Synthesis, characterization and properties of covalently linked porphyrin-naphthalimide pentamer and its metal complexes

A novel porphyrin-naphthalimide pentamer 5 was synthesized by linking four light-harvesting naphthalimide units to a central porphyrin, via ether bridges. The corresponding metal (Zn²⁺, Cu²⁺, Mn²⁺, Fe³⁺, Co²⁺) complexes 6a-6e were obtained under different reaction conditions. The structures of these compounds were characterized by ¹H and ¹³C NMR spectra, FT-IR, MALDI-TOF-MS and elemental analysis. The photophysical properties have been investigated by UV-Vis absorption and steady-state fluorescence spectroscopy. It is found that the excitation energy transfer occurs from the naphthalimide units to the porphyrin core, and that the free base porphyrin 5 and Zn porphyrin 6a can emit intense red light. Additionally, the electrochemical properties of compounds 5 and 6a-6e were also investigated by cyclic voltammetry.