Ligand–metal interactions and the Huang-Rhys factor modulating the photophysical properties of tetradentate square-planar Pt(II) complexes

Abstract

By alternating the heterocycle fragments (Het) of tetradentate ligand in square-planar Pt(II) complexes, the intramolecular ligand–metal interactions and electron-vibration coupling of phosphorescent emission are significantly regulated. Back-donation is identified as the greatest source of stability in these complexes, significantly reinforcing the metal–ligand bonds. The energy of back-donation interactions, ${\text{d}}_{\text{Pt}}\to {\pi }_{\text{Het}}^{\ast }$ and ${\text{d}}_{\text{Pt}}\to {\pi }_{\text{Ph}}^{\ast }$, exhibits an inverse linear relationship with the energy level of the lowest unoccupied molecular orbital (LUMO), while the donation energy of ${\pi }_{\text{Cz}}\to {\text{d}}_{\text{Pt}}^{\ast }$ shows an inverse linear correlation with the highest occupied molecular orbital (HOMO) energy level. Additionally, back-donation amplifies the term $2Re\left(C_1^{\ast }{C}_2〈{\psi }_m|\hat{L}|{\psi }_l〉\right)$ in angular momentum, enhancing spin–orbit coupling (SOC) in the $T_1\to S_0$ transition and accelerating the radiative decay rate ($k_r$), with Pt-9 exhibiting the highest $k_r$ due to its strong back-donation. Additionally, the back-donation ${\text{d}}_{\text{Pt}}\to {\pi }_{\text{Het}}^{\ast }$ suppresses high-frequency stretching vibrations ($\text{> 1300}{\text{cm}}^{\text{-1}}$) involving the Het fragment and promotes low-frequency (<$\text{200}{\text{cm}}^{\text{-1}}$) bending vibrations of the core backbone. Consequently, the reorganization energy (${\lambda }_{\mathrm{re}}$) is reduced, while the Huang–Rhys factor ($S$) at low frequencies is greatly enlarged. All maximum S values are less than one for the nine complexes, indicating that the Franck–Condon factors ($\mathrm{FC}$) are effectively enhanced due to favorable electron–vibrational coupling. These findings underscore the critical role of intramolecular interactions in tuning photophysical properties, offering valuable insights for the rational design of high-performance phosphorescent materials and advanced optoelectronic devices.