Fluorescence properties of simple N-substituted aldimines with a B-N interaction and their fluorescence quenching by a cyanide ion

Article abstract of DOI:10.1021/jo901733b

(Chemical Equation Presented) N-Aryl, N-alkyl, N-alkoxy, and N-amino derivatives of 2-[bis(pentafluorophenyl)boryl]benzylideneamine were synthesized by the condensation reactions of 2-[bis(pentafluorophenyl)boryl]-benzaldehyde with the corresponding amine

Full text of DOI:10.1021/jo901733b

pubs.acs.org/joc
Fluorescence Properties of Simple N-Substituted Aldimines with a B-N
Interaction and Their Fluorescence Quenching by a Cyanide Ion
Junro Yoshino, Naokazu Kano,* and Takayuki Kawashima*
Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-0033, Japan
takayuki@chem.s.u-tokyo.ac.jp
Received August 10, 2009
N-Aryl, N-alkyl, N-alkoxy, and N-amino derivatives of 2-[bis(pentafluorophenyl)boryl]benzylid-
eneamine were synthesized by the condensation reactions of 2-[bis(pentafluorophenyl)boryl]-
benzaldehyde with the corresponding amines. Their structures were investigated by NMR and X-
ray crystallographic analysis. Their properties were investigated by UV-vis and fluorescence
spectroscopy. The boryl-substituted N-arylimines show blue or green fluorescence in hexane at
room temperature, and their fluorescence efficiency is much higher than that of N-benzylideneani-
line. In particular, the boryl-substituted N-(4-dimethylaminophenyl)imine showed strong green
emissions with at least 7000 times higher fluorescence quantum yield (0.73) compared with that of
N-benzylideneaniline. The boryl-substituted N-(1-indolyl)- and N-(9-carbazolyl)imines showed dual
emissions, one of which was assignable as arising from the lowest singlet excited state and the other
from the local excited state of the substituent on the imine nitrogen. The fluorescent properties of
the boryl-substituted N-butyl- and N-methoxyimines were also investigated. Reactions of the
N-arylimine derivatives with cyanide ion gave the corresponding cyanide adducts and quenched
the fluorescence, indicating that these 2-[bis(pentafluorophenyl)boryl]benzylideneamine derivatives
have a potential as a cyanide ion sensor.
Introduction
nonfluorescent compounds. For example, the fluorescence
quantum yield of N-benzylideneaniline is less than 10-4 2
.
Imine derivatives have been widely used as coloring ma-
terials and functional molecular units.1 Imine derivatives
can be synthesized readily by condensation reactions of
aldehydes with amine derivatives. The two parts of the
π-conjugated systems in an imine that is formed from aro-
matic aldehydes and aromatic amine derivatives can be
connected through the π-orbital of the CdN double bond.
These structures constructed by a simple synthetic method
can provide highly diverse π-conjugated systems. Although
such high diversity of their structures may provide a big
advantage as fluorescence materials, imines are essentially
Although several highly fluorescent imines bearing a metal-
nitrogen interaction have been reported,3 most of them re-
quired tri- or higher-dentate ligand structures (Figure 1). A
restriction of their molecular designs to make their structures
rigid results in difficulty in further modification of their
structures to tune their fluorescence properties. In simpler
imines, some salicylideneamine derivatives bearing an
O,N-bidentate ligand structure have been reported as fluores-
cent imines. However, most of their fluorescence quantum
(2) Belletete, M.; Durocher, G. Can. J. Chem. 1982, 60, 2332.
(3) (a) Morishige, K. J. Inorg. Nucl. Chem. 1978, 40, 843. (b) Hamada, Y.;
Sano, T.; Fujita, M.; Fujii, T.; Nishio, Y.; Shibata, K. Jpn. J. Appl. Phys.
1993, 32, L511. (c) Wang, P.; Hong, Z.; Xie, Z.; Tong, S.; Wong, O.; Lee,
C.-S.; Wong, N.; Hung, L.; Lee, S. Chem. Commun. 2003, 1664.
(1) Yamamoto, K.; Higuchi, M.; Shiki, S.; Tsuruta, M.; Chiba, H. Nature
2002, 415, 509.
7496 J. Org. Chem. 2009, 74, 7496–7503
Published on Web 09/01/2009
DOI: 10.1021/jo901733b
r
2009 American Chemical Society

Yoshino et al.
JOCArticle
SCHEME 1. New Design for Boron-Substituted Imine Deriva-
tives
FIGURE 1. Previously reported highly fluorescent imines.
5-9 were synthesized by dehydrative condensation of 3
and the corresponding amines in good to moderate yields
(Scheme 2 and Table 1).
Structures. X-ray crystallographic analyses were per-
formed for 4a-c and 5-9, and their crystal structures are
shown in Figures S1-S3 in Supporting Information. In
hydrazone 8, there are two independent molecules of 8 in
the unit cell, having almost identical structures. There is an
intramolecular coordination from the imine nitrogen to the
boron in all cases in the crystalline state. The bond lengths of
FIGURE 2. Fluorescent 2-borylazobenzenes.
yields are low. There are some fluorescent simple imines
bearing a B-N interaction, but there have been only a few
examples, and their fluorescence properties have scarcely
been shown4 except for those CdN double-bond compounds
whose CdN double bond consists of part of a heterocycle.5
Therefore, the relationship between structures and fluores-
cence properties of the boron-substituted imines is still
unclear. Development of new methods to provide imines
with fluorescence by simple modification should bring high
flexibility to the molecular designs, make their fluorescence
properties more diverse, and increase their usefulness as
easily synthesizable and tunable fluorescent materials.
