2,2'-Diborylazobenzenes with double N-B coordination: Control of fluorescent properties by substituents and redox reactions

Article abstract of DOI:10.1002/ejic.201200069

2,2'-Bis[bis(pentafluorophenyl)boryl]azobenzenes were synthesized. X-ray crystallographic analysis exhibits a planar core structure of the azobenzene moiety, double intramolecular N-B coordination, and two tetracoordinate boron atoms. The 2,2'-diborylazobenzenes show fluorescence emission with orange and red colors upon irradiation. The double N-B coordination causes redshifts in both the absorption and emission maxima and a decrease in the Stokes shifts relative to those of the 2-borylazobenzene derivative. On the basis of the density functional theory calculations of the molecular orbitals of 2,2'-diborylazobenzene, the π* orbital (LUMO) was found at a much lower energy level (-4.67 eV) than that in 2-borylazobenzene (-3.70 eV). A reversible reduction wave was observed at a low reduction potential in the cyclic voltammograms of the 2,2'-diborylazobenzenes. Single-electron reduction of one of the 2,2'-diborylazobenzenes generates an azobenzene radical anion, which was confirmed by an active ESR signal. The fluorescence was quenched by the reduction and recovered by air oxidation in this azobenzene. Copyright

Full text of DOI:10.1002/ejic.201200069

SHORT COMMUNICATION
DOI: 10.1002/ejic.201200069
2,2Ј-Diborylazobenzenes with Double N–B Coordination: Control of
Fluorescent Properties by Substituents and Redox Reactions
Naokazu Kano,*[a] Akiko Furuta,[a] Tetsuya Kambe,[a] Junro Yoshino,[a] Yusuke Shibata,[a]
Takayuki Kawashima,*[a][‡] Naomi Mizorogi,[b] and Shigeru Nagase[b]
Keywords: Boron / Fluorescence / Redox chemistry / Radical ions
2,2Ј-Bis[bis(pentafluorophenyl)boryl]azobenzenes were syn-
thesized. X-ray crystallographic analysis exhibits a planar
core structure of the azobenzene moiety, double intramolecu-
lar N–B coordination, and two tetracoordinate boron atoms.
The 2,2Ј-diborylazobenzenes show fluorescence emission
with orange and red colors upon irradiation. The double N–
B coordination causes redshifts in both the absorption and
emission maxima and a decrease in the Stokes shifts relative
to those of the 2-borylazobenzene derivative. On the basis of
the density functional theory calculations of the molecular
orbitals of 2,2Ј-diborylazobenzene, the π* orbital (LUMO)
was found at a much lower energy level (–4.67 eV) than that
in 2-borylazobenzene (–3.70 eV).
A reversible reduction
wave was observed at a low reduction potential in the cyclic
voltammograms of the 2,2Ј-diborylazobenzenes. Single-elec-
tron reduction of one of the 2,2Ј-diborylazobenzenes gener-
ates an azobenzene radical anion, which was confirmed by
an active ESR signal. The fluorescence was quenched by the
reduction and recovered by air oxidation in this azobenzene.
coordination can achieve the task by extension of the π-
conjugation and perturbation of the molecular orbitals
(Figure 1).[9] We report here the synthesis of 2,2Ј-diboryl-
azobenzenes that have double intramolecular N–B coordi-
nation and control of their fluorescent properties.
Introduction
Some popular organic dyes can be used as fluorescent
dyes, which are important for light-emitting devices, fluo-
rescent probes of living tissues, and molecular detection.[1]
Azo dyes are the most frequently used dyes and popular
chromophores;[2] however, azobenzenes, the chromogen of
the azo dyes, usually do not fluoresce,[3] except for a few
faintly fluorescent examples such as ortho-metallated azo-
benzenes[4] and self-assembled aggregates of azobenzene de-
rivatives.[5] Intramolecular coordination from nitrogen to
boron, as seen in BODIPY,[6] conveniently changes nonflu-
orescent heteroaromatics to fluorescent molecules.[7] In-
deed, we synthesized green-fluorescent 2-[bis(pentafluoro-
phenyl)boryl]azobenzene [(E)-1a] (Figure 1), in which the
intramolecular N–B coordination effectively restricts con-
formational freedom of one benzene ring and changes the
HOMO and LUMO energy levels.[8] Fluorescence emission
of other colors and control of the emission by redox reac-
Figure 1. Molecular design for conformational fixation of an azo-
benzene.
