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The molecular structure of thioxanthone has been determined from gas electron-diffraction data. The molecular parameter values and their uncertainties are rg(C–C)=1.401±0.001 Å, rg(C–H)=1.099±0.007 Å, rg(C–S)=1.751±0.002 Å, rg(C–Cc)=1.498±0.004 Å, rg(Cc–O)=1.232±0.006 Å, ∠CSC=103.4±0.3°, ∠CCcC=119.4±0.6°, and θ(dihedral angle)=169.0±1.6°, where the angle parameters are rα parameters. The dihedral angle of thioxanthone is much larger than those of related molecules.

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The photooxygenation reaction of large polycyclic aromatic hydrocarbons (PAHs) composed of more than six benzene rings was investigated by using the semiempirical molecular orbital method. The optimization of the PAHs and their endo-peroxides revealed that oxygen attaches to a pair of carbon atoms which have the largest n-electron density of the highest occupied molecular orbital. The structure of the endo-peroxide of tetrabenzo [de, hi, op,st]pentacene, determined experimentally, could be reproduced by the calculation. The heats of formation for the PAHs, endo-peroxides, and transition states were also calculated, and an energy correlation diagram of the reaction was obtained. The experimental results for the rate of photooxygenation were explained in terms of the energy difference between the initial and transition states in the profile.

The gas and solid phase UV photoelectron spectra of rubrene were measured. In the molecule there is little conjugation of π-electrons between four substituent phenyl groups and the naphthacene skeleton owing to the effect of the steric hindrance of the phenyl groups. Therefore, the lower IP bands in the gas phase spectrum could be correlated to the π-bands in the photoelectron spectra of benzene and naphthacene. The features of the solid phase spectrum are similar to those of the gas phase one except for a large peak due to inelastically scattered electrons. The polarization energy due to the molecular ion in the solid, the peak to peak difference between the gas and solid phase spectra, was observed to be 0.6 eV, which is about half the polarization energy for naphthacene. This means that rubrene molecules are loosely packed in the solid as a result of the steric hindrance of four phenyl groups.

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