Novel DPP derivatives functionalized with auxiliary electron-acceptor groups and characterized by narrow bandgap and ambipolar charge transport properties

Article abstract of DOI:10.1016/j.dyepig.2020.109026

Four novel diketopyrrolopyrrole (DPP) derivatives have been synthesized and characterized: the dyes are based on a DPP electron acceptor core symmetrically functionalized with donor bi-furyl moieties and end capped with four different auxiliary electron-a

Full text of DOI:10.1016/j.dyepig.2020.109026

Dyes and Pigments xxx (xxxx) xxx
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
Novel DPP derivatives functionalized with auxiliary electron-acceptor
groups and characterized by narrow bandgap and ambipolar charge
transport properties
a
b
c
a,
b
d
Sandra Fusco , Mario Barra , Matteo Bonomo , Antonio Cassinese , Roberto Centore ,
b
d
d,*
Fabio Chiarella , Francesco Senneca , Antonio Carella
a
Dipartimento di Fisica “Ettore Pancini”, Universit a` Degli Studi di Napoli ‘Federico II’, Piazzale Tecchio, 80, I-80125, Napoli, Italy
CNR-SPIN, C/o Dipartimento di FIsica “Ettore Pancini”, P.le Tecchio, 80, I-80125, Napoli, Italy
Department of Chemistry and NIS Interdepartmental Centre, University of Torino, Via Pietro Giuria 7, 10125, Torino, Italy
Dipartimento di Scienze Chimiche, Universit a` Degli Studi di Napoli ‘Federico II’, Complesso Universitario Monte Sant’Angelo, Via Cintia 21, I-80126, Napoli, Italy
b
c
d
A R T I C L E I N F O
A B S T R A C T
Keywords:
Four novel diketopyrrolopyrrole (DPP) derivatives have been synthesized and characterized: the dyes are based
on a DPP electron acceptor core symmetrically functionalized with donor bi-furyl moieties and end capped with
four different auxiliary electron-acceptor groups. Because of the alternation along the molecular backbone of
electron acceptor and donor groups, all the dyes are characterized by optical absorption maxima approaching or
exceeding 700 nm. In the solid state, this optical behavior determines for all the dyes a very low optical bandgap
ranging from 1.57 eV to 1.29 eV, while electrochemical characterization shows a clear dependence of the LUMO
energies on the strength of the auxiliary electron-acceptor groups. All the dyes are characterized by stable LUMO
energies suitable for their application as n-type semiconductors. Organic field-effect transistors based on the
reported compounds display actually n-type behavior and, in three cases, a very interesting and balanced
ambipolar charge transport behavior was moreover observed.
Diketopyrrolopyrrole
Synthesis
Low bandgap
OFET
Ambipolar charge transport
1
. Introduction
photovoltaic properties. In addition, DPP fused unit is highly planar and
enables efficient intermolecular
π–π
interactions which, in turn, posi-
Nitrogen containing heterocycles have received in the last years a
tively affects the charge carrier mobility in Organic Filed-Effect Tran-
sistors (OFETs); in this application too, the synthetic versatility of this
unit allows the combination with different molecular blocks (electron
rich or poor, both in polymers and small molecules) providing a fine
tuning of frontier molecular orbitals (HOMO and LUMO) energies and
thus optimizing hole and electron mobility in, respectively, p-type
[26–29] and n-type materials [30,31]. Several examples of ambipolar
DPP derivatives are also reported [32–34].
While most of the DPP containing organic semiconductors reported
in the literature are based on polymeric materials, in the last years the
number of papers reporting DPP small molecules for optoelectronic
applications is continuously growing [9,15]. Indeed, the molecular
systems exhibit some advantages, since they don’t suffer from the
batch-to-batch variation of performance as a consequence of their
well-defined molecular structures; moreover, a proper molecular engi-
neering design could finely tune electronic properties and structural
great deal of interest as promising building blocks for the realization of
functional materials for advanced applications in electronics or opto-
electronics [1–6]. Among these, diketopyrrolo-pyrrole (DPP) derivatives
represent one of the most investigated class of materials: in a pristine,
non-alkylated form, DPP derivatives have been widely used as
high-performance pigments that, because of their intense colour,
excellent stability and low solubility, have found several industrial ap-
plications in paints, varnishes, and high-quality printing [7,8]. By means
of N-alkylation, DPP derivatives can be easily transformed in very sol-
uble dyes that in recent years, have been applied as active materials in
different field of organic electronics [9–15]. In particular, by combina-
tion with electron rich units, the electron deficient nature of DPP frag-
ment has been exploited for the realization of alternated donor-acceptor
molecular structures (polymers [16–19] or small molecules [20] [–]
[
25]) with an increased solar photon harvesting ability and promising
*
Corresponding author. .
E-mail address: antonio.carella@unina.it (A. Carella).
https://doi.org/10.1016/j.dyepig.2020.109026
Received 23 July 2020; Received in revised form 20 November 2020; Accepted 20 November 2020
Available online 25 November 2020
0
143-7208/© 2020 Elsevier Ltd. All rights reserved.
Please cite this article as: Sandra Fusco, Dyes and Pigments, https://doi.org/10.1016/j.dyepig.2020.109026

S. Fusco et al.
Dyes and Pigments xxx (xxxx) xxx
packing as well. Finally, an intrinsic higher mobility is expected because
of an increased structural order.
engineering principle to achieve low-band gap dyes; this class of mate-
rials is highly demanded in the field of OPVs, because their absorption
spectrum better overlaps the solar emission spectrum and thus a higher
solar photon harvesting is achieved [44]; this class of materials is
extremely appealing in OFETs technology as well, because of their po-
tential ambipolar charge transport [44,45]. The synthesized dyes were
characterized in terms of structural, optical and thermal properties. An
electrochemical characterization provided moreover a better insight on
the relative energies of frontier molecular orbitals of the dyes. The
compounds were finally processed as thin films and used as active layers
in OFET to investigate their charge transport properties.
In this context, we report here four novel DPP based molecules,
whose chemical structure is reported in Scheme 1.
