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Thiazolo[5,4-d]thiazoles – promising building blocks in

the synthesis of semiconductors for plastic electronics
Cite this: DOI: 10.1039/c3ra40851e
David Bevk,ab Lidia Marin,ab Laurence Lutsen,b Dirk Vanderzandeab
and Wouter Maes*a
Published on 09 April 2013 on http://pubs.rsc.org | doi:10.1039/C3RA40851E

The thiazolo[5,4-d]thiazole fused (bi)heterocycle is an electron deficient system with high oxidative
stability and a rigid planar structure, enabling efficient intermolecular p–p overlap. The parent thiazolo[5,4-
d]thiazole moiety hence already possesses some appealing features towards applications in organic
electronics. Moreover, aryl-functionalized thiazolo[5,4-d]thiazole derivatives – expanding the conjugated
backbone of the semiconducting material – are easily prepared. Surprisingly, interest in thiazolo[5,4-
d]thiazole-based materials has been rather low, until quite recently, when the high potential of these
molecules was widely recognized, notably in the field of organic photovoltaics. Despite the almost
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exponential growth of activity, the synthetic chemistry behind these molecules has only been explored to a
Received 20th February 2013,
minor extent and there is plenty of room for improvement and broadening of the material scope. In this
Accepted 7th April 2013
review, an overview of the currently available synthetic methods and the range of materials prepared to
DOI: 10.1039/c3ra40851e
date, together with their basic material properties and main applications, is provided, with the aim to
www.rsc.org/advances facilitate and stimulate further progress in thiazolo[5,4-d]thiazole-based materials science.

1 Introduction electron and hole mobilities, which are crucial properties for
efficient charge transfer in optoelectronic applications.1,2
The majority of organic semiconducting materials reported To date, most synthetic procedures towards TzTz-based
and employed to date contains (one or more, sometimes materials are still based on the pioneering work done by
fused) thiophene entities, taking benefit from the electron Ketcham et al.3 However, the ‘classical’ route does not allow to
donating power of the heterocycle. One of the main drawbacks achieve high yields and is associated with troublesome
of thiophene-based systems is their sensitivity towards isolation and purification protocols. A few attempts were
oxidation. By substituting one carbon by a nitrogen atom, a made to improve the preparation process, but most of them
thiazole ring is obtained, which is more electron deficient and are nowadays not being used.
therefore more resistant to oxygen. In the early 60’s, a new In the present review, an overview of the state-of-the-art of
bicyclic system based on the thiazole ring was discovered, i.e. TzTz-based materials is provided, including the reported
the thiazolo[5,4-d]thiazole (TzTz). This fused heteroaromatic synthetic protocols affording TzTz derivatives (small mole-
system has recently become subject of several studies in the cules, polymers, and metal complexes), their structural and
field of organic electronics, in which its electronic character-
istics are highly appreciated, with a particularly strong
emphasis on organic photovoltaics (OPVs). In the last decade,
the number of publications based on TzTz-type materials has
increased almost exponentially (Fig. 1). For these applications,
the TzTz is either incorporated into polymer structures or
employed as a building block towards small molecules. Its
main advantage is the higher oxidative stability compared to
its thiophene analogues. Additionally, strong p–p stacking and
overlapping of the orbitals in the solid state affords high

a
Design & Synthesis of Organic Semiconductors (DSOS), Institute for Materials
Research (IMO-IMOMEC), Hasselt University, Agoralaan 1-Building D, B-3590
Diepenbeek, Belgium. E-mail: wouter.maes@uhasselt.be; Fax: +32 11 268399;
Tel: +32 11 268312 Fig. 1 Number of publications on thiazolo[5,4-d]thiazoles in the period 1956–
b
IMEC, IMOMEC, Wetenschapspark 1, B-3590 Diepenbeek, Belgium 2012 (Scifinder search September 2012).

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Scheme 2 Synthesis of parent TzTz system 6.

solubility, unless long alkyl chains are attached to the

Scheme 1 Classical synthesis of 2,5-diaryl-substituted TzTz’s.
aromatic rings, so they readily precipitate from the reaction
Published on 09 April 2013 on http://pubs.rsc.org | doi:10.1039/C3RA40851E

mixture.
Upon checking TzTz literature, it becomes immediately clear
that the vast majority of published synthetic procedures are
electronic characteristics, and main applications in organic still based on the classical procedure as discovered by
photovoltaics, organic light-emitting diodes (OLEDs) and Ketcham.7 Reactions between dithiooxamide and (hetero)aro-
organic field-effect transistors (OFETs). As such, this overview matic aldehydes have been carried out in different solvents,
can be employed as a springboard for scientists active in one e.g. phenol and nitromethane, but most often DMF, or without
of these domains, fostering further progress in TzTz-based any solvent. The vigorous conditions required, i.e. heating up
to 200 uC, often led to black reaction mixtures with tarry by-
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material design and applications.

