Quinoxaline-based poly(aryleneethynylene)s

Article abstract of DOI:10.1021/ma0257200

The synthesis, ionochromic effect and electrochemistry of quinoxaline containing poly(aryleneethynylene) (PAE) was discussed. Hetero-PAEs are regarded as interesting emitting and n-type semiconducting materials. The high bathochromic shift observed in the

Full text of DOI:10.1021/ma0257200

546
Macromolecules 2003, 36, 546-548
Qu in oxa lin e-Ba sed P oly(a r ylen eeth yn ylen e)s
Ca r lito G. Ba n gcu yo, J oe M. Ellsw or th , Un a Eva n s,
Mich a el L. Myr ick , a n d Uw e H. F . Bu n z*
USC NanoCenter and Department of Chemistry and
Biochemistry,The University of South Carolina,
Columbia, South Carolina 29208
Received October 8, 2002
Revised Manuscript Received December 8, 2002
Poly(aryleneethynylene)s (PAE)1-3 are a class of
conjugated polymers in which aryl groups are inter-
spersed by alkyne units. Their high stability and pho-
tophysical properties make PAEs attractive. PAEs have
found interest as semiconductors, polarizers, and sen-
sory materials.4-6 While the parent dialkyl/dialkoxy-
substituted poly(p-phenyleneethynylene)s (PPE) are
well-known and thoroughly investigated, heterocyclic
PAEs7-9 have been less explored despite the expecta-
tion of these materials to behave differently. J enekhe,
Schanze, Yamamoto, Klemm, and our group have
started to explore the field of hetero-PAEs as interesting
emitting and n-type semiconducting materials.10-12
Herein we report the synthesis, ionochromic effect, and
electrochemistry of the first quinoxaline-containing
PAEs, 3 and 6.
Starting from 4,7-dibromo-2,1,3-benzothiadiazole,13
we prepared 1,4-dibromo-2,3-diaminobenzene, which
furnished the quinoxalines 1 and 4 in high yields by
condensation with benzil or acenaphthenequinone under
acidic conditions. First attempts to alkynylate 1 utilizing
(Ph3P)2PdCl2 and CuI in triethylamine failed. The
addition of THF as a cosolvent, described by Krause,14
allowed the coupling of trimethylsilylacetylene to 1 in
excellent yields. The monomer 2 was isolated as color-
less but highly fluorescent material after removal of the
TMS groups by K2CO3 in an overall yield of 86%. Trans-
fer of this protocol to 4 furnished 5 in an 83% yield.
With the two monomers in hand we performed Pd-
catalyzed couplings of 2 and 5 respectively to 2,5-bis-
(2′-ethylhexyloxy)-1,4-diiodobenzene. Utilizing standard
coupling conditions (5 mol % (Ph3P)2PdCl2, 5 mol % CuI
in piperidine), 2 furnished the desired PAE 3 in 95%
isolated yield (aqueous workup, precipitation into metha-
nol, Scheme 1) and a degree of polymerization (Pn) of
110 repeating units according to gel permeation chro-
matography. In the case of 6, we coupled the soluble
bis(trimethylsilyl)-substituted monomer 5 to the diio-
dide. Module 5 was deprotected in situ (KOH in ethanol)
and gave the sparingly soluble polymer 6 (1-2 mg in
10 mL of chloroform) in an 86% isolated yield and a Pn
of 36. While 3 dissolved easily in organic media such as
chloroform and dichloromethane, the acenaphthalene-
substituted derivative 6 was considerably less soluble.
Consequently, we were only able to obtain the solution-
phase 1H and 13C NMR spectra of 3 and a solution-phase
1H NMR spectrum of 6. All of the collected spectroscopic
data are in agreement with the proposed polymeric
structure.
F igu r e 1. Top: Solution UV-vis spectra (λmax 460 nm,
chloroform), and aggregation UV-vis spectra of PAE 3 upon
addition of methanol vs thin film UV-vis (λmax 534 nm)
spectrum of 3. Middle: Addition of AgOTf (10 mmol L-1) to
polymer 3 (10 mg L-1). Bottom: Addition of trifluoroacetic acid
to polymer 3 (10 mg L-1) (bottom).
thick preparations are golden-green and lustrous. The
polymers’ appearances resemble that of benzothiadia-
zole-based PAEs 7.10a,12
The polymers 3 and 6 show a λmax of absorption in
chloroform at 462 (3) and 474 nm (6). The small
differences in absorption are probably due to the in-
creased size of the conjugated heterocyclic unit in 6.
Polymer 3 was spun cast onto a quartz slide. Its UV-
vis spectrum (Figure 1, top, λmax ) 534 nm) is different
from that in solution. The observed bathochromic shift
(62 nm) is considerable, and probably due to planariza-
tion of the chains in the solid state. Aggregation and
interchain interactions of the charge-transfer type will
likewise play a role here. This aggregation behavior is
distinctive and similar to that observed for the PPEs
as well as for the poly(fluorenyleneethynylene)s.15 The
optical behavior of 3 in thin films can be effectively
X-ray powder diffraction shows both polymers to be
amorphous, with no distinct ordering present. In thin
films, both polymers appear reddish translucent, while
* Corresponding author. E-mail: Bunz@mail.chem.sc.edu.
10.1021/ma0257200 CCC: $25.00 © 2003 American Chemical Society
Published on Web 01/15/2003