tions are desirable for application to functional materials Results and Discussion
such as organic light-emitting diodes. Doubling the N–B
Successive reactions of (E)-2,2Ј-diiodoazobenzenes (E)-
2a–d with nBuLi and (C6F5)2BOEt gave 2,2Ј-diborylazo-
benzenes (E)-3a–d as air-stable solids (Scheme 1). Their 11
[a] Department of Chemistry, Graduate School of Science,
The University of Tokyo,
B
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
E-mail: kano@chem.s.u-tokyo.ac.jp
NMR spectra in CDCl3 [δ = –2.9 to –1.6 ppm] show that
they have tetracoordinate boron atoms.[8a]
[b] Department of Theoretical and Computational Molecular
Science,
X-ray crystallographic analysis of (E)-3a reveals its al-
most C2-symmetrical structure and the double intramolecu-
lar N–B coordination [N–B: 1.634(3), 1.635(3) Å] (Fig-
ure 2).[10] The N1–N2 bond length [1.292(3) Å] is within the
range of a double bond. The torsion angles around the azo
moiety [N1–N2–C6–C5: 176.5(2)°, N2–N1–C24–C23:
Institute for Molecular Science
Myodaiji, Okazaki 444-8585, Japan
[‡] Present address: Faculty of Science, Gakushuin University,
1-5-1 Mejiro, Toshima-ku, Tokyo, 171-8588, Japan
E-mail: Takayuki.Kawashima@gakushuin.ac.jp
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201200069.
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Eur. J. Inorg. Chem. 2012, 1584–1587

2,2Ј-Diborylazobenzenes with Double N–B Coordination
Scheme 1.
175.3(2)°] show that its coplanarity is greater than that in
(E)-1a.[8a,8c] This structural motif, where two main group
elements bridge the azo group to form fused five-membered
rings, is quite rare.[9,11]
Figure 4. Photographs of (E)-3a–d in hexane under room light (top)
and UV light (bottom).
Table 1. Optical data for (E)-3a–d and related azobenzene deriva-
tives in hexane.
R
λabs
(nm)
logε
λFL
(nm)
ΦF
Stokes shift
(cm–1)
(E)-1a[a]
(E)-4[a]
(E)-3a
(E)-3b
(E)-3c
(E)-3d
H
–
H
nBu
Br
386
315
521, 552
547, 582
549, 584
4.28
4.35
4.30, 4.20 572
4.41, 4.33 597
4.72, 4.76 602
4.74, 4.85 663
503
–
0.20
–
0.26
0.73
0.39
0.62
6000
–
630
430
510
400
OnBu 603, 646
[a] Ref.[8c]
Figure 2. ORTEP drawing of (E)-3a (thermal ellipsoid plot with
50% probability).
Stokes shifts (400–630 cm–1) are much smaller than that of
(E)-1a (6000 cm–1), which reflects their rigid conformation.
Their fluorescence quantum yields, ΦF, range from 0.26 to
0.73 (Table 1), which are much higher than the value of
2.53ϫ 10–5 for (E)-4.[12] Considering that (E)-3a is formed
by additional N–B bond formation between (E)-1a and the
boryl group (a Lewis acid moiety), the fluorescence emis-
sion of (E)-3a remarkably contrasts the fluorescence
quenching of (E)-1a by N–H bond formation upon addition
of a Brønsted acid.[8b] Moreover, in (E)-3a–d, all alkyl,
bromo, and alkoxy groups at the 4- and 4Ј-positions cause
both redshifts and an increase in ΦF values.
