Benoît Heinrich

Non-volatile organic memory with sub-millimetre bending radius

High-performance non-volatile memory that can operate under various mechanical deformations such as bending and folding is in great demand for the future smart wearable and foldable electronics. Here we demonstrate non-volatile solution-processed ferroelectric organic field-effect transistor memories operating in p- and n-type dual mode, with excellent mechanical flexibility. Our devices contain a ferroelectric poly(vinylidene fluoride-co-trifluoroethylene) thin insulator layer and use a quinoidal oligothiophene derivative (QQT(CN)4) as organic semiconductor. Our dual-mode field-effect devices are highly reliable with data retention and endurance of >6,000 s and 100 cycles, respectively, even after 1,000 bending cycles at both extreme bending radii as low as 500 μm and with sharp folding involving inelastic deformation of the device. Nano-indentation and nano scratch studies are performed to characterize the mechanical properties of organic layers and understand the crucial role played by QQT(CN)4 on the mechanical flexibility of our devices.

Dendromesogens: liquid crystal organizations of poly(amidoamine) dendrimers versus starburst structures.

A new series of liquid crystalline poly(amidoamine) (PAMAM) dendrimers is described. These dendrimers are made by attaching to the 0-, 1-, 2-, 3-, and 4-generation of PAMAM-terminal promesogenic units that carry two decyloxy chains in the 3- and 4-positions of their peripheral aromatic ring. X-ray diffraction studies show that all the compounds display a hexagonal columnar mesophase. A high density of aliphatic chains imposes a curved interface with the promesogenic units that forces the molecules to adopt a radial conformation, and therefore, the columnar structure. A model for the supramolecular organization of the different generations within the columnar mesophase is proposed based on the variation of some of the structural parameters.

A Liquid Crystalline Supramolecular Complex of C60 with a Cyclotriveratrylene Derivative

Cyclotriveratrylene (CTV) derivatives substituted with 9 (1) or 18 (2) long alkyl chains have been prepared. Whereas no liquid crystalline behavior has been observed for 1, the CTV derivative 2 has mesomorphic properties. Indeed, at room temperature compound 2 exhibits a nematic phase characterized by cybotactic groups with a local lamello-columnar order. Both CTV derivatives 1 and 2 are able to form supramolecular complexes with C60 in the solid state. In both cases, the 2:1 host-guest species have been obtained as brown compounds. No liquid crystalline behavior was observed for the supramolecular complex [C60 is included in (1)2]. In contrast, observation of the brown product obtained from C60 and the CTV derivative 2 directly after preparation by polarized optical microscopy revealed a fluid birefringent phase at room temperature. When the sample is heated above 70 degrees C, the birefringence of the texture under the microscope disappears and the X-ray diffraction pattern is transformed into a pattern characteristic of a cubic phase. For the first time in thermotropic liquid crystals, the space group of this cubic phase can be assigned as I4(1)32.

A Mixed Fullerene - Ferrocene Thermotropic Liquid Crystal: Synthesis, Liquid-Crystalline Properties, Supramolecular Organization and Photoinduced Electron Transfer

Grafting of a ferrocene-containing liquid-crystalline malonate derivative to C60 led to the mixed fullerene-ferrocene material 1 which gave rise to a smectic A phase. Cholesterol was used as liquid-crystalline promoter. X-ray diffraction experiments and volumetric measurements indicated that 1 is organized in double layered structures. The corresponding supramolecular organization within the mesomorphic lamellar phase is characterized by a microsegregation of the different units (ferrocene, fullerene, and cholesterol) in distinct sublayers. In such a smectic A phase, C60 imposes the arrangement of the other molecular moieties. Photophysical studies revealed that electron transfer occurs from the donor ferrocene to the electron accepting fullerene. The formation of a long-lived radical pair, with lifetimes of the order of several hundred nanoseconds, was confirmed by time-resolved spectrometry, especially in the near infrared region, in which the radical anion of the fullerene moiety displays its characteristic fingerprint absorption.