The molecules share a common molecular fragment constituted by a
DDP electron acceptor core (A), symmetrically linked to two electron
donor bifuran moieties (D). This common molecular motif is end func-
tionalized, on both sides, with auxiliary electron acceptor groups (A’). A
similar approach has been followed in other recent works based on
thienyl-substituted DPP derivatives, with promising results [35–37]: in
those reported cases auxiliary acceptors as cyano acetic acid derivatives,
dicyanovinyl and rhodamine groups were used and a maximum electron
ꢀ 2
2
mobility in the order of 10 cm /V⋅s was measured. In the present
work, the novel dyes are end capped with four different electron
acceptor groups of different strength, specifically a dicyanovinyl group
2. Experimental section
2.1. Synthesis
(
DCV), a thiobarbituric acid (TB), an indandione (ID) and a dicyano-
vinyl functionalized indandione (IDM). The end capping of conjugated
donor fragment with strong electron acceptor (in particular, indandione
derivatives) have been widely used in the last years as terminal groups in
non-fullerene acceptors for bulk heterojunction solar cells [38]. The
prepared dyes are moreover characterized by the presence of furan rings
rather than thiophenes in the molecular backbone. While thiophenes
rings are typically inserted in small molecules (or polymers) investigated
in the field of organic electronics, their close analogues furan rings have
been studied so far in a much lower extent; in the last years, anyway,
furans containing small molecules for organic electronics have been
experienced a growing interest [39,40], first of all because furan can be
obtained from renewable resources and, in addition, for the proved
biodegradability of many furan derivatives [41,42]. Moreover, as
compared to thiophene analogues, furan-based derivatives are known to
be characterized by a higher solubility while being more tightly packed
All reagents were purchased by Sigma Aldrich or Alfa Aesar and used
without any further purification, unless otherwise specified. All air- or
moisture-sensitive reactions were carried out under nitrogen
atmosphere.
2.1.1. Synthesis of 3,6-di(furan-2-yl)pyrrolo[3,4-c]pyrrole-1,4 (2H,5H)-
dione (1)
4.81 g (209 mmol) of Na metal and a catalytic amount of FeCl were
3
placed in 100 ml of anhydrous t-amyl alcohol. The system was taken at
◦
90 C under nitrogen atmosphere until to complete oxidation of sodium
(around 1 h). Then, 15 g of 2-cyanofuran (161 mmol) were added and
◦
the temperature was increased to 120 C. After 1 h, 9.24 g (63 mmol) of
dimethyl succinate were added dropwise. The temperature was further
◦
increased to 130 C and the reaction continued for 3 h, during which
solution colour turned to dark red. The system was then cooled down to
◦
[
43]. Finally, replacement of a sulphur atom with oxygen lowers the
80 C and quenched with a solution of 60 mL of methanol and 20 mL of
oxidation potential and make HOMO levels more accessible [43]: in the
framework of the molecular structures we are here proposing this could
be beneficial to achieve ambipolar charge transport properties by fa-
voring hole transport in systems where electron transport properties are
fueled by the several electron acceptor moieties present.
acetic acid. After cooling down the mixture to room temperature the
product was collected by suction filtration and washed with 300 ml of a
hot methanol/water solution (2/1 v/v). The washed product was again
recovered by suction filtration. Yield: 56%.
1
H NMR (DMSO‑d , 400 MHz): δ: 6.83 (s, 2H); 7.67 (m,2H); 8.05 (m,
6
The strength of several terminal electron acceptor groups on the
dye’s properties is investigated as well: the four novel dyes differ in fact
for the nature of the auxiliary acceptor groups, whose strength increases
in the order: DCV < TB < ID < IDM. The resulting A′-D-A-D-A′ structure
is characterized by an alternation of electron poor and electron rich
units along the molecular backbone and this is one of the molecular
2H); 11.18 (s,2H).
2.1.2. Synthesis of 3,6-di(furan-2-yl)-2,5-di(2-ethylhexyl)pyrrolo[3,4-c]
pyrrole-1,4(2H,5H)-dione (2)
4.31 g (16 mmol) of 1 and 6.85 g (49.6 mmol) of K CO were put in
2
3
50 mL of anhydrous DMF and the system was taken under nitrogen at-
mosphere. The about 30 mg of dibenzo-18-crown-6-ether were added,
◦
and the temperature was increased up to 100 C. After 1 h, 7.4 g (38.4
mmol) of 2-bromoethylhexan were added dropwise. The system was
◦
finally increased to 130 C and reacted overnight at this temperature for
1
8 h. The mixture is then cooled down to room temperature and the
temperature was further decreased by using means of an ice-salt bath.
The solid was collected by filtration and washed in 200 mL of HCl(aq)
0
.5 M in order to remove the excess potassium carbonate. The remaining
solid was recovered by filtration, solved in chloroform and re-filtered to
remove the unreacted insoluble compound 1. After removal of chloro-
form by means of a rotary evaporator, 3.02 g of crystalline product was
◦
recovered. M.p. 153 C. Yield: 38%.
1
3
H NMR (CDCl , 400 MHz): 0.91 (m, 12H); 1.32 (m,16H); 1.75 (m,
2
7
H); 4.04 (d, 4H, J = 6.7 Hz); 6.70 (dd,4H, J1 = 3.6 Hz, J2 = 1.6 Hz);
.62 (d,2H, J = 1.6 Hz); 8.34 (d, 2H, J = 3.6 Hz).
2
.1.3. Synthesis of 3,6-bis(5-bromofuran-2-yl)-2,5-di(2-ethylhexyl)pyrrolo
[
3,4-c]pyrrole-1,4(2H,5H)-dione (3)
.2 g (4.47 mmol) of 2 were dissolved in a mixture of 50 ml of
2
chloroform and conditioned in an ice bath. The solution was put under
nitrogen atmosphere and in the darkness in order to protect it from light.
Then, 1.91 g (10.7 mmol) of N-bromosuccinimide (NBS) were added
portion wise. The reaction was left under stirring and inert atmosphere
Scheme 1. Chemical structures of the synthesized dyes.
