Regioisomers of Organic Semiconducting Dumbbell-Shaped Molecules: Synthesis and Structure-Properties Relationship

Article abstract of DOI:10.1002/ejoc.202100473

Two new dumbbell-shaped molecules based on two solubilizing and structuring triazatruxene (TAT) units linked by a central chromophore were synthesized and studied. The central chromophore was an electro-deficient fluorene-malononitrile (FM) unit, that can be functionalized symmetrically on two different positions, giving rise to two positional isomers, called TAT-pFM and TAT-mFM, when the TATs are connected to the 2,7- and 3,6-positions, respectively. The two isomers exhibited different electronic conjugation pathways that drastically affect their absorption properties and energy levels. Moreover, while TAT-pFM was organized in a stable 3D mesomorphic structure from room-temperature to the melting point, TAT-mFM remained crystalline and decomposed before melting. Finally, despite a lower hole mobility, the TAT-mFM exhibited the highest Power Conversion Efficiency (PCE) of about 2 % in organic solar cells. This higher PCE was attributed essentially to the pronounced internal charge transfer band contribution to the charge photogeneration observed in TAT-mFM solar cells.

Full text of DOI:10.1002/ejoc.202100473

European Journal of Organic Chemistry
Accepted Article
Accepted Article
Title: Regioisomers of organic semiconducting dumbbell-shaped
molecules: synthesis and structure-properties relationship
Authors: Chaima Mahmoudi, Ibrahim Bulut, Jiang Jing, Sadiara Fall,
Benoît Heinrich, Stéphane Méry, Thomas Heiser, Patrick
Lévêque, Emilie Steveler, Mustapha Majdoub, and Nicolas
Leclerc
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content of this Accepted Article.
To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.202100473
Link to VoR: https://doi.org/10.1002/ejoc.202100473
01/2020

10.1002/ejoc.202100473
European Journal of Organic Chemistry
Regioisomers of organic semiconducting dumbbell-shaped
molecules: synthesis and structure-properties relationship
Chaima Mahmoudi,[a,b] Ibrahim Bulut,[a] Jiang Jing,[c] Sadiara Fall,[c] Benoît Heinrich,[d] Stéphane
Méry,[d] Thomas Heiser,[c] Patrick Lévêque,[c] Emilie Steveler,[e] Mustapha Majdoub,[b] Nicolas
Leclerc*[a]
[a] C. Mahmoudi, Dr. I. Bulut, Dr. N. Leclerc
Institut de Chimie et Procédés pour l’Énergie, l’Environnement et la Santé (ICPEES), UMR
7515-CNRS, Université de Strasbourg, ECPM, 25 rue Becquerel, 67087 Strasbourg, France. E-
mail: leclercn@unistra.fr; https://icpees.unistra.fr/ingenierie-des-polymeres/synthese/;
@N_Leclerc_Org
[b] C. Mahmoudi, Dr. M. Majdoub
Laboratoire des Interfaces et Matériaux Avancés (LIMA), Faculté des Sciences de Monastir
(Université de Monastir), Bd. De l'Environnement, 5019 Monastir, Tunisia.
[c] Dr. S. Fall, Dr. T. Heiser, Dr. P. Lévêque
Laboratoire ICube, UMR 7357-CNRS, Université de Strasbourg, 23 rue du Loess, 67037
Strasbourg, France.
[d] Dr. B. Heinrich, S. Méry
Institut de Physique et de Chimie des Matériaux de Strasbourg (IPCMS), UMR 7504-CNRS,
Université de Strasbourg, 23 rue du Loess, 67034 Strasbourg, France.
[e] Dr. E. Steveler
Laboratoire ICube, UMR 7357-CNRS, INSA Strasbourg, 24 Boulevard de la victoire, 67084
Strasbourg Cedex, France.
Abstract
Two new dumbbell-shaped molecules based on two solubilizing and structuring triazatruxene
(TAT) units linked by a central chromophore were synthesized and studied. The central
chromophore was an electro-deficient fluorene-malononitrile (FM) unit, that can be
functionalized symmetrically on two different positions, giving rise to two positional isomers,
called TAT-pFM and TAT-mFM, when the TATs are connected to the 2,7- and 3,6-positions,
respectively. The two isomers exhibited different electronic conjugation pathways that
drastically affect their absorption properties and energy levels. Moreover, while TAT-pFM was
organized in a stable 3D mesomorphic structure from room-temperature to the melting, TAT-
mFM remained crystalline and decomposed before melting. Finally, despite a lower hole
mobility, the TAT-mFM exhibited the highest Power Conversion Efficiency (PCE) of about 2%
in organic solar cells. This higher PCE was attributed essentially to the pronounced internal
charge transfer band contribution to the charge photogeneration observed in TAT-mFM solar
cells.
