Combined experimental-theoretical NMR study on 2,5-bis(5-aryl-3- hexylthiophen-2-yl)-thiazolo[5,4-d]thiazole derivatives for printable electronics

Article abstract of DOI:10.1002/mrc.3812

Four 2,5-bis(5-aryl-3-hexylthiophen-2-yl)thiazolo[5,4-d]thiazole derivatives have been synthesized and thoroughly characterized. The extended aromatic core of the molecules was designed to enhance the charge transport characteristics, and solubilizing hex

Full text of DOI:10.1002/mrc.3812

Research Article
Received: 5 November 2011
Revised: 12 January 2012
Accepted: 21 February 2012
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/mrc.3812
Combined experimental–theoretical NMR
study on 2,5-bis(5-aryl-3-hexylthiophen-2-yl)-
thiazolo[5,4-d]thiazole derivatives for
printable electronics
S. Van Mierloo,a V. Liégeois,b J. Kudrjasova,a E. Botek,b L. Lutsen,c
B. Champagne,b D. Vanderzande,a,c P. Adriaensensa* and W. Maesa,d*
Four 2,5-bis(5-aryl-3-hexylthiophen-2-yl)thiazolo[5,4-d]thiazole derivatives have been synthesized and thoroughly characterized.
The extended aromatic core of the molecules was designed to enhance the charge transport characteristics, and solubilizing hexyl
side chains were introduced on the thiophene subunits to enable possible integration of these semiconducting small molecules in
printable electronics. Complete elucidation of the chemical structures by detailed one-dimensional/two-dimensional NMR spectros-
copy is described, providing interesting input for chemical shift prediction software as well, because limited experimental data on
these types of compounds are currently available. Furthermore, theoretical calculations have assisted experimental observations—
giving support for the chemical shift assignment and providing a springboard for future screening and predictions—
demonstrating the benefits of a coordinated theoretical–experimental approach. Copyright © 2012 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: NMR; 1H and 13C NMR chemical shift assignment; density functional theory calculations; thiazolo[5,4-d]thiazoles; chemical
shift prediction software; printable electronics
i.e. D1–D4 (Scheme 1). Functionalization, purification, and char-
acterization were considerably simplified by the introduction of
Introduction
The development of organic semiconductors for the active layers
of field effect transistors (FETs) has received significant atten-
tion during recent years.[1] Carefully designed high-performance
organic materials are required to achieve high charge carrier
mobilities, on/off current ratios, stability, and processability. p-
conjugated small molecules such as thiophene oligomers have
been shown to possess excellent charge transport characteristics
in FETs. A number of structural modifications have been carried
out on such (oligo)thiophene compounds so far.[2] Replacement
of some of the thiophene units by thiazolo[5,4-d]thiazole (TzTz)
fused ring systems is reported to be rather effective, resulting in
high charge carrier mobilities.[3] The TzTz moiety possesses some
important features toward electronic applications. Thiazolo[5,4-d]
thiazoles are electron-deficient fused heterocycles with a rigid pla-
nar structure, which enables efficient intermolecular p-p overlap,
and they show enhanced stability toward oxygen.[4,5] Moreover,
functionalized TzTz derivatives are easily prepared starting from
the corresponding arylcarbaldehydes (e.g. 3-hexylthiophene-2-
carbaldehyde) and dithiooxamide.[6,7]
the solubilizing 3-hexylthiophene subunits, and these molecules
can hence be utilized in solvent-based processing techniques to-
ward printable electronics. Furthermore, such TzTz materials are
also of particular appeal for integration (as acceptor components)
in low bandgap copolymers toward highly efficient organic solar
cells. The interest in the TzTz structure in this respect has in-
creased spectacularly, noticeably in the last year.[9]
*
Correspondence to: P. Adriaensens, Hasselt University, Institute for Materi-
als Research (IMO-IMOMEC Ass. Lab.), Design & Synthesis of Organic Semi-
conductors (DSOS), Agoralaan 1 - Building D, B-3590 Diepenbeek, Belgium.
E-mail: peter.adriaensens@uhasselt.be
* Correspondence to: W. Maes, Hasselt University, Institute for Materials Re-
search (IMO-IMOMEC Ass. Lab.), Design & Synthesis of Organic Semicon-
ductors (DSOS), Agoralaan 1 - Building D, B-3590 Diepenbeek, Belgium.
E-mail: wouter.maes@uhasselt.be
a Hasselt UniversityInstitute for Materials Research (IMO-IMOMEC Ass. Lab.),
Design & Synthesis of Organic Semiconductors (DSOS), Agoralaan 1 - Building
D, B-3590 Diepenbeek, Belgium
Unfortunately, non-alkylsubstituted TzTz compounds are very
poorly soluble and require vacuum deposition techniques for de-
vice fabrication. So far, only a few soluble TzTz semiconducting
materials have been reported.[4,5] In a previous work, we have
decorated the TzTz core with two 3-hexylthiophene moieties.[8]
The substitution with alkyl chains obviously leads to a signifi-
cantly improved solubility in common organic solvents. Here,
an extended series of expanded TzTz chromophores is presented,
b Laboratoire de Chimie Théorique, Facultés Universitaires Notre-Dame de la
Paix, Rue de Bruxelles 61, B-5000 Namur, Belgium
c
IMEC, IMOMEC Ass. Lab., Universitaire Campus – Wetenschapspark 1, B-3590
Diepenbeek, Belgium
d Molecular Design and Synthesis, Department of Chemistry, Katholieke Univer-
siteit Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium
Magn. Reson. Chem. 2012, 50, 379–387
Copyright © 2012 John Wiley & Sons, Ltd.

