Synthesis of π-extended thiadiazole (oxides) and their electronic properties

Article abstract of DOI:10.1021/ol1018213

The syntheses of extended thiadiazole, thiadiazole oxide, and thiadiazole dioxide heterocycles are described. The electron-accepting heterocycles were investigated by X-ray crystallography and optical as well as electrochemical measurements and supported by DFT calculations. The thiadiazole dioxide heterocycles have reduction potentials of -0.7 V vs ferrocene/ferrocenium, suggesting a viable building block for n-type organic materials.

Full text of DOI:10.1021/ol1018213

ORGANIC
LETTERS
2010
Vol. 12, No. 20
4520-4523
Synthesis of π-Extended Thiadiazole
(Oxides) and Their Electronic Properties
Thomas Linder, Eider Badiola, Thomas Baumgartner,* and Todd C. Sutherland*
Department of Chemistry, UniVersity of Calgary, 2500 UniVersity DriVe NW,
Calgary, Alberta, Canada T2N 1N4
thomas.baumgartner@ucalgary.ca; todd.sutherland@ucalgary.ca
Received August 4, 2010
ABSTRACT
The syntheses of extended thiadiazole, thiadiazole oxide, and thiadiazole dioxide heterocycles are described. The electron-accepting heterocycles
were investigated by X-ray crystallography and optical as well as electrochemical measurements and supported by DFT calculations. The
thiadiazole dioxide heterocycles have reduction potentials of -0.7 V vs ferrocene/ferrocenium, suggesting a viable building block for n-type
organic materials.
Conjugated polymers and oligomers1 have shown utility in
organic electronic applications, such as organic light-emitting
diodes (OLEDs),2 organic field effect transistors (OFETs),3
and organic photovoltaics (OPVs).4 Several hole-transporting
(p-type) polymers show competitive conductivities with
inorganic analogues; however, there are only few examples
of electron-transporting (n-type) polymers.5
The limited number of organic n-type materials is due
to the availability of monomers with low-lying LUMO
energy levels. Representative n-type building blocks
include benzothiadiazole (BTD),6 perylene bisimide,7 and
PCBM (phenyl-C61-butyric acid methyl ester).8 A common
theme in the design of new n-type species is the
incorporation of fused heterocycles into the materials’
backbone. BTD has been utilized in a variety of copoly-
mers because of its n-type character, synthetic access, and
lower cost. To date, BTD has been an essential acceptor
component in the design of donor polymers for bulk
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10.1021/ol1018213  2010 American Chemical Society
Published on Web 09/16/2010