products, decreasing the yields considerably to an average of
around 50% and creating problems for up-scaling. Among
some other disadvantages, there is also a limitation for the
2 Material synthesis reaction to (hetero)aromatic aldehydes. Condensation pro-
ducts with aliphatic aldehydes were not reported yet. The
2.1. The parent TzTz heterocycle – small molecules limited scope and strong conditions applied for a rather
The thiazolo[5,4-d]thiazole heterocyclic system was initially simple two-step condensation and oxidation process should
discovered in 1960 by Ketcham, who was studying the reaction open many possibilities for improving the reaction. The
between dithiooxamide (or rubeanic acid) (1) and aromatic unsubstituted ‘parent’ TzTz system cannot be prepared in
aldehydes (Scheme 1).3 Crystalline products were obtained, one single step. It was done in two steps starting from 2,5-
which were first thought to be 2,29-diaryl-4,49-bithiazetines difuryl-TzTz 4, which was oxidized to dicarboxylic acid 5 and
(39).4 Extensive study and analysis of the reaction products further decarboxylated to afford the unsubstituted thia-
using elemental analysis, IR, UV-VIS, and other methods zolo[5,4-d]thiazole (6) (Scheme 2).6,8
showed that the correct structures were actually 2,5-diarylthia- Over the years, a number of alternative approaches to
zolo[5,4-d]thiazoles 3, which are thermodynamically favoured prepare TzTz derivatives via simpler and more efficient routes
over the bithiazetines. have been introduced, although they have found limited use.
After this initial discovery of the novel biheterocycle, the The first procedure was reported by Seybold and Eilingsfeld in
biological activity of several 2,5-diarylthiazolo[5,4-d]thiazoles 1979, who synthesized a series of bicyclic systems, of which
(Ar = phenyl, naphthyl, thienyl, indolyl or quinolyl) was tested. one was a TzTz material.9 They started from 2,5-bis(acetyla-
In an extensive study for anti-viral/amebicidal activity and mino)thiazole (7) which was treated with sodium thiocyanate
against various strains of bacteria, these molecules did not in the presence of bromine to prepare thiocyanatothiazole 8.
show significant effects, apart from a 20% increase in body This was then further cyclized by a thermal treatment in
cholesterol when administered to rats and highly selective methyl benzoate at 160 uC, affording the corresponding TzTz 9
effects on the central nervous system, manifested almost in 60% yield (Scheme 3).
purely by the induction of sleep.5 Similar results were obtained
by Ketcham and Mah, who found that certain TzTz derivatives
prolongue barbiturate sleeping time and decrease locomotor
activity. However, relatively high toxicity was associated with
most of these compounds, with LD50 values only just above 0.5
g/Kg.6
The classical Ketcham reaction proceeds at high tempera-
ture without any solvent (Scheme 1).3 The first step involves a
two-fold condensation reaction with the elimination of water,
often seen as sputtering. In the next step, ring closure occurs,
followed by oxidation to form the TzTz fused aromatic system.
2,5-Diaryl-substituted TzTz systems have a rather poor Scheme 3 Synthesis of TzTz 9.

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Scheme 6 Alternative procedure towards aryl-substituted TzTz’s 3.

products were purified by simple recrystallization.

Scheme 4 Unusual one-pot synthesis of ethyl thiazolo[5,4-d]thiazole-2,5-dicar-
boxylate 12. Surprisingly enough, this method has not been applied by
other people ever since.
Another synthetic approach involves closing of one thiazole
ring at a time, which enables preparation of asymmetrically
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A completely different approach was used by Valle et al.10 In substituted TzTz’s.13 Reaction between 5-amino-2-aryl-4-mer-
continuation of their study on the reactivity of alkyl isocya- captothiazoles 16 and orthoesters 17 in boiling alcohols with
nates towards compounds carrying an S–Cl group, they an acid catalyst for several hours furnished diaryl-TzTz’s 3 in
attempted the synthesis of 5,59-diethyloxy-2,29-bis(oxazolyl)di- 40–75% yields (Scheme 7). Similar procedures using aldehydes
sulphide (10) starting from ethyl isocyanoacetate (11) and instead of orthoesters (in refluxing toluene) were also used to
dichlorodisulphane in the presence of triethylamine afford TzTz compounds in 50–60% yields.14 Another patent
(Scheme 4). Instead of the expected product, TzTz 12 was from the same authors describes cyclization of 2-aryl-5-
isolated as colourless needles in 52% yield. This procedure (benzylidenamino)-4-thiazolethiols 18 in high boiling solvents
enables preparation of thiazolo[5,4-d]thiazole-2,5-dicarboxy- such as nitrobenzene, DMF or diethylene glycol to afford
lates 12 in a much more convenient way than the standard one diaryl-TzTz products 3 in 86–96% yield (Scheme 8).15
via a tedious multi-step procedure starting from furfural and Although the classical method is clearly most often
dithiooxamide. An extension to this procedure was published employed, there seems to be plenty of options for increasing
a few years later by Rössler and Boldt, who started from ethyl yields and enable easier processing, either by optimizing the
thioisocyanatoacetate (13) and used chlorine as the chlorinat- condensation/oxidation steps or by adapting stepwise proto-
ing agent.11 They reported a simplified synthetic protocol cols (e.g. based on Scheme 6).
affording a similar yield of 49% (Scheme 5).
Among the stepwise protocols reported so far, the procedure 2.2. Extension of the TzTz chromophore
by Hansen and Liebich looks particularly interesting Extension of the TzTz chromophore, in particular with
(Scheme 6).12 The starting compounds additional aromatic rings, is a very efficient way to extend
N,N9-bis(dialkylaminobenzyl)-dithiooxamides 14 and phenyli- the conjugation length (shifting the absorption spectrum
sothiocyanate (15) are heated under milder conditions in towards longer wavelengths) and change the charge mobility
either acetone or diglyme. Using this protocol, the authors features. There are two main synthetic methods that have been
reported a significant improvement in the yields for the employed to achieve this. One of them is based on Ketcham’s
synthesis of diaryl-TzTz’s 3, ranging from 65–90%. The method and uses the corresponding extended aldehydes to
build the TzTz system (Scheme 9, Path A).16 In the second
method (Scheme 9, Path B), the extension of the TzTz core is
achieved by Pd-catalyzed cross coupling reactions, e.g. Suzuki
or Stille.17,18