Introduction
Organic semiconducting (OSC) materials have experienced a dramatic increase of interest in
the last three decades thanks to their many interesting properties for optoelectronic
[1-4]
applications.
Several hundreds of articles are published every year that describe the
design, synthesis and characterization of new OSC small molecules and polymers. Considering
the enormous impact of electronic conjugation and self-assembly features on the
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10.1002/ejoc.202100473
European Journal of Organic Chemistry
optoelectronic properties of these solid-state materials, it is obvious that regioisomers of a
same molecular structure are likely to exhibit very different properties. Surprisingly, so far,
the impact of regioisomers on the OSC material properties has not been much addressed as
compared to other (macro-)molecular architectural features. Regioisomers arise essentially
from variations in the conjugated backbone or in the side chains. Regarding the latter, by way
of example, Lincker et al. reported the influence of the side-chains grafting positions on
[5]
thiophene units used as end-capper of liquid crystalline small molecules.
They clearly
highlighted that both molecules exhibit different polymorphisms and charge transport
properties, with a significant increase of the hole mobility (as measured by organic field-effect
transistor (OFET)) for the regioisomer exhibiting the largest number of structural phases.
Concerning the isomers of conjugated backbone, the regioisomers essentially arise from a
variation of the bond positions between aromatic units, of the location of the heteroatoms in
the conjugated core and of the sequence of aromatic units in extended fused-ring building
[6-
blocks. Spirobifluorene (SBF) is a chemical structure known to afford several regioisomers.
8] In particular, some publications have reported the study of the regioisomers resulting from
the ortho or para linkage between the SBF unit and the conjugated backbone. Thus, Bulut et
[9]
al. reported the two regioisomers with the SBF used as end-capper,
while Yang et al.
implemented the SBF unit as a conjugated bridge between two perylene diimide (PDI) units.
[10]
In both cases, the solid-state structures and optoelectronic properties are significantly
changed from one isomer to the other, resulting in substantially different device performance.
Finally, very few examples have been reported studying the influence of the different isomeric
structures on the solid-state structures and optoelectronic properties of more or less fused
extended conjugated molecules. [11-13] One can cite the recent work reported by Jia et al. that
describes the synthesis and study of three isomers leading to new extended fused-ring non-
[14]
fullerene acceptor (NFA) molecules.
By resolving the single crystal structure of the three
isomers, the authors noticed a huge difference in the solid-state packing arrangement, as one
isomer gives a triangular arrangement, the second one an interlinking shackle pattern and the
third one exhibits a linear head-to-tail stacking. Combined with slight variations in the energy
levels, the best power conversion efficiencies (PCEs) of these NFAs range from 9 to 12%,
considering the same electron-donor polymer.
Recently, we reported a new family of solution processable OSC molecules based on the use
of the planar triazatruxene (TAT) unit. This family of materials is characterized by its dumbbell
shape with two TAT units bridged together by an elongated π-conjugated core, also called
central dye. We highlighted that the TAT units act as structuration platforms in thin films while
the optoelectronic properties are mainly driven by the central dye composition. [15] Therefore,
by adjusting the nature of the central dye and the solubilizing side chains grafted on the TAT,
allows a fine tuning of the optical parameters and energy levels on one side, and of the self-
assembling properties on the other side. This two-level fine-tuning enabled at the end to
achieve high charge transport and/or photovoltaic performances. [16-18]
Herein, we report new TAT-based dumbbell-shaped molecules, containing a 9-
(dicyanomethylene)fluorene electron deficient central unit. Two regioisomers of this
molecule could be produced by connecting the TAT units to the 3,6- or 2,7 positions of the
fluorene-malonitrile (FM) core. These two molecules are found to show large variations in
physico-chemical and optoelectronic properties that can be explained by the different
conjugation pathways (through the meta- or para-linkage), as well as quite different solid-
state structures.
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10.1002/ejoc.202100473
European Journal of Organic Chemistry
Results and Discussion
The synthesis strategy of both TAT-mFM and TAT-pFM is illustrated in Scheme 1. Synthetic
procedure details can be found in the experimental section. 3,6-dibromofluorenone and 2,7-
dibromofluorenone have been synthesized by following procedures reported in the literature.
[19,20] The 2-tetramethyldioxoborolane TAT has been functionalized with ramified 2-ethylhexyl
side-chains in order to ensure enough solubility to the final compounds. It has been
synthesized by following an already reported procedure. [17]
Both molecules have been obtained through a Suzuki–Miyaura cross-coupling between the
monoboronated triazatruxene moiety and the corresponding dibromofluorenone units,
followed by a Knoevenagel condensation between the ketone groups (compounds 1 and 2)
[21]
and an excess of malononitrile.
