Synthetic control of spectroscopic and photophysical properties of triarylborane derivatives having peripheral electron-donating groups

Article abstract of DOI:10.1002/chem.201304207

The spectroscopic and photophysical properties of triarylborane derivatives were controlled by the nature of the triarylborane core (trixylyl- or trianthrylborane) and peripheral electron-donating groups (N,N-diphenylamino or 9H-carbazolyl groups). The triarylborane derivatives with and without the electron-donating groups showed intramolecular charge-transfer absorption/fluorescence transitions between the π orbital of the aryl group (π(aryl)) and the vacant p orbital on the boron atom (p(B), π(aryl)-p(B) CT), and the fluorescence color was tunable from blue to red by the combination of peripheral electron-donating groups and a triarylborane core. Detailed electrochemical, spectroscopic, and photophysical studies of the derivatives, including solvent dependences of the spectroscopic and photophysical properties, demonstrated that the HOMO and LUMO of each derivative were determined primarily by the nature of the peripheral electron-donating group and the triarylborane core, respectively. The effects of solvent polarity on the fluorescence quantum yield and lifetime of the derivatives were also tunable by the choice of the triarylborane core. Tunable fluorescence: The spectroscopic and photophysical properties of triarylborane derivatives can be controlled by the nature of the triarylborane core and peripheral electron-donating groups. The fluorescence color was tunable from blue to red owing to charge-transfer (CT) interactions between the triarylborane core and the peripheral electron-donating groups (see figure).

Full text of DOI:10.1002/chem.201304207

DOI: 10.1002/chem.201304207
Full Paper
&
Fluorescence
Synthetic Control of Spectroscopic and Photophysical Properties
of Triarylborane Derivatives Having Peripheral Electron-Donating
Groups
Akitaka Ito,*[a, c] Kazuyoshi Kawanishi,[b] Eri Sakuda,[a, b, d] and Noboru Kitamura*[a, b]
Chem. Eur. J. 2014, 20, 3940 – 3953
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Abstract: The spectroscopic and photophysical properties of
triarylborane derivatives were controlled by the nature of
the triarylborane core (trixylyl- or trianthrylborane) and pe-
ripheral electron-donating groups (N,N-diphenylamino or
9H-carbazolyl groups). The triarylborane derivatives with and
without the electron-donating groups showed intramolecu-
lar charge-transfer absorption/fluorescence transitions be-
tween the p orbital of the aryl group (p(aryl)) and the vacant
p orbital on the boron atom (p(B), p(aryl)–p(B) CT), and the
fluorescence color was tunable from blue to red by the com-
bination of peripheral electron-donating groups and a triaryl-
borane core. Detailed electrochemical, spectroscopic, and
photophysical studies of the derivatives, including solvent
dependences of the spectroscopic and photophysical prop-
erties, demonstrated that the HOMO and LUMO of each de-
rivative were determined primarily by the nature of the pe-
ripheral electron-donating group and the triarylborane core,
respectively. The effects of solvent polarity on the fluores-
cence quantum yield and lifetime of the derivatives were
also tunable by the choice of the triarylborane core.
Introduction
ical properties, a variety of triarylborane derivatives, including
polymers/dendrimers[8] and transition-metal complexes,[9] have
been hitherto synthesized for the development of lumines-
cent-,[10] optoelectronic- (OLED/EL, nonlinear optical, and two-
photon absorption),[11] and sensing materials,[12] and for other
applications.[13]
Highly emissive multicolor chromophores that emit in a broad
UV/Vis wavelength region are possible candidates for materials
applied to organic light-emitting diodes (OLEDs),[1] and imag-
ing/sensing devices;[2] therefore, their exploitation and devel-
opment are of primary importance. We have focused on triaryl-
borane derivatives as a promising material.[3] A class of these
derivatives possesses both a planar sp2-hybridized structure
and the vacant p orbital on the central three-coordinate boron
atom (p(B)) in the electronic ground state.[4] Owing to such
electronic structures, triarylborane derivatives with relatively
large p-electron systems show characteristic absorption and
fluorescence that largely differ from those of the aryl group
itself. Tri(9-anthryl)borane, a representative compound report-
ed by Yamaguchi et al. in 2000,[5] shows new absorption/fluo-
rescence bands in the visible region in addition to those of an-
thracene itself. The new absorption/fluorescence bands ob-
served for tri(9-anthryl)borane are ascribed to the intramolecu-
lar charge transfer (CT) transitions from the p orbital of the an-
thryl group, p(aryl), to p(B), p(aryl)–p(B) CT, as demonstrated
by solvent dependences of the absorption/fluorescence[6] and
electroabsorption/electrofluorescence spectra of the deriva-
tive.[7] Owing to such fascinating spectroscopic and photophys-
The electron-accepting ability of p(B) will make the excited-
state characteristics of a triarylborane controllable by the intro-
duction of an electron-donating group(s) to the periphery of
the aryl group(s) in the triarylborane.[14] One remarkable exam-
ple is tris{[p-(N,N-dimethylamino)phenylethynyl]duryl}borane
(TMAB) having three N,N-dimethylamino groups in the periph-
ery of the tridurylborane core.[15] It is worth emphasizing that
TMAB shows an extremely large fluorescence solvatochromic
shift from blue to red, owing to the electric dipole moment in
the singlet excited state, as large as ~60 D (1 D=
10ꢀ18 esucm).[16] Such a large excited-state dipole moment is
due to the presence of the peripheral electron-donating N,N-
dimethylamino groups; for the fluorescent excited state of
TMAB, it has been demonstrated that almost one electron is
transferred from the N,N-dimethylamino group to p(B).[16] Be-
cause introduction of electron-donating groups to the periph-
ery of a triarylborane core brings about large effects on the
spectroscopic and excited-state properties of the derivative, as
observed for TMAB, further systematic study on a series of de-
rivatives is worth conducting to develop new materials in the
relevant research fields. Herein, we report the synthesis and ex-
cited-state properties of a series of triarylboranes having elec-
tron-donating groups, 1a–c and 2a–c, whose structures are
shown in Scheme 1. To control the spectroscopic and photo-
physical properties of the triarylborane, we employed trixylyl-
borane (1a–c) and trianthrylborane units (2a–c) as relatively
small and large p-electron systems, respectively, and intro-
duced three electron-donating 9H-carbazolyl (1b, 2b) or N,N-
diphenylamino groups (1c, 2c) to the periphery of the triaryl-
borane core. Tri(2-m-xylyl)borane (1a) and tri(9-anthryl)borane
(2a) were also studied as references. We report the spectro-
scopic and photophysical properties of these six triarylborane
derivatives in special references to the solvent-polarity depend-
ences of these characteristics.
[a] Dr. A. Ito, Dr. E. Sakuda, Prof. Dr. N. Kitamura
Department of Chemistry
Faculty of Science, Hokkaido University
Kita-10, Nishi-8, Kita-ku, Sapporo 060-0810 (Japan)
Fax: (+81)11-706-4630
E-mail: akito@sci.osaka-cu.ac.jp
kitamura@sci.hokudai.ac.jp
[b] K. Kawanishi, Dr. E. Sakuda, Prof. Dr. N. Kitamura
Department of Chemical Sciences and Engineering
Graduate School of Chemical Sciences and Engineering
Hokkaido University
Kita-10, Nishi-8, Kita-ku, Sapporo, 060-0810 (Japan)
[c] Dr. A. Ito
Department of Chemistry
Graduate School of Science, Osaka City University
3-3-138, Sugimoto, Sumiyoshi-ku, Osaka 558-8585 (Japan)
[d] Dr. E. Sakuda
PRESTO (Japan) Science and Technology Agency (JST)
4-1-8, Honcho, Kawaguchi, Saitama 332-0012 (Japan)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201304207.
Chem. Eur. J. 2014, 20, 3940 – 3953
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Scheme 1. Molecular structures of 1a–c and 2a–c.
Figure 1. Cyclic (solid curves) and differential pulse voltammograms (broken
curves) of 1a–c and 2a–c. Dichloromethane and THF were used as solvents
in the positive and negative potential regions, respectively.
