Oxidative cyclization of bis(biaryl)acetylenes: Synthesis and photophysics of dibenzo[g,p]chrysene-based fluorescent polymers [2]

Full text of DOI:10.1021/ja016692o

J. Am. Chem. Soc. 2001, 123, 12087-12088
12087
Scheme 1
Oxidative Cyclization of Bis(biaryl)acetylenes:
Synthesis and Photophysics of
Dibenzo[g,p]chrysene-Based Fluorescent Polymers
Shigehiro Yamaguchi† and Timothy M. Swager*
Department of Chemistry
Massachusetts Institute of Technology
Cambridge, Massachusetts 02139
ReceiVed July 23, 2001
Annelation of π-conjugated frameworks has proved to be an
effective way to enhance conjugation in π-conjugated polymers
by eliminating conformational disorder. Upon the basis of this
approach a number of ladder polymers having small band gaps
and intense fluorescence have been developed.1-3 Similarly
isolated fused polycyclic aromatics also serve as attractive building
units.4 Considering their rigid structures and symmetrically
forbidden or weakly allowed transitions,5 the incorporation of
these units into π-conjugated main chains would realize unique
fluorescence properties such as small Stokes shifts and extended
excited-state lifetimes. These features may allow for long-range,
through-bond exciton migration, a key requisite for amplified
sensing of ultra-trace analytes such as 2,4,6-trinitrotoluene (TNT),6
using the molecular-wire approach.7,8 As a proof of this concept,
we have recently reported investigations of a series of triph-
enylene-based polymers.9 As an extension, we have been inter-
ested to incorporate dibenzo[g,p]chrysene, recognized as a
benzeno-annelated stilbene, into conjugated polymers. However,
the conventional routes to this skeleton have been limited to only
a few types of classical syntheses.10 We report herein a conceptu-
ally new oxidative acetylene cyclization method that provides a
facile and versatile route to dibenzo[g,p]chrysenes and a dem-
onstration of the utility of the chromophore in the design of novel
π-conjugated polymers.
dibenzo[g,p]chrysene skeleton, we have extended this scheme to
a double intramolecular cyclization as shown in eq 2. The
difficulty of this type of cyclization was how to achieve the doubly
selective 6-endo mode cyclization. Our strategy was to oxidize
the acetylene moiety, which may change the hybridization of the
acetylene carbons from sp to sp2, facilitating the subsequent
electrophilic and radical cyclization in 6-endo mode. We find that
this unusual cyclization proceeds well with the use of SbCl5 as
an oxidant,12 as shown in Scheme 1.
The addition of 1.5 mol amount of SbCl5 (1.0 M CH2Cl2
solution) to a CH2Cl2 solution of bis(biaryl)acetylene 1a at room
temperature immediately produced a deep purple solution.
Quenching the reaction with methanol followed by chromatog-
raphy gave the desired dibenzo[g,p]chysene derivative 2a in 73%
yield.13 Considering the optimized stoichiometry of SbCl5 as an
oxidant,12 the principle skeleton-forming reaction likely proceeds
by a one-electron oxidation. Other one-electron oxidants such as
FeCl3 and NOSbF6 also produced 2a but resulted in lower yields
(21 and 50%, respectively). As for other substrates, dimethoxy-
substituted 1b also underwent this cyclization to give a desired
product 2b in good yield. However, the use of 1c with isomeric
methoxy groups afforded an unexpected dimerized product 3 as
the sole characterizable product (vide infra).
Our group has previously reported powerful routes to fused
polycyclic aromatics as shown in eq 1, where the electrophilic
6-endo mode cyclization directed by electron-donating substituents
provides a variety of phenanthrene derivatives.11 To construct a
† Present address: Institute for Chemical Research, Kyoto University, Uji,
Kyoto 611-0011, Japan.
(1) Martin, R. E.; Diederich, F. Angew. Chem., Int. Ed. 1999, 38, 1350.
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Molecules; Academic Press: New York, 1971.
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Yang, J.-S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120, 11864. (c) Sohn,
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100, 2537. (b) Swager, T. M. Acc. Chem. Res. 1998, 31, 201.
(9) Rose, A.; Lugmair, C. G.; Swager, T. M. J. Am. Chem. Soc. 2001,
123, 11298-11299.
(10) (a) Bergmann, F.; Eschinazi, E. J. Am. Chem. Soc. 1944, 66, 183. (b)
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64, 6499.
Considering our observations, a plausible mechanism of the
present cyclization involves a one-electron oxidation of the
diphenylacetylene moiety of 1 to form the acetylene cation radicals
4. Subsequent electrophilic and radical cyclization with the biaryl
moieties produces radical cation intermediate 5. A PM3 calcula-
(12) SbCl5 is an excellent oxidant for the preparation of aromatic cation
radicals according to the following equation: 2Ar + 3 SbCl5 f 2Ar+•‚SbCl6
-
+ SbCl3 (b) Matuura, A.; Nishinaga, T.; Komatsu, K. J. Am. Chem. Soc.
2000, 122, 10007. (c) Blomgren, G. E.; Kommandeur, J. J. Chem. Phys. 1961,
35, 1636.
(13) X-ray crystal structural analysis of 2a has revealed its significantly
twisted structure. For the detail, see the Supporting Information. A similar
structure has been reported for parent dibenzo[g,p]chrysene: Herbstein, F.
H. Acta Crystallogr. 1979, B35, 1661. Despite this twisted structure, the
dibenzo[g,p]chrysene monomer 2 and polymers 7 still have high fluorescence
quantum yields.
10.1021/ja016692o CCC: $20.00 © 2001 American Chemical Society
Published on Web 11/08/2001

