Through-space charge-transfer emitting biphenyls containing a boryl and an amino group at the o, o ′-positions

Article abstract of DOI:10.1021/ol201909r

A new class of organoboron compounds containing a boryl and an amino group at the o,o′-positions of biphenyls display bright through-space intramolecular charge transfer fluorescence owing to the close contact between the boryl and the amino groups. Bindi

Full text of DOI:10.1021/ol201909r

ORGANIC
LETTERS
2011
Vol. 13, No. 18
4830–4833
Through-Space Charge-Transfer Emitting
Biphenyls Containing a Boryl and an
Amino Group at the o,o0-Positions
Hong Pan, Guang-Liang Fu, Yi-Hong Zhao, and Cui-Hua Zhao*
School of Chemistry and Chemical Engineering, Shandong University, Shanda Nanlu 27,
Jinan 250100, People’s Republic of China
chzhao@sdu.edu.cn
Received July 14, 2011
ABSTRACT
A new class of organoboron compounds containing a boryl and an amino group at the o,o0-positions of biphenyls display bright through-space
intramolecular charge transfer fluorescence owing to the close contact between the boryl and the amino groups. Binding of the fluoride ions
results in the remarkable blue shift and color change of the fluorescence, enabling colorimetric and ratiometric fluoride ion sensing.
Tricoordinate boron-containing π-conjugated materials
have attracted increasing attention due to their intriguing
electronic and photophysical properties, which arise from
their unique ability to accept electrons through the pπꢀπ*
conjugation between the vacant p orbital on the boron atom
and the π* orbital of the π-conjugated framework.1 When an
appropriate electron donor is present, the electron-accepting
ability of the boron center makes organoboron compounds
display intense intramolecular charge transfer (CT)
transitions, which have been exploited extensively in a wide
range of applications, such as nonlinear optical materials,2,3
two-photon absorption and emission materials,4,5 organic
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r
10.1021/ol201909r
Published on Web 08/17/2011
2011 American Chemical Society