heterojunction solar cells9 because of its absorption
properties, high mobilities, and excellent photovoltaic
performance. However, the electron-accepting properties
of BTD are only moderate, and more electron-deficient
building blocks would enable access to LUMO tuning that
could be incorporated into Donor-Acceptor (D-A)
polymers. Lowering of the LUMO level through modifi-
cation of the acceptor component in these D-A copoly-
mers will enable improved light-harvesting characteristics,
and adding fused aromatics should lead to a higher degree
of crystallinity in the eventual polymer, which could
provide superior charge-transport properties.
toluene. Heterocycles 2a,b are further reduced with PPh3
in CCl4 to give thiadiazoles 3a,b or, alternatively,
10
oxidized with peroxy acid to give thiadiazole dioxides
4a,b.10b,11 Note that others have synthesized thiadiazole12
and thiadiazole dioxide13 heterocycles using different ap-
proaches. The advantage of the present method is that it
provides easy synthetic access to each oxidation state of the
thiadiazole, which is essential if tuning of the reduction
potential for a specific material application is needed. All
heterocycles were characterized by NMR, EA, MS, and
X-ray crystallography (see Supporting Information).
The molecular structure of 2a in the solid state has been
previously reported,14 and the structures of the remaining
compounds are reported here. The heterocyclic rings of 2
show slight deviations from planarity, whereas the het-
erocyclic rings of both 3 and 4 are coplanar to the aromatic
carbon backbone. Thiadiazole oxides 2 and 4 show solid-
state C-N-S-O dihedral angles between 113° and 128°,
and each compound shows similar heterocyclic ring bond
lengths. The heterocyclic C-N bond distances for 2-4
are ∼1.3 Å, which is consistent with C-N double bond
character. Both the C1-C2 and N-S bond lengths are
consistent with single bonds resulting in a structure that
is more accurately described as a diimine. Crystal packing
diagrams of all compounds are included in the Supporting
Information. Compounds 2-4 stack into columns with
overlapping π-systems. Heterocycles 3 pack into parallel
stacks with heterocyclic rings overlapping, as opposed to
2 and 4, which form antiparallel π-stacks, most likely
because of dipole-dipole interactions. Intermolecular
interactions for the oxides are supported by decomposition
temperatures of 316 °C for 4b compared to 200 °C for
3b.
In this contribution, we explore two effects on the
thiadiazole heterocycle: first, the consequence of changing
the fused phenyl ring in BTD to either a phenanthrene or
pyrene backbone and second, changing the oxidation state
of the sulfur in the thiadiazole heterocycle from S(II) via
S(IV) to S(VI).
The synthesis of six thiadiazoles begins with the
phenanthryl and pyrenyl bis-imines 1a,b (Scheme 1) that
Scheme 1. Synthesis of Thiadiazole Heterocycles
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Org. Lett., Vol. 12, No. 20, 2010
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have the highest energy optical transitions followed by S(IV)
heterocycles, 2a,b, and finally S(VI) heterocycles, 4a,b.
Second, the pyrenyl systems possess red-shifted absorptions
compared to analogous phenanthryl heterocycles (Table 1).
Normalized fluorescence spectra of heterocycles 2-4 in
xylenes are summarized in Table 1 and included in the
Supporting Information. Expectedly, PyrSO (3b) shows a
red-shifted emission peak compared to its phenanthryl
analogue 3a. However, PhenSO2 (4a) has a red-shifted
solution emission compared to PyrSO2 (4b), suggesting an
intermolecular interaction in xylenes for 4a. The solid-state
emission spectra of heterocycles 2-4 are included in the
Supporting Information, and all heterocycle emissions are
different from solution emissions because they exhibit
vibronic bands and are red-shifted. All oxides 2a,b and 4a,b
have similar solid-state emission peaks around 610 nm,
supporting the role that the oxide(s) play(s) in intermolecular
interactions, as supported by the crystal structures. These
solid-state emission properties are encouraging for use in
D-A polymers for OPV applications because of the en-
hanced absorption profile and added crystallinity for charge
carrier ability.9a,b,16
Figure 1. Molecular structures of the pyrenyl series b in the solid state
(H atoms omitted for clarity; see Supporting Information for details).
The absorption spectra of heterocycles 2-4 are shown in
Figure 2. PhenSO, 2a, has a broad π-π* transition centered
The critical electron-accepting abilities of heterocycles
2-4 were assessed by cyclic voltammetry (CV). Both series
a and b display similar electrochemical features, and the
discussion below pertains to both phenanthryl and pyrenyl
systems. The CVs of 2 and 3 (see Supporting Information)
show quasi-reversible redox behavior consistent with the one-
electron reduction to the radical anion as assessed by the
separation between anodic and cathodic peak potentials. Both
dioxides, 4a and 4b, show two stepwise one-electron
reductions to the radical anion and dianion. As expected,
there is a clear trend of redox potential with the heterocyclic
sulfur oxidation state. Compounds 3a, 2a, and 4a have their
first reduction potentials at -2.07, -0.95 and -0.64 V vs
Fc/Fc+, respectively. Similarly, 3b, 2b, and 4b have their
first reduction potentials of -2.08, -0.88 and -0.66 V vs
Fc/Fc+, respectively. Pyrenyl series b also shows electrode
(glassy carbon) adsorption features, which appear as pre- and
postwaves to the main redox peaks in the CV experiments.
Despite the high electrolyte concentration, surface adsorption
occurs and supports the aggregation behavior observed in
the optical measurements. As a comparison, BTD has a
reduction potential of -2.12 V and a related P(V)-based
phenanthrene at -0.95 V under the same electrochemical
conditions.17 Interestingly, the electron transfer rates of the
phenanthryl series a were approximately 1 order of magni-
tude faster than those of the pyrenyl analogues (Table 1).
Figure 2. Absorption spectra of compounds 2-4 in xylenes. Molar
absorptivites of 2a (green solid), 2b (green dashed), 3a (blue solid),
3b (blue dashed), 4a (red solid), and 4b (red dashed).
at 430 nm with a moderate absorptivity of 1300 M-1 cm-1.
The corresponding peak in PyrSO, 2b, is red-shifted to 475
nm (ε ) 3400 M-1 cm-1), as expected by the additional
ethylene conjugation. Pyrenyl compounds 2b and 4b also
show intermolecular interactions in DMF resulting in broad
absorption features between 600 and 800 nm (Supporting
Information). The optically determined HOMO-LUMO gap
is quantified by the absorption onset and two clear trends
are evident (Table 1). First, both series a and b show a red-
Table 1. Electrochemical and Photophyiscal Properties of 2-4
c
ket,b
cm s-1
absonset
eV (nm) eV (nm)
,
emission,
ELUMO,
eV
a
E1/2
,
V
2a -1.16
2b -0.88
3a -2.07
3b -2.08
14.3
2.1
12.9
3.3
2.6 (483)
-3.64
2.4 (510) 3.0 (416) -3.92
3.4 (365) 3.3 (380) -2.73
3.0 (416) 3.0 (419) -2.72
The tunable reduction potentials, by simply varying the S
oxidation state, highlight the utility of these compounds as
candidate n-type functional monomers. The oxidation tuning
can be exploited in the design of tailored low band gap D-A
copolymers with ELUMO values near the ideal -3.9 eV for
4a -0.64/-1.40 36.8/35.9 2.4 (525) 2.1 (580) -4.16/-3.40
4b -0.66/-1.51 2.5/3.7 2.3 (542) 2.2 (553) -4.18/-3.29
a Average of anodic and cathode peak potentials. b Calculated using
method reported by Nicholson.15 c Calculated using the Ferrocene HOMO
level at -(4.8 + E1/2) eV.
(15) Nicholson, R. S. Anal. Chem. 1965, 37, 1351–1355.
(16) Ong, B. S.; Wu, Y.; Liu, P.; Gardner, S. J. Am. Chem. Soc. 2004,
126, 3378–3379
.
shift in absorption that correlates with sulfur oxidation state
of the thiadiazole heterocycle. The S(II) heterocycles, 3a,b,
(17) Linder, T.; Sutherland, T. C.; Baumgartner, T. Chem.sEur. J. 2010,
16, 7101–7105.
4522
Org. Lett., Vol. 12, No. 20, 2010