S. Van Mierloo et al.
C6H13
C6H13
Br
S
S
N
N
S
S
S
N
N
S
NBS
DMF
RT, 48 h
(71%)
S
S
Br
C6H13
C6H13
P2
P1
ArB(OR)2, Pd(PPh3)4, DME
NaHCO3/K3PO4 or
ArB(OH)2, Pd(PPh3)4,
THF, Na2CO3
(thiophen-2-yl)Sn(Bu)3
Pd(PPh3)4, THF
S
C6H13
C6H13
N
Ar
S
S
N
N
S
S
S
S
N
S
S
Ar
C6H13
C6H13
S
D0 (71%)
D1 Ar = 4-CF3-Ph (64%)
D2 Ar = 4-CN-Ph (53%)
D3 Ar = 4-F-Ph (58%)
D4 Ar = Ph (84%)
Scheme 1. Synthetic procedures toward TzTz derivatives D0–D4.
In this paper, we report the synthesis, detailed hydrogen-1 (1H)
and carbon-13 (13C) NMR chemical shift assignment and the cor-
relation of experimental NMR results with theoretical chemical
shift calculations for a number of functionalized 2,5-bis(5-aryl-3-
hexylthiophen-2-yl)thiazolo[5,4-d]thiazole derivatives.
1H = 7.24 ppm and 13C = 77.70 ppm. The proton spectra were ac-
quired with a 90ꢁ pulse of 4.3 ms, a spectral width of 4500 Hz, an
acquisition time of 3.5 s, a preparation delay of 8 s and 20 accu-
mulations, processed with a line broadening of 0.2 Hz. The con-
centration of the samples was ~2 mg/0.7 ml. The carbon spectra
were acquired with a 90ꢁ pulse of 12 ms, a spectral width of
16500 Hz, an acquisition time of 0.8 s, a preparation delay of
60 s and 2500 accumulations, processed with a line broadening
of 3 Hz. Higher concentrations were used for the one-dimensional
and two-dimensional spectra with 13C detection. The 13C spin-lattice
relaxation time (T1C) decay times were measured by the inver-
sion recovery method, using 17 recovery time increments
between 0.01 and 70 s (80 accumulations; preparation delay of
60 s). With regard to DEPT, a full ‘ADEPT’ editing was performed
on the basis of an array of spectra taken with θ = 45ꢁ, 90ꢁ and
135ꢁ as tip angles for the final proton pulse (40 accumulations;
preparation delay of 20 s). COSY spectra were taken with 256 in-
direct time increments (eight accumulations; preparation delay
of 2 s). Short-range (direct) HETCOR experiments were acquired
with 96 indirect time increments (eight accumulations; prepara-
tion delay of 2 s; optimized for 1JC–H = 140 Hz). Long-range HET-
COR experiments were acquired with 96 indirect time increments
(120 accumulations; preparation delay of 10 s; optimized for
nJC–H = 8 Hz).
Experimental Section
Synthesis
Detailed synthetic procedures toward TzTz derivatives D1, D2,
and D3 will be reported elsewhere.[10]
2,5-Bis(3-hexyl-5-phenylthiophen-2-yl)thiazolo[5,4-d]thiazole (D4)[11]
Under nitrogen atmosphere, a small amount of Pd(PPh3)4 (6 mg,
5 mmol) was dissolved in tetrahydrofuran (4 ml). 2,5-Bis(5-bromo-3-
hexylthiophene-2-yl)thiazolo[5,4-d]thiazole (P2) (100 mg, 158 mmol),
a Na2CO3 solution (2 M, 0.5 ml) and phenylboronic acid (42 mg,
347 mmol) were added to the stirring solution in the mentioned se-
quence. After heating at reflux temperature for 5 h under nitrogen
atmosphere and protected from light, no starting material was
observed any more (by thin layer chromatography analysis). After
cooling to room temperature, the crude reaction mixture was con-
centrated under reduced pressure and subsequently diluted with
water and CH2Cl2. The organic layer was separated, and the aque-
ous layer was extracted with CH2Cl2 (3 ꢀ 50 ml). The combined
organic layers were washed with a saturated NaHCO3 solution and
brine, dried over MgSO4, filtered, and concentrated by evaporation
in vacuo. The reaction product was purified by gradient column
chromatography (silica, eluent CH2Cl2 to CH2Cl2:MeOH 95:5) and
recrystallized from ethyl acetate, resulting in 80 mg (84% yield) of
red crystalline needles. Gas chromatography–mass spectrometry
electron impact (EI): m/z = 626 (M+); UV–vis (CHCl3, nm) lmax (log e)
434 (4.656).
