The emission properties of a series of flavin (FL) decorated Ru (II) polyimine complexes were investigated by extensive time-dependent (TD) density functional theory (DFT) and DFT based calculations. We attributed the moderate emission properties of FL decorated Ru(II) polyimine complex (Ru-1), such as triplet lifetime and luminescence quantum yield, to the dominant fast nonradiative decay due to the small adiabatic energy gap between the ground state and the lowest lying triplet state (Δ Ead) and the slow radiative decay owing to the ligand localized triplet (3IL) nature of the emissive state. Electron withdrawing groups such as F and Cl were attached to the FL moiety of Ru-1 to alter Δ Ead. Both the radiative and nonradiative decay rates were found to be sensitive to Δ Ead and may result in a drastic change of the photophysical properties of the Ru(II) complexes. Specifically, substitution with F leads to an increase in the Δ Ead from 1.85 to 1.93 eV, resulting in a nearly doubled phosphorescent quantum yield and triplet lifetime with respect to Ru-1. These findings are vital for the rational design of phosphorescent transition metal complexes.