Organisms must adapt to survive, necessitating regulation of molecular and subcellular processes. Green plant photosynthesis responds to potentially damaging light levels by downregulating the fraction of excitation energy that drives electron transfer. Achieving adaptive, self-regulating behaviour in synthetic molecules is a critical challenge that must be met if the promises of nanotechnology are to be realized(1). Here we report a molecular pentad consisting of two light-gathering antennas, a porphyrin electron donor, a fullerene electron acceptor and a photochromic control moiety. At low white-light levels, the molecule undergoes photoinduced electron transfer with a quantum yield of 82%. As the light intensity increases, photoisomerization of the photochrome leads to quenching of the porphyrin excited state, reducing the quantum yield to as low as 27%. This self-regulating molecule modifies its function according to the level of environmental light, mimicking the non-photochemical quenching mechanism(2-8) for photoprotection found in plants.