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

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 2TPP) 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 H2TPP tautomers in a first elementary logic device is proposed. Frozen tautomers at room temperature are exploited in a first logic device prototype. Following a newly proposed growth strategy, a purely 2D ordered array of metal-free porphyrins shows all the tautomers aligned along a direction settled a priori. This occurrence characterizes the anisotropic optical property of the system. The quenching/renewing of the porphyrin anisotropy defines two distinct logic states (0/1) easily readable by optics. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.