Tridurylboranes extended by three arylethynyl groups as a new family of boron-based π-electron systems

Article abstract of DOI:10.1021/ol006660q

(equation presented) A series of tris(phenylethynylduryl)boranes (R-C₆H₄-C≡C-duryl)₃B with various substituents R have been prepared as air-stable solids owing to the steric protection of the boron atom by the three bulky

Full text of DOI:10.1021/ol006660q

ORGANIC
LETTERS
2000
Vol. 2, No. 26
4129-4132
Tridurylboranes Extended by Three
Arylethynyl Groups as a New Family of
Boron-Based π-Electron Systems
Shigehiro Yamaguchi,* Toshiaki Shirasaka, and Kohei Tamao*
Institute for Chemical Research, Kyoto UniVersity, Uji, Kyoto 611-0011, Japan
tamao@scl.kyoto-u.ac.jp
Received September 26, 2000
ABSTRACT
A series of tris(phenylethynylduryl)boranes (R-C H -CtC-duryl)3B with various substituents R have been prepared as air-stable solids owing
6
4
to the steric protection of the boron atom by the three bulky duryl groups. These compounds show unique photophysical properties due to
the pπ−π* conjugation through the p-orbital on the boron atom. In particular, a push−pull type derivative with R ) NMe2 exhibits a significant
solvatochromism of fluorescence from blue to orange colors.
Boron-containing π-conjugated systems have recently at-
tracted increasing attention as a new class of π-electron
materials for optoelectonics.1-6 The pπ-π* conjugation7 of
the vacant p-orbital on boron with the π* orbital of the
attached carbon π-conjugated moieties is responsible for
some outstanding properties such as unique absorption and
emission,1,2,4,5 a low reduction potential susceptible to
n-doping,1-3,8 and high electron-transporting properties.3 The
photophysics of triphenylboranes has been well documented
to include (1) the substituent effects on the phenyl rings9
and, recently, (2) the fact that the push-pull types of
π-electron systems having both amino and dimesitylboryl
groups have been disclosed as potent nonlinear optical
materials.4 We have recently reported trianthrylborane 1 and
its derivatives based on the concept that the introduction of
three identical π-conjugated moieties (i.e., three anthryl
groups) onto a boron atom allows divergently extended
π-conjugation through the vacant p-orbital of the boron in
the LUMO.2 Consequently, the highly delocalized low-lying
(1) (a) Matsumi, N.; Naka, K.; Chujo, Y. J. Am. Chem. Soc. 1998, 120,
5112. (b) Matsumi, N.; Naka, K.; Chujo, Y. J. Am. Chem. Soc. 1998, 120,
10776.
(2) Yamaguchi, S.; Akiyama, S.; Tamao, K. J. Am. Chem. Soc. 2000,
122, 6793.
(3) (a) Noda, T.; Shirota, Y. J. Am. Chem. Soc. 1998, 120, 9714. (b)
Noda, T.; Ogawa, H.; Shirota, Y. AdV. Mater. 1999, 11, 283.
LUMO significantly influences their intramolecular charge-
transfer transitions,9 causing their long-wavelength absorp-
tions. In line with this concept, we have now designed new
boron-based π-electron systems: the tridurylborane deriva-
10.1021/ol006660q CCC: $19.00 © 2000 American Chemical Society
Published on Web 11/28/2000

