Design, synthesis, photophysics and self-assembly study of platinum(II) 2,6-bis(benzimidazol-2'-yl)pyridine complexes with conjugated polyelectrolytes and their utilization as luminescent probes for molecules of biological interest

Abstract

Several series of cationic platinum(II) 2,6-bis(benzimidazol-2′-yl)pyridine (bzimpy) complexes have been synthesized and characterized. Water-soluble conjugated polyelectrolytes (CPEs), PPE-SO3– and mPPE-Ala, have also been synthesized and characterized. The photophysical and electrochemical properties of all the complexes and CPEs have been studied. Some complexes have been demonstrated to show ground state aggregation in aqueous solutions at high concentrations. The induced self-assembly studies of [Pt(bzimpy-Et){C≡CC6H4(CH2NMe3-4)}]Cl2 and [Pt(bzimpy-Et)(C≡C–CH2NMe3)](OTf)2 by poly(sodium p-styrene sulfonate) (PSS) have been reported. The photophysical, self-assembly and Fӧrster resonance energy transfer (FRET) properties of [Pt(bzimpy-Et){C≡CC6H4(CH2NMe3-4)}]Cl2 and [Pt(bzimpy-Et)(C≡CCH2NMe3)](OTf)2 with the two CPEs in aqueous media have been investigated. The photophysical properties of the two-component ensemble formed by [Pt(bzimpy-Et){C≡CC6H4(CH2NMe3-4)}]Cl2 and PPE-SO3– have been found to be significantly different from that of the ensemble formed by [Pt(bzimpy-Et){C≡CC6H4(CH2NMe3-4)}]Cl2 and mPPE-Ala.

The formation of supramolecular assemblies between [Pt(bzimpy-Et){C≡CC6H4(CH2NMe3-4)}]Cl2 and mPPE-Ala and their FRET properties in aqueous buffer solution have been examined. The ensemble has been employed in a “proof-of-principle” concept for label-free detection of G-quadruplex DNAs with the intramolecular propeller parallel folding topology, such as c-myc, in aqueous buffer solution. By the modulation of the aggregation / deaggregation of the polymer–metal complex aggregates and hence the FRET from mPPE-Ala to the aggregated complexes, the ensemble has been demonstrated for sensitive and selective label-free detection of c-myc via the monitoring of emission spectral changes of the ensemble. Ratiometric emission of the ensemble has been shown to distinguish the intramolecular propeller parallel G-quadruplex folding topology of c-myc from other G-quadruplex-forming sequences of different folding topologies. The formation of high-order intermolecular multimeric G-quadruplexes from c-myc and the conformational and topological transition of human telomeric DNA under molecular crowding conditions have been successfully probed by ratiometric emission.

A water-soluble dinuclear alkynylplatinum(II) bzimpy complex has been synthesized and characterized. Water-soluble CPEs, bPPE-SO3–, PFE-SO3–, PFP-OSO3–, PF-CO2– and HBPF-CO2– have also been synthesized and characterized. UV-Vis absorption and emission spectroscopies of all the platinum(II) complexes and CPEs have been performed in organic solvents and / or aqueous buffer solutions. The photophysical properties and the ground state aggregation of the complexes have been examined. The dinuclear alkynylplatinum(II) bzimpy complex has been found to undergo supramolecular self-assembly as well as intramolecular and / or intermolecular aggregation in aqueous solution, leading to low-energy metal–metal-to-ligand charge transfer (MMLCT) absorption and triplet MMLCT (3MMLCT) emission. The extent of induced self-assembly of the dinuclear complex by PSS has also been revealed. Moreover, the photophysical, self-assembly and FRET properties of [Pt(bzimpy-Et){C≡CC6H4(CH2NMe3-4)}]Cl2 and [Pt(bzimpy-Et)(C≡CCH2NMe3)](OTf)2 with the various CPEs under aqueous conditions have been studied. With the modifications of the structures, geometries and architectures of the CPEs, different association constants as well as quenching efficiencies on CPE fluorescence have been obtained. Distinctively different FRET behaviors and emission spectral changes have also been observed. Through the comprehensive comparative study on the different supramolecular architectures of CPEs and the different structural, geometrical and electronic properties of the ensembles, the factors that dictated the FRET efficiency and the resultant spectroscopic changes have been discussed.