Transition-metal-free synthesis of poly(phenylene ethynylene)s with alternating aryl-perfluoroaryl units

Article abstract of DOI:10.1021/ja710564b

A catalytic amount of fluoride salt is all that is required to generate high molecular weight poly(phenylene ethynylene)s from silylacetylene-functionalized monomers and C₆F₆. The sole side product is gaseous fluorotrimethylsilane an

Full text of DOI:10.1021/ja710564b

Published on Web 12/19/2007
Transition-Metal-Free Synthesis of Poly(phenylene Ethynylene)s with
Alternating Aryl-Perfluoroaryl Units
Tanmoy Dutta, Kathy B. Woody, and Mark D. Watson*
UniVersity of Kentucky, Department of Chemistry, Lexington, Kentucky, 40506-0055
Received November 23, 2007; E-mail: mdwatson@uky.edu
Scheme 1 Synthesis of Alternating Poly(phenylene ethynylene)s
Poly(arylene ethynylene)s (PAE) may be prepared with a broad
range of (opto)electronic properties to serve as active components
in solar cells,1a sensors,1b and field effect transistors (oligomers).1c
The main routes for their syntheses are Hagihara-Sonogashira and
related palladium-catalyzed couplings,2 or acyclic diyne metathesis
(ADIMET).3 We report here a simple alternative, transition-metal-
free synthesis which delivers PAEs in high molecular weight,
structural fidelity, purity, and yield with minimal required purifica-
tion.
Many nucleophiles react regioselectively with perfluorobenzene
(PFB), replacing two fluorides at its 1,4-positions.4 A number of
polymers have been produced this way,5 but very few were built
from carbon-centered nucleophiles.6 Carbon-centered nucleophiles
can be conveniently generated in situ from easily handled orga-
nosilanes.7 Specifically, activated silyl acetylenes react with a
number of electrophiles,8 and we report here their reaction with
PFB to prepare high molecular weight poly(phenylene ethynylene)s
(PPE). The repeating structures composed of perfectly alternating
fluorinated and non-fluorinated π-electron units can be expected
to provide fine control over self-assembly9 and bulk optoelectronic
properties.10
a GPC vs polystyrene. babs:10-6 M THF; PL: 10-8 M THF. cDifferential
scanning calorimetry. danhydrous, room temp. eTBAF‚3H2O, benchtop, room
f
temp. anhydrous, 60 °C. All in toluene, except reactions using TMAF
(THF).
Polymerization requires only a catalytic amount of fluoride ion
to generate nucleophilic pentacoordinate silicate ions from 1, which
form new bonds with PFB (Scheme 1). The fluoride lost as volatile
fluorotrimethylsilane (TMSF) with each new C-C bond is regener-
ated by substitution. Reactions of silyl acetylenes with perfluoro-
pyridine are also catalytic in fluoride (not shown), unlike published
reports8b using molar excesses of fluoride.
with step-growth termination by unreactive CtCH end-groups,
formed via protiodesilylation by water and protic impurities
resulting from Hofmann elimination of TBAF. The ratio of C6F5
to CtCH end-groups can be controlled by using a molar excess of
PFB under anhydrous conditions (1H and 19F NMR, Supporting
Information).
The maximum attainable number-average degree of polymeri-
zation (Pn) for step polymerizations falls precipitously with
comonomer stoichiometric imbalance as described by the Carothers
equation (Figure 1). In cases where bifunctional monomer produces
a more reactive intermediate, i.e., the first bond-formation is the
rate-limiting step, Pn can be enhanced over practical reaction periods
by using an excess of that monomer.12 Three pieces of evidence
indicate that this polymerization proceeds through a more reactive
intermediate, that is, the second fluoride displacement from PFB
is faster than the first:
(I) The relationship between Pn and C6F6:1c stoichiometry is
plotted in Figure 1. The distinct deviation from Carothers equation
allows for relatively high molecular weight despite a high molar
excess of PFB, which in turn leads to well-defined C6F5 end-groups.
Descriptions of Pn estimations by NMR and GPC are provided in
the Supporting Information.
(II) During polymerization, 19F NMR shows growing signals
from TMSF and 1,2,4,5-tetrafluorophenyl repeating units, but the
steady-state concentration of C6F5 end-groups remains low owing
to their much higher reactivity compared to PFB.
To minimize competing protiodesilyation, we began the synthetic
study using freshly calcined CsF with 18-crown-6, which proved
most effective for preparing copolymers from silylthiophenes.11
Above 60 °C, this catalyst system is effective with monomers 1b,c
but with 1a the surface of the CsF particles simply darkened.
Tetramethyl- and tetrabutyl ammonium fluoride (TMAF, TBAF)
are effective for all three monomers at room temperature. The
differing reactivity of monomer 1a from 1b,c could be due to
solubility of reactive intermediates or to subtle differences in
electron donating/withdrawing ability of the alkoxy substituents
dictated by rotational freedom/steric demand of the alkyl chains.
The dark color fouling the CsF surface in reactions with monomer
1a may arise from trapped reactive intermediates. Homogeneous
polymerizations with 1b,c catalyzed by TBAF also initially become
very dark, but the solutions assume the yellow color of the polymers
at high conversion when the concentration of reactive intermediates
becomes low (e.g., pentacoordinate silicates or Meisenheimer
complexes).
Number-average molecular weights (Mn, GPC) of a few kDa
are obtained with TBAF‚3H2O and off-the-shelf solvents. Mn is
increased when using a commercial “anhydrous” TBAF solution
(1 M THF, 5% H2O v/v), and further still with anhydrous TMAF
in anhydrous solvents (Mn > 100 kDa). This trend is consistent
(III) Model study (Scheme 2): Although the initial molar ratio
of 2:PFB is 1:1, ∼ 80% of 2 is converted to 5, similar to reactions
of PFB with lithium acetylides.13 The phenyl-acetylene group of 4
activates the para-position toward further substitution, perhaps via
9
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10.1021/ja710564b CCC: $40.75 © 2008 American Chemical Society

