Low optical bandgap polythiophenes by an alternating donor/acceptor repeat unit strategy [16]

Full text of DOI:10.1021/ja9640399

J. Am. Chem. Soc. 1997, 119, 5065-5066
5065
Low Optical Bandgap Polythiophenes by an
Alternating Donor/Acceptor Repeat Unit Strategy
Qing T. Zhang and James M. Tour*
Department of Chemistry and Biochemistry
UniVersity of South Carolina
Columbia, South Carolina 29208
ReceiVed NoVember 22, 1996
ReVised Manuscript ReceiVed February 18, 1997
tion afforded 2.7 Stannylation yielded the requisite electron rich
thiophene monomer 3 which was ready for the modified Stille
cross coupling with monomer 1.
The synthesis of low optical bandgap polymers has been an
area of intense interest due to their unique optoelectronic
properties. The optical bandgaps of conjugated polymers have
often been lowered by maximizing the extended π-conjugation
through promotion of a near-planar conformation between the
consecutive repeat units. Generally, irreversible ladder-type
linkages between the consecutive repeat units of the polymers
8
After screening various coupling conditions, the optimal
molecular weights (by size exclusion chromatography (SEC)
9
relative to polystyrene (PS) standards) of 4 (Mn ) 7000, Mw
)
10,200) were obtained using a mixed Pd(0)/CuI pre-catalyst
8c,10
system (eq 2).
Triphenylarsine was used as the supporting
1
have been used to effect the coplanar arrangements. More
2
recently, reversible or noncovalent linkages, specifically H-
3
bonding, have provided the desired twist inhibition. We report
here a far less-exploited strategy to induce minimally twisted
arrangements in conjugated polymers: constructing an alternat-
ing [AB] polymer where the A-unit has strong electron-donating
moieties and the B-unit has strong electron-withdrawing moi-
4
eties. This results in a consecutive zwitterion-like interaction
with high double bond character between the repeat units,
stabilizing the low optical bandgap quinoidal forms of the
polymers.5
Two required monomers for an [AB] polymerization were
obtained from a common intermediate as shown in eq 1.
Thiophene was selectively brominated at the 2,5-positions and
then nitrated at the 3,4-positions to afford the electron deficient
monomer 1.6 Reduction of the nitro moieties with concomitant
dehalogenation followed by tert-butoxycarbonyl (Boc) protec-
(
1) (a) Overberger, C. G.; Moore, J. A. AdV. Polym. Sci. 1970, 7, 113.
(
b) Schl u¨ ter, A.-D. AdV. Mater. 1991, 3, 282. (c) Yu, L.; Chen, M.; Dalton,
ligand of the catalyst, since triphenylphosphine is known to
L. R. Chem. Mater. 1990, 2, 649. (d) Hong, S. Y.; Kertesz, M.; Lee, Y. S.;
Kim, O.-K. Chem. Mater. 1992, 4, 378. (e) Godt, A.; Schl u¨ ter, A.-D. AdV.
Mater. 1991, 3, 497. (f) Yu. L.; Dalton, L. R. Macromolecules 1990, 23,
8c,d
undergo aryl transfer acting as a chain terminator.
Compound
4
was purified by dissolution in acetone and fractional precipita-
3
439. (g) Scherf, U.; M u¨ llen, K. Synthesis 1992, 23. (h) Lamba, J. J. S.;
tion with hexane. Deprotection of 4 with trifluoroacetic acid
afforded 5 which, due to its solubility in water, was purified by
dialysis (added to aqueous NaHCO3 in a cellulose membrane
tube with a pore size molecular weight cutoff of 3500 Da,
suspension of the tube in deionized water, and changing the
water three times per day for 7 days whereupon the resistivity
of the water remained constant). There were no residual Boc
Tour, J. M. J. Am. Chem. Soc. 1994, 116, 11723. (i) Schl u¨ ter, A.-D.;
Schlicke, B. Synlett 1996, 425. (j) Kertesz, M.; Hong, S. Y. Macromolecules
1
992, 25, 5424. (k) Kintzel, O.; M u¨ nch, W.; Schl u¨ ter, A.-D.; Godt, A. J.
Org. Chem. 1996, 61, 7304.
2) (a) Marsella, M. J.; Swager, T. M. J. Am. Chem. Soc. 1993, 115,
(
1
1
1
2
2214. (b) McCullough, R. D.; Williams, S. P. J. Am. Chem. Soc. 1993,
15, 11608. (c) Brockmann, T. W.; Tour, J. M. J. Am. Chem. Soc. 1995,
17, 4437. (d) McCullough, R. D.; Williams, S. P. Chem. Mater. 1995, 7,
001.
