Synergic effect of unsaturated inner bridges and polymorphism for tuning the optoelectronic properties of 2,3-thieno(bis)imide based materials

Article abstract of DOI:10.1039/c4tc01792g

2,3-Thieno(bis)imide (N) ended oligomers are emerging as valuable molecular materials for applications in organic electronics. Here, we report the synthesis and characterization of three new 2,3-thieno(bis)imide ended oligothiophenes (T) bearing unsaturated ethylene (E), azomethine (I) and ethinyl (A) inner bridges (NTE, NTI and NTA, respectively). The effect of the unsaturated bridge on the π-conjugation extent, molecular conformation and overall aromaticity is related to the functional optoelectronic and morphological properties and compared to the properties of the linear analogue (NTT) with a bithiophene inner moiety. Optical spectroscopy and cyclovoltammetry analysis show a strong red shift of the absorption and an increased energy band gap on going from NTI and NTE to NTA. The HOMO level decreases in the order NTE > NTI > NTA. Moreover, while the LUMO of NTE and NTA have almost the same energy, NTI has a LUMO energy about 0.1 eV lower, likely due to the electron withdrawing effect of the azomethine moiety. Morphological investigation of solution cast thin deposits shows that the unsaturated bridges promote the formation of concomitant polymorphs with the simultaneous presence of microcrystals with different morphology and fluorescence properties. Moreover, irreversible conversion of one polymorph to the other was achieved by thermal treatments for NTA and NTE and by exploiting this feature, we realized a time temperature integrator (TTI) device based on NTE material. This device allowed to monitor temperature evolutions in the range between RT and 200 °C by means of a red to yellow fluorescence switch that was detectable by optical microscopy. This journal is

Full text of DOI:10.1039/c4tc01792g

Journal of
Materials Chemistry C
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PAPER
Synergic effect of unsaturated inner bridges and
polymorphism for tuning the optoelectronic
properties of 2,3-thieno(bis)imide based materials†
Cite this: J. Mater. Chem. C, 2015, 3,
21
1
a
a
a
b
Massimo Zambianchi, Laura Favaretto, Margherita Durso, Cristian Bettini,
a
a
a
c
d
Alberto Zanelli, Ilse Manet, Massimo Gazzano, Lucia Maini, Denis Gentili,
d
e
df
d
Stefano Toffanin, Federico Gallino, Michele Muccini, Massimiliano Cavallini*
and Manuela Melucci*a
2,3-Thieno(bis)imide (N) ended oligomers are emerging as valuable molecular materials for applications in
organic electronics. Here, we report the synthesis and characterization of three new 2,3-thieno(bis)imide
ended oligothiophenes (T) bearing unsaturated ethylene (E), azomethine (I) and ethinyl (A) inner bridges
(NTE, NTI and NTA, respectively). The effect of the unsaturated bridge on the p-conjugation extent,
molecular conformation and overall aromaticity is related to the functional optoelectronic and
morphological properties and compared to the properties of the linear analogue (NTT) with a
bithiophene inner moiety. Optical spectroscopy and cyclovoltammetry analysis show a strong red shift of
the absorption and an increased energy band gap on going from NTI and NTE to NTA. The HOMO level
decreases in the order NTE > NTI > NTA. Moreover, while the LUMO of NTE and NTA have almost the
same energy, NTI has a LUMO energy about 0.1 eV lower, likely due to the electron withdrawing effect
of the azomethine moiety. Morphological investigation of solution cast thin deposits shows that the
unsaturated bridges promote the formation of concomitant polymorphs with the simultaneous presence
of microcrystals with different morphology and fluorescence properties. Moreover, irreversible
conversion of one polymorph to the other was achieved by thermal treatments for NTA and NTE and by
exploiting this feature, we realized a time temperature integrator (TTI) device based on NTE material.
Received 12th August 2014
Accepted 16th October 2014
DOI: 10.1039/c4tc01792g
ꢀ
This device allowed to monitor temperature evolutions in the range between RT and 200 C by means
www.rsc.org/MaterialsC
of a red to yellow fluorescence switch that was detectable by optical microscopy.
1
2
(
OFETs) and photovoltaic devices (OPVs) or as coadjutants of
Introduction
3
polymeric based devices.
Multifunctional p-conjugated small molecules are experiencing
A key issue of producing materials with optimized opto-
a new revival as active components of eld effect transistors electronic performances is molecular design allowing to achieve
the desirable electronic structure, solid-state packing and
1–4
physical properties. These features highly rely on the choice
of proper p-conjugated building blocks, molecule size/shape
and peripheric substituents. Among the recently emerged
building blocks, the electron decient 2,3-thienoimide group
(N) is gaining increasing attention due to the promotion of n-
type and ambipolar charge transport properties in the resulting
a
Consiglio Nazionale delle Ricerche, Istituto per la Sintesi Organica e la Fotoreattivit a` ,
CNR-ISOF), via P. Gobetti 101, 40129, Bologna, Italy. E-mail: manuela.melucci@isof.
