IntroductionExtended polyaromatic hydrocarbons (PAH) are finding increasing interest among other self-assembled supramolecular systems due to their pronounced tendency for formation of extended π-stacked assemblies. These have many potential properties, such as a high electron density which allows their use in promising applications in molecular electronics. 1 However, the main drawback of these rigid polycondensed molecules resides in the difficulty of their synthesis and their very low solubility which prevents their isolation and manipulation. Recently, Müllen and co-workers 2 succeeded in finding a versatile synthesis of hexa-perihexabenzocoronene (HBC), a polycondensed hexagonal molecule containing 13 benzene rings which shows a very high chemical stability 3 and which self-organizes into columnar stacks, by π-stacking, with an outstanding degree of order in the solid state. 4,5 Additionally, its solubility was improved by the addition of peripheral long aliphatic chains which at the same time bestows the molecule with a ditopic property: These HBC derivatives show a liquid crystalline (LC) state over a wide temperature range depending on the length of the aliphatic chain. 6 Moreover, these HBC derivatives exhibit the highest charge carrier mobility recorded so far for an organic molecule both in the solid and in the liquid crystalline states, 7 paving the way toward their application in the domain of electronic 8 and optoelectronic 9-11 devices.Nevertheless, the lateral interaction between the adjacent HBC stacks caused by the crystalline packing of their aliphatic side chains 12 constitutes a major drawback since it interferes with the formation of very long conducting π-stacks. Additionally, most of the HBCs bearing aliphatic chains are liquid crystalline just above room temperature, which leads to a second disadvantage since it induces a drastic decrease of their charge carrier mobilities, 13 which is reported to be due to "lateral" conduction and as a reason for the perturbation or the deterioration of an ideal liquid crystalline mesophase that dominates the conduction process. 14 * To whom correspondence should be addressed. Tel. (Office): +41 (0) Synthesis and characterization of new hexabenzocoronene (HBC) derivatives bearing perfluoroalkylated chains are described. These disc-shaped polycondensed aromatic hydrocarbons are known to self-assemble by π-stacking into columnar architectures. The peripheral decoration with perfluorinated chains, well-known for their low van der Waals forces, leads as expected to reduced intercolumnar interactions. Powder X-ray diffraction and differential scanning calorimetry of these perfluoroalkylated HBCs proved liquid crystalline (LC) properties whose transition temperatures, mesophase stability, and nature depend on the detailed structure of these side chains, i.e., the ratio between aliphatic and perfluorinated parts. On the other hand, the insertion of a phenyl spacer between the aromatic core and the lateral chains increases the LC transition temperature and, sur...

Synthesis of VLCFAs (very long chain fatty acids) and biosynthesis of DHS (dihydrosphingosine) both are of vital importance for Saccharomyces cerevisiae. The bulk of VLCFAs and DHS are used for ceramide synthesis by the Lag1p (longevity-assurance gene 1)/Lac1p (longevity-assurance gene cognate 1)/Lip1p (Lag1p/Lac1p interacting protein) ceramide synthase. LAG1 and LAC1 are redundant but LIP1 is essential. Here we show that 4Delta (lag1Deltalac1Deltaypc1Deltaydc1Delta) cells devoid of all known endogenous ceramide synthesis pathways are unviable but can be rescued by the expression of Lass5, a mouse LAG1 homologue. Ceramide synthase activity of 4Delta.Lass5 cells only utilizes C16 and C18 fatty acids and does not require the help of Lip1p, an essential cofactor of Lag1p/Lac1p. HPLC-electrospray ionization-MS/MS analysis demonstrated that in IPCs (inositolphosphorylceramides) of 4Delta.Lass5, the very long chain fatty acids (C26 and C24) account for <1% instead of the normal >97%. Notwithstanding, IPCs incorporated into glycosylphosphatidylinositol anchors of 4Delta.Lass5 show normal mobility on TLC and the ceramide- and raft-dependent traffic of Gas1p (glycophospholipid-anchored surface protein) from endoplasmic reticulum to Golgi remains almost normal. Moreover, the biosynthesis of C24:0 fatty acids remains essential. Thus, C(24:0) and dihydrosphingosine are both necessary for survival of yeast cells even if they utilize C16 and C18 fatty acids for sphingolipid biosynthesis.