12088 J. Am. Chem. Soc., Vol. 123, No. 48, 2001
Communications to the Editor
Scheme 2a
Figure 1. UV-vis absorption spectra and fluorescence spectra of dibenzo-
[g,p]chrysene monomer and polymers: 2d (dotted line), 7a (solid line),
7b (broken line), 7c (broken-dotted line).
a Reagents and conditions: i) t-BuLi, Et2O, rt; ii) ICH2CH2I, -78 °C;
iii) appropriate diethynylarene, Pd(PPh3)4/CuI, toluene/i-Pr2NH.
Table 1. Photophysical Data for Dibenzo[g,p]chrysene Derivatives
and Polymersa
tion showed that the highest occupied molecular orbital of 1 is
localized on the central diphenylethyne, suggesting that the initial
oxidation occurs at this moiety. The formation of the radical cation
5 is supported by the exclusive formation of dimer 3 in the case
of 1c which has no substituents at the R1 positions. This scheme
represents a new approach to polycyclic aromatic compounds,
and this reaction is, to our knowledge, the first example of an
oxidative acetylene cyclization. However, if this reaction is
considered as an acetylene-extended version of oxidative aryl-
aryl coupling for the biaryl synthesis,14 an alternative mechanism
involving the oxidation of one of the biaryl moieties followed by
the cascade electrophilic or radical cyclization to form 5 is
possible.
UV-vis absorption
fluorescence
max/nmc
cmpd
λ
max/nmb
log ꢀ
λ
Φfd
2a
2b
2d
7a
7b
7c
397
394
412
450
444
453
3.63
3.73
3.91
4.89e
4.83e
4.78e
415
423
431
474
480
478
0.24
0.30
0.29
0.31
0.25
0.35
a In dichloromethane. b Only the longest λmax are given. c Excited at
the absorption maximum wavelengths. d Determined with quinine
sulfate as a standard. e Per monomer unit.
To incorporate the dibenzochrysene into π-conjugated polymer
chain, we need to introduce reactive functional groups into its
skeleton. The introduction of an ortho-directing group for lithiation
affords the functionalized dibenzochrysene as shown in Scheme
2. Thus, 2-methoxyethoxy-substituted dibenzochrysene 2d pre-
pared by the above cyclization was dilithiated using tert-BuLi
followed by the treatment with 1,2-diiodoethane to give diiodod-
ibenzochrysene 6 in 47% yield. Novel π-conjugated polymers 7
are synthesized as air-stable yellow solids by Pd/Cu-catalyzed
cross-coupling reactions15 of 6 with diethynylbenezene monomers,
including a diethynylpentiptycene monomer recently developed
for TNT sensing.6ab
The dibenzochrysene-based polymers 7 are highly fluorescent
and their spectra are shown in Figure 1 and summarized in Table
1 together with the data for monomer. Monomeric dibenzochry-
senes show their longest absorption maxima around 390-412 nm
with moderate intensity and the polymers exhibit intense absorp-
tion maxima around 444-453 nm. The red-shift from monomer
2d to polymers 7 and the significant increase in molar extinction
coefficients suggest an extension of π-conjugation along the
polymer main chain. In the fluorescence spectra, the polymers
show intense emission bands around 474-480 nm with high
quantum yields comparable to those of monomeric compounds
2.13 These polymers show small Stokes shifts of only 24-36 nm
despite the involvement of the phenylene-vinylene structure in
the π-conjugated framework. Most importantly, these polymers
have considerably longer fluorescence lifetimes (7a, 2.5 ns; 7b,
2.6 ns; 7c, 1.4 ns) than the subnanosecond lifetimes typical of
π-conjugated polymers such as poly(p-phenyleneethynylene)s and
poly(p-phenylenevinylene)s.
In summary, we report a new oxidative cyclization method to
dibenzo[g,p]chrysenes and demonstrate their utility in the syn-
thesis of novel π-conjugated polymers. The polymers thus
prepared show attractive fluorescence properties such as high
quantum yields, small Stokes shifts, and long excited-state
lifetimes. These features make them promising candidates for
sensory applications. As an initial proof of this fact, we have
conducted preliminary experiments indicating that polymer 7a
displays higher sensitivity to TNT than the triphenylene- or
pentiptycene-based polymers under investigation in our group.6ab,9
Further comprehensive study on the relationships of the structure-
sensing abilities for these polymers is now in progress.
Acknowledgment. This work was supported by a MURI grant from
the Army Research Office and the Office of Naval Research. We thank
A. Rose for the lifetime measurements.
Supporting Information Available: Experimental procedures and
data for all new compounds and crystal structural data for 2a (PDF).
This material is available free of charge via the Internet at http://pubs.acs.org.
(14) Whiting, D. A. ComprehensiVe Organic Synthesis: SelectiVity, Strategy
and Efficiency in Modern Organic Chemistry: Trost, B. M., Fleming, I., Eds.;
Pergamon Press: Oxford, 1991; Vol. 3, pp 659-703.
(15) Sonogashira, K.; Tohda, Y. Hagihara, N. Tetrahedron Lett. 1975, 4467.
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