(
cnr.it
b
MIST-ER Laboratory, via P. Gobetti 101, 40129, Bologna, Italy
c
Dipartimento di Chimica “G. Ciamician”, Universit a` di Bologna, Via F. Selmi 2,
4
0126, Bologna, Italy
5
N-ended oligomers. This new class of materials is being tested
d
Consiglio Nazionale delle Ricerche, Istituto per lo Studio dei Materiali
Nanostrutturati (CNR-ISMN), via P. Gobetti 101, 40129, Bologna, Italy. E-mail: m.
cavallini@bo.ismn.cnr.it
6
as active layer in ambipolar light emitting transistors (OLETs),

5
eld-effect transistors (OFETs), and both as donor and acceptor
in photovoltaic cells (OPV). Due to its peculiar electron de-
cient feature, the thienoimide group strongly inuences the
7
e
SAES-GETTERS S. p. A., Viale Italia 77, 20020, Lainate, MI, Italy
f
E.T.C. s.r.l., via P. Gobetti 101, 40129, Bologna, Italy
†
Electronic supplementary information (ESI) available: Comprehensive optoelectronic properties of the nal oligomers. In particular,
experimental details on the synthesis and characterization of the intermediate
2,3-thienoimide impacts the LUMO energy level that results
compounds in Schemes 1–3, cyclic voltammetry at different scan rates,
insensitive of the number of thienoimide moieties and mainly

uorescence lifetime imaging, photoluminescence in thin deposits, differential
scanning calorimetry, theoretical calculations, X-ray diffraction. See DOI:
0.1039/c4tc01792g
5,6a
located on the oligomer periphery.
Moreover, the interplay
between intermolecular interactions enabled by the heteroatom
1
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rich 2,3-thienoimide chemical structure, and p–p stacking,
strongly inuences molecular conformation and solid state
packing, two properties that are intimately related to optical
and charge transport functionalities as well as to the nal device
performances. A highly crystalline morphology has been
5,6
observed for thin lms of 2,3-thieno(bis)imide derivatives.
Several structure–property relationship studies have shown that
the N-end substitution promotes p-stack or herringbone
packing – the latter more typical for conventional oligothio-
6
a,b
phenes – depending on the molecular structure.
Moreover,
polymorphic behaviour evidences were found for some N
6
b
8
derivatives in single crystal and thin lms.
9
Polymorphism is known for many commonly studied
10
organic molecular semiconductors such as pentacene,
11
12
13
rubrene, oligothiophenes and anthradithiophene among
others. If on one hand, polymorphism allows a deeper
comprehension of the structure–property relationships in
organic semiconductors, it is also highly controversial since it
Fig. 1 Molecular structure of compounds 1–3.
can markedly affect the device performance and response Experimental
reproducibility.
Synthesis
Despite this, we demonstrated the potential of polymorphic
General information and the synthesis of all precursors are
organic materials, such as a small trimeric 2,3-thieno(bis)imide
8
reported in the ESI le.†
derivative, for the application of time temperature integrators
0
0
14
(E)-2,2 -(5,5 -(Ethene-1,2-diyl)bis(thiophene-5,2-diyl))bis(5-
hexyl-4H-thieno[2,3-c]pyrrole-4,6(5H)-dione), NTE (1). To a
reuxing toluene solution (6 mL) of 6 (184 mg, 0.58 mmol) and
in situ prepared catalyst Pd(AsPh ) (10 mol%, i.e. 30 mg of
(
TTI), i.e. device capable of monitoring its thermal history
through an irreversible variation of morphological and/or
optical properties of a thin deposit.
3
4
Achieving control of polymorphism is therefore crucial not
only for charge transport based devices, but also for novel
applications based on the controlled conversion of functional
polymorphs in thin lms. In this respect, molecular design and
structure–property relationship investigations play a key role.
Ethylene (E) bridge insertion in oligothiophenes has been
associated to a reduced band gap derived from enhanced
molecular planarity and a lower overall aromatic character than
Pd dba and 71 mg of AsPh3 in 13 mL toluene) under N2
2
3
atmosphere, bis-stannane 5 (204 mg, 0.26 mmol) in toluene (2
mL), was added dropwise. The solution was reuxed for 48 h,
then n-pentane was added at room temperature, and the
precipitate was puried by FC (silica gel, eluent mixture DCM–
pentane 30 : 70 to 100 : 0). The fractions containing the product
were combined, the solvent evaporated, and the residue crys-
tallized from hot toluene to give a dark red solid (100 mg, 57%).
2
15
oligothiophenes containing the same number of sp carbons.
ꢀ
c+
M.p. 255 C; DE-MS (m/z): 662 (M ); absorption maximum, 465
Similarly, an azomethine (I) bridge promotes an even greater
1
nm, emission maximum, 588 nm in DCM; H NMR (CDCl ), d
[ppm]: 7.31 (s, 2H), 7.23 (d, J ¼ 4.0 Hz, 2H), 7.04 (d, J ¼ 4.0 Hz,
H), 7.02 (s, 2H), 3.60 (t, 4H), 1.64 (m, 4H), 1.31 (m, 12H), 0.88
3
bathocromic effect and has also been investigated as a struc-
3
3
16
tural tool to tune the electronic properties of oligothiophenes.
2
Contrarily, the main effect observed for acetylenic (A) bridged
1
3
2
(m, 6H); C NMR (CDCl
3
), d [ppm]: 163.9, 162.7, 149.8, 145.3,
43.7, 137.3, 134.3, 128.0, 126.7, 122.1, 116.5, 38.6, 31.4, 28.8,
6.5, 22.5, 14.0. Anal. calcd for C34 (662.90): C, 61.60;
oligothiophenes with respect to their fully sp carbon based p-
1
2
conjugated analogues is an increased band gap originating
34 2 4 4
H N O S
from the mismatch of the sp carbon orbitals with the p-delo-
17
H, 5.17. Found: C, 61.54; H, 5.22.
