Giant meso-meso-linked porphyrin arrays of micrometer molecular length and their fabrication

On the basis of the Ag^I-promoted coupling reaction of zinc(II)-5,15-bis(3,5-dioctyloxyphenyl)porphyrin Z1, chain elongation has been attempted by using a stepwise doubling approach, which provides Z2, Z4, Z8, Z16, Z32, Z64, Z128, Z256, Z384, and Z512. The porphyrin arrays up to Z128 are sufficiently soluble in CHCl₃ and THF despite their very long molecular lengths and rodlike structures, while the arrays over Z128 show a significant drop in solubility and stability. The discrete porphyrin arrays thus isolated were characterized by means of ¹H NMR spectroscopy, matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry, UV/Vis spectroscopy, gel-permeation chromatography (GPC), cyclic voltammetry (CV), single-crystal X-ray crystallography, scanning tunneling microscopy (STM), and atomic force microscopy (AFM). Contrary to expected linear conformations of the arrays Z n (where n is the number of porphyrins), the single molecular images of Z128, Z256, and Z512 revealed largely bent structures; this finding indicates the substantial conformational flexibility of Zn. We also exploited an effective synthetic route by means of which Zn can be fabricated with a thiol-protected aryl group to provide ZnS₂ through ZnBr₂, by bromination with N-bromosuccinimide and subsequent Pd-catalyzed Suzuki-Miyaura arylation. Finally, the reaction of Z256 provided Z512, Z768, and Z1024. Collectively, this work provides an important milestone in the preparation of sub-microscale discrete organic molecules and the fabrication of molecular-based materials, hence significantly contributing to device applications.