DFT/TDDFT insight into the impact of ring size of the NHC chelating unit of high effective phosphorescent Platinum (II) complexes

摘要

Uncovering the photodeactivation mechanisms of unique N-heterocyclic carbene (NHC)-based transition metal complexes is favorable for designing more high-efficiency phosphorescent materials. In this work, four bidentate platinum (II) complexes with NHC-chelate are investigated by the density functional theory (DFT) and time-dependent density functional theory (TDDFT) to probe into how the ring size of NHC-chelate unit influences on electronic structures and the phosphorescent properties. To illustrate the photodeactivation mechanisms clearly, three significant photodeactivation processes (radiative decay process, temperature-independent and temperature-dependent nonradiative decay processes) were taken into consideration. We stated that radiative decay rate constants k(r) slightly increased with declined number of NHC-chelate ring, owing to the gradually larger SOC matrix elements between the T-1 state and S-i, states. Combining the temperature-independent with temperature-dependent nonradiative decay processes, the nonradiative decay rate k(nr) is Pt-4 (five-membered) Pt-3 (six-membered) Pt-2 (seven-membered) Pt-1 (eight-membered). The calculated results testify that the decrease of size of the NHC chelating unit is a reliable insurance to improve the quantum yield. The designed complex Pt-4 with five-membered NHC-ring can serve as a highly efficient phosphorescent material in the future. The results indicated controlling the ring size of NHC-chelate is a feasible method to tune phosphorescence properties of Pt (II) complexes.