2

S. Fusco et al.
Dyes and Pigments xxx (xxxx) xxx
′
′′′
overnight. The day after, the solvent was removed by using a rotary
evaporator. The obtained product was, finally, purified by chromatog-
2.1.6. Synthesis of 5,5’-((5 ,5 -(2,5-di(2-ethylhexyl)-3,6-dioxo-2,3,5,6-
′
′
tetrahydropyrrolo[3,4-c]pyrrole-1,4-diyl)bis([2,2 -bifuran]-5,5 -diyl))bis
(methanylylidene))bis(1,3-diethyl-2-thioxodihydropyrimidine-4,6(1H,5H)-
dione) (TB)
raphy column with silica gel as stationary phase and CH
2
Cl
2
as eluent.
◦
M.p. 167 C. Yield: 51%.
1
H NMR (CDCl
3
, 400 MHz): δ = 0.86–0.93 (m. 12H); 1.27–1.37
TB was synthetized according to the same procedure adopted for
DCV, with the only exception that N,N-diethyl-thiobarbituric acid, was
(
(
m,16H); 1.56 (s broad,2H); 1.72 (m, 2H); 3.98 (d, 4H, J = 6.7 Hz); 6.62
d,2H, J = 3.6 Hz); 8.03 (d, 2H, J = 3.6 Hz).
◦
used instead of malononitrile. M.p. 279 C. Yield: 73%.
1
H NMR (CDCl
3
, 400 MHz): δ = 0.84 (t, 6H, J = 7.3 Hz); 0.94 (t, 6H,
′
′′′
2
.1.4. Synthesis of 5 ,5 -(2,5-di(2-ethylhexyl)-3,6-dioxo-2,3,5,6-
J = 7.3 Hz); 1.24–1.43 (m, 28 H); 1.86 (m, 2H); 4.12 (d, 4H, J = 7.6 Hz);
4.57 (q, 4H, J = 7.1 Hz); 7.03 (d, 2H, J = 4.0 Hz); 7.19 (d, 2H, J = 4.0
Hz); 8.36 (s, 2H); 8.54 (d, 2H, J = 4.0 Hz); 8.85 (d, 2H, J = 4.0 Hz).
′
tetrahydropyrrolo[3,4-c]pyrrole-1,4-diyl)bis(([2,2 -bifuran]-5-
carbaldehyde)) (4)
1
3
7
50 mg (1.15 mmol) of 3 were dissolved in 20 mL of anhydrous THF.
The system was degassed and 965 mg (6.9 mmol) of 5-formylfuranylbor-
onic acid and 4.5 ml of 2 M Na CO solution (9 mmol) were added.
Then, after a further degassing, 245 mg of Pd[P(Ph) (0.212 mmol)
C NMR (CDCl
3
, 500 MHz): δ = 10.5; 12.4; 14.0; 23.5; 28.4; 29.7;
30.1; 39.6; 43.4; 44.1; 46.7; 108.9; 112.4; 113.1; 114.4; 123.0; 130.6;
132.1; 139.1; 146.0; 146.7; 151.4; 152.2; 160.4; 160.7; 178.4.
2
3
]
3 4
FTIR (KBr, cm 1): 2961, 2933, 2871, 1671, 1551, 1381, 1284, 1216,
ꢀ
were added and the mixture reacted for 18 h under reflux. In few hours
the mixture colour turned from purple to blue. The system was then
cooled down to room temperature and the solvent is evaporated in
vacuo. The residue was collected and extracted with 50 mL of chloro-
form and 80 mL of water. The organic phase is dried over anhydrous
1107, 1030.
Anal. Calcd. for C56
64.55, H 6.32, N 7.94.
ESI-MS: Calcd. for MH : 1045.42; Found: 1045.34.
64 6 10 2
H N O S : C 64.35, H 6.17, N 8.04; found: C
+
′
′′′
Na
2
SO
4
and the solvent was evaporated again by rotary evaporator. The
Cl and
2.1.7. Synthesis of 2,2’-((5 ,5 -(2,5-di(2-ethylhexyl)-3,6-dioxo-2,3,5,6-
′
′
product in the round-bottom flask is dissolved in 15 ml of CH
2
2
tetrahydropyrrolo[3,4-c]pyrrole-1,4-diyl)bis([2,2 -bifuran]-5 ,5-diyl))bis
diluted with 30 mL of methanol. By setting rotary evaporator bath at
room temperature and reducing the pressure, dichloromethane (solvent)
was distilled off causing at the same time a decrease of the temperature
in the round bottom flask: a sort of solvent/non-solvent recrystallization
of the product at low temperature occurred in this way. The obtained
(methanylylidene))bis(1H-indene-1,3-(2H)-dione) (ID)
ID was synthetized according to the same procedure adopted for
DCV, with the only exception that 1,3-indandione was used instead of
◦
malononitrile. M.p. 328 C. Yield: 84%.
1
H NMR (CDCl3, 500 MHz): δ = 0.84 (t, 6H, J = 7.2 Hz); 0.93 (t, 6H,
◦
solid was finally recovered by suction filtration. M.p. 208 C. Yield: 60%.
J = 7.2 Hz); 1.24–1.42 (m, 16H); 1.88 (m, 2H); 4.14 (d, 4H, J = 7.6 Hz);
7.01 (d, 2H, J = 3.9 Hz); 7.16 (d, 2H, J = 4.0 Hz); 7.77 (s, 2H); 7.78 (m,
2H); 7.96 (m, 2H); 8.52 (d, 2H, J = 4.0 Hz); 8.69 (d, 2H, J = 4.0 Hz).
1
H NMR (CD
2
Cl
2
, 400 MHz): δ = 0.87 (t, 6H, J = 7.2 Hz); 0.93 (t, 6H,
J = 7.4 Hz) 1.26–1.41 (m,16H); 1.86 (m, 2H); 4.09 (d, 4H, J = 7.6 Hz);
1
3
6
8
.92 (d,2H, J = 3.6 Hz); 7.17 (d,2H, J = 3.8 Hz); 7.38 (d,2H, J = 3.8 Hz);
.46 (d, 2H, J = 3.8 Hz); 9.68 (s, 2H).
