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

Evidence of exciton-libron coupling in chirally adsorbed single molecules

Interplay between motion of nuclei and excitations has an important role in molecular photophysics of natural and artificial structures. Here we provide a detailed analysis of coupling between quantized librational modes (librons) and charged excited states (trions) on single phthalocyanine dyes adsorbed on a surface. By means of tip-induced electroluminescence performed with a scanning probe microscope, we identify libronic signatures in spectra of chirally adsorbed phthalocyanines and find that these signatures are absent from spectra of symmetrically adsorbed species. We create a model of the libronic coupling based on the Franck-Condon principle to simulate the spectral features. Experimentally measured librational spectra match very well the theoretically calculated librational eigenenergies and peak intensities (Franck-Condon factors). Moreover, the comparison reveals an unexpected depopulation channel for the zero libron of the excited state that can be effectively controlled by tuning the size of the nanocavity. Our results showcase the possibility of characterizing the dynamics of molecules by their low-energy molecular modes using µeV-resolved tip-enhanced spectroscopy.

Vibronic coupling in molecules plays an essential role in photophysics. Here, the authors observe optical fingerprints of the coupling between librational states and charged excited states in a single phthalocyanine molecule chirally absorbed on a surface.

Making and breaking of chemical bonds in single nanoconfined molecules

Nanoconfinement of catalytically active molecules is a powerful strategy to control their chemical activity; however, the atomic-scale mechanisms are challenging to identify. In the present study, the site-specific reactivity of a model rhenium catalyst is studied on the subnanometer scale for complexes confined within quasi-one-dimensional molecular chains on the Ag(001) surface by scanning tunneling microscopy. Injection of tunneling electrons causes ligand dissociation in single molecules. Unexpectedly, while half of the complexes show only the dissociation, the confined molecules show also the reverse reaction. On the basis of density functional theory calculations, this drastic difference can be attributed to the limited space in confinement. That is, the split-off ligand adsorbs closer to the molecule and the dissociation causes less structural disruption. Both of these facilitate the reverse reaction. We demonstrate formation and disruption of single chemical bonds of nanoconfined molecules with potential application in molecular data storage.

Energy funnelling within multichromophore architectures monitored with subnanometre resolution

The funnelling of energy within multichromophoric assemblies is at the heart of the efficient conversion of solar energy by plants. The detailed mechanisms of this process are still actively debated as they rely on complex interactions between a large number of chromophores and their environment. Here we used luminescence induced by scanning tunnelling microscopy to probe model multichromophoric structures assembled on a surface. Mimicking strategies developed by photosynthetic systems, individual molecules were used as ancillary, passive or blocking elements to promote and direct resonant energy transfer between distant donor and acceptor units. As it relies on organic chromophores as the elementary components, this approach constitutes a powerful model to address fundamental physical processes at play in natural light-harvesting complexes.

Plasmon-Driven Motion of an Individual Molecule

We demonstrate that nanocavity plasmons generated a few nanometers away from a molecule can induce molecular motion. For this, we study the well-known rapid shuttling motion of zinc phthalocyanine molecules adsorbed on ultrathin NaCl films by combining scanning tunneling microscopy (STM) and spectroscopy (STS) with STM-induced light emission. Comparing spatially resolved single-molecule luminescence spectra from molecules anchored to a step edge with isolated molecules adsorbed on the free surface, we found that the azimuthal modulation of the Lamb shift is diminished in case of the latter. This is evidence that the rapid shuttling motion is remotely induced by plasmon-molecule coupling. Plasmon-induced molecular motion may open an interesting playground to bridge the nanoscopic and mesoscopic worlds by combining molecular machines with nanoplasmonics to control directed motion of single molecules without the need for local probes.

Probing Molecular Excited States by Atomic Force Microscopy

By employing single charge injections with an atomic force microscope, we investigated redox reactions of a molecule on a multilayer insulating film. First, we charged the molecule positively by attaching a single hole. Then we neutralized it by attaching an electron and observed three channels for the neutralization. We rationalize that the three channels correspond to transitions to the neutral ground state, to the lowest energy triplet excited states and to the lowest energy singlet excited states. By single-electron tunneling spectroscopy we measured the energy differences between the transitions obtaining triplet and singlet excited state energies. The experimental values are compared with density functional theory calculations of the excited state energies. Our results show that molecules in excited states can be prepared and that energies of optical gaps can be quantified by controlled single-charge injections. Our work demonstrates the access to, and provides insight into, ubiquitous electron-attachment processes related to excited-state transitions important in electron transfer and molecular optoelectronics phenomena on surfaces.

