Maximilian Zander

Fluorescence quenching of alternant and non-alternant polycyclic hydrocarbons by electron transfer

Rate constants of fluorescence quenching by electron acceptors are greater for alternant than for non-alternant hydrocarbons with equal Eoo. The reverse is true for quenching by electron donors. This is consistent with the lowering of the π-orbital energies of non-alternant compared to alternant polycyclic hydrocarbons of equal Eoo.

Structural features and mesophase formation of coal-tar pitch fractions obtained by preparative size exclusion chromatography

Abstract High-temperature coal-tar pitch was fractionated by preparative size exclusion chromatography on Sephadex LH-20 in pyridine at 60 °C. The fractions obtained were characterized by elemental analysis, molecular weight determination (vapour pressure osmometry), i.r. spectroscopy, and TGA. The linear correlation observed between log molecular weight (range 200–2500 Dalton) and elution volume shows that fractionation of the pitch was mainly due to size exclusion and that interfering polar adsorption on the gel was essentially avoided. This is supported by the similarity between the i.r. spectra and elemental analyses of the fractions, respectively. The molecular weight distribution of the nitrogen-containing pitch constituents is also discussed. Selected samples were heat-treated using hot-stage microscopy. The reactivity of the fractions towards polymerization is lower than that of the unfractionated pyridine-soluble portion of pitch. This leads to a slower increase in viscosity of the fractions, thus allowing the formation of larger mesophase spheres. Addition of a small amount of a low molecular weight fraction to a high molecular weight fraction enhanced polymerization.

On the composition of pitches

Abstract The composition of pitches is discussed in terms of defined chemical compounds and classes of compounds that have been identified in these materials. The main emphasis is placed on polycyclic aromatic hydrocarbons, systems containing the acenaphthylene moiety and sulphur heterocyclic systems.

Recent advances in pitch characterization

Abstract Some recent advances in pitch characterization are reviewed. Topics covered include solvent fractionation of pitches, extrography, size exclusion chromatography, charge-transfer fractionation of pitches, n.m.r. spectroscopy, differential pulse voltametry, reaction analysis and X-ray diffraction as applied to carbomesophase.

Topological effects on mo energies

Abstract By different linkages between bi- or multi-valent molecular partial structures, pairs of isomers are generated which are denoted by ∫ and ⊥, e.g. o -(∫) and p -benzoquinodi-methane (⊥). Using analytical methods, it is shown that the MO patterns of ∫ and ⊥ are generally related to one another so that in any interval bounded by two eigenvalues of ∫, there are alternately two and zero eigenvalues of ⊥. Furthermore, unifying relationships concerning the HOMO-LUMO separation of the systems are obtained. All these findings are equally valid for the π- and the σ-MOs of alternant and non-alternant, and benzoid and non-benzoid systems. The analytically derived conclusions are verified by comparison with experimental data and ab initio calculations.

A review of the significance of polycyclic aromatic chemistry for pitch science

Abstract As polycyclic aromatic hydrocarbons (PAHs) and structurally related hetero-aromatic systems are the main constituents of pitches, there are many relations between polycyclic aromatic chemistry and pitch science. Examples are discussed in this overview, which deals inter alia with more recent observations and considerations regarding the composition and compositional complexity of coal-tar pitch, the underlying principles of structure-property relations of PAHs, the mechanism of PAH carbonization, a quantitative relation between molecular topology and carbonization rates of PAHs, solid-state carbonization of PAHs, and inadequacies of solvent fractionation as a pitch characterization method.

Fluorescence of bromoperylenes and the requirements of heavy-atom quenching

Abstract The quantum yields and decay times of bromo-substituted perylenes are nearly identical with those of perylene. The lack of an expected internal heavy atom effect is due to the absence of a triplet energetically close to the fluorescing state.

On the nitrogen containing constituents of coal-tar pitch

Abstract Experimental evidence for a statistical distribution of the nitrogen present in coal-tar pitch among pitch constituents is presented. It is shown that the concentration of basic nitrogen compounds in pitch fractions increases with increasing molecular weight of the fractions. The general suggestion is derived that the high-molecular weight portion of coal-tar pitch is probably not predominantly a mixture of polycyclic aromatic hydrocarbons but rather a mixture of heterocyclic systems. Ion exchange chromatography, charge-transfer fractionation using picric acid as the complexing agent as well as precipitation of hydrochlorides with gaseous HCl have been used for isolation of basic pitch constituents.

Relation between dewar localization energies and rates of thermally induced carbonization of polycyclic aromatic hydrocarbons

Abstract A Hammett analogous free energy relationship was found to exist between thermally induced carbonization rates (measured at 430 °C) of polycyclic aromatic hydrocarbons and minimum values of Dewar localization energies as a reactivity index.

Phosphorescence of [n]paracyclophanes and their ground-state complexes with silver perchlorate

Abstract The red-shift of the phosphorescence transition of [n]paracyclophanes (n = 7–10) compared to planar benzene homologues is almost entirely determined by the bending angle of the benzene rings. The same is assumed for [2.2]paracyclophane. The higher stability of the [2.2]paracyclophane/AgClO4 ground-state complex compared with that of p-xylene is not due to benzene ring bending.