C NMR (1,1–2,2-C
2
D
2
Cl
4
, 400 MHz): δ = 10.1; 13.6; 22.7; 23.0;
27.8; 29.6; 39.0; 46.2; 107.9; 112.0; 112.8; 122.3; 122.4; 124.7; 126.6;
131.7134.5; 134.7; 138.0; 139.6; 141.6; 144.8; 146.4; 149.4; 151.4;
154.4; 160.1; 189.0.
1
3
2 2
C NMR (CD Cl , 400 MHz): 10.2; 13.8; 23.1; 28.4; 29.7; 30.1; 39.6;
4
1
6.4; 108.2; 109.7; 112.6; 122.0; 132.5; 134.5145.4; 146.8; 149.4;
52.5; 160.7; 176.9.
FTIR (KBr, cm 1): 2956, 2928, 2855, 1681, 1577, 1508, 1456, 1349,
ꢀ
1
282, 1221, 1099, 1023.
′
′′′
2
.1.5. Synthesis of 2,2’-((5 ,5 -(2,5-di(2-ethylhexyl)-3,6-dioxo-2,3,5,6-
Anal. Calcd. for C58
52 2
H N O10: C 74.34, H 5.59, N 2.99; found: C
′
′
tetrahydropyrrolo[3,4-c]pyrrole-1,4-diyl)bis(([2,2 -bifuran]-5,5 -diyl))bis
74.67, H 5.75, N 2.90.
+
(
methanylylidene))dimalononitrile (DCV)
ESI-MS: Calcd. for MH : 937.37; Found: 937.33.
In a round-bottom flask 100 mg (0.147 mmol) of 4 were dissolved in
′
′
′′′
a solution consisting of 3 mL of absolute anhydrous ethanol and 12 mL of
anhydrous dichloroethane. Afterwards, 39 mg (0.588 mmol) of malo-
nonitrile and 39 mg (0.441 mmol) of β-alanine were added. The mixture
reacted under nitrogen atmosphere and refluxed overnight and solution
colour turned from blue to green. Then the system was allowed to cool
down to room temperature and the formed solid was recovered by
filtration. The crude product was dissolved in chloroform (5 mL),
filtered and the collected solution treated by rotary evaporator in order
to remove the solvent. Afterwards, the solid was dissolved in the mini-
mum amount of dichloromethane, poured into boiling heptane dropwise
and filtered from hot heptane. Finally, the product was dissolved in the
minimum amount of dichloromethane and poured in boiling ethanol:
the formed precipitate was recovered by vacuum filtration from hot
2.1.8. Synthesis of 2,2’-((2Z,2 Z)-((5 ,5 -(2,5-dioctyl-3,6-dioxo-2,3,5,6-
′
′
tetrahydropyrrolo[3,4-c]pyrrole-1,4-diyl)bis([2,2 -bifuran]-5 ,5-diyl))bis
(methanylylidene))bis(3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))
dimalononitrile (IDM)
IDM was synthetized according to the same procedure adopted for
DCV, with the only exception that, 1,1-dicyanomethylene-3-indanone
was used instead of malononitrile. M.p. no detectable. Yield: 86%.
1
H NMR (CDCl
3
, 500 MHz): δ = 0.86 (t, 6H, J = 7.5 Hz); 0.93 (t, 6H,
J = 7.5 Hz); 1.26–1.43 (m, 16H); 1.84 (m, 2H); 4.10 (d, 4H, J = 8.0 Hz);
7.03 (d, 2H, J = 4.0 Hz); 7.23 (d, 2H, J = 3.5 Hz); 7.73 (m, 2H); 7.90 (m,
2H); 8.45 (s, 2H); 8.56 (d, 2H, J = 4.0 Hz); 8.85 (d, 2H, J = 4.0 Hz).
1
3
C NMR (CDCl
3
, 500 MHz): δ = 10.5; 14.0; 23.1; 23.5; 28.5; 29.7;
30.2; 39.6; 46.8; 70.8; 109.4; 113.4; 114.0; 114.3; 114.5; 123.1; 123.9;
124.6; 125.2; 127.8; 129.7; 132.2; 134.6; 135.2; 137.0; 139.6; 146.0;
146.7; 151.0; 151.7; 187.1.
◦
ethanol. M.p. 292 C. Yield: 72%.
1
H NMR (CDCl
3
, 500 MHz): δ = 0.85 (t, 6H, J = 6.5 Hz); 0.91 (t, 6H,
FTIR (KBr,cm 1): 2959, 2928, 2857, 2221, 1708, 1669, 1561, 1540,
ꢀ
J = 7.5 Hz), 1.25–1.41 (m, 16H); 1.82 (m, 2H); 4.12 (d, 4H, J = 7.5 Hz);
6
.99 (d, 2H, J = 3.5 Hz); 7.24 (d, 2H, J = 4.0 Hz); 7.41 (broad, 2H); 7.46
1501, 1371, 1343, 1292, 1239, 1127, 1091, 1023.
(
s, 2H); 8.47 (d, 2H, J = 4.0 Hz).
Anal. Calcd. for C64
52 6 8
H N O : C 74.40, H 5.07, N 8.13; found: C
1
3
C NMR (CD
2
Cl
2
, 500 MHz): d = 11.0; 14.0; 23.0; 23.5; 28.3; 30.1;
74.58, H 5.13, N 8.03.
+
3
1
9.5; 46.4; 76.4; 108.5; 108.9; 111.4; 113.1; 114.2; 114.7; 122.6; 132.5;
45.8; 146.0; 148.0; 150.5; 160.6.
FTIR (KBr, cm 1): 3032, 2958, 2873, 2221, 1665, 1570, 1529, 1434,
ESI-MS: Calcd. for MH : 1033.39; Found: 1033.34.
ꢀ
2.2. Chemical-physical characterization
1
7
386, 1286, 1095, 1029.
Anal. Calcd. for C46
H
44
N
6
O
6
: C 71.12, H 5.71, N 10.82; found: C
The compounds identity was confirmed by NMR analysis performed
on Bruker 400 MHz and 500 MHz Varian spectrometers. ESI (Electro
Spray Ionization) experiments were conducted with a Thermo Finnigan
1.29, H 5.57, N 10.71.