Results and Discussion
Electrochemical properties
idation waves at ca. +0.5 and +0.9 V (versus SCE), respectively,
and that these waves are due to the oligomerization of the
carbazole groups, as reported for 1b by Lambert et al.[17] Be-
cause this behavior is not relevant to the investigation herein,
the data on these irreversible oxidation waves, as observed for
1b and 2b, are not included in Table 1 and this behavior will
not be commented on further.
To confirm the introduction of the electron-donating 9H-carba-
zolyl (1b, 2b) or N,N-diphenylamino groups (1c, 2c) into a tri-
xylylborane (1a–c) or trianthrylborane core (2a–c), we studied
the redox potentials of the derivatives. Figure 1 shows the
cyclic (CV) and differential pulse voltammograms (DPV) of the
derivatives. The voltammograms in the positive and negative
potential regions were measured separately by employing di-
chloromethane and tetrahydrofuran (THF), respectively, as sol-
vents. The redox potentials determined are summarized in
Table 1. It is worth noting that 1b and 2b show irreversible ox-
The reduction waves (E1/2(red)) observed for 1b and 2a–
c are ascribed to those of the triarylborane cores; the trixylyl-
borane and trianthrylborane cores exhibited E1/2(red) at ꢀ1.94
and in the range, ꢀ1.25–ꢀ1.45 V, respectively. On the other
hand, 1a showed an oxidation wave (E1/2(ox1)) at +1.79 V,
while 2a exhibited three consecutive oxidation waves at
Table 1. Redox potentials of 1a–c and 2a–c in dichloromethane and THF
(0.1m TBAPF6) in the positive and negative potential regions, respectively.
E
1/2(ox1, ox2, ox3)= +1.23, +1.39, and +1.57 V, correspond-
ing to the oxidation of the three anthryl groups. The potentials
of the first three oxidation waves of 1c (E1/2(ox1, ox2, ox3)=
+0.92, +1.05, and +1.17 V) were almost comparable with
those of 2c (E1/2(ox1, ox2, ox3)= +0.93, +1.04, and +1.17 V),
indicating the sequential oxidation of the N,N-diphenylamino
groups in 1c similar to that of the anthryl groups in 2a. Be-
cause the oxidation potentials observed for 1b (E1/2(ox3)= +
1.75 V) and 1c (E1/2(ox4)= +1.76 V) were very similar to that
of 1a (E1/2(ox1)= +1.79 V), the potentials correspond to the
Derivative
E1/2 [V] vs. SCE
red
N.D.[a]
ox1
ox2
ox3
ox4
ox5
1.72
1a
1b
1c
2a
2b
2c
1.79
1.19
0.92
1.23
1.24
0.93
ꢀ1.94
1.30
1.05
1.39
1.36
1.04
1.75
1.17
1.57
1.60
1.17
N.D.[a]
1.76
1.52
ꢀ1.45
ꢀ1.25
ꢀ1.26
[a] Potential more negative than ꢀ2.0 V vs. SCE.
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oxidation of the xylyl group. On the other hand, the oxidation
behavior of 2b was somewhat different from those of other
derivatives and the E1/2(ox1, ox2, ox3) values of +1.24, +1.36,
and +1.60 V were not in accord with the oxidation potentials
of the 9H-carbazolyl groups in 1b (+1.19 and +1.30 V), but
similar to those of the anthryl groups in 2a (+1.23, +1.39,
and +1.57 V). Because our DFT calculations (see below) indi-
cate electron delocalization among the anthryl and carbazolyl
groups in the highest-energy occupied molecular orbital
(HOMO) in 2b, the oxidation potentials observed for 2b would
be the admixture of those of the anthryl and carbazolyl
groups.
would vary by the nature of both the electron-donating group
and the triarylborane core, a dependence that will be de-
scribed in detail in the following sections.
Absorption and fluorescence spectra in THF
We studied the absorption and fluorescence spectra of the six
derivatives in a series of solvents; the spectroscopic properties
of the derivatives are summarized in Tables 2 and 3. Typically,
the absorption and fluorescence spectra of the derivatives in
THF are shown in Figure 2. All of the derivatives exhibited sev-
eral absorption bands in the range 17000–37000 cmꢀ1 (270–
590 nm) and the spectra were characterized by broad lowest-
energy and relatively structured higher-energy bands. The
higher-energy absorption band observed for each derivative
can be ascribed to the p–p* transition of the aryl group as
judged by the close similarities of the absorption energy and
spectral band shapes with those of the aryl group itself; the
spectral comparison between 1b, 2a, 2c, and the relevant
model compounds can be found in the Supporting Informa-
tion, Figures S1–S3. To elucidate the nature of the broad
lowest-energy absorption bands of 1b, 2a, and 2c, we studied
Although the nature of the E1/2(ox1) value of 2b is ambigu-
ous, as described above, the negative shift of E1/2(ox1) in the
sequence 1b (+1.19 V)>1c (+0.92 V) demonstrates clearly
that the electron-donating ability of the N,N-diphenylamino
group is higher than that of the 9H-carbazolyl group. Further-
more, the E1/2(red) value is in the range ꢀ1.25–ꢀ2.0 V, depend-
ing on the nature of the triarylborane core. Because E1/2(ox1)
and E1/2(red) correspond to the HOMO and lowest-energy un-
occupied molecular orbital (LUMO) energies, respectively, the
absorption and fluorescence characteristics of the derivatives
Table 2. Solvent parameters and spectroscopic properties of 1a–c.
Solvent Parameters
n[a] f(Ds,n)[b]
1a
1b
1c
Solvent
Ds[a]
g(n)[b]
na (e)
nf
na (e)
nf
na (e)
nf
[cmꢀ1 (103 mꢀ1 cmꢀ1)] [cmꢀ1
]
[cmꢀ1 (103 mꢀ1 cmꢀ1)] [cmꢀ1
]
[cmꢀ1 (103 mꢀ1 cmꢀ1)] [cmꢀ1
]
n-hexane
cyclohexane
toluene
chloroform
ethyl acetate
THF
dichloromethane
1,2-dichloroethane 10.36
acetone
CH3CN
DMSO
1.8799 1.37226 ꢀ0.00078 0.25328 31000 (11.3)
2.023
1.42623 ꢀ0.00259 0.28920 30700 (9.8)
2.3779 1.49413
27400 26100 (36.7)
27300 26000 (36.1)
27200 26300 (31.5)
27000 26700 (29.3)
27000 26900 (29.9)
27000 26700 (28.2)
26900 26900 (29.4)
26900 26900 (31.0)
26500 27100 (33.6)
26700 27200 (N.D.)[d]
26500 26900 (27.2)
24400 24500 (64.4)
24300 24400 (58.6)
23300 24400 (51.0)
22900 24600 (49.4)
21900 24700 (49.5)
21700 24500 (53.9)
22100 24600 (47.4)
22000 24500 (45.8)
21000 24700 (47.4)
20600 insoluble[d]
20300 24500 (47.1)
22800
22700
22000
21300
20700
20600
20500
20300
19600
19100
19000
0.03041 0.33358 30700 (11.3)
0.37211 0.30021 30600 (11.3)
0.49026 0.25163 30700 (10.3)
0.55004 0.27511 30800 (11.9)
0.59155 0.28584 30700 (10.5)
0.62301 0.29966 30600 (10.3)
0.79124 0.24234 competing[c]
0.86305 0.23435 30700 (9.6)
0.84102 0.32267 30500 (8.4)
4.806
6.02
7.58
8.93
1.44293
1.36979
1.40496
1.42115
1.4421
20.7
37.5
46.68
1.35596
1.34411
1.4773
[a] Data taken from ref. [34]. [b] Calculated by using Equations (1). [c] Could not be determined due to superposition of absorption by solvent itself.
[d] Could not be determined due to the low solubility of the derivative.
Table 3. Spectroscopic properties of 2a–c.