Computational procedure
The ground state geometries were optimized at the density func-
tional theory (DFT) level of approximation by employing the
B3LYP exchange-correlation (XC) functional and the 6-31 G* basis
set. For convenience, the n-hexyl substituents have been
replaced by smaller methyl groups. The chemical shifts (d) of all
systems were calculated as the difference of isotropic shielding
constants (s) with respect to TMS. All s values were obtained
with the B3LYP XC functional and the 6-311 + G(2d,p) basis set to-
gether with the Gauge invariant (or including) atomic orbital
method to ensure origin independence, following the approach
that was employed and tested recently for polyvinyl chloride oli-
gomers, fluoroionophores, and poly(thienylene vinylene) model
compounds.[12] As shown in these previous studies as well as in
NMR characterization
All 1H and 13C liquid-state NMR experiments were performed
at room temperature on a Varian Inova 300 spectrometer (Agilent
Technologies, Santa Clara, California, US) in a 5-mm four-nucleus
PFG probe. The chemical shift scales are calibrated to CDCl3:
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2,5-Bis(5-aryl-3-hexylthiophen-2-yl)-thiazolo[5,4-d]thiazoles: combined experimental–theoretical NMR study
related ones,[13] linear fits between experimental and theoretical
d values of representative model compounds can facilitate the in-
terpretation of the NMR spectra of more complex and larger
compounds and can therefore provide a way of correcting the
calculated properties from systematic errors. Preliminary
investigations on PTV model compounds have been reported
before,[12d] and the theoretically estimated d values were
obtained from the calculated values by using the following
relationships (in ppm):
the B3LYP/6-31 G* level of theory with the inclusion of solvent
effects. Each torsion angle between adjacent rings is char-
acterized by two minima, close to values of 0ꢁ and 180ꢁ. There-
fore, taking into account the molecular symmetry, we end
up with 16 conformers for D0, but only four for D1–D4. The
Maxwell–Boltzmann weights are given in the supplemen-
tary material Table S1. All calculations were performed using
Gaussian09.[15]
Results and Discussion
sp2 in a 13C : dðestimatedÞ ¼ 0:8099d½IEFꢂPCM=B3LYP=6
(1a)
ꢂ311þGð2d; pÞꢃþ15:39
Synthesis
The common precursor for the reported TzTz semiconductors, 2,5-
bis(3-hexylthiophen-2-yl)thiazolo[5,4-d]thiazole (P1), was synthe-
sized in moderate yield (47%) via a condensation reaction at
elevated temperature (200 ꢁC) between dithiooxamide and an ex-
cess of 3-hexylthiophene-2-carbaldehyde,[8] prepared according
to a modified literature procedure (Scheme 1).[6,7] The resulting
product mixture required extensive purification (column chro-
matography and two successive recrystallizations from ethanol
and acetonitrile) to convert the black crude residue into pure
yellow crystals of P1. After subsequent dibromination with N-
bromosuccinimide (71% yield), the final TzTz derivatives D1, D2,
D3, and D4 were obtained through Suzuki cross-coupling reac-
tions with the respective boronic acids or esters (Scheme 1).[10]
The dithienyl-substituted TzTz counterpart D0 has previously
been prepared by an analogous Stille protocol.[8] The cross-
coupling reactions were in general very effective, main product
losses occurring during the tedious material purification (by re-
petitive recrystallization and column chromatography), affording
highly pure materials for device applications.
1H : dðestimatedÞ ¼ 0:8653d½IEFꢂPCM=B3LYP=6
ꢂ311þGð2d; pÞꢃþ0:67
(1b)
(2a)
(2b)
(3a)
(3b)
sp2 in b 13C : dðestimatedÞ ¼ 0:7618d½IEFꢂPCM=B3LYP=6
ꢂ311þGð2d; pÞꢃþ27:61
1H : dðestimatedÞ ¼ 0:8845d½IEFꢂPCM=B3LYP=6
ꢂ311þGð2d; pÞꢃþ0:61
other sp2 13C : dðestimatedÞ ¼ 0:6510d½IEFꢂPCM=B3LYP=6
ꢂ311þGð2d; pÞꢃþ40:36
1H : dðestimatedÞ ¼ 0:8911d½IEFꢂPCM=B3LYP=6
ꢂ311þGð2d; pÞꢃþ0:52
NMR characterization
To confirm the identity and purity of the novel thiazolo[5,4-d]thi-
azole derivatives, they were analyzed by mass spectrometry and
NMR.[10] NMR spectroscopy confirmed that TzTz’s D0–D4 were
obtained in excellent purity. Complete assignment of the 1H
and 13C resonances was achieved by a combination of regular
1H and 13C NMR spectra with attached proton test (APT), DEPT,
T1C inversion recovery experiments, COSY, and short/long-range
HETCOR experiments. Table 1 presents an overview of all signal
assignments for D0–D4. The complete NMR analysis of TzTz D0
has previously been reported.[8] Such a detailed analysis is useful
because the obtained chemical shift information can be helpful
in further research on TzTz moieties, for instance by implemen-
tation in NMR-based prediction software. To date, experimental
NMR data on TzTz compounds, and resonance assignments
in particular, are very scarce. The chemical structures of TzTz
compounds D1–D4 differ only in the aryl substituents on the
thiophene rings. Figure 1 provides an arbitrary numbering
scheme for the different carbon and hydrogen atoms of the TzTz
derivatives D0–D4.
other atoms 13C : dðestimatedÞ ¼ 0:9618d½IEFꢂPCM=B3LYP=6
ꢂ311þGð2d; pÞꢃꢂ2:66 (4a)
1H : dðestimatedÞ ¼ 0:9451d½IEFꢂPCM=B3LYP=6
ꢂ311þGð2d; pÞꢃþ0:12
(4b)
As discussed in Ref. 12d, a single equation for correcting all 1H
or all 13C chemical shifts turns out to be insufficient. First, we have
made the distinction between the sp2 C atoms (and the H atoms
linked to them) and the other hybridizations (other atoms). Then,
the sp2 atoms (C and the H attached to them) have been classi-
fied into three categories, those of the thiophene rings (where
a and b refer to the position with respect to the S atoms) and
the other ones (other sp2). Therefore, for both the H and C atoms,
we have four equations, associated with different combinations
of hybridization/position, and all the theoretical chemical shifts
reported in this paper have been corrected by the appropriate
linear regression equation. Solvent (here CHCl3) effects were
taken into account within the integral equation formalism of
the polarizable continuum model (IEF-PCM)[14] for the calcu-
lations of the isotropic shielding constants of all compounds
(including TMS). Τhe d[IEF-PCM/B3LYP/6-311 + G(2d,p)] values
consist in the Maxwell–Boltzmann average chemical shifts over
the stable conformers on the basis of the energies calculated at
Assignment of the resonances of TzTz D1
In a previous work, all 1H and 13C NMR resonances of the precur-
sors P1 and P2, and TzTz derivative D0 were assigned.[8] This
knowledge readily allowed the assignment of the carbon signals
of D1 at 161.5, 150.9, 144.7 and 133.0 ppm to the non-protonated
carbon atoms C2, C1, C4 and C3 of the fixed core, respectively.