+
ESI-MS: Calcd. for MH : 777.34; Found: 777.24.
3

S. Fusco et al.
Dyes and Pigments xxx (xxxx) xxx
Advantage Max Ion trap spectrometer in positive ion acquiring mode:
samples are dissolved in chloroform and slightly acidified with formic
the related MOSFET equation used to fit linearly the square root of the
measured current by:
+
acid to improve the MH detectability. Sheath gas flow rate was set at 25
ꢀ̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅
√
̅̅̅̅̅
W
2L
(
arbitrary unit), auxiliary gas flow rate at 5 (arbitrary unit), spray
I
ds
=
C
OX
μ
*(VGS ꢀ Vth
)
(1)
sat
◦
voltage at 3.25 (KV), capillary temperature at 270 C, capillary voltage
at 7 (V), and tube lens offset at 60.00 (V). Nitrogen was used as sheath
and auxiliary gases.Thermogravimetric analysis (TGA) was carried by
2
where Cox is the oxide capacitance per unit area (Cox = 17.25 nF/cm ).
◦
TA4000 Perkin- Elmer instrument at a heating rate of 20 C/min under
3
. Results and discussion
air in the temperature range 30–900 ◦C. The reported decomposition
temperature refers to 5% weight loss. Differential scanning calorimetry
3
.1. Synthesis and chemical-physical properties
(
DSC) analysis was performed on a PerkinElmer Pyris 1 instrument
◦
under a nitrogen flow at a heating rate of 10 C/min. Absorption spectra
were recorded on Jasco V-530 UV–vis spectrophotometer at room
temperature with a scanning rate of 200 nm/min. FTIR spectra were
recorded dispersing the dye in KBr pellets and using a Jasco F-430
spectrophotometer. Thin films of the dyes have been prepared by spin-
coating a chloroform solution of the dyes on glass substrates for opti-
cal characterization by means of a Laurell WS-650Mz-23NPP spin pro-
cessor. Electrochemical characterization was performed by means of
cyclic voltammetry (CV) using was recorded with a BioLogic sp150. The
samples were drop-casted onto a Pt working electrode from dichloro-
methane solution in a three-electrode set-up, being a Pt wire the
counter-electrode and Ag/AgCl the reference electrode. Electrolyte
consisted in a tetrabutylammoiun hexafluorophosphate 0.1 M solution
in anhydrous acetonitrile. CV were conducted at 100 mV/s. The found
potential values were referred to Fc/Fc + acting as internal standard.
Before each measurement, the electrolyte was degassed with Argon to
avoid the presence of oxygen.
The novel DPP dyes were synthesized through a multistep synthetic
procedure that is graphically sketched in Scheme 2.
DPP unit was obtained by the base catalyzed reaction between 2-cya-
◦
noforan and dimethyl succinate at 130 C under inert atmosphere. DPP
core was then N-functionalized by reacting 1 with 1-bromo-2-ethylhex-
ane in dimethylformamide (DMF) and in presence of K
2 3
CO as base, with
3
8% yield. Afterwards, bromuration in darkness with N-bromosuccini-
mide (NBS) allowed to get the dibromo derivative 3. Once dibromo
difuran DPP was obtained, it was combined with 5-formyl-2-furanbor-
onic acid by Pd(0) catalyzed Suzuki coupling, affording a symmetrical
formyl terminated precursor. This common diformyl precursor was
finally functionalized with four different acceptor groups to afford the
reported DPP dyes: the last synthetic step consisted in a Knoevenagel
condensation of 4 with compound containing acid methylene group,
specifically malononitrile (DCV dye), N,N-diethyl thiobarbituric acid
(
TB), indandione (ID) and 1,1-dicyanomethylene-3-indanone (IDM).
β-alanine was used as catalytic base in this condensation reaction.
NMR analysis provided an indication of the relative strength of the
auxiliary terminal acceptor groups of the four dyes, as shown in Fig. 1. In
particular, the resonance value of the proton of the methine group is
influenced by the adjacent electron acceptor moiety: values of 7.46,
2
.3. Transistor fabrication and electrical characterization
For the transistor fabrication, DPP dyes films were prepared by spin
coating starting from a 5 mg/ml chloroform/tetrachloroethane 99/1 v/v
8
.36, 7.77 and 8.45 were found for, respectively, DCV, TB, ID and IDM;
solution (filtered through a 0.2
μ
m syringe filter), at 1500 rpm for 1 min;
this trend suggests that the electron acceptor strength is in the order
DCV < ID < TB < IDM.
◦
the obtained films were dried in a vacuum oven, at 80 C, for 1 h. Typical
thickness values were comprised in the range between 50 and 150 nm.
The organic dyes were spin-coated specifically on standard multilayer
Thermal properties of the dyes were investigated by means of DSC
and TGA analysis and are summarized in Table 1. All the dyes but IDM
+
+
structures consisting of a 500-
acting as a gate electrode, a 200-nm thick SiO
50-nm thick pre-patterned source/drain gold electrodes with inter-
μm thick highly-doped silicon (Si
)
◦
showed, during DSC scan, a melting point at 292, 279 and 328 C (see
2
dielectric barrier and
Fig. S1). The thermal stability of the dyes was assessed by TGA analysis:
all the dyes featured a fair good thermal stability in air with decompo-
1
digitated layout. The considered electrode geometry allows the defini-
tion of semiconducting active channels characterized by a ratio between
width (W) and length (L) of about 550. For our experimental set-up, this
feature provides a reliable estimation of the field-effect mobility values
◦
◦
sition temperature ranging from 337 C (TB) to 375 C (DCV). TGA scan
is reported in Fig. S2.
Optical characterization was performed on the reported dyes both in
solution and as thin films; dyes optical properties are summarized in
Table 2, while their optical absorption spectra in chloroform solution are
reported in Fig. 2.
