Investigation of mechanism of photo induced polymerization excited by two-photon absorption

Abstract

Three kinds of the two-photon chromophores are used to investigate the process of electron transfer between the initiator and the multifunctional acrylate monomer in two-photon-induced polymerization. Their molecular structures are simplified as D- -d- -D, D- -D and D- -a- -D, where D is the strong donor terminal group, is a -conjugated core, d and a the electron-donating and electron-accepting side group linked with the -conjugated core, respectively. We noticed that the emission of those chromophores is very sensitive to the solvent polarity. Additionally, the red-shift, structureless and long-lived emission was observed in the monomer. Correlated with quantum chemical calculation (MOPAC-PM5), we proposed a process of electron transfer in two-photon-induced polymerization (TPIP). Following the non-irradiation from the high excited state (S2), intramolecular charge transfer (intra-CT) takes place from the terminal group to the core in the primary step, where intra-CT state is named as intra-exciplex. Then the rapid electron hopping from the ion radical core of the chromophore to the monomer was initiated. The electron transfer produces the ion radical pair, attributed to the photopolymerization. Therefore, the rate of two-photon-induced polymerization is determined by an equilibrium of electron transfer and back electron transfer between initiator and monomer to some extent in spite of the contribution of highly sensitivity in two-photon absorption to the number of the excited chromophore.