The presence of the three 19F nuclei on the para-trifluoromethyl
Magn. Reson. Chem. 2012, 50, 379–387
Copyright © 2012 John Wiley & Sons, Ltd.
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S. Van Mierloo et al.
Table 1. Chemical shifts (ppm) and assignments of the proton and carbon resonances for thiazolo[5,4-d]thiazole derivatives D0–D4. The chemical
shift scales are calibrated to CDCl3: 1H = 7.24 ppm and 13C = 77.70 ppm. The assignment of the chemical shifts for TzTz D0 was retrieved from
[8]
previous work
D0
D1
D2
D3
D4
D0
D1
D2
D3
D4
Ar Thien-2-yl Ar p-CF3-Ph Ar p-CN-Ph Ar p-F-Ph
Ar Ph
Ar = Thien-2-yl Ar p-CF3-Ph Ar p-CN-Ph Ar p-F-Ph
Ar Ph
Carbon
C1
Proton
150.7
161.5
131.0
144.5
127.6
137.3
31.1
150.9
161.5
133.0
144.7
128.3
143.9
31.2
151.3
161.6
133.8
145.0
129.2
143.5
31.1
150.9
161.9
131.9
145.2
127.4
144.9
31.1
150.7
161.8
131.7
144.7
—
—
—
—
—
—
—
—
—
—
C2
C3
—
—
—
—
—
C4
—
—
—
—
—
C5
127.3 H-5
146.1
7.00 (s)
—
7.27 (s)
—
7.30 (s)
—
7.12 (s)
—
7.20 (s)
—
C6
C7
31.2 H-7
30.5 H-8
30.1 H-9
32.3 H-10
23.3 H-11
14.8 H-12
134.1
2.88 (t)
1.71 (p)
1.45 (p)
1.34 (m)
1.34 (m)
0.90 (t)
—
2.97 (t)
1.76 (p)
1.48 (p)
1.35 (m)
1.35 (m)
0.90 (t)
—
2.96 (t)
1.75 (p
1.47 (p)
1.35 (m)
1.35 (m)
0.90 (t)
—
2.94 (t)
1.74 (p)
1.47 (p)
2.96 (t)
1.75 (q)
1.47 (q)
C8
30.5
30.4
30.5
30.7
C9
30.1
30.1
30.1
30.1
C10
C11
C12
C13
C14
C15
C16
C17
32.3
32.3
32.3
32.4
1.35 (m) 1.35 (m)
1.35 (m) 1.35(m)
23.3
23.3
23.3
23.3
14.8
14.8
14.8
14.8
0.90 (t)
0.90 (t)
139.4
125.2
128.8
126.0
137.3
126.2
126.6 (q)
130.4 (q)
124.7 (q)
138.3
126.6
133.5
112.0
119.3
130.7 (d)
128.3 (d)
116.8 (d)
163.6 (d)
—
—
126.4 H-14
129.7 H-15
128.9 H-16
7.22 (dd)
7.02 (dd)
7.24 (dd)
—
7.73 (d)
7.64 (d)
—
7.72 (d)
7.67 (d)
—
7.60 (dd) 7.64 (dd)
7.08 (dd) 7.39 (t)
—
—
7.31 (t)
—
—
—
substituent, affording quadruplet 13C resonance patterns under
broadband proton-decoupling, was very helpful for further
assignments. The nonprotonated carbon atom C17 has a chemi-
cal shift at 124.7 ppm (split into a quadruplet with lines at 130.1,
126.5, 122.9, and 119.2 ppm due to the direct 1JC–F coupling with
coupling constant of 272 Hz). The 2JC–F coupling of 33 Hz was also
clearly identifiable in the quadruplet with chemical shift of
130.4 ppm (131.1, 130.6, 130.2, and 129.8 ppm) arising from
C16. Finally, a 3JC–F coupling of 3.5 Hz allowed to locate the chem-
ical shift of C15 at 126.6 ppm (Fig. 2). The proton signal at
7.64 ppm could be assigned to H-15 via a short-range (1J or direct
coupling) HETCOR experiment.
