Pyrylium and thiopyrylium salts as electron transfer photosensitizers for the [2π+2π] cyclodimerization of poly(vinyl cinnamate) in solution

The photochemical cross-linking of poly(vinyl cinnamate) (PVCin, 1a-d) mediated by pyrylium salts in solution has been investigated. 2,4,6-Triphenylpyrylium tetrafluoroborate (TPT, 2a), a well-known electron-transfer photosensitizer, produces highly selective [2 pi+2 pi] cyclodimerization between cinnamate units of PVCin: head-to-head dimers (truxinates) account for 98-99% of the cycloaddition products. Among all the identified dimers, zeta-truxinate (8) seems to be specifically associated with the presence of TPT, since not even traces of this compound can be detected when no sensitizer is employed. delta-Truxinate (5) and beta-truxinate (7) are the major products, but their percentages are inverted in comparison with the unsensitized reaction. 2,4,6-Triphenylthiopyrylium perchlorate (2b) has been shown to be 3 to 4 times more efficient than TPT, as evidenced by UV-vis and H-1 NMR experiments. The use of several cross-linking additives such as 1,4-butanediol dicinnamate (3a), pentaerithritol tetracinnamate (3b), dimethyl p-phenylenediacrylate (3c), diethyl cinnamylidenemalonate (3d) and pentaerithritol tetraacrylate (3e), results in a significant enhancement of the intermolecular cycloaddition. (C) 1998 Elsevier Science S.A.