Self-Assembly of Luminescent Alkynylplatinum(II) Terpyridyl Complexes: Modulation of Photophysical Properties through Aggregation Behavior

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Abstract

Complexes of platinum(II) with polypyridine (that is, the multidentate ligands related to pyridine, such as bipyridine or terpyridine) have rich photophysical properties. These compounds are able to give different crystal forms in the solid state: this polymorphism is evident in the broad range of colors that can be observed in solid samples. Because of the square-planar coordination geometry of the metal center, Pt···Pt as well as π-π interactions between the chromophoric polypyridyl platinum(II) moieties are thought to contribute to the polymorphism. Owing to limited solubility, metal···metal interactions in platinum(II) polypyridyl systems had been mainly studied in the solid state, but our preparation of more soluble complexes has enabled detailed spectroscopic examinations in solution. In this Account, we describe our development of these alkynylplatinum(II) terpyridyl complexes and their unique spectral properties. A series of square-planar platinum(II) terpyridyl complexes with enhanced solubility due to the presence of the alkynyl group exhibited intense emission in solution. The lowest energy absorption and emission bands are suggested to originate from the dπ(Pt) → π*(terpy) metal-to-ligand charge transfer (MLCT) and π(C≡C-C(6)H(4)-R) → π*(terpy) ligand-to-ligand charge transfer (LLCT) transitions. In addition to polymorphism and a wide range of spectral properties, these complexes also exhibit "solvatochromism" and "solvatoluminescence". They show remarkable color changes and luminescence enhancement when the diethyl ether content in a solvent mixture is varied, even as the concentration of the platinum(II) complex is held constant. The dramatic color changes and luminescence enhancement are tentatively suggested to originate from a metal-metal-to-ligand charge transfer (MMLCT) transition: reduced solvation (caused by an increase in the fraction of diethyl ether, which is the nonsolvating component of the liquid) is thought to increase Pt···Pt and π-π stacking interactions that arise from ground-state self-assembly or aggregate formation. The absorbance and luminescence wavelengths in these solvent-induced self-assemblies are also found to be dependent on the nature of the anions. Thus, counterions play an important role in governing the degree of self-assembly and the extent of interactions within these aggregates. Several polymers carrying multiple negatively charged functional groups (under basic conditions) as well as oligonucleotides have been shown to induce the aggregation and self-assembly of the positively charged water-soluble alkynylplatinum(II) terpyridyl complexes. The driving force for the induced aggregation and self-assembly is electrostatic binding of the complex molecules to the polymer, which brings the cations into a close proximity that induces Pt···Pt and π-π interactions and gives rise to remarkable color changes and luminescence enhancement. The spectral changes are shown to be related to the properties of both the complexes and the polymers. Upon electrostatic interaction, the platinum(II) complex cations are also found to stabilize the polymers and biopolymers in a helical conformation through Pt···Pt and π-π interactions. The influence on their secondary structure is revealed by significant circular dichroism (CD) signal enhancement.

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