Disentangling the components of a multiconfigurational excited state in isolated chromophore with light-scanning-tunneling microscopy.

Molecular radicals are efficient electroluminescent emitters due to the spin multiplicity of their electronic states. The excited states often exhibit a complex composition with multiple significant electronic configurations, which are essential for their optoelectronic properties but difficult to probe directly. Here we use light-scanning tunneling microscopy to investigate such an excited state by visualizing the response of a single radical molecule to a laser excitation. We observe characteristic atomic-scale spatial photocurrent patterns that can be tuned by applied bias voltage. We interpret these patterns as resulting from decay of an excited doublet state through sequential electron transfers with the tip and the substrate. The relative contributions of two dominating electronic configurations involved in this excited state are tuned by the applied voltage. This approach thus allows for disentangling the components of multiconfigurational excited states in single molecules.