2a
2b
2c
Solvent
na (e)
nf
[cmꢀ1
na (e)
nf
[cmꢀ1
na (e)
nf
[cmꢀ1
[cmꢀ1 (103 mꢀ1 cmꢀ1)]
]
[cmꢀ1 (103 mꢀ1 cmꢀ1)]
]
[cmꢀ1 (103 mꢀ1 cmꢀ1)]
]
n-hexane
cyclohexane
toluene
chloroform
ethyl acetate
THF
dichloromethane
1,2-dichloroethane
acetone
21400 (26.7)
21300 (25.2)
21100 (23.2)
21200 (20.3)
21300 (20.9)
21300 (21.3)
21100 (19.6)
21200 (20.3)
21300 (20.2)
21300 (21.2)
21100 (21.0)
19500
19300
19000
18600
18700
18500
18300
18200
18000
17800
17600
20500 (N.D.)[a]
20500 (N.D.)[a]
20300 (N.D.)[a]
20500 (N.D.)[a]
20500 (N.D.)[a]
20500 (25.2)
20400 (N.D.)[a]
20300 (N.D.)[a]
20500 (N.D.)[a]
20500 (N.D.)[a]
20000 (N.D.)[a]
18500
18400
18000
17800
17700
17600
17600
17500
17300
17000
decomp.[b]
19400 (19.4)
19300 (36.0)
19100 (32.7)
19300 (28.9)
19300 (32.2)
19300 (32.5)
19300 (31.7)
19200 (31.5)
19300 (31.5)
insoluble[c]
17400
17400
16800
16500
16200
16100
15800
15800
15500
15200
14900
CH3CN
DMSO
19000 (27.2)
[a] Could not be determined due to the low isolation yield of the derivative. [b] Could not be determined due to decomposition of the derivative.
[c] Could not be determined due to the low solubility of the derivative.
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band observed for the carbazolyl-type derivative 1b or 2b (see
also Tables 2 and 3). These characteristics are due to the stron-
ger electron-donating ability of the N,N-diphenylamino group
in 1c or 2c relative to that of the 9H-carbazolyl group in 1b or
2b, thus agreeing very well with the oxidation potential of the
N,N-diphenylamino or 9H-carbazolyl group, as discussed
above. The lower-energy shifts of the p(aryl)–p(B) CT absorp-
tion bands of the 9H-carbazolyl- and N,N-diphenylamino-type
derivatives compared to that of 1a or 2a is due to destabiliza-
tion of the ground state by the presence of the lone-pair elec-
trons on the nitrogen atoms as well as stabilization of the ex-
cited-states of 1b, 1c, 2b, or 2c (a more detailed discussion
on the basis of the solvent polarity dependences of the ab-
sorption and fluorescence spectra of the derivatives is given
below). The increase in the e value of the p(aryl)–p(B) CT band
of the 9H-carbazolyl- (1b, 2b) or N,N-diphenylamino-type de-
rivative (1c, 2c) compared to that of 1a or 2a is due essential-
ly to the presence of the electron-donating groups and, thus,
to charge transfer from the peripheral electron-donating group
to the vacant p orbital on the boron atom.
All of the derivatives showed broad and structureless fluo-
rescence spectra, which were completely different from those
of the relevant model compounds, as shown in Figure 2 and
Figures S1–S3 in the Supporting Information. Fluoride-addition
experiments, as illustrated in Figures S4–S6 in the Supporting
Information, have revealed that the fluorescence observed for
the derivatives comes from the p(aryl)–p(B) CT excited states.
In THF, the fluorescence maximum energies (nf) of the 9H-car-
bazolyl- (1b and 2b: 21700 and 17600 cmꢀ1 (461 and
568 nm), respectively) and N,N-diphenylamino-type derivatives
(1c and 2c: 20600 and 16100 cmꢀ1 (485 and 621 nm), respec-
tively) were much lower than that of the relevant parent mole-
cule (1a or 2a: 27000 and 18500 cmꢀ1 (370 and 541 nm), re-
spectively) without an electron-donating group; see also
Tables 2 and 3. Such behaviors are quite similar to those for
the p(aryl)–p(B) CT absorption band and will be explained by
destabilization and stabilization of the ground- and excited-
state energies, respectively, and the redox potential of the de-
rivative. The fluorescence energies of the derivatives in THF
within the range 16100–27000 cmꢀ1 (370–621 nm) demon-
strate control of the fluorescence energy in the visible region
by the nature of the electron-donating substituents in a trixylyl-
borane- or trianthrylborane-core unit. Detailed photophysical
properties of the derivatives will be discussed in sections
below.
Figure 2. Absorption (broken curves) and fluorescence spectra (solid curves)
of 1a–c (upper panel) and 2a–c (lower panel) in THF. The colors correspond
to those indicated in Figure 1.
the effects of a fluoride ion (derived from tetra-n-butylammoni-
um fluoride, TBAF) on the absorption spectra (see the Support-
ing Information, Figures S4–S6). A fluoride ion is known to act
as a Lewis base for p(B) and coordinates with the boron atom
in a triarylborane derivative. As seen in Figures S4–S6 in the
Supporting Information, the lowest-energy absorption bands
of 1b, 2a, and 2c in THF with energies of 26700, 21300, and
19300 cmꢀ1 (375, 469, and 518 nm), respectively, were attenu-
ated upon addition of TBAF and the absorption spectra in the
presence of sufficient amounts of TBAF agreed very well with
those of the relevant model compounds, 9H-(2-m-xylyl)carba-
zole, anthracene, and 9-N,N-diphenylaminoanthracene. Similar
TBAF effects on the absorption spectra of 1a, 1c, and 2b were
also confirmed. The results demonstrate that the lowest-
energy absorption bands of these triarylboranes are strongly
influenced by p(B) and we confidently assign the bands to
p(aryl)–p(B) CT transitions.
Time-dependent density-functional-theory (TD-DFT)
calculations
The absorption maximum energy of the p(aryl)–p(B) CT
band (na) observed for 1c or 1b, 24500 or 26700 cmꢀ1 (408 or
375 nm), respectively, in THF, the compound having N,N-diphe-
nylamino or 9H-carbazolyl groups, respectively, is lower than
that of 1a (30800 cmꢀ1 (325 nm)). The molar absorption coeffi-
cients of the bands (e) for 1c and 1b are also larger than that
of 1a in THF, the values being 53900, 28200, and
11900mꢀ1 cmꢀ1, respectively. Furthermore, 1c or 2c, which has
N,N-diphenylamino groups, showed more intense (that is,
larger e value) and lower-energy absorption compared to the
To investigate the electronic structures in the ground and ex-
cited states of the six derivatives, we conducted TD-DFT calcu-
lations. The optimized molecular structures of the derivatives
are shown in Figure S7 in the Supporting Information. All the
derivatives possess planar BC3 core structures; the results are
indicative of no significant effects of the triarylborane core and
peripheral electron-donating groups on the ground-state geo-
metries of the derivatives. The details of the five lowest-energy
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Figure 3. Molecular-orbital contours (0.03 eꢁꢀ3) of 1a–c and 2a–c in HOMO (bottom) and LUMO (top) obtained by DFT calculations.
absorption transitions of the derivatives are summarized in
Tables S1–S6 in the Supporting Information. The lowest-energy
absorption transition in each derivative is best characterized
by a transition from the HOMO to the LUMO. The molecular-
orbital contours of the derivatives in the HOMO and LUMO
levels are shown in Figure 3. In the LUMO, the electron is local-
ized primarily on the vacant p orbital on the boron atom, irre-
spective of the derivative; however, the HOMO distributes in
different fashions among the six derivatives. The HOMO elec-
tron in 1a or 2a without an electron-donating group, distrib-
utes among the three p orbitals of the aryl groups. In the case
of 1b, 1c, or 2c, on the other hand, the electron resides on
the electron-donating groups whereas that in 2b almost deloc-
alizes over the one carbazolyl–anthracene unit. The results on
2b suggest mixing of the p orbitals of the anthryl and carba-
zolyl groups, thus agreeing very well with the results of the
electrochemical measurements described above. These results
demonstrate that the introduction of the three electron-donat-
ing groups to the triarylborane core greatly influences the
electronic structures of the derivative.
the spectroscopic/photophysical properties and electronic
structures of the derivatives.
