Synthesis of thiophene 1,1-dioxides and tuning their optoelectronic properties

Article abstract of DOI:10.1021/ol4024024

A 2,5-bis(tributylstannyl)thiophene 1,1-dioxide was prepared from 2,5-bis(trimethylsilyl)thiophene 1,1-dioxide, bis(tributyltin) oxide, and tetrabutylammonium fluoride (TBAF). The 2,5-bis(tributylstannyl)thiophene 1,1-dioxide and a 2,5-diiodothiophene 1,1

Full text of DOI:10.1021/ol4024024

ORGANIC
LETTERS
2013
Vol. 15, No. 20
5230–5233
Synthesis of Thiophene 1,1-Dioxides and
Tuning Their Optoelectronic Properties
Chia-Hua Tsai, Danielle N. Chirdon, Andrew B. Maurer, Stefan Bernhard, and
Kevin J. T. Noonan*
Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh,
Pennsylvania, 15213, United States
noonan@andrew.cmu.edu
Received August 20, 2013
ABSTRACT
A 2,5-bis(tributylstannyl)thiophene 1,1-dioxide was prepared from 2,5-bis(trimethylsilyl)thiophene 1,1-dioxide, bis(tributyltin) oxide, and tetrabutyl-
ammonium fluoride (TBAF). The 2,5-bis(tributylstannyl)thiophene 1,1-dioxide and a 2,5-diiodothiophene 1,1-dioxide were utilized in a series of Stille
cross-coupling reactions to afford thiophene 1,1-dioxides with either electron-donating or electron-withdrawing substituents. Electron-withdrawing
groups greatly facilitate the reduction of these sulfone heterocycles, and ÀC6H4-p-NO2 substituents produce a 510 mV shift as compared to a
thiophene 1,1-dioxide with two phenyl groups.
Organic electronics are altering the technology land-
scape with diverse applications from sensing1 to mimicking
human skin.2 The solution processability and modular struc-
ture of conjugated materials make them attractive for orga-
nic photovoltaics (OPVs),3 organic light-emitting devices
(OLEDs),4 and organic field effect transistors (OFETs).5
Organic semiconductors are generally classified as either
hole-transport (p-type) or electron-transport (n-type) ma-
terials. The n-type systems are not as well developed,
particularly in OFETs, due to challenges with stability
under ambient conditions and lower charge mobility.5 In
the late 1990s, Barbarella pioneered the unique strategy of
converting p-type oligothiophenes into n-type materials by
inserting thiophene 1,1-dioxides.6 Judicious placement of
one or more oxidized heterocycles within the conjugated
architecture produced materials that exhibit much more
facile reductions than their unoxidized analogues.6c,d More-
over, some thiophene dioxide oligomers are highly lumines-
cent in the solid state.6b,7
Barbarella typically utilized a coupling strategy to
connect the thiophene dioxide with other aromatics
(e.g., benzene, thiophene).8 Subsequent work by Rozen
(1) (a) Roberts, M. E.; Sokolov, A. N.; Bao, Z. J. Mater. Chem. 2009,
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and co-workers highlighted HOF CH3CN as an oxidant
to completely oxidize a variety of different thiophene oligo-
mers and obtain [all]-S,S-dioxides.9 Additionally, Rozen
3
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(7) (a) Mazzeo, M.; Vitale, V.; Della Sala, F.; Pisignano, D.; Anni,
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G. Adv. Mater. 2003, 15, 2060–2063. (b) Tedesco, E.; Kariuki,
B. M.; Harris, K. D. M.; Johnston, R. L.; Pudova, O.; Barbarella, G.;
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121–128. (c) Antolini, L.; Tedesco, E.; Barbarella, G.; Favaretto, L.;
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r
10.1021/ol4024024
Published on Web 10/03/2013
2013 American Chemical Society