We recently reported the synthesis of fluorescent boron-
substituted azobenzenes (Figure 2)6 in the continuation of
our work on 2-borylazobenzene derivatives.7 Taking into
consideration the similarity in structures between imines and
azobenzenes, the simple introduction of a diarylboryl group
to benzylideneamines was expected to provide a new class of
fluorescent imines with a B-N interaction (Scheme 1). Di-
rect introduction of a boryl group to the phenylimine π-
conjugated system is considered to affect the system much
more than its introduction through an oxygen atom in the
case of salicylideneamine derivatives. Here, we report the
synthesis of 2-(diarylboryl)phenylimine derivatives, the re-
lationship between their structures and fluorescence, and
their reactivities.
˚
the coordination bonds shown in Table 2 [1.60-1.65 A] are
close to the sum of the covalent radii of boron and nitrogen
˚
(1.57 A), indicating the strong B-N interactions in 4a-c and
˚
5-9. Their imine C-N bond lengths [1.29-1.30 A] are
slightly longer than the CdN double bond length of N-
8
benzylideneaniline (10) (1.276(1) A), and much shorter than
˚
˚
the usual C-N single bond lengths (1.38 A), indicating that
the imine C-N bonds in 4a-c and 5-9 maintain their
double-bond character. In anthrylimine 6, an anthryl group
and a C6F5 group of another molecule are stacked inter-
molecularly. In hydrazone 8, intramolecular π-π stacking
between the indolyl group and one of the C6F5 groups
(Figure S2, Supporting Information) and intermolecular
π-π stackings between two neighboring indolyl groups,
two C6F5 groups, or the indolyl group and one of the C6F5
groups are found (see Supporting Information). In hydra-
zone 9, there is only intramolecular π-π stacking between
the carbazolyl group and a C6F5 group (Figure S2, Support-
ing Information). Compounds 4a-c, 5, and 7 have neither
intermolecular nor intramolecular stacking. The existence of
the intramolecular B-N interactions is unaffected by
whether the π-π stackings exist or not. The 11B NMR
chemical shifts of 2-borylphenyl-substituted imines 4a-c
and 5-9 in CDCl3 were observed between -3.2 and -0.6
ppm, showing the tetracoordination state of the boron atoms
in solution as in the crystal structures.
Results and Discussion
The CdN double-bond planes of hydrazones 8 and 9 are
approximately perpendicular to their nitrogen-containing
heterocycles, probably because of the aforementioned intra-
molecular π-π stackings. The indolyl and carbazolyl
groups are almost parallel to one of the C6F5 groups, and
their benzene rings are overlapped with the C6F5 group.
In contrast, imines 4a-c and 5-7 have a relatively planar
N-substituted phenylimine moiety, judging from the tor-
sion angles ω1 [torsion angle between C7-N1-X-Y;
4a-c, 5, and 6: (X, Y) = (C8, C9), 7: (X, Y) = (O1, C8), 8
and 9: (X, Y) = (N2, C8)] and ω2 [torsion angle between
C1-C6-C7-N1] shown in Table 2.
Synthesis of 2-Borylphenyl-Substituted Imines. Successive
reactions of 2-bromobenzaldehyde diethyl acetal 1 with n-
butyllithium and ethoxyborane 2 followed by hydrolysis
under acidic conditions gave 2-borylbenzaldehyde 3 in
50% yield. 2-Borylphenyl-substituted imines 4a-c and
(4) (a) Padmanathan, T. DE1926925, 1970. (b) Allmann, R.; Hohaus, E.;
Olejnik, S. Z. Naturforsch. 1982, 37B, 1450. (c) Mir, J. M.; Martinez, C. Talanta
1986, 33, 541. (d) Riddle, J. A.; Lathrop, S. P.; Bollinger, J. C.; Lee, D. J. Am.
Chem. Soc. 2006, 128, 10986.
(5) (a) Wakamiya, A.; Taniguchi, T.; Yamaguchi, S. Angew. Chem., Int.
Ed. 2006, 45, 3170. (b) Son, H.-J.; Han, W.-S.; Wee, K.-R.; Chun, J.-Y.; Choi,
K.-B.; Han, S.-J.; Kwon, S.-N.; Ko, J.; Lee, C.; Kang, S. O. Eur. J. Inorg.
Chem. 2009, 1503.
(6) (a) Yoshino, J.; Kano, N.; Kawashima, T. Chem. Commun. 2007, 559.
(b) Yoshino, J.; Kano, N.; Kawashima, T. Chem. Lett. 2008, 37, 960.
(7) (a) Kano, N.; Yoshino, J.; Kawashima, T. Org. Lett. 2005, 7, 3909.
(b) Yoshino, J.; Kano, N.; Kawashima, T. Tetrahedron 2008, 64, 7774.
(8) Harada, J.; Harakawa, M.; Ogawa, K. Acta Crystallogr. B 2004, 60,
578.
J. Org. Chem. Vol. 74, No. 19, 2009 7497

Yoshino et al.
