Organic field-effect transistor based on paramagnetic Cu(II) neutral complexes coordinated by Schiff base-type TTF ligands

Article abstract of DOI:10.1016/j.poly.2017.03.006

We have succeeded in preparation of novel paramagnetic Cu(II) complexes composed of tetrathiafulvalene (TTF)-based ligands as electrical conducting units, [Cu^II(Bz-OMe-^tBu-sae-TTF)₂], [Cu^II(EDT-bissae-TTF)] and [Cu^II(Bz-bis(OMe-^tBu-sae)-TTF)]. Electrochemical measurements of these neutral complexes revealed that they exhibited two pairs of redox waves attributed to the oxidation of TTF moieties. Among the newly synthesized TTF complexes, only [Cu^II(EDT-bissae-TTF)] gave a radical salt by electrochemical crystallization, but its resistive behavior was insulating. Field-effect transistors (FET) were fabricated with spin-coated films of [Cu^II(Bz-OMe-^tBu-sae-TTF)₂] and [Cu^II(EDT-bissae-TTF)], and their FET performances were very low compared with the conventional organic thin film transistors (OTFT) based on TTF derivatives. Atomic force microscopy (AFM) images of the spin-coated films indicated the non-uniform film surfaces with many small grains leading to the large grain boundaries, which probably causes the low carrier mobility of the FET devices.

Full text of DOI:10.1016/j.poly.2017.03.006

Accepted Manuscript
Organic Field-Effect Transistor Based on Paramagnetic Cu(II) Neutral Com-
plexes Coordinated by Schiff Base-Type TTF Ligands
Atsushi Wachi, Yusuke Kudo, Aoba Kanesaka, Hiroyuki Nishikawa, Takuya
Shiga, Hiroki Oshio, Masayuki Chikamatsu, Reiko Azumi
PII:
S0277-5387(17)30183-3
DOI:
http://dx.doi.org/10.1016/j.poly.2017.03.006
POLY 12518
Reference:
Polyhedron
To appear in:
Received Date:
Revised Date:
Accepted Date:
28 December 2016
23 February 2017
6 March 2017
Please cite this article as: A. Wachi, Y. Kudo, A. Kanesaka, H. Nishikawa, T. Shiga, H. Oshio, M. Chikamatsu, R.
Azumi, Organic Field-Effect Transistor Based on Paramagnetic Cu(II) Neutral Complexes Coordinated by Schiff
Base-Type TTF Ligands, Polyhedron (2017), doi: http://dx.doi.org/10.1016/j.poly.2017.03.006
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Organic Field-Effect Transistor Based on Paramagnetic Cu(II) Neutral Complexes
Coordinated by Schiff Base-Type TTF Ligands
Atsushi Wachi a, Yusuke Kudo a, Aoba Kanesaka a, Hiroyuki Nishikawa a, *
a Department of Chemistry, Faculty of Science, Ibaraki University, 2-1-1 Bunkyo, Mito,
Ibaraki 310-8512, Japan
Takuya Shiga b, Hiroki Oshio b
b Deartment of Chemistry, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba, Ibaraki
305-8571, Japan
Masayuki Chikamatsu c
c Research Center For Phtovoltaic Technologies, National Institute of Advanced Industrial
Science and Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan
Reiko Azumi d
d Electronics and Photonics Research Institute, National Institute of Advanced Industrial
Science and Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan
* Correspond author. Tel. / fax: +81 29 228 8371.
E-mail: hiroyuki.nishikawa.sci@vc.ibaraki.ac.jp
Key Words: molecular conductor; paramagnetic metal complex; tetrathiafulvalene;
field-effect transistor
Abstruct
We have succeeded in preparation of novel paramagnetic Cu(II) complexes
composed of tetrathiafulvalene (TTF)-based ligands as electrical conducting units,
[CuII(Bz-OMe-tBu-sae-TTF)2],
[CuII(EDT-bissae-TTF)]
and
1

