Three 3,5-dichloro-4H-1,2,6-thiadiazines, which differ according to the electron withdrawing nature of their substituent at C-4: (a) 3,5-dichloro-4-methylene-4H-1,2,6-thiadiazine (5a), (b) 3,5-dichloro-4H-1,2,6-thiadiazin-4-one (5b) and (c) 2-(3,5-dichloro-4H-1,2,6-thiadiazin-4-ylidene)malononitrile (5c), are characterized using resonance Raman (RR), absorption (UV/vis) and photoluminescence (PL) spectroscopies. These weakly aromatic and electron-deficient heterocycles are potential components as acceptors in donor–acceptor systems for organic electronics. Experimental results, which include the synthesis, characterization and single crystal X-ray structure of 3,5-dichloro-4-methylene-4H-1,2,6-thiadiazine (5a), combined with theoretical calculations of their orbitals and vibrational frequencies, provide an understanding of the optical properties, on the basis of molecular geometry and electron distribution.