Elsevier

Polyhedron

Volume 136, 4 November 2017, Pages 70-73
Polyhedron

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

https://doi.org/10.1016/j.poly.2017.03.006Get rights and content

Abstract

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 [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 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.

Graphical abstract

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.

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Introduction

In addition to unique applications of organic semiconductors in low-cost, large-area and flexible electronics [1], they have 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], and graphene [4], 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 π-electrons and the localized d-spins interacts (π–d system), and several interesting phenomena such as negative magnetic resistance and magnetic field induced superconductivity have been found in the π–d system [5], [6]. In most cases, however, the π–d interaction is usually very weak due to spatial separation between conducting π-electrons 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 π–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 a 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 π–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 the 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)] (Fig. 1), are reported. Fabrication of FET devices based on the TTF-based Cu(II) complexes was also examined.

Section snippets

Experimental

New Schiff base-type TTF-ligands and their Cu(II) complexes were prepared according to the reported method [8], [9]. Cyclic voltammetry was carried out by an 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

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 1. 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

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, 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

Acknowledgement

This work was supported by Grant-in-Aid for Challenging Exploratory Research (Grant Number JP15K13811) from the Japan Society for the Promotion of Science (JSPS) – Japan.

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