Reported here is a molecular dipole that self-assembles into highly ordered patterns at the liquid-solid interface, and it can be switched at room temperature between a bright and a dark state at the single-molecule level. Using a scanning tunneling microscope (STM) under suitable bias conditions, binary information can be written at a density of up to 41 Tb cm⁻² (256 Tb/in²). The written information is stable during reading at room temperature, but it can also be erased at will, instantly, by proper choice of tunneling conditions. DFT calculations indicate that the contrast and switching mechanism originate from the stacking sequence of the molecular dipole, which is reoriented by the electric field between the tip and substrate.