Triangulene leuco derivatives bearing phenylethynyl group at the central carbon atom show a photocleavage reaction affording their cationic form with the quantum yield (Phi(r)) of 0.33-0.67 in ethanol. The photoreaction can be detected not only by the change of absorption and fluorescence due to the generation of the cation, but also by scanning tunneling microscopy (STM) at the single-molecule resolution at the air/Au(111) interface.