All compounds were characterized by standard NMR
spectroscopy. However, despite the use of ramified side chains on the TAT units, both final
molecules are poorly soluble in organic solvents, in particular at room temperature.
1
Consequently, if H NMR at high temperature gave nice resolved spectra, it has not been
possible to record readable 13C NMR spectra. Therefore MALDI-TOF spectrometry has been
used to confirm the final molecules.
Scheme 1. Synthetic routes towards the preparation of TAT-based dyes.
DFT and TD-DFT calculations have been performed, using Gaussian 09 software, in order to
support the experimental characterizations. Details regarding calculation methods and
functional used can be found in the Supporting information.
A first geometry optimization has been performed through the minimization of energy (Table
S1 and S2). Interestingly, both molecules adopt a cis-cis geometry (see Figure S1 in SI) in which
both dihedral angles, between the central FM unit and the TAT moieties, are equivalent and
minimum, ca. around 32-35°.
The electrochemical properties of TAT-mFM and TAT-pFM have been investigated by cyclic
voltammetry (CV) in CH2Cl2 (Figure 1). Potentials are given versus a saturated calomel
electrode (SCE). The ionization potentials (IP or HOMO level) and electron affinities (EA or
푟푒푑
LUMO level) were calculated using the following formulas: EA(eV) = 퐸
+ 4.4; IP(eV) =
표푛푠푒푡
표푥
퐸
표푛푠푒푡
+ 4.4. These formulas are based on the assumptions that the energy level of SCE relative
[22]
to vacuum is 4.4 eV as suggested by Jenekhe and coworkers.
Ferrocene was used as
internal reference for calibration. The voltammograms display several well-defined and
reversible waves, which can be assigned, by comparison with reference to isolated synthons,
to the constitutive molecular fragments of the compounds.
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10.1002/ejoc.202100473
European Journal of Organic Chemistry
Figure 1. Top. Cyclic voltammograms of TAT-mFM (left) and TAT-pFM (right) (CH2Cl2 + Bu4NPF6 0.2 M); Sweep-rate: 150 mV.s-1, platinum wire
working electrode). Fe* corresponds to the redox wave of ferrocene. Bottom. Frontier molecular orbitals LUMO and HOMO and CV extracted
energy levels for TAT-mFM (left) and TAT-pFM (right).
As previously observed, the TAT fragment is not electroactive in the anodic window, whereas
the central FM moiety display two reversible monoelectronic reduction waves, corresponding
to the reduction of each nitrile group, as seen usually in other 9-(dicyanomethylene)fluorene
derivatives. [23, 24] The TAT-mFM exhibits positively shifted reduction processes as compared
to the TAT-pFM derivative, leading to a slightly stabilized EA of -3.7 eV vs -3.6 eV, respectively.
In oxidation, both compounds exhibit three reversible monoelectronic waves, in agreement
with previous reported results. [16] Similarly to the cathodic processes, the TAT-mFM exhibits
positively shifted oxidation processes as compared to the TAT-pFM derivative. Consequently,
HOMO levels are calculated at -4.9 and -4.7 eV for TAT-mFM and TAT-pFM, respectively. The
slightly larger difference between the HOMO levels, compared to the LUMO levels, can be
explained in part by the large difference in electronic delocalization of each HOMO and LUMO
level, as calculated by TD-DFT (Figure 1 bottom). Indeed, in the TAT-pFM molecule, thanks to
the para-linkage, the HOMO is fully delocalized on the overall molecule (although the electron
density is higher on TAT units), while in the TAT-mFM, the HOMO level is almost entirely
distributed on the TAT units only. In contrast, the LUMO level is in both cases, mainly centered
on the FM unit.
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European Journal of Organic Chemistry
UV-visible absorption has been performed in chlorobenzene (CB) solution and as thin-films.
The absorption spectra are given in Figures 2 and 3, and the peak maxima of the energy
transitions are reported in Table 1.
Table 1. Electrochemical and optical parameters of TAT-mFM and TAT-pFM.

max (103
M-1.cm-1)
[ICT
Eg-opt
(eV)
Eox (V) [HOMO (eV)]a ERed (V) [LUMO (eV)]a
λmax [λonset] (nm)b
]
Solution 326 ; 364 ; 589 [725] 1.71
Film 323 ; 364 ; 581 [770] 1.61
Solution 320 ; 395 ; 665 [780] 1.57
Film 322 ; 399 ; 640 [890] 1.39
140 [27]
TAT-mFM 0.52 ; 0.70 ; 1.19 [-4.9] -0.69 ; -1.23 [-3.7]
TAT-pFM 0.33 ; 0.61 ; 1.10 [-4.7] -0.78 ; -1.32 [-3.6]
158 [3.7]
a. Electrochemistry has been performed in dichloromethane. b. Solution absorption measurements have been
performed in dichlorobenzene solvent.
