The present invention provides a compound, a photoelectric converter and a photoelectrochemical cell. When the control signal is in a first state and the voltage applied to the input is higher than the voltage at the output, the circuit provides a low resistance path between its input and output terminals thereby enabling current to flow from the input to the output. The assembly permits the location of a tumour to be verified in the irradiation position, thus rendering a relocation of the patient unnecessary.