Functionalizable polycyclic aromatics through oxidative cyclization of pendant thiophenes

Article abstract of DOI:10.1021/ja0262636

We present a general strategy for obtaining large sulfur-containing polycyclic aromatics from thienyl precursors through iron(Ill) chloride mediated oxidative cyclizations. By placing thienyl moieties in close proximity to adjacent arenes, we have directed the oxidized intermediates into controlled cyclization pathways, effectively suppressing polymer formation. Utilizing these cyclized compounds and their thienyl precursors, we have studied cyclization/polymerization pathways of polymers such as poly(2). The unsubstituted positions α to the sulfur atoms within these aromatic cores allowed for efficient halogenation and further functionalization. As a demonstration, we prepared a series of arylene-ethynylene polymers with varying degrees of chromophore aromatization and used them to probe the effects of synthetically imposed rigidity on polymer photophysical behavior. The symmetries and effective conjugation pathways within the monomers play a key role in determining photophysical properties. We observed that rigid, aromatized chromophores generally led to increased excited-state lifetimes by decreasing radiative rates of fluorescence decay.

Full text of DOI:10.1021/ja0262636

Published on Web 06/06/2002
Functionalizable Polycyclic Aromatics through Oxidative
Cyclization of Pendant Thiophenes
John D. Tovar, Aimee Rose, and Timothy M. Swager*
Contribution from the Department of Chemistry and the Center for Materials Science and
Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Received March 21, 2002
Abstract: We present a general strategy for obtaining large sulfur-containing polycyclic aromatics from
thienyl precursors through iron(III) chloride mediated oxidative cyclizations. By placing thienyl moieties in
close proximity to adjacent arenes, we have directed the oxidized intermediates into controlled cyclization
pathways, effectively suppressing polymer formation. Utilizing these cyclized compounds and their thienyl
precursors, we have studied cyclization/polymerization pathways of polymers such as poly(2). The
unsubstituted positions R to the sulfur atoms within these aromatic cores allowed for efficient halogenation
and further functionalization. As a demonstration, we prepared a series of arylene-ethynylene polymers
with varying degrees of chromophore aromatization and used them to probe the effects of synthetically
imposed rigidity on polymer photophysical behavior. The symmetries and effective conjugation pathways
within the monomers play a key role in determining photophysical properties. We observed that rigid,
aromatized chromophores generally led to increased excited-state lifetimes by decreasing radiative rates
of fluorescence decay.
Introduction
received little attention when compared to the reliance on this
method for the synthesis of polythiophenes.1c Arene cyclizations
Thiophene-based electronic materials have engendered sig-
nificant interest from the standpoint of new synthetic method-
ologies and improved material properties.1 Research initially
focused on polythiophenes since a “bridging sulfur” could
effectively provide aromatic stability to polyacetylene while
preserving desirable physical properties such as high conductiv-
ity.2 The facile functionalization of a thiophene monomer offers
relatively efficient synthetic solutions to band-gap tuning,
solubility, and processability.1b Earlier research on charge-
transfer conductors revealed the role of sulfur as an atom having
greater radial extension in its bonding thus enhancing cofacial
electronic interactions between stacked molecules. The incor-
poration of sulfur into polycyclic aromatic frameworks has also
met considerable success as applied to the design and synthesis
of unique organic semiconductors that exhibit gate-induced field
effects. For example, synthetic efforts led by Katz and by Mu¨llen
have utilized variants of polycyclic aromatic hydrocarbon
chemistry to obtain fully aromatized, sulfur-containing aromatics
exhibiting p-type field-effect mobilities of up to 0.15 cm2/Vs.3,4
The study of oxidative carbon-carbon bond formation as a
means of constructing discrete thiophene-based polycyclics has
induced by chemical oxidants have successfully led to the
construction of aromatized ring structures as diverse as four-
ring triphenylenes and 91-ring planar graphitic sheets.5,6 On the
other hand, prior to our studies, the highly reactive nature of
oxidized thiophene moieties has not allowed for the development
of well-defined, thiophene-centered oxidative cyclizations. Such
oxidized monomers often polymerize rather than undergo
discrete monomer cyclizations.1c,7 Photochemical Mallory-type
cyclizations in the presence of oxidants such as iodine, an
important variant of this strategy, often provide poor yields of
aromatized thiophenes when compared to their benzo bretheren
due to aryl migrations and formation of reactive thiyl radicals.8
Recently, we described a tandem cyclization/polymerization
strategy from pendant monomer 1 where we could selectively
form an aromatized naphtho[2,1-b:3,4-b′]dithiophene such as
2 through chemical or electrochemical oxidation of an ap-
propriately substituted thienyl monomer prior to further polymer
growth (Scheme 1).9 We report herein that this general procedure
allows for the synthesis of a variety of electroactive thiophene-
based materials. We present the synthesis of model compounds
(5) Matheson, I. M.; Musgrave, O. C.; Webster, C. J. Chem. Commun. 1965,
278-279.