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SCHEME 2. Synthesis of 4a-c and 5-9 via 3 and Structural Formula of N-Benzylideneaniline (10)
TABLE 1. Yields of 2-Borylphenyl-Substituted Imines 4a-c and 5-9
TABLE 2. Selected Bond Lengths (d) and Torsion Angles (ω) of Imines
4a-c and 5-9
compound
d1(B-N), A
d2(C-N), A
ω1a, deg
ω2b, deg
˚
˚
4a
4b
4c
5
1.619(2)
1.6222(18)
1.6160(18)
1.602(3)
1.294(2)
1.2981(17)
1.3003(18)
1.287(2)
25.6(2)
13.66(18)
28.96(19)
17.0(3)
4.05(18)
1.23(15)
2.87(16)
1.2(2)
6
1.620(4)
1.605(3)
1.6132(19)
[1.618(2)]
1.646(3)
1.294(3)
1.285(3)
1.2952(18)
[1.2948(18)]
1.295(2)
33.5(4)
5.7(3)
61.73(18)
[72.88(17)]
72.0(2)
2.8(4)
1.2(2)
3.72(17)
[4.71(17)]
5.8(2)
7
8c
9
aTorsion angle between C7-N1-X-Y. 4a-c, 5, and 6: (X, Y) = (C8,
C9). 7: (X, Y) = (O1, C8). 8 and 9: (X, Y) = (N2, C8). bTorsion angle
between C1-C6-C7-N1. cValues for another independent molecule
are in brackets.
aMeONH2 HCl, EtOH, pyridine, rt. bEt2O, rt.
3
Absorption and Fluorescence Properties of N-Alkyl- and
N-Arylimines. In the UV-vis absorption spectra in hexane,
imines 4a-c, 5, and 6 showed a strong absorption in the UV
region, as shown in Table 3 and Figure 3. These absorption
maxima are assignable to the π-π* transition of the imine
moiety. The wavelengths of the π-π* absorption maxima of
N-phenylimine 4a, N-butylimine 5, and N-anthrylimine 6
become longer in the order of 5 < 4a < 6, reflecting the
extension of the π-conjugation. In the cases of 4a-c, the
electron-donating groups in the π-conjugated systems also
effectively produced a red shift of the absorption maxima.
Fluorescence spectra and fluorescence properties of 4a-c,
5, and 6 are shown in Figure 4 and Table 3, respectively.
Imine 4a showed blue emission in hexane at rt upon UV
irradiation (Figure 5A). In the fluorescence spectrum, imine
4a showed an emission maximum at 460 nm and the fluo-
rescence quantum yield was determined to be 0.0050, which is
a higher fluorescence efficiency than nonfluorescent N-ben-
zylideneaniline (10) (ΦF < 0.0001).9 Imines bearing a more
extended π-conjugated system and/or an electron-donating
aryl group on the nitrogen atom showed fluorescence that
was more red-shifted with higher fluorescence efficiency. For
example, N-[4-(dimethylamino)phenyl]imine 4c and N-an-
thrylimine 6 showed strong green emission (Figure 5B) with
fluorescence quantum yields (0.73 and 0.39, respectively)
very much higher than that of N-benzylideneaniline (10). It is
considered that introduction of the bis(pentafluorophenyl)-
boryl group and the eventual B-N interaction played
important roles in providing imines 4a-c and 6 with such
FIGURE 3. UV-vis absorption spectra of imines 4a-c, 5, and 6 in
hexane.
FIGURE 4. Fluorescence spectra of imines 4a-c, 5, and 6 in
hexane.
high fluorescence efficiencies. These increased fluorescence
quantum yields would be caused by suppression of non-
radiative relaxation of the excited state by increasing the
rigidity of the molecular structure with a tight B-N bond
and a rigid five-membered ring framework. An electron-
donating group at the end of the imine π-conjugated
system, such as dimethylamino group of 4c, is also consid-
ered to contribute to increasing the rigidity of the molecular
structure by planarization of the diarylimine moiety due
to the delocalization of the lone pair to the imine π-con-
jugated system. Furthermore, masking of the lone pair of the
(9) Belletete, M.; Durocher, G. Can. J. Chem. 1982, 60, 2332.
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JOCArticle
FIGURE 5. Photographs of (A) 4a and (B) 6 in hexane upon UV
irradiation.
FIGURE 7. Calculated energy levels of frontier orbitals of 10 and
4a at the B3PW91/6-31þG(d) level of theory.
FIGURE 6. Calculated energy levels of singlet and triplet excited
states of (A) 10 and (B) 4a at the B3PW91/6-31þG(d) level of
theory. The 3(n, π*) state of 4a was not detected between T1 and T12
(4.2 eV).
nitrogen atom by coordination to the boron atom is con-
sidered to contribute to removing the factors for the absence
of fluorescence in 10, that is, the high rate of nonradiative
relaxation process with the intersystem crossing from the
lowest singlet excited state to triplet excited states10 and
the existence of a lone pair spatially perpendicular to the
π-orbitals, which undergoes rapid intersystem crossing.
To clarify this point, theoretical calculations using DFT
and time-dependent DFT (TD-DFT) methods at the
B3PW91/6-31þG(d) level of theory were performed. Opti-
mized structures of 4a and 7-10 and energy levels of their
molecular orbitals were obtained by the DFT calculations.
The energy levels of their singlet and triplet excited states and
identification of each excitation were obtained by the TD-
DFT calculations. In N-benzylideneaniline (10), its two
triplet excited states, T1 (2.6 eV) and T2 (3.3 eV), were at
lower energy levels than the S1 state (3.7 eV) (Figure 6A).
Both the S1 and T1 states consist of an excitation from the
HOMO to the LUMO. The T2 state consists of an excitation
from the HOMO-2 to the LUMO.11 The LUMO (-1.9 eV),
HOMO (-6.2 eV), and HOMO-2 (-7.2 eV) of 10 are
mainly made up by the π*, π, and n orbitals of the azo-
methine group, respectively (Figure 7). Therefore, the S1, T1,
and T2 states should be 1(π, π*), 3(π, π*), and 3(n, π*) excited
states. The intersystem crossing 1(π, π*)f3(n, π*) would be
relatively fast because of the large spin-orbit interaction
between the lone pair on the nitrogen atom and the π orbital
FIGURE 8. Fluorescence spectra of imine 4a in the solid state (bold
line) and in the solution state (thin line).