ꢀ 6
2
down to 10 cm /V⋅s, making possible the accurate electrical charac-
terization of films achieved by evaporated small molecules [46],
solution-processed polymers [47] and even semi-insulating bio-oriented
compounds [48]. Before the film deposition, all multilayer
All the dyes absorb at very high wavelengths because of the alter-
nation of electron donor and electron acceptor groups along the mo-
lecular backbone. The wavelength of the absorption maxima of the dyes
is clearly influenced by the terminal electron acceptor groups func-
tionalizing the common molecular fragment: in particular, as the
strength of the electron acceptor end group increases, the dye’s ab-
sorption moves to higher wavelengths. We thus observed λmax values of
++
2
Si /SiO /Au substrates were functionalized by applying HMDS (hex-
amethyldisilazane) self-assembling monolayers using a process lasting 7
days (see Refs. [48] for more detail). In this way, the final water-contact
◦
angle (θ
from an initial θ
of the hydrophobic degree of SiO
C
) of the SiO
2
surface was increased up to about 110 (starting
◦
C
~ 60 ). It is well known, indeed, that the enhancement
surface allows strongly reducing the
2
6
83, 705, 724 and 768 nm, respectively, for DCV, ID, TB and IDM. This
occurrence of charge trapping mechanisms related to the presence of
water molecules absorbed at the dielectric/semiconductor interface. The
deposited films surfaces were investigated by atomic force microscopy
trend is consistent with the previously discussed NMR analysis. As
shown in previous papers [50,51], functionalizing a specific molecular
fragment with electron acceptor groups of different strength have a
relevant effect on tuning optical absorption properties of the molecular
systems, occurring mainly through the stabilization of LUMO energies of
the dyes. All the dyes present a second optical feature at higher energy
ranging from 444 nm (DCV) to 488 nm (IDM) that eventually contrib-
utes to the final colour of the solutions, being green for all the dyes but
IDM, which affords midnight blue colored solutions. The optical ab-
sorption of IDM is particularly interesting being widely spread in all
visible range and hence appealing for light harvesting application.
(
AFM) in non-contact mode, in order to prevent surface damages, using
a Park system XE100 microscope. The electrical measurements were
carried out mounting the samples in a Janis cryogenic probe station,
connected to a Keithely 2612A dual-channel source-meter in order to
apply contemporarily the VGS and VDS voltages and to record, in parallel,
the related IDS and IGS currents. Field-effect measurements were sys-
ꢀ
5
tematically carried out in vacuum, with a base pressure P
The values of the field-effect mobility ( sat) and of the threshold voltage
th) were extracted in the saturation regime (VDS = + or - 50 V) through
r
~10 mbar.
μ
(
V
4

S. Fusco et al.
Dyes and Pigments xxx (xxxx) xxx
◦
Scheme 2. Synthetic procedure followed for the preparation of the dyes; i) Na (1.3 eq.), FeCl
3
, dimethylsuccinate (0.8 eq.) in t-amyl alcohol, 130 C, 3 h, in N
2
; ii)
◦
Na
2
CO
3
, 1.2 eq. of 1-bromo-2-ethylhexane, DMF, 130 C, N2, 20 h; iii) 1.2 eq. of N-bromosuccinimide, chloroform, in the dark, N
2
, RT, 16 h; iv) 3 eq. of 5-formyl-2-
furanylboronic acid, Pd[P(PH
3
)]
4
, dry THF, N
2
, reflux, 20 h; v) 2 eq. of acceptor, dichloroethane-ethanol 4:1, β-alanine, reflux, 20 h, N
2
.
Table 1
Thermal properties of DPP dyes.
◦
a
◦
d
T ( C)
b
Dye
T
m
( C)
DCV
TB
292
279
328
–
375
337
374
363
ID
IDM
(
a)
◦
Melting temperature determined by DSC analysis at 10 C/min under
nitrogen atmosphere.
(
b)
Decomposition temperature determined as the temperature corre-
sponding to the 5% weight loss of the samples in TGA measurement per-
◦
formed at 20 C/min in air.
Table 2
Optical properties of the dyes in chloroform solution and as thin films.
(cm 1⋅Mꢀ 1
ꢀ
)
λ
FILM
MAX(nm)
a
b
Dyes
λ
MAX(nm)
ε
g
E (eV)
4
DCV
TB
683
724
705
768
7.27 ⋅ 10
9.84 ⋅ 10
8.15 ⋅ 10
8.28 ⋅ 10
759
779
782
861
1.57
1.45
1.41
1.29
4
4
4
ID
IDM
(
a)
UV–vis analysis on film obtained by CHCl
3
solution by spin coating and
◦
1
annealed at 85 C in vacuum for 1 h.
(
Fig. 1. H NMR spectra of the dyes in CDCl
3
; the peak corresponding to the
b)
Optical Bandgap extrapolated by Tauc plot [49].
methine resonance is starred to highlight its dependence from the strength of
the terminal acceptor groups.
5

S. Fusco et al.
Dyes and Pigments xxx (xxxx) xxx
Fig. 2. Normalized absorption spectra of the synthesized dyes in chloroform solution.
Quantitatively, all compounds show a fair high molar extinction coef-
reported in Fig. 3. It is possible to observe that, in solid state, optical
absorption became broader and red shifted as compared to solution
optical spectra.
ꢀ 1
ꢀ 1
ficient ranging from 73000 (DCV) to 98000 cm
M (TB) as shown in
Table 2.
Optical characterization was also performed on thin films of the dyes
spin coated from chloroform solution and the optical spectra are
The absorption of the dye’s films extends significantly in the NIR part
of the spectrum. The optical bandgap of the dyes has been graphically
Fig. 3. Normalized absorption spectra of the synthesized dyes as thin films spin coated from chloroform solution.
6

S. Fusco et al.
Dyes and Pigments xxx (xxxx) xxx
evaluated by applying Tauc relationship [49]. All the dyes are charac-
terized by a very low bandgap with values arriving to the extremely
interesting value of 1.29 eV for IDM.
Table 3
+
Electrochemical potentials vs FC /FC and estimated HOMO and LUMO energies
vs vacuum.
a
a
b
CV
The dyes were electrochemically characterized by means of cyclic
voltammetry. CV experiments were performed on the dyes drop casted
as thin films on platinum working electrode, as described in the
Experimental Section. CV graphs are reported in Fig. 4.