1
moieties in 5 as determined by H NMR and IR analyses.
(
3) (a) Jenekhe, S. A.; Osaheni, J. A. Macromolecules 1992, 25, 5828.
b) Tarkka, R. M.; Zhang, X.; Jenekhe, S. A. J. Am. Chem. Soc. 1996, 118,
There were several pieces of evidence which suggested that
5 had a high degree of zwitterionic and quinoidal character.
First, unlike 4, polymer 5 was only soluble in polar solvents
such as MeOH, DMSO, and H2O. Second, the optical spectral
shift on the conversion of 4 to 5 was profound; for 4, λmax (THF)
) 407 nm and (MeOH) ) 395 nm, while for 5, λmax (MeOH)
(
9
438. (c) van Mullekom, H. A. M.; Vekemans, J. A. J. M.; Meijer, E. W.
Chem. Commun. 1996, 2163. (d) Delnoye, D. A. P.; Sijbesma, R. P.;
Vekemans, J. A. J. M.; Meijer, E. W. J. Am. Chem. Soc. 1996, 118, 8717.
(4) Although there have been a few reported examples of this strategy,
none, to our knowledge, have employed multiple intensely donating and
withdrawing groups as described here, and the reported corresponding
bandgap decreases have usually been be considerably less intense. See: (a)
Yamamoto, T.; Zhou, Z.-h.; Kanbara, T.; Shimura, M.; Kizu, K.; Maruyama,
T.; Nakamura, Y.; Fukuda, T.; Lee, B.-L.; Ooba, N.; Tomaru, S.; Kurihara,
T.; Kaino, T.; Kubota, K.; Sasaki, S. J. Am. Chem. Soc. 1996, 118, 10389.
)
662 nm (Figure 1). The optical bandgap decrease, evidenced
by the strong bathochromic shift of 267 nm in the conversion
of 4 to 5, suggested that there was significant quinoidal character
(
b) Yamabe, T.; Bakhski, A. K.; Yamaguchi, A. K.; Ago, H. Synth. Met.
996, 79, 115. (c) Demanze, F.; Yassar, A.; Garnier, F. Macromolecules
996, 29, 4267. (d) Yamamoto, T.; Kanbara, T.; Ooba, N.; Tomaru, S. Chem.
1
(7) Galvez, C.; Garcia, F.; Garcia, J.; Soldevila, J. J. Heterocycl. Chem.
1986, 23, 1103.
1
Lett. 1994, 1709. (e) Greenham, N. C.; Moratti, S. C.; Bradley, D. D. C.;
Friend, R. H.; Holmes, A. B. Nature 1993, 365, 628. (f) Zhou, Z.-h.;
Maruyama, T.; Kanbara, T.; Ikeda, T.; Ichimura, K.; Yamamoto, T.; Tokuda,
K. J. Chem. Soc., Chem. Commun. 1991, 1210. (g) Pan, M.; Bao, Z.; Yu,
L. Macromolecules 1995, 28, 515. For an example of a zwitterionic polymer
with a very low optical bandgap, see: (h) Havinga, E. E.; ten Hoeve, W.;
Wynberg, H. Polym. Bull. 1992, 29, 119.
(8) (a) Bao, Z.; Chan, W. K.; Yu, L. J. Am. Chem. Soc. 1995, 117, 12426.
(b) Stille, J. K. Angew. Chem., Int. Ed. Engl. 1986, 25, 508. (c) Farina, V.
Pure Appl. Chem. 1996, 68, 73. (d) Farina, V.; Krishnan, B. J. Am. Chem.
Soc. 1991, 113, 9585.
(9) Since SEC is a measure of the hydrodynamic volume and not the
molecular weight, significant yet consistent errors in Mn and Mw usually
result when comparing rigid rod polymers to the flexible coils of PS
standards. Therefore, the values recorded here are given simply as a
reference. For the degree of errors that could be generated, see: Tour, J.
M. Chem. ReV. 1996, 96, 537.
(
5) (a) Handbook of Conducting Polymers; Skotheim, T. J., Ed.;
Dekker: New York, 1986. (b) Zollinger, H. Color Chemistry; VCH: New
York, 1991.
