Synthesis, Structure, and Photochemistry of 5,14-Diketopentacene

5,14-alpha-Diketopentacene, a structural isomer of 6,13-alpha-diketopentacene, was prepared from pentacene in three steps. In addition to the typical n-pi* absorption of the diketone moiety at around 468 nm and the anthracene-like absorption at 333, 349, and 367 nm, a broad absorption was observed at around 386 nm, which could be assigned to an intramolecular charge-transfer absorption from anthracene to the diketone moiety. 5,14-alpha-Diketopentacene could be converted into pentacene quantitatively by photoirradiation at 405 and 468 nm in toluene with quantum yields of 2.3 and 2.4%, respectively, and these values are higher than the quantum yield of 1.4% obtained for 6,13-alpha-diketopentacene irradiated at 468 nm. The quantum yields in acetonitrile were lowered to 0.33 and 0.28% with irradiation at 405 and 468 nm. The crystal structure of 5,14-alpha-diketopentacene showed a CH-pi interaction and pi-pi stacking between neighbouring anthracene and benzene moieties. The lower solubility of 5,14-alpha-diketopentacene compared with the 6,13-isomer could be explained by this crystal structure.