Final Report Summary - RADICALELECTRONICS (Paramagnetic metal complex transistors)
The aim of this project was the design, synthesis and characterisation of small molecules suitable for application in the field of molecular electronics such as transistors and / or spintronics in the case of paramagnetic compounds. In particular, metal complexes have been selected as small molecules in order to study their properties for possible field effect transistor (FET) applications. Ligands bearing nitrogen and sulphur atoms as donor atoms, such as dithiolenes, diimines and quinolinethiole, have been used to prepare the complexes.
Several diimine-dithiolato mixed-ligand nickel complexes have been prepared and characterised by elemental analysis, cyclic voltammetry and UV-vis spectroscopy:
(Ni(Indoledt)(4,4'-EtCO2-2,2'-bipy)) (1);
(Ni(bdt)(4,4'-MeCO2-2,2'-bipy)) (2);
(Ni(dddt)(4,4'-EtCO2-2,2'-bipy)) (3)
; (Ni(dmit)(4,4'-EtCO2-2,2'-bipy)) (4);
(Ni(Indoledt)(3,4,7,8-Cl4-1,10-phen)) (5);
(Ni(dmit)(3,4,7,8-Cl4-1,10-phen)) (6);
(Ni(dddt)(3,4,7,8-Cl4-1,10-phen)) (7);
(Ni(bdt)(3,4,7,8-Cl4-1,10-phen)) (8).
The compound 2 has been structurally characterised by X-Ray diffraction showing a square planar coordination for the nickel atom. Computational investigations at the density functional theory (DFT) level have been done on the complexes 1-8. The HOMO-LUMO transition will have ligand-to-ligand charge transfer (CT). While the compounds 4, 6, 7 and 8 seem to be unstable in a high polar solvent as DMF, the other complexes show a strong negative solvatochromism which suggests that these complexes are potential second-order Non-Linear Optic chromophores. NLO measurements on complexes 1, 2, 3 and 5 have been done in collaboration with the group of Prof. Maddalena Pizzotti in Milan (Italy). The obtained values of (first hyperpolarisability) are higher than those reported in the literature for similar compounds. FET measurements have been done on the compounds 2 and 4 but they do not show any conducting properties. Unfortunately, attempts to synthesise the corresponding copper (II) paramagnetic complexes in a similar manner to that of 1-8 proved unsuccessful.
The (Ni(bdt)(benzo-1,2-diimine)) (9), another nickel mixed-ligand complex has been also prepared and characterised. The crystal structure shows that several short solid state interactions are present between the molecules, which are of crucial importance for the conductivity properties. DFT calculations show that the frontier orbitals (HOMO and LUMO) are delocalised on both ligands and the energy gap between them is quite small (1.69 eV). Electrochemistry and electronic spectroscopy measurements are also in agreement with the calculations. This compound has not been tested so far.
The complexes (M(8-quinolinethiolate)2) (M = nickel (10) or palladium (11); see Chart1) which show interesting intermolecular short interactions, have been prepared and tested to study their electronic properties, after have been deposited as thin films (nanoscale thickness) by physical vapour deposition (PVD) onto chips with gold inter-digitated electrodes; this work has been done in collaboration with the group of Prof. Kunio Awaga at the Nagoya University in Japan. While the palladium compound does not exhibit any conductivity, the nickel one presented a small charge mobility (5·10-5 cm2V-1S-1) with a good on / off ratio of 104 (ratio between the current which passes through the device when the electric field at the gate is on and when it is off, it is a measurement of the so called gate effect). This complex in thin film form has been fully characterised by X-ray diffraction (XRD), magnetic measurements and electronic microscopies in collaboration with the group of Dr Sandrine Heutz at the Imperial College in London. These studies shown that this compound is present as a new ferromagnetic phase, different from that presented in the bulk form. Mobility measurements in the presence of a magnetic field, in order to investigate a magnetoresistance effect, have been done at the Nagoya University. Also the phase present in the bulk form has been studied and it shows good conductivity but not gate effect. This compound exhibits a giant negative magnetoresistance effect (a change in the conducting property of a material when it is measured in a magnetic field) which is very important for spintronic applications.
Another class of molecules which has been investigated is that of dibenzo(14)annulene macrocycles bearing halogen atoms (fluorine or chlorine) at the benzene rings, in order to increase the solid state intermolecular interactions and, consequently, the conducting properties in these compounds. The nickel complexes of both ligands have been successfully synthesised (12, 13) as well as the copper analogous with the macrocycle fluorinated (14), while attempts to prepare cobalt (with both ligands) and copper (chlorinated ligand) complexes failed; moreover, complex 12 has been structurally characterised. The fluorinated complexes of both, nickel and copper have been deposited by PVD onto chips with gold or platinum electrodes and their transport properties studied. They show conducting properties but, unfortunately, any gate effect. Being the copper compound a paramagnetic molecule, magnetoresistance measurements are scheduled in Nagoya. The complex 13 decompose during the sublimation; moreover, the attempts to synthesise the corresponding cobalt complexes, as well as the copper compound with the chlorinated ligand, in a similar manner to that of 12 - 14 proved unsuccessful.
The complexes (Pt(tetrafluorobenzo-1,2-diimine)2) (15) and (Co(tetrachlorobenzo-1,2-diimine)2) (16) have been also prepared and characterised. Calculation, electrochemistry and electron-spectroscopy results, comparable with to those found for similar compounds which exhibit very interesting FET properties, show that these complexes are promising. PVD deposition and FET characterisation measurements are scheduled.
