Supramolecular Coordination Polymer Formed from Artificial Light-Harvesting Dendrimer

We report the formation of supramolecular coordination polymers formed from multiporphyrin dendrimers (PZₙPM; M = FB or Cu), composed of the focal freebase porphyrin (PFB) or cupper porphyrin (PCᵤ) with eight zinc porphyrin (PZₙ) wings, and multipyridyl porphyrins (PyPM; M = FB or Cu), PFB or PCᵤ with eight pyridyl groups, through multiple axial coordination interactions of pyridyl groups to PZₙs. UV–vis absorption spectra were recorded upon titration of PyPFB to PZₙPFB. Differential spectra, obtained by subtracting the absorption of PZₙPFB without guest addition as well as the absorption of PyPFB, exhibited clear isosbestic points with saturation binding at 1 equiv addition of PyPFB to PZₙPFB. Job’s plot analysis also indicated 1:1 stoichiometry for the saturation binding. The apparent association constant between PZₙPFB and PyPFB (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 PZₙPFB and PyPFB was visualized by atomic force microscopy and transmission electron microscopy (TEM). When a 1:1 mixture solution of PZₙPFB and PyPFB (20 μM) 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 PZₙPFB and PyPFB (0.1 wt %) in toluene clearly showed the formation of nanofibers with a regular diameter of ca. 6 nm. Fluorescence emission measurement of PZₙPM indicated efficient intramolecular energy transfer from PZₙ to the focal PFB or PCᵤ. By the formation of supramolecular coordination polymers, the intramolecular energy transfer changed to intermolecular energy transfer from PZₙPM to PyPM. When the nonfluorescent PyPCᵤ was titrated to fluorescent PZₙPFB, fluorescence emission from the focal PFB was gradually decreased. By the titration of fluorescent PyPFB to nonfluorescent PZₙPCᵤ, fluorescence emission from PFB in PyPFB was gradually increased due to the efficient energy transfer from PZₙ wings in PZₙPCᵤ to PyPFB.