Theoretical Insights into the Phosphorescence Quantum Yields of Cyclometalated (C<sup>∧</sup>C*) Platinum(II) NHC Complexes: π‑Conjugation Controls the Radiative and Nonradiative Decay Processes

In this article, the radiative and nonradiative decay processes of four cyclometalated (C<sup>∧</sup>C*) platinum­(II) N-heterocyclic carbene (NHC) complexes were unveiled via density functional theory and time-dependent density functional theory. In order to explore the influence of π-conjugation on quantum yields of (NHC)­Pt­(acac) (NHCN-heterocyclic carbene, acac = acetylacetonate) complexes, the factors that determine the radiative process, including singlet–triplet splitting energies, transition dipole moments, and spin–orbit coupling (SOC) matrix elements between the lowest triplet states and singlet excited states were calculated. In addition, the SOC matrix elements between the lowest triplet state and the ground state as well as Huang–Rhys factors were also computed to describe the temperature-independent nonradiative decay processes. Also, the triplet potential energy surfaces were investigated to elucidate the temperature-dependent nonradiative decay processes. The results indicate that complex <b>Pt-1</b> has higher radiative decay rate than complexes <b>Pt-2–4</b> due to the larger SOC matrix elements between the lowest triplet states and singlet excited states. However, complexes <b>Pt-2–4</b> have smaller Huang–Rhys factors, smaller SOC matrix elements between the lowest triplet and the ground states, and higher active energy barriers than complex <b>Pt-1</b>, indicating that complexes <b>Pt-2–4</b> have smaller nonradiative decay rate constants. According to these results, one may discern why complex <b>Pt-2</b> has higher phosphorescence quantum efficiency than complex <b>Pt-1</b>; meanwhile, it can be inferred that the nonradiative decay process plays an important role in the whole photodeactivation process. In addition, on the basis of complex <b>Pt-2</b>, <b>Pt-5</b> was designed to investigate the influence of substitution group on the photodeactivation process of rigid (NHC)­Pt­(acac) complex.