Scheme 5 Improved synthesis of TzTz 12 starting from a thioisocyanatoacetate. Scheme 7 Roetling’s procedure towards diaryl-TzTz’s 3.

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Scheme 8 Synthesis of diaryl-TzTz’s 3 by cyclization of 2-aryl-5-(benzylidena-

mino)-4-thiazolethiols 18.

2.3. TzTz-based polymers

In the greater part of recent publications, the thiazolo[5,4-
d]thiazole system is introduced in a semiconducting polymer Scheme 10 Synthesis of fluorene-TzTz copolymer 20 by a polycondensation
reaction.
structure. Some of these polymers have afforded improved
Published on 09 April 2013 on http://pubs.rsc.org | doi:10.1039/C3RA40851E

solar cell efficiencies as well as increased polymer stability in

bulk heterojunction blends,2 both key factors for the success-
ful commercialization of organic solar cells. The TzTz is TzTz copolymers 20 by a simple polycondensation reaction
incorporated into the polymer backbone in either homopoly- between fluorene-dicarbaldehyde 21 and dithiooxamide, in
mer or copolymer configurations. To lower the optical 59% yield (Scheme 10). Unfortunately, no data on the
bandgap of the copolymers and broaden the absorption molecular weight of the obtained polymer were reported.
window, donor–acceptor (D–A) type copolymers are often The first successful Suzuki polycondensation reaction
pursued. In these cases, the TzTz acts as the acceptor part, resulting in a TzTz-containing polymer was also performed
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and is combined with electron rich donor units such as in 2005. Using standard Suzuki conditions, fluorenes 22 and
4H-cylopenta[2,1-b;3,4-b9]dithiophene (CPDT), benzo[1,2-b;4,5- 23 were coupled to TzTz 24 in the presence of tetrakis(triphe-
b9]dithiophene (BDT), or other similar mostly thiophene-based nylphosphine)-palladium(0) catalyst to prepare different copo-
moieties. As solubility is of high importance for the proces- lymers 25a–e (Mw = 8–32 kDa) (Scheme 11).22 On the other
sability of such polymers, the heterocyclic building blocks are hand, McCullough and co-workers synthesized thiophene-
generally substituted with long linear or branched alkyl TzTz copolymers 28 and 30 using Stille cross-coupling
chains. reactions, catalyzed by Pd2(dba)3-P(o-Tolyl)3 (Scheme 12).2a
For the preparation of TzTz-based polymers, various poly- A different polymerization approach was used by Wudl et al.,
condensation reactions have been used, such as palladium- who prepared rather simple polymers 32a,b consisting of
catalyzed Stille and Suzuki cross-coupling reactions, and alkylthiophenes and TzTz units by oxidative polymerization in
oxidative polymerization reactions employing iron(III) chlor- moderate yields between 30 and 60% (Scheme 13).1 The
ide. The first syntheses of TzTz-containing polymers were molecular weights of the obtained polymers were very high, in
based on simple polycondensation methods, resulting in the range of 1–4 6 106, and the materials exhibited strong
polyesters, polycarbonates, etc.,19,20 whereas Stille and Suzuki fluorescence in solution.
reactions were almost exclusively used for the preparation of All successive publications using TzTz-based conjugated
polymers for optoelectronic applications. polymers have used palladium-catalyzed polycondensation
An interesting polymerization approach was reported by reactions, due to the excellent regiocontrol and high purity
Belfield et al. in 2005.21 They prepared alternating fluorene-

Scheme 9 Synthetic pathways to extend the TzTz chromophore (exemplified for thienyl units).