The aromatic region of the H NMR spectrum presents three
1
resonance patterns corresponding to the ten aromatic protons
of D1: an AB-system[16] consisting of H-15 (7.64 ppm) and H-14
3
(7.73 ppm; J = 9.0 Hz) and a singlet resonance at 7.27 ppm from
the thiophene protons H-5. A short-range HETCOR experiment
then allowed to determine the chemical shifts of C5 (128.3 ppm)
and C14 (126.2 ppm). This left only two unidentified chemical
shifts (137.3 and 143.9 ppm) for two nonprotonated aromatic car-
bons, C6 and C13. A long-range HETCOR experiment (Fig. 3)
showed a clear correlation between H-15 and the carbon signal
at 137.3 ppm which can only arise from the 3J-coupling between
H-15 and C13. Indeed, H-15 is too far (four bonds) from C6 to show
Ar
Ar
6
1
N
S
S
N
S
S
3
5
2
4
7
8
9
10
12
11
D2:
16
NC
17
D1:
D0:
S
13
14
13
14
13
14
16
16
F3C
15
15
15
17
D3:
16
F
D4:
13
14
13
16
14
15
15
Figure 1. Arbitrary numbering scheme for 2,5-bis(5-aryl-3-hexylthiophen-
2-yl)thiazolo[5,4-d]thiazole derivatives D0–D4. The different carbon atoms
are numbered from C1 to C17, and the different hydrogen atoms are
labeled accordingly from H-5 to H-16.
Figure 2. Part of the aromatic region of the 13C NMR spectrum of D1.
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2,5-Bis(5-aryl-3-hexylthiophen-2-yl)-thiazolo[5,4-d]thiazoles: combined experimental–theoretical NMR study
carbon atoms (C1, C2, C3, C4, and C6—with similar tc), the decay
times are determined by the spatial distance to the nearest pro-
tons (a longer distance corresponding to a longer decay time).
With regard to the carbon atoms of the mobile hexyl side chains
(with a similar distance to protons), the closer the carbon is situ-
ated toward the end of the side chain, the longer its relaxation de-
cay time will become because of less restricted conformational
motions.[13] This is obvious for C7 and C12 which have the short-
est and longest T1C decay time of the side-chain carbon atoms,
respectively. The short-range HETCOR spectrum allowed the
assignment of the proton signals of the bonded protons, and this
was confirmed by a COSY experiment.
Assignment of the resonances of TzTz D2
Carbon peaks at 151.3, 161.6, 133.8, and 145.0 ppm could be
assigned to C1, C2, C3, and C4, respectively, on the basis of the
previous attributions for the precursors P1 and P2 and the TzTz
derivative D0.[8] The aromatic region of the 1H NMR spectrum
again showed three resonance patterns corresponding to
the ten aromatic protons of D2: an AB-system consisting of
H14/H15 (7.72/7.67 ppm; 3 J = 9 Hz) and a singlet resonance at
7.30 ppm for the thiophene protons H-5. A short-range HETCOR
experiment allowed to assign the carbon signal at 129.2 ppm to
C5 (128.3 ppm). This left six aromatic carbon atoms at 143.5,
138.3, 126.6, 133.5, 112.0, and 119.3 ppm, for which an APT spec-
trum (and integration) showed that the signals at 126.6 and
133.5 ppm arose from methine carbon atoms. On the basis
of the chemical shift and T1C decay time, the carbon signal
at 143.5 ppm could be assigned to C6. Indeed, the long-range
Figure 3. Long-range HETCOR (J = 8 Hz) spectrum of D1.
a correlation.[17] The remaining carbon signal at 143.9 ppm corre-
lates with H-5 and H-14, confirming its assignment to C6. Figure 3
shows additional confirmative correlations (e.g. C4–H5, C3–H5,
C16–H14, C17–H15) which are not discussed in detail. The assign-
ment of the alkyl side chain carbon atoms was based on COSY and
short-range HETCOR experiments as described in our previous pa-
per.[8] Most assignments could be confirmed by T1C experiments
(Table 2). Taking the through-space dipole–dipole interaction as
the main mechanism for relaxation, the T1C relaxation of small
molecules in solution (situated in the so-called extreme narrowing
region) is mainly determined by the correlation time of motion
(tc) and the distance to surrounding proton magnetic dipoles
(1/T1C has a 1/r6 dependency to proton spins). Proton magnetic
moments induce, due to molecular motions, local oscillating mag-
netic fields in the neighborhood of the carbon atoms, the source
of T1C relaxation.[18] With regard to the nonprotonated TzTz core
3
HETCOR experiment (Fig. 4) revealed a J coupling between this
carbon resonance and the proton resonance at 7.72 ppm, next
to a 2J coupling with the proton resonance at 7.30 ppm (H-5). This
allowed to assign the proton signal at 7.72 ppm to H-14. Confir-
mation could be found in C4 (145.0 ppm), which only correlates
to H-5 because it is too far (5 bonds) from H-14. The remaining
Table 2. 13C spin-lattice relaxation time (T1C) relaxation decay times (s), determined by the inversion recovery method, for the carbon atoms of
thiazolo[5,4-d]thiazole derivatives D0–D2 and D4. T1C data for TzTz D0 were retrieved from previous work[8]
D0
T1C
D1
T1C
D2
T1C
D4
T1C
Ar = Thien-2-yl
(s)
Ar p-CF3-Ph
(s)
Ar p-CN-Ph
(s)
Ar Ph
(s)
Carbon
C1
150.7
161.5
131.0
144.5
127.6
137.3
31.1
9.3
150.9
161.5
133.0
144.7
128.3
143.9
31.2
8.55
6.00
6.82
2.74
0.23
3.85
0.39
0.51
0.73
1.24
1.95
2.95
2.51
0.41
0.46
3.42
4.26
151.3
161.6
133.8
145.0
129.2
143.5
31.1
11.32
150.7
161.8
131.7
144.7
127.3
146.1
31.2
10.28
C2
6.7
7.29
6.81
C3
7.8
8.67
3.64
0.35
5.48
0.43
0.55
0.86
1.50
2.28
2.97
4.05
0.52
0.46
3.98
1.32
8.32
3.80
0.43
4.65
0.48
0.69
0.96
1.58
2.35
2.98
3.62
0.76
0.69
0.41
C4
3.3
C5
0.40
6.1
C6
C7
0.50
0.71
1.00
1.67
2.32
3.12
4.00
0.63
0.63
0.41
C8
30.5
30.4
30.5
30.5
C9
30.1
30.1
30.1
30.1
C10
C11
C12
C13
C14
C15
C16
C17
32.3
32.3
32.3
32.3
23.3
23.3
23.3
23.3
14.8
14.8
14.8
14.8
139.4
125.2
128.8
126.0
137.3
126.2
126.6 (q)
130.4 (q)
124.7 (q)
138.3
126.6
133.5
112.0
119.3
134.1
126.4
129.7
128.9
Magn. Reson. Chem. 2012, 50, 379–387
Copyright © 2012 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/mrc

