Breaking Degeneracy of Tautomerization—Metastability from Days to Seconds

Abstract

We present a detailed study of the tautomerization, that is, the switching of hydrogen protons, between different sites in the molecular frame of phthalocyanine (HPc) on a Ag(111) substrate by means of scanning tunneling microscopy (STM) and STM-based pump-and-sample techniques. Our data reveal that the symmetry mismatch between the substrate and the molecular frame lifts the energetic degeneracy of the two HPc tautomers. Their energy difference is so large that only one tautomer can be found in the ground state. Tip-induced tautomerization was triggered at sufficiently high bias voltages. The excited metastable HPc tautomer was found to exhibit a lifetime of at least several days, as derived from the fact that the molecule did not change back to the ground state within experimentally accessible time scales as long as noninvasive tunneling parameters were used to probe the state of the molecule. By the controlled removal of a hydrogen proton from the molecule, a four-level system was created. Pump-and-sample experiments reveal that the lifetime of the metastable positions amounts to seconds only. Current- and bias-dependent studies indicate that the presence of the STM tip modifies the potential barrier, thereby allowing for a controlled tuning of the metastable tautomer's lifetime.

Spin in a Closed‐Shell Organic Molecule on a Metal Substrate Generated by a Sigmatropic Reaction

“…The high current densities and electrical field strength present in a STM junction have been widely used to induce chemical processes on surfaces with molecular precision. In particular, intramolecular hydrogen transfers and dehydrogenations have been already reported from N−H or C−H bonds of porphyrinic molecules, melamine, and nanographene‐like molecules . The proposed two‐step process depicted in Scheme (right) is motivated by the general concept of hydrogen lability in conjugated molecules but selects a pathway associated to a major topological change (left).…”

Tip‐Induced Inversion of the Chirality of a Molecule's Adsorption Potential Probed by the Switching Directionality

“…The understanding of the switching behavior of molecular systems1 is of specific importance for the realization of active units in future information processing and storage devices. In particular, switching processes at surfaces have been studied intensely by scanning probe techniques, including investigations of switching based on rotational motion of molecules,2–7 molecular conformational changes,8–11 intramolecular hydrogen transfer,12–20 chemical changes in molecules,21,22 and charge‐state transitions of single molecules 23. The switching process itself can be driven either by thermal activation2,4 or by external means, including energy transfer from inelastically scattered electrons2,12,14–16 or photons,8,24 as well as the application of mechanical forces 17,25,26.…”

Single-molecule switching via controlled tautomerization at room temperature

“…Over the past decade, tautomerization has been demonstrated in the surface-based characterization of tetrapyrrole macrocycles, such as porphyrins and naphthalocyanine [4][5][6]10,11 using scanning tunnelling microscopy (STM) and atomic force microscopy (AFM). Although distinct states have been observed for these macrocycles upon their absorption on at surfaces, the resulting pairs of tautomeric states are generally degenerate or quasi-degenerate with a low isomerization barrier 12 . Due to the low barrier, the interconversion between the tautomeric states is in dynamic balance and spontaneously occurs even under cryogenic conditions, which prevents the potential application of the single-molecule tautomerization in the construction of controllable molecular devices.…”