Solvent dependences of spectroscopic and photophysical
properties
We studied solvent dependences of the spectroscopic and
photophysical properties of the derivatives to evaluate the
roles of the peripheral electron-donating group in the ground-
and excited-state characteristics. Figure 4 shows the absorption
and fluorescence spectra of the derivatives in a series of the
solvents, and their spectroscopic properties are summarized in
Tables 2 and 3, together with the static dielectric constants (Ds)
and refractive indices (n) of the solvents used.
The trixylylborane- (1a–c) and trianthrylborane-type deriva-
tives (2a–c) showed opposite solvent polarity effects on the
p(aryl)–p(B) CT absorption band. The p(aryl)–p(B) CT absorption
bands of 2a–c showed bathochromic shifts with an increase in
the solvent polarity. Such solvent effects are often observed for
an intramolecular CT-type compound owing to stabilization of
the charge-separated Franck–Condon excited-state energy. In
contrast, 1a–c showed hypsochromic shifts of the p(aryl)–p(B)
CT absorption bands with an increase in Ds, suggesting larger
stabilization of the ground-state energy relative to the stabili-
zation energy in the excited state. A homoleptic triarylborane
in the ground state generally possesses D3-symmetry and the
highly symmetric structure in the ground state would not be
affected largely by a solvent. Therefore, the solvent-polarity-in-
duced hypsochromic shifts of the CT absorption bands of 1a–
c will be due to symmetry-breaking structures in the ground
states. While the solvent polarity effects on the p(aryl)–p(B) CT
absorption band were dependent on the nature of a triarylbo-
rane core, the fluorescence spectra of the derivatives showed
similar solvent-polarity dependences. The fluorescence maxi-
mum energies of the derivatives (nf) shifted to the lower
energy (that is, longer wavelength) with an increase in the sol-
vent polarity. Such solvent effects have been sometimes ob-
served for an intramolecular CT-type compound and are due
The decrease in the calculated HOMO–LUMO transition
energy in the order 1a>1b>1c or 2a>2b>2c agrees very
well with the electron-donating ability of the peripheral group
(E1/2(ox1): 1b (+1.19 V)>1c (+0.92 V) or 2b (+1.24 V)>2c (+
0.93 V)) and shows a quite good linear relationship with the
experimentally observed na values in THF, as shown in Fig-
ure S8 in the Supporting Information. The slope value of 1.11
between theoretical and experimental absorption energies
supports that the calculations successfully reproduce the ex-
perimental observations because TD-DFT calculations are
known to estimate the transition energy smaller than experi-
mental value by 10%.[18] The calculated oscillator strength
(f(calculated)) of the lowest-energy absorption transition
(Tables S1–S6 in the Supporting Information) also relates pro-
portionally to the molar absorption coefficient of the p(aryl)–
p(B) CT band in THF (Figure S8 in the Supporting Information).
These results indicate that the introduction of electron-donat-
ing groups into the triarylborane core should influence largely
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Figure 4. Solvent dependences of the absorption (broken curves) and fluorescence spectra (solid curves) of 1a–c and 2a–c. The fluorescence intensities are
normalized to those at the maximum energies. The vertical axes of the absorption spectra of 1a–c, 2a, and 2c represent the molar absorption coefficients,
and absorbance of 2b is normalized to those at the maximum energies of the p(aryl)–p(B) CT bands.
ꢀ
ꢁ
2n2 þ 1 Ds ꢀ 1 n2 ꢀ 1
to stabilization of the fluorescent CT excited-state energy. In
the present case, the solvatochromic shift of nf from n-hexane
to dimethyl sulfoxide (DMSO), Dnf, was dependent on the
nature of the electron-donating groups (see Tables 2 and 3). In
practice, the Dnf values of 1b, 1c, and 2c were ꢀ4100, ꢀ3800,
and ꢀ2500 cmꢀ1, respectively, thus showing larger solvent po-
larity shifts than that of the parent derivative, 1a or 2a (ꢀ1100
or ꢀ1900 cmꢀ1, respectively). The results indicate that the de-
rivatives having the peripheral electron-donating groups pos-
sess larger electric dipole moments in the excited states and
their excited states are stabilized in energy largely compared
to the derivatives without an electron-donating group. One ex-
ceptional case was 2b possessing electron-donating 9H-carba-
zolyl groups and 2b showed a comparable fluorescence shift
to 2a, Dnf =ꢀ1500 cmꢀ1 (from n-hexane to CH3CN). It will be
due to electron delocalization among the carbazolyl and an-
thryl groups in the ground state of 2b as described above.
To investigate further the molecular and electronic structures
of 1a–c and 2a–c, we evaluated the ground- (mg) and excited-
state dipole moments (me) of the derivatives. The mg and me
values of a molecule can be determined by the solvent-polarity
dependences of the absorption and fluorescence spectra on
the basis of the simple quantum-mechanical second-order per-
turbation theory by Kawski et al. by using the following solvent
polarity parameters, f(Ds,n) and g(n),[19]
ð1aÞ
ð1bÞ
fðDs; nÞ ¼
ꢀ
n2 þ 2 Ds þ 2 n2 þ 2
ꢀ
ꢁ
3
2
n4 ꢀ 1
gðnÞ ¼
2
ðn2 þ 2Þ
where the terms involving Ds and n imply the contributions of
the dipole orientation and electronic polarizability of a solvent
molecule, respectively, to solvent-induced stabilization of the
ground- and excited-state energies of a molecule concerned.
The f(Ds,n) and g(n) values used in the present study are in-
cluded in Table 2. Furthermore, it has been known that the
(naꢀnf) and (na +nf) values should correlate linearly with the
solvent parameters as given in Equations (2a) and (2b), respec-
tively:
naꢀnf ¼ m1fðDs,nÞ þ constant
ð2aÞ
ð2bÞ
na þ nf ¼ ꢀm2½fðDs,nÞ þ 2gðnÞꢁ þ constant
where m1 and m2 are the slope values of the relations in Equa-
tions (2a) and (2b), respectively. The mg and me values can be
then evaluated by Equations (3):
sffiffiffiffiffiffiffiffiffiffi
m2 ꢀ m1 hca3O
ð3aÞ
mg ¼
2
2m1
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(mg =0.36–0.80 D), 1a–c showed
the relatively large (1.7 D for 1a)
and/or negative dipole moments
in the ground states (ꢀ2.9 or
ꢀ0.51 D for 1b or 1c, respec-
tively) compared to 2a–c. The
results for 1b and 1c suggest
non-symmetrical
ground-state
structures of the derivatives pos-
sessing the trixylylborane-core
unit and are consistent with the
solvent dependences of the ab-
sorption spectra (hypsochromic
solvent polarity shift). On the
other hand, the excited-state
Figure 5. Solvent-parameter dependences of the spectroscopic properties of 1a–c and 2a–c. The colors corre-
spond to those indicated in Figure 1 and the data are taken from Tables 2 and 3. Solid lines correspond to the
linear regression based on Equations (2).
sffiffiffiffiffiffiffiffiffiffi
m2 þ m1 hca3O
dipole moment, me, was significantly influenced by the intro-
duction of the electron-donating groups to the triarylborane
core. In practice, 1b, 1c, and 2c in the excited states possess
large and comparable dipole moments with me =11—13 D, in-
dicative of charge transfer from the peripheral electron-donat-
ing group(s) to the vacant p orbital on the boron atom. How-
ever, the me values of 2a (8.0 D) and 2b (8.5 D) were somewhat
smaller than those of other derivatives and, in particular, the
electron-donating 9H-carbazolyl groups in 2b played a minor
role in determining me. Because the oxidation potentials of the
anthryl and carbazolyl groups in 2b comparable to that of 2a
are very close to each other and the p orbitals of the anthryl
and carbazolyl group are mixed, as suggested by the TD-DFT
calculations described above, the me value of 2b will be the
reasonable consequence.
ð3bÞ
me ¼
2
2m1
where h, c, and aO are the Planck’s constant, the speed of light,
and the Onsager radius of a molecule, respectively. As shown
in Figure 5, the solvent dependences of the (naꢀnf) and (na +nf)
values followed Equations (2) (correlation coefficient >0.93).