FIGURE 9. UV-vis absorption spectra of imines 7-9 in hexane.
perpendicular to it.12 These high rates of the relaxation
processes including the intersystem crossing process should
effectively suppress the S1fS0 radiative relaxation process
and cause the absence of fluorescence in 10. In 4a, there is a
B-N dative bond, and the orbital formed from the bonding
orbital between the N and B atoms and the π orbital of the
C6F5 groups in 4a is greatly stabilized by the B-N interac-
tion and becomes HOMO-10 (-9.7 eV), while the π and π*
orbitals of the imine moiety of 4a become the HOMO
(-7.0 eV) and the LUMO (-3.0 eV), respectively
(Figure 7). Although two triplet excited states, T1 (2.4 eV)
and T2 (3.1 eV), were at lower energy levels than the S1 state
(10) Knyazhanskii, M,. I.; Stryukov, M. B.; Minkin, V. I. Opt. Spectrosc.
1972, 33, 484.
(11) The TD-DFT calculations indicate that the T2 state also consists of
an excitation from the HOMO-1 to the LUMO, but its contribution is
smaller than that of the excitation from the HOMO-2.
(12) El-Sayed, M. A. J. Chem. Phys. 1963, 38, 2834.
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FIGURE 10. Molecular structures and molecular orbital diagrams for (A) 8 and (B) 9 calculated at the B3PW91/6-31þG(d) level of theory.
Gray, white, pink, aqua, and blue balls in molecular structures indicate carbon, hydrogen, boron, fluorine, and nitrogen atoms, respectively.
(3.4 eV) for 4a, neither the S1, T1, nor T2 states had the
character of the n-π* excited state (Figure 6B), because both
S1 and T1 states consist of excitation from the HOMO to the
LUMO and the T2 state consists of excitation from the
HOMO-5 (-7.6 eV), which is mainly made up by the
π-orbital on the benzene ring of 4a, to the LUMO. The S1 state
should be 1(π, π*) excited state. The T1 and T2 states should
be 3(π, π*) excited states. As a result, the rate of the
relaxation paths from excited 4a including S1fT2 and/or
S1fT1 intersystem crossings would be lower than those of
10, so that the S1fS0 radiative relaxation process of 4a can
compete with its nonradiative relaxation processes. There-
FIGURE 11. Fluorescence spectra of hydrazones 8 and 9 in hexane.
Thin lines indicate spectra upon excitation at wavelengths of their
absorption maxima (8, 398 nm; 9, 423 nm), and bold lines indicate
fore, this not-suppressed, allowed π-π* transition would
provide 4a with high emitting ability.
Compound 4a also showed fluorescence in the solid state.
A blue emission was observed upon UV irradiation. In the
fluorescence spectra at room temperature, it showed an
emission maximum at 461 nm (Figure 8). This emission
wavelength is almost the same as that in hexane, indicating
that intermolecular interaction of 4a in the solid state is
comparable with that in the solution state.
Absorption and Fluorescence Properties of an Oxime and
Hydrazones. N-Methoxyimine 7 showed its absorption max-
imum at 296 nm in hexane (Figure 9). This blue shift of the
absorption maximum from that of N-phenylimine 4a (324
nm) is due to the existence of the oxygen lone pair, which is
significantly conjugated with the π-orbitals of the benzyli-
deneamine moiety. This contribution of the methoxy group
in 7 makes its absorption blue-shifted by producing a higher
energy level of the LUMO (-2.6 eV) compared with that of
4a (-3.0 eV), while the HOMO energy levels of both 7 and 4a
were almost equal (-7.0 eV for each). In the fluorescence
spectrum in hexane, 7 showed no fluorescence, in contrast to
the aforementioned N-aryl and N-alkylimines, which showed
fluorescence. This may also be an effect of the lone pair on
the oxygen atom.
those upon excitation at shorter wavelengths (8, 290 nm; 9, 320 nm).
In the spectra of 8 upon irradiation at 290 nm, the spike-shaped
peak at 580 nm (cross-marked) is not a part of fluorescence peak of 8
but the second-order scattering light.
investigated by DFT and TD-DFT calculations. The opti-
mized structures of 8 and 9 match well with their crystal
structures. The moderately perpendicular structures at their
N-N bonds disconnect the conjugation between the π-
orbitals of the substituent on the imine nitrogen and the
benzylideneamine moiety. According to the TD-DFT calcu-
lations, the transitions between the ground state and the
lowest singlet excited states of 8 and 9 were assignable to the
transitions between the HOMOs and the LUMOs. Their
HOMOs and LUMOs consist mainly of the π-orbitals of the
substituent on the imine nitrogen and the benzylideneamine
moiety, respectively (Figure 10). The transition between
these spatially separated HOMOs and LUMOs would have
charge-transfer character. Therefore, the charge-transfer
character of the transitions for the longest absorption max-
ima of 8 and 9 suggest that the greatly twisted structures
shown by the X-ray crystallographic analysis and the theo-
retical calculations were also dominant in the solution state.