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Scheme 11 Fluorene-TzTz copolymers 25a–e prepared by Suzuki reactions.

of the obtained polymers, both of great importance for di(2-pyridyl)thiazolo[5,4-d]thiazole (33) as well as a copper (Cu)
achieving good results in optoelectronic applications. complex 34 (Fig. 2). The Cu complex was prepared simply by
adding a methanolic solution of copper(II) nitrate to a solution
2.4. Metal complexes of the TzTz derivative. Green plates precipitated after two days.
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The presence of S and N heteroatoms in the TzTz structure On the other hand, the Ru complex 35 was prepared by
allows construction of coordinative compounds and several refluxing the same TzTz ligand with [Ru(bpy)2Cl2].xH2O in a
examples of this have been reported. The first ones were mixture of ethanol and water for two days and it was isolated
prepared in 2004 by Steel and co-workers.23 They prepared as red crystals. Both complexes, including the isolated
diastereomeric dinuclear ruthenium (Ru) complexes of 2,5- diastereomeric form, were analyzed by X-ray diffraction
analysis. It was shown that the metal atom is bonded
coordinatively between the two nitrogen atoms of the thiazole
and pyridine rings, which are almost coplanar (dihedral angle
4.9u). The complexes exhibited a rather low bandgap, in the
range between 2.00–2.06 V, as well as weak metal–metal
interactions.
Similar complexes involving the nitrogen atoms of the TzTz
core and the attached aromatic moiety were prepared by Zhang
et al. using boron as the central atom (Scheme 14).24 After
lithiation of 2,5-bis(2-bromophenyl)-TzTz’s 36 and subsequent
reaction with triphenylborane or dimesitylfluoroborane, a
series of diboron ladder compounds 37–40 was obtained. In
these complexes, boron is covalently attached to the phenyl
ring and via a coordinative bond to the nitrogen atom of the
thiazole ring (Fig. 3).
In another publication from the same group, similar
complexes 42–45 were prepared (Scheme 14).25 In this case,

Scheme 12 Stille polycondensations towards thiophene-TzTz copolymers 28

Scheme 13 Oxidative polymerization affording thiophene-TzTz polymers 32a,b.
and 30.

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Fig. 3 Single-crystal X-ray structure of boron ladder complex 39 (Reproduced

based upon the data in the CCDC database).
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Another type of complexes has been created based on TzTz-

Fig. 2 Single-crystal X-ray structures of dinuclear copper (right) and ruthenium
dicarboxylic acid 5. Both the groups of Maspero and Wudl
(left) complexes 34 and 35 based on 2,5-di(2-pyridyl)thiazolo[5,4-d]thiazole (33) used this precursor to prepare a series of different metal
(Reproduced from ref. 23). complexes (Fig. 4).26,27 Using standard conditions, complexes
with Mg, Ca, Sr, Co, Cu, and Zn were formed as 1-dimensional
coordination polymers, in which the first oxygen atom of the
the boron atom was connected to the phenyl ring via a carboxylic group is attached to the metal, while the second
hydroxyl bridge. Such ladder type structures give good thermal oxygen atom from another TzTz molecule is attached axially.
stability to the complexes, accompanied by high fluorescence Ba atoms form a 2-dimensional network, based on a face- and
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quantum yields and strong electron affinities. edge-sharing Ba polyhedral. One carboxylate oxygen atom and
water molecules act as bridges between each Ba and its three
neighbouring atoms. Ag atoms form more complex 3-D
networks, in which each Ag atom is connected to five different
atoms, thus forming a framework without cavities capable of
hosting water molecules. Since the ligand contains two
carboxylic acid groups, it can easily be decarboxylated at
temperatures around 70 uC. The metal complexes are much
more stable, up to about 150 uC, when water elimination
usually starts.

3. Structural and electronic characteristics

3.1. Crystal structures
As mentioned before, the correct structures of the products
obtained from the reaction between aromatic aldehydes and
dithiooxamide were determined only after a detailed study
using spectroscopic methods by Ketcham in 1960.3 For the
first X-ray confirmation of the TzTz structure, it took a few
more years until Porzio et al. obtained suitable single crystals

Fig. 4 Single-crystal X-ray structures of TzTz-based magnesium (left) and cobalt

(right) complexes (Reprinted with permission from ref. 26 and 27. Copyright
Scheme 14 Synthesis of boron ladder complexes 37–40 and 42–45. 2010 Elsevier/2008 American Chemical Society).

