Design and synthesis of platinum(II) complexes with multidentate n-donor ligands : from photophysics to the control of supramolecular self-assembly properties

Abstract

Square-planar platinum(II) complexes represent ideal candidates for the formation of nanostructures with potential applications due to their intriguing spectroscopic properties and propensity to exhibit metalmetal interactions. Despite these attractive features, the construction of higher-ordered morphologies remains challenging due to the need for a rational molecular design strategy, a careful control of the microenvironment and a delicate balance of non-covalent interactions. With the high versatility of this class of complexes, several series of platinum(II) complexes with multidentate N-donor ligands have been synthesized and characterized. Their photophysical and spectroscopic properties as a result of the self-organization processes have been explored. Through a judicious choice of counter-anions and rational design, some of the triethylene glycol-containing alkynylplatinum(II) terpyridine complexes were found to be soluble in aqueous media and would give rise to fibrous aggregates at high concentrations. The amphiphilicity of the complexes has been fine-tuned through changes in counter-anions and the incorporation of alkyl chains of various lengths. An optimization in the alkyl chain lengths on the terpyridine moiety has been achieved for the formation of ordered and stable twisted fibrous structures. In addition, a series of triazine-containing alkynylplatinum(II) terpyridine complexes has been further designed to construct higher-ordered nanostructures such as helices and tubes. In sharp contrast to the more common strategies such as the incorporation of chiral centers or the use of chiral additives, the bent molecular structures and the directional Pt⸱⸱⸱Pt interactions have been exploited for the formation of helices or tubes. The formation of sophisticated helical ribbons from these alkynylplatinum(II) terpyridine amphiphiles in mixed-solvent compositions through a delicate balance of multiple non-covalent interactions would provide alternative routes to the construction of these higher-ordered supramolecular architectures.

Apart from the alkynylplatinum(II) terpyridine complexes, a series of dicyanoplatinum(II) complexes with L-amino acid-containing 2,2'-bipyridine ligands has been synthesized, which combined the self-assembly properties of amino acid units and the intriguing spectroscopic properties of the platinum(II) complexes. Not only did the incorporation of the L-valine moieties lead to the formation of chiral aggregates at low temperatures, but also gave rise to interesting circular dichroism and circularly polarized luminescence properties detectable by various spectroscopic techniques. Further modifications of the hydrophobic motifs appended to the L-valine was found to result in a morphological transformation from disordered aggregates to well-ordered rod-like structures, suggesting the high tunability of the supramolecular nanostructures through simple changes in the molecular design.

The self-assembly properties of a series of dicyanoplatinum(II) 2-(2-pyridyl)benzimidazole complexes have been investigated upon changes in concentration, solvent compositions and the addition of silver(I) ions. In particular, the dicyanoplatinum(II) complex bearing the second generation poly(benzyl) ether moiety was found to give vesicular structures in mixed solvent compositions due to the optimal generation and suitable surface groups at the periphery. Dramatic color and spectral changes have also been observed and detected for the complex upon the addition of silver(I) ions. The interesting spectroscopic changes and self-assembly behaviors of the complexes in response to various stimuli would provide further insights into the engineering of multi-addressable transition metal complexes for potential applications.