Accessing a Charged Intermediate State Involved in the Excitation of Single Molecules

Intermediate states are elusive to many experimental techniques due to their short lifetimes. Here, by performing single-electron alternate charging scanning tunneling microscopy of molecules on insulators, we accessed a charged intermediate state involved in the rapid toggling of individual metal phthalocyanines deposited on NaCl films. By stabilizing the transient species, we reveal how electron injection into the lowest unoccupied molecular orbital leads to a pronounced change in the adsorption geometry, characterized by a different azimuthal orientation. This observation allows clarifying the nature of the toggling process, unveiling the role of transient ionic states involved into fundamental processes occurring at interfaces.

Figure 1

$\mathrm{MgPc}/\mathrm{NaCl}$ $(2\mathrm{ML})/\mathrm{Cu}(100)$. (a) Chemical structure of a metal phthalocyanine (Pc). (b) Constant-current STM image of the NIR ($I=1\mathrm{pA}$, $V=0.65\mathrm{V}$). Arrows highlight the position of nodal planes. (c),(d) Constant-current in-gap STM images ($I=1\mathrm{pA}$, $V=0.1\mathrm{V}$). Solid blue lines indicate the directions of a molecular axis. Scale bar: 10 Å.

Figure 2

(a) $\mathrm{Pc}/\mathrm{NaCl}(2\mathrm{ML})/\mathrm{Cu}$. Electron tunneling through the ultrathin (2 ML) insulating layer, required for conventional STM operation, allows only transient charging of Pc for a very short lifetime ($\approx 0.2\mathrm{ps}$) [20]. (b) $\mathrm{Pc}/\mathrm{NaCl}$ $(\ge 20\mathrm{ML})/\mathrm{Cu}$. On thick NaCl films, tunneling is suppressed, allowing for charge-state manipulation and imaging of electronic transitions by AC-STM [33].

Figure 3

$\mathrm{CuPc}/\mathrm{NaCl}$ $(\ge 20\mathrm{ML})/\mathrm{Cu}(111)$. (a),(b) Constant-height AC-STM images measured using a Cu-terminated tip ($V_{\mathrm{dc}}=1.0\mathrm{V}$, oscillation amplitude $A=1\AA$): (a) $0\to 1^{-}$ ($V_{\mathrm{ac}}=0.75$ $V_{\mathrm{pp}}$, $\mathrm{\Delta }z=3.7\AA$); (b) $1^{-}\to 0$ ($V_{\mathrm{ac}}=1.00{\mathrm{V}}_{\mathrm{pp}}$, $\mathrm{\Delta }z=4.4\AA$). $\mathrm{\Delta }z$ are given with respect to an AFM set point of $\mathrm{\Delta }f=-1.5\mathrm{Hz}$ at $V_{\mathrm{dc}}=0\mathrm{V}$. Solid lines indicate nodal planes. Scale bar: 10 Å. (c) Energy scheme of the neutral and anionic molecule. (d) Schematic energy diagram of the excitation mechanism.

Figure 4

(a) Potential energy as a function of azimuthal angle for ${\mathrm{CuPc}}^0$ (blue) and ${\mathrm{CuPc}}^{-}$ (red). (b),(c) Top and side view of the optimized geometry of ${\mathrm{CuPc}}^0$ at 12°. (d),(e) Top and side view of the optimized geometry of ${\mathrm{CuPc}}^{-}$ at 0°. (c),(e) The vertical coordinates of both species are displayed with respect to the average plane of the molecule and magnified by a factor 4. (f),(g) Vertical displacement of the ${\mathrm{Na}}^{+}$ (smaller circles) and ${\mathrm{Cl}}^{-}$ (larger circles) ions for ${\mathrm{CuPc}}^0$ and ${\mathrm{CuPc}}^{-}$, respectively. (f) A dotted ellipse highlights one of the ${\mathrm{Na}}^{+}{\mathrm{Cl}}^{-}$ ion pairs below a peripheral C-H group exhibiting a strong relaxation toward the molecule.