Synthesis, Structure, and Optical Properties of a Bis-Macrocycle Derived from a Highly Emissive 1,3,6,8-Tetra(1H-pyrrol-2-yl)pyrene.

A tetra-functionalized pyrene precursor 4b is prepared using the Suzuki-Miyaura coupling of 1,3,6,8-tetrabromopyrene with N-Boc-2-pyrroleboronic acid. 4b displayed a blue emission with a high quantum yield (ϕF = 0.89). 4b is subjected to [3 + 2] Lewis acid-catalyzed condensation with 2,2'-bithiophene-dialcohol 5, affording a planar bis-N2S2 internally linked with pyrene. The single-crystal X-ray structure of bis-N2S2 revealed a planar conformation with all of the pyrrolic nitrogens and thiophenic sulfurs pointing toward the macrocyclic core. Further, the reduction of bis-N2S2 was attempted in the presence of Zn/NH4Cl at room temperature in CHCl3. A sharp color change from pink to brown was observed presumably due to the formation of its reduced congener bis-N2S2-2H. However, the reduced species was found to revert back to its oxidized form over a period of 25 min in CHCl3. Density functional theory (DFT) studies reveal that the two monocyclic halves of bis-N2S2-2H exhibit differences in aromaticity depending on amino and imino pyrroles present inside each individual core. Such a conversion was also monitored by ultraviolet-visible (UV-vis) absorption spectral studies, and the exact composition of bis-N2S2-2H was confirmed by High-resolution/mass spectrometry (HR/MS) analysis. Experimental and theoretical studies reveal a weak aromatic character of bis-N2S2 due to the absence of global conjugation.