reported oxidations of fused thiophenes and star thiophenes
using HOF CH3CN.10 To date, a variety of different small
molecules and oligomers bearing oxygenated thiophene
rings have been synthesized and characterized.11 Thiophene
dioxide moieties have also been incorporated into conju-
gated polymer architectures.12
co-workers (Scheme 1).16 Compound 2b and other substi-
tuted thiophene 1,1-dioxides have been successfully em-
ployed as coupling partners in palladium-catalyzed Stille
reactions.8,17 However, to take advantage of widely available
aryl halides, we also envisioned a novel distannylated thio-
phene 1,1-dioxide. A previous report by Buchwald and co-
workers highlighted the potential of TBAF to convert
alkynylsilanes to alkynyl stannanes,18 and Burton illustrated
that a similar method could convert vinylsilanes to vinyl-
stannanes.19 We found that the combination of compound 1
with bis(tributyltin) oxide in the presence of TBAF produced
the desired distannylated precursor 3 (yield: 49%, Scheme 1).
3
These heterocycles have attracted attention for electron
transport,13 but to the best of our knowledge, electron
withdrawing groups have not yet been investigated as a
method to influence their electronic properties. In other
families of n-type materials such as arylene diimides,14
substitution of the conjugated core with electron-
withdrawing groups (e.g., CN) has resulted in stability
and high electron mobility.15 Inspired by the potential of
thiophene 1,1-dioxides as n-type materials, we developed a
new synthetic scheme for coupling these sulfone hetero-
cycles with electron-withdrawing groups and explored the
effects of electronic modulation on their properties.
We sought a thiophene 1,1-dioxide that could be employed
in cross-coupling reactions to attach various aryl groups.
Compound 1 can be converted to 2,5-dihalo derivatives
(2a and 2b) following a published report by Furukawa and
Scheme 1. Synthesis of Thiophene 1,1-Dioxide Precursors
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J. H.; Collins, B. A.; Tumbleston, J. R.; Toney, M. F.; Ade, H.; Hawker,
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Org. Lett., Vol. 15, No. 20, 2013
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Compounds 2a and 3were utilized in a series of Stille cross-
coupling reactions to afford compounds 4aÀh(Table1). The
diiodo precursor (2a) was employed to couple more electron-
rich substituents (Ph, ÀC6H4-p-OMe, 2-thienyl, 2-furyl) to
the central sulfone ring (reaction A, Table 1). Electron-
withdrawing substituents were connected to the dioxide
moiety by coupling compound 3 with the appropriate aryl
bromide (reaction B, Table 1). All compounds except 4b were
synthesized using Pd2dba3 and AsPh3, similar to a previous
report by Barbarella and co-workers.8
Though compounds 4a6b and 4f8 have been prepared
previously, no cyclic voltammetry was reported. Here, all
synthesized thiophene 1,1-dioxides were probed electrochemi-
cally to investigate redox properties and frontier orbitals. The
oxidation signal observed for 4a (1.85 V, Table 2) is similar
to the oxidations reported previously for 5 and 6 (1.88 and
1.85 V, respectively, Figure 1).6b The first reduction of 4a
(À1.16 V, Table 2) is easier than the first reductions of 5and 6,
which are likely inhibited by electron donation from the
Figure 2. Cyclic voltammograms of 4a (blue), 4b (red), and 4e
(black). All voltammograms were collected in 0.10 M N(n-Bu)4PF6
(MeCN) solution.
and 8) have been characterized electrochemically, and 8
was polymerized in MeCN by repeated cyclic voltammetry
scans.6b,c
additional R groups (5, Ered = À1.30 V and 6, Ered
=
À1.57 V).6b
Compound 4b, with the electron-donating ÀC6H4-p-
OMe groups, exhibits a lower oxidation (1.39 V) and more
negative reduction (À1.30 V) when compared to 4a
(Table 2). In direct contrast, substitution of the thiophene
1,1-dioxide core with electron-withdrawing groups has the
opposite effect. Compounds 4c and 4d (ÀC6H4-p-CF3 and
ÀC6H4-p-CN groups) exhibit no oxidation within the
solvent window, and both are more easily reduced than
4a (À0.90 for 4c and À0.77 for 4d). Reduction is most
positively shifted for 4e giving an impressive first reduction
potential of À0.65 V (Figure 2). This reveals strong electron
accepting ability and may stem from significant LUMO
delocalization over 4e’s ÀC6H4-p-NO2 groups observed via
DFT computation (Supporting Information). The reduc-
tion of 4e is also fully reversible which is unique as compared
to the other phenyl substituted derivatives (4aÀd).
Negative scan polarity was usedto delay scanning highly
positive potentials in 4f and 4g, and the resulting cyclic
voltammograms resemble that of 4a. Notably, oxidation
is easier in 4f and 4g than in 4a, while reduction occurs at
a similar potential. Compound 4h meanwhile exhibits
the most complex electrochemical behavior showing
three reductions within the solvent window (Supporting
Information).
In previous studies on oligothiophenes with a central
thiophene dioxide, reduction potentials were not tuned
by increasing the number of thiophenes. Computations
attributed this to concentration of the LUMO on the
dioxide.6c Here, calculations of the LUMOs for 4aÀh
still show strong contributions from the dioxide ratio-
nalizing the similarity of reduction in 4a, 4f, and 4g.
However, the LUMOs also extend to the substituents,
particularly with para substituted phenyl groups which
explains the exciting reduction modulation achieved
in 4aÀe.
For all phenyl derivatives, the observed oxidation and
first reduction processes are consistently reproduced in
multiple potential sweeps. Compounds 4f and 4g do not
show the same electrochemical stability. The redox pro-
cesses of 4f are altered by scanning to high potentials
(∼2 V), while all of 4g’s redox processes diminish following
oxidation. In both cases, slight deposition was visually
observed on the surface of the working electrode and may
signify either degradation or electro-polymerization. Sev-
eral substituted analogues of 4f (Figure 1, Compounds 7
Poor correlation is observed when comparing the
LUMO energies (DFT) to the reduction potentials of
4aÀh. The radical anion SOMO energies from DFT also
exhibited poor correlation to the reduction potentials.
However, the SCF energy difference between the calcu-
lated radical anion and the calculated neutral molecule
reveals a strong correlation with the first reductions of
4aÀh (Figure 3). Consequently, DFT calculations should
be helpful in identifying other thiophene 1,1-dioxides as
good candidates for n-type charge transport.
The λabs for the synthesized family of heterocyclic
sulfones ranges from 373 to 423 nm, and the absorption
has been assigned as a πÀπ* transition using time depen-
dent DFT calculations. All derivatives except 4g lumi-
nesce in acetonitrile solution upon excitation at 390 nm
Figure 1. Previously prepared thiophene 1,1-dioxides that have
been probed electrochemically.
ꢁ
(20) Jaffe, H. H.; Orchin, M. Theory and Applications of Ultraviolet
Spectroscopy; John Wiley and Sons Inc.: New York, 1962.
5232
Org. Lett., Vol. 15, No. 20, 2013

