π-conjugated sulfonium-based photoacid generators: an integrated molecular approach for efficient one and two-photon polymerization

The cationic photoinitiating abilities of a series of 'push-pull' sulfonium-based photoacid generators (PAGs) have been investigated. In this linear pi-conjugated series, a 4-N,N-diphenylaminostilbene subunit is associated with different types of sulfonium substituents, which are connected to the stilbene moiety either in the 4' position or in the 3' position. This para-to-meta substitution effect leads to a strong increase of the quantum yield for acid generation with a maximum value of ca. 0.5. Such a positioning effect has a strong influence on the efficiency of the S-C bond cleavage. A detailed photolysis mechanism has been proposed. In contrast to commercially available sulfonium salts, these highly reactive pi-conjugated PAGs all exhibit large absorption in the visible range as well as large two-photon absorption cross-sections (delta(max) > 600 GM) in the near-infrared region. As a consequence, efficient one and two-photon polymerization reactions are observed at 405 nm and 800 nm, respectively, using typical monomers such as cyclohexene oxide, n-butyl vinyl ether or SU-8 photoresists. By the fabrication of well resolved two-dimensional microstructures, we finally demonstrate the potential use of these new generation PAGs in the fields of one and two-photon lithography.