Gradient sublimation

Gradient sublimation has been used for many years as a means to purify materials and many variations exist and have been proven effective for organic purification. A detailed description of the vapor phase transport method applied to organic materials was given by Ch. Kloc,... 

Firsich, D. W. (1984). Energetic material separations and specific polymorph preparation via thermal gradient sublimation. J. Hazard. Mater., 9, 133-7. [294]... 

Pentacene (Pc) was obtained from Fluka in 98% purity and purified by temperature gradient sublimation under high vacuum. Characterisation of the purified Pc by mass spectroscopy revealed that the main impurities are dihydro-pentacene, pentacenequinone and pentaceneol. The sum of the concentrations of these impurities was estimated from the intensity ratio of the mass signals to be below 2%. In contrast to Jurchescu et al. [12], we find dihydropentacene (instead of pentacenequinone) as the main impurity. 

The other films were produced to study the influence of the interface between the metal top contact and the organic semiconductor. Here, silicon wafers with a native oxide were used as substrates. The samples were provided by the group of Pflaum at the University of Stuttgart. To minimise impurities, like in the case of DIP, Pc (purchased from Fluka) is purified twice by gradient sublimation before being used as starting material. The films were prepared in UHV at a base pressure of about 7 x 10 mbar from a graphite effusion cell. The evaporation rate was about 3 A/s it was controlled by a quartz microbalance located next to the sample [8]. 

Gradient sublimation To ensure a high material yield in combination with a high selectivity, sublimation over an extended temperature gradient ( 500 K/m) is the preferred method if the sublimation temperature of the host material is known. Inside a glass tube separation across the temperature gradient takes place and, in most cases, the purified fraction occurs spatially well-separated from the contaminants. The efficiency can be improved in terms of yield and stability in the presence of an additional inert carrier gas, e.g. Ar or N2, which at low pressure (10 Torr) reduces the molecular mean free path and equilibrates temperature fluctuations across the glass tube. 

Rubrene was purchased from Aldrich (elemental purity > 98%) and additionally purified by gradient sublimation Freshly cleaved mica (001) was used as substrate. Rubrene thin films were deposited by hot wall epitaxy in a vacuum chamber with a base pressure below 10 Pa at different deposition rates and substrate temperatures (Ts). Pole figures were measured with a Philips X pert x-ray diffractometer using CrKa radiation and a secondary side graphite monochromator. Specular scans were performed on a Bruker D8-Discover diffractometer using CuKa radiation. POWDER CELL and STEREOPOLE were used for the evaluation of the specular scans and simulation of pole figures. 

Fig. 3.1 Gradient sublimation. Here, a suitable temperature gradient is produced using a conical metal tube which is heated at one end - the inlet - and held at a constant, lower temperature at its other end. Molecules of the starting material which is initially in the inlet region sublime and are transported by a...

A mixture of 140 mg 4,8-dimethoxy-l,5-naphthyridine and 34 mg methyl iodide (molar ratio = 3 1) was heated at 220°C for 12 h. The resulting crude product was boiled with benzene to remove any 1,5-dimethyl-l,5-naphthyridine-4(Iff),8(5//)-dione present and then gradient sublimed at 275°C. The white crystalline sublimate was dissolved in warm dilute aqueous NaOH, and the resulting violet solution was adjusted to pH 2 with concentrated HCl. The precipitate was collected and recrystallized from a large volume of water to afford 74 mg pure 3,7-dimethyl-l,5-naphthyridine-4(lff),8(5ff)-dione, in a yield of 53%, m.p. > 300°C. However, when 280 mg pure 4,8-dimethoxy-1,5-naphthyridine was heated at 226°C for 10 h, and the crude product was dissolved in boiling benzene and Altered hot through a thin pad of Norit A on Celite, 174 mg 1,5-dimethyl-l,5-naphthyridine-4(lff),8(5ff)-dione was obtained as a white crystalline after cooling and Altration, in a yield of 62%, m.p. 268.5-272°C. 

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