[CuII(Bz-bis(OMe-tBu-sae)-TTF)]. Electrochemical measurements of these neutral complexes
revealed that they exhibited two pairs of redox waves attributed to the oxidation of TTF
moieties. Among the newly synthesized TTF complexes, only [CuII(EDT-bissae-TTF)] gave a
radical salt by electrochemical crystallization, but its resistive behavior was insulating.
Field-effect transistors (FET) were fabricated with spin-coated films of
[CuII(Bz-OMe-tBu-sae-TTF)2] and [CuII(EDT-bissae-TTF)], and their FET performance was
very low compared with the conventional organic thin film transistors (OTFT) based on TTF
derivatives. Atomic force microscopy (AFM) images of the spin-coated films were indicated
the non-uniform film surfaces the existence of many small grains, which probably causes the
low carrier mobility of the FET devices. The large grain boundaries are attributable to the low
carrier mobility.
1. Introduction
In addition to unique applications of organic semiconductors in low-cost, large-area
and flexible electronics [1], they have been attracted much attention as a candidate for a
spin-related device, so-called spintronics. Organic spintronic materials are generally
considered to have smaller spin-orbit interaction leading to a longer spin relaxation time of
spin carriers compared with the inorganic spintronic materials, and organic spin-valve devices
have been reported by using organic molecules such as pentacene [2], rubrene [3], graphene
[4] and so on as a carrier transport layer.
On the other hands, one of the recent research trends in the field of molecular
conductors is to develop molecular based magnetic conductors where the conductive
-electrons and the localized d-spins interacts (-d system), and several interesting
phenomena such as negative magnetic resistance, magnetic field induced superconductivity
and so on have been found in the -d system [5, 6]. In most cases, however, the -d
interaction are usually very weak due to spatial separation between conducting -electrons
2

and localized d-spins lying on the different molecules, which results in the emergence of
interesting phenomena only at extremely low temperatures. In order to enhance the -d
interaction, we prepared
a
TTF-based metal complexes, [CuII(EDT-sae-TTF)2]
(EDT-Hsae-TTF, 1a; 4-(2-salicylideneiminoethylthio)-5-methyl-4’,5’-ethylenedithio-TTF), in
which TTF-ligands directly coordinate to the paramagnetic Cu(II) ion through the Schiff-base
type coordination site. Because of the neutral valence state of [CuII(EDT-sae-TTF)2], it is
soluble to common organic solvents. Thus, [CuII(EDT-sae-TTF)2] can be utilized to the active
layer of OTFT involving the localized spins. Organic spintronic devices that have localized
spins in their carrier transport layers have rarely been studied because spin current may be
scattered by the local moments. Using the -d interaction in the spin-containing carrier
transport layer would suppress the spin scattering by applying external magnetic field,
resulting in elongation of the spin relaxation time of the spin current. In previous work, we
fabricated a thin-film FET device of [CuII(EDT-sae-TTF)2] and found it operative although its
FET performance was very low [7]. The low performance probably results from the low
quality of the thin film composed of small grains. In order to improve the device properties,
we prepared new TTF-based metal complexes which show large intermolecular interaction to
obtain larger grains in the thin films. In this paper, synthesis and electrochemical properties of
new TTF-based Cu(II) complexes, [CuII(Bz-OMe-tBu-sae-TTF)2], [CuII(EDT-bissae-TTF)]
and [CuII(Bz-bis(OMe-tBu-sae)-TTF)] (Figure 1), are reported. Fabrication of FET devices
based on the TTF-based Cu(II) complexes was also examined.
3

