140.9, 139.2, 137.9, 137.4, 131.5, 128.5, 128.4, 125.9, 120.1, 93.9,
89.6, 23.3, 21.4, 21.2 ppm; m/z: calcd for C60H60B2: 802.4881; found:
802.4879.
ent effects on their absorption and fluorescence properties.
Linear relationships were obtained not only between the
Hammett sp constants of the p-substituents and the ab-
+
sorption and fluorescence properties in solutions, but also
1,4-Bis(4-trifluoromethylphenylethynyl)-2,5-bis(dimesitylboryl)benzene
(5): This compound was prepared essentially in the same manner as de-
scribed for 2 by using 9 (62 mg, 0.1 mmol), 4-trifluoromethyl-1-iodoben-
+
between the sp constants and the fluorescence quantum
yields in the solid state. In addition, most of the synthesized
derivatives showed enhanced fluorescence in the solid state
compared to solution. These findings imply that the present
diborylphenylene unit is a very effective and useful core
skeleton to prevent fluorescence quenching in the solid
state, irrespective of the character of p-conjugated skeletons
(namely, electron-donating or -accepting). We also investi-
gated the fluorescence fluoride ion sensing ability of the
most emissive 1 and demonstrated the intense fluorescence
properties irrespective of the binding modes. This is an im-
portant and advantageous feature of the present system
over the precedent boron-based fluoride sensors. We believe
that our current results would provide an important basis
for the further rational design not only of highly emissive or-
ganic materials, but also of new boron-based p-conjugated
functional materials.
zene (82 mg, 0.3 mmol), [PdACHTNUGTRNEGNU(PPh3)4] (11.6 mg, 0.01 mmol), and CuI
(3.8 mg, 0.02 mmol) in (iPr)2NEt/THF 1:3 (4 mL) at 508C. The purifica-
tion by silica-gel column chromatography (hexane/CHCl3 5:1, Rf =0.30)
afforded 76 mg (0.083 mmol) of 5 in 84% yield as a pale-yellow solid.
M.p. 291–2928C; 1H NMR (CDCl3): d=7.50 (s, 2H), 7.42 (d, J=8.0 Hz,
4H), 7.07 (d, J=8.0 Hz, 4H), 6.77 (s, 8H), 2.26 (s, 12H), 2.04 ppm (s,
24H); 13C NMR (CDCl3): d=152.4, 142.3, 140.9, 139.5, 137.8, 131.7, 129.1
(q, J=31 Hz), 128.5, 126.9, 125.7, 124.6 (q, J=3.3 Hz), 119.9 (q, J=
270 Hz), 92.6, 92.3, 23.3, 21.2 ppm; HRMS (FAB): m/z: calcd for
C64H54B2F6: 910.4316; found: 910.4312.
1,4-Bis(4-cyanophenylethynyl)-2,5-bis(dimesitylboryl)benzene (6): This
compound was prepared essentially in the same manner as described for
2 by using 9 (124 mg, 0.2 mmol), 4-iodobenzonitrile (183 mg, 0.8 mmol),
[PdACHTUNGRTNEUNG(PPh3)4] (11.6 mg, 0.01 mmol), and CuI (3.9 mg, 0.02 mmol) in
(iPr)2NEt/THF 1:3 (8 mL) at 508C. Purification by silica-gel column
chromatography (hexane/CHCl3 5:1, Rf =0.30) afforded 153 mg
(0.186 mmol) of 6 in 93% yield as a pale-yellow solid. M.p.> 3008C;
1H NMR (CDCl3): d=7.53 (s, 2H), 7.47 (d, J=8.4 Hz, 4H), 7.06 (d, J=
8.4 Hz, 4H), 6.79 (s, 8H), 2.28 (s, 12H), 2.05 ppm (s, 24H); 13C NMR
(CDCl3): d=152.4, 142.2, 140.9, 139.6, 137.8, 131.9, 131.4, 128.5, 127.9,
125.6, 118.5, 111.1, 94.1, 92.4, 23.3, 21.2 ppm; HRMS (FAB): m/z: calcd
for C60H54B2N2: 824.4473; found: 824.4475 [M]+..
Experimental Section
X-ray crystal structure analysis of compound 4:[28] Single crystals of 4
suitable for X-ray crystal analysis were obtained by recrystallization from
CH2Cl2/hexane. Intensity data were collected at 173 K on a Rigaku
Single Crystal CCD X-ray diffractometer (Saturn 70 with MicroMax-007)
with MoKa radiation (l=0.71070 ꢃ) and graphite monochromater. A
total of 15390 reflections were measured at a maximum 2q angle of
50.08, of which 4092 were independent reflections (Rint =0.0386). The
structure was solved by direct methods (SHELXS-97)[29] and refined by
the full-matrix least-squares on F2 (SHELEXL-97).[29] All the non-hydro-
gen atoms were refined anistropically and all hydrogen atoms were
placed by using AFIX instructions. The crystal data are as follows:
General procedure: Melting points were measured on a Yanaco MP-S3
instrument. 1H and 13C NMR spectra were recorded with a JEOL A-400
spectrometer (400 MHz for 1H, 100 MHz for 13C NMR). UV/Vis absorp-
tion spectra and fluorescence spectra measurements were performed at
room temperature with a Shimadzu UV-3150 spectrometer and a Hitachi
F-4500 spectrometer, respectively, in degassed spectral grade solvents.
