Jean-Jacques André

Synthesis and physico-chemical studies of neutral and chemically oxidized forms of bis(octaalkyloxyphthalocyaninato) lutetium

Abstract The synthesis of three differently substituted bis(octaalkyloxyphthalocyaninato) lutetium complexes and of their chemically oxidized hexachloroantimonate derivatives is described. Their thermotropic polymorphism is studied using polarizing microscopy, differential scanning calorimetry, and X-ray diffraction. A tentative model describing the columnar (discotic) structure of one of the observed mesophases is proposed. The magnetic behavior of the neutral complexes has been determined by ESR as a function of temperature, showing the presence of one spin per molecule. The structural transitions are associated with the change of shape and width of the ESR signal.

Preparation and study of a lithium phthalocyanine radical: optical and magnetic properties

Abstract A metallophthalocyanine radical, PcLi, was prepared by electrochemical oxidation of the corresponding dianion. The redox properties in solution show that PcLi is both easily reduced and easily oxidized (ΔE = Ered1/2-E1/2ox = 0.83 V). This property is of the utmost importance for obtaining intrinsic molecular semiconductors. The optical properties are similar to those for oxidized phthalocyanines and involve two low-lying π-π transitions at 430 and 810 nm. Despite the tendency to aggregation, the ESR spectrum of the isolated PcLi molecule has been obtained. It is consistent with an unpaired electron delocalized over the whole phthalocyanine ring.

Lutetium bisphthalocyanine: the first molecular semiconductor. Conduction properties of thin films of p- and n-doped materials

Abstract Thin films of bisphthalocyaninatolutetium derivatives (Pc 2 Lu) have been doped using a co-sublimation technique with an electron acceptor (dichlorodicyanoquinodimethane, DDQ) and an electron donor (tetraphenyldithiapyranylidene, DIPSΦ 4 ). The conduction properties of Pc 2 Lu and of p- and n-type doped materials have been determined and correlated with the optical characteristics of the films. Pc 2 Lu is the first molecular semiconductor that satisfies the following three criteria: (i) it is a molecular material, (ii) its intrinsic conductivity is in the range 10 −6 −10 −1 Ω −1 cm −1 , (iii) it can be doped with both electron acceptors and electron donors.

The first field effect transistor based on an intrinsic molecular semiconductor

Abstract An insulated gate field effect transistor based on an intrinsic molecular semiconductor, bis(phthalocyaninato)lutetium (Pc2Lu), is described for the first time.

A well-behaved field effect transistor based on an intrinsic molecular semiconductor

Abstract The electrical characteristics of an insulated gate field effect transistor (FET) based on an intrinsic molecular semiconductor, bis(phthalocyaninato)lutetium (Pc 2 Lu), are described. All the usual parameters of conventional devices are determined; the threshold voltage (−2 v), the transconductance (0.5X 10 −9 Ω −1 ) and the amplification factor (15) are compared with the values found for silicon-based devices.

Electrical properties of condensed phases of the mesogen bis (octa-octadecyloxymethylphthalocyaninato) lutetium

Abstract The electrical properties of the mesogen bis (octa-octadecyloxymethylphthalocyaninato)lutetium are studied in the isotropic liquid crystalline and solid phases. An electronic conductivity is demonstrated in the two ordered phases. The addition of known amounts of chemically oxidized species increases the overall conductivity. The condensed phases are made of columns formed by superposition of the rigid aromatic cores. The conductivity measurements are discussed in terms of intra- and inter-columnar processes.

Effects of solvent on the morphology and crystalline structure of lithium phthalocyanine thin films and powders

Abstract The effect of acetone vapours on the phase transformation x → α was studied by means of electron microscopy, X-ray diffractometry, electron spin resonance (ESR) and optical spectroscopy on thin films and powders of lithium phthalocyanine (PcLi). The exposure of amorphous films of PcLi, deposited at a substrate temperature of 15 °C onto glass slides, to solvent vapours of acetone at room temperature, induces important changes in the morphology and crystalline structure of the films. Exposure to acetone vapours under low pressure (200 mbar)leads to an x-like structure with low crystallinity, uniform coverage and no preferential ordering. Longer exposure in a saturated acetone vapour atmosphere results in an important phase transformation to the α form which leads to crystallites up to several micrometres large lying in the plane of the substrate and forming a discontinuous coverage. In the case of x powders treated in liquid acetone, a phase transformation to the α form of PcLi is observed. This x → α phasechange induced by the acetone vapour treatment is interpreted in terms of crystal size effects and compared with the phase transformation observed in thin films deposited at various substrate temperatures in the range 25–225 °C.

