Photochemical Synthesis of Pentacene and its Derivatives

• Published in: Chemistry : a European journal 2005-10, Vol.11 (21), p.6212-6220
• Main Authors: Yamada, Hiroko, Yamashita, Yuko, Kikuchi, Makoto, Watanabe, Hikaru, Okujima, Tetsuo, Uno, Hidemitsu, Ogawa, Takuji, Ohara, Keishi, Ono, Noboru
• Format: Article
• Language: English
• Subjects: diketones; organic semiconductors; pentacene; photolysis; singlet oxygen
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200500564

A novel α‐diketone precursor of pentacene, 6,13‐dihydro‐6,13‐ethanopentacene‐15,16‐dione, was prepared and converted successfully to pentacene in 74 % yield by photolysis of the precursor in toluene: Irradiation of the diketone solution in toluene with light of 460 nm under an Ar atmosphere caused the solution to change from yellow to fluorescent orange‐pink within a few minutes, after which, purple precipitates appeared. After 35 min, the solution changed to colorless and the purple precipitates were filtered to give pentacene in 74 % yield. By contrast, in the presence of oxygen, the color of the solution changed from yellow to pale yellow, and only 6,13‐endoperoxide of pentacene was quantitatively obtained. The rate of the reaction upon photolysis was measured by observing the decay of n–π* absorption of the precursor at 460 nm, and was found to be similar in both the presence and absence of oxygen. Therefore, the photoreaction of the α‐diketone precursor seemed to occur via the singlet excited state. Because the T–T absorption of pentacene was observed upon photolysis of the precursor in the nanosecond transient absorption measurement under an Ar atmosphere, the excited triplet state of the pentacene generated singlet oxygen by sensitization, and it reacted with the ground‐state pentacene to give the 6,13‐endoperoxide. The α‐diketone deposited on glass was also converted successfully to pentacene film by photoirradiation. In addition, diketone precursors of a mixture of 2,8‐ and 2,9‐dibromopentacene and 2,6‐trianthrylene were also prepared and their photoconversion was performed. Converted by the light! A series of novel α‐diketone precursors of pentacenes were prepared and photochemically converted to pentacenes in toluene under an Ar atmosphere (see picture). An α‐diketone precursor deposited on glass was also converted successfully to pentacene film by means of photoirradiation.