Synthesis of Amphiphilic Fullerene Derivatives and Their Incorporation in Langmuir and Langmuir-Blodgett Films

Various amphiphilic fullerene derivatives were prepared by functionalization of [5,6]fullerene-C60-Ih (C60) with malonate or bis-malonate derivatives obtained by esterification of the malonic acid mono-esters 5–7. Cyclopropafullerene 10 was obtained by protection of the carboxylic acid function of 6 as a tert-butyl ester, followed by Bingel addition to C60 and a deprotection step (Scheme 2). The preparation of 10 was also attempted directly from the malonic acid mono-ester 6 under Bingel conditions. Surprisingly, the corresponding 3′-iodo-3′H-cyclopropa[1,9][5,6]fullerene-C60-Ih-3′-carboxylate 11 was formed instead of 10 (Scheme 3). The general character of this new reaction was confirmed by the preparation of 15 and 16 from the malonic acid mono-esters 13 and 14, respectively (Scheme 4). All the other amphiphilic fullerene derivatives were prepared by taking advantage of the versatile regioselective reaction developed by Diederich and co-workers which led to macrocyclic bis-adducts of C60 by a cyclization reaction at the C-sphere with bis-malonate derivatives in a double Bingel cyclopropanation. The bis-adducts 37–39 with a carboxylic acid polar head group and four pendant long alkyl chains of different length were prepared from diol 22 and acids 5–7, respectively (Scheme 9). In addition, the amphiphilic fullerene derivatives 45, 46, 49, 54, and 55 bearing different polar head groups and compound 19 with no polar head group were synthesized (Schemes 11–13, 15, and 5, resp.). The ability of all these compounds to form Langmuir monolayers at the air-water interface was investigated in a systematic study. The films at the water surface were characterized by their surface pressure vs. molecular area isotherms, compression and expansion cycles, and Brewster-angle microscopy. The spreading behavior of compound 10 was not good, the two long alkyl chains in 10 being insufficient to prevent aggregation resulting from the strong fullerene-fullerene interactions. While no films could be obtained from compound 19 with no polar head group, all the corresponding amphiphilic fullerene bis-adducts showed good spreading characteristics and reversible behavior upon successive compression/expansion cycles. The encapsulation of the fullerene in a cyclic addend surrounded by four long alkyl chains is, therefore, an efficient strategy to prevent the irreversible aggregation resulting from strong fullerene-fullerene interactions usually observed for amphiphilic C60 derivatives at the air-water interface. The balance of hydrophobicity to hydrophilicity was modulated by changing the length of the surrounding alkyl chains or the nature of the polar head group. The best results in terms of film formation and stability were obtained with the compounds having the largest polar head group, i.e.45 and 46, and dodecyl chains. Finally, the Langmuir films obtained from the amphiphilic fullerene bis-adducts were transferred onto solid substrates, yielding high-quality Langmuir-Blodgett films.

Peripherally Fused Porphyrins via the Scholl Reaction: Synthesis, Self-Assembly, and Mesomorphism

Oxidative coupling of activated aryl groups attached to β-positions of the porphyrin ring provides convenient access to derivatives containing peripherally fused phenanthrene and benzo[g]chrysene units. Tetra(benzochryseno)porphyrin, reported here for the first time, contains a nonplanar, sterically locked π system and shows very intense electronic absorptions in the Q range of the electronic spectrum. Tetraphenanthroporphyrins show a tendency to aggregate in solution. In one case, a discrete dimer is formed, whose structure was investigated spectroscopically and theoretically. Derivatives bearing long alkyl chains are mesomorphic and exhibit columnar phases (tetraphenanthroporphyrins) and a monoclinic 3D phase (tetrabenzochrysenoporphyrin). The symmetry of column packing in the columnar phases is dependent on the number of alkyl chains per molecule. X-ray diffraction measurements show that, in spite of their nonplanarity, the aromatic cores in the mesophases are tightly stacked within the column. The corresponding stacking patterns were derived from the structure of the dimer, on the basis of geometrical analysis and molecular modeling.

