In this study, density functional theory (DFT) and time dependent DFT were employed to elucidate the photo-deactivation mechanisms of (C<^>N)Pt(O<^>O) complexes 1-4 (where C<^>N = 2-phenylpyridine derivatives, O<^>O = dipivolylmethanoate). To make thorough understanding of the radiative decay, the singlet triplet splitting energies Delta E(S-n-T-1) (n = 1; 2, 3, 4,...), transition dipole moment mu(S-n) for S-0-S-n transitions and the spin orbit coupling (SOC) matrix elements < T-1 vertical bar H-SOC vertical bar S-n > were all calculated. Moreover, the spin orbit coupling between T-1 and S-0 < T-1 vertical bar H-SOC vertical bar S-0 > and Huang Rhys factors were calculated to estimate the temperature-independent nonradiative decay processes. Meanwhile, the thermal deactivation via metal-centered (MC)-M-3 was described to analyze the temperature dependent nonradiative decay processes. As a result, the effective SOC interaction between the lowest triplet and singlet excited states successfully rationalize why complexes 1 and 3 have higher radiative decay rate constant than that of complex 2, while the larger < T-1 vertical bar H-SOC vertical bar S-0 > and lower energy barrier for thermal deactivation in 3 reasonably explains why 3 has larger nonradiative rate than that of 1 and 2. Consequently, it can be concluded that it is the < T-1 vertical bar H-SOC vertical bar S-0 > and thermal population. of (MC)-M-3 that account for the nonemissive behavior of (C<^>N)Pt(O<^>O) complexes, and controlling pi-conjugation is an efficient method for tuning phosphorescence properties of transition-metal complexes.