Synthesis, photophysics and reverse saturable absorption of bipyridyl platinum(ii) bis(arylfluorenylacetylide) complexes

Abstract

A series of Pt(II) bipyridyl complexes with different aryl substituents (Ar = naphthyl (1a), anthryl (1b), pyrenyl (1c) and phenothiazyl (1d)) on the fluorenylacetylide ligands are synthesized and investigated. The influence of the aryl substituent on the photophysics of these complexes is systematically investigated by spectroscopic methods and simulated by time dependent density functional theory (TD-DFT). All complexes exhibit ligand-centered ¹π,π* transitions significantly admixed with the metal-to-ligand charge transfer (¹MLCT)/ligand-to-ligand charge transfer (¹LLCT) transitions in the UV and blue spectral region, and broad, structureless ¹MLCT/¹LLCT absorption bands in the visible spectral region. All complexes are emissive in solution at room temperature, with the fluorescence originating predominantly from the ¹MLCT/¹LLCT states. The triplet emitting state is dominated by the ³π,π* state localized on the fluorenylacetylide motif and mixed with some ³ML′CT character (metal-to-fluorenylacetylide ligand charge transfer) for 1a, 1b and 1d. For 1c, the phosphorescence predominantly originates from the pyrene localized ³π,π* state. The variation in the photophysical properties is related to the twisting angle of the aryl substituent from the fluorenyl plane, which defines the conjugation between the substituents and fluorenylacetylide ligands and, consequently, alters the energy and intensity of absorption and emission in these complexes. 1a–1d also exhibit broadband triplet excited-state absorption in the visible spectral region. Therefore, they show strong reverse saturable absorption at 532 nm for nanosecond laser pulses as demonstrated by the nonlinear transmission experiment.