In the fluorescence spectra in hexane, 8 and 9 showed a
single emission with the emission maximum at 525 and
545 nm, respectively, as shown in Figure 11, upon excitation
at wavelengths of their longest absorption maximum, 398 and
423 nm, respectively. In contrast, they showed dual emis-
sions as shown in Table 3 and Figure 11 upon excitation at
shorter wavelengths, 290 nm for 8 and 320 nm for 9,
The UV-vis and fluorescence spectra of N-indolylimine 8
and N-carbazolylimine 9 in hexane showed a different aspect
from the other imines described above. Their longest absorp-
tion maximum appeared as a broad peak at 398 and 423 nm,
respectively (Figure 9), suggesting that the transition be-
tween the ground state and the lowest singlet excited state
has a charge-transfer character. The molecular orbitals and
the nature of the absorption of hydrazones 8 and 9 were
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TABLE 3. Absorption Maxima (λabs), Molar Absorption Coefficients
(ε), Fluorescence Wavelengths (λem), and Fluorescence Quantum Yields
(ΦF) of Imines 4a-c and 5-9 in Hexane at Room Temperature
a
compound
λabs, nm
ε, M-1 cm-1
λema, nm
ΦF
4a
4b
4c
5
6
7
324
366
439
290
442
296
398
1.0 ꢀ 104
1.5 ꢀ 104
2.5 ꢀ 104
1.3 ꢀ 104
9.1 ꢀ 103
7.6 ꢀ 103
6.8 ꢀ 103
460
466
0.0050b
0.074b
0.73c
500, 527
398
497, 531
0.011b
0.39c
d
8
525
(365, 531)e
545
0.0092b
(f)e
9
423
2.7 ꢀ 103
0.0052b
(380, 396, 545)g
(0.020)b,g
FIGURE 12. Fluorescence spectra of 4c (4.1 μM) in DMF before
(thin line) and after (bold line) addition of sodium cyanide.
aUnless otherwise noted, excited at the longest absorption maxima.
bBased on 9,10-diphenylanthracene in cyclohexane (ΦF = 1.0). cBased
on fluorescein in 0.1 M aq NaOH (ΦF = 0.85). dNo fluorescence.
eExcited at 290 nm. fNot determined. gExcited at 320 nm.
excitation from the HOMO to the LUMO, and this process
involves the intramolecular electron transfer from the sub-
stituent on the imine nitrogen to the benzylideneamine
moiety of 9 (Scheme 3). Finally, emission from the S1 state
gives the lower energy emission band. The case of 8 is
explained analogously.14 The dependence of emission behav-
ior on the excitation wavelength is explained as follows:
excitation at short enough wavelengths gives the local excited
states of the carbazolyl or indolyl moieties and produces dual
emissions. In contrast, excitation at longer wavelengths only
gives the lowest excited state and produces only single, low-
energy emission. Therefore, the spatially separated π-orbit-
als of hydrazones 8 and 9, which are the result of the
moderately perpendicular structures, would provide them
with the dual emission phenomenon.
Reaction of a Boron-Substituted Diphenylazomethine with
a Cyanide Source. Imines are known to form the correspond-
ing R-aminonitriles by addition of cyanide ion in the presence
of a proton source, as represented by the Strecker reaction.15
Aldimines, such as N-benzylideneaniline (10), are also
known to give aldimine-coupling products via cyanide ad-
ducts at imine carbons.16 Boron compounds also react with a
cyanide ion, resulting in formation of cyanoborates.17 These
reactions of imines or boranes with a cyanide source would
greatly change their optical properties. Because the fluores-
cent boron-substituted imines have two reactive sites for a
cyanide ion, a CdN double bond and a boron center, both
reactions are possible and are expected to change the fluo-
rescence properties. To determine which site reacts with a
cyanide ion, reactions of imines 4a and 4c with a cyanide ion
were investigated. In the absorption spectra in DMF, the
absorption maximum of 4c (440 nm) completely disappeared
after addition of excess sodium cyanide, and new absorption
maxima (302 and 328 nm) appeared instead. This large blue
suggesting that two radiative relaxation paths for 8 and 9
exist. In the cases of 8 and 9, the wavelengths of the lowest
energy emission bands upon excitation at both longer and
shorter wavelengths are almost the same, indicating that they
originate from the same excited states. Their excitation
spectra for these emission bands matched with their absorp-
tion spectra, and the shapes of these emission bands con-
sisted of the mirror images of the longest absorption peaks.
Therefore, these lowest energy emission bands are consid-
ered to be emission from the lowest singlet excited states,
those from the CT states. The excitation spectrum of 9 for the
higher energy emissions (396 nm) showed maxima at 326 and
340 nm. The peak shapes of this higher energy emission and
the corresponding absorption resemble those of the emission
and absorption of carbazole. In addition, their emission and
absorption wavelengths are close to each other (λabs of
carbazole = 292 and 332 nm; λem of carbazole = 335 and
347 nm in cyclohexane at 298 K).13 Judging from these
observations, this higher energy emission of 9 is considered
to be an emission from the local excited state of the carba-
zolyl moiety. Lifetimes of fluorescence of 9 for the emissions
at 396 and 545 nm upon excitation at 320 nm were measured
as 18.15(8) and 5.27(7) ns, respectively. A lifetime of fluo-
rescence of 9 at 545 nm upon excitation at 395 nm was
determined as 5.39(3) ns, and it was considered to be
identical to that upon excitation at 320 nm. These two
fluorescence lifetimes of 9 for the higher and the lower energy
emissions were different but not so largely different. Fluo-
rescence lifetimes of 8 showed the same tendency. The higher
energy emission of 8 is considered to be an emission from the
local excited state of the indolyl moiety by analogy with the
case of 9. In the TD-DFT calculations of 9, the transition
giving the local excited state of the substituent on the imine
nitrogen was found as the transition between the HOMO and
the LUMOþ1 (Figure 10B). On the basis of these calcula-
tions, the dual fluorescence behavior of 9 is explained as
shown in Scheme 3. First, UV irradiation at the absorption
band of the carbazolyl moiety gives the local excited state of
the carbazolyl moiety, consisting of the excitation from the
HOMO to the LUMOþ1. One fate of this excited state is the
direct radiative relaxation to the ground state, giving the
higher energy emission band. Another fate is relaxation to
the lowest singlet excited state, formally consisting of the
(14) The transition giving the local excited state of the substituent on the
imine nitrogen in 8 was detected as the transition between the HOMO and the
LUMOþ2.