C O M M U N I C A T I O N S
In summary, we have shown that high molecular weight PPEs
can be obtained by nucleophilic aromatic substitution. The polymers
are easily purified as the only side product is a gas (TMSF), and
the catalyst can be removed with a simple aqueous treatment. Their
well-defined C6F5 endgroups might be selectively functionalized
with other nucleophiles, or the polymers can be used as mac-
romonomers. This synthetic route may enjoy broad applicability
in synthesis of acetylenic scaffolds of varying geometries.15 It may
show particular importance for incorporating alkynyl monomers
that are unstable16 if desilylated prior to Hagihara-Sonogashira
coupling. For example we have coupled 9,10-trialkylsilylacetylene-
functionalized anthracene and the analogous 6,13-functionalized
pentacene with perfluoro-pyridine and -toluene in 60-80% isolated
yields (unoptimized).
Figure 1. Pn vs C6F6:1c molar ratio (TMAF catalyst), estimated by NMR
end-group analysis ([) and GPC (4), compared to Carothers equation
(dashed line). See Supporting Information for treatment of GPC data. The
inset shows the Carothers equation with r ) molar ratio of excess monomer
to limiting monomer.
Acknowledgment. This work was supported in part by the
American Chemical Society (Grant PRF 43047-G), Kentucky
Science and Engineering Foundation, and the National Science
Foundation (Grant CHE 0616759). This work is dedicated in
memory of Prof. George B. Butler and his contributions to polymer
chemistry.
Scheme 2. Model Reactiona
Supporting Information Available: Experimental procedures,
spectroscopic and analytical data. This material is available free of
charge via the Internet at http://pubs.acs.org.
References
(1) (a) Mwaura, J. K.; Zhao, X.; Jiang, H.; Schanze, K. S.; Reynolds, J. R.
Chem. Mater. 2006, 18, 6109-11. (b) Liao, J. H.; Swager, T. M. Langmuir
2007, 23, 112-5. (c) Roy, V. A. L.; Zhi, Y.-G.; Xu, Z.-X.; Yu, S.-C.;
Chan, P. W. H.; Che, C.-M. AdV. Mater. 2005, 17, 1258-61.
(2) Babudri, F.; Farinola, G. M.; Naso, F. J. Mater. Chem. 2004, 14, 11-34.
(3) (a) Bunz, U. H. F. Chem. ReV. 2000, 100, 1605-44. (b) Zhang, W.; Moore,
J. S. Macromolecules 2004, 37, 3973-5.
a Isolated yields; R ) CCPh.
additional cumulene-like resonance forms of the resulting inter-
mediate Meisenheimer complex.
(4) Brooke, G. M. J. Fluor. Chem. 1997, 86, 1-76.
(5) (a) Budd, P. M.; Ghanem, B. S.; Makhseed, S.; McKeown, N. B.; Msayib,
K. J.; Tattershall, C. E. Chem. Commun. 2004, 230-1. (b) Kim, J.-P.;
Lee, W.-Y.; Kang, J.-W.; Kwon, S.-K.; Kim, J.-J.; Lee, J.-S. Macromol-
ecules 2001, 34, 7817-21. (c) Kimura, K.; Tabuchi, Y.; Nishichi, A.;
Yamashita, Y.; Okumura, Y.; Sakaguchi, Y. Polymer J. 2001, 33, 290-
6.
(6) (a) Woody, K. B.; Bullock, J. E.; Parkin, S. R.; Watson, M. D.
Macromolecules 2007, 40, 4470-3. (b) Deck, P. A.; Lane, M. J.;
Montgomery, J. L.; Slebodnick, C.; Fronczek, F. R. Organometallics 2000,
19, 1013-24. (c) Deck, P. A.; Maiorana, C. R. Macromolecules 2001,
34, 9-13.
(7) Prakash, G. K. S.; Yudin, A. K. Chem. ReV. 1997, 97, 757-86.
(8) (a) Bosse, F.; Tunoori, A. R.; Maier, M. E. Tetrahedron 1997, 53, 9159-
68. (b) Kovalenko, S. V.; Alabugin, I. V. Chem. Commun. 2005, 1444-
6.
(9) (a) Bacchi, S.; Benaglia, M.; Cozzi, F.; Demartin, F.; Filippini, G.;
