Synthesis of nanometer-scale porphyrin wheels of variable size

Starting from 1.3-phenylene linked diporphyrin zinc(II) complex 2ZA. repeated stepwise Ag¹-promoted coupling reactions provided linear oligomers from 2nZA up to 128ZA. Of these zigzag shaped porphyrin arrays. the Ag'-promoted intramolecular cyclization reaction of 2nZA (n = 5. 6. 8. 9, 12, and 16) under dilute conditions gave the corresponding cyclic porphyrin wheels C2wZA (n=5, 6. 8. 9, 12, and 16), whereas large arrays 2nZA (n = 24, 32. and 48) did not providecyclic porphyrin products. These large discrete porphyrin arrays and wheels were fully characterized by means of ¹HNMR spectroscopy, MALDI-TOF mass spectrometry, UV/Vis absorption spectroscopy, GPC-HPLC analysis, and the scanning tunneling microscopy (STM) technique. The STM images of C12ZA and C18ZA reveal their large circular structures. In the cyclic structures of C2nZA in solution, however, the gradual decrease in fluorescence quantum yields and fluorescence lifetimes are observed, reflecting some conformational heterogeneities. Collectively, the present work provides an important contribution to the construction of fully covalently linked large cyclic arranged porphyrin arrays with ample electronic interactions as a model of light-harvesting antenna.