Thermally and Vibrationally Induced Tautomerization of Single Porphycene Molecules on a Cu(110) Surface

Abstract

We report the direct observation of intramolecular hydrogen atom transfer reactions (tautomerization) within a single porphycene molecule on a Cu(110) surface by scanning tunneling microscopy. It is found that the tautomerization can be induced via inelastic electron tunneling at 5 K. By measuring the bias-dependent tautomerization rate of isotope-substituted molecules, we can assign the scanning tunneling microscopy-induced tautomerization to the excitation of specific molecular vibrations. Furthermore, these vibrations appear as characteristic features in the dI/dV spectra measured over individual molecules. The vibrational modes that are associated with the tautomerization are identified by density functional theory calculations. At higher temperatures above ∼75 K, tautomerization is induced thermally and an activation barrier of about 168 meV is determined from an Arrhenius plot.

Direct observation and control of hydrogen-bond dynamics using low-temperature scanning tunneling microscopy

“…The degeneracy of tautomerization is lifted in the triclinic TPP crystal, but co-crystallization with a small amount of Ni-TPP can restore the degeneracy because of the tetragonal structure [74]. In Section 6, direct observation and control of tautomerization within a single porphycene molecule on Cu(1 1 0) [75,76] are presented. The tautomerization is induced through thermal activation or vibrational excitation via inelastic electron tunneling process in STM, and also the impact of local environmental perturbation on the tautomerization can be also examined either by placing a single Cu atom close to a single molecule, or by changing the orientation of neighboring molecules in molecular assemblies.…”

“…6.6(i) (the relative error is larger at lower set-point currents because of less events measured than at higher setpoint current). From [76]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)…”

“…STM-induced cis-cis tautomerization of a porphycene molecule on Cu(1 1 0) at 5 K[76]. (a)-(h) Voltage dependent STM images of a single porphycene molecule.…”

“…The slopes (n) were determined as 1.67 (±0.05), 1.35 (±0.03), 1.05 (±0.11), 0.91 (±0.02), 0.81 (±0.04) and 0.95 (±0.09) for 200 mV, 250 mV, 275 mV, 300 mV, 325 mV, and 350 mV, respectively (note that fitting to obtain the slopes was implemented to the logarithmic values, thus ln R vs. ln I t ). From[76]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.…”

“…A = 10 9.6(±0.8) and E a = 168(±12) meV were obtained from the best fitted curve to the Arrhenius equation. From[76].…”