* To whom correspondence should be addressed. E-mail: tswager@
mit.edu.
(6) Iyer, V. S.; Wehmeier, M.; Brand, J. D.; Keegstra, M. A.; Mu¨llen, K. Angew.
Chem., Int. Ed. Engl. 1997, 36, 1604-1607.
(1) (a) Handbook of Conducting Polymers, 2nd ed.; Skotheim, T. A.,
Elsenbaumer, R. L., Reynolds, J. R., Eds.; Marcel Dekker: New York,
1998. (b) Roncali, J. Chem. ReV. 1997, 97, 173-205. (c) McCullough, R.
D. AdV. Mater. 1998, 10, 93-116.
(7) See for example: Naudin, E.; El Mehdi, N.; Soucy, C.; Breau, L.; Be´langer,
D. Chem. Mater. 2001, 13, 634-642.
(8) (a) Mallory, F. B.; Mallory, C. W. Org. React. 1984, 30, 1-456. For specific
examples of 3-thienyl decomposition and photoisomerizations, see: (b)
Wynberg, H.; van Driel, H.; Kellogg, R. M.; Buter, J. J. Am. Chem. Soc.
1967, 89, 3487-3494. (c) Kellogg, R. M.; Groen, M. B.; Wynberg, H. J.
Org. Chem. 1967, 32, 3093-3100.
(2) Tourillon, G.; Garnier, F. J. Electroanal. Chem. 1982, 135, 173-178.
(3) Laquindanum, J. G.; Katz, H. E.; Lovinger, A. J. J. Am. Chem. Soc. 1998,
120, 664-672.
(4) Sirringhaus, H.; Friend, R. H.; Wang, C.; Leuninger, J.; Mu¨llen, K. J. Mater.
Chem. 1999, 9, 2095-2101.
(9) Tovar, J. D.; Swager, T. M. AdV. Mater. 2001, 13, 1775-1780.
9
7762
J. AM. CHEM. SOC. 2002, 124, 7762-7769
10.1021/ja0262636 CCC: $22.00 © 2002 American Chemical Society

Functionalizable Polycyclic Aromatics
A R T I C L E S
Scheme 1
Scheme 3a
Scheme 2
a R ) n-C14H29, Ar ) 2,5-Me2C6H3-. Reagents and conditions: (a) FeCl3,
CH2Cl2; (b) Br2, CH2Cl2; (c) PhI(OCOCF3)2, I2, CCl4 (no light); (d)
ArB(OH)2, Pd(PPh3)4, Na2CO3, PhMe/EtOH/H2O; (e) i. n-BuLi, THF, -78
°C; ii. AcOH, -78 °C to room temperature.
polymerizable 3 as an example of model A. As outlined in
Scheme 3, bromination occurred regioselectively at the R-posi-
tions of 1, and subsequent iodination occurred at the remaining
R′-positions.11 Although the high reactivity of both thienyl
halides provided low chemoselectivity for the Suzuki cross-
coupling, we obtained modest yields of the desired iodo-coupled
product corresponding to aryl installation at the R′-site. Finally,
debromination via lithium-halogen exchange provided the
R-unsubstituted 3. To assess the specificity of the oxidative
cyclization under chemical conditions, we subjected 3 to FeCl3
oxidation and obtained 4 in high yield. We could employ several
molar excesses of oxidant to drive the reaction to completion
without apparent dimerization or polymerization. Remarkably,
this reaction provided only one product, indicating that the R-R
sites of 3 had participated in the cyclization.12 To verify this
assignment, we performed a parallel synthesis from 2. Bromi-
nation of the R′-sites of 2 followed by Suzuki coupling yielded
an identical product to that obtained through oxidative cycliza-
tion of 3. This verifies that in the presence of both vacant R-
and â-positions of 3, the more reactive R-sites indeed participate
in the cyclization to afford 4 selectively and in high yield. We
observed no evidence for the formation of the hypothetical â-â
coupling product shown in Scheme 3.
that provide convincing electrochemical evidence in support of
our cyclization/polymerization scheme. These polycyclic aro-
matics allow for further functionalization at positions R to the
sulfur atoms and ultimately for incorporation into new extended
molecular and polymeric systems. Through such elaboration,
we will present the effects of chromophore aromatization on
polymer photophysical behavior and utilize molecular models
to further corroborate this effect.
