Synthesis of the most intensely fluorescent azobenzene by utilizing the B-N interaction

Article abstract of DOI:10.1039/b615966d

A boron-substituted azobenzene, (E)-[2-(4-methoxyphenylazo)phenyl] bis(pentafluorophenyl)borane, presented the most intense fluorescence among the azobenzene derivatives. The Royal Society of Chemistry.

Full text of DOI:10.1039/b615966d

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Synthesis of the most intensely fluorescent azobenzene by utilizing the
B–N interaction{
Junro Yoshino, Naokazu Kano and Takayuki Kawashima*
Received (in Cambridge, UK) 2nd November 2006, Accepted 8th December 2006
First published as an Advance Article on the web 20th December 2006
DOI: 10.1039/b615966d
spectroscopic properties, and theoretical calculations of this
azobenzene and related compounds.
A boron-substituted azobenzene, (E)-[2-(4-methoxyphenylazo)-
phenyl]bis(pentafluorophenyl)borane, presented the most
intense fluorescence among the azobenzene derivatives.
Successive reaction of (E)-2-iodoazobenzene ((E)-1) with n-BuLi
and fluoroborane 2a in Et2O at 2112 uC gave a boron-substituted
azobenzene (E)-3a (28%) as a brown solid, which can be handled
in the air (Scheme 1). Compounds (E)-3b (31%) and (E)-3c (27%)
were synthesized similarly from (E)-1 and 2b and from (E)-4 and
2a, respectively. Tetracoordination states of the boron atoms of
(E)-3a–c were confirmed by the 11B NMR spectra in CDCl3 [(E)-
3a: dB 20.8, (E)-3b: dB 7.0{, (E)-3c: dB 20.6]. The X-ray
crystallographic analysis of (E)-3a revealed that (E)-3a has a
Azobenzene is one of the most common and frequently-used
chromophores.1 Many azobenzene derivatives have been synthe-
sized and used as dyes. Azobenzenes are also used as photo-
responsive molecular switches by taking advantage of their
photoisomerization.2 Their photoisomerization features inhibit
another important property of chromophores, fluorescence. Since
the photoisomerization process is very efficient in photoexcited
azobenzenes, their radiative deactivation processes, which are
slower, are not competitive.3 Hence, azobenzene itself virtually
does not emit light.4 There have been only limited exceptional
azobenzene derivatives that show fluorescence, however, most of
them fluoresce only in a rigid matrix at low temperature and their
fluorescence quantum yields, WF, are quite low.5 Ortho-metalated
azobenzene complexes6 and self-assembled aggregates of azoben-
zene derivatives7 have also been reported as emissive azobenzene
derivatives and most of their fluorescence quantum yields at
ambient temperature in the solution state are about 1023, which
are still low compared with organic fluorescent materials. In
general, azobenzenes have remained as non-fluorescent chromo-
phores while many other organic dye molecules such as
rhodamines and fluoresceins have been developed as highly
emissive chromophores and utilized for the labeling of biomole-
cules such as proteins to make them detectable.8 Azobenzenes
would have promise to be useful as fluorescent materials, with
applications in light-emitting devices, fluorescent probes, and
molecular detection, if they emitted fluorescence because of the
variety of easy methods for synthesizing azobenzenes and for
tuning their properties.
NLN bond length [1.2704(18) A]11 which is almost the same as that
˚
12
˚
of the previously reported (E)-azobenzenes (avg. 1.25 A).
In the UV–vis spectrum in hexane, (E)-3a showed an absorption
maximum at 386 nm (e = 1.9 6 104), assignable to the p–p*
transition of the azo group, and no resolved band corresponding
to the n–p* transition (Fig. 1(a)). This large bathochromic shift
from the p–p* transition of unsubstituted (E)-azobenzene ((E)-5)
(l = 315 nm) suggests a considerable perturbation of the electronic
structure of the azobenzene moiety of (E)-3a by the B–N
interaction.