light-emitting diodes,6ꢀ8 and chemical sensors for fluoride
and cyanide anions.9ꢀ13
The syntheses of the boryl-substituted biphenyls 1ꢀ2
were accomplished via the lithiation of the corresponding
bromo-substituted biphenyls with n-BuLi followed by
treatment with dimesitylfluoroborane (see Supporting In-
formation, SI). These two compounds are stable to air and
water and can be purified by flash column chromatogra-
phy. The structure of 1 was determined by X-ray crystal-
lography. As shown in Figure 2, the biphenyl unit is
significantly twisted with a torsion angle of 70.7° between
two benzene rings. This nonplanar structure apparently
arises from the steric hindrance of the lateral boryl and
amino groups. It is worth noting that despite the remark-
able steric congestion between the boryl and the amino
groups and the rotation flexibility of single bond in the
biphenyl framework, the biphenyl moiety is arranged in
The majority of organoboron compounds exhibiting
intramolecular CT transition generally have a linear geo-
metry, in which the donor and the acceptor are attached at
the terminal positions of the conjugated spacer. We and
others have recently disclosed a class of charge-transfer
emitting organoboron materials, in which the electron-
accepting boryl groups were introduced at the lateral
positions instead.14 These organoboron materials exhibit
extremely intense fluorescence even in the solid state
owing to the suppressed fluorescence quenching as a result
of the steric bulkiness of the boryl groups and the large
Stokes shift induced by the intramolecular CT transition.
In the above-mentioned systems, the intramolecular CT
transition takes place through the corresponding aromatic
linker. In contrast, when the donor and acceptor are con-
nected by a rigid bridge in the U-shaped system,11 the rigidity
of the linker forces the π-systems of the chromophores out of
coplanarity with that of the linker. As a consequence, the
charge transfer occurs through space rather than through
bond, promoting the through-space intramolecular CT fluor-
escence, which is switchable to πꢀπ* fluorescence by the
addition of fluoride. Encouraged by the unusual photophy-
sical properties and the various potential applications of the
intramolecular CT emitting organoboron materials, here we
now disclose a new class of tricoordinate organoboron
compounds 1 and 2, in which a boryl group and an amino
group are incorporated at o- and o0-positions of the biphenyl
framework (Figure 1). We envisioned the steric effect of the
lateral boryl and amino groups would make the biphenyl
moiety exhibit a very twisted structure and permit a very close
contact between them. The special molecular structure and
the electronic interaction between the boryl and the amino
groups would lead to unique properties. In fact, we found
that such molecules display intense fluorescence in both
solution and the solid state with much longer emission
wavelength compared to those of the regioisomers. Binding
of 1 with fluoride ions can lead to a 165 nm blue shift of the
fluorescence and a significant emission color change, en-
abling colorimetric and ratiometric fluoride ion sensing.
˚
the geometry with a B N distance of 3.59 A, much
shorter than those of the through-space CT emitting
organoboron compounds reported so far.11
3 3 3
Figure 2. ORTEP drawing of 1. Thermal ellipsoids are drawn at the
50% probability level. Hydrogen atoms are omitted for clarity.
In cyclohexane, the UVꢀvis absorption spectrum of 1
features an intense band at 306 nm (log ε = 4.74) (Figure
3). In the fluorescence spectrum, it displays a strong green
emission at 521 nm (ΦF = 0.47 in cyclohexane). Two
notable features are the particularly large Stokes shift
(Δλ = 215 nm, Δν = 13485 cmꢀ1) and very long emission
wavelength for this biphenyl unit with such a limited
conjugation length. The TDDFT calculation (B3LYP/6-
31G(d)) indicates that the intense absorption band at 306 nm
is assignable to the transition from HOMOꢀ1 located on
the filled π-orbitals of the mesityl and phenyl rings to the
Figure 1. (a) Schematic representation and structures of biphe-
nyls 1, 2 containing a boryl group and an amino group at o,o0-
positions and (b) structures of the related compounds 3, 4.
(10) (a) Deng, Y.-L.; Chen, Y.; Cao, D.-X.; Liu, Z.-Q.; Li, G.-Z.
Sens. Actuators, B 2010, 149, 165. (b) Liu, Z.-Q.; Shi, M.; Li, F.-Y.;
Fang, Q.; Chen, Z.-H.; Yi, T.; Huang, C.-H. Org. Lett. 2005, 7, 5481.
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5713. (b) Liu, X.-Y.; Bai, D.-R.; Wang, S. Angew. Chem., Int. Ed. 2006,
45, 5475. (c) Hudson, Z. M.; Liu, X.-Y.; Wang, S. Org. Lett. 2011, 13,
300.
Figure 3. Absorption and fluorescence spectra of 1ꢀ4 in
cyclohexane.
Org. Lett., Vol. 13, No. 18, 2011
4831

