Dibenzoborole-containing φ-electron systems: Remarkable fluorescence change based on the "on/off" control of the pφ-φ* conjugation

Article abstract of DOI:10.1021/ja026689k

A series of dibenzoborole derivatives with various groups such as (N,N-diphenylamino)phenyl, thienyl, and bithienyl groups at the 3,7-positions have been synthesized and their photophysical properties studied. These new π-electron systems show significant solvatochromism in the fluorescence spectra. Thus, about 100-140 nm blue shifts in the emission maxima and 20-30-fold increments in the quantum yields are observed upon changing the solvent from THF to DMF. Similar fluorescence changes are observed upon the addition of n-Bu₄NF to their THF solutions, demonstrating their sensing abilities toward a fluoride ion. These fluorescence changes result from the "on/off" control of the p_π-π* conjugation in their LUMO by the coordination of donor solvents or fluoride ion to the boron atom in the dibenzoborole skeleton. Copyright

Full text of DOI:10.1021/ja026689k

Published on Web 07/09/2002
Dibenzoborole-Containing π-Electron Systems: Remarkable Fluorescence
Change Based on the “On/Off” Control of the pπ-π* Conjugation
Shigehiro Yamaguchi,*,†,‡ Toshiaki Shirasaka,†,‡ Seiji Akiyama,† and Kohei Tamao*,†
Institute for Chemical Research, Kyoto UniVersity, and PRESTO, Japan Science and Technology Corporation,
Uji, Kyoto 611-0011, Japan
Received April 26, 2002
Boron-containing π-electron systems have recently attracted
much attention1-5 because of their intriguing electronic and
photophysical properties due to the pπ-π* conjugation through the
vacant p-orbital of the boron atom. One notable aspect may be the
control of this conjugation, which provides an opportunity to modify
the properties of these systems.6 We recently reported a new fluoride
sensor using tri(9-anthryl)borane, in which the binding of a fluoride
ion onto a boron atom turns off the pπ-π* conjugation in the
LUMO, resulting in a color change from orange to colorless.7 We
now apply this concept to a new π-electron system consisting of a
Figure 1. The “on/off” control of the pπ-π* conjugation in the LUMO
dibenzoborole skeleton. In this system, the “on/off” control of the
pπ-π* conjugation by the addition of a donor molecule would
of dibenzoborole-containing π-electron systems.
Scheme 1 a
change the LUMO delocalization mode from “mode A” to “mode
B”, as shown in Figure 1. This change would significantly increase
the HOMO-LUMO gap and cause a hypsochromic shift in the
absorption and fluorescence maxima, but the resulting LUMO in
mode B would still be delocalized over the carbon framework as a
normal π-conjugated system. Thus, we expect that this change
would lead to a photophysical response in the visible region.
To examine this idea, we employed a series of dibenzoborole
derivatives, including the parent dibenzoborole 1 and its extended
derivatives with (diphenylamino)phenyl (2), thienyl (3), and bithie-
nyl (4) groups at the 3,7-positions (Scheme 1); a bulky 2,4,6-
triisopropylphenyl (Tip) group is introduced onto a boron atom for
kinetic stabilization.
The parent 1 was prepared by the metalation of dibromobiphenyl
5a, followed by treatment with TipB(OMe)2. Similarly, the use of
diiodo-dibromobiphenyl 5b allowed us to synthesize the 3,7-
dihalogenated dibenzoboroles 6 and 7, and the Kosugi-Migita-
Stille coupling of the diiodide 7 with appropriate arylstannanes in
the presence of Pd2(dba)3/P(furyl)3 catalyst8 successfully afforded
the extended compounds 2-4. All the synthesized dibenzoborole
derivatives are air- and moisture-stable due to steric protection by
the Tip group.
a Reagents and conditions: (i) (1) Mg (2.1 mol amount), THF, or n-BuLi
(2.2 mol amount) in THF then MgBr2 (3.0 mol amount), (2) TipB(OMe)2
(1.0-1.4 mol amount); (ii) (1) t-BuLi (4.4 mol amount) Et2O, (2) ICH2CH2I
(4.4 mol amount); (iii) ArSnBu3 (3.0 mol amount) Pd2(dba)3 (0.05 mol
amount), P(furyl)3 (0.2 mol amount), THF.
to the photophysical properties. Actually, the shoulder band of 1
in the absorption spectrum can be ascribed to the transition from
the HOMO delocalized over the biphenyl moiety to the LUMO
delocalized over the dibenzoborole skeleton through the pπ-π*
conjugation.11
Based on this fundamental feature of the dibenzoborole skeleton,
the extended π-conjugated compounds 2-4 also show notable
photophysical properties, as summarized in Table 1. In THF, while
these compounds have weak absorption bands around 480-504