In solution, the absorption spectra of the two molecules display three maxima. Two of them
are located below 450 nm and the third one appears between 500 and 800 nm. Thanks to TD-
DFT calculations, the transitions associated to the different bands can be attributed.
Figure 2. Left. UV-vis absorption spectra in CB solution (full line) and calculated (dashed line) of TAT-mFM (blue) and TAT-pFM (red). Inset.
Zoom on the ICT bands. Right. Representation of the energy levels and the main molecular orbitals involved in the electronic transitions of
TAT-mFM and TAT-pFM.
For TAT-mFM, the calculated UV-visible spectrum exhibits only two bands, but the first one
results from the merging of the two first bands displayed in the experimental spectrum (see
Table S3 reporting the calculated optical transitions for each compounds). Thus, the first high
energy band, centered around 325 nm, can be mainly attributed to a transition from the
HOMO-3 localized on the TAT units to the LUMO+2 localized on the full molecule. The second
band, centered on 375 nm, is mainly assigned to a charge transfer transition from the HOMO
orbital localized on the TAT units to the LUMO+1 level localized mainly on the FM group.
Finally, the band at higher wavelength with a maximum at 590 nm, corresponds to an ICT
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10.1002/ejoc.202100473
European Journal of Organic Chemistry
transition between the HOMO and LUMO levels. It is worth noting that this low energy ICT
band is relatively intense, as it exhibits an extinction coefficient of 2.7 104 M-1.cm-1. This
transition is geometrically favored by the fact that the dicyanomethylene group is located in
para position from the TAT unit substitutions, leading to an efficient conjugation between the
TAT electron donating groups and the electron deficient dicyanomethylene fragment.
In contrast, for TAT-pFM which also displays three bands, the ICT transition exhibits a very low
intensity with a molar extinction coefficient limited to 3750 M-1.cm-1. This ICT band is also
significantly shifted towards longer wavelengths, the maximum being around 655 nm. The
very low intensity of this band is explained by the TAT substitution in meta position as regards
to the dicyanomethylene unit, greatly reducing the conjugation between electron rich and
electron poor fragments. This meta substitution however, leads to an extension of the
electronic conjugation between the TAT units through the fluorene core, which translates into
the overall bathochromic shift of the bands. The first bands at lower wavelengths are
attributed to several transitions between energy levels of type HOMO-1 and HOMO-2 to
excited energy levels above the LUMO (see Table S3). The transition centered at 400 nm
corresponds mostly to a π-π* transition between HOMO-2 and LUMO+1.
Figure 3. UV-vis spectra in thin-films of TAT-mFM (blue) and TAT-pFM (red).
The main characteristics of the absorption spectra in solution are found in the solid-state film
spectra. However, a slight bathochromic shift of the onset of absorption can be noted for TAT-
mFM (760 nm) and TAT-pFM (890 nm) leading to calculated optical bandgaps, measured from
the absorption onsets, of 1.63 eV and 1.39 eV, respectively.
A combination of differential scanning calorimetry (DSC), powder X-Ray diffraction/scattering
(PXRD/S) and polarized optical microscopy (POM) allows the comparison of the thermal and
structural properties of both molecules. By DSC, TAT-pFM exhibits clear melting and
crystallization transition peaks at 275°C and 248°C, respectively. However, PXRD/S
measurements, combined with POM, clearly highlight the mesomorphic nature of this dye.
Actually, at room temperature, TAT-pFM is frozen in a 3D mesomorphic solid state, that
gradually flows to a fluid and birefringent mesophase on heating and finally melts into the
isotropic liquid at about 270°C (Figure S2). In contrast, TAT-mFM is highly crystalline and
decomposes before melting above 300°C (Figure S3).
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European Journal of Organic Chemistry
Charge carrier mobilities have been determined by using commercially available bottom-
gate/bottom contact (BG/BC) OFETs. The detailed elaboration and characterization procedure
of devices can be found in the supporting information (see also Figures S4-S6 for output
characteristics). Hole mobility values have been extracted from the linear regime. Data are
summarized in the Table 2. The mesomorphic organization in TAT-pFM turned to be more
favorable for the charge-carrier mobility as this para-derivative exhibits an almost one order
of magnitude higher mobility than the crystalline TAT-mFM. Overall though, the OFET hole
mobilities obtained remain lower than for other previously reported TAT-based dumbbell-
[16,18]
shaped molecules.