Results and Discussion
Synthetic and Electrochemical Studies. Our initial work
sought to verify the connectivity of polymeric materials such
as poly(2) and to shed light on the tandem cyclization/
polymerization sequence shown in Scheme 1. To accomplish
this, we needed models that would participate in electrochemi-
cally mediated cyclization events without undergoing oxidative
polymerization. We envisioned that the generic structural motifs
presented in Scheme 2 would serve in this capacity. Model A
has pendant 3-thienyl rings with blocking groups at the
5-position R-sites to prevent polymerization. For the substitu-
tional isomer, we chose model B, where the pendant 2-thienyl
rings also have blocking groups on the polymerizable R-sites.
If polymer growth indeed propagates through the sites now
blocked in these models, cyclic voltammetry (CV) should detect
the redox activities of any new electroactive species formed at
the electrode rather than irreversible follow-up chemical po-
lymerization. Furthermore, these models would shed light on
the propensities (if any) toward coupling defects through vacant
aryl or â-thienyl sites on the aromatized models. Systematically
addressing these reactive sites allows us to probe the observed
reactivity while maintaining a close degree of structural similar-
ity to 1 and the resulting poly(2). In our previous studies, we
found that monomer 1 exhibited two anodic peaks during
successive cyclic sweeping, while chemically R-R cyclized
monomer 2 displayed only one signal that corresponded well
to the second peak observed for 1.9 Here, we electrochemically
examine the first step of the polymerization with substrates that
should not polymerize.
Analysis of 3 and 4 by CV revealed corroborating oxidation
activity. Both 3 and 4 shared reversible redox characteristics
when swept at 100 mV/s (Figure 1): 3 exhibited two redox
waves with half-wave potentials (E1/2) of 0.86 and 1.31 V while
4 displayed two redox waves with E1/2 values of 0.88 and 1.28
V. The lower potential anodic wave observed for 3, with an
anodic peak potential (Epa) of 0.93 V, appeared much stronger
and broader than that for 4, indicating a possible superposition
of an irreversible oxidation and a reversible redox couple.
Indeed, slower scans (25 mV/s) through the first half-wave
potential of both compounds helped to better resolve this
difference (insets of Figure 1). These data clearly show two
distinct oxidations occurring within the first broad redox wave
of 3 (top inset): one irreversible oxidation with an Epa of 0.85
Using standard thiophene halogenation chemistry and Suzuki
(11) D’Auria, M.; Mauriello, G. Tetrahedron Lett. 1995, 36, 4883-4884.
(12) Five aromatic resonances appeared in the 1H NMR of 4, three of which
we assigned to the pendant 2,5-dimethylphenyl moieties. The two remaining
singlets, reflecting a high degree of molecular symmetry, indicated either
an R-R or a â-â coupling.
cross-coupling techniques,10 we obtained the R′-blocked, non-
(10) Miyaura, N.; Yanagi, T.; Suzuki, A. Synth. Commun. 1981, 11, 513-519.
9
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A R T I C L E S
Tovar et al.
Scheme 4a
a Reagents and conditions: (a) 5-methyl-2-tributylstannyl thiophene,
(Ph3P)2PdCl2, DMF, 80 °C; (b) FeCl3, CH2Cl2, room temperature.
Figure 1. Cyclic voltammetry of 3 (top, 1.2 mM) and 4 (bottom, 1.0 mM)
at a 2 mm2 Pt button electrode in 0.1 M n-Bu4PF6 (CH2Cl2) scanned at 100
mV/s; inset scans were taken at 25 mV/s. E1/2(Fc/Fc+) ) +222 mV.
V and one reversible couple with an E1/2 of 0.85 V (Epa ) 0.89
V, peak-peak separation ∆Ep ) 88 mV). In comparison, 4 only
exhibits the redox wave with an E1/2 of 0.85 V (bottom inset;
Epa ) 0.90 V, ∆Ep ) 98 mV).13
In line with the independent chemical syntheses of 4, the
initial oxidation of 3 leads to an intramolecular R-R cyclization.
Subsequent deprotonation and rearomatization leads to 4, and
this product undergoes reversible redox chemistry at a higher
potential essentially identical with that displayed by the chemi-
cally synthesized 4. With the R′-sites blocked, we observed no
evidence for irreversible chemical follow-up reactions upon
oxidation (e.g. dimerization). In the absence of aryl groups
blocking the reactive R′-positions of 4, radical cations derived
from such an electrode-aromatized product (e.g. 2•+) then engage
in follow-up chemical couplings and subsequent growth of poly-
(2) as opposed to the reversible redox chemistry observed for
the blocked model.