comparable environment that has arisen in the solid
state and in the dilute cyclohexane solution.
To elucidate the uniqueness of the photophysical prop-
erties for the current through-space intramolecular CT
organoboron π-system, we compared the photophysical
properties of 1 with those of its regioisomers 3 and 4. In
contrast to 1, 3 and 4 display distinct intramolecular CT
absorption bands at 388 and 369 nm respectively, with 4
having the highest absorption intensity (log ε = 3.48 for 3;
4.50 for 4). The assignments of the CT absorption bands
have been corroborated by the theoretical calculations
(Figure 4) and the large solvent dependence of the fluo-
rescence spectra (see SI). Although the calculated first
excited energies of these three compounds are not remark-
ably different, the oscillator strength increases greatly
from 0.0048 for 1 to 0.0821 for 3 and 0.616 for 4, which
is in good agreement with the order of the experimentally
obtained molecular absorption coefficients for these three
molecules. These results suggest that the nature of the
transition significantly depends on the geometry of the
molecular structure. In compound 1, the benzene rings of
the biphenyl main chain are approximately perpendicular
to each other due to the large steric congestion of the
lateral boryl and amino groups, which in return results in
the poor conjugation of the biphenyl. From 1 to 3 and
4, with the decrease in the steric congestion of the lateral
groups, the biphenyl moiety presumably becomes in-
creasingly planar, which makes the conjugation more
efficient. As a result, the HOMO is most delocalized in
4, facilitating the CT transition from the amino group to
the boryl group. It is most noteworthy that 3 and 4 exhibit
blue and purple blue fluorescence at much shorter
wavelengths compared to that of 1 (λem = 477 nm for
3; 409 nm for 4 in cyclohexane). Similarly, the emission
wavelength of 1 is the longest among these three
compounds for their spin-coated films (λem = 519
nm for 1; 507 nm for 3; and 478 for 4). Based on the
LippertꢀMataga plots (see SI), 1 displays the smallest
dipole moment change from the ground state to the
excited state. And thus the long emission wavelength
of 1 may arise from its extremely twisted structure,
which would cause a significant structural relaxation
in the excited state and thus substantial structure
differences between the twisted structure in the ground
state and presumably planar structure in the excited
state. The current molecular design may provide a new
method to achieve fluorescence at long wavelengths.
The through-space charge transfer is a general feature
for the present biphenyl π systems containing a boryl and
an amino group at the o,o0-positions. Compound 2 also
shows a characteristic through-space emission, which is
also highly dependent on the polarity of the solvents
(see SI). Due to the lower electron-donating ability of the
diphenylamino group relative to that of the dimethyl-
amino group, a significant blue shift in the fluorescence
of 2 was observed compared to that of 1, ca. 90 nm in
cyclohexane. Notably, the absorption of 2 is very similar to
that of 1, confirming that the intense absorption band of 1
at 306 nm is assignable to the transition from HOMOꢀ1 to
Figure 4. Plot of the KohnꢀSham HOMO and LUMO energy
levels and pictorial drawings of the HOMOs and LUMOs for (a)
1, (b) 3, and (c) 4. The transition energies and oscillator strengths
were calculated at the B3LYP/6-31G(d) level of theory.
LUMO located on the dimesitylborylphenyl moiety (see
SI). The lowest electronic transition in fact mainly consists
of a charge transfer from the HOMO localized on the
dimethylaminophenyl unit to the LUMO with a quite
small oscillator strength (f = 0.0048), which presumably
makes the lowest transition band too weak to be distin-
guished (Figure 4). Since there is almost no overlap
between the two frontier orbitals, the charge transfer from
the HOMO to the LUMO takes place most likely through
space rather than through bond. The through-space CT
transition was further supported by a substantial fluores-
cence solvatochromism, from 521 nm in cyclohexane to
540 nm in benzene, 547 nm in chloroform, 551 in THF, and
580 nm in MeCN (see SI). Another notable feature for 1 is
that it shows increased fluorescence intensity in the
solid state, similar to other organoboron compounds
containing lateral boryl groups.14c The quantum yield
for its spin-coated film is 0.86, as determined by a
calibrated integrating sphere. Such a fluorescence
behavior may be ascribed to the easily exchangeable
multiple conformations of the nonplanar structure of
the main chain, which facilitate the nonradiative decay
of the excited state. In the solid state, the exchanges
between multiple conformations are greatly sup-
pressed due to the spatial congestion of the molecular
stacking in the solid state.15 The emission spectrum is
quite similar to that in cyclohexane, indicative of a
€
(12) Li, H.; Jakle, F. Macromol. Rapid Commun. 2010, 31, 915.
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Am. Chem. Soc. 2006, 128, 15934. (b) Wakamiya, A.; Mori, K.;
Yamaguchi, S. Angew. Chem., Int. Ed. 2007, 46, 4273. (c) Zhao, C.-H.;
Sakuda, E.; Wakamiya, A.; Yamaguchi, S. Chem.;Eur. J. 2009, 15,
10603.
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J.; Sung, H. H. Y.; Willams, I. D.; Kwok, H. S.; Tang, B. Z. Chem.
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Commun. 2007, 3255. (b) Tong, H.; Hong, Y.; Dong, Y.; Hauβler, M.;
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4832
Org. Lett., Vol. 13, No. 18, 2011

