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

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 $\sim 75\mathrm{K}$, tautomerization is induced thermally and an activation barrier of about 168 meV is determined from an Arrhenius plot.

Figure 1

(a) Molecular structure of porphycene (the hydrogen atom transfer during the cis-cis tautomerization is shown by arrows). The distances between the N atoms in the cavity [7] are indicated. (b) Typical STM image of a single porphycene molecule on Cu(110) ($1.49\times 1.42{\mathrm{nm}}^2$ in size; $I_t=10\mathrm{nA}$ and $V=100\mathrm{mV}$). (c) The simulated STM image of the cis-1 configuration with the optimized structure in (d). (e)–(l) Voltage dependence of the STM image of a single porphycene molecule at 5 K. The bias voltage is indicated in each image and the tunneling current is set at $I_t=5\mathrm{nA}$. The scan area is $2\times 2{\mathrm{nm}}^2$ with a scanning time of about $9\sec /\mathrm{image}$.

Figure 2

(a) Current dependence of the tautomerization rate at different temperatures and bias voltages: 78 K and 50 mV (red squares) and 5 K and 200 mV (blue circles). The insets show STM images of a single molecule at 78 and 86 K. (b) Arrhenius plot of the thermally activated tautomerization (see text).

Figure 3

(a) A typical current trace during a voltage pulse of 300 mV while the STM tip is fixed over a molecule (indicated by the black star in the image) with gap conditions of 100 mV and 10 pA. (b) Schematics of the tautomerization process. (c) Current dependence of the tautomerization rate at various bias voltages. 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, 250, 275, 300, 325, and 350 mV, respectively. (d) Voltage dependence of the tautomerization yield for $h$- (red circles) and $d$-porphycene (blue circles) at 5 K (the latter is sketched in the inset). Yields above 380 mV cannot be determined due to our STM time-resolution of about 1 ms. A magnification of both bias polarities is shown on top. (e) $dI/dV$ conductance spectra of single $h$- and $d$-porphycene molecules (after subtracting the background measured over the clean surface). The spectra were obtained at the same position as (a) with set points of 100 mV and 20 nA and 30 nA for $h$- and $d$-porphycene, respectively.

Figure 4

Calculated projected vibrational density of states of the $h$-porphycene and the $d$-porphycene adsorbed on a rigid Cu(110) surface [the optimized structure is shown in the Fig. 1d]. The in-plane projection is on the displacements in the molecular plane of the two inner H atoms and the four N atoms. The discrete vibrational modes have been broadened by a Gaussian with a width of 2 meV [39].