(15) Strecker, A. Ann. Chem. Pharm. 1850, 75, 27.
(16) (a) Walia, J. S.; Singh, J.; Chattha, M. S.; Satyanarayana, M.
Tetrahedron Lett. 1969, 10, 195. (b) Walia, J. S.; Guillot, L.; Singh, J.;
Chattha, M. S.; Satyanarayana, M. J. Org. Chem. 1972, 37, 135. (c) Reich,
B. J. E.; Greenwald, E. E.; Justice, A. K.; Beckstead, B. T.; Reibenspies, J. H.;
North, S. W.; Miller, S. A. J. Org. Chem. 2005, 70, 8409.
(17) (a) Badugu, R.; Lakowicz, J. R.; Geddes, C. D. Anal. Biochem. 2004,
327, 82. (b) Badugu, R.; Lakowicz, J. R.; Geddes, C. D. Anal. Chim. Acta
2004, 522, 9. (c) Badugu, R.; Lakowicz, J. R.; Geddes, C. D. Dyes Pigm. 2005,
64, 49. (d) Badugu, R.; Lakowicz, J. R.; Geddes, C. D. J. Am. Chem. Soc.
2005, 127, 3635. (e) Huh, J. O.; Do, Y.; Lee, M. H. Organometallics 2008, 27,
1022. (f) Chiu, C.-W.; Kim, Y.; Gabbai, F. P. J. Am. Chem. Soc. 2009, 131,
60. (g) Chiu, C.-W.; Gabbai, F. P. Dalton Trans. 2008, 814. (h) Agou, T.;
Sekine, M.; Kobayashi, J.; Kawashima, T. Chem.;Eur. J. 2009, 15, 5056.
(13) Bonesi, S. M.; Erra-Balsells, R. J. Lumin. 2001, 93, 51.
J. Org. Chem. Vol. 74, No. 19, 2009 7501

Yoshino et al.
JOCArticle
SCHEME 3. Plausible Mechanism of the Dual Emissions in 9a
a(a) At the absorption step. (b) At the relaxation step from the carbazolyl-group-localized excited state. (c) At the final emission step.
SCHEME 4. Reactions of 4a and 4c with NaCN
shift of the absorption maxima suggests disconnection of the
π-conjugated system of imine 4c, meaning the formation of
the corresponding cyanide adduct 11c (Scheme 4). In the
fluorescence spectra of the same solution, the emission of 4c
was completely quenched synchronically to the change in the
absorption spectra (see Figure 12).
of the 3(n, π*) excited state of the imine, resulting in
suppression of the nonradiative deactivation via triplet states
and hence fluorescence emission. Imines containing a more
extended π-conjugated system and/or an electron-donating
group on the π-system showed fluorescence that was more
red-shifted with higher fluorescence efficiency. In the N-
amino derivatives, dual emissions were observed. This phe-
nomenon is explained as a result of the moderately perpen-
dicular structure of the imine moieties. These findings
concerning the relationship between structures and proper-
ties, especially fluorescence, in the fluorescent imine deriva-
tives will expand the possibilities of utilizing imines as more
versatile light-emitting devices. The dual emissions of hy-
drazones based on this new design can be applied to single-
component white color emission material by further tuning
of the substituents. Changing the fluorescence property of
the N-arylimines by the formation of cyanide adducts
showed that these 2-[bis(pentafluorophenyl)boryl]benzy-
lideneamine derivatives have potential as a cyanide ion
sensor in future investigations.
In the 19F NMR spectra, two nonequivalent C6F5 groups
of the products from 4a and 4c were observed. In the 13C
NMR spectra of the reaction solution of 4a, the aldimine car-
bon signal (172.21 ppm) disappeared and new signals of a ter-
tiary carbon (56.76 ppm) and a quaternary carbon (121.16 ppm)
appeared instead (see Supporting Information). Finally, the
cyanide adduct salt 11a0 was isolated in 88% yield by
exchange of a countercation of 11a with tetrabutylammo-
nium bromide in water (Scheme 4) and fully characterized by
spectroscopic methods and elemental analysis. These results
clearly showed that the imine double-bond moiety of the
fluorescent imines was a more favored site for reaction with a
cyanide ion rather than the boron center. These results show
that the imine CdN double bond is essential to the fluores-
cence of imines 4a and 4c and that changing the CdN double
bond into the C-N single bond by addition of a cyanide ion
can turn off their fluorescence. It is suggested that these
2-[bis(pentafluorophenyl)boryl]benzylideneamine deriva-
tives, which do not release the toxic cyanide ion after bind-
ing, can be utilized as a cyanide ion sensor in the future.
Experimental Section
Synthesis of (2-Formylphenyl)bis(pentafluorophenyl)borane
(3). n-BuLi (1.5 M in hexane, 7.3 mL, 11 mmol) was added to
an Et2O solution (40 mL) of 2-bromobenzaldehyde diethyl
acetal (2.20 mL, 10.9 mmol) at -78 °C. The reaction mixture
was stirred for 1.5 h, added to an Et2O solution (10 mL) of
ethoxyborane 2 (3.00 mL, 11.4 mmol) at -78 °C, and allowed to
warm gradually to rt. The reaction mixture was treated with
water at rt and then with diluted hydrochloric acid, stirred for
1 h, and extracted with Et2O. The organic layer was dried over
anhydrous magnesium sulfate and evaporated. Recrystalliza-
tion of the residue from Et2O/hexane afforded 3 (2.45 g, 50%):
pale brown crystals (Et2O/hexane), mp 138.5-140.6 °C (dec).