As the cyclization/polymerization reactivity depicted in
Scheme 1 also applied to the substitutional R-linked thiophene
isomers, we sought to explore the electrochemical activity of
nonpolymerizable R-thienyl derivatives such as 5 (model B),
readily obtained through Stille cross-coupling (Scheme 4).14
Unlike 3, methyl-blocked 5 only has one vacant site available
for oxidative cyclization that would lead to an aromatized
system. Accordingly, chemical oxidation of 5 furnished the
naphthodithiophene 6 in modest yield. Both 5 and 6 provided
Figure 2. Cyclic voltammetry of 5 (s, 1.1 mM) and 6 (- -, 1.2 mM) at a
2 mm2 Pt button electrode in 0.1 M n-Bu4PF6 (CH3CN) scanned at 100
mV/s (top) and at 750 mV/s (bottom); E1/2(Fc/Fc+) ) +84 mV.
distinct electrochemical signatures that also shed light into the
general mechanism of Scheme 1 (Figure 2, top).
The CV of 5 displayed two strong oxidation peaks at 100
mV/s scan rates (solid line): one irreversible wave with an Epa
of 0.74 V attributed to oxidation of the pendant thienyl ring of
5 followed by a â-â chemical cyclization and deprotonation
process, and one well-defined and reversible redox wave with
an E1/2 of 0.82 V (Epa ) 0.87 V, ∆Ep ) 110 mV) attributed to
electrochemical formation of a stable radical cation species.
Subjecting 6 to the same conditions provided the reversible
redox process only with an E1/2 of 0.82 V (dashed line: Epa
)
0.89 V, ∆Ep ) 130 mV). Furthermore, repetitive fast CV scans
of 5 at 750 mV/s resulted in a decrease in intensity for the initial
oxidative peak (Epa ) 0.79 V) followed by a pseudo-steady-
state buildup of an electrochemically active product in the
vicinity of the electrode (Figure 2, bottom). This quasi-reversible
redox couple, with an E1/2 at 0.82 V (Epa ) 0.89 V, ∆Ep ) 130
mV), resembled that obtained independently through fast scan
sweeps of 6 (Figure 2, bottom; dashed line). As with 3 and 4,
we observed no evidence for dimerization or polymer growth
with continued cyclic sweeping through oxidizing potentials.
In total, the data for 3-4 and for 5-6 support a coupled and
electrochemically mediated cyclization/polymerization process,
(13) Although a formally reversible one-electron redox couple requires a peak-
to-peak separation of 59 mV, we observed similar broadening behavior
under identical conditions for the known, reversible one-electron ferrocene/
ferrocenium couple. For another recent observation of such nonnernstian
behavior in aprotic solvents, see: Lai, R. Y.; Fabrizio, E. F.; Lu, L.; Jenekhe,
S. A.; Bard, A. J. J. Am. Chem. Soc. 2001, 123, 9112-9118.
(14) Milstein, D.; Stille, J. K. J. Am. Chem. Soc. 1979, 101, 4992-4998.
9
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Functionalizable Polycyclic Aromatics
A R T I C L E S
Scheme 5a
Figure 3. CVs of 8 (s, 1.0 mM) and 9 (- -, 1.1 mM) scanned at 750 mV/s
with conditions as in Figure 1; E1/2(Fc/Fc+) ) +230 mV.
phenylene spacer placed between the dialkoxy arene and the
thienyl moiety. After cyclization, 9 then exhibits two reversible
oxidations at less positive potentials than in 8 due to the
stabilization offered by a completely aromatized system.
To demonstrate generality, we examined if extended benz-
annulation on the thiophene moiety would affect the cyclization
efficiency. We prepared model compound 10 through similar
cross-coupling strategies used for 8, where benzo[b]thiophen-
2-yl groups in 10 replace the thienyl groups of 8 (Scheme 5,
bottom). Similar oxidative conditions provided the doubly
cyclized 11 in moderate yield. As with the conversion to 9,
extensive decomposition occurred during FeCl3-promoted cy-
clization, and the reaction mixture proved difficult to purify.
Although efforts to further functionalize 11 have not yet met
with success, the utilization of this chemistry onto larger
heteroaromatics indicated that such oxidative cyclizations extend
beyond simple thiophene derivatives. Owing to the favorable
cofacial interactions possible between such large planar π-sys-
tems, fused compounds 9 and 11 exhibited concentration-
dependent 1H NMR shifts as expected for a strongly aggregating
system. We anticipate that this propensity will allow for
extended intermolecular organization, making more crystalline
derivatives of 9 and 11 suitable for organic field-effect transistor
(OFET) applications.