Long-Range Surface-Assisted Molecule-Molecule Hybridization

Metalated phthalocyanines (Pc's) are robust and versatile molecular complexes, whose properties can be tuned by changing their functional groups and central metal atom. The electronic structure of magnesium Pc (MgPc)-structurally and electronically similar to chlorophyll-adsorbed on the Ag(100) surface is investigated by low-temperature scanning tunneling microscopy and spectroscopy, non-contact atomic force microscopy, and density functional theory. Single, isolated MgPc's exhibit a flat, fourfold rotationally symmetric morphology, with doubly degenerate, partially populated (due to surface-to-molecule electron transfer) lowest unoccupied molecular orbitals (LUMOs). In contrast, MgPc's with neighbouring molecules in proximity undergo a lift of LUMOs degeneracy, with a near-Fermi local density of states with reduced twofold rotational symmetry, indicative of a long-range attractive intermolecular interaction. The latter is assigned to a surface-mediated two-step electronic hybridization process. First, LUMOs interact with Ag(100) conduction electrons, forming hybrid molecule-surface orbitals with enhanced spatial extension. Then, these delocalized molecule-surface states further hybridize with those of neighbouring molecules. This work highlights how the electronic structure of molecular adsorbates-including orbital degeneracies and symmetries-can be significantly altered via surface-mediated intermolecular hybridization, over extended distances (beyond 3 nm), having important implications for prospects of molecule-based solid-state technologies.

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.

Sub-cycle atomic-scale forces coherently control a single-molecule switch

Scanning probe techniques can leverage atomically precise forces to sculpt matter at surfaces, atom by atom. These forces have been applied quasi-statically to create surface structures^1-7 and influence chemical processes^8,9, but exploiting local dynamics^10-14 to realize coherent control on the atomic scale remains an intriguing prospect. Chemical reactions^15-17, conformational changes^18,19 and desorption²⁰ have been followed on ultrafast timescales, but directly exerting femtosecond forces on individual atoms to selectively induce molecular motion has yet to be realized. Here we show that the near field of a terahertz wave confined to an atomically sharp tip provides femtosecond atomic-scale forces that selectively induce coherent hindered rotation in the molecular frame of a bistable magnesium phthalocyanine molecule. Combining lightwave-driven scanning tunnelling microscopy^21-24 with ultrafast action spectroscopy^10,13, we find that the induced rotation modulates the probability of the molecule switching between its two stable adsorption geometries by up to 39 per cent. Mapping the response of the molecule in space and time confirms that the force acts on the atomic scale and within less than an optical cycle (that is, faster than an oscillation period of the carrier wave of light). We anticipate that our strategy might ultimately enable the coherent manipulation of individual atoms within single molecules or solids so that chemical reactions and ultrafast phase transitions can be manipulated on their intrinsic spatio-temporal scales.

Quantifying the evolution of atomic interaction of a complex surface with a functionalized atomic force microscopy tip

Terminating the tip of an atomic force microscope with a CO molecule allows data to be acquired with a well-known and inert apex. Previous studies have shown conflicting results regarding the electrostatic interaction, indicating in some cases that the negative charge at the apex of the CO dominates, whereas in other cases the positive charge at the end of the metal tip dominates. To clarify this, we investigated [Formula: see text](111). [Formula: see text] is an ionic crystal and the (111) surface does not possess charge inversion symmetry. Far from the surface, the interaction is dominated by electrostatics via the negative charge at the apex. Closer to the surface, Pauli repulsion and CO bending dominate, which leads to an unexpected appearance of the complex 3-atom unit cell. We compare simulated data in which the electrostatics are modeled by point particles versus a charge density calculated by DFT. We also compare modeling Pauli repulsion via individual Lennard-Jones potentials versus a total charge density overlap. In doing so, we determine forcefield parameters useful for future investigations of biochemical processes.

Spontaneous Symmetry Breaking in Cyclo[18]Carbon

After the experimental evidence of polyynic as the stable form of cyclo[18]carbon, in the present paper, using ab initio electronic structure calculations, we show that this result is a symmetry breaking event, a consequence of the second-order Jahn-Teller effect. We show that the eigenfunctions associated with lowest unoccupied molecular orbitals (LUMO) and LUMO + 1, the excited states of this ring molecule, interact with the eigenfunctions associated with the ground state (occupied states), and this interaction stabilizes the less symmetric polyynic form of cyclo[18]carbon with D_9h symmetry, instead of the cumulenic form. The frontier state interactions are responsible for the distortions in the symmetry in the electronic structures, lowering the energy and making the polyynic form the stable one with alternating triple and single bonds.