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 ( E 0–0 ) and reorganization energy. Frequency reduction suppresses nonradiative decay rates ( k nr ), 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 ( k r ). Selective deuteration of the Cz ring effectively increases k r, suppresses k 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.