Repetitive fast scan CV cycling appeared to indicate an
efficient conversion of 10 to 11 although undesired electrode
passivation at higher potentials complicated CV analysis. The
CVs of very dilute samples of 10 (ca. 0.09 mM) provided only
capacitive current until sweeping through the initial Epa at 1.11
V. Subsequent scans revealed a less-positive potential redox
couple with an E1/2 of 0.78 V (Epa ) 0.84 V, ∆Ep ) 120 mV)
and a concurrent decrease in anodic current of the higher
potential oxidation step, analogous to 8 (Figure 4, top). In
comparison, we observed a similar redox couple for very dilute
solutions of 11 with an E1/2 at 0.75 V (Epa ) 0.83 V, ∆Ep )
150 mV; Figure 4, bottom); again, sweeping in solutions on
the order of 1 mM at more positive potentials also led to
passivation. Future efforts will incorporate more stable units
with stronger aromaticity to foster greater electrochemical
reversibility.
a R ) 2-ethylhexyl. Reagents and conditions: (a) 2-tributylstannyl
thiophene, (Ph3P)2PdCl2, DMF, 80 °C; (b) 2-tributylstannyl benzo[b]-
thiophene, (Ph3P)2PdCl2, DMF, 80 °C; (c) 3,4-di(2-ethylhexyloxy)phen-
ylpinacolatoborane, Pd(PPh3)4, Na2CO3, PhMe, EtOH, H2O, 90 °C; (d)
FeCl3, CH2Cl2, room temperature; (e) i. n-BuLi, TMEDA, THF -78 °C;
ii. I2, -78 °C to room temperature.
where the oxidized form of a monomer initially cyclized in situ
(such as 2) participates in intermolecular couplings leading to
polymer growth through the remaining unsubstituted sites R to
the sulfur atoms.
Using the examples of efficient thienyl-based carbon-carbon
bond formation from above, we utilized oxidative cyclizations
to construct larger systems via thienyl-aryl carbon-carbon bond
formation. From the versatile halide scaffold 7,15 chemoselective
cross-coupling under palladium catalysis provided the differ-
entially substituted pendant monomer 8 via sequential Stille and
Suzuki cross-couplings (Scheme 5, top). In an experimentally
similar oxidation protocol used for constructing 2, 4, and 6
(FeCl3/CH2Cl2), pendant compound 8 readily cyclized to
construct the extended aromatic system 9 in respectable yield;
however, the significant excesses of iron oxidant required to
drive the cyclization to completion also led to some decomposi-
tion.
Analogous to 5 and 6, repetitive fast scan CVs at 750 mV/s
with 8 indicated one Epa at 1.32 V, and successive scans
eventually reached a pseudo-steady-state voltammogram ex-
hibiting two reversible waves with E1/2 values of 0.84 (Epa
)
0.95 V, ∆Ep ) 220 mV) and 1.11 V (Epa ) 1.23 V, ∆Ep ≈ 200
mV; Figure 3). For comparison, the CV of 9 under identical
conditions exhibited two reversible processes with E1/2 values
of 0.83 (Epa ) 0.96 V, ∆Ep ) 240 mV) and 1.11 V (Epa ) 1.23
V, ∆Ep ) 230 mV; Figure 3, dashed line). This indicates the
buildup of a new electroactive species at the working electrode
resulting from an oxidatively induced aryl-thienyl bond forma-
tion. When compared to 3, the more positive initial Epa for 8
reflects the lesser degree of electronic induction afforded by
the pendant alkoxy groups. This results from the additional
Incorporation into Conjugated Polymers. We found that
unsubstituted positions R to the sulfur provided synthetic handles
to allow for incorporation into even larger systems in analogy
to the readily halogenated core of 2.9 For example, the R-sites
(15) Hart, H.; Harada, K.; Du, C.-J. F. J. Org. Chem. 1985, 50, 3104-3110.
9
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A R T I C L E S
Tovar et al.
Scheme 6a
a R ) 2-ethylhexyl. Reagents and conditions: (a) 13, Pd(PPh3)4, CuI,
PhMe, i-Pr2NH, 85 °C; (b) i. Hg(OAc)2, CH2Cl2; ii. I2, room temperature.
dominant conjugation pathway through a dibenzochrysene or a
triphenylene core has different geometric and electronic proper-
ties than that for 14.
A systematic study of such large aromatic chromophores, one
goal of the present work, would assess the relative photophysical
contributions of molecular symmetry and planarity within
conjugated polymer systems and would help to outline param-
eters required for more efficient sensory materials. Prior to our
investigations, no molecular design precedents have established
definite relations between chromophore rigidity and photophysi-
cal properties.20 We sought to study these trends within
polymeric systems of varied symmetries and structural con-
straints. The facile halogenations of both the pendant 8 and
cyclized 9 prompted us to examine the effects of chromophore
aromatization on the polymer’s photophysical behavior. To this
end, we synthesized the flexible, noncyclized polymer 15 after
iodinating the meta precursor 8 directly (Scheme 6, bottom;
Mn ) 21 000). This polymer serves to maintain similar structural
connectivity to 14 thereby allowing for comparison of the effects
of chromophore aromatization. As a further application of
thiophene-based controlled oxidative cyclizations, we examined
the related para family of polymers (16 and 17 in Scheme 7)
with backbone electronic properties that again differ in the
degree of comonomer aromatization imparted by two oxidatively
formed carbon-carbon bonds.