E)-5-Hexyl-2-(5-(((5-(5-hexyl-4,6-dioxo-5,6-dihydro-4H-thieno-
2,3-c]pyrrol-2-yl)thiophen-2-yl)imino)methyl)thiophen-2-yl)-4H-
thieno[2,3-c]pyrrole-4,6(5H)-dione, NTI (2). According to the
calized system.
(
On these bases, herein we focus on the insertion of ethylene,
azomethine and acetylene bridges in N-ended materials as a
strategy to alter a p-conjugated system, thus, to ultimately tune
their optoelectronic and packing properties. The effect of the
unsaturated bridges in NTE, NTI, NTA (Fig. 1) compounds on
the functional properties has been evaluated and compared to
those of conventional oligothiophenes or to a bridge-free 2,3-
[
16a
method described by Skene and coworkers, to a mixture of the
aminothiophene derivative 12 (50 mg, 0.15 mmol) and carbal-
dehyde 14 (62 mg, 0.18 mmol) in iPrOH (5 mL), a catalytic
amount of TFA was added under stirring. The reaction mixture
was then heated to reux for 6 h under nitrogen atmosphere,
while magnetic stirring was maintained. Complete removal of
the solvent led to a dark red solid, which was washed in n-
pentane. Crystallization from toluene–pentane mixture gave the
6b
thieno(bis)imide isostructural derivative. During the thin lm
deposition, all compounds crystallized with concomitant poly-
18
morphs which were characterized by a different morphology
and uorescence emission. The polymorphic nature of these
compounds prompted us to exploit the functional poly-
morphism of these materials combined with the possibility of
thermal phase control to produce a NTE based TTI device.
ꢀ
product as a red powder (89 mg, 90% yield). M.p. 256 C; DE-MS
c+
(
m/z): 663 (M ); absorption maximum, 471 nm, emission
1
3
maximum, 544 nm in DCM; H NMR (CDCl ), d [ppm]: 8.48 (s,
122 | J. Mater. Chem. C, 2015, 3, 121–131
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3
3
1
1
1
H), 7.44 (d, J ¼ 4.0 Hz, 1H), 7.43 (s, 1H), 7.35 (d, J ¼ 4.0 Hz, compartment glass cell under Ar pressure. The working elec-
3
3
2
H), 7.32 (s, 1H), 7.24 (d, J ¼ 4.0 Hz, 1H), 7.10 (d, J ¼ 4.0 Hz, trode was semi-spherical Pt (area 0.05 cm ), the auxiliary elec-
1
3
H) 3.60 (m, 4H), 1.64 (m, 4H), 1.31 (m, 12H), 0.88 (m, 6H);
C
trode was a Pt wire in a separate compartment, and the
NMR (CDCl ), d [ppm]: 163.9, 163.7, 162.7, 162.5, 155.1, 150.3, reference electrode was an aqueous KCl saturated calomel
3
ꢀ
1
1
49.1, 145.2, 143.1, 140.0, 138.7, 137.4, 133.3, 131.2, 126.5, 125.8, electrode (Eferrocene/ferricinium ¼ 0.47 V vs. SCE) separated from
25.8, 117.7, 116.5, 38.7, 38.6, 31.4, 28.8, 28.7, 26.5, 22.5, 14.0. the working electrode compartment by a liquid bridge con-
33 3 4 4
Anal. calcd for C33H N O S (663.89): C, 59.70; H, 5.01. Found: taining the same solvent and support electrolyte.
C, 59.63; H, 5.07.
0
0
2
,2 -(5,5 -(Ethyne-1,2-diyl)bis(thiophene-5,2-diyl))bis(5-hexyl-
Thin deposits
Thin deposits were prepared by drop casting 20 mL cm of a 1 g
4
H-thieno[2,3-c]pyrrole-4,6(5H)-dione), NTA (3). To a reuxing
toluene solution (15 mL) of bromo thienoimide 6 (348 mg, 1.10
mmol) and in situ prepared catalyst Pd(AsPh ) (10 mol%, i.e. 52
ꢁ
2
ꢁ
1
L
2
solution in toluene on Si/SiO wafers. The solvent was slowly
3
4
evaporated at room temperature in a solvent-saturated atmo-
mg of Pd
2
dba
3
and 122 mg of AsPh
3
2
) under N atmosphere, bis-
2
sphere or in air. The substrates consist of a 10 ꢂ 10 mm piece
stannane 15 (384 mg, 0.50 mmol) was added dropwise. The
solution was reuxed for 24 h, then the solvent was removed
under vacuum, and the crude product was washed in pentane.
The residue was puried by ash chromatography performed
on an automated system using a 24 gram silica RediSep column
and cyclohexane–EtOAc solvent gradients. Due to its strong
retention on silica gel, the title compound was nally eluted
with hot toluene. The fractions containing compound 3 were
combined, the solvent evaporated, and the residue crystallized
from toluene gave an orange solid (225 mg, 68% yield). M.p. 228
of silicon covered by 200 nm of thermal oxides or pre-patterned
Si/SiO
2
substrates fabricated by photolithography. The
substrates were washed by sonication for 2 min. in electronic-
grade water (Milli-Q-pure quality), acetone and 2-propanol.
Optical microscopy
Optical micrographs were recorded with a Nikon i-80 micro-
scope equipped with epi-illuminator and cross polars (POM). All
the images presented were recorded using objective: LU Plan
ELWD 20X/0.40 objective.
