Imaging excitons in single molecules
Excitons – bound electron and hole pairs in molecules – are at the heart of numerous ultrafast processes in nature, such as charge and energy transfer in photosynthetic light-harvesting complexes. Experiments in the bulk phase provide an ensemble averaged view of the exciton dynamics, where both the atomic-scale quantum dynamic properties of the excitons and the intermolecular interactions are smeared out. By performing excitonic wavepacket interferometry at the single molecule level, researchers from Max Planck Institute for Solid State Research (MPI-FKF), Università della Calabria and Universidad Autónoma de Madrid have now succeeded in capturing the atomic-scale properties of exciton dynamics in molecules and demonstrated how dark and bright excitons emerge in interacting molecules.
Scanning tunneling microscopy (STM) as a probe for excitons dynamics
The pursuit to track and control optically-induced exciton dynamics in single molecules is a long-standing one, as it is the key to understanding numerous ultrafast processes in nature, e.g., energy conversion and charge transfer in photosynthetic systems. Ultrafast experiments performed in the bulk phase provide an ensemble averaged view of molecular exciton dynamics where several crucial factors influencing their dynamics, such as electronic inhomogeneity within a single molecule and intermolecular interactions are smeared out. The capability to directly monitor molecular exciton dynamics at its intrinsic length (~ Ångström-scale) and timescales (~ femtoseconds) has remained challenging until date.
The unification of scanning tunneling microscopy (STM) with ultrashort light pulses provides a unique avenue to directly visualize ultrafast exciton dynamics in single molecules at the orbital-level. Utilizing such a quantum microscope coupled with a sequence of two ultrashort pulses excitonic wavefunctions were tracked in single complex organic molecules by wavepacket interferometry. The generation of atomically localized photocurrents in single molecules enabled orbital-resolved imaging and investigation of excitonic coherences.
Contrary to spatially unresolved measurements, intermolecular interactions in molecular dimers lead to local coherences involving both bright and dark states, which were selectively excited and probed at the atomic scale by changing the position of the STM nanotip over the molecules. The excitonic coherence times in coupled molecules is state dependent and is shorter compared to the isolated single molecules, highlighting the importance of intermolecular interactions in exciton dynamics, which were hitherto not possible to explore. Furthermore, the investigation of the molecular dark states is often complicated by the need of external electric or magnetic fields to enforce their blending with the bright states, nevertheless, in the atomistic near fields of the nanotip, they can be easily accessed and their dynamics probed.
Orbital-resolved imaging of electronic coherences will help resolve the long-standing debate about the role of quantum coherence in photosynthesis by providing the direct visualization of these fundamental processes. Artificial donor-acceptor systems and molecular chains can be realized in an STM by tip manipulation techniques and space-time resolved orbital imaging as demonstrated in the current work could be utilized to address these questions.