OPV applications,18 assuming a band gap of 1.5 eV, in efforts
to achieve 10% OPV efficiency.18
identical coefficients and energies because the ethylene bridge
does not play a role (see Supporting Information). Both the
electrochemical reduction (ELUMO) and the absorption onset
(HOMO-LUMO gap) are consistent with the calculations,
suggesting the DFT results correctly describe the FMOs in
solution. The FMOs of 3 and 4 are similar with the LUMOs
confined to the heterocyclic ring and are included in the
Supporting Information.
Heterocycles 2-4 were subjected to DFT calculations19
(B3LYP-6-31G+d,p) to assess the orbitals involved in the
redox reactions, as well as their energies (Figure 3). The
To further assess the synthetic potential of these building
blocks as monomers for polymerizations the 2,7-dibromo-
PhenSO, 5, accessible in a similar protocol via the dibromo-
phenanthrene quinone, was representatively subjected to a
Stille cross-coupling reaction to provide the bis(aniline)
species 6 (Scheme 2). Due to the extended π system and its
Scheme 2. Cross-Coupling of Dibromo-thiadiazole(oxide) 5
Figure 3. Calculated FMO energy levels of compounds 2-4.
trend in LUMO energy levels mirror the results found by
the electrochemically determined LUMO energy levels. A
lowering of the LUMO energy level is found when compar-
ing heterocycles S(II) 3 with S(IV) 2 and S(VI) 4, whereas
the backbone has little influence on the LUMO. Compounds
3a and 2a also contain nearly degenerate HOMO energy
levels, whereas 3b and 2b have higher energy, discrete
occupied FMOs because of the added conjugation. Again,
the higher HOMOs of series b, in conjunction with similar
LUMO energies, results in the pyrenyl systems having
smaller HOMO-LUMO energy gaps, which is consistent
with the observed absorption onsets. The energy-optimized
structures of 2-4 are nearly identical to the experimental
crystal structures lending support to the calculated geom-
etries. The HOMO and HOMO-1 are nearly degenerate for
2a with electron density confined to the heterocyclic ring in
the HOMO-1 and localized on the phenanthrene backbone
for the HOMO. In 2b, the HOMO and HOMO-1 energies
are split. The 2b HOMO shares features with the HOMO-1
of 2a, and the HOMO-1 of 2b is similar to the HOMO of
2a. The additional ethylene bridge in 2b raises the HOMO
energy level. Note the LUMOs of 2b and 2a have nearly
D-A character, 6 shows a red-shifted emission at 595 nm
(cf. 538 nm for 2a in CH2Cl2). The absorption maximum of
6, on the other hand, remains the same as 2a at 430 nm.
These observations are in line with the D-A nature of 6.
In summary, we have synthesized a series of thiadiazole
heterocycles that show strong electron-accepting character-
istics, particularly for the S(IV) and S(VI) congeners, which
are supported by DFT, absorption spectroscopy, and elec-
trochemical measurements. The results emphasize the high
value of these building blocks for the generation of n-type
conjugated organic materials.
Acknowledgment. The authors thank NSERC of Canada
and the Institute for Sustainable Energy, Environment, and
Economy.
Supporting Information Available: Synthetic details and
NMR spectra, X-ray crystallographic details, absorption,
fluorescence and excitation spectra, CVs, TGAs, FMOs, and
computational details. This material is available free of
charge via the Internet at http://pubs.acs.org.
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(b) Zhou, H.; Yang, L.; Stoneking, S.; You, W. ACS Appl. Mater. Interfaces
2010, 2, 1377–1383.
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Wallingford, CT, 2004.
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