The monomer synthetic scheme allowing entry into the para
systems paralleled that used for constructing the meta system 8
(Scheme 7). Sequential Stille and Suzuki cross-couplings onto
the isomeric para scaffold 1815 afforded the thienyl-substituted
terphenyl 19 (X ) H), and halogenation proceeded in high yield
to provide dibromide 20 or diiodide 21. At this point, 21
copolymerized with 13 under Sonogashira conditions to afford
the pendant polymer 16 (Mn ) 15 000). The oxidative conver-
sion of 19 directly into the aromatized system failed due to
unavoidable polymer formation,21 in stark contrast to the
Figure 4. CVs of 10 (repetitive scans, top, ca. 0.09 mM) and 11 (bottom,
ca. 0.22 mM) taken at 500 mV/s, with conditions as in Figure 1; E1/2(Fc/
Fc+) ) +231 mV.
of 9 iodinated in very high yield after double lithiation and
iodine trapping to provide 12 (Scheme 5). Halogenation
proceeded regioselectively to the R-sites as determined by
ROESY spectroscopy. From a molecular standpoint, facile
functionalization of a large aromatic core has important implica-
tions for the development of new molecular architectures.
Additional thiophene couplings will tune the redox potential
while substitution with an unsaturated residue such as an alkynyl
moiety will further delocalize the chromophore. For example,
12 readily participated in Sonogashira-type polymerizations with
the diethynyl pentiptycene 13 to provide the fluorescent
conjugated polymer 14 (Scheme 6, Mn ) 16 500).16,17
Our recent studies on polymers comprised of triphenylene
and dibenzo[g,p]chrysene units showed that larger aromatic
cores facilitated exciton migration and provided longer excited-
state lifetimes,18 two key improvements necessary for optimizing
sensory responses.19 This earlier work specifically targeted the
triphenylene chromophore due to its weakly forbidden low-
energy transitions, a property often shared with other larger
polycyclic aromatics. We anticipated that the extended planarity
and rigidity of meta-oriented comonomer 12 would also offer
a long-lived excited state; indeed, polymer 14 exhibited a
lifetime of 1.06 ns in solution (CH2Cl2). When coupled with
our earlier findings, this provides an additional and empirical
demonstration of excited-state lifetime extension in that the
(16) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975, 4467-
4470.
(20) For examples concerning biphenyl and terphenyl derivatives of varying
degrees of geometric restrictions, see: Nijegorodov, N. I.; Downey, W. S.
J. Phys. Chem. 1994, 98, 5639-5643. While the rate of fluorescence decay
in “planar” methylene-bridged biphenyl (fluorene) increased relative to
biphenyl, the methylene-bridged terphenyl systems displayed the opposite
effect with enhanced planarity.
(17) Yang, J.-S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120, 11864-11873.
(18) (a) Rose, A.; Lugmair, C. G.; Swager, T. M. J. Am. Chem. Soc. 2001, 123,
11298-11299. (b) Yamaguchi, S.; Swager, T. M. J. Am. Chem. Soc. 2001,
123, 12087-12088.
(19) Swager, T. M. Acc. Chem. Res. 1998, 31, 201-207.
9
7766 J. AM. CHEM. SOC. VOL. 124, NO. 26, 2002

Functionalizable Polycyclic Aromatics
A R T I C L E S
Scheme 7a
Figure 5. Absorption and emission spectra for 15 (top) and 14 (bottom)
acquired at room temperature in CH2Cl2. Quantum yields were measured
relative to quinine sulfate in 0.1 N H2SO4 (f ) 0.55); lifetimes were
determined by phase modulation. ꢀ was reported per monomer repeat unit.
a R ) 2-ethylhexyl. Reagents and conditions: (a) 2-tributylstannyl
thiophene, (Ph3P)2PdCl2, DMF, 80 °C; (b) 3,4-di(2-ethylhexyloxy)phen-
ylpinacolatoborane, Pd(PPh3)4, Na2CO3, PhMe, EtOH, H2O, 90 °C; (c) NBS,
DMF, CH2Cl2, room temperature; (d) i. Hg(OAc)2; ii. I2, CH2Cl2, room
temperature; (e) 13, Pd(PPh3)4, CuI, PhMe, i-Pr2NH, 85 °C; (f) FeCl3,
CH2Cl2, room temperature.
relatively clean cyclization of meta system 8 into 9. Therefore,
we hoped to prevent this by employing the halogens to block
the reactive and polymerizable R-sites. The carbon-iodine bond
lability of 21 apparently led to significant dehalogenation and
dimerization during chemical cyclization as observed by mass
spectrometry; however, bromide 20 cleanly cyclized under the
FeCl3 oxidation conditions to afford 22. Despite the sparingly
soluble nature of 22, exhaustive purification of the crude product
(and concomitant material losses) provided modest isolated
yields. From 22, we used Sonogashira conditions to obtain
polymer 17 (Mn ) 20 000).
