Design and synthesis of d⁸ square-planar metal complexes and their inorganic-organic hybrids : from photophysics to supramolecular assembly and luminescence chemosensing

Abstract

A series of alkynylplatinum(II) terpyridine complexes with sulfonate and methylsulfonyl group has been synthesized and characterized. Particularly, water-soluble anionic alkynylplatinum(II) terpyridine complexes were found to demonstrate a high propensity to assemble in water to offer spherical nanostructures. In addition, their self-assembly behaviors in different solvent compositions have also been investigated. Interestingly, it has been demonstrated that subtle changes in the molecular design of the complexes could result in different assembly pathways. Upon increasing the acetone content in aqueous solutions, these complexes exhibited a single aggregation-deaggregation or an aggregation-deaggregation-aggregation process. The color and luminescence changes have been studied using UV-vis absorption and emission spectroscopy and their spectroscopic changes were also correlated to the morphological transformation from spherical nano-aggregates to nanorods, as evidenced by transmission electron microscopy (TEM) and confocal fluorescence microscopy. On the other hand, side-by-side assembly of gold nanorods (GNRs) has been demonstrated to be directed by the supramolecular scaffolds formed by sulfonate-containing alkynylplatinum(II) terpyridine complexes, in which the sulfonate groups on the terpyridine ligand at the peripheral position were preferentially bound to the sides of the GNRs. Interestingly, the extent of the assembly of GNRs into ladder rung-like nanostructures was shown to be modulated by the concentration of the platinum(II) complex. The Pt···Pt interaction assisted formation of supramolecular scaffolds and its directed side-by-side assembly of GNRs have been characterized by UV-vis absorption spectroscopy, computational modeling, electron microscopy, energy-dispersive X-ray (EDX) analysis and surface-enhanced Raman scattering (SERS). An alkynylplatinum(II) terpyridine complex functionalized with dipicolylamine moiety has been synthesized and characterized. This complex was employed as a dual-selective probe for the detection of cations and anions. Such complex was shown to exhibit a strong binding affinity towards Zn2+, whereas its zinc-bound adduct was found to exhibit substrate-induced aggregation as a result of the addition of pyrophosphate (PPi). As evidenced by molecular modeling and various spectroscopic and spectrometric studies, a PPi anion was found to be capable of bridging two zinc-bound platinum(II) complex molecules in a clip-shaped fashion, which was further oligomerized through intermolecular Pt···Pt and π−π stacking interactions. Apart from the study of the precious metal complexes of platinum(II), a series of cyclometalating tridentate and tetradentate ligand-containing complexes of earth-abundant nickel(II) has been designed and synthesized. Among them, the carbazolylnickel(II) complex demonstrated an orange color room-temperature luminescence. Such a complex was also found to exhibit intense luminescence with excited state lifetimes in the submicrosecond regime at 77 K, suggesting the triplet nature of the emissive state. Meanwhile, the self-assembly properties of the tetradentate ligand-containing nickel(II) complex in solution has been investigated. Owing to its nearly perfect square-planar geometry, it was found to exhibit self-assembly properties with the aid of π−π interactions and possibly weak Ni···Ni interactions, which have been supported by density functional theory (DFT) calculations and non-covalent interaction (NCI) plot. Indeed, the ground-state aggregation behavior of this complex has been confirmed by concentration-dependent UV-vis absorption spectroscopy. Moreover, in the solution-induced aggregation studies, upon the addition of non-solubilizing solvents, the emergence of low-energy absorption bands has been realized and such a complex was found to demonstrate intriguing self-assembly behaviors to offer well-defined and highly ordered supramolecular architectures.