4-boratastyrylstilbene and 1,4-bis(boratastyryl)benzene: Synthesis, structural characterization, and photophysics [10]

Full text of DOI:10.1021/ja001873w

J. Am. Chem. Soc. 2000, 122, 8577-8578
8577
Scheme 1
4-Boratastyrylstilbene and
1,4-Bis(boratastyryl)benzene: Synthesis, Structural
Characterization, and Photophysics
Bun Yeoul Lee and Guillermo C. Bazan*
Department of Chemistry, UniVersity of California
Santa Barbara, California 93106
ReceiVed May 30, 2000
Recent reports highlight the potential use of boron-containing
conjugated polymers in emerging optoelectronic applications.1 To
understand the effect of conjugation in these materials, a proper
description of the electronic structure in the monomeric or
oligomeric constituents is required. This “building up” approach
for extrapolating the properties of molecules to those of polymeric
materials has proven successful in several families of conjugated
polymers and forms the basis of an ever growing area of materials
chemistry.2
Scheme 2
In connection to boron-containing polymers, we recently
reported the synthesis, structure, and photophysics of boratastil-
bene (M-1, where M ) Li or Na).3 When compared to stilbene,
the isoelectronic B- for C substitution allows one to examine the
effect of a negative charge and an asymmetric charge distribution
on excited-state properties.
excess NaH, followed by crystallization from a THF/ether solution
provides Na-2 in 75% yield.
The synthesis of Na2-3 (Scheme 2) requires two transmetalation
steps. Addition of 1,4-diethynylbenzene to 2 equiv of Cp2Zr(H)-
Cl provides the binuclear species 6. Reaction of 6 and BCl3 in
toluene gives (1,4-phenylenedi-1,2-ethenediyl)bis(dichloroborane)
(7), which can be separated from Cp2ZrCl2 by extraction with
pentane. Transmetalation with 1,1-dibutyl-1-stannacyclohexa-2,5-
diene6 gives 1,1′-(1,4-phenylenedi-1,2-ethenediyl)bis(1-boracy-
clohexa-2,5-diene) (8) in 46% overall yield from 1,4-diethynyl-
benzene. It should be noted that the direct reaction of 6 with
1-chloro-1-boracyclohexa-2,5-diene gives 8. However, 8 and Cp2-
ZrCl2 have similar solubility properties and cannot be separated
easily. The desired Na2-3 is obtained by addition of 8 to a NaH/
THF slurry and is an exceptionally air- and moisture-sensitive
orange solid.
Figure 1a shows that the absorption maximum of Na-2 in THF
appears at 404 nm (ꢀ404 ) 2.7 × 104 L mol-1 cm-1). Compared
against Na-1, this band is 47 nm red-shifted, a consequence of
the extended conjugation length. Addition of 10 equiv of dibenzo-
18-crown-6 to Na-2 in THF causes a bathochromic shift to 447
nm (ꢀ447 ) 2.1 × 104 L mol-1 cm-1, Figure 1b). The photolu-
minescence (PL) spectrum of Na-2 in THF upon excitation at
404 nm is shown in Figure 1c, and the calculated overall PL
quantum efficiency (ΦPL) is approximately 2%. Addition of
dibenzo-18-crown-6 causes no change in the fluorescence spectra
or ΦPL.
Compounds of the type M-1 show aggregation-dependent
photophysics. In nonpolar solvents the solution structure is a tight
ion pair, and low photoluminescence quantum yields are observed.
In polar solvents, or when crown ethers that encapsulate the
countercations are added, the solvent-separated species is highly
emissive as a result of an intramolecular charge-transfer process.