Quantum yields were determined with a Hamamatsu C9920–01 calibrat-
ed integrating sphere system. TLC was performed on plates coated with
0.25 mm thick silica gel 60F-254 (Merck). Column chromatography was
performed by using PSQ 60B (Fuji Silysia). All experiments were carried
out under an argon atmosphere.
C58H56B2;
F
W =774.65; crystal size=0.20ꢂ0.20ꢂ0.20 mm3; monoclinic;
P21/a; a=11.285(3), b=16.755(2), c=12.332(3) ꢃ; b=93.3667(14)8; V=
2327.7(10) ꢃ3; Z=2; 1calcd =1.105 gcmꢀ3. The refinement converged to
R1 =0.0572, wR2 =0.1433 (I>2s(I)); GOF=1.084.
Computational methods: All calculations were conducted by using the
Gaussian 98 program.[27]
1,4-Bis(dimesitylboryl)-2,5-bis(4-methoxyphenylethynyl)benzene
(iPr)2NH/THF 1:3 (4 mL) degassed mixed solvent was added to a mix-
ture of 9 (62 mg, 0.1 mmol), 4-iodoanisole (70 mg, 0.3 mmol), [Pd(PPh3)4]
(2):
X-ray crystal structure analysis of compound 6:]28] Single crystals of 6
suitable for X-ray crystal analysis were obtained by recrystallization from
CH2Cl2/hexane. Intensity data were collected at 173 K on a Rigaku
Single Crystal CCD X-ray diffractometer (Saturn 70 with MicroMax-007)
with MoKa radiation (l=0.71070 ꢃ) and a graphite monochromater. A
total of 15383 reflections were measured at a maximum 2q angle of
50.08, of which 4143 were independent reflections (Rint =0.0558). The
structure was solved by direct methods (SHELXS-97)[29] and refined by
the full-matrix least-squares on F2 (SHELEXL-97).[29] All the non-hydro-
gen atoms were refined anistropically and all hydrogen atoms were
placed by using AFIX instructions. The crystal data are as follows:
C60H54B2N2; FW =824.67; crystal size=0.20ꢂ0.15ꢂ0.05 mm3; monoclinic;
P21/n; a=12.418(3), b=8.274(2), c=22.965(6) ꢃ; b=90.0707(16)8, V=
2359.5 (11) ꢃ3; Z=2; 1calcd =1.161 gcmꢀ3. The refinement converged to
R1 =0.0709, wR2 =0.1492 (I>2s(I)); GOF=1.149.
AHCTUNGTRENNUNG
(11.6 mg, 0.01 mmol), and CuI (3.9 mg, 0.02 mmol) at room temperature
under a stream of argon. The reaction mixture was stirred at 508C for
12 h. The solvents were removed under reduced pressure. After addition
of CHCl3, the mixture was washed successively with a 5% NH4OH aque-
ous solution, 1n HCl aqueous solution, and brine. The organic layer was
dried over anhydrous MgSO4, filtered, and concentrated. The resulting
mixture was subjected to silica-gel column chromatography (hexane/
CHCl3 2:1, Rf =0.32) to afford 44 mg (0.053 mmol) of 2 in 53% yield as a
green solid. M.p.>3008C; 1H NMR (CDCl3): d=7.42 (s, 2H), 6.91 (d,
J=8.4 Hz, 4H), 6.76 (s, 8H), 6.68 (d, J=8.4 Hz, 4H), 3.78 (s, 6H), 2.26
(s, 12H), 2.03 ppm (s, 24H); 13C NMR (CDCl3): d=159.3, 152.0, 142.6,
140.9, 139.1, 137.4, 133.0, 128.4, 125.9, 115.4, 113.4, 93.7, 89.1, 55.2, 23.3,
21.2 ppm; HRMS (FAB): m/z: calcd for C60H60B2O2: 834.4779; found:
834.4766.
1,4-Bis(dimesitylboryl)-2,5-bis(4-methylphenylethynyl)benzene (3): This
compound was prepared essentially in the same manner as described for
2 by using 9 (93 mg, 0.15 mmol), 4-iodotoluene (98 mg, 0.45 mmol), [Pd-
Acknowledgements
ACHTUNGTRENNUNG(PPh3)4] (17 mg, 0.015 mmol), and CuI (5.7 mg, 0.03 mmol) in (iPr)2NEt/
THF 1:3 (6 mL) at 508C. Purification by silica-gel column chromatogra-
phy (hexane/CHCl3 10:1, Rf =0.18) afforded 53 mg (0.066 mmol) of 3 in
44% yield as a green solid. M.p.>3008C; 1H NMR (CDCl3): d=7.48 (s,
2H), 6.99 (d, J=8.1 Hz, 4H), 6.91 (d, J=8.1 Hz, 4H), 6.79 (s, 8H), 2.32
This work was partly supported by Grants-in-Aid (no. 19675001 and
17069011) from the Ministry of Education, Culture, Sports, Science, and
Technology (Japan).
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Chem. Eur. J. 2009, 15, 10603 – 10612

Highly Emissive Diborylphenylene-Containing Bis(phenylethynyl)benzenes
FULL PAPER
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Received: April 2, 2009
Published online: September 8, 2009
10612
www.chemeurj.org
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 10603 – 10612