Theoretical insight into the photodeactivation pathway of the tetradentate Pt (II) complex with different inductive substituents

Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) both were used to explore the impacts of different inductive substituents on the photophysical properties, radiative/nonradiative processes and photodeactivation mechanism for the Pt (II) complex with novel spiro-arranged tetradentate ligand. Spectrum simulations show that the electron donor methoxyl (-OCH3) group can cause the emission wavelength to red-shift but have little effect on the absorption spectrum. In the simulation of the radiative decay process for the tetradentate Pt (II) complex, the singlet-triplet splitting energy is reduced by the introduction of substituents with strong electron-releasing capability (i.e., from the original trifluoromethyl (-CF3) group to -OCH3 group), accompanied with a lower radiative rate constant (k(r)). The analyses of non-radiative decay processes show that the substitution of -OCH3 group on azole rings reduces the energy barriers of thermally activated non-radiative photodeactivation pathway, which in turn increases the temperature-dependent non-radiative rate constants (k(nr)(T)). In addition, the substitution of -CF3 by -OCH3 group slightly weakens molecular rigidity and enhances the Huang-Rhys factor, but decreases the SOC between the triplex excited (T) state and the ground (S-0) state. Thereby, the two complexes may have the similar temperature-independent non-radiative rate constant (k(nr)'). This work offers theoretical guidance for the design and optimization of the efficient organic light emitting diode (OLED) materials based on the structure of tetradentate Pt (II) complexes.