  Tip-enhanced photoluminescence (TEPL) measurements are performed with
sub-nanometer spatial resolution on individual molecules decoupled from a
metallic substrate by a thin NaCl layer. TEPL spectra reveal progressive
fluorescence quenching with decreasing tip-molecule distance when electrons
tunneling from the tip of a scanning tunneling microscope are injected at
resonance with the molecular states. Rate equations based on a many-body model
reveal that the luminescence quenching is due to a progressive population
inversion between the ground neutral (S$_0$) and the ground charge ($D_0^-$)
states of the molecule occurring when the current is raised. We demonstrate
that both the bias voltage and the atomic-scale lateral position of the tip can
be used to gate the molecular emission. Our approach can in principle be
applied to any molecular system, providing unprecedented control over the
fluorescence of a single molecule.
