Direct Observation and Control of Single‐Molecule Tautomerization by Low‐Temperature Scanning Tunneling Microscopy

Abstract

This chapter focuses on scanning tunneling microscopy (STM) studies of tautomerization within a single molecule adsorbed on a surface. First, the operation principle and theory of STM is briefly described and then several examples from recent studies are discussed. STM can be used to manipulate single atoms and molecules, thereby enabling to build artificial structures on surfaces. The trans-trans tautomerization of a single naphthalocyanine molecule, that is, the transfer of both H atoms in the cavity among the pyrrole rings can be induced if a voltage pulse larger than 1.4V is applied to the molecule. Tautomerization within porphyrin and phthalocyanine derivatives has been studied at cryogenic temperatures and induced by a voltage pulse of the STM. The tautomerization mechanism has been attributed to inelastic electron tunneling. Time-resolved STM methods with the order of nanoseconds or picoseconds can be achieved by combining a short electric or optical pulse

Reversible Switching Based on Truly Intramolecular Long-Range Proton Transfer─Turning the Theoretical Concept into Experimental Reality

“…A first working example of a tautomeric switch, where the switching is mediated by PT, was proposed by IBM Zurich Research Laboratory 15 years ago. , Using a low-temperature scanning tunneling microscope (STM), a voltage pulse at the STM tip can induce a change in the orientation of the hydrogen atom pair at the center of naphthalocyanine, leading to tautomeric switching between low and high conductance. , It sounds very exciting, indeed, and opens new horizons because most of the tautomeric compounds, where the proton(s) exchange happens over a short distance between two sites (as in naphthalocyanines or in similar macrocycles) along an intramolecular hydrogen bond, are switchable either at the conditions applied there, i.e., at the low temperature in a vacuum, or even at room temperature in solution under a variety of stimuli. , However, the relatively low PT barriers in such systems provide population of the on -state only while the external stimulus (light or voltage) is turned on. In this respect, a long-range PT potentially creates conditions for bistable switching.…”

Field-Induced Conformational Change in a Single-Molecule-Graphene–Nanoribbon Junction: Effect of Vibrational Energy Redistribution

“…With semiconductor-based electronics entering the nanoscale, devices using molecules as active components increasingly appear as promising alternatives to conventional silicon technology. A variety of switching mechanisms have been investigated over the past decade (see, for example, refs − and references therein), ranging from isomerization − to ring-opening − and hydrogen transfer reactions , via STM-induced tautomerization − and redox reactions at finite bias voltage. , The conductance properties of these molecules bound to metal electrodes have been thoroughly studied, − while embedding between graphene nanoribbons (GNR) leads have also received a fare share of attention in recent years. − The latter are very promising materials as molecular wires and field-effect transistors, since they are virtually one-dimensional building blocks and have advantageous electronic transport properties even at room temperature . Further, they bind covalently to organic molecules, which may affect their conducting properties.…”