No abstract

The ability of electric fields to align nonpolar semiconducting molecules was demonstrated using hexa(para-n-dodecylphenyl)hexabenzocoronene (HBC-PhC12) as a model compound. A solution of HBC-PhC12 was applied to a glass surface by drop-casting and the molecules were oriented into highly ordered structures by an electric field during solvent evaporation. Atomic force microscopy (AFM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) showed a long-range alignment where the disclike molecules were organized in columns perpendicular to the direction of the imposed electric field. The high anisotropy of the uniaxially aligned films was characterized by cross-polarized light microscopy. The birefringence of the HBC-PhC12 films was related to the presence of extended domains of unidirectionally aligned columns in which the aromatic cores of the HBC-PhC12 molecules were perpendicular to the columnar axis. The packing and the arrangement of the molecules in the field-force ordered films were proven by electron diffraction and X-ray analyses.

The desorption/ionization behaviour of polycyclic aromatic hydrocarbons (PAHs) in matrix-assisted laser desorption/ionization (MALDI) and laser desorption (LD) mass spectrometry was studied by the solvent-free sample preparation method. As the understanding of the desorption/ionization mechanism in MALDI is normally hampered by the different ionization and desorption efficiencies of the analytes, this work was focused on the analyses of a homologous series of four hexabenzocoronenes (HBCs) possessing virtually the same ionization efficiency: HBC parent, hexamethyl-hexabenzocoronene (HBC-C1), hexapropyl-hexabenzocoronene (HBC-C3) and hexakis(dodecyl)-hexabenzocoronene (HBC-C12). The different signal intensities obtained in their mass spectra can be related to differences in their desorption efficiencies, which are attributed to the different strengths of the intermolecular interactions between unsubstituted and alkylated HBCs in the solid state. The influence of the aromatic structure of PAHs on their photoionization/desorption probability was investigated. As a model system, an equimolar mixture composed of HBC-C12 and hexakis(dodecyl)-hexaphenylbenzene (HPB-C12) was chosen. The aromatic structures of both molecules and thus their absorption coefficients at the laser wavelength differ substantially and have a huge influence on their photoionization efficiency. The combined effect of laser light absorption and intermolecular interactions on the desorption/ionization behaviour of giant PAHs was further studied by using an equimolar mixture composed of a larger PAH (C(222)H(42)) and its dendritic precursor (C(222)H(150)). This mixture shows the opposite behaviour to that of the former example, because the balance between desorption and ionization efficiency has changed significantly. The present investigation should be of interest for providing a better understanding of MALDI and LD spectra obtained from natural PAH-containing samples, such as heavy oils, asphaltenes or pitches, for which our artificial mixtures represent suitable model systems.

We explore the feasibility of reliable quantitative matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) analyses via solvent-free sample preparation, as this procedure provides the unique convenience of being applicable also to insoluble samples. As quantitative MALDI measurements are even more complicated for species ionized by cation attachment, we investigated model systems, such as polycyclic aromatic hydrocarbons (PAHs) and fullerenes, which undergo photoionization and do not require additional cationizing salts. Our quantitative approach rests upon applying the standard-addition method in MALDI for the quantitative characterization of binary mixtures. Two different systems are tested. Set 1 is composed of hexakis(dodecyl)hexabenzocoronene and hexakis(dodecyl)hexaphenylbenzene, which represent the product and precursor of a cyclodehydrogenation reaction, and Set 2 is a mixture of C60 and C70 fullerenes. In Set 1, severe anomalies could be detected due to a strong influence of the matrix/analyte ratio on the correlation between signal intensity and analyte amount. This can be related to the strong intermolecular interactions among the hexabenzocoronene (HBC) aromatic cores hampering the desorption step and to intermolecular charge transfers, which influence the ionization probability. Minor interferences to the quantitative MALDI characterization are encountered in the analysis of C60 and C70 fullerenes. The spherical shapes of C60 and C70 buckyballs prevent strong aggregation. Thus, no moleculedependent anomalies in their desorption-photoionization behaviour are recognized. Copyright # 2008 John Wiley & Sons, Ltd.The acquisition of reliable quantitative information via matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) represents a big challenge. The qualitative characterization of polymers and macromolecules is straightforward, as the MALDI approach is sensitive, fast and flexible. 4,5 In addition, the use of structurally modified compounds as internal standards has been successfully employed for the relative quantification of low molecular weight compounds. 6 To date, however, standard molecules with properties resembling those of the analyte have been used to quantify a single component in a mixture. The method proposed here also has the aim of allowing quantification of both components of a binary mixture by using one of them as an internal standard. This is of special value for a slightly soluble or insoluble component not obtainable as a pure sample. Most quantitative MALDI-based investigations have used the traditional solvent-based technique for sample preparation. However, this approach often leads to the so-called hot-spot formation due to severe inhomogeneities caused by solvent evaporation, leading to a large variation in relative signal intensities. Recently, it has been shown that improved homogeneity can be achieved by employing an alternative preparation technique, which avoids the use of a solvent, the so-called solvent-free sample preparation. 7,8...