Macromolecules, Vol. 36, No. 3, 2003
Communications to the Editor 547
Sch em e 1. Syn th esis of th e P AEs 3 a n d 6
Ta ble 1. Com p a r ison of P M3 Ba n d Ga p Ca lcu la tion s of Sever a l Con ju ga ted Oligom er s
simulated by addition of methanol to a solution of the
polymer in chloroform (Figure 1, top).
other with an emission maximum at 538 nm for 3 and
540 nm for 6. Addition of a small amount of TFA or of
Ag+ ions quenches their fluorescence. Addition of metha-
nol to the polymer 3 does change its emission, and a
slight red shift (Figure 2, bottom) from 540 to 550 nm
is recorded. The golden luster and presence of the
electron withdrawing quinoxaline units in 3 and 6
suggests that these polymers are electronically different
from the PPEs. Quantum chemical calculations reveal
that the HOMO-LUMO gap (Hartree-Fock, 6-31G**,
implemented as Spartan Pro, Wavefunction Inc., on
Windows 2000) of diethynylbenzothiadiazole (HO )
-8.45 eV, LU ) 0.69 eV) is lower than that of diethy-
nylquinoxaline (HO ) -8.11 eV, LU ) 1.31 eV). The
polymer 3 is more electron rich than 7 as indicated by
both the ab inition calculation of the monomer and the
semiempirical PM3 (Spartan Pro) calculations of model
octamers (Table 1). The quinoxaline building block
seems thus less electron accepting than the benzothia-
diazole unit. However, the PM3 results are qualitative
and should not be over-interpreted, but the trend seen
in Table 1 is mirrored in the cyclic voltammograms of
Addition of a solution of lithium trifluorosulfonate in
acetone does not have an effect on the absorption
spectrum. However, silver(I) salts and trifluoracetic acid
(TFA) have a distinct influence of the absorption spec-
trum of 3 (Figure 1, middle and bottom). In both cases,
the primary absorption peak disappears and a sloping
but featureless shoulder appears. For TFA this band
extends well into the 700+ nm range. Dilute solutions
of 6 show a similar behavior as their absorption goes
asymptotically to zero at wavelengths above 700 nm
upon addition of TFA or Ag+. It is noticeable that thin
films of 3 show a bathochromic shift in absorption when
exposed to a solution of AgNO3 in methanol (534 nm to
560 nm), but are essentially unchanged when exposed
to fumes of HCl or trifluoroacetic acid. The reason for
this surprising behavior is the great hydrophobicity of
the polymers 3 and 6 mediated through the ethylhexyl
side chains.
The emission peaks of the polymers 3 and 6 in dilute
chloroform solution are almost indiscernible from each

548 Communications to the Editor
Macromolecules, Vol. 36, No. 3, 2003
cal Society, for financial support. U.B. is a Camille
Dreyfus Teacher-Scholar (2000-2004). We thank Prof.
Haskell W. Beckham for the solid state CP MAS 13C
NMR spectrum of polymer 6.
Su p p or tin g In for m a tion Ava ila ble: Text giving experi-
mental procedures, including structures for 1 and 2, and
detailed spectroscopic characterization of the monomers and
the polymers, and figures showing NMR spectra for polymer
1. This material is available free of charge via the Internet at
http://pubs.acs.org.
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that was spin cast onto an indium tin oxide slide.16
While 7 is reduced at -1.42 V,10a reversible reduction
of 3 occurs at -1.54 V (at -1.66 V reoxidation) under
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quinoxaline-containing PAEs 3 and 6. These materials
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Supporting Information.
Ack n ow led gm en t. We thank the Vice President for
Research, Dr. W. Harris, the National Science Founda-
tion (CHE 0138-659), and the donors of the Petroleum
Research Fund, administered by the American Chemi-
MA0257200