Oxidative coupling of parent free-base or zinc porphyrin on the surface of an aqueous phase in a Langmuir-Blodgett trough yields sheets of free-base porphene (C20N4H2)∞, a regular fully conjugated two-dimensional polymer of monoatomic thickness, composed of meso-meso and beta-beta fused porphyrin rings. The presence of some meso-beta coupled defects cannot be excluded. As initially formed, the polymer is metal-free, is doped with positively charged holes (radical cations), and its charge is compensated by a diffuse layer of negative counterions in the aqueous surface layer. Addition of iodide ions to the subphase removes the positive holes and thereafter metal cations such as Zn2+ or Fe2+ can be permanently inserted into the macrocyclic rings. Depending on reaction conditions, single or double sheets of dimensions up to tens of micrometers across are obtained. The structure of porphene was determined using in-situ synchrotron grazing incidence X-ray diffraction, X-ray reflectance, and, after transfer to a solid substrate, by scanning tunneling, atomic force, and transmission electron microscopy, and UV-visible, IR, and X-ray photoelectron spectroscopy. Results of density functional calculations agree with the observed properties and suggest that the most stable tautomer of the free base contains a checkerboard arrangement of NH bonds in which their directions alternate from any macrocycle to the four neighbors with which it shares sides.

Properties of porphene, a heterocyclic analog of graphene

ACS Spring Meeting 2021

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