1H NMR (400 MHz, CDCl3) δ 7.55 (t, 3J = 7.6 Hz, 1H), 7.87
(t, 3J = 7.6 Hz, 1H), 8.13-8.19 (m, 2H), 9.84 (s, 1H). 11B NMR
(128 MHz, CDCl3) δ 8.5 (line width h1/2 = 169 Hz). 13C{1H}
NMR (100 MHz, CDCl3) δ 114.0 (br s, C), 128.5 (s, CH), 130.3
Conclusion
Several boron-substituted imine derivatives, N-substi-
tuted 2-[bis(pentafluorophenyl)boryl]benzylideneamines, were
synthesized very easily and in high yields by condensation
of the corresponding formyl derivative and amines. In
the boryl-substituted N-aryl and N-alkylimines, the B-N
interaction provided them with fluorescence. Thus, non-
fluorescent N-benzylideneaniline was converted into a
blue-fluorescent material. The DFT and TD-DFT calcula-
tions showed that the B-N interaction raised the energy level
7502 J. Org. Chem. Vol. 74, No. 19, 2009

Yoshino et al.
JOCArticle
(s, CH), 132.0 (quint, 5JCF = 3.3 Hz, CH), 135.6-138.57 (m, C),
138.64-141.7 (m, C), 139.0 (s, CH), 146.2-149.1 (m, C), 171.3
(br s, C), 201.2 (s, CH). A signal due to the carbon attached to
the formyl group could not be detected. 19F NMR (376 MHz,
1H NMR (400 MHz, CDCl3) δ 0.92 (t, 3J = 7.3 Hz, 12H), 1.27
(sext, 3J = 7.3 Hz, 8H), 1.36-1.47 (m, 8H), 2.80-2.88 (m, 8H),
5.76 (s, 1H), 6.45 (t, 3J = 7.1 Hz, 1H), 6.75 (d, 3J = 8.1 Hz, 2H),
7.01-7.06 (m, 2H), 7.15-7.18 (m, 2H), 7.37-7.41 (m, 1H),
7.50-7.54 (m, 1H). 11B NMR (128 MHz, CDCl3) δ -4.9 (line
width h1/2 = 125 Hz). 13C{1H} NMR (126 MHz, CDCl3) δ 13.4
(s, CH3), 19.5 (s, CH2), 23.6 (s, CH2), 56.4 (s, CH), 58.3 (s, CH2),
113.7 (s, CH), 114.2 (s, CH), 121.4 (s, C), 121.6 (s, CH), 125.4 (s,
CH), 127.7 (s, CH), 128.3 (s, CH), 128.7 (s, CH), 135.1-139.9
(m, two signals overlapped each other, C), 139.4 (s, C),
146.5-149.9 (m, two signals overlapped each other, C), 149.3
(s, C), 156.4 (br, C). A signal due to the carbon attached to the
boron could not be detected. 19F NMR (376 MHz, CDCl3) δ
3
CDCl3) δ -162.99 to -162.80 (m. 4F), -156.01 (t, JFF = 20
Hz, 2F), -132.05 to -131.90 (m, 4F). Anal. Calcd for
C19H5BF10O: C, 50.71; H, 1.12. Found: C, 50.85; H, 1.28%.
Typical Procedure for the Synthesis of {2-[Bis(pentafluoro-
phenyl)boryl]benzylidene}amine Derivatives 4a-c and 5-9. A
toluene (25 mL) solution of 3 (205 mg, 0.454 mmol) and aniline
(42.4 mg, 0.455 mmol) was refluxed for 4 h, during which water
˚
generated in the reaction was removed with molecular sieves 4A
using a Soxhlet extractor. Evaporation of the solvent gave 4a
quantitatively. Recrystallization from benzene/hexane gave 4a
(199 mg, 83%): yellow crystals (benzene/hexane), mp
191.7-195.5 °C (dec). 1H NMR (400 MHz, CDCl3) δ
7.34-7.47 (m, 6H), 7.54 (ddd, 3J = 7.6 Hz, 3J = 7.3 Hz, 4J =
0.7 Hz, 1H), 7.67 (d, 3J = 7.3 Hz, 1H), 7.86 (d, 3J = 7.6 Hz, 1H),
9.04 (s, 1H). 11B NMR (128 MHz, CDCl3) δ -1.8 (line width
3
-166.14 to -165.72 (m, 4F), -162.96 (t, JFF = 22 Hz, 1F),
-161.72 (t, 3JFF = 20 Hz, 1F), -134.77 to -133.27 (m, 4F). MS
(FABþ) m/z 242 (Bu4Nþ). MS (FAB-) m/z 551 ([M - Bu4N]-).
UV-vis (THF) λ (ε) 257 (1.1 ꢀ 104), 278 nm (1.3 ꢀ 104). Anal.
Calcd for C42H46BF10N3: C, 63.56; H, 5.84; N, 5.29. Found: C,
63.35; H, 5.82; N, 5.14%.
h
1/2 = 152 Hz). 13C{1H} NMR (100 MHz, CDCl3) δ 115.8 (br s,
X-ray Crystallographic Analysis. X-ray diffraction data for
single crystals of 4a-c and 5-9 were collected using
Rigaku MERCURY CCD. The structures were solved by the
direct method and refined by full-matrix least-squares using
SHELX-97.18
Theoretical Calculations. All calculations were carried out at
the DFT level using the B3PW9119,20 exchange-correlation
functional implemented in the Gaussian 03 (Revision D.02)21
suite of programs. All calculations were performed with the
6-31þG(d) basis set. The energy levels of excited states were
obtained by a time-dependent DFT (TD-DFT) method. Zero
point energy corrections are not included in all the calculations.
