The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Pentacene is recognized as a good organic semi-conducting material for use in p-type organic thin film transistors (OTFT), due to its high charge mobility. It is currently known that pentacene can be prepared by volatile decarbonylation of a 6,13-bridged-6,13-dihydropentacene precursor, e.g., by heating to 150° C. to liberate carbon monoxide. This precursor is reported to have some degree of solubility in a limited number of organic solvents such as chloroform and might therefore be amenable to spin coating using multiple castings, e.g., coats.
The characteristics of pentacene can be attributed to its high degree of crystallinity in the solid state. However, the high tendency of crystallization also renders its low degree of solubility in organic solvents. Therefore it becomes difficult to prepare thin films through solution processes. In an effort to address this shortcoming, scientists at Philips and IBM have developed a strategy of preparing precursors of pentacene that are soluble in organic solvents, and are able to produce pentacene after the films are prepared. The structures of pentacene precursor are usually cyclo-adducts of pentacene itself (acting as a diene) with another small volatile fragment (acting as a dienophile). After a film is produced by spin coating processes, relatively pure pentacene can be regenerated upon heating through a retro-cyclization process.
Since the purity of pentacene is crucial to the conductivity of films, the volatile fragment(s), which is released during the thermal process, should be expelled out of the crystal lattice as much as possible. The yield of retro-cyclization must be high, and the smaller the leaving fragment the better. Heavy elements such as sulfur and chlorine, which have been present in many leaving groups of previous examples, are known to readily contaminate the resultant films. The preparation of a pentacene precursor, i.e. compound 1, which extrudes a unit of carbon monoxide at 150° C., has been described by Chen et al. The small size and inert nature of carbon monoxide render it the best candidate for serving as a leaving fragment. The pentacene film thus produced exhibited typical OTFT characteristics, i.e. an on/off current ratio about 1.2×105 and field-effect mobility μ close to 0.01 cm2V−1s−1.

The solubility of compound 1 is a relatively low ˜0.7 mg/mL in some common organic solvents, e.g. methylene chloride and THF. However, in order to prepare a film with required thickness therefrom, repeated casting processes are necessary.
However, it would be advantageous to provide a pentacene precursor having improved solubility in various organic solvents, as well as being capable of conversion to pentacene below 360° C. in a step that leaves little, it any, non-pentacene residue in the product. This would permit pentacene to be used in a larger range of commercial applications. Ideally, pentacene processes should be highly soluble in organic solvents such that a limited number of castings are required to obtain useful end-product. In addition to improved pentacene processes, anthracene and tetracene type-oligoacenes useful in organic semiconductor and/or photovoltaic applications are also disclosed and shall be readily processable in accordance with the teachings of the present invention.