ꢀ
c+
C; DE-MS (m/z): 660 (M ); absorption maximum, 433 nm,
emission maximum, 547 nm in DCM; H NMR (CDCl
1
3
), d [ppm]:
3
3
7
3
.34 (s, 2H), 7.25 (d, J ¼ 4.0 Hz, 2H), 7.24 (d, J ¼ 4.0 Hz, 2H),
13
.60 (t, 4H), 1.62 (m, 4H), 1.31 (m, 12H), 0.88 (m, 6H); C NMR Laser scanning confocal uorescence microscopy
(
CDCl ), d [ppm]: 163.8, 162.6, 148.9, 145.2, 138.1, 137.1, 133.5,
3
Confocal imaging was performed on a Nikon A1 laser scanning
confocal microscope equipped with a CW Argon ion laser
1
25.9, 124.0, 117.1, 88.1, 38.7, 31.4, 28.7, 26.5, 22.5, 14.0. Anal.
calcd for C34 (660.89): C, 61.79; H, 4.88. Found: C,
1.72; H, 4.95.
32 2 4 4
H N O S
(excitation at 488 nm, Melles Griot), and a 485 nm pulsed/CW
6
diode laser (Picoquant, Germany). The system mounts an
inverted Ti-E Nikon microscope. Confocal uorescence imaging
was carried out on the samples at RT. The 521 ꢂ 512 or 1024 ꢂ
Optical spectroscopy
Quantum yield measurements of thin-deposits and solutions 1024 pixel images were collected using a Nikon PLAN APO VC
were carried out by a stand-alone and automatic absolute PL 60ꢂ oil immersion objective with NA 1.40. With this imaging
quantum yield measurements system (C9920-02, Hamamatsu). conguration, pixel side dimension ranged from 100 to 250 nm.
The measurements were performed in air, exciting the PL Hexagonal pinhole dimension was set to 0.8 au corresponding
emission absorption maximum wavelength.
to a dimension of 25 mm and optical thickness of 330 nm. In the
scan head, a dichroic mirror reecting 405 nm, 488 nm, 541 nm
and 640 nm was used, as well as a 20/80 beam splitter. Fluo-
rescence was collected in several spectral windows. In front of
the photomultiplier, we used band-pass lters centered at 525
nm (50), 595 nm (50) and 700 nm (75). Spectral imaging has
been performed using the Nikon A1 spectral detector consisting
of a multi-anode photomultiplier with an array of 32 anodes. A
wavelength range of 10 nm per anode has been applied.
Computational details
The molecular structure geometry of all the compounds has
been optimized in their ground state in vacuum using the
1
9a,b
B3LYP
set as implemented in the Gamess code.
of the lowest singlet–singlet excitation S0 / S1 have been
performed to compute the optical gap E . LUMO energy levels
were obtained by summing the optical gap to the HOMO levels.
Overlap integrals S and S /4
exchange-correlation functional and 6-31+G* basis
19c,d
TDDFT calculations
g
h
e
have been obtained as Sij ¼ h4
i
j
i
Fluorescence microscopy
where 4i,j is the HOMO or LUMO orbital of the i,j-molecule of
Fluorescence images were recorded with a Nikon i-80 micro-
scope equipped with epi-uorescence (FM) using FM lter
Nikon Ex 420, DM 435, BA 475 and Ex 535, DM 570, BA 590. FM
images were recorded using a commercial CCD camera (Nikon
the pair.
Cyclic voltammetry
ꢁ1
1
mmol L solution of compounds 1–3 in Ar purged DCM CCD DS-2Mv). The illumination was performed by a 100 W
(
0
Aldrich, distilled over P O and stored under Ar pressure) with halogen lamp at a xed power (i.e. tension 12 V) and with a xed
2
5
ꢁ1
.1 mol L (C
4
H
9
)
4
NClO
4
3
(Fluka, crystallized from CH OH and time of acquisition of the CCD (500 ms). Image color compo-
vacuum dried). Analysis was performed in a home-made three- sition was analyzed by Graphic Converter X soware.
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Scheme 1 Synthetic route to compound 1.
Scheme 2 Synthetic route to compound 2.
Thermal annealing
NTI (2) was obtained in nearly quantitative yield by forming
the imine bridge between compound 12, with an amino end
group and the formyl ended 14 (Scheme 2).
Firstly, 2-thiophenecarboxylic acid 7, was protected as
methyl ester (7a) in 80% yield and then stannylated to give
derivative 8 in 40% yield. A [Pd(0)] catalyzed Stille cross-
coupling reaction between 8 and the brominated 2,3-thienoi-
mide 6 gave the dimer 9 in 80% yield. The hydrolysis of the
methyl ester occurred by treating compound 9 with NaOH, and
Thermal annealing was performed under the optical micro-
scope using a heating stage Linkham TMHS600 connected to a
TP94 controller (control of 0.1 C).