Dyes
Eox1(V)
Eox2(V)
Ered(V)
HOMO
LUMO
E
g
c
c
(eV)
(eV)
(eV)
DCV
TB
0.53
0.45
0.41
0.48
0.74
0.61
0.59
0.70
ꢀ 1.00
ꢀ 0.96
ꢀ 0.97
ꢀ 0.78
ꢀ 5.63
ꢀ 5.55
ꢀ 5.51
ꢀ 5.58
ꢀ 4.08
ꢀ 4.14
ꢀ 4.13
ꢀ 4.28
1.55
1.41
1.38
1.30
ID
The analysis of these graphs provides important electrochemical
parameters which, in turn, allow the estimation of HOMO and LUMO
energies of the dyes: HOMO and LUMO energies can be related,
respectively, to oxidation and reduction potentials of the dyes, if these
IDM
a)
Calculated as the half wave potential of the anodic and cathodic peaks.
Calculated as the onset of the reduction potential.
Calculated according to ref. 52.
b)
c)
+
are measured relative to ferrocene/ferrocenium (Fc /Fc) potential, by
applying the following equations:
ꢁ
)
dyes so that the reduction potentials were calculated by considering the
E
E
HOMO = ꢀ Eox vs Fc+/Fc + 5.1 (eV)
red
E
onset. While electrochemically determined HOMO energies do not
ꢁ
)
present a clear trend, LUMO energies are consistent with the strength of
the electron acceptor terminal group: in fact, the more and the less
stabilized LUMO are presented by, respectively, IDM and DCV, the two
dyes bearing the stronger and the weaker electron acceptor terminal
groups. TB and ID showed similar and intermediate LUMO energies. The
electrochemical bandgap is in a quite good accord with the observed
optical bandgap.
LUMO = ꢀ Ered vs Fc+/Fc + 5.1 (eV)
+
where Fc /Fc redox potential is assumed 5.1 eV below vacuum level
[
52].
HOMO and LUMO energies along with oxidation and reduction po-
tentials of the dyes are reported in Table 3.
Two oxidation processes are evident for all the dyes and being these
processes semi-reversible it has been possible to calculate the relative
oxidation potentials as the halfwave potential of the anodic and cathodic
peaks (E ½). The single reduction event was not reversible for all the four
Fig. 4. CV graphs of the synthesized dyes drop casted as thin film on a Pt working electrode.
7

S. Fusco et al.
Dyes and Pigments xxx (xxxx) xxx
3
.2. Electrical characterization of dye based OFETs
electrode).
The curves in Fig. 6 reveal also that, in the low (i.e VDS <5 V) region,
the transistor response is far from being perfectly linear, as predicted by
the MOSFET model. This feature is commonly observed for n-type
bottom-contact organic field-effect transistors based on both evaporated
[54] and solution-processed active layers [55]. This undesired effect is
related to the so-called contact resistance phenomenon whose physical
reasons can be ascribed both to the deteriorated film morphology close
the electrodes and/or to the mismatch between the gold Fermi level and
the energy levels (i.e. LUMO or HOMO) of the frontier orbitals involved
in the charge transport phenomenon.
The synthesized dyes have been spin coated as active layers of OFET
(
see the experimental section from more details about the fabrication
process) in order to assess their electrical properties by estimating the
corresponding field-effect response parameters. Different batch of the
synthesized molecules were characterized, all giving comparable results;
in this report we will discuss in particular the transistors affording the
best performance. The results ot the electrical characterization of these
transistors are reported in Table 4. The OFET characterization was
specifically pursued by recording in vacuum both transfer (IDS vs VGS at
fixed VDS) and output (IDS vs VDS at fixed VGS) curves. Fig. 5, in partic-
ular, reports a set of transfer curves acquired in the saturation regime for
both hole (negative VGS and VDS = ꢀ 50 V) and electron (positive VGS
and VDS = + 50 V) accumulation regions. As expected from the low lying
LUMO [53], all the dyes presented a significant electron mobility. As
shown, field-effect transistors based on the TB, ID and IDM molecules
exhibited a clear ambipolar behavior and the IDS current could be
enhanced by both positive and negative VGS voltages. On the contrary,
the DCV device displayed a pure n-type response and no hole accumu-
lation phenomenon was observed.
In parallel with the electrical characterization, semiconducting film
surfaces were investigated by atomic force microscopy (AFM). In Fig. S3,
representative morphologies are reported for all the analyzed com-
pounds. All the film surfaces result compact and slightly wrinkled with
the presence of sparse precipitates. Surfaces of the samples DCV in
Fig. S18(a) and ID in Fig. S18(b) seem rather similar, being very flat and
characterized by the presence of small holes of about 100 nm for DCV
and 200 nm in the case of ID. Moving on to the other two samples, the
surface morphologies became more structured and small grains and
clusters can be more easily identified. In the case of TB samples (Fig. S18
(d)), in particular, the microstructure is composed of uniformly
distributed grains with a few hundreds of nm size. In the Table S1, the
Table 4 summarizes the mobility and the threshold voltage values
extracted from the transfer curves recorded in the saturation regime.
From this, it is immediately clear that TB is the dye providing the best
charge transport properties, with mobility values, for both electrons and
holes, largest by far. ID and IDM molecules, on the other hand, behave
rather similarly, with the former showing, however, significantly higher
roughness (
σ) and the correlation length (ξ) of the images are reposted
for completeness. Typically, the correlation length is an underestimation
of the average size of the grains. As a whole, TB sample seems to be
morphologically the best with a low roughness and a relative high
correlation length. These features should explain the superior electrical
performances of this compound as compared to the other dyes.
(
by about a factor 2x) electron mobility values. As aforementioned, for
DCV no hole accumulation effect could be detected. This compound was
characterized by the worst performances in terms of low mobility values
and high threshold voltages also as far as the n-type response is con-
cerned. In particular, for DCV, the mean Vth value was very close to 40
V. Only IDM showed similar values of Vth, which, on the contrary, was
found to be significantly reduced for the other two molecules. For all the
analyzed compounds, specific field-effect tests carried out in air sug-
gested a considerable reduction of the mobility values by a factor
exceeding, typically, one order of magnitude. As expected, mobility
degradation was more severe for electrons.