Irradiation (l = 360 nm) of (E)-3a and (E)-3c in C6D6 at r.t.
with a super-high-pressure mercury lamp (500 W) did not cause
their photoisomerization to (Z)-3a and (Z)-3c, but caused strong
green fluorescence, in contrast to the behavior of most azobenzene
derivatives (Fig. 1(b)). In the fluorescence spectra in hexane, (E)-3a
and (E)-3c showed emission maxima at 503 nm (excited at 386 nm)
and 524 nm (excited at 439 nm), respectively (Fig. 1(a)). Matching
Recently, Yamaguchi et al. reported that the fixation of
p-conjugate systems of boryl-substituted thienylthiazoles into a
planar conformation with the B–N interaction provided them with
fluorescence.9 Meanwhile, during the course of our study on
azobenzene derivatives bearing a main-group-element substitu-
ent,10 we have found the most intensely fluorescent azobenzene by
utilizing the B–N interaction. We report here the synthesis,
Department of Chemistry, Graduate School of Science, The University
of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
E-mail: takayuki@chem.s.u-tokyo.ac.jp; Fax: +81-3-5800-6899;
Tel: +81-3-5800-6899
{ Electronic supplementary information (ESI) available: Synthesis and
characterization of (E)-3a–c, the X-ray crystallographic analysis for (E)-3a,
Cartesian coordinates and molecular orbital diagrams for (E)-3a–c. See
DOI: 10.1039/b615966d
Scheme 1 Synthesis of (E)-3a–c.
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Fig. 1 (a) Absorption (blue line) and fluorescence (red line) spectra of
(E)-3a and (E)-3c in hexane at r.t. (b) Photograph of hexane solution (ca.
5 mM) of (E)-3c under irradiation by 360 nm light.
between the peak of the excitation spectra corresponding to the
emission at the wavelength of the emission maxima and the
absorption maxima due to the p–p* transition for both (E)-3a and
(E)-3c indicates that these emissions are assignable to the transition
from the 1(p, p*) state to the ground state. The fluorescence
quantum yields of (E)-3a and (E)-3c in hexane at r.t. were
determined as WF = 0.23 and 0.76, respectively,13 which are
extremely high compared with 2.53 6 1025 for the unsubstituted
azobenzene (E)-5;14 the latter value is higher than those of any
azobenzene derivatives reported previously.5–7 In contrast, (E)-3b
showed almost no fluorescent emission. The substituents on the
boron atom should be responsible for the intensity of the
fluorescence, considering the above results.
Fig. 2 Calculated (a) energy levels of frontier molecular orbitals and (b)
singlet excitation energies of (E)-azobenzene ((E)-5) and (E)-3a–c at the
B3PW91/6-31+G(d)//B3PW91/6-31G(d) level of theory. In (a), the orbitals
indicated by gray–black and gray levels originate from the p (p*) and n
orbitals of the azo group, respectively. In (b), gray–black and gray excited
1
1
states have the character of (p, p*) and (n, p*) states, respectively.
The electronic states and excitation energies of (E)-3a–c and
unsubstituted azobenzene ((E)-5) were investigated by theoretical
calculations using density-functional theory (DFT) and a time-
dependent DFT (TD-DFT) method at the B3PW91/6-31+G(d)//
B3PW91/6-31G(d) level of theory.15 The optimized structure of
(E)-3a matches well with its crystal structure. In (E)-5, the p*, n
and p orbitals are assignable to the LUMO, HOMO, and HOMO-
1, respectively. However, the energy levels of the orbitals of (E)-3a–
c that originate from the n orbital of the azo group (28.9, 28.5,
and 28.7 eV, respectively) are lower than those originating from
the p orbital of the azo group (27.1 to 27.3, 26.9 to 27.1, and
26.6 eV, respectively) (Fig. 2(a)). Lowering of the energy level of
the n orbital would be caused by the B–N bonding interaction. As
a reflection of this anomaly, the 1(p, p*) excited states of (E)-3a–c
(2.8 to 3.2, 3.0 to 3.2, and 2.8 eV, respectively) are below the 1(n,
p*) excited states (3.9, 4.1, and 4.0 eV, respectively) (Fig. 2(b)).
This order of the energy levels is in contrast to that of (E)-5.
