Star-Shaped Fluorescent Porphyrins with Impressive Two-Photon-Absorption Cross-Sections for Oxygen Sensitization: The Impact of Periphery-Conjugated Electron-Releasing Groups

While pursuing the exploration of porphyrin/fluorene combinations at the molecular level and their application as luminescent photosensitizers for two-photon photodynamic therapy (2P-PDT), we wondered about the impact of adding electron-releasing endgroups to these molecular structures. Furthermore, we also wondered about the influence of spacers within the arms of these star-shaped architectures and about the impact of the metalation of these free-base porphyrins by zinc. To address these questions, we synthesized a series of ten novel star-shaped meso-tetraarylporphyrins, featuring various aromatic rings with 1,2-ethenyl fluorenyl and/or 1,2-ethynyl spacers, capped by diphenylamine endgroups. The linear and nonlinear optical properties of these compounds were characterized through absorption, emission, and two-photon excited fluorescence studies, thereby screening the impact of endgroups, metalation and arm composition on the optical parameters of interest. Subsequently, their singlet oxygen-photosensitizing properties were also evaluated. The enhanced properties observed for compounds presenting diphenylamino (DPA) endgroups in comparison to analogs with none, along with the impact of metalation by Zn(II) and that of 1,2-ethenyl versus 1,2-ethynyl linkers on these, were eventually analyzed using classical figures of merit. This contribution highlights the strong potential of DPA-capped star-shaped free-base porphyrins as fluorescent photosensitizers for combined 2P-PDT and imaging applications.