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 (<i>E</i>_0-0) and reorganization energy. Frequency reduction suppresses nonradiative decay rates (<i>k</i>_nr), boosting photoluminescence quantum yield (PLQY). Analyses of Huang-Rhys factors (<i>S</i>) 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 (<i>k</i>_r). Selective deuteration of the Cz ring effectively increases <i>k</i>_r, suppresses <i>k</i>_nr, 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.