Stable Alignment of Tautomers at Room Temperature in Porphyrin 2D Layers

TLDR

Molecular electronics seeks intramolecular switching, but tautomerization in porphyrins has only been used at low temperatures and is unpredictable, hindering circuit integration. The authors propose using uniaxially oriented H₂TPP tautomers to build a first elementary logic device. Buckling of the anchored H₂TPP molecules removes tautomer degeneracy, enabling directional alignment. They demonstrate that 2D layers of metal‑free H₂TPP exhibit room‑temperature frozen tautomerization with macroscopic, pre‑aligned hydrogen atoms.

Abstract

A major challenge in molecular electronics is to develop logic devices based on a truly intramolecular switching mechanism. Recently, a new type of molecular device has been proposed where the switching characteristic is mediated by the bistability in the position of the two hydrogen atoms which can occupy different, energetically equivalent positions (tautomerization) in the inner cavity of porphyrins and naphthalocyanines. Up to now, such a reaction has only been exploited at low temperatures and induced or detected through atomic scale manipulation. In addition, the unpredictability of the tautomer orientation currently excludes molecular interconnection to functional electronic circuits. Here, full evidence is provided that, following a newly proposed growth strategy, 2D layers of metal‐free tetraphenylporphyrins (H 2 TPP) show frozen tautomerization even at room temperature on macroscopic domains, with the H atoms aligned along a direction settled a priori. This behavior is ascribed to the buckling of the molecule, anchored to the substrate, which removes the degeneracy between the two tautomer alignments. On this basis, a new way to exploit uniaxially oriented H 2 TPP tautomers in a first elementary logic device is proposed.