These moderate mobility values are partly attributed to the
dicyanomethylene unit which drastically lower the solubility of the dyes and lead to
inhomogeneous thin-films.
Table 2. hole mobilities and OPV parameters of TAT-mFM and TAT-pFM.
µ
h+ (cm2.V-1.s-1)
2.5x10-6
Voc (V)
0.95
0.59
Jsc (mA.cm2)
5.41
FF (%)
38.1
47.0
PCE (%)
1.95
0.83
TAT-mFM
TAT-pFM
1.5x10-5
2.99
Interestingly, the processing of these two molecules are improved when they are blended
with PC71BM in chloroform using diiodooctane (DIO) as an additive. Thus, with more
homogeneous thin films, the photovoltaic properties of these new dyes could be investigated
in the following reversed configuration: ITO/PEIE/Active layer/MoOx/Ag. After a careful
optimization, we found the best D:A ratio to be 1:1.5 for both molecules. Optimal power
conversion efficiencies (PCEs) were found by using DIO as additive in wt% of 0.2 and 0.3% for
TAT-pFM and TAT-mFM, respectively. Despite its lower hole mobility, TAT-mFM is the most
performing dye of this study with almost 2% of PCE in average (Table 2). Except FF, all
photovoltaic parameters are higher for TAT-mFM devices (Table 2 and Figure 4). In line with
its deeper HOMO level, this dye shows the best Voc with a value as high as 0.95 V. It should be
noticed that between the two dyes, the Voc improvement for TAT-mFM devices is much higher
(0.36 V) than the measured HOMO offset (0.2 eV). This difference may be due to the presence
of efficient free-carrier traps in TAT-pFM devices that should lower significantly the Voc value
[25,26]
compared to the expected one based on the frontier energy-level position.
This
hypothesis is consistent with the current density in TAT-mFM devices which is almost twice
the one measured for TAT-pFM.
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10.1002/ejoc.202100473
European Journal of Organic Chemistry
Figure 4. J(V) curves under illumination (full line) and under dark (dashed line)
of TAT-mFM (blue) and TAT-pFM (red).
This significantly better Jsc could also be partly explained by the higher absorbance of the TAT-
mFM molecule in the ICT range as compared to the TAT-pFM dye. In order to validate this
point, Figure 5 shows a comparison of the UV-vis absorption spectra of thin films of each of
the dyes pure and mixed with PC71BM as well as the EQE spectra measured on organic solar
cells. The absorption spectra of the mixtures were measured directly on the previously
characterized solar cells. The active layers have the same thickness of about 90-100 nm.
Comparison of the absorption spectra of the electron-donor molecules and the mixtures
reveals the contribution of PC71BM to the light absorption. Indeed, while the thin films
absorption spectrum of TAT-mFM and TAT-pFM are quite different in solid-state (Figures 3
and 5) the two blend spectra in solid-state are very similar (Figure 5). The difference between
the two blends comes from the absorption intensity which is higher for the TAT-mFM-based
blend between 450 and 650 nm compared to TAT-pFM-based blend. This difference is
consistent with the position of the ICT absorption band of TAT-mFM which overlaps with the
absorption of PC71BM in the region between 500 and 700 nm. In the same wavelength-range,
TAT-pFM hardly absorbs at all. Finally, the EQE clearly show that the charge photogeneration
mainly occurs in both blends between 400 and 700 nm. In this area the EQE of the TAT-mFM
is approximately twice the one of TAT-pFM. Considering that both blends have the same
PC71BM content and approximately the same thickness, one can conclude that the main
difference in between both dyes is originating from the highest ICT absorption band in the
TAT-mFM material, which contributes significantly to the photocurrent and to the presence
of charge-carrier traps in TAT-pFM blends that hinder charge-extraction.
Figure 5. Comparison of absorbance spectra of active layers (full line),
pure electron donor dye films (dash and dot line) and EQE (dashed line)
for TAT-mFM (blue) and TAT-pFM (red).
Conclusion
In summary, two regioisomers of a donor-acceptor-donor (D-A-D) triad have been synthesized
and characterized. They are made of two electron-rich triazatruxene (TAT) units connected to
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10.1002/ejoc.202100473
European Journal of Organic Chemistry
a central electron-deficient fluoren-9-ylidene malononitrile (FM) core, via a grafting either at
para (2,7-fluorenyl, TAT-mFM) or meta (3,6-fluorenyl, TAT-pFM) positions.