tives 2 extended by three arylethynyl groups. The present
design relies on the introduction of three bulky duryl groups
onto a boron atom to obtain sufficient chemical stability10
and the employment of the ethynylene spacer to effectively
extend the π-conjugation. Reported herein are their syntheses,
structures, and photophysical properties including solvato-
chromism.
A series of tridurylborane derivatives 2 has been synthe-
sized using tris(ethynylduryl)borane 6 as the key precursor,
as shown in Scheme 1. Thus, the mono-lithiation of dibro-
t-BuLi in THF and iodine produced tris(iododuryl)borane
4. The Pd/Cu-catalyzed cross-coupling11 of 4 with trimeth-
ylsilylacetylene gave 5, which was desilylated under alkaline
conditions to give the tris(ethynylduryl)borane 6. Finally, the
Pd/Cu-catalyzed cross-coupling11 of 6 with aryl halides
successfully afforded a series of 2 bearing various terminal
aryl groups. In this synthesis, the transformation from
tribromide 3 to triiodide 4 is crucial, because of the low
reactivity of the tribromide toward the Pd/Cu-catalyzed
coupling reaction. All tridurylborane derivatives 2 are
significantly stable toward air and water due to the steric
protection of the central boron atom by three bulky duryl
groups.12
Scheme 1a
Among the obtained tridurylboranes, the structure of 2a
has been determined by X-ray crystallography.13 The ORTEP
drawing is shown in Figure 1. The molecule has a crystal-
a (a) i, n-BuLi (1.0 molar amount), Et2O, -78 °C; ii, BF3‚OEt2
(0.3 molar amount), -78 °C to rt.; (b) i, t-BuLi (6.0 molar amount),
THF, -78 °C to rt; ii, I2 (4.5 molar amount), rt; (c) Me3SiCtCH
(4.5 molar amount), PdCl2(PPh3)2 (0.15 molar amount), CuI (0.30
molar amount), Et2NH, rt; (d) KOH, MeOH/THF, rt; (e), ArX (3.0
molar amount), PdCl2(PPh3)2 (0.05-0.15 molar amount), CuI
(0.10-0.30 molar amount), piperidine at rt for 2a-2d and Et2NH
in reflux for 2e. As the ArX, appropriate ArI was used for 2a-2c
and ArBr for 2d-2e.
Figure 1. ORTEP drawing of 2a (50% probability for thermal
ellipsoids). Selected bond lengths [Å] and bond angles [deg]: B1-
C1 1.589(3), C1-C2 1.420(3), C2-C3 1.397(4), C3-C4 1.409-
(4), C4-C7 1.448(4), C7-C8 1.161(4), C1-B1-C1* 119.6(3),
C1-B1-C19 120.2(1), C4-C7-C8 179.7(3), C7-C8-C9 179.4-
(3).
lographic C2 axis along with the B1-C19 bond. The central
boron is completely trigonal planar. Three duryl groups are
arranged in a propeller-like fashion, and the dihedral angles
between the boron plane and the duryl planes are 53-55°.14
The outer benzene rings maintain a high coplanarity with
the duryl groups with 17-18° dihedral angles, suggesting
that the ethynylene spacer is effective for extending the
π-conjugation.
The photophysical properties of the tridurylborane deriva-
tives 2 in THF solutions are summarized in Table 1, together
with those of some related compounds including diarylacety-
lenes 7 and the mono-arylethynyl group-substituted dimesi-
tylboranes 8,15 for comparison.
modurene followed by reaction with BF3‚OEt2 gave tris-
(bromoduryl)borane 3. The successive treatments of 3 with
(4) (a) Yuan, Z.; Taylor, N. J.; Marder, T. B.; Williams, I. D.; Kurtz, S.
K.; Cheng, L.-T. J. Chem. Soc., Chem. Commun. 1990, 1489. (b) Yuan,
Z.; Taylor, N. J.; Williams, I. D.; Kurtz, S. K.; Cheng, L.-T. Organic
Materials for Non-linear Optics II; Hahn, R. A., Bloor, D., Eds.; Spec.
Publ. No. 91; Royal Society of Chemistry: Cambridge, 1991; p 190. (c)
Yuan, Z.; Taylor, N. J.; Sun, Y.; Marder, T. B.; Williams, I. D.; Cheng,
L.-T. J. Organomet. Chem. 1993, 449, 27. (d) Yuan, Z.; Taylor, N. J.;
Ramachandran, R.; Marder, T. B. Appl. Organomet. Chem. 1996, 10, 305.
(e) Yuan, Z.; Collings, J. C.; Taylor, N. J.; Marder, T. B.; Jardin, C.; Halet,
J.-F. J. Solid State Chem., in press. (f) Lequan, M.; Lequan, R. M.; Chane-
Ching, K. J. Mater. Chem. 1991, 1, 997. (g) Branger, C.; Lequan, M.;
Lequan, R. M.; Barzoukas, M.; Fort, A. J. Mater. Chem. 1996, 6, 555. (h)
Branger, C.; Lequan, M.; Lequan, R. M.; Large, M.; Kajzar, F. Chem. Phys.
Lett. 1997, 272, 265.
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Org. Lett., Vol. 2, No. 26, 2000