Hexane solutions of (E)-3a–d show orange, pink, purple,
and blue colors, respectively, with longer absorption max-
ima, λabs, in the UV/Vis spectra than those for PhN=NPh
[(E)-4] and (E)-1a (Figures 3 and 4, Table 1). The large red-
shifts are caused by effective π-conjugation, which reflects
good coplanarity.
The molecular orbitals of (E)-3a, studied by density
functional theory (DFT) calculations at the B3PW91/6-
31+G(d) level of theory, were compared with those of (E)-
1a and (E)-4.[8a,8c,13] The n orbital, which is the HOMO
(energy level: –6.45 eV) in (E)-4, is stabilized considerably
by N–B coordination, which corresponds to HOMO–19
(–10.7 eV) in (E)-3a and HOMO–8 (–8.9 eV) in (E)-1a. In
addition, the π* orbital (LUMO) in (E)-3a is found at a
Figure 3. UV/Vis absorption and fluorescence spectra of (E)-3a–d
in hexane.
Irradiation of (E)-3a–d in hexane causes orange or red lower energy level (–4.67 eV) than those in (E)-1a
fluorescence emission, unlike the nonfluorescent behavior (–3.70 eV) and (E)-4 (–2.55 eV).
of most normal azobenzene derivatives (Figure 4). In the
The electrochemical properties of (E)-3a,b were studied
fluorescence spectra of (E)-3a–d (excitation at 500 nm), the by cyclic voltammetry. The cyclic voltammogram of (E)-3a
emission maxima λFL are between 572 and 663 nm, much exhibits a reversible reduction wave at E1/2 = –0.13 V (Epc
longer than that of (E)-1a (Figure 3).[8a] Moreover, their = –0.17 V, Epa = –0.09 V) vs. Ag/Ag+ in CH2Cl2, while (E)-
Eur. J. Inorg. Chem. 2012, 1584–1587
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N. Kano, T. Kawashima et al.
SHORT COMMUNICATION
4 shows an irreversible reduction wave (Epc = –1.07 V). In duction and oxidation reactions. Control of the fluorescent
addition, (E)-3b exhibits a similar reversible reduction wave properties of an azobenzene by doubling the N–B coordina-
[(E)-3b: E1/2 = –0.28 V]. In accordance with the calculated tion and the redox reactions is significant for further appli-
low LUMO level, the reduction potentials suggest that (E)- cation.
3a,b can be reduced more easily than (E)-4 and (E)-1a
(E1/2 = –0.81 V). Hence, (E)-3b, which fluoresces most
brightly among (E)-3a–d, was reduced chemically by treat-
ment with decamethylcobaltocene (1 equiv.) in THF to gen-
Conclusions
erate an azobenzene radical anion, an intermediate in the
We synthesized four 2,2Ј-diborylazobenzenes with a
double intramolecular N–B coordination, which results in
the planar structure of the azobenzene core. The azoben-
zenes fluoresce with orange and red colors with fluores-
cence quantum yields (ΦF) ranging from 0.26 to 0.73. These
fluorescence colors and small Stokes shifts are in contrast
to those of 2-borylazobenzene derivatives. The LUMO of
the 2,2Ј-diborylazobenzene is the π* orbital of the azobenz-
redox interconversion between an azobenzene and a
hydrazobenzene.[14] Formation of the azobenzene radical
anion 5b was confirmed by the color change from red to
blue [5b: λabs (THF) = 613 nm] and an active ESR signal
(Scheme 2, Figure 5). The ESR spectra of 5b in THF at
both 4.2 and 100 K show a broad signal with a g value of
2.0037 without a resolved hyperfine splitting (peak-to-peak
width 32 G). Such broadening of the signal without any
ene moiety and is found at a much lower energy level than
hyperfine coupling was also found in the ESR spectra of 2-
that in the 2-borylazobenzene. The single-electron re-
duction of one of the 2,2Ј-diborylazobenzenes generated an
azobenzene radical anion, which does not fluoresce at all,
and air oxidation recovers its fluorescence emission. The
(arylazo)pyridine radical anions and arylazoimine radical
anions with nitrogen–metal coordination.[15] In these litera-
ture cases, the 14N hyperfine splitting is small in these π-
radicals, and it is difficult to resolve it in the ESR spectrum
reversible changes in both the absorption and emission col-
because of dominant anisotropic contributions. The DFT
ors by electrochemical processes may open perspectives for
calculations at the UB3PW91/6-31+G(d) level show that
its use in optoelectronic devices.
