We have recently performed high field ALC measurements on the polycrystalline Metal Tris(Hydroxy-quinolate), MQ3, series in order to further investigate the intrinsic electron spin re- laxation (eSR) in organic semiconductors, where the M is Al, Ga In and Bi. We observe a linear dependency to the eSR. To probe whether the results are unique to the MQ3 compounds and investigate whether the hyperfine or spin orbit (SO) interactions are responsible, we have also studied a series of triethylsylil functionalised molecules, which indicate that the mechanism of eSR in many molecules is SO based. To investigate SO effects further and clarify the generality of the linear dependence, we wish to compare the effect of increasing the mass on the quinolate ligand, rather than the central ion, by substituting two of the hydrogens in Alq3 with bromine, chlorine and iodine.