Photo- and Thermal-Induced Isomerization of Diels-Alder Adducts of Pentacene and TCNE

• Published in: European journal of organic chemistry 2012-05, Vol.2012 (14), p.2707-2710
• Main Authors: Johns, Valentine K., Shi, Zheng, Hu, Wenfang, Johns, Jaime B., Zou, Shengli, Liao, Yi
• Format: Article
• Language: English
• Subjects: Chemistry; Condensed benzenic and aromatic compounds; Cycloaddition; Exact sciences and technology; General and physical chemistry; Isomerization; Organic chemistry; Pentacene; Photochemistry; Physical chemistry of induced reactions (with radiations, particles and ultrasonics); Preparations and properties; Retro ­reactions
• ISSN: 1434-193X; 1099-0690
• DOI: 10.1002/ejoc.201200195

Herein we report the formation of the second‐ring Diels–Alder adduct together with the central‐ring adduct in a reaction of pentacene and tetracyanoethylene at room temperature. DFT calculations showed that the difference between the free energy of the two isomers is about 3.9 kcal/mol. Photo‐ and thermal‐induced isomerization between the central‐ring adduct and the second‐ring adduct were studied in solution and in polymer films. In solution, the less‐stable second‐ring adduct can be converted into the more‐stable central‐ring adduct either thermally or photochemically, but the reverse transformation does not occur. In a polymer matrix, isomerization can be photochemically induced in both directions at different wavelengths, which results in a photoswitchable system. Formation of pentacene in the photochemical experiments was also observed, which supports an isomerization process involving a photo‐retro‐Diels–Alder reaction. Isomerization of the second‐ring Diels–Alder adduct to the central‐ring adduct in the reaction of pentacene and tetracyanoethylene is reported. Photoisomerization can be induced in both directions, whereas thermal isomerization can be induced only from the second‐ring adduct to the central‐ring adduct.