Simple inspection of the absorption and emission spectra for
both the meta family (14 and 15) and the para family (16 and
17) reveals the electronic effects of chromophore planarization
(Figures 5 and 6). When compared to the conformationally
flexible 15 and 16, the more rigid, aromatized units present in
14 and 17 offer more defined vibronic structure and significant
reduction in energy between the (0,0) absorption and the
emission λmax. The relative oscillator strengths for the low-
energy absorbances of meta polymer 14 have lower intensities
when compared to the analogous low-energy absorptions for
para 17. While significantly altering the absorption profiles,
monomer aromatization within a family only results in a slightly
red-shifted emission maximum (meta: 14 nm; para: 10 nm).
Despite the differences in ground-state geometries within both
Figure 6. Absorption and emission spectra for 16 (top) and 17 (bottom)
acquired at room temperature in CH2Cl2. Measurements as in Figure 5.
(21) For FeCl3 polymerizations of the base 1,4-di(2-thienyl)benzene moiety,
see: Reynolds, J. R.; Ruiz, J. P.; Child, A. D.; Nayak, K.; Marynick, D. S.
Macromolecules 1991, 24, 678-687.
families of polymers, these close resemblances in emission λmax
indicate similar excited-state energetics among the flexible and
9
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A R T I C L E S
Tovar et al.
Scheme 8a
Figure 7. Absorption and emission spectra for 23 (top) and 24 (bottom)
acquired at room temperature in CH2Cl2. Measurements as in Figure 5.
To further investigate the interplay between chromophore
aromaticity and symmetry, we synthesized alkynylated meta
models 23 and 24 and the related para derivatives 25 and 26 as
shown in Scheme 8. Unlike the polymers, these systems possess
shorter, well-defined conjugation lengths thus allowing for a
clear assessment of planarization effects within molecular
systems without significant contributions from conformational
variations in effective conjugation lengths. While we initially
anticipated the use of oxidative cyclizations to obtain the
alkynyl-appended polycyclic aromatics (e.g. 23 to 24), extensive
decomposition during the reaction led to unidentifiable materials,
apparently a result of alkyne degradation.23 However, despite
the problematic purification of the para bromide 22, Sonogashira
cross-coupling of the respective dihalides with alkyne 2724
provided all four alkynylated models in high yields.
a R ) 2-ethylhexyl, Ar ) 4-(n-C12H25O)C6H4-. Reagents and condi-
tions: (a) i. Hg(OAc)2, ii. I2, CH2Cl2, room temperature; (b) 27, (Ph3P)2PdCl2,
CuI, THF/ DIPA, 50 °C.
the rigidified polymers. This supports the fact that the pendant
arenes of noncyclized 15 and 16 become more planarized after
initial excitation in order to allow for greater excited state
delocalization. The slightly blue-shifted emissions observed in
the pendant systems (15 and 16) indicate a minor persistence
of localized poly(phenylene) character in their excited states.
This would suggest that charge localization still exists on
phenylene (or thienylene) segments despite any excited-state
planarization effects.
With both isomeric sets of polymers displaying absorption
and emission characteristics expected after incorporating a more
rigid aromatic chromophore, we measured the fluorescence
lifetimes and quantum yields of these polymers. Pendant meta
polymer 15 had a solution lifetime of 0.30 ns (Φ ) 0.40)
whereas aromatized 14 displayed a lifetime of 1.06 ns (Φ )
0.27), a 3.5-fold larger value. A correlation of the lifetimes with
the quantum yield measurements indicated a decreased rate of
radiative decay for the aromatized 14 relative to 15. In contrast,
para polymers 16 and 17 exhibited identical solution lifetimes
(0.57 and 0.61 ns, respectively) and quantum yields (Φ ) 0.29
and 0.31, respectively), suggesting that the inclusion of the
aromatized system effects no significant change in the radiative
rate. We have assumed that the meta systems exist in disordered
chain conformations; however, the geometric constraints im-
posed by the meta orientation may possibly allow for intrachain
interactions. We therefore needed to examine systems where
chain dynamics would not offer complications.22
In agreement with the optical trends observed for the polymer
systems, the aromatized systems 24 and 26 displayed sharper
vibronic structure and decreased Stokes’ shifts when compared
to the pendant analogues 23 and 25 (Figures 7 and 8). Again in
(22) At this time, we cannot rule out macromolecular conformations within the
meta polymer systems that may allow for intrapolymer cross-talk by way
of helical “kinked” segments in solution analogous to the work of Moore.
