Theoretical Insights into the Photo-Deactivation of Emitting Triplet Excited State of (C^N)Pt(O^O) Complexes: Radiative and Nonradiative Decay Processes

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 <b>1</b>–<b>4</b> (where C^N = 2-phenylpyridine derivatives, O^O = dipivolylmethanoate). To make thorough understanding of the radiative decay, the singlet–triplet splitting energies Δ<i>E</i>(S_<i>n</i>–T₁) (<i>n</i> = 1, 2, 3, 4, ...), transition dipole moment μ­(S_<i>n</i>) for S₀–S_<i>n</i> transitions and the spin–orbit coupling (SOC) matrix elements ⟨T₁|H_SOC|S_<i>n</i>⟩ were all calculated. Moreover, the spin–orbit coupling between T₁ and S₀ ⟨T₁|H_SOC|S₀⟩ and Huang–Rhys factors were calculated to estimate the temperature-independent nonradiative decay processes. Meanwhile, the thermal deactivation via metal-centered ³MC 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 <b>1</b> and <b>3</b> have higher radiative decay rate constant than that of complex <b>2</b>, while the larger ⟨T₁|H_SOC|S₀⟩ and lower energy barrier for thermal deactivation in <b>3</b> reasonably explains why <b>3</b> has larger nonradiative rate than that of <b>1</b> and <b>2</b>. Consequently, it can be concluded that it is the ⟨T₁|H_SOC|S₀⟩ and thermal population of ³MC that account for the nonemissive behavior of (C^N)­Pt­(O^O) complexes, and controlling π-conjugation is an efficient method for tuning phosphorescence properties of transition-metal complexes