Charged optical excitations (trions) generated by charge carrier injection are crucial for emerging optoelectronic technologies as they can be produced and manipulated by electric fields. Trions and neutral excitons can be efficiently induced in single molecules by means of tip-enhanced spectromicroscopic techniques. However, little is known of the exciton-trion dynamics at single molecule level as this requires methods permitting simultaneous subnanometer and subnanosecond characterization. Here, we investigate exciton-trion dynamics by phase fluorometry, combining radio frequency modulated scanning tunnelling luminescence with time-resolved single photon detection. We generate excitons and trions in single Zinc Phthalocyanine (ZnPc) molecules on NaCl/Ag(111), and trace the evolution of the system in the picosecond range. We explore the dependence of effective lifetimes on bias voltage and describe the conversion mechanism from neutral excitons to trions, via charge capture, as the primary pathway to trion formation. We corroborate the dynamics of the system by a causally deterministic four-state model.