ꢀ
X-ray diffraction analysis was carried out by means of a
PANalytical X'Pert diffractometer equipped with a copper anode
mean ¼ 0.15418 nm) and a fast X'Celerator detector. Step 0.05ꢀ
(
(
l
2 theta scale), counting time 400 s.
by reacting the resulting crude with SOCl
the ester, the strongly basic treatment also opened the thie-
noimide ring, so SOCl was used in order to dehydrate the
2
. Besides hydrolyzing
Results and discussion
2
intermediate and thus close again the thienoimide, giving
compound 10 with a semiquantitative yield. By using diphenyl
phosphoryl azide (DPPA), compound 10 was then converted
into its azide derivative, which underwent Curtius rearrange-
ment to the corresponding isocyanate. Since the reaction was
carried out in the presence of tert-butanol, the nal product of
this synthetic step was the corresponding tert-butyl carbamate
Synthesis
NTE (1) was prepared starting from (E)-1,2-di(thiophen-2-yl)-
ethene 4. This compound was synthesized according to an
already described procedure leading to the trans derivative as
20
majority product. Trans-4 was then bis-stannylated (nBuLi,
Bu SnCl) to give 5 in 98% yield. Subsequent coupling with
3
bromothienoimide 6 gives the target NTE in 57% isolated yield.
The corresponding H-NMR spectrum conrms the exclusive
presence of only one isomer.
11. This compound was obtained in 42% yield and converted to
1
compound 12 by TFA treatment (99% yield).
124 | J. Mater. Chem. C, 2015, 3, 121–131
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Table 1 Optoelectronics properties
a
ꢀ
ꢀ
Item
l
abs (nm)
l
em (nm)
E
ox (V)
E
red (V)
HOMO (eV) exp/th
LUMO (eV) exp/th
g
E (eV) exp/th
b
c
c
NTT
NTE
NTI
451
465
471
433
568
588
544
547
—
—
ꢁ5.69 /ꢁ5.77
ꢁ3.16 /ꢁ3.34
2.53/2.44
2.38/2.36
2.37/2.37
2.66/2.47
d
1.17, 1.41
1.28 , 1.43
1.46
ꢁ1.21
ꢁ5.54/ꢁ5.57
ꢁ5.65/ꢁ5.73
ꢁ5.83/ꢁ5.74
ꢁ3.16/ꢁ3.21
ꢁ3.28/ꢁ3.36
ꢁ3.17/ꢁ3.27
d
e
ꢁ1.09, ꢁ1.49
ꢁ1.20
e
NTA
a
b
c
Excitation at the maximum absorption wavelength. Taken from ref. 5a and corrected by CH
butyl and hexyl ends is expected. Half-wave potential. Peak potential at 0.1 V s
2
Cl
2
,/H
2
O liquid junction. No difference between
d
e
ꢁ1
.
2
achieved by (sp ) CC, CN and TT moieties. Regarding emission
spectra, NTE emission is red-shied with respect to NTT (588
nm vs. 568 nm), NTI and NTA compound. The quantum effi-
ciencies (PLQY) in solution were also measured. NTE and NTA
displayed comparable PLQY, i.e. 19% and 20%, respectively,
while a lower value was found for NTI (2%).
Scheme 3 Synthetic route to compound 3.
Conversely, dimer 14 was prepared starting from 2-thio-
Cyclic voltammetry (CV) analysis results are reported in
phenecarboxaldehyde that was rst protected and then Fig. 2c and Table 1. The voltammogram of NTE (Fig. 2c) shows
stannylated to give the organotin derivative 13 (details in ESI†). two quasi-reversible oxidation waves (1.17, 1.41 V vs. SCE) and a
Then, Stille cross-coupling between 13 and brominated thie-
reduction one centred at ꢁ1.21 V (details in ESI†). The
noimide 6 gave 14 in almost quantitative yield (98%, Scheme 2). replacement of ethylene by azomethine induces the shi of the
Finally, the two dimeric species 12 and 14 were coupled in rst oxidation wave that partially overlaps the second one (see
iPrOH in the presence of catalytic amounts of TFA to give the slower scan rate in Fig. S1, ESI†) generating an irreversible
target azomethine functionalized NTI in 90% yield. This process.
synthetic route generally led to the trans isomer. Accordingly,
On the other hand, the reduction wave shis to less negative
DFT calculations show a good match between experimental and potentials (ꢁ1.09 V) and a second reduction process appears
calculated HOMO and LUMO energy levels for the trans isomer into the electrochemical window (ꢁ1.49 V) indicating the
(Table 1).‡
possibility to host two electrons in the LUMO. Finally, NTA not
NTA, which is 3 with the ethinyl spacer, was synthesized by only shows an irreversible oxidation wave shied to the highest
Sonogashira coupling between 2-iodothiophene and trime- potential (1.46 V), but also a quasi-reversible reduction wave at
thylsilylacetylene followed by stannylation and Stille cross- the potential characteristic for the TBI moiety (about ꢁ1.2 V).
5c
coupling reaction between 15 (ref. 21) and the brominated
The HOMO energy levels decrease in the order NTE > NTI >
thienoimide 6 provided the target material NTA in 68% yield NTA. On the other hand, the LUMOs of NTE and NTA have
(
Scheme 3).
almost the same energy, but that of NTI is about 0.1 eV lower,
likely due to the electron withdrawing effect of the azomethine
moiety. In agreement with the spectroscopic data, the energy
Optoelectronic properties
gap (E ) of NTA is signicantly larger than that of NTE and NTI.
g
The electronic properties of the newly synthesized materials
were investigated by combined optical, electrochemical and
theoretical analysis and compared to those of a reference 2,3-
thieno(bis)imide bridge-free system having the same number of
thienyl rings, named NTT (Fig. 2a). The data are summarized
in Table 1.
