Probing molecular excited states by atomic force microscopy

TL;DR

This study uses atomic force microscopy to probe and quantify excited states of molecules through controlled single-charge injections, providing insights into electron transfer and optoelectronic processes.

Contribution

It demonstrates a novel method to access and measure molecular excited states via AFM-based charge injection and spectroscopy, linking experimental results with theoretical calculations.

Findings

Identification of three neutralization channels corresponding to different excited states

Quantification of triplet and singlet excited state energies

Validation of experimental results with density functional theory calculations

Abstract

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.