C), 121.8 (s, CH), 127.2 (s, CH), 127.8 (s, CH), 128.6 (s, CH),
129.4 (s, CH), 129.7 (s, CH), 134.7 (s, CH), 135.5-141.1 (m, C,
two signals overlapped each other), 136.6 (s, C), 140.9 (s, C),
146.4-149.3 (m, C), 165.8 (br s, C), 167.6 (s, CH). 19F NMR
(376 MHz, CDCl3) δ -162.98 to -162.79 (m, 4F), -157.05 (t,
3JFF = 20 Hz, 2F), -131.68 to -131.52 (m, 4F). MS (EI, 70 eV)
m/z 525 (37, Mþ), 506 (21), 376 (24), 358 (100), 328 (31), 309 (65),
237 (69), 210 (63), 77(57), 51 (24%). UV-vis (hexane) λmax (ε)
215 (1.4 ꢀ 104), 324 nm (1.0 ꢀ 104). Anal. Calcd for
C25H10BF10N: C, 57.18; H, 1.92; N, 2.67. Found: C, 57.07; H,
2.15; N, 2.59%.
Reaction of 4a with Sodium Cyanide. To a DMF solution
(0.5 mL) of 4a (47.6 mg, 90.6 μmol) in an NMR tube with a sealed
tube of CDCl3 for NMR locking was added sodium cyanide
(5.2 mg, 0.11 mmol), and the mixture was shaken to make it homo-
geneous. Disappearance of 4a and quantitative formation of
cyanide adduct 11a were observed by 11B, 13C, and 19F NMR
spectroscopy. 11a: 11B NMR (128 MHz, DMF) δ -4.5 (line
width h1/2 = 81 Hz). 13C{1H} NMR (126 MHz, DMF) δ 56.8 (s,
CH), 114.6 (s, CH), 114.7 (s, CH), 121.2 (s, C), 122.4 (s, CH),
126.1 (s, CH), 126.4 (br s, C), 128.1 (s, CH), 128.65 (s, CH),
128.73 (s, CH), 135.0-139.8 (m, two signals overlapped each
other, C), 140.9 (s, C), 146.7-150.3 (m, two signals overlapped
each other, C), 149.7 (s, C), 156.2 (br, C). 19F NMR (376 MHz,
DMF) δ -166.47 to -166.24 (m, 2F), -166.81 to -166.59 (m,
2F), -163.16 (t, 3JFF = 20 Hz, 1F), -162.61 (t, 3JFF = 20 Hz,
1F), -133.30 (br s, 2F), -133.00 (d, 3JFF = 18 Hz, 2F).
Reaction of 4c with Sodium Cyanide. Similarly, the reaction of
4c (4.5 mg, 7.9 μmol) with sodium cyanide (29 mg, 0.59 mmol) in
DMF (1.0 mL) gave observation of disappearance of 4c and
quantitative formation of cyanide adduct 11c in 11B and 19F
NMR. 11c: 11B NMR (128 MHz, DMF) δ -4.4 (line width
Acknowledgment. We thank Dr. Tomohisa Takaya and
Mr. Hiroaki Itoi (The University of Tokyo, Japan) for their
measurement of fluorescence lifetimes. We thank Tosoh
Finechem Corp. and Central Glass Co., Ltd. for gifts of
alkyllithiums and fluorine compounds, respectively. This
work was partially supported by Grants-in-Aid for the
Global COE Program for Chemistry Innovation and for
Scientific Researches from the Ministry of Education, Cul-
ture, Sports, Science and Technology, Japan and the Japan
Society for the Promotion of Science. J.Y. was granted a
Research Fellowship of Japan Society for the Promotion of
Science for Young Scientists. N.K. thanks Shorai Founda-
tion for Science and Technology, Iketani Science and Tech-
nology Foundation, and Mistubishi Chemical Corporation
for research grants.
Supporting Information Available: General experimental
methods. Compound characterization data for 4b, 4c, and 5-9.
Crystal data for 4a-c and 5-9. ORTEP drawings of 4a-c and
5-9. A part of the packing structures of 8 in the unit cell.
Cartesian coordinations of optimized structures by DFT calcu-
lations. Full list of authors for ref 21. NMR spectra of new
compounds. Crystallographic data in CIF format. This material
is available free of charge via the Internet at http://pubs.acs.org.
h
1/2 = 80 Hz). 19F NMR (376 MHz, DMF) δ -166.84 to -166.64
(m, 2F), -166.53 to -166.32 (m, 2F), -163.39 (t, 3JFF = 20 Hz,
1F), -162.76 (t, 3JFF = 20 Hz, 1F), -133.64 to -132.74 (m, 4F).
UV-vis (DMF) λ (ε) 302 (1.6 ꢀ 104), 328 nm (1.2 ꢀ 104).
Isolation of Tetrabutylammonium Salt of a Cyanide Adduct of
4a. A DMF solution (1.0 mL) of 4a (101 mg, 0.192 mmol) was
added to sodium cyanide (52.7 mg, 1.08 mmol) at rt. The
reaction mixture was stirred for 30 min, treated with aqueous
solution (100 mL) of tetrabutylammonium bromide (517 mg,
1.60 mmol), and stirred for additional 10 min. Filtration of the
precipitate followed by washing with water afforded 11a0 (134 mg,
88%): colorless solid (DMF/water), mp 51.2-62.8 °C (dec).
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Wallingford, CT, 2004.
J. Org. Chem. Vol. 74, No. 19, 2009 7503