Stokes shifts except for 2b, which has about a 80-100 nm
longer emission maximum than those of the others. These
unique properties of 2b are due to its highly solvent-
dependent emission. The data for the solvent effect for 2b
are summarized in Table 2 together with those for the parent
Table 1. Photophysical Properties of Tridurylborane
Derivatives and Related Compounds
UV-vis,a
λ
FL,a
max/nmc
d
compound
max/nmb
log ꢀ
λ
ΦF
2a (R ) H)
364
393
371
369
379
299
329
345
376
4.77
4.83
4.75
4.91
5.08
4.35
4.47
4.34
4.37
415
512
432
412
415
337
380
391
496
0.35
0.16
0.45
0.48
0.54
n.d.
n.d.
0.45
0.27
2b (R ) NMe2)
2c (R ) OMe)
2d (R ) CN)
2e (R ) Ar-CC)e
7a (R ) H)
7b (R ) NMe2)
8a (R ) H)
8b (R ) NMe2)
Table 2. Solvent Effect on the Emission Wavelengths of
Tridurylborane Derivatives 2a and 2b and a Related Compound,
8b
FL, λmax/nma
compound
benzene
THF
DMF
2a (R ) H)
2b (R ) NMe2)
8b (R ) NMe2)
409
457
443
415
512
496
432
534
530
a In THF. b Only the longest absorption maxima are shown. c Emission
maximum wavelength excited at the absorption maximum. d Determined
using anthracene as a standard. e Ar ) 4-n-BuC6H4.
a Excited at 366 nm.
2a and the mono-arylethynyl-substituted analogue 8b. A
bathochromic shift of about 80 nm is observed for the
emission of 2b from 457 nm in benzene to 534 nm (with a
shoulder at 570 nm) in DMF (Figure 2a), whereas 2a shows
only a 20 nm red shift. This spectral change for 2b
corresponds to the change in the emission color from blue
In the UV-visible absorption spectra, all compounds 2
show unique and intense bands with their maxima around
360-400 nm. Some notable features are summarized as
follows. (1) The λmax of 2a is about 60 nm longer than that
of diphenylacetylene 7a and, more importantly, about 20 nm
longer than that of 8a. This result represents one of the
notable effects of the incorporation of the three arylethynyl
π-systems. The absorption of 2 can be interpreted as the
intramolecular charge-transfer transition from the diaryl-
acetylene moiety to the triarylborane moiety9 and, therefore,
the extent of the π-conjugation through the vacant p-orbital
of boron in the LUMO is relevant for the charge-transfer
transition energy.2 Thus, the red shift from 8a to 2a is
conceivably due to the difference in the extent of the π-
conjugation in the LUMO. (2) From 2a to 2e, the λmax shifts
to longer wavelengths by only 15 nm, despite the introduction
of three additional arylethynyl moieties as the R groups in
2e. This suggests that the extension of the π-conjugation in
the LUMO of the triarylboranes has a certain limitation and
that the additional arylethynyl moieties in 2e are not
significantly effective in extending the π-conjugation. (3)
Among 2a-2d with various substituents (R ) H, NMe2,
OMe, and CN), compound 2b bearing the electron-donating
NMe2 group has the longest absorption maximum. This
should be rationalized in terms of the strongest π-donor
ability of the amino-substituted diarylacetylene moiety to the
boron center.4
In the fluorescence spectra in THF, all compounds 2 show
strong emissions around 415-430 nm with 50-60 nm
Figure 2. Fluorescence of 2b: (a) the emission spectra in various
solvents (benzene, blue line; THF, green line; DMF, orange line)
and (b) a picture of the solutions under irradiation of light at 365
nm. The spectrum in DMF is magnified 10× in intensity.
(5) (a) Lee, B. Y.; Wang, S.; Putzer, M.; Bartholomew, G. P.; Bu, X.;
Bazan, G. C. J. Am. Chem. Soc. 2000, 122, 3969. (b) Lee, B. Y.; G. C.
Bazan, J. Am. Chem. Soc. 2000, 122, 8577.
(6) Itoh, T.; Matsuda, K.; Iwamura, H.; Hori, K. J. Am. Chem. Soc. 2000,
122, 2567.
Org. Lett., Vol. 2, No. 26, 2000
4131