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Fig. 5 ORTEP drawing of the single-crystal X-ray structure of parent thia-

zolo[5,4-d]thiazole (6) (Reproduced from ref. 28).

of the parent thiazolo[5,4-d]thiazole (6) in 1987 (Fig. 5).28 X-ray

diffraction analysis showed that the condensed rings lie
around an inversion centre in the middle of the C–C bond
and within one plane, with a maximum displacement of just
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0.002 Å. Compared to single thiazole or thiophene rings, a

slight decrease in aromaticity was observed after ring fusion,
as indicated by the slightly longer bond lengths.
Furthermore, work done by Benin and co-workers on mono-
as well as dihalogenated TzTz’s showed a slight difference in
bond lengths and bond angles between the two fused thiazole
rings.8 A difference in the length between the two formally
single C–S bonds was observed. Such difference, combined
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with the fact that the C–C bond is longer than a typical double
Fig. 7 Structures of TzTz compounds 46–48.
bond, shows resonance delocalization. The same study also
showed, from the measured angles between both thiazole
rings across the central C–C bond (180.00u and 179.33u for the
dibrominated and monobrominated TzTz, respectively), near When thiophene rings were coupled to the TzTz core (see
to perfect planarity of the fused bicyclic ring system. Scheme 13), X-ray analysis showed perfect coplanarity of the
additional heterocycles (Fig. 6).1 p–p face-to-face stacking of
the molecules could also be seen from the crystal structures of
both monomers and polymers, with an interplanar distance of
3.47 Å. A short intermolecular S–N interaction (3.20 Å) was
observed, significantly shorter than the sum of the van der
Waals radii (3.35 Å). On the other hand, the X-ray study
performed by Wagner and Kubicki on 2,5-bis(thien-2-yl)thia-
zolo[5,4-d]thiazole showed a small dihedral angle of 1.68u
between the thiazole and thiophene rings.29 Similarly, also in
this case a short intermolecular distance (3.58 Å between the S
atoms in the planes) was observed and attributed to weak S…S
and C–H…S interactions. Distortion in planarity was likewise
reported by Pope et al. for 2,5-bis(2-hydroxy-3,5-di-tert-butyl-
phenyl)-TzTz (46),30 as well as by Yamashita et al. for 2,5-bis[5-
(4-trifluoromethylphenyl)thiazol-2-yl]-TzTz (47) and 2,5-bis[2-
(4-trifluoromethylphenyl)thiazol-5-yl]-TzTz (48) (Fig. 7).16c

3.2. Electronic characteristics

3.2.1. Absorption spectra. An extensive spectroscopic study

on the unsubstituted thiazolo[5,4-d]thiazole (6) has been
conducted by Zanirato and co-workers in 1983.31 The ultra-
violet spectrum of the TzTz vapours starts at 290 nm and
consists of a broad absorption peaking at approximately 250
nm and a weaker absorption with some structure around 200
nm (Fig. 8). The main absorption starts with two shoulders at
272 and 275 nm, then rises to a first intense shoulder at 260
nm and a second strong maximum peaks at 248 nm. In the
Fig. 6 (a) top (left) and side (right) views of the X-ray structure of TzTz 31a (b)
molecular stacking of the monomer units (left; two different views) and
region of 200 nm a different electronic transition was
schematic representation of the arrangement of two polymer chains for TzTz observed, most likely to be a Rydberg transition. The
32a (right) (Reprinted with permission from ref. 1. Copyright 2008 American absorption spectrum for the solution state corresponds in its
Chemical Society). major features to that of the vapour, except for the fine

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Fig. 10 Typical UV-VIS absorption spectra of TzTz-based conjugated polymers

(in this case thiophene-TzTz 30) (black – CHCl3 solution; green – warm CHCl3
Fig. 8 UV spectra of parent TzTz 6 in the gas phase (top) and in solution solution; blue – as cast film; red – annealed film) (Reproduced from ref. 2a).
(bottom) (Reprinted from ref. 31 by permission of the publisher, Taylor and
Francis Ltd, http://www.tandf.co.uk/journals).

between the thiazole ring and a 3-methyl-substituted thio-

phene ring). Although the coplanar form is energetically most
structure in the region around 200 nm, which was not
stable, according to the theoretical calculations several
observed (Fig. 8). All these data suggested the presence of
conformers can coexist at room temperature, thus causing
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two intense pAp* electronic transitions, overlapping in the