On a first hand, the two regioisomers exhibit different electronic conjugation pathways that
drastically affect the absorption properties and energy levels of the molecules. Actually, in this
series, the architectural key parameter is the conjugation pathway that connects the TAT
donor units to the dicyanomethylene acceptor moiety of the FM core. Thus, in TAT-mFM, the
meta substitution enables a good conjugation between the donor and acceptor units that
exacerbates the ICT band, responsible for the higher absorption, and deepens the HOMO
level. TD-DFT calculations are in very good agreement with the experimental data and
highlight the strong HOMO-LUMO transition of these ICT bands.
On the other hand, the two regioisomers present quite different structural properties. Thus,
while TAT-pFM exhibits a stable 3D mesomorphic organization from room-temperature to the
melting, TAT-mFM remains crystalline and decomposes before melting.
Finally, despite a lower hole mobility, the TAT-mFM exhibits the highest PCE of about 2 %
essentially from the positive ICT band contribution to the charge photogeneration. Overall,
these results contribute to a better understanding of architectural key parameters in such D-
A-D dumbbell-shape conjugated molecules, and highlight the insufficiently systematically
studied influence of regioisomers on optoelectronic properties.
Experimental Section
General Methods.
Synthesis. All reactions were performed under an atmosphere of dried argon using standard
Schlenk tube techniques. All chemicals were used as received from commercial sources unless
stated otherwise. Chromatographic purifications were performed using silica gel (40-63 µm).
TLC was performed on silica gel plates coated with fluorescent indicator.
1
NMR analysis. H NMR (400 MHz) and 13C NMR (100 MHz) spectra were recorded at room
temperature and at high temperatures (90°C) on a Bruker Advance III HD 400 MHz
spectrometer.
MALDI-TOF analysis. The mass molar MALDI-TOF spectrometry was given by Bruker Daltonics
flex Analysis.
UV-Visible Spectroscopy. Absorption spectra in solution and in thin films were recorded on a
Shimadzu UV-2600 spectrophotometer. In solid state, the absorption spectra were measured
on thin films drop-casted on glass substrates from a 0.5 mg/mL chloroform solution of organic
dyes.
Electrochemical measurements. Cyclic voltammetry (CV) was recorded with a VSP BioLogic
potentiostat/galvanostat with a scan rate of 100 mV/s using 0.1 M tetrabutylammonium
tetrafluoroborate (Bu4NBF4) in CH2Cl2 as supporting electrolyte, an Ag/AgCl electrode as the
reference electrode, a Pt wire electrode as working electrode, a Pt wire as counter electrode
and ferrocene/ferrocenium (Fc/Fc+) as a reference. The cell was briefly deoxygenated with
argon before each scan.
Differential Scanning Calorimetry. DSC measurements were performed with TA Instruments
Q1000 instrument, operated at scan rate of 5°C/min on heating and on cooling.
Thermogravimetric analysis. TGA measurements were carried out on a SDTQ 600 apparatus
at scanning rate of 10 °C/min. A differential thermal analysis (DTA) signal was simultaneously
measured.
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European Journal of Organic Chemistry
Powder X-Ray diffraction/scattering. PXRD/S measurements were performed with a
transmission Guinier-like geometry. A linear focalized monochromatic Cu Kα1 beam (λ = 1.5405
Å) was obtained using a sealed-tube generator (600 W) equipped with a bent quartz
monochromator. The samples were filled in 1 mm Lindemann capillaries. The temperature
was controlled within ±0.01 °C and exposure times were of 5 h (20°C) or 1 h (110°C). The
patterns were recorded with a curved Inel CPS120 counter gas-filled detector.
Computational methods. All calculations of the compounds were performed using the
Gaussian 09 software package1 and Gauss View 5.0 molecular visualization program Package.
The theoretical simulations were carried out using density functional theory (DFT) with B3LYP
(Becke three-parameter Lee–Yang–Parr) exchange-correlation functional.2 6-31G(d,p) was
used as a basis set for all atoms (C, N, H, O). Therefore, in this work, TD-B3LYP method has
been used to simulate the electronic absorption spectra and the orbitals molecular. It is
important to consider the solvent effect on theoretical calculations when seeking to predict
the experimental spectra with a reasonable accuracy. The oscillator strengths and excited
state energies were investigated using TD-DFT calculations on the fully DFT optimized
geometries.
