Large Porphyrin Squares from the Self-Assembly of meso-Triazole-Appended L-Shaped meso-meso-Linked Zn-II-Triporphyrins: Synthesis and Efficient Energy Transfer

meso-Triazolyl-appended Zn-II-porphyrins were readily prepared by Cu-I-catalyzed 1,3-dipolar cycloaddition of benzyl azide to meso-ethynylated Zn-II-porphyrin (click chemistry). In noncoordinating CHCl3 solvent, spontaneous assembly occurred to form tetrameric array (3)(2) from meso-meso-linked diporphyrins 3, and dodecameric porphyrin squares (4)(4) and (5)(4) from the L-shaped meso-meso-linked triporphyrins 4 and 5. The structures of these assemblies were examined by H-1 NMR spectra, absorption spectra, and their gel permeation chromatography (GPC) retention time. Furthermore, the structures of the dodecameric porphyrin squares (4)(4) and (5)(4) were probed by small- and wide-angle X-ray scattering (SAXS/WAXS) measurements in solution using a synchrotron source. Excitation-energy migration processes in these assemblies were also investigated in detail by using both steady-state and time-resolved spectroscopic methods, which revealed efficient excited-energy transfer (EET) between the meso-meso-linked Zn-II-porphyrin units that occurred with time constants of 1.5 ps(-1) for (3)(2) and 8.8 ps(-1) for (5)(4).