TBAF increased, the emission band at 551 nm decreased
and the blue-shifted band at 386 nm increased gradually.
This change became saturated when the concentration of
TBAF amounted to 39.7 μM. The binding constant was
determined to be 1.55 ꢁ 105 Mꢀ1, which is comparable to
those of other tricoordinate organoboron compounds. It is
most noteworthy that the ratios of emission intensities at
386 and 551 nm (I386/I551) exhibit a dramatic change from
0.008:1 to 12.7:1 and increase linearly almost within the
whole region. Moreover, a dramatic color change of emis-
sion was observed from yellow to blue (see SI), thus
enabling colorimetric fluoride ion sensing by the naked
eye. While it has been well documented that tricoordinate
organoboron compounds can act as efficient fluorescent
sensors for fluoride ions, little ratiometric fluorescence
sensing has been reported because of the trivial fluores-
cence spectra changes or the low fluorescence intensity of
either organoboron compounds or their complexes with
fluoride ions.13 Our current molecular design may also
provide an efficient strategy to obtain organoborons cap-
able of the ratiometric fluorescencesensingoffluoride ions.
In summary, we have disclosed a new class of tricoordi-
nate organoboron compounds, in which the electron-
accepting boryl group and the electron-donating amino
groupare incorporated atthe o,o0-positions of the biphenyl
framework. The close contact between boryl and amino
groups permits a characteristic through-space intramole-
cular CT fluorescence at long wavelengths. Binding of
fluoride ions leads to the remarkable fluorescence spectra
and color changes, enabling colorimetric and ratiometric
fluoride ion sensing.
Figure 5. Fluorescence spectra change of 1 (1.08 μM in THF)
upon addition TBAF. Inset: Plot of fluorescence intensity ratios
beween 386 and 551 nm (I386/I551) versus concentration of Fꢀ in
THF.
LUMO, which is independent of the electron-donating
ability of the amino group.
Considering the characteristic intramolecular CT emis-
sion of the present tricoordinate organoboronπ system, we
next investigated their fluorescence sensing ability for
fluoride ions using 1 as a representative compound. The
addition of excess fluoride ions to a THF solution of 1 led
to the appearance of a new fluorescence band at 386 nm,
blue-shifted by 165 nm. Such a large blue shift is very rare
for the tricoordinate compounds when complexed with
fluoride. This is accompanied by a blue shift of the
absorption to a much shorter wavelength, which is hardly
measured and can be assigned to a charge transfer from the
HOMO localized on the dimesityl rings to the LUMO
delocalized over the biphenyl part (see SI). The corre-
sponding charge-transfer emission presumably tallies with
the fluorescence band at 386, with a much higher energy
than that of the through-space charge-transfer emission of
1. Moreover, the complexation of 1 with fluoride resulted
in a significant increase in the fluorescence efficiency
(ΦF = 0.27 for 1; ΦF = 0.98 for 1•Fꢀ in THF). The
remarkable blue shift and high fluorescence intensity of 1
irrespectiveof the binding mode promptedus toinvestigate
its ratiometric fluorescence sensing ability for fluoride ions.
The titration experiment of 1 with the fluoride ion was
carried out using n-Bu4NF as the fluoride source. The
fluorescence spectral change of 1 (1.08 μM) upon addition
of TBAF is shown in Figure 5. As the concentration of
Acknowledgment. The authors gratefully acknowledge
the National Nature Science Foundation of China (Grant
Nos. 21072117, 20802041), Nature Science Foundation of
Shandong Province (No Q2008B02), Science Foundation
of Ministry of Education of China (No 200804221009),
and 973 Program (2010CB933504) for financial support.
Supporting Information Available. Experimental de-
tails, photophysical data, results of theoretical calcula-
tions, and crystallographic data in CIF format. These
materials are available free of charge via the Internet at
http://pubs.acs.org.
Org. Lett., Vol. 13, No. 18, 2011
4833