The slope values of the plots in Figure 5 and the mg/me values
of each derivative calculated by Equations (3) are summarized
in Table 4, together with the aO values predicted from the
ground-state geometries by DFT calculations. The minus sign
of mg represents the dipole moment with the reverse vector di-
rection from that of me. While the ground-state dipole mo-
ments of 2a–c were almost comparable with one another
Solvent dependences of the photophysical properties of the
derivatives were also evaluated except for those of 1a, which
could not be excited by the present experimental setup
(405 nm laser pulses). The fluorescence decay profiles of the
derivatives shown in Figures S9–S13 in the Supporting Infor-
mation can be analyzed by single-exponential functions, and
the fluorescence quantum yields (Ff) and lifetimes (tf) deter-
mined are summarized in Tables 5 and 6, together with the
fluorescence (kf) and nonradiative decay rate constants (kd) cal-
culated by the equations, Ff =kf/(kf+kd) and tf =1/(kf+kd),
where kd is the sum of the rate constants of internal conver-
Table 4. Solvent-dependent parameters (m1 and m2) and ground-/excit-
ed-state electric dipole moments of 1a–c and 2a–c.
Derivative
aO
[ꢁ]
m1
[cmꢀ1
m2
[cmꢀ1
mg
me
]
]
[D]
[D]
1a
1b
1c
2a
2b
2c
5.95
7.35
7.73
6.76
7.91
7.92
600
1180
3100
3710
2070
1460
2630
+1.7
5.3
11
13
8.0
8.5
11
5190
4000
1700
1340
2310
ꢀ2.9
ꢀ0.51
+0.80
+0.36
+0.73
Table 5. Solvent dependences of the photophysical properties of 1b and 1c.
1b
1c
Solvent
Ff
tf
kf
kd
[107 sꢀ1
Ff
tf
kf
kd
[107 sꢀ1
[ns]
[107 sꢀ1
]
]
[ns]
[107 sꢀ1
]
]
n-hexane
cyclohexane
toluene
chloroform
ethyl acetate
THF
dichloromethane
1,2-dichloroethane
acetone
0.53
0.58
0.58
0.70
0.60
0.95
0.61
0.69
0.90
0.94
0.82
2.1
1.9
2.6
3.2
5.6
6.0
4.9
5.0
9.2
11
25
30
22
22
11
16
12
14
9.8
8.6
8.0
23
22
16
0.39
0.38
0.36
0.48
0.45
0.53
0.51
0.45
0.53
0.52
0.66
2.0
2.1
2.1
3.0
4.5
4.4
4.8
5.0
7.7
10
20
18
17
16
10
12
11
8.9
6.9
5.2
7.6
31
29
30
17
12
11
10
11
6.1
4.7
3.9
9.3
7.2
0.77
8.0
6.2
1.1
0.55
1.8
CH3CN
DMSO
10
8.7
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Table 6. Solvent dependences of the photophysical properties of 2a–c.
2a
2b
2c
Solvent
Ff
tf
kf
kd
[107 sꢀ1
Ff
tf
kf
kd
[107 sꢀ1
Ff
tf
kf
kd
[107 sꢀ1
[ns]
[107 sꢀ1
]
]
[ns]
[107 sꢀ1
]
]
[ns]
[107 sꢀ1
]
]
n-hexane
cyclohexane
toluene
chloroform
ethyl acetate
THF
dichloromethane
1,2-dichloroethane
acetone
0.47
0.53
0.21
11
12
4.2
4.5
2.9
2.5
2.1
2.2
2.0
2.1
1.5
1.5
1.7
4.8
4.0
11
0.39
0.50
0.17
0.12
0.062
0.093
0.059
0.056
0.037
decomp.[a]
decomp.[a]
9.4
10
4.2
5.0
2.9
3.0
2.4
2.4
2.4
2.7
2.2
6.5
5.0
14
22
36
23
38
45
57
0.38
0.45
0.23
7.1
8.1
4.7
2.2
1.7
1.9
1.0
1.0
5.3
5.6
4.9
3.1
2.8
3.6
1.9
2.8
1.5
8.8
6.7
16
42
58
48
98
98
127
7.2
3.2
3.5
4.3
2.3
1.8
1.6
1.1
1.3
5.9
4.0
2.6
3.9
2.5
2.1
1.7
0.081
0.075
0.093
0.045
0.037
0.024
0.016
0.022
29
26
21
42
54
61
89
75
0.070
0.047
0.069
0.019
0.028
0.012
0.010
0.084
0.78
decomp.[a]
decomp.[a]
CH3CN
DMSO
decomp.[a]
decomp.[a]
[a] Could not be determined due to decomposition of the derivative.
sion to the ground state and intersystem crossing to the excit-
ed triplet state. All of the derivatives showed relatively intense
fluorescence, Ff >0.1 for 1b–c in all of the solvents studied
and 2a–c in less polar solvents than toluene.
As one of the important findings, 1b–c and 2a–c exhibited
different trends of the solvent polarity effects on Ff and tf.
With an increase in the solvent polarity, the Ff and tf values of
1b–c having the trixylylborane-core unit increased (for exam-
ple, Ff =0.39 and tf =2.0 ns in n-hexane, Ff =0.53 and tf =
7.7 ns in acetone for 1c), while those of 2a–c with the trian-
thrylborane core remarkably decreased (for example, Ff =0.38
and tf =7.1 ns in n-hexane, Ff =0.012 and tf =0.78 ns in ace-
tone for 2c). It has been known that the tf and Ff values of
a CT-type compound in general decrease with an increase in
Ds because the singlet excited-state energy is lowered in
a high-polarity solvent, thus giving rise to fast nonradiative
decay to the ground state, as predicted by the energy-gap (nf)
law of a nonradiative-decay rate constant (kd): lnkd =ꢀaꢂnf +
constant.[20] In practice, energy-gap dependences of kd have
been reported for various organic compounds and transition-
metal complexes.[21] Therefore, the solvent dependences of the
photophysical properties observed for 1b–c are quite unusual
as expected from those of ordinary organic compounds. It is
worth emphasizing here that such anomalous solvent effects
on Ff and tf (and, thus, kd) are neither fortuitous nor due to
our experimental error because similar solvent effects have
been also observed for tris{[p-(N,N-dimethylamino)phenylethy-
nyl]duryl}borane (TMAB) by the present authors[16] as well as
for 1b by Lambert et al.[17]
Figure 6 shows the energy-gap (nf) dependences of lnkd ob-
served for the derivatives. As clearly seen in the figure, the lnkd
values of 2a–c linearly decrease with an increase in nf, while
those of 1b–c exhibit opposite trends to what is observed for
2a–c. In the case of 2a–c, the Ff and tf values are determined
primarily by kd; kf <kd, except for the values of 2a in n-hexane
and cyclohexane. In a polar solvent, the CT excited states of
these derivatives are stabilized in energy and undergo fast
nonradiative decay to the ground state as in the case of a typi-
cal CT excited state. In the case of 1b and 1c, on the other
hand, the kd values decrease by ~10-fold on going from n-
Figure 6. Energy gap plots for 1a–c and 2a–c. The colors correspond to
those indicated in Figure 1 and the data are taken from Tables 2, 3, 5, and 6.
hexane to DMSO and the kf/kd ratios become larger with an in-
crease in the solvent polarity, as seen in Table 5. Therefore, the
Ff and tf values of 1b and 1c become larger and longer, re-
spectively, with an increase in Ds. The nonradiative reverse-CT
process from the excited CT state of 1b or 1c having me of
+11 or +13 D, respectively, to the ground state with the op-
posite dipole moment (mg =ꢀ2.9 or ꢀ0.51 D, respectively)
should accompany both relatively large structural change and
solvent reorganization energy and, thus, these factors will
impede the nonradiative decay process, showing reverse
energy-gap dependence of lnkd. Such ground- and excited-
state characteristics will be the origin of the reverse energy-
gap dependence of kd observed for 1b and 1c. We are cur-
rently studying more detailed solvent dependences of the
photophysical properties of 1b–c, which will be reported in
a separate publication.
