  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.