However, we would expect that such helical conformations would contribute
minimally to the macromolecular architecture given our choice of spec-
troscopic solvent. The model systems (compounds 23-26) also affirm the
photophysical effects of chromophore rigidity in the absence of macro-
molecular conformational issues or effective conjugation length disparities.
For a seminal study of solvent-driven “zigzag” to helix conformational
changes with related m-phenylene ethylene oligomers, see: Nelson, J. C.;
Saven, J. G.; Moore, J. S.; Wolynes, P. G. Science 1997, 277, 1793-1796.
(23) Cyclic voltammetric studies of 23 and 25 show a lack of new electroactive
products formed near the electrode surface as well as complete anodic
irreversibility over a range of potential scan rates (0.1-10 V/s). Please see
the Supporting Information for CVs taken at 100 mV/s for 23 and 25
(Supplemental Figure 1).
(24) Goldfinger, M. B.; Swager, T. M. J. Am. Chem. Soc. 1994, 116, 7895-
7896.
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7768 J. AM. CHEM. SOC. VOL. 124, NO. 26, 2002

Functionalizable Polycyclic Aromatics
A R T I C L E S
much longer excited-state conjugation pathways localized
through the thienyl-aryl-thienyl segments despite any struc-
tural rigidification. As such, the differences in radiative rates
in the planarized para 17 are not as severe as are those in the
less delocalized meta systems. Ridigification provides a con-
sistent explanation for the observed changes in optical properties
upon chromophore aromatization, but the lack of significant
excited-state lifetime enhancement within the planarized para
17 indicates that simple rigidification of a given chromophore
does not necessarily couple to an increase in lifetime. Our
ongoing studies will assess the physical properties of these
molecular ensembles as well as continue to investigate the
effects of repeat unit aromatization on enhancing excited-state
lifetimes in systems of other symmetries. Future work will
measure the extent of exciton migration through these and other
related polymers in an effort to study and exploit such systems
as new sensory materials.
Conclusions
We have shown that oxidative couplings of pendant thienyl
moieties allow for construction of discrete molecular structures
as a complement to well-established arene-based molecular
cyclizations and to thiophene-based polymerization strategies.
The ease of synthesis for these large polycyclic aromatics and
their pendant aromatic precursors has allowed us to further study
and expand upon a cyclization/polymerization strategy we
recently described by providing a powerful route to electroactive
and emissive aromatic structures. We have demonstrated both
the R- and the â-sites of pendant monomers may undergo
cyclization under oxidative conditions and have applied this to
the synthesis of a variety of large thiophene-based polycyclics
from readily synthesized pendant aromatics. By incorporating
the thiophene moieties, we included a synthetic handle to access
even more elaborate scaffolds. We then used this chemistry as
a platform to demonstrate the effects of chromophore aroma-
tization on photophysical properties of conjugated polymers.
From these studies, we have established further structural
groundwork to aid in the design of new sensory materials. We
will report ongoing studies related to the liquid crystallinity and
OFET behavior of these new materials in due course.
Figure 8. Absorption and emission spectra for 25 (top) and 26 (bottom)
acquired at room temperature in CH2Cl2. Measurements as in Figure 5.
line with the polymer electronic properties, the low oscillator
strength of the (0,0) absorption observed for 24 (436 nm, log
ꢀ ) 3.63) when compared to that in 26 (also at 436 nm, log
ꢀ ) 4.69) indicates a much more weakly allowed process. We
then measured the fluorescence lifetimes and quantum yields
for these molecular systems of shorter and more defined
conjugation lengths. Consistent with the polymeric systems, we
found a near 9-fold increase in lifetime in the cyclized meta
system (23: 0.58 ns; 24: 5.00 ns) while the quantum yield
remained equal (Φ ) 0.17). The cyclized para system displayed
a much less pronounced extension upon planarization (25: 0.80
ns; 26: 1.12 ns) while the quantum yield decreased (25: 0.54;
26: 0.41). For both systems, the decreases in radiative rates
upon planarization within an isomeric family give rise to the
lifetime extensions.
Acknowledgment. We appreciate financial support provided
by the Army Research Office through a MURI award (Tunable
Optical Polymer Systems). We thank Dr. Jeff Simpson (MIT
DCIF) for assistance and advice with ROESY experiments.
These results stand in agreement with similar lifetime
enhancements resulting from symmetry-forbidden transitions in
conjugated polymers containing triphenylene cores.18b In line
with our earlier findings related to the electrochromicity and
conductivity of poly(2),9 the set of para model systems (25 and
26) and the related para polymers (16 and 17) appear to maintain
Supporting Information Available: Experimental procedures
and characterization data for all new compounds and CVs of
23 and 25 (PDF). This material is available free of charge via
the Internet at http://pubs.acs.org.
JA0262636
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