Conclusion
The present study demonstrated that the spectroscopic and
photophysical properties of a triarylborane derivative were
controllable by the nature of a triarylborane core (tri(2-m-xylyl)-
borane or tri(9-anthryl)borane) and peripheral electron-donat-
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ing groups (N,N-diphenylamino or 9H-carbazolyl groups). All of
the six derivatives showed the p(aryl)–p(B) CT absorption/fluo-
rescence, and the fluorescence wavelength was successfully
tuned over the visible region by the combination of the pe-
ripheral electron-donating groups and a triarylborane core. The
detailed electrochemical, spectroscopic and photophysical
measurements on the derivatives provided several important
insights into the control of the spectroscopic and excited-state
characteristics.
Experimental Section
Chemicals
Tris[2-{5-(9H-carbazolyl)-m-xylyl}]borane (1b) was synthesized ac-
cording to the literature[17] and identified by IR, H NMR, and ele-
1
mental analysis.[22] Derivative 2a was the same sample with that re-
ported earlier.[6–7,23] Acetonitrile (Wako Pure Chemical Industries) for
spectroscopic measurements was distilled prior to the use.[24] Other
spectroscopic-grade solvents purchased from Wako Pure Chemical
Industries, Kanto Chemical Company, or Dojindo Molecular Tech-
nologies were used without further purification. As the solvents for
electrochemical measurements, anhydrous CH2Cl2 and THF (Wako
Pure Chemical Industries) were used as supplied.
Introduction of peripheral electron-donating groups to a
triarylborane core
Synthesis
The HOMO and LUMO energies of triarylborane are deter-
mined primarily by the nature of the peripheral electron-do-
nating group and a triarylborane core, respectively. As a result,
an introduction of a stronger electron-donating group(s) to
a given triarylborane core makes the fluorescent excited state
lower in energy and, thus, the fluorescence energy of the de-
rivative is controllable. Distributions of the HOMO electron
from the electron-donating groups to p(B) resulted in a relative-
ly large excited-state electric dipole moment of the derivative
(me =8.5—13 D). Owing to such me values, the derivatives
showed large fluorescence solvatochromic shifts. Furthermore,
the change in the dipole moment upon photoexcitation of the
derivative (that is, meꢀmg) brought about enhancement of the
molar absorption coefficient of the p(aryl)–p(B) CT absorption
band. These are the effects of the electron-donating groups in-
troduced to the periphery of a triarylborane core and an ap-
propriate choice of the nature of an electron-donating
group(s) would lead to synthetic tuning of both spectroscopic
and excited-state properties of the derivatives.
Derivatives 1c, 2b, and 2c were synthesized through the copper-
catalyzed coupling reaction of a xylene or anthracene derivative
with diphenylamine or carbazole and subsequent reaction of the
product with n-butyllithium (nBuLi) or tert-butyllithium (tBuLi). Al-
though 1a has been reported to be synthesized by a Grignard re-
action,[25] it has been successfully synthesized by using tBuLi. Syn-
thetic details are described below.
The reagents for synthesis, purchased from Wako Pure Chemical In-
dustries, Tokyo Chemical Industry Company, Kanto Chemical Com-
pany. or Fluorochem, were used as supplied. The synthesized com-
pounds were purified by column chromatography (Merck silica gel
60, particle size: 0.063–0.020 mm) or Recycling Preparative HPLC
(Japan Analytical Industry Company, LC-9201) with a JAIGEL-1H
gel-permeation chromatography (GPC) column equipped with UV-
3740 UV/Vis and RI-50s RI detectors. Thin-layer chromatography
was conducted by using Merck silica gel 60 F245 aluminum sheets
and the sample spots were visualized by a UVP UVGL-25 compact
UV lamp. Electron ionization (EI-MS) and electrospray ionization
mass spectrometry (ESI-MS) were performed by direct injection
into the mass detector of a Shimadzu GCMS-QP5000 system and
a Waters micromass ZQ detector, respectively. Melting points of
the compounds were determined using a Yanaco MP-500D or
1
SANSYO Meltingpoint SMP-300 apparatus. H NMR spectra in CDCl3
Role of a triarylborane core in the spectroscopic/
photophysical properties
or CD2Cl2 (both from Cambridge Isotope Laboratories) were record-
ed on a JEOL LME-EX270 FT-NMR system (270 MHz). The chemical
shifts of the spectra were given in ppm with tetramethylsilane as
an internal standard (0.00 ppm). IR spectra were recorded on a Shi-
madzu FTIR-8300 spectrometer. Elemental analyses were conduct-
ed in the Instrumental Analysis Division, Equipment Management
Center, Creative Research Institution, Hokkaido University. Spectro-
scopic and photophysical purities of 1b–1c and 2a–2c were con-
firmed by single-exponential fluorescence decays in toluene (see
Figures S9–S12 in the Supporting Information).
The trixylylborane-core derivatives showed hypsochromic shifts
of the p(aryl)–p(B) CT absorption bands with an increase in the
solvent polarity probably owing to the symmetry-breaking
structures in the ground states, while the CT absorption bands
of the trianthrylborane-core derivatives exhibited opposite
(bathochromic) solvent-polarity effects.
We suppose that these results originate from the structural
rigidity of the derivative in the ground state and the trixylyl-
borane-core derivatives having less rigid/crowded structures,
compared to the trianthrylborane-core derivatives, deviate
from D3 symmetry in the ground states. This might be one of
the possible reasons for different solvent polarity responses to
the Ff and tf values between the trixylylborane- and
trianthrylborane-core derivatives, and these photophysical
properties can be also tuned by a choice of a triarylborane
core.
Synthesis of tri(2-m-xylyl)borane (1a): An oven-dried Schlenk
tube was evacuated and filled subsequently with argon gas. Into
the vessel were added 2-bromo-m-xylene (0.86 mL, 6.5 mmol) and
dry diethyl ether (11 mL) and, then, the mixture was cooled to
ꢀ788C in an acetone/dry ice bath for 30 min. tBuLi (1.57m in n-
pentane, 9.5 mL, 15 mmol) was added dropwise to the reaction
mixture at ꢀ788C. After stirring at ꢀ788C for 30 min, the mixture
was allowed to warm to 08C and, then, stirred for further 30 min.
Boron trifluoride diethyl etherate (0.26 mL, 2.1 mmol) in dry diethyl
ether (2.0 mL) was added to the reaction mixture at 08C. After stir-
ring at 08C for 30 min followed by warming to ambient tempera-
ture, the mixture was stirred for 16 h. Ethyl acetate (5 mL) was
added to the mixture and it was poured into dichloromethane
(30 mL). The solution was washed with brine (10 mLꢂ3), dried
These insights obtained in this study will greatly contribute to
molecular design toward future triarylborane-based materials,
devices, and other fascinating systems.
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over anhydrous MgSO4, and evaporated to dryness under reduced
pressure. The crude product was recrystallized from n-hexane to
give pure 1a as colorless crystals (420 mg, 62%). Rf =0.75 (SiO2;
copper powder (44 mg, 0.71 mmol), 18-crown-6 (190 mg,
0.71 mmol), potassium carbonate (3.9 g, 28 mmol), and o-dichloro-
benzene (15 mL). The reaction mixture was heated at 1808C for
20 h. After the reaction was completed, the mixture was suspend-
ed in chloroform (200 mL) and, then, insoluble solids were re-
moved by filtration. The filtrate was washed with brine (100 mLꢂ
3), dried over anhydrous MgSO4, and evaporated to dryness under
reduced pressure. Two chromatographic purifications (SiO2; chloro-
form/n-hexane, 4:1 and, then, SiO2; chloroform/n-hexane, 1:4) fol-
lowed by recrystallization from chloroform/n-hexane afforded 9-
bromo-10-(9H-carbazolyl)anthracene as yellow needles (430 mg,
14%). Rf =0.58 (SiO2; chloroform/n-hexane, 1:4); 1H NMR (CDCl3):
d=8.72 (2H, td, J=0.9, 8.9 Hz; 4,5-Ar-H of An), 8.29 (2H, td, J=1.3,
7.0 Hz; 1,8-Ar-H of An), 7.67–7.61 (2H, m; 3,6-Ar-H of An), 7.35–7.24
(8H, m; 2,7-Ar-H of An and 1’,2’,3’,6’,7’,8’-Ar-H of Cz), 6.72 ppm
(2H, td, J=1.0, 7.0 Hz; 4’,5’-Ar-H of Cz); MS (EI): m/z: 421 [M]+.
