Synergistic Singlet-Triplet Regulation in Platinum(II)-Acetylide Triads with Strong Two-Photon Absorption and Optical Power Limiting.

Unveiling the intramolecular photophysical transitions and intrinsic excited-state dynamics of organometallic complexes is of critical importance yet remains challenging. Herein, two platinum (II)-acetylide triads, namely Pt-1 bearing the triethylphosphine ligands and Pt-2 with the triphenylphosphine ligands, were rationally synthesized and investigated comprehensively. The NMR shielding phenomena of Pt(II) atoms with dense extranuclear electrons are elucidated properly through comparative studies on a reference analogue, BDT-TPA. The rigid planar molecular geometries endow the triads with extended π-conjugation, which promotes efficient ligand-to-metal charge transfer (LMCT) and ligand-to-ligand charge transfer (LLCT). Furthermore, a significant effect of Pt(II)-acetylide d-π coordination, ligand aromaticity, and steric hindrance on the steady state and nonlinear optical properties are also elucidated comparatively. Benefiting from strong spin-orbit coupling and ligand-mediated conjugation, both Pt(II)-acetylide triads exhibit enhanced intersystem crossing rates and prolonged triplet-state lifetimes. Notably, Pt-2 displays a maximum two-photon absorption cross-section of 1830 GM in THF upon femtosecond excitation at 650 nm, which is obviously higher than that of 980 GM at 775 nm observed for BDT-TPA. High-performance optical power limiting assessments based on the two-photon absorption mechanism reveal that both triads possess low onset thresholds around 0.06 J·cm-2 and favorable limiting thresholds around 0.25 J·cm-2. Theoretical calculations further correlate the superior nonlinear optical performance with efficient excited-state absorption as well as prominent LMCT characteristics. This study not only affords mechanistic insights into the intrinsic photophysics of the Pt(II)-acetylide complexes but also sheds light on the potential nonlinear optical applications.