A natural progression for extending the conjugation length of
these “boratachromophores” is the synthesis and study of the
anionic 4-boratastyrylstilbene (Na-2) and the dianionic 1,4-bis-
(boratastyryl)benzene (Na2-3). These molecules are isoelectronic
to distyrylbenzene chromophores, which find use in optoelectronic
devices and supramolecular engineering.4
It is likely that, as in the case of Na-1, the crown coordinates
to sodium and generates the solvent-separated ion pair. Removing
the interactions between the boratabenzene ring and sodium allows
for a transition that ultimately transfers electron density from
boratabenzene to the rest of the molecule. The lower ΦPL of Na-2
vis-a´-vis Na-1 (16% in THF and 66% in THF with dibenzo-18-
Scheme 1 shows the synthesis of Na-2. Reaction of 4-ethyn-
ylstilbene with Cp2Zr(H)Cl in toluene provides the zirconium
compound 4, which is subsequently treated in situ with 1-chloro-
1-boracyclohexa-2,5-diene.5 Crystallization from a CH2Cl2 solu-
tion affords 1-[2-(4-styrylphenyl)ethenyl]-1-boracyclohexa-2,5-
diene (5) in 57% overall yield. Deprotonation of 5 in THF using
(4) (a) Colaneri, N. F.; Bradley, D. D. C.; Friend, R. H.; Burn, P. L.;
Holmes, A. B.; Spangler, C. W. Phys. ReV. B 1990, 42, 11670. (b) Renak, M.
L.; Bartholomew, G. P.; Wang, S.; Ricatto, P. J.; Lachicotte, R. J.; Bazan, G.
C. J. Am. Chem. Soc. 1999, 121, 7787. (c) Dottinger, S. E.; Hohloch, M.;
Hohnholz, D.; Segura, J. L.; Steinhuber, E.; Hanack, M. Synth. Met. 1997,
84, 267. (d) Lupton, J. M.; Hemingway, L. R.; Samuel, I. D. W.; Burn, P. L.
J. Mater. Chem. 2000, 10, 867. (e) Sun, R. G.; Wang, Y. Z.; Wang, D. K.;
Zheng, Q. B.; Kyllo, E. M.; Gustafson, T. L.; Epstein, A. J. App. Phys. Lett.
2000, 76, 634. (f) Sarker, A. M.; Strehmel, B.; Neckers, D. C. Macromolecules
1999, 32, 7409.
(5) Maier, G.; Hankelmann, J.; Reisenauer, H. P. Angew. Chem., Int. Ed.
Engl. 1985, 24, 1065.
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Jutzi, P.; Buumgartner, J. J. Organomet. Chem. 1978, 148, 257.
(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. (c) Corriu, R. J.-P.; Daforth, T.; Douglas, W. E.; Guerrero, G.; Siebert,
W. S. Chem. Commun. 1998, 963. (d) Matsumi, N.; Miyata, M.; Chujo, Y.
Macromolecules 1999, 32, 4467.
(2) Mu¨llen, K., Wegner, G., Eds. Electronic Materials: The Oligomer
Approach; Wiley-VCH: New York, 1998.
(3) Lee, B. Y.; Wang, S.; Putzer, M.; Bartholomew, G. P.; Bu, X.; Bazan,
G. C. J. Am. Chem. Soc. 2000, 122, 3969.
10.1021/ja001873w CCC: $19.00 © 2000 American Chemical Society
Published on Web 08/17/2000

8578 J. Am. Chem. Soc., Vol. 122, No. 35, 2000
Communications to the Editor
Figure 3. ORTEP drawing of [Na(THF)3]2[3].
Figure 1. Spectra of Na-2 in THF. (a) Absorption of Na-2, (b) absorption
of Na-2 with 10 equiv of dibenzo-18-crown-6, (c) emission of Na-2.
Figure 4. ORTEP drawing of [Na(dibenzo-18-crown-6)THF2]2[3].
Figure 4 shows the structure of a single crystal obtained from
a solution containing Na2-3 and dibenzo-18-crown-6. The mo-
lecular composition corresponds to [Na(dibenzo-18-crown-6)-
THF2]2[3]. In this arrangement, the sodium ions are insulated from
3 by the crown and two “axial” THF molecules. It is surprising
to us that 3 is present in the lattice without external Coulombic
stabilization of the dianionic charge. That the sodium ions are
insulated from 3 is consistent with the blue shift in fluorescence
when dibenzo-18-crown-6 is added. Under these conditions, the
ions cannot stabilize charge redistribution upon photoexcitation,
and a normal Stokes shift is observed.