The work carried out under this project is a part of a research field that could have profound consequences for the future application of low-cost electronic materials with enhanced, or even unique, functional properties. This could have evident and very important socio-economic impacts; indeed, organic electronics is today a USD 1 billion industry and is projected to be worth USD 30 billion by 2027.
Several diimine-dithiolato mixed-ligand nickel complexes have been prepared and characterised by elemental analysis, cyclic voltammetry and UV-vis spectroscopy:
(Ni(Indoledt)(4,4'-EtCO2-2,2'-bipy)) (1);
(Ni(bdt)(4,4'-MeCO2-2,2'-bipy)) (2);
(Ni(dddt)(4,4'-EtCO2-2,2'-bipy)) (3)
; (Ni(dmit)(4,4'-EtCO2-2,2'-bipy)) (4);
(Ni(Indoledt)(3,4,7,8-Cl4-1,10-phen)) (5);
(Ni(dmit)(3,4,7,8-Cl4-1,10-phen)) (6);
(Ni(dddt)(3,4,7,8-Cl4-1,10-phen)) (7);
(Ni(bdt)(3,4,7,8-Cl4-1,10-phen)) (8).
The compound 2 has been structurally characterised by X-Ray diffraction showing a square planar coordination for the nickel atom. Computational investigations at the density functional theory (DFT) level have been done on the complexes 1-8. The HOMO-LUMO transition will have ligand-to-ligand charge transfer (CT). While the compounds 4, 6, 7 and 8 seem to be unstable in a high polar solvent as DMF, the other complexes show a strong negative solvatochromism which suggests that these complexes are potential second-order Non-Linear Optic chromophores. NLO measurements on complexes 1, 2, 3 and 5 have been done in collaboration with the group of Prof. Maddalena Pizzotti in Milan (Italy). The obtained values of (first hyperpolarisability) are higher than those reported in the literature for similar compounds. FET measurements have been done on the compounds 2 and 4 but they do not show any conducting properties. Unfortunately, attempts to synthesise the corresponding copper (II) paramagnetic complexes in a similar manner to that of 1-8 proved unsuccessful.
The (Ni(bdt)(benzo-1,2-diimine)) (9), another nickel mixed-ligand complex has been also prepared and characterised. The crystal structure shows that several short solid state interactions are present between the molecules, which are of crucial importance for the conductivity properties. DFT calculations show that the frontier orbitals (HOMO and LUMO) are delocalised on both ligands and the energy gap between them is quite small (1.69 eV). Electrochemistry and electronic spectroscopy measurements are also in agreement with the calculations. This compound has not been tested so far.
The complexes (M(8-quinolinethiolate)2) (M = nickel (10) or palladium (11); see Chart1) which show interesting intermolecular short interactions, have been prepared and tested to study their electronic properties, after have been deposited as thin films (nanoscale thickness) by physical vapour deposition (PVD) onto chips with gold inter-digitated electrodes; this work has been done in collaboration with the group of Prof. Kunio Awaga at the Nagoya University in Japan. While the palladium compound does not exhibit any conductivity, the nickel one presented a small charge mobility (5·10-5 cm2V-1S-1) with a good on / off ratio of 104 (ratio between the current which passes through the device when the electric field at the gate is on and when it is off, it is a measurement of the so called gate effect). This complex in thin film form has been fully characterised by X-ray diffraction (XRD), magnetic measurements and electronic microscopies in collaboration with the group of Dr Sandrine Heutz at the Imperial College in London. These studies shown that this compound is present as a new ferromagnetic phase, different from that presented in the bulk form. Mobility measurements in the presence of a magnetic field, in order to investigate a magnetoresistance effect, have been done at the Nagoya University. Also the phase present in the bulk form has been studied and it shows good conductivity but not gate effect. This compound exhibits a giant negative magnetoresistance effect (a change in the conducting property of a material when it is measured in a magnetic field) which is very important for spintronic applications.
Another class of molecules which has been investigated is that of dibenzo(14)annulene macrocycles bearing halogen atoms (fluorine or chlorine) at the benzene rings, in order to increase the solid state intermolecular interactions and, consequently, the conducting properties in these compounds. The nickel complexes of both ligands have been successfully synthesised (12, 13) as well as the copper analogous with the macrocycle fluorinated (14), while attempts to prepare cobalt (with both ligands) and copper (chlorinated ligand) complexes failed; moreover, complex 12 has been structurally characterised. The fluorinated complexes of both, nickel and copper have been deposited by PVD onto chips with gold or platinum electrodes and their transport properties studied. They show conducting properties but, unfortunately, any gate effect. Being the copper compound a paramagnetic molecule, magnetoresistance measurements are scheduled in Nagoya. The complex 13 decompose during the sublimation; moreover, the attempts to synthesise the corresponding cobalt complexes, as well as the copper compound with the chlorinated ligand, in a similar manner to that of 12 - 14 proved unsuccessful.
The complexes (Pt(tetrafluorobenzo-1,2-diimine)2) (15) and (Co(tetrachlorobenzo-1,2-diimine)2) (16) have been also prepared and characterised. Calculation, electrochemistry and electron-spectroscopy results, comparable with to those found for similar compounds which exhibit very interesting FET properties, show that these complexes are promising. PVD deposition and FET characterisation measurements are scheduled.
The work carried out under this project is a part of a research field that could have profound consequences for the future application of low-cost electronic materials with enhanced, or even unique, functional properties. This could have evident and very important socio-economic impacts; indeed, organic electronics is today a USD 1 billion industry and is projected to be worth USD 30 billion by 2027.