S. Van Mierloo et al.
Figure 5. Aromatic region of the 13C NMR spectrum of D3.
Figure 4. Long-range HETCOR (J = 8 Hz) spectrum of D2.
range HETCOR correlated C5 to the proton singlet of H-5 at
7.12 ppm.
The alkyl side-chain assignment was again based on COSY,
short-range HETCOR, and T1C experiments. The only remaining
carbon signal at 144.9 ppm therefore corresponded to C6.
Besides the H-5 singlet, the aromatic region of the 1H NMR
spectrum showed two additional resonance patterns, originating
from the p-F-Ph ring protons, which could be assigned on the ba-
sis of the J coupling patterns: a doublet of doublets at 7.60 ppm
aromatic proton signal at 7.67 ppm could then be assigned to H-
15. The carbon signals at 126.6 and 133.5 ppm could be attrib-
uted to C14 and C15, respectively, on the basis of a short-range
HETCOR experiment. Taking the electron-withdrawing effect
of the –CN functional group into account, the reduced electron
density at C15 corresponds with its downfield chemical shift (as
compared with C14). In the long-range HETCOR experiment, the
13C signal at 112.0 ppm presented a correlation with H14
(7.72 ppm; 3J coupling) and H-15 (7.67 ppm; 2 J coupling) and
therefore could be assigned to C16 (Fig. 4). In agreement with
previous TzTz derivatives, C13 could be assigned to the carbon
signal at 138.3 ppm, whereas C17 resonates at 119.3 ppm. The
latter is fully in agreement with literature.[19] Figure 4 shows addi-
tional confirmative correlations (e.g. C17-H15, C14-H14, C15-H15,
C3-H5, C13-H5) which are not discussed further in detail. The
alkyl side-chain assignment was again based on COSY, short-
range HETCOR, and T1C experiments, as reported earlier and
previously.[8]
4
(3JH–H = 9 Hz, JH–F = 6 Hz) and an ‘apparent’ triplet at 7.08 ppm
3
(3JH–H and JH–F = 9 Hz; overlap with H-5 at 7.12 ppm). The short-
range HETCOR experiment (Fig. 6) confirmed these assignments:
the carbon signal at 116.8 ppm (C15) correlated with the
7.08 ppm ‘triplet’ of H-15, whereas the 128.3 ppm peak (C14) cor-
related with the double doublet at 7.60 ppm of H-14.
Assignment of the resonances of TzTz D4
The carbon resonances at 161.8, 150.7, 131.7, and 144.7 ppm are
again attributed to the fixed TzTz core carbon atoms C1, C2, C3,
and C4, respectively, based on the T1C relaxation data (Table 2)
and previous assignments for D0.[8] The aromatic region of the
1H NMR spectrum showed four resonance patterns originating
from the four aromatic protons of D4, which could easily be inter-
preted on the basis of J coupling patterns and integration values;
the doublet at 7.65/7.62 ppm (3 J = 7.3 Hz, 4H) to H-14, the triplet
centered at 7.39 ppm (3 J = 7.3 Hz, 4H) to H-15 and the triplet
Assignment of the resonances of TzTz D3
As 4-fluorophenyl-substituted TzTz D3 was found to be some-
what less soluble in CDCl3, NMR spectra were acquired at a
slightly elevated temperature (45 ꢁC). The carbon peaks at
150.9, 161.9, 131.9, and 145.2 ppm could be attributed to the
fixed core carbon atoms C1, C2, C3, and C4, respectively, based
on the T1C relaxation data (Table 2) and previous assignments.[8]
The 19F nucleus gave rise to doublet resonance patterns in the
proton-decoupled 13C NMR spectrum (Fig. 5). The nonprotonated
carbon atom C16 appeared as a doublet (165.2 and 161.9 ppm)
with chemical shift centered at 163.6 ppm because of a direct
1JC–F coupling of 248 Hz (remark that the signal of C2 overlaps
with the high-field line of the doublet). C15 also showed a dou-
blet with chemical shift centered at 116.8 ppm (116.9 and
2
116.6 ppm) because of a JC–F coupling of 22 Hz. Furthermore,
C14 and C13 could be attributed to the doublets with chemical
shifts at 128.3 and 130.7 ppm, based on their 3JC–F and 4JC–F cou-
plings of 8.5 and 3.4 Hz, respectively (Fig. 5). Taking the electron-
donating effect of the –F functional group into account, the
increased electron density at C15 corresponds with its low-
frequency chemical shift (as compared with C14). C5 was the only
protonated aromatic carbon atom remaining and could hence
easily be assigned by DEPT to the signal at 127.4 ppm. A short-
Figure 6. Short-range HETCOR (J = 140 Hz) spectrum of D3.
