Imaging Charge Localization in a Conjugated Oligophenylene

Polaron formation in conjugated polymers has a major impact on their optical and electronic properties. In polyphenylene, the molecular conformation is determined by a delicate interplay between electron delocalization and steric effects. Injection of excess charges is expected to increase the degree of conjugation, leading to structural distortions of the chain. Here we investigated at the single-molecule level the role of an excess charge in an individual oligophenylene deposited on sodium chloride films. By combining sub-molecular-resolved atomic force microscopy with redox-state-selective orbital imaging, we characterize both structural and electronical changes occurring upon hole injection. While the neutral molecule exhibits a delocalized frontier orbital, for the cationic radical the excess charge is observed to localize, inducing a partial planarization of the molecule. These results provide direct evidence for self-trapping of the excess charge in oligophenylenes, shedding light on the interplay of charge localization and structural distortion.

Figure 1

(a) Chemical structures of neutral (top) and three resonance structures for the cationic species of $6P$. For the cationic radical, a transition to a quinoid structure is directly linked to the separation of hole and unpaired electron, as is highlighted in red. (b) Calculated HOMO contour of gas-phase $6P^0$.

Figure 2

(a),(b) Constant-height AFM $\mathrm{\Delta }f$ images of $6P^0$ on $\mathrm{NaCl}(2\mathrm{ML})/\mathrm{Cu}(111)$, acquired with a CO-functionalized tip at $\mathrm{\Delta }z=-0.9$ (a) and $\mathrm{\Delta }z=-2.6\AA$ (b). $\mathrm{\Delta }z$ is given with respect to opening the feedback loop at an STM set point of $I=0.6\mathrm{pA}$, $V=0.12\mathrm{V}$ above the clean NaCl surface. AFM oscillation amplitude, $A=1\AA$. (c) Model of gas-phase $6P$, where the torsional angle ($\theta$) is indicated. Gray and white spheres represent carbon and hydrogen atoms, respectively. (d),(e) Determination of the torsional angle for $6P^0$. (d) Vertical image of $6P^0$ acquired with a CO-functionalized tip along the direction indicated in the inset of (e). The white crosses in (d) mark the minimum ($z^{*}$) of each $\mathrm{\Delta }f(z)$ spectrum. (e) $\mathrm{\Delta }f(z)$ spectra acquired above the tilted phenyl rings. Inset: an arrow indicates the direction of the vertical image, superimposed to the panel (b). (d),(e) The feedback was opened at an STM set point of $I=1.5\mathrm{pA}$, $V=0.30\mathrm{V}$. A background $\mathrm{\Delta }f(z)$ spectrum acquired on the bare NaCl surface has been subtracted from all spectra in (d),(e).

Figure 3

Charge-state manipulation of $6P$ on $\mathrm{NaCl}(\ge 20\mathrm{ML})/\mathrm{Cu}(111)$. (a) $\mathrm{\Delta }f(V_{\mathrm{dc}})$ spectrum acquired on an isolated molecule ($A=1\AA$, $\mathrm{\Delta }z=-2.5\AA$). (b) Constant-height AFM $\mathrm{\Delta }f$ image acquired with a metal tip apex ($V_{\mathrm{dc}}=-3.45\mathrm{V}$, $V_{\mathrm{ac}}=0\mathrm{V}$, $A=1\AA$, $\mathrm{\Delta }z=-2.7\AA$). $\mathrm{\Delta }z$ is given with respect to opening the feedback loop at an AFM set point of $\mathrm{\Delta }f=1\mathrm{Hz}$, $V_{\mathrm{dc}}=0\mathrm{V}$ above the clean NaCl surface.

Figure 4

Charge-state controlled imaging of $6P$ on $\mathrm{NaCl}(\ge 20\mathrm{ML})/\mathrm{Cu}(111)$. (a),(b) Constant-height AFM $\mathrm{\Delta }f$ images: $6P^0$ [(a) $V_{\mathrm{dc}}=-3.20\mathrm{V}$, $V_{\mathrm{ac}}=0{\mathrm{V}}_{\mathrm{pp}}$, $\mathrm{\Delta }z=-2.7\AA$] and $6P^{+}$ [(b) $V_{\mathrm{dc}}=-3.55\mathrm{V}$, $V_{\mathrm{ac}}=0{\mathrm{V}}_{\mathrm{pp}}$, $\mathrm{\Delta }z=-2.7\AA$]. (c),(d) Constant-height AC-STM images: $0\to 1^{+}$ [(c) $V_{\text{dc}}=-3.25\mathrm{V}$, $V_{\text{ac}}=1.40{\mathrm{V}}_{\mathrm{pp}}$, $\mathrm{\Delta }z=-2.5\AA$]; $1^{+}\to 0$ [(d) $V_{\mathrm{dc}}=-3.32\mathrm{V}$, $V_{\mathrm{ac}}=1.40{\mathrm{V}}_{\mathrm{pp}}$, $\mathrm{\Delta }z=-2.7\AA$]. All the images have been acquired with the same metal tip apex and $A=1\AA$. $\mathrm{\Delta }z$ is given with respect to opening the feedback loop at an AFM set point of $\mathrm{\Delta }f=-1\mathrm{Hz}$, $V_{\mathrm{dc}}=0\mathrm{V}$ above the clean NaCl surface. (e) Line profiles of AFM (top) and AC-STM (bottom) images.