R3
R3
R2
S
R1
R1
S
S
S
S
R4
R4
N
O
O
N
S
S
N
N
O
O
R1
R1
S
S
S
S
Cu
S
Cu
S
S
S
S
R1
R1
R4
R4
R2
R3
R3
[CuII(EDT-sae-TTF)2 : R1-R1 = S(CH2)2S, R2 = CH3, R3 = R4 = H
[CuII(Bz-OMe-tBu-sae-TTF)2 : R1-R1 = (CH=CH)2,
R2 = CH3, R3 = OCH3, R4 = tBu
[CuII(EDT-bissae-TTF)] : R1-R1 = S(CH2)2S, R2 = R3 = H
[CuII(Bz-bis(OMe-tBu-sae)-TTF)] : R1-R1 = (CH=CH)2,
R3 = OCH3, R4 = tBu
Figure 1. TTF-paramagnetic metal complexes
2. Experimental
New Schiff base-type TTF-ligands and their Cu(II) complexes were prepared by
according to the reported method [8, 9]. Cyclic voltammetry was carried out by an
o
electrochemical analyzer (ALS model 620A or HOKUTO DENKO HZ-5000), at 25 C in
dichloromethane containing n-Bu4NPF6 as a supporting electrolyte at a scan rate of 0.1 V/s,
using a glassy carbon working electrode, Pt wire counter electrode and a saturated calomel
electrode (SCE) as a reference. The solution was deaerated with N2 bubbling before
measurements.
Thin films of the newly synthesized TTF-metal complexes were prepared from
chloroform by spin-coat method with rotating speed of 1500 rpm onto a highly doped n-type
silicon wafers covered with 300 nm SiO2 layer. The surface of the thin films was washed with
acetone and ethanol, and then treated with hexamethyldisilazane before spin-coating.
Morphology of the thin film was investigate by the atomic force microscopy (AFM) using a
Seiko Instruments, SPI 3800 and SPA300. The FET devices were fabricated by top-contact
method with Au contacts deposited on to the spin-coated films on the silicon substrate. The
FET properties were evaluated by use of a semiconductor chatacterization system (Keithley
4200) under nitrogen atmosphere.
4

3. Results and Discussion
3.1. Synthesis and Electrochemical Properties
New Schiff-base type TTF-ligands, Bz-OMe-tBu-Hsae-TTF (1b), EDT-bisHsae-TTF
(2a) and Bz-bis(OMe-tBu-Hsae)-TTF (2b), were synthesized by the condensation reaction of
salicylaldehyde (5) or 3-methoxy-5-tert-butylsalicylaldehyde (6) and aminoethylthio-TTF
derivatives
(3b,
4a-b)
as
shown
in
Scheme.
The
Cu(II)
complex,
[CuII(Bz-OMe-tBu-sae-TTF)2], was prepared by mixing a methanolic solution of CuII(AcO)2･
H2O and a CH2Cl2 solution of Bz-OMe-tBu-Hsae-TTF. The Cu(II) complexes with the
tetradentate ligands, [CuII(EDT-bissae-TTF)] and [CuII(Bz-bis(OMe-tBu-sae)-TTF)], were
prepared by the addition of a methanolic solution of CuII(ClO4)2･6H2O to a CH2Cl2 solution
of the corresponding TTF-ligands in the presence of triethylamine.
Scheme
R3
S
R3
R1
R1
S
S
S
S
N
S
R1
R1
S
S
S
S
NH2
R2
HO
+
CH2Cl2
R4
R2
R4
1a-b, 2a-b
OH
O
3a-b, 4a-b
S
R3
N
5: R3 = R4 = H
6: R3 = OCH3, R4 = C(CH3)3
1:R2 = CH3, 2:R2 =
3:R2 = CH3, 4:R2 = S(CH2)2NH2
HO
a: R1-R1 = S(CH2)2S
b: R1-R1 = (CH=CH)2
R4
a: R1-R1 = S(CH2)2S, R3 = R4 = H
b: R1-R1 = (CH=CH)2, R3 = OCH3, R4 = C(CH3)3
The Cu(II) complexes with tetradentate TTF-ligands showed two reversible redox
waves originating from the oxidation of the TTF moieties ([CuII(EDT-bissae-TTF)]: E1 = 0.49
V, E2 = 0.86 V (vs. SCE); [CuII(Bz-bis(OMe-tBu-sae)-TTF)]: E1 = 0.55 V, E2 = 0.96 V). On
the other hands, the redox waves corresponding to the oxidation of the TTF moieties of
[CuII(Bz-OMe-tBu-sae-TTF)2)] were observed at 0.46 V and 0.54 V as reversible one-electron
oxidation wave, and at 1.00 V as reversible two-electron oxidation. Separation of the first
oxidation wave of TTF moieties indicates the existence of some interaction between two TTF
moieties. In addition, reversible one-electron oxidation wave was observed at 0.85 V, which
probably corresponds to the oxidation of phenolato coordination site. Further investigation on
the redox of this complex is ongoing.
5