(
6) Mozingo, R.; Harris, S. A.; Wolf, D. E.; Hoffhine, C. E., Jr.; Easton,
(10) (a) Sa a´ , J. M.; Martorell, G. J. Org. Chem. 1993, 58, 1963. (b)
Liebeskind, L. S.; Fengl, R. W. J. Org. Chem. 1990, 55, 5359.
N. R.; Folkers, K. J. Am. Chem. Soc. 1945, 67, 2092.
S0002-7863(96)04039-5 CCC: $14.00 © 1997 American Chemical Society

5066 J. Am. Chem. Soc., Vol. 119, No. 21, 1997
Communications to the Editor
In an effort to further confirm the need for the repeating
donor/acceptor structure of the polymers for these unusually
large wavelength absorptions, trimers 9-12 were prepared that
had one electron-donating group per electron-withdrawing
group, much like in the polymers 4, 5, and 8.
Figure 1. Optical absorbance spectra of 4 (THF) (- - -), 5 (MeOH)
s), and 8 (THF) (---- - - -).
(
in 5. This change was even more striking in view of the fact
that highly regiochemically pure 3-substituted polythiophenes
have λmax values of 450-460 nm in solution1 while 3,4-
1
disubstituted polythiophenes generally have λmax < 300 nm due
to the unavoidable 3-3′ (head-to-head) interactions.1
j,12
Here,
we have a fully substituted polythiophene, albeit non-alkyl-
substituted, with an unusually low bandgap by exploitation of
the donor/acceptor repeat unit strategy.
We also prepared an analogous donor/acceptor system that
had an imide rather than a dinitro acceptor unit. Tetrabromi-
nation of thiophene followed by selective 2,5-debromination
In each case, the efficacy of the extended polymeric systems
were verified since the polymers had much lower optical
bandgaps. Even more striking is the fact that 8 is 128 nm
bathochromically shifted relative to 12, even though 8 had
sterically larger and electronically poorer donors units than 12.
In summary, we have prepared low optical bandgap conju-
gated polymers based on an [AB] polymer using alternating
donor and acceptor moieties within the repeat unit to maximize
extended π-conjugation. The polymers have significant quinoi-
dal and/or zwitterionic character as evidenced by their optical
absorbance spectra and solubility properties.
13
afforded 3,4-dibromothiophene. Cyano-substitution, hydroly-
14
15
sis, and rebromination at the 2,5-positions afforded 6. Diacid
chloride formation and treatment with n-butylamine afforded
the desired imide monomer 7 which could be polymerized with
3
to afford the donor/acceptor polymer 8 (Mn ) 5900, Mw)
1
6 800) (eq 3). Remarkably, even in its sterically encumbered
N,N′-di-Boc-protected form, 8 had an intensely bathochromically
shifted optical absorbance: λmax (THF) ) 526 nm and (MeOH)
)
548 nm (Figure 1). Attempts to remove the Boc group
yielded insoluble polymer. This may be due to ensuing amide
formation and concomitant loss of n-butylamine, although we
were successful in accomplishing the efficient deprotection
Acknowledgment. Support came from the Office of Naval Re-
search, the National Science Foundation (DMR-9158315, EHR-91-
(80%) in small oligomeric systems (Vide infra).
0
8772), and industrial contributors to the NSF Presidential Young
(
11) (a) McCullough, R. D.; Lowe, R. D.; Jayaraman, M.; Anderson, D.
Investigator Award Program (1991-96): Hercules, IBM, Albemarle,
Ethyl, Shell, Eli Lilly, Polaroid, Farchan, and Exxon Corporations. We
thank Molecular Design Ltd. for the use of their synthetic data base.
L. J. Org. Chem. 1993, 58, 904. (b) Chen, T.-A.; Wu, X.; Rieke, R. D. J.
Am. Chem. Soc. 1995, 117, 233.
(
12) Kankre, J.; Lukkari, J.; Pasanen, P.; Sillap a¨ a¨ , R.; Laine, H.; Harmaa,
K. Macromolecules 1994, 27, 4327.
Supporting Information Available: Synthetic and characterization
details for the new compounds 3-8 (4 pages). See any current
masthead page for ordering and Internet access instructions.
(
(
13) Gronowitz, S. Acta Chem. Scand. 1959, 13, 1045.
14) MacDowell, D. W. H.; Wisowaty, J. C. J. Org. Chem. 1972, 37,
1
5
712.
(
15) Pomerantz, M.; Yang, H.; Cheng, Y. Macromolecules 1995, 28,
706.
JA9640399