Molecular semiconductors and junction formation: Phthalocyanine derivatives☆

Abstract Three complementary aspects of the semiconducting behavior of molecular materials have been examined : the intrinsic semiconductivity, the possibility of doping and the realization of an extended space charge layer in a junction. This study is focused on the phthalocyanine derivatives. Molecular materials are all insulators when undoped. In contrast an intrinsic semiconducting behavior has been observed on two radicalar phthalocyanines: Pc2Lu (substituted or not). From their magnetic, optical and electrical properties, they may be described as intrinsic semiconductors (for Pc 2 Lu : σ RT ⋍ 10 −5 ohm −1 cm −1 ; activation energy : 0.33 eV). Controlled doping can be achieved by cosublimation of acceptors or donors (DDQ, TCNQ, TTF, DIPS). The formation of a space charge region in M1/PcM/M2 cells (M = Zn, Ni ; M1 = Au; M2 = Al, Cr) has been shown to be occur by transferring electrons from the rectifying electrode M2 to the oxygen molecules bound at the surface of the crystallites of PcM. The extent of the space charge region cannot be varied when an external voltage is superimposed. This uncontrolled rectifying behavior is linked to the migration of the ionized dopant within the molecular material. By using, at a low doping level, larger molecular weight acceptor molecules as dopant to prevent migration at room temperature, it has been possible to observe a diode behavior in strict absence of oxygen. A localized molecular model of the formation of junctions is proposed and the potentialities of molecular materials in the realization of such devices are discussed.

Polymorphism in powders and thin films of lithium phthalocyanine. An X-Ray, optical and electron spin resonance study☆

Abstract The polymorphism of lithium phthalocyanine (PcLi) was studied for powders as well as for thin films deposited on glass substrate. Powders were yielded by two electrochemical synthesis routes. While the powders obtained by electrochemical oxidation of PcLi 2 in acetonitrile and acetone lead to an x structure, the synthesis route proposed by M.A. Petit leads apparently to the α form of PcLi. Their very different sensitivity to oxygen, observed using electron spin resonance (ESR) spectroscopy, is explained by means of the crystallographic structure of both polymorphi. Thin films were prepared by vacuum deposition and phase transitions as well as preferential ordering of the crystallites are studied as a function of increasing substrate temperature. While films deposited at low substrate temperatures, below 150°C, lead to an x-like form with the molecular columns lying in the plane of the substrate, the higher substrate temperatures favour the growth of the α form and a loss in preferential ordering. Accordingly, changes in the sensitivity to oxygen are followed as a function of substrate temperature.

Anisotropic spin transport in oriented lithium phthalocyanine thin films

Thin films of monolithium phthalocyanine (PcLi) were prepared by vacuum deposition on various substrates: cleaved mica, quartz glass, and drawn sheets of poly(tetrafluoroethylene). The highly anisotropic spin transport in these films was studied by electron spin resonance (ESR) as a function of film preparation (thickness and substrate temperature), orientation, and temperature. Transmission electron microscopy confirms the existence of enhanced orientation which can be controlled by adjusting the substrate temperature Ts during film deposition. For low substrate temperatures, homogeneous films with no azimuthal orientation are formed. Above 400 K, epitaxial growth on mica leads to rectangular wedge-shaped crystals. Raising the temperature Ts does not improve the orientation but creates a more discontinuous coverage of the substrate and reduces the electronic relaxation times T1 and T2, as monitored by ESR. When increasing the film thicknesses, a steady change towards the bulk properties is demonstrated by the decreasing ESR linewidth.