to orange, as visualized in Figure 2b. As observed in the
UV-visible absorption spectra, the emission maximum of
2b in each solvent is slightly longer than that of the
monosubstituted analogue 8b. It should also be noted that
no significant solvatochromism is observed in the UV-
visible absorption spectra of 2b: λmax 394 nm in benzene,
393 nm in THF, and 393 nm in DMF. This suggests the
highly polarized excited state produced by the intramolecular
charge-transfer transition in 2b.
derivatives which may be useful as a new core unit for boron-
based π-electron systems with substantial chemical stability.
The unique absorption and emission properties of the present
π-electron systems are of interest for applications to opto-
electronics such as nonlinear optical materials and organic
electroluminescent devices. Further study in this line is now
in progress in our laboratory.
Acknowledgment. This work was partly supported by a
Grant-in-Aid (No. 11740349) from the Ministry of Education,
Science, Sports and Culture of Japan.
In summary, we have disclosed a series of tridurylborane
(7) π-Conjugation through vacant p-orbital of boron, for examples, see:
(a) Zweifel, G.; Clark, G. M.; Leung, T.; Whitney, C. C. J. Organomet.
Chem. 1976, 117, 303. (a) Eisch, J. J.; Galle, J. E.; Kozima, S. J. Am. Chem.
Soc. 1986, 108, 379. (b) Budzelaar, P. H. M.; van der Kerk, S. M.; Krogh-
Jespersen, K.; Schleyer, P. v. R. J. Am. Chem. Soc. 1986, 108, 3960. (c)
Eisch, J. J.; Shafii, B.; Odom, J. D.; Rheingold, A. L. J. Am. Chem. Soc.
1990, 112, 1847. (d) Byun, Y.-G.; Saebo. S.; Pittman, C. U., Jr. J. Am.
Chem. Soc. 1991, 113, 3689. (e) Sugihara, Y.; Yagi, T.; Murata, I.; Imamura,
A. J. Am. Chem. Soc. 1992, 114, 1479. (f) Salzner, U.; Lagowski, J. B.;
Pickup, P. G.; Poirier, R. A. Synth. Met. 1998, 96, 177.
Supporting Information Available: Experimental pro-
cedures and data for all new compounds and crystal structural
data for 2a. This material is available free of charge via the
Internet at http://pubs.acs.org.
OL006660Q
(8) (a) Leffler, J. E.; Watts, G. B.; Tanigaki, T.; Dolan, E.; Miller, D. S.
J. Am. Chem. Soc. 1970, 92, 6825. (b) Dupont, T. J.; Mills, J. L. J. Am.
Chem. Soc. 1975, 97, 6375. (c) Kaim, W.; Schultz, A. Angew. Chem., Int.
Ed. Engl. 1984, 23, 615. (d) Schultz, A.; Kaim, W. Chem. Ber. 1989, 122,
1863. (e) Okada, K.; Sugawa, T.; Oda, M. J. Chem. Soc., Chem. Commun.
1992, 74.
(9) UV absorption spectra of triarylboranes: (a) Ramsey, B. G.; Leffler,
J. E.J. Phys. Chem. 1963, 67, 2242. (b) Ramsey, B. G. J. Phys. Chem.
1966, 70, 611. (c) Miller, D. S.; Leffler, J. E. J. Phys. Chem. 1970, 74,
2571. See also refs 4.
(10) Brown, H. C.; Dodson, V. H. J. Am. Chem. Soc. 1957, 79, 2302.
(11) (a) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975,
4467. (b) Alami, M.; Ferri, F.; Linstrumelle, G. Tetrahedron Lett. 1993,
34, 6403.
(12) All tridurylboranes were isolated through usual aqueous workup
followed by silica gel column chromatography without any decomposition.
(13) Crystal data of 2a (instrument; Rigaku RAXIS-IV): C54H51B1, FW
) 710.81, crystal size 0.30 × 0.30 × 0.20 mm, monoclinic, C2/c (No. 15),
a ) 16.212(1) Å, b ) 22.153(1) Å, c ) 14.9162(9) Å, â ) 127.956(2)°,
V ) 4224.0(4) Å3, Z ) 4, Dc ) 1.118 g cm-3, µ(Mo KR) ) 0.62 cm-1
,
number of unique reflections ) 4461, temperature -110 °C, R ) 0.068,
Rw ) 0.089, and GOF ) 1.20.
(14) These values are comparable to those observed for the crystal
structure of trianthrylborane 1, see ref 2.
(15) Compounds 8 were prepared essentially in a manner similar to the
synthesis of 2 using (bromoduryl)dimesitylborane instead of tris(bromo-
duryl)borane 3.
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Org. Lett., Vol. 2, No. 26, 2000