wileyonlinelibrary.com/journal/mrc
Copyright © 2012 John Wiley & Sons, Ltd. Magn. Reson. Chem. 2012, 50, 379–387

2,5-Bis(5-aryl-3-hexylthiophen-2-yl)-thiazolo[5,4-d]thiazoles: combined experimental–theoretical NMR study
centered at 7.31 ppm (3 J = 7.3 Hz, 2H) to H-16. The COSY spec-
trum revealed cross-peaks confirming these assignments. The
only remaining aromatic proton signal could be observed as a
singlet resonating at 7.20 ppm and corresponded to the thio-
phene protons (H-5). From the short-range HETCOR experiment
(Fig. 7), it could be derived that the methine carbon signals at
129.7, 128.9, 127.3, and 126.4 ppm belong to C15, C16, C5, and
C14, respectively. This left two unassigned aromatic carbon sig-
nals at 146.1 and 134.1 ppm. A long-range HETCOR experiment
(Fig. 8) further revealed that the aromatic carbon signal at
146.1 ppm is coupled with both H-14 and H-5, allowing to assign
it to C6 as C4 is too remote (five bonds) from H-14. C4 only corre-
lates to the aromatic proton H-5. The remaining carbon signal at
134.1 ppm could then be attributed to C13. The assignment of
the alkyl side-chain carbon resonances was based on the same
protocol as described earlier for D1–D3.
In general, only subtle changes in NMR chemical shifts were
observed for the TzTz core and hexyl side-chain carbon
atoms of the differently substituted 2,5-dithienyl-TzTz derivatives
D0–D4 (Table 1). The most noticeable difference is the deshield-
ing of the thiophene proton for p-CF3-phenyl and p-CN-phenyl
derivatives D1 and D2, as compared with the reference com-
pound D4, and the upfield shift for the p-F-phenyl TzTz D3
(Table 1). More pronounced differences were, of course, observed
within the respective aryl substituents, most notably for the ipso
and ortho aromatic carbon resonances, reflecting the nature of
the appended functional groups.
Theoretical characterization
Table 3 presents the calculated chemical shifts as well as the cor-
rected mean absolute errors (CMAE), obtained after using Eqs 1–4
(in the Experimental section), with respect to the experimental
values given in Table 1. The calculated values are in good agree-
ment with the experimental data and confirm the chemical shift
assignments. More precisely, when the chemical shifts display a
dependence with respect to the substituents (aromatic carbon
atoms C5, C6, C13, C14, C15, and C16), the calculations correctly
reproduce the chemical shift trends, demonstrating they account
for different electron donating and withdrawing effects. This is
further illustrated in Table 4 where the differences of the calcu-
lated chemical shifts for derivatives D1–D3 are reported (the
differences between the experimental data are also given in
parentheses) with respect to derivative D4 (Ar = Ph), which is
taken as a reference. For instance, C16 becomes more deshielded
in derivative D3 because of the inductive attractor effect of the
F atom, whereas it becomes more shielded in derivative D2
because of anisotropy of the triple C–N bond. The change of
chemical shifts of C15 can be explained by the mesomeric donor
(derivative D3) and attractor (derivative D2) effects. There are also
a number of cases where the differences between the experimen-
tal and calculated values remain large, even after linear regression
correction, but they can be explained to a large extent. d(C4) is
underestimated because the hexyl chain is replaced by a methyl
group in the calculations. For C16 and C17, the predictions are
also of lesser quality as a result of the close proximity to hetero-
atoms, especially the electron withdrawing F and CN groups,
and the fact that our approach has not been fitted specifically
for these carbon atoms. However, the CMAE values are smaller
than 3.0 ppm for 13C NMR and smaller than 0.1 ppm for 1H NMR,
which is a double support for the experimental assignment and
the method reliability.
Figure 7. Short-range HETCOR (J = 140 Hz) spectrum of D4.
Conclusion
A series of soluble aryl-substituted 2,5-dithienylthiazolo[5,4-d]thiazole
semiconductors was efficiently synthesized by Suzuki cross-coupling
reactions. Complete NMR chemical shift assignment for these
materials was accomplished by means of a number of one-
dimensional and two-dimensional NMR experiments. Computa-
tional chemistry has been used as an additional tool to confirm
the experimental NMR data. The calculated chemical shift trends
are generally consistent with the inductive, mesomeric, and aniso-
tropic effects imposed by the substituents, and a few signatures
were identified enabling straightforward screening of various
(unknown) TzTz compounds in the future. The gathered chemical
shift data are of relevance for chemical shift prediction software,
because limited experimental data on TzTz derivatives are cur-
rently available. This will be particularly interesting because a
spectacular increase in the application of TzTz semiconducting
molecules can be seen from literature in the last year, notably in
organic photovoltaics. The acquired knowledge will also strongly
support further (ongoing) research within our groups on TzTz-
Figure 8. Long-range HETCOR (J = 8 Hz) spectrum of D4.
