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Simultaneous differential scanning calorimetry (DSC) and luminescence (LUM) measurements of polyacrylonitrile revealed that a precursory reaction progresses below the well-known endothermic cyclization reaction (i.e., peak temperature ca. 280 °C). The intensity of LUM increased above 90 °C, which is around the glass transition temperature. After the peak at 180–190 °C, the LUM intensity decreased with temperature up to 250 °C, i.e., below the onset temperature of cyclization reaction. Thermal signals cannot be seen on the DSC curve in the temperature range of the LUM peak. The activation energy (Ea) estimated from the heating rate dependence of the LUM peak temperatures was 122 kJ mol−1. However, the Ea obtained from the well-known exothermic peak temperatures of DSC measurements was 190 kJ mol−1. Electron spin resonance and nuclear magnetic resonance also support the chemical structure of the luminescence species. FT-IR and X-ray diffraction were also performed to confirm the change in chemical and physical structural. Finally, ab initio calculations suggest that thermally stable heterocyclic radical species with two to four fused six-membered rings are the main products of the precursory reaction.

To fabricate organic field-effect transistors (OFETs) with high carrier mobility, we attempted to grow 5,6,11,12-tetraphenylnaphthacene (rubrene) single crystals from solution and to improve their quality. Investigations into solvents in which rubrene was highly soluble proved that its solubility depended on the presence or absence of aromatic rings and chloro groups rather than on the polarity of the solvents. Rubrene crystals were grown from aromatic solvents, specifically from toluene, p-xylene, and aniline solvents, as well as from 1,2-dichloroethane (DCE) solvent. As a result, rubrene single crystals larger than 1 mm were obtained. Powder X-ray diffraction (XRD) analysis showed that the crystals obtained from the p-xylene and toluene solvents were rubrene, and 1H-nuclear magnetic resonance (1H-NMR) measurement proved that the crystals had not incorporated the solvent at the detection level. In addition, atomic force microscopy (AFM) confirmed that the rubrene crystals grown from the p-xylene and aniline solvents had flat faces and that the crystal from the p-xylene solvent had monomolecular steps on parts of the surfaces. Rubrene single crystal OFETs with graphite electrodes and parylene as an insulator showed carrier mobilities of ~0.75 cm2 V-1 s-1.