4. Conclusions
In this paper, we reported on the synthesis of four novel organic dyes,
characterized by a common molecular fragment based on a DPP electron
acceptor core symmetrically functionalized with two electron donor
bifuran moieties and differing only for the terminal units constituted by
auxiliary electron acceptor groups of different strength. The dyes were
characterized by a fair good thermal stability in air, with decomposition
◦
Fig. 6 reports the output curves recorded for a TB transistor
considering both hole (negative VGS and VDS) and electron (positive VGS
and VDS) accumulation regimes. Similar sets of curves acquired for the
other compounds are included in the Supplementary Information File
temperature higher than 337 C. The optical properties of the synthe-
sized dyes were, moreover, analyzed both in solution and as thin film.
All the dyes showed
a very high molar extinction coefficients
ꢀ 1
M and a clear dependence of the absorption
5
ꢀ 1
approaching 10 cm
(
see Fig. S15–17). The ambipolar nature of TB is confirmed by the
maximum wavelengths from the strength of the auxiliary electron
acceptor groups. As expected, the alternation of electron donor and
electron acceptor groups along the molecular backbone determines an
optical absorption significantly extending in the NIR zone of the spec-
trum; this feature is particularly enhanced in the optical absorption of
thin films which is broader as compared to that of dyes in solution. Thin
films of the dyes are characterized by very low optical bandgaps, ranging
from 1.57 eV (DCV dye) to the very striking value of 1.29 eV for IDM.
Electrochemical characterization performed by Cyclic Voltammetry on
drop casted dye films provided some information about HOMO and
LUMO energies of the reported molecules. As far as LUMO energies are
concerned it was found that the LUMO stability becomes higher with
increasing the strength of the auxiliary electron acceptor groups. All the
dyes show sufficiently low LUMO energies to act as n-type semi-
conductors. Films spin-coated from chloroform/tetrachloroethane so-
lutions of the synthesized dyes were then used as active layers in OFETs
in order to assess the nature and the quality of their charge transport
properties. As expected, all the dyes show electron-transporting prop-
erties. Moreover, three of them exhibit also a quite balanced hole-
transporting capability resulting thus in ambipolar properties: ambipo-
lar behavior can emerge also because of the extremely low bandgap of
these materials. TB is the dye with the best charge transport properties,
observation that, in the p-type response (i.e. output curves on the left), at
low VGS and high VDS (both considered in absolute value), the IDS
behavior is basically ruled by the injection of electrons from the drain
electrode (i.e. in these conditions, VGD is largely positive and electrons
can be accumulated in the semiconducting region near the drain
Table 4
Electron and hole mobility (
μ
) and threshold voltage (Vth) values estimated by
Eq. (1) for the transistors based on the reported dyes.
av
2
MAX
e
av
h
2
MAX
h
Dyes
μ
e
* (cm /
μ
V
th
/
μ
*(cm /
μ
Vth/p-
2
2
V⋅s)
(cm /V⋅s)
n-
type
V)
V⋅s)
(cm /
type
(V)
V⋅s)
(
ꢀ
ꢀ
2
3
ꢀ 3
2.1*10
TB
(1.7 ±
2.2*10
1.2*10
15 ±
3
(1.6 ±
ꢀ 3
0.4)*10
ꢀ 22 ±
1.5
ꢀ
ꢀ
2
3
0
.4)*10
DCV
(1.0 ±
.2)*10
39.5
± 1.7
No Field
effect
No Field
effect
No
0
Field
effect
ꢀ 16 ±
2
ꢀ
3
ꢀ 4
ID
(3.7 ±
4.8*10
25 ±
3
(5.2 ±
ꢀ 4
6.1*10
8.1*10
ꢀ
ꢀ
3
3
0
.8)*10
(1.9 ±
.2)*10
0.6)*10
(7.4 ±
ꢀ
3
ꢀ 4
IDM
2.05*10
36 ±
1.4
ꢀ 16.5
± 5
ꢀ 4
0
1.0)*10
*
ꢀ 2
Mean values were achieved by considering at least four transistors for any
molecule. Error bars represent the standard deviation values.
with maximum electron and hole mobility of, respectively, 2.2⋅10 and
ꢀ 3
2
2.1⋅10 cm /V⋅s. In conclusion, this work demonstrates the possibility,
8

S. Fusco et al.
Dyes and Pigments xxx (xxxx) xxx
Fig. 5. Transfer curves recorded in saturation regime for the four different DPP dye-based transistors. The p-type (negative VGS, VDS = ꢀ 50 V) and n-type (positive
V
GS, VDS = 50 V) responses are reported on the left and on the right, respectively.
Fig. 6. Output curves recorded for a TB based transistor under negative (on the left) or positive (on the right) VGS and VDS voltages.
in relatively easy way, of engineering the molecular structures of DPP
derivatives to tune optical properties and electrical performances by
properly modifying electron acceptor end groups in a common molec-
ular motif. The good electrical properties along with a significantly
narrow bandgap suggest these dyes as interesting candidate in the field
of organic photovoltaics and this aspect will be addressed in upcoming
works.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
5. Acknowledgements
CRediT authorship contribution statement
This research was supported by the Italian MIUR through Progetto
PON “E-DESIGN - Combinazione di design, elettronica e materiali
multifunzionali per nuovi componenti estetici - ARS01_01158. Authors
kindly acknowledge Dr. Roberto Buscaino (University of Turin) for the
support in mass spectroscopy analyses.
Sandra Fusco: Investigation, Data curation, Writing - original draft.
Mario Barra: Investigation, Writing - review & editing. Matteo
Bonomo: Investigation. Antonio Cassinese: Conceptualization, Re-
sources. Roberto Centore: Conceptualization, Supervision. Fabio
Chiarella: Investigation. Francesco Senneca: Investigation. Antonio
Carella: Conceptualization, Supervision, Writing - review & editing.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
9

S. Fusco et al.
Dyes and Pigments xxx (xxxx) xxx
org/10.1016/j.dyepig.2020.109026.
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of small molecule solar cells via the more polarized DPP-attached donor units. Phys
Chem Chem Phys 2012;14:14238–42. https://doi.org/10.1039/c2cp42050c.
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