Organic Field Effect Transistors. Bottom contact bottom gate OFET structures were fabricated
using commercially available silicon substrates. Lithographically defined Au (30 nm)/ITO (10
nm) bilayers were used as source and drain electrodes and 230 nm thick SiO2 layer as a gate
dielectric. Channel length and width were L = 20 µm and W = 10 mm, respectively. Substrates
were cleaned consecutively in ultrasonic baths at 45°C for 15 min each step using soapsuds,
acetone, and isopropanol and followed by 15 min UV-ozone treatment. Then substrates were
transferred into a nitrogen filled glove box where hexamethyldisilazane (HMDS) was
spincoated on top of the SiO2 followed by annealing at 135°C for 10 min. Finally, solutions of
pure TAT-mFM and TAT-pFM were spincoated substrates to complete the FET devices. For
each molecule, a concentration of 4 mg/mL in CHCl3 was used. Prior to characterization,
completed devices were dried under high vacuum (≈10-5-10-6 mbar). Transistor output and
transfer characteristics were measured using a Keithley 4200 semiconductor characterization
system. Hole mobilities were extracted in the linear regime using a standard device model.
Organic Photovoltaic devices. ITO coated glass substrates were cleaned in ultrasonic bath of
deionized water, acetone and 2-propanol at 45°C for 15 minutes for each step. Then, they
were dried by nitrogen and were treated in a UV/Ozone oven to remove residual organic
contaminants. PEIE (MW = 70000 g/mol) was spin coated (5000 rpm, 60s) to obtain a 5 nm
thick layer and thermally annealed for 10 minutes at 100°C under nitrogen atmosphere. The
photoactive layer was prepared from different solutions containing TAT-based dye:PC71BM
blends with a TAT-based dye concentration of 5 mg/ml in CHCl3. The ratio of donor and
acceptor was varied from 1:1 to 1:3. The solution were spin-coated onto the PEIE/ITO
substrates, and the resulting films were annealed at 120°C for 10 min under a nitrogen
atmosphere. The 7 nm thick MoO3 and 120 nm thick Ag layers were thermally evaporated
(Pressure ≈ 1x10-6 mbar). The effective area of each cell was 12 mm2.
OPV measurements. The (J-V) curves of the OPV devices were measured under AM 1.5G-100
mW/cm2 simulated sunlight irradiation. (J-V) characteristic measurements of photovoltaic
devices were conducted using LabView-controlled Keithley 2400 SMU. The performance of
solar cells under the dark and illumination were measured using a BET Technologies Sun 3000
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10.1002/ejoc.202100473
European Journal of Organic Chemistry
solar simulator with an AM1.5G filter. All the photovoltaic parameters of solar devices (VOC,
JSC, FF and PCE) were extracted using the LabView software.
Acknowledgements
This work has been supported by the Interreg IV-A program under Rhin-Solar project (nr
C25) and the French National Research Agency (ANR ORION project ANR-13-PRGE-0001).
Conflict of interest
The authors declare no conflict of interest.
Keywords: Organic semiconductor • regioisomer • absorption spectroscopy • solar cells •
triazatruxene
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Graphical Abstract:
Table of Contents text:
Two dumbbell-shaped molecules isomers based on triazatruxene units sandwiching a
fluorene-malononitrile unit have been synthesized and studied. The isomers exhibited
different solid-state structures and electronic conjugation pathways that drastically affect
their absorption properties and energy levels. Photovoltaic performances have been
investigated and correlated to the internal charge transfer band contribution to the charge
photogeneration.
Author biographies:
Chaima Mahmoud received her M.S degree in organic chemistry (2017) from the
Faculty of sciences of Monastir (Tunisia). She is now pursuing her PhD in the
University of Monastir under the supervision of Prof. Mustapha Majdoub.
She visited the institute of chemistry and processes for energy, environment and
health (ICPEES, Strasbourg, France) for an internship, under the supervision of Dr.
Nicolas Leclerc. Her interests are the synthesis of organic semiconductor
materials (polymers and small molecules), investigation of their structural and
optoelectronics proprieties as well as theoretical calculation (DFT and TD-DFT).
This article is protected by copyright. All rights reserved.

10.1002/ejoc.202100473
European Journal of Organic Chemistry
Ibrahim Bulut obtained his Ph.D. in 2015 from the University of Strasbourg under
the supervision of Stéphane Méry and Nicolas Leclerc. After a first postdoctoral
fellow at the University of Oxford with Prof. H. L. Anderson he continued in 2017
at the Institut National de l’Energie Solaire (INES, CEA) before to be appointed as
a senior research scientist at Brilliant Matters (Québec, Canada) and finally as
R&D engineer at Isorg. His current research focuses on OPD on CMOS.
Jiang Jing obtained his M.S degree in chemistry (2018) from the Beijing
University of Chemical Technology. He is now Ph.D. acndidate at the University
of Strasbourg and under the supervision of Dr. Emilie Steveler and Prof. Thomas
Heiser. He is interested in the charge transport and exciton dynamics of organic
semiconductor materials.