Absorption spectra (Fig. 2b) show a red shi of the absorp-
tion maxima from 433 nm to 465 nm and to 471 nm on going
from NTA to NTE to NTI. On the other hand, the emission
maximum of NTE (588 nm) is strongly red shied with respect
to those of NTA (547 nm) and NTI (544 nm). Similarly, a strong
red shi of both absorption and emission is observed for NTE
and NTI in comparison to NTT (465 nm, 471 nm vs. 451 nm for
NTT) while a blue shi is observed for NTA with respect to NTT
To gain an insight on the optoelectronic properties, theo-
retical calculations were performed. Firstly, the most stable
molecular conformation and the HOMO and LUMO energy
levels and charge distribution of the new compounds were
investigated by density functional theory (DFT) calculations (see
Fig. 3).
6b
All the molecules adopt an almost planar conformation with
dihedral angles between the N-moiety and the inner thiophene
ꢀ
ring of about 160 and between the thiophene ring and the
ꢀ
ꢀ
inner spacer of about 180 (NTE) and 165 (NTA). While NTE
and NTA are centrosymmetric (C point group), NTI is non-
i
centrosymmetric because of the azomethine spacer. This results
in an asymmetric set of dihedral angles and in a charge distri-
bution of frontier molecular orbitals. In NTE and NTA, the
HOMO orbital is homogeneously distributed over the inner
conjugated thiophene rings, while the LUMO orbital extends
also to the electron decient thienoimide units, in agreement
with what is generally observed for N-ended systems as the
(433 nm vs. 451 nm). This can be likely ascribed to the lower
overlap with the p-conjugated system of the (sp) C–C than that
‡
HOMO and LUMO energy levels for the cis-isomer are blue shied by about 0.10
eV (ꢁ5.81 eV and ꢁ3.46 eV).
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large crystals. No continuous lms were observed in between
the crystals by AFM and laser confocal microscopy. As expected,
thin deposits obtained from high boiling point solvents, such as
toluene, show larger crystals than thin deposits obtained from
low boiling point solvents, such as chloroform or dichloro-
methane. All thin deposits exhibit crystalline microstructure
with strong birefringence under polarized optical microscopy
(POM) and an intense photoluminescence (PL). Interestingly,
concomitant polymorphs with markedly different uorescence
colour emission were clearly evident for all compounds.
When deposited from toluene, NTE forms a mixture of rod-
like and platelet-like crystals (Fig. 4a, red emitting and orange
emitting polymorphs, respectively). Rod-like crystals appear as
long crystallites (#100 mm) randomly distributed on the surface
with a width ranging from 5 mm to 10 mm. Platelet-like crystals
show a regular shape with a long axis from 20 mm to 100 mm.§
Both polymorphs exhibit a strong birefringence under POM
with the characteristic behaviour of optically anisotropic
materials with extinction at precise orientations (rod-like crys-
tals extinguish when the polarizers are oriented parallel or
perpendicular to the long axis of the crystal, while platelets-like
crystals extinguish when the polarizers are oriented along
parallel or perpendicular directions to the border of the crystal).
The evidence of light extinction at the same orientations in each
crystal implies that they are either single crystals or formed by
iso-oriented domains.
Fluorescence spectra performed by confocal microscopy
(Fig. 4, bottom) show a luminescence spectrum peaking at 640
nm for the rod-polymorph and two emission peaks at 560 nm
and 600 nm for the platelet polymorph. The polymorphic nature
of platelets- and rod-like crystals was conrmed by XRD analysis
carried out on samples specically prepared in different
conditions in order to strongly enhance the presence of one of
the two forms. Rod- and platelets-like crystals show different
Fig. 2 (a) Molecular structure of reference bridge free 2,3-thieno(bis)
6
b
ꢀ
ꢀ
ꢀ
imide system NTT (b) UV-vis and PL spectra of all compounds in patterns with the most intense peaks at 6.1 , 18.3 , 24.5 for the
ꢁ1
ꢀ
ꢀ
ꢀ
CH
2 2 2 2 4 9 4 4
Cl , (c) voltammograms in CH Cl 0.1 mol L (C H ) NClO of the
former and at 2.7 , 5.4 and 8.1 for the latter (the full list is
reported as ESI†).
ꢁ1
three molecules (1 mmol L ).
NTI deposits show needle shaped crystals (length < 500 mm)
randomly distributed on the surface. Their width ranges from
6
b
reference NTT compound. On the contrary, for the non-
symmetric NTI molecule, the LUMO is mainly localized on the
thienoimide unit of the NC-bound molecular branch. DFT
calculations, in agreement with experimental data, show a lower
band gap for the NTE and NTI with respect to NTT. On the other
hand, NTA shows a higher band gap and a signicant shi of
the HOMO level, likely due to a lower overlap of the sp carbons
with the p-conjugated system, while the LUMO levels are much
less affected by the unsaturated bridges since they are mainly
located on the 2,3-thienoimide groups. This behaviour is in
1
0 mm to 100 mm resulting in an aspect ratio (length/width)
between 1 : 5 and 1 : 50. Occasionally, some small aggregates
size < 2 mm) appear on the surfaces decorating the rod-like
(
crystals (Fig. 4b). Crystals exhibit a strong birefringence under
POM with the typical behaviour of optically anisotropic mate-
rials with extinction at precise orientations meaning that they
are either single crystals or formed by domains oriented in the
same direction.
The uorescent image shows a strong red emission peaked
at 660 nm from rod-like crystals, while the decorating aggre-
gates emit in the green region.