3.2 Field-Effect Transistors of TTF-Metal Complexes
The bottom-gate top-contact FET devices of [CuII(Bz-OMe-tBu-sae-TTF)2] and
[CuII(EDT-bissae-TTF)] showed p-type characteristics corresponding to the TTF-ligands
owing to the hole mobility. Figure 2, 3 shows the source-drain current (Isd)-drain voltage (Vd)
and the Isd-gate voltage (Vg) performance of [CuII(Bz-OMe-tBu-sae-TTF)2] and
[CuII(EDT-bissae-TTF)], respectively. The value of hole mobility and on/off ratio was
estimated to be 1.5 x 10-7 and 1.1 x102 ([CuII(Bz-OMe-tBu-sae-TTF)2]), and 1.4 x 10-7 cm2/Vs,
and 1.0 x103 ([CuII(EDT-bissae-TTF)]). The FET devices of a spin-coated thin film of
[CuII(Bz-bis(OMe-tBu-sae)-TTF)] was not operative.
100
50
15
0
-50
-100
-150
-200
10
0 V
-20 V
-40 V
-60 V
-80 V
5
-80
-60
-40
-20
0
20
-80
-60
-40
-20
0
Vsd / V
Vg / V
Figure 2. Isd-Vsd (left) and Isd-Vg graph (right) of a device with [CuII(Bz-OMe-tBu-sae-TTF)2]
thin film. A inset (left) shows a gate-voltage. Vsd (right) is -80 V as constant.
0
-100
-200
-300
-400
20
15
10
5
0 V
-20 V
-40 V
-60 V
-80 V
0
-80
-60
-40
-20
0
-80
-60
-40
-20
0
20
Vg / V
Vsd / V
Figure 3. Isd-Vsd (left) and Isd-Vg graph (right) of a device with [CuII(EDT-bissae-TTF)2] thin
film. A inset (left) shows a gate-voltage. Vsd (right) is -80 V as constant.
6

3.3 Atomic Force Microscopy (AFM)
We investigated the surface morphology of the spin-coated thin films of
[CuII(Bz-OMe-tBu-sae-TTF)2] and [CuII(EDT-bissae-TTF)] by AFM. As shown in Figure 4
(left), the morphology of [CuII(EDT-bissae-TTF)] film for scan range of 3 m2 films was not
uniform. In the enlarged views (scan range of 300 nm2, Figure 4 (right)), we did not find any
grains probably because they were too small to detect by the AFM cantilever. We also
obtained similar results for the surface of [CuII(Bz-OMe-tBu-sae-TTF)2] film. In general, the
carrier mobility in the FET depends on the grain size. The device with larger grains exhibits
relatively higher performances. In the present case, the thin films of
[CuII(Bz-OMe-tBu-sae-TTF)2] and [CuII(EDT-bissae-TTF)] had extremely small grains and
then large grain boundaries, which resulted in the quite low FET performance [10].
Figure 4. AFM images of [CuII(EDT-bissae-TTF)] thin film (left: 3 m2, right: 300 nm2)
Summary
In summary, we synthesized novel Cu(II) complexes coordinated by Schiff base-type
TTF-ligands, and their redox properties were investigated with electrochemical measurements.
We fabricated FET devices with spin-coated films of the TTF-metal complexes and the thin
films of [CuII(Bz-OMe-tBu-sae-TTF)2] and [CuII(EDT-bissae-TTF)] showed FET properties,
7

although the performances were very low. The AFM images of the surfaces of
[CuII(Bz-OMe-tBu-sae-TTF)2] and [CuII(EDT-bissae-TTF)] thin films revealed the small
grains and large grain boundaries, which is probably the reason for the low device
performances.
Acknowledgement
This work was supported by a Challenging Exploratory Research (Grant Number
JP15K13811) from the Japan Society for the Promotion of Science.
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8

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9

Synopsis
New paramagnetic Cu(II) complexes coordinated by Schiff base-type TTF-ligands have
been synthesized and field-effect transistor (FET) devices with spin-coated films of the
TTF-based metal complexes were fabricated. Thin film devices of new TTF-based metal
complexes showed FET properties. AFM measurements revealed the surface morphologies of
the thin films.
11