Fig. 3 Molecular orbital diagrams for the HOMO of (a) (E)-3a, (b) (E)-
3b, and (c) (E)-3c (B3PW91/6-31+G(d)//B3PW91/6-31G(d)).
quantum yield to quite low. Therefore, the characteristic emission
behavior of (E)-3a and (E)-3c should be attributed to the inverted
order of the 1(n, p*) and 1(p, p*) states and the low-lying occupied
orbitals of the strong electron-withdrawing C6F5 group.
In addition, the tight B–N coordination caused by fixation of
the boron atom and the nitrogen atom at a suitable position to
interact can provide more rigidity for the structure around the azo
group than protonation of azo groups and substitution with amino
groups.5a,c–e The enhanced rigidity of the structure of (E)-3a and
(E)-3c suppresses the conformational change around the azo group
like boryl-substituted thienylthiazoles.9 This suppression locks the
photoisomerization process around the NLN double bond, which
is the mechanism responsible for the very efficient radiationless
deactivation in the unsubstituted azobenzene,3 and helps provide
(E)-3a and (E)-3c with the extremely high fluorescence quantum
yield.
1
1
Inversion of the order between the (n, p*) and (p, p*) states on
the energy scale increases the transition probability from excited
states to the ground state based on the orbital symmetries, and this
inversion has been considered as one of the causes of fluorescence
of protonated azobenzenes and 4-(dialkylamino)azobenzenes.5a,c–e
In addition, the p orbitals of the azo group of (E)-3a and (E)-3c lie
on their HOMO (Fig. 3(a) and (c)). As a result, the 1(p, p*) excited
state of (E)-3a and (E)-3c occupies the lowest singlet excited state
S1, and the transition between S1 and ground states is allowed. In
contrast, the HOMO of (E)-3b is not on the azobenzene moiety
but localized on the aryl groups attached to the boron atom
(Fig. 3(b)). The relatively high energy level of the occupied orbitals
of p-FC6H4 groups prevents the p orbital of the azo group of (E)-
3b from occupying the HOMO and restricts its fluorescence
In conclusion, we have succeeded in synthesizing the most
intensely fluorescent azobenzene. Both the intramolecular B–N
dative bond and the strong electron-withdrawing C6F5 groups
have been revealed to play an important role in providing the
azobenzene with extremely high light-emitting ability, which could
560 | Chem. Commun., 2007, 559–561
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assembled aggregation. This new design to provide azobenzene
with fluorescence will expand the possibilities of utilizing
azobenzenes for more versatile fluorescent probes of chemical
sensors and light-emitting devices. Tuning of the emission
maximum wavelengths and study on excited states and decay
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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 21st
Century COE Program for Frontiers in Fundamental Chemistry
(T.K.) and for Scientific Researches from the Ministry of
Education, Culture, Sports, Science and Technology, Japan and
the Japan Society for the Promotion of Science.
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Notes and references
{ The VT-NMR revealed that the equilibrium in the solution between the
tricoordinate and tetracoordinate boron state of (E)-3b is negligible because
its chemical shift was almost unchanged in the temperature range from
260 uC to +60 uC. Our preliminary theoretical calculations on the 11B
NMR chemical shifts of (E)-3a and (E)-3b support the proposal that the
difference between their observed chemical shifts should be attributed not
to the equilibrium but to the difference in the substituents on the boron
atom.
11 Crystallographic data for (E)-3a: C24H9BF10N2, M = 526.14, triclinic,
˚
a = 9.861(7), b = 10.732(6), c = 10.822(7) A, a = 99.976(7), b =
3
˚
113.714(8), c = 91.109(7)u, U = 1027.7(11) A , T = 120 K, space group
P1, Z = 2, m(Mo-Ka) = 0.164 mm21, 6375 reflections measured, 3488
¯
unique (Rint = 0.0191) which were used in all calculations. The final R1
(I . 2s(I)) and wR2 (all data) were 0.0371 and 0.1068, respectively.
GOF = 1.089. CCDC 607217. For crystallographic data in CIF or other
electronic format see DOI: 10.1039/b615966d.
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