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aThe self-assembled architectures in solution of a new HBC derivative bearing perfluoroalkylated side chains were investigated by optical excitation and emission spectroscopy and correlated to cryo-SEM, a new technique in organic chemistry.The ability of molecules to self-assemble into highly ordered architectures has gained increasing importance in the past decade, as these nano-objects are applicable in the field of optical and electronic devices. 6 (1), bearing six side chains each consisting of an alkylated and a perfluoroalkylated section. The alkylated section acts as a four-carbon spacer between the core structure and the six-carbon perfluorinated tail. Preliminary investigations on aliphatic spacers of variable length n (n 5 2-6) indicate that the four-carbon aliphatic spacer chosen for this investigation presents an optimum for the following parameters: i) total isolation of the electron withdrawing effect of the perfluoro chain from the core structure; ii) allowing for maximum flexibility between the rigid core structure and the semi-rigid perfluoroalkyl tail; 11 iii) short enough to prevent crystalline alkyl chain-alkyl chain interactions. The seemingly very minor change in the length of the perfluorinated part, six carbons instead of eight as in our previous work, 12 had a tremendous impact on the solubility of the new compound in a range of solvents such as 1,2,4-trichlorobenzene (TCB), benzotrifluoride (BTF) and hexafluorobenzene (HFB), whereas the compound with (CH 2 ) 4 (CF 2 ) 8 F side chains showed very low solubility in all solvents, severely hampering its purification. Nevertheless, the most important property of HBC-[(CH 2 ) 4 (CF 2 ) 8 F] 6 , i.e. its tendency to form very long monostranded molecular stacks, was fully retained in the new variant. Scheme 1 illustrates the six step synthesis of 1 which is obtained as a bright yellow solid.13 Compound 1 was investigated by cryo scanning electron microscopy (cryo-SEM) and optical luminescence and excitation spectroscopy. Cryo-SEM, 14 a new technique in organic chemistry, was chosen instead of regular SEM, since it allows the examination of supramolecular structures pre-existing in solution, which would otherwise aggregate to solids in the usual target preparation involving application of a solution containing the analyte to the target followed by evaporation of the solvent. In cryo-SEM the applied BTF solution is shock frozen in liquid nitrogen further cooled below 63 K and transferred in this state to the cryo transfer system (Gatan ALTO 2500) of the microscope. Subsequently the solvent is sublimed off the sample in the high vacuum of the instrument.

The investigation of two hexa-peri-hexabenzocoronene (HBC) derivatives carrying linear or branched perfluoroalkylated side chains is reported. Polycondensed aromatic hydrocarbons (PAH) such as HBC derivatives are well known to self-organize to form highly ordered monomolecular stacks, which in turn show a concentration-and solvent-dependent lateral aggregation. However, possible applications of self-assembled HBC derivatives require linear, laterally non-aggregated columnar structures. According to powder X-ray diffraction (XRD) and differential scanning calorimetry (DSC), HBC derivatives with linear perfluoroalkylated side chains show liquid crystalline (LC) properties whereas those with branched perfluoroalkylated side chains have an amorphous structure. The stacking behaviour and the lateral aggregation are found to be greatly influenced by changes in the medium, as shown by fluorescence spectroscopy and cryo-scanning electron microscopy (cryo-SEM).

Abstract:The synthesis of two new triarylamine compounds bearing perfluoroalkylated side chains is described. Good thermal stabilities combined with a blue emission make these compounds promising candidates for materials applications.

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Polycondensed aromatic hydrocarbons (PAH), which can be regarded as two-dimensional subsections of graphite, have begun to attract increasing interest in supramolecular chemistry. Substituted hexa-peri-hexabenzocoronenes (HBC), an outstanding class of PAH, are well-known to self-organize in solution into highly ordered columnar molecular stacks. The formed structures are very sensitive to any variation of the medium as well as the lateral substituents of the HBC derivatives. Various perfluoroalkylated HBC compounds have been prepared and investigated by powder XRD, DSC, fluorescence and cryo-SEM in order to gain certain control over the selfassembling behavior of this class of compounds.