We show that the emission efficiency of blue-green phosphorescent emitters can be controlled through coupling of the excited state to vibrational modes. We controlled this vibrational coupling through choice of different ligands and as a result, complexes with CF₃-groups on the ancillary ligand were essentially non-emissive (Φ_PL < 1%), whereas with isosteric CH₃-groups the complexes were strongly emissive (Φ_PL > 50%). Emission of the complexes can be drastically improved (30 times higher Φ_PL compared to degassed solution for the CF₃-containing complexes) by blending them with an inert solid host such as PMMA, which mitigates metal-ligand vibrations. Solution-processed organic light-emitting diodes made from these materials showed efficiency as high as 6.3%.