Supramolecular Coordination Polymer Formed from Artificial Light-Harvesting Dendrimer

We report the formation of supramolecular coordination polymers formed from multiporphyrin dendrimers (P_ZnP_M; M = FB or Cu), composed of the focal freebase porphyrin (P_FB) or cupper porphyrin (P_Cu) with eight zinc porphyrin (P_Zn) wings, and multipyridyl porphyrins (PyP_M; M = FB or Cu), P_FB or P_Cu with eight pyridyl groups, through multiple axial coordination interactions of pyridyl groups to P_Zns. UV-vis absorption spectra were recorded upon titration of PyP_FB to P_ZnP_FB. Differential spectra, obtained by subtracting the absorption of P_ZnP_FB without guest addition as well as the absorption of PyP_FB, exhibited clear isosbestic points with saturation binding at 1 equiv addition of PyP_FB to P_ZnP_FB. Job's plot analysis also indicated 1:1 stoichiometry for the saturation binding. The apparent association constant between P_ZnP_FB and PyP_FB (2.91 × 10⁶ M^-1), estimated by isothermal titration calorimetry, was high enough for fibrous assemblies to form at micromolar concentrations. The formation of a fibrous assembly from P_ZnP_FB and PyP_FB was visualized by atomic force microscopy and transmission electron microscopy (TEM). When a 1:1 mixture solution of P_ZnP_FB and PyP_FB (20 M) in toluene was cast onto mica, fibrous assemblies with regular height (ca. 2 nm) were observed. TEM images obtained from 1:1 mixture solution of P_ZnP_FB and PyP_FB (0.1 wt %) in toluene clearly showed the formation of nanofibers with a regular diameter of ca. 6 nm. Fluorescence emission measurement of P_ZnP_M indicated efficient intramolecular energy transfer from P_Zn to the focal P_FB or P_Cu. By the formation of supramolecular coordination polymers, the intramolecular energy transfer changed to intermolecular energy transfer from P_ZnP_M to PyP_M. When the nonfluorescent PyP_Cu was titrated to fluorescent P_ZnP_FB, fluorescence emission from the focal P_FB was gradually decreased. By the titration of fluorescent PyP_FB to nonfluorescent P_ZnP_Cu, fluorescence emission from P_FB in PyP_FB was gradually increased due to the efficient energy transfer from P_Zn wings in P_ZnP_Cu to PyP_FB.