Table 2. Electrochemical and Photophysical Properties of 2,5-Bis(aryl)thiophene 1,1-Dioxides
b
compd λabsb (nm) (ε MÀ1 cmÀ1
)
λem
φb (%) Eoxc (V vs SCE) E1/2
1 (V vs SCE) ΔE (mV) Ered 2 (V vs SCE) ΔE (mV)
red
4a
378 (11700)
407 (13200)
373 (16200)
383 (17300)
392 (18800)
421 (17300)
423 (18600)
397 (11300)
483
452
471
477
479
476
3.9
8.7
1.85
1.39
À1.16e
66.7
66.9
63.8
60.3
58.9
70.8
72.4
71.9
À1.73g
4b
À1.30e
À0.90e
À0.77e
À0.65f
À1.10f
À1.11e
À0.92e
4c
7.6
À1.41e
À1.16f
157.3
82.4
4d
15.2
15.3
1.3
4ea
4fa
4ga
4hi
À1.65e,h
À1.64c
À1.76g,h
À1.47c
189.2
1.46
1.42d
460
4.5
a Negative scan polarity. b Spectra were collected in MeCN. Emission was measured versus quinine sulfate. c Irreversible. d Process seems to cause
reaction or degradation. e Quasi-reversible. f Fully reversible. g Irreversible process with lower intensity than 1st reduction. h Cannot be cycled.
i Compound 4h also has a third quasi-reversible reduction at À1.85 (ΔE = 117.8), and none of its processes can be cycled if the anodic scans exceed ∼1.6 V.
breaks this pattern. Emission quantum yields vary widely
among the dioxides. In the phenyl-substituted derivatives
(4aÀe), functionalization enhances quantum yield (Table 2).
Notably, compound 4e (Figure 4) demonstrates the best
emission efficiency (15.3%) despite its nitro groups which
normally accept electron density during excitation of π
systems to create nonluminescent charge-transfer excited
states.20 For 4e, computations show that the excitation has
πÀπ* character rather than charge transfer character
because the LUMO delocalizes over both the electron
deficient thiophene dioxide and nitrophenyl substituent
(Supporting Information).
In conclusion, we have prepared a 2,5-bis(tributyl-
stannyl)thiophene 1,1-dioxide using TBAF and bis-
(tributyltin)oxide. The distannylated thiophene 1,1-diox-
ide and a diiodo precursor were utilized to prepare a family
Figure 3. Calculated SCF energy difference (B3LYP/6-31G(d,
of 2,5-bis(aryl)thiophene 1,1-dioxides. The electrochemi-
p)) between the radical anion and neutral thiophene 1,1-dioxides
cal and photophysical properties of these molecules were
correlated to the first reduction potentials.
evaluated using cyclic voltammetry and fluorescence
spectroscopy. Several molecules show significant quan-
tum yields including the compound with nitro groups.
Reduction and oxidation potentials are readily tuned
using electron-donating and -withdrawing substituents.
The nitro substituent ÀC6H4-p-NO2 most dramatically
faciliates reduction enhancing the possibility for n-type
behavior.
Acknowledgment. We are grateful to Carnegie Mellon
for financial support of this work and to Prof. Roberto Gil
(CMU) for help with NMR assignments. Financial support
from the NSF is gratefully acknowledged (CHE-0130903,
CHE-1039870, CHE-1055547, and DMR-0819860). D.N.
C. gratefully acknowledges the support of the U.S. DOE
Office of Science Graduate Research Fellowship.
Supporting Information Available. NMR spectra, cyclic
voltammograms, absorption and emission spectra, calcu-
lated energies, coordinates, and orbital images. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Figure 4. Absorption spectrum (blue) and luminescence spec-
trum (red) of 4e collected for a 10 μM MeCN solution. Lumi-
nescence was measured following excitation at 390 nm.
(Table 2). Emission from most phenyl-substituted deriva-
tives (4bÀe) is red-shifted as the substituent becomes more
electron withdrawing and lowers the LUMO, but 4a
The authors declare no competing financial interest.
Org. Lett., Vol. 15, No. 20, 2013
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