1
chloroform/n-hexane, 1:2); m.p. 179.5–182.58C; H NMR (CDCl3) d=
7.14 (3H, t, J=7.6 Hz; 4-Ar-H), 6.91 (6H, dd, J=0.54, 7.7 Hz; 3,5-Ar-
H), 2.02 ppm (18H, s, CH3); MS (ESI): m/z: 345 ([M+F]ꢀ); IR (KBr):
n˜ =3049 (Ar), 2958 (CH3), 2926 (CH3), 1588 (Ar), 1448 (Ar), 1241 (Ar),
1201 (Ar), 870 (Ar), 776 (Ar), 767 (Ar), 672 cmꢀ1 (Ar); elemental
calcd (%) for C36H27B: C 88.35, H 8.34; found: C 88.31, H 8.58.
Synthesis of tris{2-(5-N,N-diphenylamino-m-xylyl)}borane (1c): As
the first step, 2-bromo-5-N,N-diphenylamino-m-xylene was synthe-
sized similar to the literature method.[17] An oven-dried three-
necked flask equipped with a Dimroth condenser was evacuated
and filled subsequently with argon gas. Into the vessel were added
diphenylamine (1.2 g, 6.8 mmol), copper(I) iodide (64 mg,
0.34 mmol), 2,2’-bipyridine (55 mg, 0.35 mmol), and potassium tert-
butoxide (1.5 g, 13 mmol), followed by 2,5-dibromo-m-xylene
(2.1 g, 8.0 mmol) in dry toluene (5 mL). The reaction mixture was
heated at 1258C for 24 h. After the reaction was completed, the
mixture was suspended in dichloromethane (50 mL) and washed
with brine (50 mLꢂ2). The aqueous layer was extracted with di-
chloromethane (50 mL). The organic phase was dried over anhy-
drous MgSO4 and evaporated to dryness under reduced pressure.
Purification by column chromatography (SiO2; dichloromethane/n-
hexane 1:3) afforded 2-bromo-5-(N,N-diphenylamino)-m-xylene as
colorless solids (760 mg, 32%). Rf =0.65 (SiO2; dichloromethane/
An oven-dried Schlenk tube was evacuated and filled subsequently
with argon gas. Into the vessel were added 9-bromo-10-(9H-carba-
zolyl)anthracene (210 mg, 0.50 mmol) and dry diethyl ether
(2.0 mL) and, then, the mixture was cooled to ꢀ788C in an ace-
tone/dry ice bath for 30 min. tBuLi (1.57m in n-pentane, 0.67 mL,
1.1 mmol) was added dropwise to the reaction mixture at ꢀ788C.
After stirring at ꢀ788C for 30 min, the mixture was allowed to
warm to ambient temperature and, then, stirred for 30 min. Boron
trifluoride diethyl etherate (20 mL, 0.17 mmol) in dry diethyl ether
(0.4 mL) was added to the reaction mixture at 08C using an ice
bath. After stirring at 08C for 60 min and at ambient temperature
for 16 h, the mixture was poured into dichloromethane (30 mL), in-
soluble solids were removed by filtration and, then, the filtrate was
evaporated to dryness under reduced pressure. The crude product
was purified by column chromatography (SiO2; chloroform/n-
hexane, 1:2) followed by preparative HPLC (GPC, chloroform) to
give pure 2b as reddish-orange solids (ca. 3 mg, 2%). Rf =0.14
1
n-hexane, 1:3); m.p. 158.7–164.68C; H NMR (CDCl3): d=7.27–7.21
(4H, m; 3,5-Ar-H of Ph), 7.05–7.01 (6H, m; 2,4,6-Ar-H of Ph), 6.81
(2H, s; 3’,5’-Ar-H of Xy), 2.31 ppm (6H, s; CH3); MS (EI): m/z: 352
[M]+; IR (KBr): n˜ =3032 (Ar), 2920 (CH3), 1942 (Ar), 1582 (Ar), 1489
(Ar), 1467 (Ar), 1342 (Ar), 1280 (Ar), 1232 (Ar), 1171 (Ar), 1155 (Ar),
1075 (Ar), 1016 (Ar), 859 (Ar), 759 (Ar), 695 cmꢀ1 (Ar).
An oven-dried Schlenk tube was evacuated and filled subsequently
with argon gas. Into the vessel were added 2-bromo-5-N,N-diphe-
nylamino-m-xylene (0.35 g, 1.0 mmol) and dry diethyl ether
(4.0 mL) and, then, the mixture was cooled to ꢀ788C in an ace-
tone/dry ice bath for 30 min. tBuLi (1.57m in n-pentane, 1.3 mL,
2.0 mmol) was added dropwise to the reaction mixture at ꢀ788C.
After stirring at ꢀ788C for 30 min, the mixture was allowed to
warm to 08C and, then, stirred for 30 min. Boron trifluoride diethyl
etherate (40 mL, 0.33 mmol) in dry diethyl ether (0.4 mL) was added
to the reaction mixture at 08C. After stirring at 08C for 30 min, the
reaction mixture was allowed to warm to ambient temperature fol-
lowed by stirring for 16 h. The mixture was poured into dichloro-
methane (30 mL), washed with brine (10 mLꢂ3), dried over anhy-
drous MgSO4, and evaporated to dryness under reduced pressure.
The crude product was purified by column chromatography (SiO2,
chloroform/n-hexane=1/3) and recrystallized twice from chloro-
form/n-hexane to give pure 1c as yellow needles (41 mg, 16%).
Rf =0.49 (SiO2; chloroform/n-hexane, 1:3); m.p. 266.1–268.28C;
1H NMR (CDCl3): d=7.31–7.21 (12H, m; 3,5-Ar-H of Ph), 7.09 (12H,
td, J=1.3, 7.5 Hz; 2,6-Ar-H of Ph), 7.02 (6H, tt, J=1.2, 7.3 Hz; 4-Ar-
H of Ph), 6.63 (6H, s; 3’5’-Ar-H of Xy), 1.98 ppm (18H, s; CH3); MS
(ESI): m/z: 851 [M+Na]+; IR (KBr): n˜ =3034 (Ar), 2949 (CH3), 2910
(CH3), 1584 (Ar), 1492 (Ar), 1334 (Ar), 1296 (Ar), 1212 (Ar), 1142 (Ar),
858 (Ar), 752 (Ar), 697 cmꢀ1 (Ar); elemental analysis cald for
C60H54BN3: C 87.04, H 6.57, N 5.08; found: C 87.18, H 6.70, N 4.83.
1
(SiO2; chloroform/n-hexane, 1:2); H NMR (CD2Cl2): d=8.43 (6H, d,
J=8.6 Hz; 1,8-Ar-H of An), 8.34–8.31 (6H, m; 4,5-Ar-H of An), 7.38–
7.14 (30H, m; 2,3,6,7-Ar-H of An and 1’,2’,3’,6’,7’,8’-Ar-H of Cz),
6.80–6.77 ppm (6H, m; 4’,5’-Ar-H of Cz); MS (ESI): m/z: 1057 [M+
F]ꢀ.
