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

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 (H2TPP) 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

Unconventional post‐deposition chemical treatment on ultra‐thin H2TPP film grown on graphite

“…All crystals are placed onto a flat 2D phase that represents a sort of organic wetting layer for the following evolution of the 3D phase in thicker films. The 2D phase is compatible with the height of a single layer of porphyrins, whose structure is reported elsewhere . To the best of our knowledge, the evidence of this clear SK growth mode in the H 2 TPP/HOPG sample has been never reported to date, yet .…”

“…Recently, some particular molecular materials, such as porphyrins and phthalocyanines, have attracted a renewed interest because they present a particular and well‐known chemical reaction, called tautomerization (i.e. the interconversion between two constitutional isomers, accompanied by the migration of hydrogen atoms) , that could be exploited in molecular switches and logic devices . Some of these prototype devices use single isolated molecules on metal surfaces (first electrode) and scanning probe tips as a second electrode.…”

“…The resulting molecular switch works along the vertical direction with respect to the surface. In order to explore scalable organic devices, the latter should be developed horizontally in a single or multi layered system . Unfortunately, organic compounds often follow a 3D or at least a Stranski‐Krastanov (SK) growth mode, which precludes obtaining a pure 2D phase.…”

Electronic structure of tetra(4-aminophenyl)porphyrin studied by photoemission, UV–Vis spectroscopy and density functional theory

“…Engineering of the opto-electronic properties of porphyrins thus continues to stimulate an intense research activity. Important modification to the porphyrin spectral response can be obtained for instance by functionalization [5,6], nano-structuration of extended porphyrin-based architectures [7,8] or adsorption on a substrate [9][10][11].…”

Reactive Dissolution of Organic Nanocrystals at Controlled pH

“…The deposition of both H 2 TPP and ZnTPP molecules under high vacuum on HOPG substrates follows a layer-plus-island growth mode (see Figure S1 in the SI). [30,31] The film surface morphology ( Figure 1) is characterized by an ensemble of many crystals (approximately 30-50 crystals/μm 2 ), showing many edges at 90°a nd is organized in domains with different orientations. The porphyrin crystal lateral size ranges from tens to hundreds of nanometers.…”