Trimeric Assembly of Dendritic Light-Harvesting Antenna with Two Kinds of Porphyrin Cores.

A trimeric assembly of light-harvesting antennas was prepared using a copper-catalyzed Hüisgen 1,3-dipolar cycloaddition reaction between a dendrimer having a zinc diethynyldiphenylporphyrin core (ZnDEDPP) with two azide terminals and two equivalents of dendrimers having a zinc tetraphenylporphyrin core (ZnTPP) with one ethynyl terminal. The absorptions of the trimer appear in a longer-wavelength region compared to monomeric references in toluene; however, there is almost no shift in wavelength in 1,1,2,2-tetrachloroethane (TCE). Fluorescence spectra of the trimer show that the singlet energy transfer from ZnTPP to ZnDEDPP takes place more effectively in toluene than in TCE. These absorption and fluorescence studies are compatible with solvent-dependent conformation; the extended forms of the trimers are favored by solvation in polar TCE, and the folded conformation is stabilized by the attractive van der Waals and dipole-dipole interactions between the dendritic chains in nonpolar toluene.