broadening of the absorption peaks.
region of the near-UV.
The absorption spectra are generally quite similar for TzTz-
The absorption spectra of substituted TzTz’s are all quite
based polymers, except for the fact that additional conjugation
similar, at least for simple compounds. Their absorption
causes an even larger bathochromic shift, resulting in
maxima can be found red-shifted (as expected due to the
absorption in the visible region and over a much wider
extended conjugation of the p system) in the range from 350–
spectral range (as shown for thiophene-TzTz polymers 30 in
400 nm, with typical log e values around 4.5, and consisting of
Fig. 10).2
two closely overlapping peaks.3 2,5-Diphenylthiazolo[5,4-
d]thiazole has been studied by Atvars et al. (Fig. 9).32 They 3.2.2. HOMO–LUMO energy levels. The energy levels of TzTz
showed that the maximum of the absorption band is centered molecules are important to judge their applicability in
at 357 nm, and they also observed overlap of two peaks. A optoelectronic applications. Careful tuning of the HOMO–
slight solvatochromic effect was seen as a function of solvent LUMO levels is for instance important to achieve high organic
polarity, without any noticeable peak broadening. A suggested solar cell efficiencies. In general, both the HOMO and LUMO
explanation for the low vibrational resolution of the TzTz levels are lowered upon introduction of electron withdrawing
spectrum is the occurrence of rotamers around the C–C bond groups. In most cases, the HOMO–LUMO values are estimated
between the thiazole and phenyl rings.32 The energy barrier from oxidation and reduction potentials, as obtained from
was theoretically estimated at 5.1/5.5 kcal mol21, using HF and cyclic voltammetry (CV) measurements, together with optical
DFT methods, respectively (compared to a value of 4.95 kcal bandgaps taken from the onset of the absorption spectra.34,35
mol21 reported by McCullough et al.33 for the energy barrier Nowadays, for almost every new compound used in organic
electronic applications, either a small molecule or a polymer,
HOMO–LUMO energy levels are experimentally derived and
often compared with calculated values. The HOMO–LUMO
values for a range of TzTz-based molecules (Fig. 11 and 12) are
gathered in Tables 1 and 2.

4. Applications
TzTz-based materials are recently mostly applied in organic
electronic devices, i.e. organic solar cells, field-effect transis-
tors and light-emitting diodes, the latter two being used in
active matrix displays. A common feature that makes these
materials so popular lately is their excellent hole mobility
throughout the molecule, as well as their high stability
Fig. 9 Electronic absorption spectra of 2,5-diphenyl-TzTz in cyclohexane (–?–?–), towards environmental influences such as oxygen and
chloroform (—), acetonitrile (–––), DMSO (–??–??–), methanol (????), and humidity.36
trifluoroacetic acid (——) (Reproduced from ref. 32).

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Fig. 11 Structures of the TzTz-based polymers listed in Table 1.

Before their recent use as organic semiconducting materi- TzTz’s in FETs, their first paper being published in 2004.16a
als, the liquid crystalline properties of a series of TzTz small They observed much higher field-effect mobilities for thio-
molecules and polymers were also studied.37–39 However, no phene-TzTz oligomers in the range of 1022 cm2 V21 s21 than
significant progress has been made ever since. After a brief for analogous thiophene-thiazole oligomers. Several other
decline in the number of publications on the TzTz system in publications then followed, all using similar small molecules
the period 1992–2000 (see Fig. 1), the TzTz revival started with containing a TzTz core with appended aromatic moieties
a series of papers by Yamashita and co-workers who studied

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Table 2 HOMO–LUMO energy levels for TzTz-based small molecules (Fig. 12)

TzTz HOMO (eV) LUMO (eV) Ref.

24
3 25.98 22.47
37 25.92 22.97 Ibid.
38 26.05 23.07 Ibid.
39 25.64 22.88 Ibid.
40 25.50 22.91 Ibid.
25
42 25.57 23.05
43 25.68 23.22 Ibid.
44 25.45 23.11 Ibid.
45 25.41 22.88 Ibid.
55
75 25.32 23.07
56
76 25.39 22.91
18
77 25.74 22.91
78 25.54 23.14 Ibid.
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57
79a 25.83 23.04
Fig. 12 Structures of the TzTz-based small molecules listed in Table 2.
79b 25.76 23.25 Ibid.

Table 1 HOMO–LUMO energy levels for a range of TzTz-based polymers

(Fig. 11)a carrying either electron accepting or electron donating groups
to form n- or p-type materials, respectively.16
TzTz HOMO (eV) LUMO (eV) Ref.
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25a 25.77 22.16 22 4.1. OFETs