nm, they show yellow to orange emissions around 550-576 nm,
with low quantum yields. The large red shifts in their absorption
and emission maxima, by about 70-100 nm and 35-60 nm,
respectively, relative to those of 1 suggest that the π-conjugation
is substantially extended along with the carbon framework. More
importantly, we found that the fluorescence properties of 2-4 are
highly dependent on the solvent. Thus, in donor solvents such as
DMF,12 the emission bands observed in THF completely disappear,
and new intense blue emission bands appear around 419-478 nm,
the quantum yields of which are dramatically increased to about
0.5-0.9. Thus, about 100-140 nm blue shifts and 20-30-fold
increments in the quantum yields are observed by changing the
Although the absorption spectra have already been reported for
a few dibenzoborole derivatives,9 little is known about the
fluorescence properties of these compounds. We now find that the
parent dibenzoborole 1 has a fluorescence property distinct from
those of the carbon and nitrogen analogues, fluorene and carbazole.
Thus, in THF, 1 shows a characteristic weak shoulder band at 410
nm (log ꢀ ) 2.39) in the UV-visible absorption spectrum and
exhibits a green fluorescence at 514 nm with a low quantum yield
of 0.09. The large Stokes shift (∼100 nm) and the considerably
longer emission maximum (∆λmax ) 160-200 nm) compared to
those of fluorene (λem ) 314 nm)10 and carbazole (λem ) 349 nm)10
suggest the significant contribution of the boron vacant p-orbital
* To whom correspondence should be addressed. E-mail: shige@scl.kyoto-
u.ac.jp; tamao@scl.kyoto-u.ac.jp.
† Kyoto University.
‡ Japan Science and Technology Corporation.
9
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J. AM. CHEM. SOC. 2002, 124, 8816-8817
10.1021/ja026689k CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Photophysical Data of Dibenzoborole π-Electron
of 3 and 4. The binding constants toward the fluoride ion determined
by these fluorescence titration experiments were 3.5 (( 0.4) × 105
M-1 for 2, 1.1 (( 0.5) × 106 M-1 for 3, and 1.4 (( 0.3) × 106
M-1 for 4 at 20 °C. Moreover, the produced borates can be changed
back to the starting dibenzoboroles without decomposition by
treatment with a stronger fluoride scavenger such as BF3‚OEt2.
These results not only support our assumption concerning the origin
of the solvatochromism but also demonstrate the significant potential
of the present boron system as a new type of fluorescent sensor
for the fluoride ion.13 It should be noted that the output of the
fluoride binding events is the “turn-on” of the fluorescence in terms
of the intensity increase along with the dramatic fluorescence color
changes in the visible region. Further detailed studies on the
photophysics of the dibenzoborole π-electron systems are now in
progress.
Systems
absorption
max/nm (log ꢀ)
fluorescence
λ
max/nm (Φf)a
compd
solvent or additive
λ
2
THF
480 (3.08), 361 (4.65)
479 (2.72), 372 (4.71)
374 (4.78)
488 (2.95), 394 (4.33)
389 (4.56)
561 (0.030)c
423 (0.89)d,e
419 (0.92)d
550 (0.041)c
419 (0.88)d
417 (0.86)d
576 (0.022)c
478 (0.55)d,e
478 (0.42)d
DMF
TBAF/THFb
THF
3
4
DMF
TBAF/THFb
THF
400 (4.65)
504 (3.51), 405 (4.67)
498 (3.11), 425 (4.72)
428 (4.82)
DMF
TBAF/THFb
a Excited at the longest absorption maximum wavelengths, unless
otherwise stated. The Φf is the average values of repeated measurements
within (5% errors. b n-Bu4NF in THF. The data for the produced borates
are those at the saturated points of the spectral changes upon addition of
TBAF. c Determined using fluorescein as a standard. d Determined using
quinine sulfate as a standard. e Excited at the second longest absorption
maximum wavelength.
Acknowledgment. This work was financially supported by the
Ministry of Education, Culture, Sports, Science and Technology,
Japan, and the PRESTO, Japan Science and Technology Corpora-
tion.
Supporting Information Available: Analytical and spectral data
for 1-4 and fluorescence spectra for the titration experiments (PDF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
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Scheme 2
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of a THF solution of n-Bu4NF as a fluoride source to a THF solution
of 2 (Scheme 2), the emission band of 2 at 561 nm indeed became
weak, and the intensity of the fluorescent band at 419 nm
significantly increased (see the Supporting Information). Eventually,
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