Uniaxial alignment of 2D layers of porphyrins onto graphite and insulating chiral substrates for single-molecule optoelectronics

Porphyrins are ubiquitous organic molecules having a pivotal role in biological processes and as active layers in thin-film based advanced technological devices. Free-base porphyrins and porphyrinic compounds are subjected to tautomerism, i.e. the reversible prototropic formation of two isomers differing only in the position of the H-atoms bonded to the pyrrole N-atoms inside the molecular cavity. Different tautomers may have substantially different molecular conformation, conferring an anisotropic character to the system. Tautomerism characterizes the isolated porphyrin molecules and is generally also active in solid state aggregates [1]. The accomplishment of a frozen tautomeric form in monomolecular aggregates of porphyrins represented the basis for the exploitation of an intra-molecular mechanism for the development of single-molecule optoelectronic devices. Here, we show the results of growth methods, which ensure at room temperature: i) the nucleation of a pure porphyrin 2D phase on a substrate without the simultaneous growth of 3D crystals; ii) the selection of a single tautomeric form; iii) the prediction of the tautomer orientation of the deposited molecules; iv) the generation of an electronically correlated 2D layer of molecules giving rise to two stable states (the two orientations of the tautomers) to be exploited in logic devices. The successful iso-orientation at room temperature of tautomers in monomolecular domains of free-base tetraphenyl porphyrin (H2TPP) was accomplished onto exfoliated HOPG(0001) [2,3]. However, the six-fold symmetry of HOPG(0001) generates an overall isotropic distribution of such domains, hindering the possibility to scale the physical properties from multidomains of the molecular film down to the single molecule. Moreover, HOPG is a conductive substrate, which represents an obstacle for the integration of the assembly into devices. These issues were recently solved by using as a substrate the triclinic and chiral cleavage surface of the mixed crystal of fumaric acid and glycine anhydride. These characteristics allow the growth of uniaxially oriented domains of frozen tautomers of H2TPP [4,5]. Free-base porphyrins are also bases able to react with Brønsted acids (e.g. HCl) reversibly, by adding up to two protons to their inner core; this removes the anisotropic character of the molecule. The possibility to control the protonation and deprotonation of up to two H-atoms, in solid state, of frozen oriented porphyrin tautomers represents a fundamental milestone for the comprehension of some biomolecular phenomena and for their exploitation for fabricating electronic devices and sensors on the molecular scale. [1] Braun et al., J. Am. Chem. Soc. 1994, 116, 6593-6604 [2] Bussetti et al. Adv. Funct. Mater. 2014, 24, 958-963 [3] Bussetti et al. J. Phys. Chem. C 2014, 118, 15649-15655 [4] Campione et al. Nano Letters 2019, 19, 5537-5543 [5] Campione et al. Crystal Growth & Des 2020, 11, 7450-7459