Synthesis of tris{9-(10-N,N-diphenylaminoanthryl)}borane (2c):
As the first step, 9-bromo-10-N,N-diphenylaminoanthracene was
synthesized similar to the synthesis of 1c. An oven-dried three-
necked flask equipped with a Dimroth condenser was evacuated
and filled subsequently with argon gas. Into the vessel were added
diphenylamine (2.5 g, 15 mmol), copper(I) iodide (280 mg,
1.5 mmol), 2,2’-bipyridine (230 mg, 1.5 mmol), potassium tert-but-
oxide (3.3 g, 30 mmol), 9,10-dibromoanthracene (5.0 g, 15 mmol),
and dry toluene (30 mL). The reaction mixture was heated at
1258C for 33 h. After the reaction, the mixture was suspended in
chloroform (500 mL), and insoluble solids were removed by filtra-
tion. The filtrate was washed with water (200 mLꢂ3), dried over
anhydrous MgSO4, and evaporated to dryness under reduced pres-
sure. Two chromatographic purifications (SiO2; chloroform/n-
hexane, 3:2 and, then, SiO2; chloroform/n-hexane, 1:4) followed by
recrystallization from chloroform/n-hexane afforded 9-bromo-10-
N,N-diphenylaminoanthracene as yellow needles (240 mg, 4%).
Rf =0.38 (SiO2; chloroform/n-hexane, 1:4); m.p. 216.8–219.68C;
1H NMR (CDCl3): d=8.61 (2H, td, J=1.0, 8.9 Hz; 4,5-Ar-H of An),
8.10 (2H, td, J=0.8, 8.9 Hz; 1,8-Ar-H of An), 7.60–7.54 (2H, m; 2,7-
Ar-H of An or 3,6-Ar-H of An), 7.45–7.39 (2H, m; 2,7-Ar-H of An or
3,6-Ar-H of An), 7.19–7.12 (4H, m; 2’,6’-Ar-H of Ph or 3’,5’-Ar-H of
Ph), 7.07–7.02 (4H, m; 2’,6’-Ar-H of Ph or 3’,5’-Ar-H of Ph),
6.88 ppm (2H, tt, J=1.3, 7.2 Hz; 4’-Ar-H of Ph); MS (EI): m/z: 424
Synthesis of tris[9-{10-(9H-carbazolyl)anthryl}]borane (2b): As
the first step, 9-bromo-10-(9H-carbazolyl)anthracene was synthe-
sized similar to the synthesis of 1c. An oven-dried three-necked
flask equipped with a Dimroth condenser was evacuated and filled
subsequently with argon gas. Into the vessel were added carbazole
(1.2 g, 7.1 mmol), 9,10-dibromoanthracene (2.4 g, 7.1 mmol),
Chem. Eur. J. 2014, 20, 3940 – 3953
www.chemeurj.org
3950
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
[M]+; IR (KBr): n˜ =3036 (Ar), 1587 (Ar), 1487 (Ar), 1351 (Ar), 1295
a fluorescence standard, respectively, and, S, A, and n are the area
of a fluorescence spectrum, the absorbance at an excitation wave-
length, and the refractive index of a solvent used, respectively.
Fluorescence decay measurements were conducted by using
a time-correlated single-photon counting system.[30] Optically para-
metric amplified 400 nm laser pulses (repetition rate: 100 kHz,
fwhm: 200 fs autocorrelation trace, Coherent, model 9400) of the
output pulses from a mode-locked Ti:sapphire laser (Coherent,
Mira model 900-F) or 405 nm laser pulses (repetition rate:
100 MHz, fwhm: 70 ps) from a picoseconds light pulser (Hamamat-
su, PLP10–040) were used as an excitation light source. By using
a Soleil–Babinet compensator (Melles Griot), the polarized direction
of the excitation laser beam was set at a magic angle (54.78) or
vertical direction for fluorescence decay measurements. Fluores-
cence from a sample was detected by a microchannel-plate photo-
multiplier (Hamamatsu, R3809U-50) equipped with a monochroma-
tor (Jobin Ybon, H-20) and analyzed by a single-photon counting
module (Edinburgh Instruments, SPC-300). The excitation laser
pulse profile was measured at the excitation wavelength by using
dye-free polystyrene resins as a scattering material. Decay profiles
were analyzed by an iterative nonlinear least-squares deconvolu-
tion method. The absorbance of a sample solution was set <0.05
at the excitation wavelength, and sample solutions were deaerated
by using an argon gas stream over 30 min.
(Ar), 1278 (Ar), 1027 (Ar), 916 (Ar), 751 (Ar), 693 cmꢀ1 (Ar).
An oven-dried Schlenk tube was evacuated and filled subsequently
with argon gas. Into the vessel were added 9-bromo-10-N,N-diphe-
nylaminoanthracene (200 mg, 0.47 mmol) and dry diethyl ether
(2.0 mL) and, then, the mixture was cooled to ꢀ788C in an ace-
tone/dry ice bath for 30 min. nBuLi (1.62m in n-hexane, 0.35 mL,
0.57 mmol) was added dropwise to the reaction mixture at ꢀ788C.
After stirring at ꢀ788C for 45 min, the mixture was allowed to
warm to 08C and, then, stirred for 20 min. Boron trifluoride diethyl
etherate (20 mL, 0.17 mmol) in dry diethyl ether (0.4 mL) was added
to the reaction mixture at 08C. After stirring at 08C for 30 min, the
reaction mixture was allowed to warm to ambient temperature
and stirred for further 12 h. The mixture was poured into chloro-
form (30 mL), insoluble solids were removed by filtration and, then,
the filtrate was evaporated to dryness under reduced pressure. The
crude product was purified by column chromatography (SiO2;
chloroform/n-hexane, 2:3) followed by recrystallization from
chloroform/n-hexane to give pure 2c as deep-red needles (46 mg,
26%). Rf =0.34 (SiO2; chloroform/n-hexane, 2:3); m.p. >3008C
1
(decomp.); H NMR (CDCl3): d=8.18 (6H, ddd, J=0.76, 1.0, 4.6 Hz;
4,5-Ar-H of An), 8.15 (6H, ddd, J=0.78, 1.2, 4.7 Hz; 1,8-Ar-H of An),
7.31–7.08 (30H, m; 2’,3’,4’,5’,6’-Ar-H of Ph), 7.00–6.88 ppm (12H, m;
2,3,6,7-Ar-H of An); IR (KBr): n˜ =3068 (Ar), 3034 (Ar), 1589 (Ar), 1493
(Ar), 1482 (Ar), 1404 (Ar), 1268 (Ar), 772 (Ar), 748 (Ar), 692 cmꢀ1
(Ar); MS (ESI): m/z: 1044 [M]+; elemental analysis calcd (%) for
C78H54BN3·CHCl3·1.5H2O: C 79.70, H 4.91, Cl 8.93, N 3.53; found: C
79.57, H 4.88, Cl 8.88, N 3.46.
Theoretical calculations
Theoretical calculations for the derivatives were conducted on the
Gaussian 09W programs.[31] The ground-state geometries of the de-
rivatives were optimized by using the B3LYP density functional
theory (DFT)[32] and 6–31G* basis set. Time-dependent DFT (TD-
DFT) calculations were then performed to estimate the energies
and oscillator strengths of the five lowest-energy electronic transi-
tions of the derivatives. All the calculations were carried out as THF
solutions by the Polarizable Continuum Model (PCM). The contours
of the electron density were plotted by using GaussView 5.0.[33]
Electrochemical measurements
Cyclic and differential pulse voltammograms were recorded on an
electrochemical analyzer (BAS, ALS-701 A) with a three-electrode
system by using glassy carbon working, Pt auxiliary, and SCE (satu-
rated calomel electrode) reference electrodes. The voltammograms
in the positive and negative potential regions were measured sep-
arately using CH2Cl2 and THF as solvents, respectively. The concen-
trations of the derivatives were set ca. 1ꢂ10ꢀ3 m, and 0.1m of
tetra-n-butylammonium hexafluorophosphate (TBAPF6) was used
as a supporting electrolyte. Sample solutions were deaerated by
using an Ar gas stream over 20 min prior to the experiments. The
potential was swept at 100 mVsꢀ1 in cyclic voltammetry. Differen-
tial pulse voltammetry was conducted with 50 mV height pulses
(0.05 s duration) being stepped by 5.0 mV intervals (2.0 s interval
between the two pulses).
Acknowledgements
This work was partly supported by Grant-in-Aid from MEXT for
Young Scientists (B) under Grant Number 23750141 to ES.
Keywords: charge transfer · fluorescence · photophysics ·
solvent effects · triarylboranes
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Received: October 19, 2013
Published online on March 18, 2014
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