Periodic Reporting for period 1 - CATNIp (Interplay of charge and energy transfer in single molecules)
Reporting period: 2023-04-01 to 2025-03-31
Molecular architectures are becoming increasingly important, for example to improve cost and energy efficiency of optoelectronic devices or to miniaturize information storage. In contrast to “classical” semiconductor devices, the properties and functionality of molecular devices are governed by the quantum properties of their nanoscopic building blocks, namely the single molecules, but also by the interactions between them. Processes such as energy conversion depend heavily on the nanoscale interactions between individual charges and the optically excited states of molecules. Investigating such things therefore requires tools that can control and probe both the charge state and the excited state of individual or interacting molecules with submolecular precision. On their own, optical spectroscopy techniques cannot meet these criteria, but in combination with high-resolution scanning probe microscopy, both submolecular spatial resolution and charge state control down to the single electron can be achieved. In this context, CATNIp has focused on understanding the interplay between single charges and optically excited states in individual and coupled molecules. To this end, the unique combination of atomically precise scanning probe methods and optical spectroscopy was used to study and control the intricate interplay of charge transfer and excited state dynamics with near-atomic precision.
In the course of the project, we have refined a unique experimental setup that allows optical excitation of individual molecules in the tunnel junction and energy- and time-resolved detection of optical signals from these molecules. We have taken the first steps towards implementing an additional scanning probe technique, atomic force microscopy, which allows the study of chromophores on fully insulating substrates and thus provides the possibility to stably localize individual charges within single molecules and molecular assemblies. In addition to the experimental advances, the project succeeded in probing the quantum nature of correlated photon emission cascades from a single molecule and the underlying charge and excited state dynamics, as well as several crucial aspects of light-matter interaction at the single molecule scale. We have studied the position and energy dependence of photoexcitation and radiative decay in coupled molecular systems with submolecular resolution. The results show that the spatial patterns of tip-enhanced photoluminescence of the coupled molecules are governed by a complex interplay of excitation and decay probabilities.
We shed light on charge and excited state dynamics in single molecules with unparalleled accuracy. The results of CATNIp show, for example, the first observation of a photon emission cascade from a single molecule and demonstrate that we can control the underlying dynamics with an unprecedented degree of control. These insights in model systems such as single molecules are essential for the development of efficient energy conversion schemes.