Perylenediimide-based donor-acceptor dyads and triads: impact of molecular architecture on self-assembling properties.

Perylenediimide-based donor-acceptor co-oligomers are particularly attractive in plastic electronics because of their unique electro-active properties that can be tuned by proper chemical engineering. Herein, a new class of co-oligomers has been synthesized with a dyad structure (AD) or a triad structure (DAD and ADA) in order to understand the correlations between the co-oligomer molecular architecture and the structures formed by self-assembly in thin films. The acceptor block A is a perylene tetracarboxyl diimide (PDI), whereas the donor block D is made of a combination of thiophene, fluorene, and 2,1,3-benzothiadiazole derivatives. D and A blocks are linked by a short and flexible ethylene spacer to ease self-assembling in thin films. Structural studies using small and wide X-ray diffraction and transmission electron microscopy demonstrate that AD and ADA lamellae are made of a double layer of co-oligomers with overlapping and strongly π-stacked PDI units because the sectional area of the PDI is about half that of the donor block. These structural models allow rationalizing the absence of organization for the DAD co-oligomer and therefore to draw general rules for the design of PDI-based dyads and triads with proper self-assembling properties of use in organic electronics.

Fluorenone core donor–acceptor–donor π-conjugated molecules end-capped with dendritic oligo(thiophene)s: synthesis, liquid crystalline behaviour, and photovoltaic applications

We have synthesized a new series of donor–acceptor–donor (D–A–D) π-conjugated molecules, consisting of fluorenone core end-capped with dendritic oligo(thiophene)s of increasing generation (abbreviated as FG0, FG1, and FG2). In view of the application of these new organic semiconductors in photovoltaic devices, we have explored their spectroscopic, redox, and structural properties. The thermal behaviour of the new organic semiconductors was investigated by differential scanning calorimetry and polarized-light optical microscopy. Liquid crystalline behaviour has been found in the case of FG1, corresponding to a smectic ordering with a triclinic symmetry (Smobl) upon heating, as confirmed by variable temperature small-angle X-ray diffraction studies. In order to evaluate their photovoltaic performances, devices with an active area of 0.28 cm2 were fabricated. Under AM1.5 simulated sunlight (100 mW cm−2) conditions, a device containing FG1/[70]PCBM blends showed a power conversion efficiency of ca. 0.8%.

Structure and photoconductive behaviour of a sanidic liquid crystal

A series of [1]benzothieno[3,2-b][1]benzothiophene-2,7-dicarboxylate derivatives consisting of a flat, conjugated aromatic moiety containing sulphur hetero-atoms and substituted by terminal chains - ethyl, decyl, Z-4-decenyl or 2-(2-butoxyethoxy)ethyl - has been synthesized. These materials exhibit a smectic liquid crystalline phase, the stability of which was found to be strongly dependent on the length and the nature of the terminal chains. In the mesophase, the molecular arrangement within the smectic layer is characterized by a local stacking of the rigid parts similar to that found in discotic columnar systems. Finally, clear photoconductive behaviour was found in mechanically aligned samples of these liquid crystals.

A nematic [60]fullerene supermolecule: when polyaddition leads to supramolecular self-organization at room temperature

The synthesis, liquid-crystalline properties and supramolecular organization of a [60]fullerene-based supermolecule are described, and the great potential of the spherical carbonated polyhedron to produce new materials through its multiple and addressable regioselective functionalization is emphasized. Indeed, hexaaddition on the carbon sphere has allowed the preparation, in 50% yield, of the first room-temperature enantiotropic nematic C60-based material, even though the promoter was not mesomorphic. Despite the high molecular weight of such a supermolecule, the resulting material was revealed to be easily oriented in its mesophase by a magnetic field; this has allowed, by small angle X-ray diffraction investigations, a fine description of the molecular organization and of the corresponding relevant interactions involved. These results open the way to multifuntional materials that can take advantage of such an easy orientation of the grafted active chromophores (donor and acceptor) to be particularly efficient (for example in the design of photovoltaic materials).