Sadiara Fall received his M.S degree in micro- and nano-electronics, physical
components and systems in 2009 from the University of Strasbourg (France).
He was promoted to PhD from the University of Strasbourg in April 2013. He
did his Ph.D. in the Organic Photovoltaic Components and Electronics group of
the Laboratory of Science Engineering, Computing and Imaging (ICube) in
Strasbourg, under the co-supervision of Dr. P. Lévêque and Prof T. Heiser. His
thesis topic was the study of charge transport in organic semiconductors and
its impact on photovoltaic performance. He then join the CINaM institute
(Marseille, France) as post-doctoral fellowship, where he conducted various
studies on charge transport, carrier lifetime in active layers of solar cells and
recombination at interfaces. He is currently a research engineer at ICube
laboratory, where he focuses on the study and development of optoelectronic
devices using organic semiconductors.
Benoît Heinrich obtained his PhD in structural investigation of liquid crystals
from the University of Strasbourg (France) in 1993. After post-doctoral
fellowships at Technical University of Berlin (Germany) and at University of
Sheffield (UK), he joined the French Scientific Research National Center (CNRS)
in 1996, to become an Engineer specialized in the structural investigation of
organic materials. His core activity is X-ray scattering and diffraction
(SAXS/WAXS, GISAXS/GIWAXS) combined to density determination, and to
thermal and optical analyses. Main fields of interest are molecular self-
assembly, mesomorphic structures and morphology of thin films.
This article is protected by copyright. All rights reserved.

10.1002/ejoc.202100473
European Journal of Organic Chemistry
Stéphane Méry obtained his PhD in 1990 from the University of Bordeaux
(France). After a two year post-doctoral fellowship at Naval Research Laboratory
in Washington DC (USA), he was appointed as CNRS researcher at IPCMS in
Strasbourg, France. His research activities cover the design of self-organized
functional materials for (opto)electronic properties and the development of
siloxane-based functional materials.
Thomas Heiser received his Ph.D. in semiconductor and device physics from the
University of Strasbourg (France) in 1987, where he was appointed a faculty
member in 1988. He joined the University of California at Berkeley (USA) as a
visiting scholar in 1995 for two years. His current research focuses on organic
semiconductors and their applications in optoelectronic devices, in particular
organic solar cells, chemical sensors, and optically addressed spatial light
modulators.
Patrick Lévêque received his Ph.D. in Material Science from the University of
Poitiers (France) in 1998. He joined the Royal Institute of Technology (KTH) in the
department of Solid States Electronics in Stockholm (Sweden) in 1999 and started
to work on organic electronics in Nantes University (France) in 2003. He became
appointed as a faculty member in Strasbourg University (France) in 2004 His
current research focuses on organic semiconductors and their applications in
solar cells and on charge-carrier dynamics in these devices.
Dr Emilie Steveler is graduated with a PhD in materials physics from Lorraine
University (France). Since 2015, she has been an assistant professor at INSA
Strasbourg (France) and works in the Materials for electronic and photovoltaic
devices group at the ICube laboratory. She is interested in the optoelectronic
properties of semiconductor materials, and more particularly in the exciton
dynamics in organic materials.
Mustapha Majdoub is Professor of Chemistry at Monastir University (Tunisia) and
he is also the Head of the Laboratory of Interfaces and Advanced Materials (LIMA)
at the same University. Prof. Majdoub obtained a Ph.D in polymer chemistry in
1985 at the University Pierre et Marie Curie (Paris, France). After two post-
doctoral years at the University of Orsay (Paris IX, France) in a pioneer laboratory
in selective organic synthesis under microwaves, he accepted a teaching-research
position at the University of Monastir in Tunisia. Prof. Majdoub's research
activities focus on the synthesis of new functional polymers such as
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10.1002/ejoc.202100473
European Journal of Organic Chemistry
semiconducting polymers and bio-based polymers for optoelectronic and sensor
applications. During his academic career, Prof. Majdoub has supervised more
than 30 masters and 20 Ph.D. candidates and co-authored more than 100
publications in specialized scientific journals and books.
Dr. Nicolas Leclerc received his PhD from the Pierre and Marie Curie University
(Paris, France) in 2003. After completing his post-doctoral research at Laval
University in Mario Leclerc's team (Québec, Canada), he joined the Institute of
Chemistry and Processes for Energy, Environment and Health (ICPEES) of the
University of Strasbourg (France) as a CNRS researcher in 2005. He has been
appointed research director in 2020. He is the Head of the organic electronic team
at ICPEES. His research interests focus on the development of new molecular and
macromolecular organic semiconductor materials and their applications in
optoelectronics.
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