25b 25.78 22.23 Ibid.
Small TzTz molecules with attached thiophenes were studied
25c 25.82 22.41 Ibid.
25d 25.85 22.45 Ibid. as p-type semiconducting materials. Several thiophene-sub-
25e 25.88 22.49 Ibid. stituted TzTz’s were prepared, i.e. 2,5-bis(thien-2-yl)-TzTz 80,
40
49 25.29 23.27 2-bithiophenyl-TzTz 81 and 4-hexyl-2-bithiophenyl-TzTz 82
41b
50 25.3 23.7
51 25.4 23.6 Ibid. (Fig. 13). TzTz 80 didn’t show any reasonable FET properties,
42a
52 25.19 22.82 due to its high ionisation potential, while the other two
43
53 25.4 23.19 derivatives both showed good hole mobilities (2.0 and 0.3 6
44
54 25.24 23.10
55 25.05 23.11 45 1022 cm2 V21 s21, respectively, with on/off ratios of 103 for
56 25.3 23.2 46 both materials). Easy derivatization of the TzTz structure
47
57 25.33 23.39 hence enabled simple tuning of the FET characteristics.
58 25.40 23.29 Ibid.
Further work from the same group focused on screening of
59 25.18 23.38 Ibid.
60 25.31 23.51 17 different TzTz molecules, derivatized by heterocyclic substi-
2c
61 25.3 NA tuents, also pursuing n-type semiconductor behaviour. In
62 25.2 NA Ibid. terms of n-type materials, the measured mobilities reached
63 25.2 NA Ibid.
64 25.2 NA Ibid. values of up to 0.35 cm2 V21 s21, while for p-type devices based
65 25.1 NA Ibid. on 2-(4-trifluoromethylphenyl)thiazol-4-yl-substituted TzTz’s,
48
66 25.31 23.24
49
mobilities up to 0.30 cm2 V21 s21 were reported.16
67a 25.65 23.83
67b 25.62 23.77 Ibid. Semiconducting TzTz-based polymers are nowadays mostly
67c 25.64 23.81 Ibid. explored in bulk heterojunction (BHJ) organic solar cells.
50
68 25.67 22.70
51
69a 25.06 23.46
69b 25.05 23.47 Ibid.
18
70a 25.26 23.43
70b 25.25 23.51 Ibid.
52
71a 25.42 23.36
71b 25.37 23.40 Ibid.
1
32a NA 24.00
32b NA 24.02 Ibid.
53
72 25.3 22.8
73 25.4 22.9 Ibid.
54
74a 25.60 22.75
74b 25.30 22.79 Ibid.
a
NA = not announced.

Fig. 13 Structures of the TzTz semiconductors used in OFETs.

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fluorene in the backbone (to achieve good electroluminescent

properties) with TzTz (to improve mobility) (Schemes 10 and
11). Indeed, this resulted in electroluminescent polymers that
were 3 times more efficient than the fluorene homopoly-
mers.22,50 Fluorene-free high molecular weight light-emitting
polymers 67a–c (Fig. 11) were also prepared and showed high
fluorescence both in solution and in the solid state.49
The only reports on the use of small TzTz molecules for
OLED’s were published recently by Zhang and co-workers.24,25
They synthesized boron-bridged ladder-type TzTz molecules
37–40 and 42–45 (Scheme 14), with bulky side groups attached
Scheme 15 Synthesis of TzTz-metallopolyyne materials 71a,b. to the boron bridge preventing p–p stacking. Consequently,
such materials exhibited high fluorescence quantum yields
and strong electron affinities. A simple electroluminescence
Published on 09 April 2013 on http://pubs.rsc.org | doi:10.1039/C3RA40851E

device thus afforded the highest brightness (up to 18 6 103 cd

However, their use in OFET applications might also have some
m22) and efficiency among all boron-containing materials
advantages compared to small molecules, especially in terms
reported at the time, as well as a good thermal stability.
of processability, since simple and low-cost solution deposi-
tion techniques can be used.18,58 Simple TzTz-thiophene 4.3. OPVs
copolymers 28/30 were synthesized by Stille polycondensation
The vast majority of publications on TzTz’s can be found in the
reactions by McCullough et al. (Scheme 12).2a Despite the
field of organic photovoltaics, especially focusing on the
rather low molecular weight of polymer 30 (Mw = 17.1 6 103),
synthesis of low bandgap copolymers for bulk heterojunction
it exhibited surprisingly high mobilities of 0.14 cm2 V21 s21
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solar cells. Organic solar cells currently receive wide interest of

