Selective Deuteration-Enhanced Phosphorescent Performance in Square-Planar Tetradentate Pt(II) Complexes: Unveiling the Role of Vibration Coupling in Electronic Transitions

Deuteration enhances the organic light-emitting diode (OLED) stability and performance. Theoretical calculations on square-planar tetradentate Pt(II) complexes show that deuteration exerts no electronic effects because deuterium has the same number of electrons as hydrogen. Consequently, the equilibrium structures (ground/excited states), frontier molecular orbital (FMO) distributions and energy levels, natural transition orbitals (NTOs), and spin-orbital coupling (SOC) remain unchanged. Deuteration-doubled mass lowers vibrational frequencies, reducing vibrational energy levels and zero-point energy (ZPE) by about 2.08 kcal/mol per deuteration, while slightly affecting the 0-0 transition energy (E0-0) and reorganization energy. Frequency reduction suppresses nonradiative decay rates (knr), boosting photoluminescence quantum yield (PLQY). Analyses of Huang-Rhys factors (S) and Franck-Condon factors (FC) show significant changes in mid-to-high-frequency vibrational coupling of particle and hole. Altered vibrational wave functions enhance the Herzberg-Teller (HT) transition dipole moment, affecting radiative decay rates (kr). Selective deuteration of the Cz ring effectively increases kr, suppresses knr, and modulates the fine structure of the spectrum, comparable to perdeuteration. C-D bond chemical degradation rate is about 3.94 times slower than C-H, further improving stability. These results establish a framework for optimizing emitters and extending OLED operating lifetimes.