Single-molecule phosphorescence and intersystem crossing in a coupled exciton-plasmon system

TL;DR

This paper investigates molecular-scale phosphorescence and intersystem crossing using a scanning tunneling microscope, revealing detailed triplet excitation pathways and potential for controlling light emission at the nanoscale.

Contribution

It demonstrates the first detailed analysis of phosphorescence and intersystem crossing at the single-molecule level using tip-enhanced emission spectroscopy.

Findings

Tip-enhanced phosphorescence spectra reveal local variations in emission.

Laser-induced phosphorescence monitors singlet-to-triplet intersystem crossing.

Insights into triplet excitation pathways at sub-molecular resolution.

Abstract

Scanning the sharp metal tip of a scanning tunneling microscope (STM) over a molecule allows tuning the coupling between the tip plasmon and a molecular fluorescence emitter. This allows access to local variations of fluorescence field enhancement and wavelength shifts, which are central parameters for characterizing the plasmon-exciton coupling. Performing the same for phosphorescence with molecular scale resolution remains a significant challenge. In this study, we present the investigation of phosphorescence from isolated Pt-Phthalocyanine molecules by analyzing tip-enhanced emission spectra in both current-induced and laser-induced phosphorescence. The latter directly monitors singlet-to-triplet state intersystem crossing of a molecule below the tip. The study contributes to a detailed understanding of triplet excitation pathways and their potential control at sub-molecular length scales. Additionally, the coupling of organic phosphors to plasmonic structures is a promising route for improving light-emitting diodes.