Efficient Complexation of Pyrrole-Bridged Dizinc(II) Bisporphyrin with Fluorescent Probe Pyrene: Synthesis, Structure, and Photoinduced Singlet-Singlet Energy Transfer

A diethylpyrrole-bridged dizinc(II) bisporphyrin (Zn2DEP) is reported that encapsulates fluorescent probe pyrene molecules through strong pi-pi interactions, which can relay information about the chemical environment in the interior of the host-guest supramolecular assembly. X-ray structures of both Zn2DEP and the encapsulated pyrene complex are reported, which provides a rare opportunity to investigate the structural changes upon guest binding. A comparative structural analysis demonstrated the exceptional ability of this bisporphyrin platform to open its binding pocket for pyrene encapsulation by a vertical displacement of more than 2.45 angstrom, although both Zn2DEP and the pyrene complex have nearly parallel porphyrin ring orientations. The H-1 NMR spectrum of the encapsulated pyrene complex in solution shows the upfield shifts of the pyrene protons due to a strong ring current effect, which demonstrates the retention of the solid-state structure in solution. To further assess the extent to which pyrene guests remain encapsulated in solution, a known fluorescence quencher, dimethylaniline, was added to the host-guest assembly, which shows no exciplex formation for days in nonpolar solvents. Thus, the assembly also retained the structural integrity in solution for a long time. The association constant (K-asso) for such a complexation process in solution was observed to be 1.78 x 10(5) M-2 for 1:2 binding. Steady-state fluorescence and lifetime studies indicate significant photoinduced singlet-singlet energy transformation from the excited state of pyrene to zinc bisporphyrin.