15a
16
good accordance to that observed for ethylene, azomethine
and acetylene bridged oligothiophenes without 2,3-thienoi-
mide end groups.
17
Furthermore, NTA lms show two polymorphs, i.e. ber-
like and rhombohedral crystals (Fig. 4c). The rhombohedral
crystals show extinction when the long axis of the crystal is
Thin deposits morphology, polymorphism and uorescence
Thin deposits of NTE, NTI and NTA, were grown by drop casting
ꢀ
oriented 45 with respect to the polarizers, typical behaviour
22
on silicon and glass surfaces in air. When deposited on a
surface, all compounds exhibited a strong propensity to form diluted solution.
§
Rod-like majority from the saturated solution, platelets-like majority from the
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Fig. 3 (a) Molecular structures with dihedral angles and (b) frontier orbitals isodensity plots and energy values of NTE, NTI, NTA.
of optically anisotropic materials. The ber-like crystals
The emission of all compounds in thin deposits was also
exhibit a strong birefringence, but they appear multicolored investigated by conventional PL spectroscopy indicative of the
and do not extinguish at any directions, indicating their macroscopic lm (Fig. S6, ESI†). According to the space resolved
polycrystalline nature. Both polymorphs exhibit a strong PL spectra (Fig. 4, bottom), red shied emission was observed

uorescence with two peaks at 535 and 575 nm for the crys- for NTE and NTI with respect to NTA. The PLQY for NTE and
tals and two maxima at 560 and 600 nm for the bundle of NTA thin deposits was 6% and 10%, respectively, i.e. lower than
bers. Time-resolved uorescence lifetime imaging those observed in solution, and 4% for NTI.
measuring emission in the 505–535 nm, 565–605 nm and DFT calculations were exploited to tentatively explain the
35–675 nm ranges conrmed that the self-assembled uo- origin of the observed polymorphism. In particular, the
rophores experienced locally different environments in the possibility of conformational polymorphism was considered.

19
6
different morphologies (details in ESI†).
However, the conformation energy plot computed by varying
the dihedral angle (q) between the thienoimide moiety and the
ꢁ1
Fig. 4 Fluorescence confocal images of the films of NTE, NTI, and NTA obtained upon evaporation of 0.5 mg mL toluene solutions on glass.
Top: images are a merge of the fluorescence collected on three PMT with bandpass filters 525/25 nm, 595/50 nm and 700/70 nm. Bottom:
confocal fluorescence spectra collected for ROI indicated by the circles.
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Table 2 Overlap integrals computed for dimer pairs, 4 A˚ -spaced
perpendicular to molecular axis. Data reported in absolute values and
meV
Item
S
h
S
e
NTT
NTE
NTI
707
702
599
688
767
381
588
283
NTA
ꢀ
present an endothermic peak at ꢃ120 C, corresponding to the
solid-solid transition at the platelet-like crystal phase. The
transition was irreversible and no morphological changes were
observed in the shape of the crystals but the POM was consis-
tent with the presence of a polycrystalline compound (Fig. 6b).
The structural evolution associated to the thermal treatment
was investigated by XRD that conrmed the evolution from one
polymorph into the other (Fig. 6c).
Fig. 5 XRD profile for rod-like (red) and platelet-like crystals (orange).
inner thiophene ring (Fig. S2, ESI†) shows two minima corre-
sponding to the ANTI and the SYN conformers with a small
ꢁ
1
energy difference (z2 kcal mol ). This, in combination with
the already observed crystallization of N-based molecules in
both ANTI and SYN forms, suggests that the observed
concomitant polymorphs might be due to the crystallization of
both conformers.
Moreover, calculations were exploited to investigate the
effect of the aromaticity variation, induced by the unsaturated
bridge substitution, on molecular p-stacking, through the
computation of the overlap integrals S and S for HOMO and
h e
LUMO orbitals, respectively. In Table 2, the overlap integrals
˚
computed for a molecular pair, 4A-spaced perpendicular to
molecular axis, are reported for NTT and the bridge substituted
derivatives. A dramatic decrease in the LUMOs overlap is
observed for all the substituted molecules with respect to NTT.
In particular, as expected, NTA, having the acetylenic spacer and
the most reduced aromatic character, shows the worst LUMO
overlap. A reduced effect is observed for HOMOs overlaps that,
however, are lower with respect to NTT. These preliminary
calculations conrm that decreasing the aromaticity of the
molecules may cause a decreasing of the intermolecular inter-
actions. Since all the structures observed for symmetric N-based
oligomers show strong p–p interactions, we will further inves-
tigate if the decreased aromaticity could lead to a different
packing motif.
Thermal effects in thin deposits
The behaviour of thin deposits upon thermal annealing was
explored for all compounds.
Fig. 6a and b show the uorescence microscopy images of
ꢀ
NTE lms before and aer the thermal cycle RT / 200 C /
RT.
Upon thermal annealing, the two polymorphs of NTE exhibit
a different behaviour. While the platelet-like crystals persisted
till the melting point (z260 C, DSC, ESI Fig. S7†), the rod-like
crystals showed an irreversible change of the emission, that
progressively changed from red to orange, which corresponded
to the emission of platelet-like crystals. The transition is
ꢀ
Fig. 6 Thin deposits observed by fluorescence microscopy for NTE.