Magn. Reson. Chem. 2012, 50, 379–387
Copyright © 2012 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/mrc

S. Van Mierloo et al.
Table 3. Theoretical chemical shifts (ppm) for thiazolo[5,4-d]thiazole derivatives D0–D4. TMS was taken as a reference and its calculated s values
1
are 31.31 ppm and 183.33 ppm for H and 13C, respectively. All values have been calculated at the IEF-PCM/B3LYP/6-311 + G(2 d,p) level of theory
with CHCl3 as a solvent and have been corrected by the linear regression Eqs 1–4. The differences between the calculated and experimental (Table 1)
chemical shifts are given in parentheses
D0
D1
D2
D3
D4
D0
D1
D2
D3
D4
Ar = Thien- Ar p-CF3-
Ar p-CN-
Ar p-F-
Ar Ph
Ar = Thien- Ar p-CF3-
Ar p-CN-
Ar p-F-
Ar Ph
2-yl Ph
Ph
Ph
2-yl
Ph
Ph
Ph
Carbon
C1
Proton
149.8 (ꢂ0.9) 150.4 (ꢂ0.5) 150.6 (ꢂ0.7) 149.8 (ꢂ1.1) 149.8 (ꢂ0.9)
158.8 (ꢂ2.7) 159.3 (ꢂ2.2) 159.3 (ꢂ2.3) 159.2 (ꢂ2.7) 159.2 (ꢂ2.6)
131.9 (þ0.9) 135.2 (þ2.2) 136.0 (þ2.2) 133.6 (þ1.7) 133.5 (þ1.8)
140.7 (ꢂ3.8) 140.1 (ꢂ4.6) 140.5 (ꢂ4.5) 140.4 (ꢂ4.8) 140.6 (ꢂ4.1)
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
C2
C3
C4
C5
129.1 (þ1.5) 130.5 (þ2.2) 131.4 (þ2.2) 129.4 (þ2.0) 129.8 (þ2.5) H-5 7.11 (þ0.11) 7.33 (þ0.06) 7.36 (þ0.06) 7.16 (þ0.04) 7.19 (ꢂ0.01)
C6
139.4 (þ2.1) 143.2 (ꢂ0.7) 142.7 (ꢂ0.8) 144.4 (ꢂ0.5) 145.5 (ꢂ0.6)
138.0 (ꢂ1.4) 135.3 (ꢂ2.0) 135.8 (ꢂ2.5) 130.3 (ꢂ0.4) 132.5 (ꢂ1.6)
—
—
—
—
—
—
—
—
—
—
C13
C14
C15
C16
C17
CMAE
126.4 (þ1.2) 126.4 (þ0.2) 126.5 (ꢂ0.1) 127.9 (ꢂ0.4) 126.9 (þ0.5) H-14 7.36 (þ0.14) 7.74 (þ0.01) 7.73 (þ0.01) 7.63 (þ0.03) 7.64 (þ0.00)
129.2 (þ0.4) 126.9 (þ0.3) 132.2 (ꢂ1.3) 119.7 (þ2.9) 128.4 (ꢂ1.3) H-15 7.03 (þ0.01) 7.63 (ꢂ0.01) 7.69 (þ0.02) 7.16 (þ0.08) 7.48 (þ0.09)
126.5 (þ0.5) 129.8 (ꢂ0.6) 115.9 (þ3.9) 153.5 (ꢂ10.1) 128.1 (ꢂ0.8) H-16 7.31 (þ0.07)
7.46 (þ0.15)
129.4 (þ4.7) 120.6 (þ1.3)
1.8 2.0
—
—
—
—
—
1.5
2.7
1.7
0.08
0.02
0.03
0.05
0.07
CMAE, corrected mean absolute errors.
Table 4. Differences of the theoretical chemical shifts (ppm) between thiazolo[5,4-d]thiazole derivatives D1–D3 and D4 (taken as a reference). All
values have been calculated at the IEF-PCM/B3LYP/6-311 + G(2d,p) level of theory with CHCl3 as a solvent and have been corrected by the linear
regression Eqs 1–4. Bold (italic) values represent important deshielding (shielding) effects with respect to the chemical shifts of D4 (Ar Ph). The
corresponding experimental values are given in parentheses
D1
D2
D3
D1
D2
D3
Ar p-CF3-Ph
Ar p-CN-Ph
Ar p-F-Ph
Ar p-CF3-Ph
Ar p-CN-Ph
Ar p-F-Ph
Carbon
C1
Proton
0.6 (þ0.2)
0.1 (ꢂ0.3)
0.8 (+0.6)
0.1 (ꢂ0.2)
0.0 (+0.2)
0.0 (+0.1)
—
—
—
C2
—
—
—
C3
1.8 (þ1.3)
ꢂ0.5 (þ0.0)
0.7 (þ1.0)
2.5 (þ2.1)
ꢂ0.1 (þ0.3)
1.6 (þ1.9)
ꢂ2.8 (ꢂ2.6)
3.3 (þ4.2)
ꢂ0.4 (þ0.2)
3.8 (þ3.8)
ꢂ12.2 (ꢂ16.9)
0.2 (þ0.2)
—
—
—
C4
ꢂ0.2 (þ0.5)
ꢂ0.4 (þ0.1)
ꢂ1.0 (ꢂ1.2)
ꢂ2.2 (ꢂ3.4)
1.1 (þ1.9)
ꢂ8.7 (ꢂ12.9)
25.4 (þ34.7)
—
—
—
C5
H-5
0.14 (þ0.07)
—
0.18 (þ0.10)
—
ꢂ0.03 (ꢂ0.08)
C6
ꢂ2.2 (ꢂ2.2)
2.9 (þ3.2)
ꢂ0.5 (ꢂ0.2)
ꢂ1.5 (ꢂ3.1)
1.7 (þ1.5)
—
C13
C14
C15
C16
—
—
—
H-14
H-15
0.09 (þ0.09)
0.15 (þ0.25)
—
0.08 (þ0.08)
0.21 (þ0.28)
—
ꢂ0.02 (ꢂ0.04)
ꢂ0.33 (ꢂ0.31)
—
based semiconductors and narrow bandgap copolymers toward
applications in organic electronics.
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