with on/off ratios of 106 after annealing. An additional
researchers all over the world, as they show some appealing
advantage is also the high oxidative and environmental features such as simple preparation, aesthetics, flexibility,
stability. On the other hand, polymer 28 was found insoluble semi-transparency, better performance in diffuse light and
in many solvents, even at elevated temperatures, and therefore reduced weight, which makes OPV particularly attractive for
could not be characterized.2a Similar results were later on portable or wearable electronics and building-integrated
published by the same group as well as by Zhan et al.,2,54 who photovoltaics (BIPV). The photoactive layer materials that are
reported carrier mobilities in the same range using similar used can be divided into two main groups, namely small
polymers, simply decorated with longer alkyl side chains to molecules (including oligomers) and polymers. Polymer solar
achieve higher solubility. cells are particularly attractive due to their ease of processing,
An interesting polymer material was prepared in a colla- which allows the use of straightforward large area printing
borative effort of different research groups from China and procedures and enables competitive pricing compared to the
Hong Kong.52 Instead of preparing standard type carbon- established silicon solar cell technology. Rapid progress has
backbone polymers, platinum acetylide polymers 71a,b were been achieved in terms of solar energy power conversion
synthesized, in which the Pt atoms are part of the main efficiency (PCE), reaching values up to 10% now, as well as
polymer chain (Scheme 15). Thus obtained polymers showed operational stability.2 However, there are still some important
impressive peak field-effect charge carrier mobilities of 2.1–2.8 issues that need to be overcome before efficient and stable
6 1022 cm2 V21 s21 and on/off ratios of 0.8–1.0 6 105 for the organic solar cells can be made widely available on the market.
holes. The high hole mobility reported for one of the polymers On the other hand, there is also a lot of prospect for solution-
is among the highest reported for metallopolyynes. processable conjugated small molecules, as they have recently
been shown to afford comparable efficiencies to the low
4.2. OLEDs
bandgap conjugated polymers.43,57
Organic light-emitting diodes are currently extensively being Thiazolo[5,4-d]thiazoles have been shown to be powerful
used in displays, mostly for portable devices, due to their lower building blocks in the design of materials for organic solar
energy consumption compared to traditional LED displays. cells, mainly due to their high environmental stability and
There are only a few publications where TzTz’s have been used charge transfer capabilities. The first synthesis of TzTz-based
for the manufacturing of LED devices. Most of them concern polymers for BHJ solar cells was performed by Choi et al. in
TzTz-based polymers, which exhibit suitable properties 2010,40 who prepared a low bandgap polymer 49 using a Stille
required for high efficiency photoluminescent materials, such coupling reaction between TzTz 84 and the extended chromo-
as amorphous or ‘solution-like’ solid state behaviour, charge phore 85 (Scheme 16). When blended with PC71BM ([6,6]-
transport ability, etc. phenyl C71 butyric acid methyl ester) in a 1 : 2 ratio (polymer
It is known from literature that fluorene-based polymers vs. PC71BM), a moderate PCE of 1.21% was achieved with this
exhibit good electroluminescent properties as well as a good polymer.
stability (among other advantages).59 However, poor electron Since then, a lot of publications have appeared on the
mobilities generally prevent efficient hole and electron preparation of new copolymers, using a similar D–A approach.
recombination, reducing the electroluminescence abilities. The record PCE of 5.7% for TzTz-based polymers was recently
Therefore, copolymers 20 and 25 were prepared, combining achieved by Takimiya et al. using a rather simple copolymer 86

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Scheme 16 Synthesis of TzTz-based D–A copolymer 49.

(Fig. 14),60 which was initially prepared for OFETs.2 The high combination with MDMO-PPV as the electron donor polymer.
molecular weight (Mw = 3 6 105; Mn = 33 6 103) obtained Although the PCE achieved was very poor (0.1%), high open-
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seems to be crucial for high PCE’s. However, even lower circuit voltage (Voc) values were obtained and useful informa-
molecular weight batches resulted in only a slight decrease in tion on the charge transfer mechanism was gathered that may
efficiency to 5.3%. help in the further design of high performance devices.58
By combining two of the most common building blocks for
polymer solar cells, namely TzTz and CPDT, and employing a
Suzuki polycondensation reaction, the low bandgap material
60 has been prepared (see Fig. 11).17 Use of asymmetrical alkyl 5. Conclusions
substitution on the CPDT unit improved solubility, while still The thiazolo[5,4-d]thiazole biheterocyclic system has been
enabling p–p stacking, resulting in a semicrystalline material. increasingly studied in recent years, mainly due to the steadily
By improving the purity of the polymer, the PCE was growing interest in its applications in organic electronics.
noticeable increased, reaching 4.03% (when blended with Materials containing the TzTz structure generally exhibit a
PC71BM in a 1 : 3 ratio).
good stability towards oxygen and light, which are rather
Besides polymers, also small molecules can be used in
detrimental for a large number of the thiophene-based
different types of solar cells. To date, there are not many
materials most often applied in devices. The excellent charge
examples of small TzTz molecules applied in organic solar
mobilities resulting from the planarity of the TzTz core and
cells. Donor–acceptor–donor type dyes 75 and 76 with
the high tendency for p–p stacking render TzTz derivatives
triphenylamine donor groups flanking the TzTz were very
recently synthesized (Fig. 12).56,57 The materials were solution- excellent candidates for both OFETs and organic solar cells.
processed in a 1 : 4 blend with PC61BM, giving excellent Tuning of the electronic properties can readily be achieved by
thermal stability and a PCE up to 3.73%. On the other hand, attaching different electron donor and/or acceptor groups. The
similar materials 78 and 79a,b (Fig. 12) with electron low solubility of the parent TzTz structure requires substitu-
withdrawing groups have been used as acceptor materials, tion with alkyl chains whenever solution-processability is
replacing the standard fullerenes in BHJ solar cells in desirable.
Looking at the synthetic chemistry that has been explored
so far for the development of TzTz compounds, it seems
plausible that the best is yet to come for these materials.
Although many novel TzTz derivatives have been synthesized
during the last decade, most of the procedures still employ the
original synthetic protocol, which is rather harsh and low-
yielding. Some of the alternative procedures listed certainly
deserve a novel more proper look. In particular pathways
towards the parent TzTz system 6 are attractive to increase the
diversity of structures. In general, there are certainly opportu-
nities ahead for creative synthetic/material chemists, and the
overview of synthetic protocols explored so far can serve as a
Fig. 14 Structure of TzTz -based D–A polymer 86 affording a PCE of 5.7%. source of inspiration. Further TzTz derivatization paths may

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