(
(
a) Fluorescence microscopy image of a drop cast thin deposit of NTE
ꢁ1
toluene 1 g L on Si/SiO
2
wafer); the bar scale is 100 mm. (b) The same
ꢀ
image after thermal treatment at 200 C for 30 s. (c) XRD profile of the
same sample before (A) and after thermal cycle (B). The B curve is that
observed also in the DSC curve of the rod-like crystals which of the platelet-like crystals shown in Fig. 5.
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The same approach was also used for a NTA thin deposit
ꢀ
which was heated up to 200 C and then cooled down to room
temperature. Upon this treatment, while the morphology and
emission features of rhombohedral crystals remained appar-
ently almost unaltered, the ber-like crystals exhibited an irre-
versible change of the PL, which switched irreversibly from
yellow to green (Fig. 7).
Although we were not able to obtain thin deposits with only
one form, XRD were performed on NTA thin deposits at room
ꢀ
temperature and aer the thermal cycle RT / 200 C / RT
(Fig. S9, ESI†). The XRD proles show signicant differences, in
particular, the disappearing of some reections aer the
thermal cycle support, the polymorphic conversion observed by
optical microscopy.
Finally, azomethine NTI did not show detectable uores-
cence differences upon thermal treatment before the melting
Fig. 8 Quantitative analysis of the fluorescence images. A commercial
CCD recorded fluorescence images. The colours of the graphs
correspond to the colour components of the CCD (red, blue, green).
point. However in POM, a solid state transition is observed for The figure shows the number of counts versus temperature obtained
the rod-like crystals, which is consistent with the solid-solid using a filter Ex 420, DM 435, BA 475, which enables the transmission
ꢀ
of all visible components except a slight portion of the blue.
transition at 170 C, observed in the DSC curve.
Time temperature integrator (TTI)
function of the temperature at which the device was exposed for
The polymorphism of NTE was exploited for the fabrication of a at least 30 s. Each image was recorded xing the intensity of
8,14
TTI device, monitoring the evolution of temperature-depen- illumination and the time of acquisition of the CCD (see details
dent uorescence properties.
in the method section). Upon heating the thin deposit at
A commercial CCD recorded uorescence images during the different temperatures (the devices were tested upon several
thermal treatment. Fig. 8 shows the number of counts (NCs) of cycles), the crystals were stable and changed their emission
the red, green and blue component recorded by the CCD as a colour at the highest temperature experienced for more than 30
s. This colour was retained aer cooling down the thin deposit
to room temperature (Fig. 8).
The quantitative analysis of the CCD counts of PL images
shows that for each temperature, the colour recorded by the
CCD is formed by a unique and dened combination of red,
green and blue (RGB) components. Therefore, the temperature
at which the device was exposed could be analytically derived
from the colour composition of the PL image. This is the
principal requirement of a TTI based on colour changes. It
ꢀ
should be noted that for NTE in the range of RT-120 C (i.e.
below the transition temperature between the two polymorphs,
therefore independent from polymorphism), higher sensitivity
ꢀ
is obtained from the red component with an accuracy of ꢄ5 C.
ꢀ
On the other hand, for temperatures higher than 120 C, i.e.
above the transition temperature of the polymorphs, the best
accuracy can be extracted from the blue component with an
ꢀ
accuracy of <ꢄ2 C, i.e. 2 times better than the interval RT-120
ꢀ
C. This behaviour suggests that the sensitivity to temperature
exposure in TTI devices based on this class of materials can be
improved by polymorphic transition and therefore tailored by
chemical design.
Conclusions
In conclusion, we have reported the synthesis and the applica-
tion in a TTI device of new thieno(bis)imide materials bearing
unsaturated ethylene, azomethine and acetylene inner bridges.
The optoelectronic and morphological properties were investi-
gated and compared to those of a bridge-free isostructural
Fig. 7 Thin deposits observed by fluorescence microscopy of NTA. (a)
Fluorescence microscopy image of a drop cast thin deposit of NTA
ꢁ
1
(
toluene 1 g L on Si/SiO
2
wafer); the bar scale is 100 mm. (b) The same
ꢀ
image after thermal treatment at 200 C for 30 s.
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that the energy gap of ethylene and azomethine based
compound NTE and NTI is lower than that of acetylene based
NTA. This was ascribed to the lower overlap of the C–C sp bond
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and is in good accordance to what was observed for N-free
systems. While the HOMO levels decrease in the order NTE >
NTI > NTA, the LUMOs of NTE and NTA have almost the same
energy. However, a slightly lower LUMO energy was found for
NTI likely due to the electron withdrawing effect of the azo-
methine moiety.
Thin deposits obtained by solution casting present high
crystallinity for all compounds showing concomitant poly-
morphs. Indeed, the simultaneous presence of crystals with
different morphology and uorescence was detected by optical
microscopy for NTE and NTA samples. XRD conrms the poly-
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structure. Nevertheless, successful control of the crystal phase
was achieved by thermal treatments of the lms. Indeed, space
resolved uorescence spectra, DSC and XRD analysis conrm
that thermal heating allows us to irreversibly convert one
polymorph into the other. Exploiting such features, we
produced a TTI device able to monitor thermal changes of the

lm by optical microscopy.
This work introduces a chemical design strategy to tune the
optoelectronic properties and polymorphism attitude of small
molecules usable for TTI devices. Concomitant polymorph
formation combined with the possibility of controlling the
polymorph type and solid state uorescence, may enable the
development of new organic devices and sensors based on
functional polymorph conversion.
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