Porphene and Porphite: Porphyrin Analogs of Graphene and Graphite

Two-dimensional (2D) organic materials offer atomic precision for optoelectronics and energy-efficient nanoelectronics, but most are not easily patterned and tuned. The long-sought porphene [C20N4H2)∞, 1] has now been prepared in a hole-doped form from the zinc salt C20N4H12Zn (Zn-2) of porphyrin (C20N4H14, 2) by oxidative polymerization on aqueous surface accompanied by loss of zinc ions. After hole removal by excess reductant in the subphase, metal ions can be introduced to form Zn-porphene, (C20N4Zn)∞ (Zn-1, Figure 1), or other metalloporphenes. Reversible insertion of metal ions promises painting on an atomic canvas with distinct metal ions and ligands without removing any π centers from conjugation. The bond pattern in 1 and Zn-1 is deduced from in-situ and ex-situ spectra and images. Early GGA DFT computations for a perfect sheet of Zn-1 predicted a P4mm (D4h) square unit cell and metallic conductivity, but hybrid DFT predicts a semiconductor with two identical but differently oriented slightly rectangular antiaromatic P2mm (D2h) unit cells with planar 8-membered rings, analogous to “Kekule” structures of a 2×2 fragment of Zn-1, and planar [4n]annulenes, (cf. a vast physics literature on 2D-Peierls distortions). The polymer sheet was transferred to solid substrates, mostly as multilayers of 1 (porphite) and semiconducting Zn-1 (Zn-porphite), analogous to graphite, but also as a monolayer.