A newly synthesized light emitting compound 9,10-dimethylsulfone-7,12-diphenylbenzo[k]fluoranthene (DSDPBF) and its synthetic intermediate 9, 10-dimethyl-7,12-diphenylbenzo[k] fluoranthene (DMDPBF) were studied to evaluate how the addition of weak electron donating methyl groups and the subsequent addition of an electron withdrawing sulfone group affect the photophysical and electrochemical properties as well as the rate of radical cation coupling of the parent compound, 7,12-diphenylbenzo[k]fluoranthene (DPBF). Although the photochemical and electrochemical properties of DSDPBF were more similar to the unsubstituted DPBF than to the DMDPBF, there was a substantial decrease in the quantum efficiency upon addition of the electron-rich sulfone group which was not observed upon addition of the methyl groups. On the other hand, the rate of radical cation coupling or dimerization observed upon electrochemical oxidation varied significantly. The addition of the electron donating methyl groups decreased the reactivity of the radical cation resulting in a 40 times slower rate of dimerization than that observed for the unsubstituted benzo[k]fluoranthene, whereas the addition of the electron withdrawing sulfone group to the methyl groups increased the radical cation reactivity resulting in a rate of dimerization that was 3 times faster than the unsubstituted parent compound. As a result, the electrogenerated chemiluininescence emission spectrum obtained from the annihilation reaction between the radical anion and radical cations of DSDPBF was dominated by emission from the dimer at 589 and 621 nm instead of emission from the monomer at ca. 485 nm.