Determination of electron transfer mechanism of two-photon-induced polymerization via an efficient way: one-photon process

Abstract

Three types of the two-photon chromophores, D-π-D, D-π-d(a)-π-D, are used to investigate the mechanism of two-photon-induced polymerization (TPIP), wherein D is the terminal amine group, d and a are an electron-donating group (MeO) and an electron-accepting group (CN) linked with π-conjugated core, respectively. Based on the fact that deactivation of two-photon absorption is through the lowest singlet excited state, we study the process of electron transfer of TPIP between the excited dye and the monomer via one-photon process. In the bulk monomer used for TPIP, the emission spectra of the chromophores were obviously red-shifted and the lifetimes were elongated relative to that in the solvents. This indicates formation of an exciplex between the excited dye and the monomer. Correlated with the quantum calculation, we conclude that even for the symmetrical molecular structure, the intramolecular electron transfer from the donor terminal group to the π-conjugated core of the chromophore takes place before intermolecular electron transfer which leads to formation of exciplex. Consistent with our experimental results obtained from the polymerized rate in TPIP, the totally large electron transfer rate of the D-π-D chromophore is attributed to both efficient intramolecular and intermolecular electron transfer. Moreover, the abstraction of proton from the terminal group produced the amine radical. That is why Et₂Nas the terminal group is more active than n-Bu₂N- for two-photon polymerization. We are expecting that the same mechanism can be realized in two-photon photodynamic therapy.