Internal Stark effect of single-molecule fluorescence

TL;DR

This study demonstrates the internal Stark effect at the single-molecule level by observing spectral shifts in fluorescence caused by internal charges within a chromophore, using cryogenic STM techniques.

Contribution

It introduces a novel experimental approach to directly measure the internal Stark effect in single molecules through STM-induced fluorescence.

Findings

Spectral shifts correlate with internal electrostatic charges.

Deprotonation induces measurable Stark shifts.

STM can manipulate and probe single-molecule electrostatic environments.

Abstract

The optical properties of chromophores can be efficiently tuned by electrostatic fields generated in their close environment, a phenomenon that plays a central role for the optimization of complex functions within living organisms where it is known as internal Stark effect (ISE). Here, we realised an ISE experiment at the lowest possible scale, by monitoring the Stark shift generated by charges confined within a single chromophore on its emission energy. To this end, a scanning tunneling microscope (STM) functioning at cryogenic temperatures is used to sequentially remove the two central protons of a free-base phthalocyanine chromophore deposited on a NaCl-covered Ag(111) surface. STM-induced fluorescence measurements reveal spectral shifts that are associated to the electrostatic field generated by the internal charges remaining in the chromophores upon deprotonation.