the SOMO of 5b has a large azo-π* character (see Support-
ing Information). The broadening of the ESR spectrum can
be explained similarly to that in the literature. Exposure of
Experimental Section
a THF solution of 5b to air leads to rapid oxidation, to
recover (E)-3b. The apparent color change is demonstrated
by a conspicuous difference in the absorption spectra [(E)-
3b: λabs (THF) = 558, 592 nm]. Moreover, 5b does not
fluoresce. The absorption and fluorescence properties of
(E)-3b can be switched reversibly by single-electron re-
nBuLi (1.5 m in hexane, 2.1 mL, 3.15 mmol) and an Et2O solution
(10 mL) of ethoxy[bis(pentafluorophenyl)]borane (1.99 mL,
3.18 mmol) were successively added to 2,2Ј-diiodoazobenzene
(600 mg, 1.38 mmol) in Et2O (300 mL) at –116 °C. After gradual
warming of the mixture to room temperature, evaporation of sol-
vents, separation by silica gel column chromatography, and
recrystallization from hexane gave (E)-3a (0.50 g, 42%). Red solid.
M.p. Ͼ400 °C. 1H NMR (400 MHz, CDCl3): δ = 7.47 (t, 3J =
3
7.7 Hz, 2 H), 7.63 (t, 3J = 7.7 Hz, 2 H), 7.79 (d, J = 7.7 Hz, 2 H),
7.93 (d, 3J = 7.7 Hz, 2 H) ppm. 11B NMR (128 MHz, CDCl3): δ =
–1.6 (line width h1/2 = 783 Hz) ppm. 13C{1H} NMR (100 MHz,
CDCl3): δ = 111.3 (br. s), 122.4 (s), 130.0 (s), 130.4 (s), 136.7 (s),
1
1
137.4 (dm, JCF = 254.9 Hz), 138.7 (dm, JCF = 245.4 Hz), 147.1
(s), 147.7 (dm, JCF = 241.7 Hz), 157.3 (br. s) ppm. 19F NMR
1
3
(376 MHz, CDCl3): δ = –162.63 (t, J = 18.4 Hz, 8 F), –155.13 (t,
3J = 18.4 Hz, 4 F), –131.65 (t, J = 18.4 Hz, 8 F) ppm. MS (FAB):
3
Scheme 2. Formation of azobenzene radical anion 5b from (E)-3b.
m/z = 898 [M+]. C36H8B2F20N2 (870.05): calcd. C 49.70, H 0.93, N
3.22; found C 49.63, H 1.19, N 3.12.
Supporting Information (see footnote on the first page of this arti-
cle): Synthetic procedures, spectroscopic data, cyclic voltammog-
rams, UV/Vis spectra, ESR measurements, and DFT calculations
are presented.
Acknowledgments
We thank Tosoh Finechem Corp. for the gift of butyllithium. This
work was supported by Grants-in-Aid for the GCOE Program and
for Scientific Researches (No. 22108508, 22685005, 08J06720) from
MEXT and JSPS. N. K. thanks JGC-S Scholarship Foundation,
Shorai Foundation for Science and Technology, and Mitsubishi
Chemical Corp. Fund for research grants.
Figure 5. An ESR spectrum of 5b at 4.2 K (left) and a photograph
of 5b in THF (right).
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Eur. J. Inorg. Chem. 2012, 1584–1587

2,2Ј-Diborylazobenzenes with Double N–B Coordination
T = 120(2) K, 18546 measured and 5525 independent reflec-
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wR2 (all data) = 0.1153. CCDC-851780 contains the supple-
mentary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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Received: January 23, 2012
Published Online: February 21, 2012
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