Gavezzotti, A. Chem. Eur. J. 2006, 12, 3538-46. (b) Smith, C. E.; Smith,
P. S.; Thomas, R. L.; Robins, E. G.; Collings, J. C.; Dai, C.; Scott, A. J.;
Borwick, S.; Batsanov, A. S.; Watt, S. W.; Clark, S. J.; Viney, C.; Howard,
J. A. K.; Clegg, W.; Marder, T. B. J. Mater. Chem. 2004, 14, 413-20.
(c) Dai, C.; Nguyen, P.; Marder, T. B.; Scott, A. J.; Clegg, W.; Viney, C.
Chem. Commun. 1999, 24, 2493-4.
(10) (a) Facchetti, A.; Yoon, M.-H.; Stern, C. L.; Marks, T. J. J. Am. Chem.
Soc. 2006, 128, 5792-801. (b) Sakamoto, Y.; Suzuki, T.; Kobayashi, M.;
Gao, Y.; Fukai, Y.; Inoue, Y.; Sato, F.; Tokito, S. J. Am. Chem. Soc.
2004, 126, 8138-40. (c) Winkler, B.; Meghdadi, F.; Tasch, S.; Mullner,
R.; Resel, R.; Saf, R.; Leising, G.; Stelzer, F. Opt. Mater. 1998, 9, 159-
62.
(11) Wang, Y.; Watson, M. D. J. Am. Chem. Soc. 2006, 128 (8), 2536-7.
(12) (a) Kihara, N.; Komatsu, S.; Takata, T.; Endo, T. Macromolecules 1999,
32, 4776-83. (b) Nomura, N.; Tsurugi, K.; RajanBabu, T. V.; Kondo, T.
J. Am. Chem. Soc. 2004, 126, 5354-5.
(13) Waugh, F.; Walton, D. R. M. J. Organomet. Chem. 1972, 39, 275-8.
(14) Mendez, J. D.; Schroeter, M.; Weder, C. Macromol. Chem. Phys. 2007,
208, 1625-36.
(15) Acetylene Chemistry: Chemistry, Biology, and Material Science; Dieder-
ich, F., Stang, P. J., Tykwinski, R. R., Eds.; Wiley-VCH: Weinheim,
Germany, 2005.
The model reaction is violently exothermic when conducted at
high concentration, producing substantial amounts of 6 and 7. At
dilutions approaching those of the polymerizations, combined
conversions to 6 and 7 are lowered to ∼6%. These side-products
correspond to branching points or cross-links in the analogous
polymerizations, which could also cause deviation from Carothers
equation. However, the actual percentage of such defects in the
polymers is less than 1/150 repeat units based on integration of
diminutive 19F NMR signals. For comparison, model “defect” 6 (1
mol %) is readily detected in a solution of model polymer repeat
unit 5 by 19F NMR after relatively few transients. The low
percentage of defects in the polymers might be attributed to steric
bulk of monomers 1. From other bis-silyl-acetylene monomers, we
have prepared hyperbranched or cross-linked PAE gels, which may
have their own merits14 and will be described in a future com-
munication.
Unlike poly1a,6a poly1b,1c are highly soluble in common organic
solvents at room temperature. The solution absorbance/photolumi-
nescence spectra of poly1b,1c differ significantly (Supporting
Information). The 2-ethyl-hexyl chains of poly1c present greater
steric bulk in the vicinity of the polymer backbone, altering
rotational freedom around backbone bonds and/or the benzene-
oxygen bonds. The former may affect backbone conjugation, while
the latter can affect the mesomeric/inductive influence of the side
chains on the chromophoric backbone. The result is a larger apparent
Stokes shift and nearly featureless absorbance and PL profiles. A
future communication will link solid-state optical and thermal
properties to packing arrangements determined by wide-angle X-ray
scattering studies.
(16) Taylor, M. S.; Swager, T. M. Angew. Chem., Int. Ed. 2007, 46, 8480-3.
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