Given its central role in photosynthesis(1-4) and artificial energy-harvesting devices(5-7), energy transfer has been widely studied using optical spectroscopy to monitor excitation dynamics and probe the molecular-level control of energy transfer between coupled molecules(2-4). However, the spatial resolution of conventional optical spectroscopy is limited to a few hundred nanometres and thus cannot reveal the nanoscale spatial features associated with such processes. In contrast, scanning tunnelling luminescence spectroscopy(8-19) has revealed the energy dynamics associated with phenomena ranging from single-molecule electroluminescence(11,12,14,17,19), absorption of localized plasmons(19) and quantum interference effects(19-21) to energy delocalization(17) and intervalley electron scattering(15) with submolecular spatial resolution in real space. Here we apply this technique to individual molecular dimers that comprise a magnesium phthalocyanine and a free-base phthalocyanine (MgPc and H2Pc) and find that locally exciting MgPc with the tunnelling current of the scanning tunnelling microscope generates a luminescence signal from a nearby H2Pc molecule as a result of resonance energy transfer from the former to the latter. A reciprocating resonance energy transfer is observed when exciting the second singlet state (S-2) of H2Pc, which results in energy transfer to the first singlet state (S-1) of MgPc and final funnelling to the S-1 state of H2Pc. We also show that tautomerization(22) of H2Pc changes the energy transfer characteristics within the dimer system, which essentially makes H2Pc a single-molecule energy transfer valve device that manifests itself by blinking resonance energy transfer behaviour.