In summary, the synthesis of Na-2 and Na2-3 are described.
The strategy includes hydrozirconation with Shwartz reagent11
followed by transmetalation from zirconium to boron.12 Differ-
ences in the optical properties of Na-2 relative to those of Na-1,
namely the red-shifted spectra and lower ΦPL values, can be
rationalized in terms of the extended conjugation length. For Na2-
3, the motion of countercations adjacent to the chromophore
allows for stabilization of the excited-state energy. Removing
these cations from 3 by addition of dibenzo-18-crown-6 results
in enhanced charge transfer and less efficient stabilization of the
excited state.
Figure 2. Spectra of Na2-3 in THF. (a) absorption of Na2-3, (b)
absorption of Na2-3 with 50 equivalents dibenzo-18-crown-6, (c) emission
of Na-2 with 50 equiv of dibenzo-18-crown-6, (d) emission of Na2-3.
crown-6) is attributed to the smaller HOMO/LUMO gap of 2 (the
energy gap law).7 That the crown ether does not change the
emission suggests that sodium does not stabilize the excited state
of Na-2. Perhaps the charge in the vinylstilbene fragment is too
diffuse for a strong Coulombic interaction.
In THF, the absorption of Na2-3 is centered at 396 nm (ꢀ396
)
3.1 × 104 L mol-1 cm-1, Figure 2). The emission band is centered
at 604 nm (ΦPL ) 16%), but its broad shape suggests more than
one emitting species. Upon addition of excess (50 equiv) dibenzo-
18-crown-6 the absorption red shifts to 447 nm (ꢀ447 ) 1.9 ×
104 L mol-1 cm-1), but the emission blue shifts to 534 nm
(ΦPL ) 44%). The change in absorption corresponds to enhanced
charge transfer from the outer charged rings into the inner ring.
The relative energies of emission are likely related to the ability
of the sodium ions to compensate and stabilize changes in the
electron distribution in 3 upon photoexcitation.8,9 The structure
of Na2-3 in a crystal grown from THF solution shows a sodium
ion adjacent to each boratabenzene ring (Figure 3). In solution,
these ions can optimize their location relative to 3 with changes
in electrostatic distribution and stabilize the excited-state species.10
Acknowledgment. We are grateful to the Department of Energy, the
ACS PRF, and Equistar Chemicals LP for financial assistance and to Dr.
Xianhui Bu for the single crystal X-ray diffraction studies. B.Y.L. is
thankful to Korea Science and Engineering Foundation for partial financial
support.
Supporting Information Available: Complete details for the syn-
thesis of all compounds and the crystallographic studies of [Na(THF)3]2-
[3] and [Na(dibenzo-18-crown-6)THF2]2[3] (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
(7) Turro, N. J. Modern Molecular Photochemistry; University Science
Books: Mill Valley, 1991.
(8) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Plenum: New
York, 1983.
JA001873W
(9) (a) Tolbert, L. M. Acc. Chem. Res. 1986, 19, 268. (b) Tolbert, L. M. In
ComprehensiVe Carbanion Chemistry, Part C; Buncel, E., Durst, T., Eds.;
Elsevier: New York, 1987.
(11) Harada, S.; Taguchi, T.; Tabuch, N.; Narita, K.; Hanzawa, Y. Angew.
Chem., Int. Ed. 1998, 37, 1698.
(12) Fagan, P. J.; Nugent, W. A.; Calabrese, J. C. J. Am. Chem. Soc. 1994,
116, 1880 and references therein.
(10) Hogen-Esch, T. E.; Plodinec, J. J. Am. Chem. Soc. 1978, 100, 7633.