RAFT Copolymerization of Pt(II) Pincer Complexes With Water-Soluble Polymer as an Efficient Way to Obtain Micellar-Type Nanoparticles With Aggregation-Induced NIR Emission

Abstract

The idea of preparation a water-soluble Pt-containing AIEgen was successfully realized by direct reversible addition-fragmentation transfer copolymerization of a Pt(II) complex (LPtPV) containing a vinyl group and polyvinylpyrrolidone (p(VP)). The resulting block-copolymer p(VP-b-LPtPV) containing 5–8 Pt(II) chromophores exhibits intriguing photophysical properties—strong solvent and concentration dependence of absorption and emission characteristics. Various physicochemical and analytical methods (NMR spectroscopy, XRD analysis, ESI-MS, AUC, DLS, ICP-OES, GPC, viscometry, TEM) were used to characterize the initial complex, its binuclear analogs, p(VP) and p(VP-b-LPtPV). The obtained data indicate that the photophysical properties of the latter are dictated by the type of aggregation process rather than solvatochromic effects. It is shown that at low concentration in organic solvents, the platinum chromophores aggregation is either absent (dimethylformamide) or occurs predominantly at intramolecular level (MeCN), whereas in aqueous media, p(VP-b-LPtPV) readily aggregates into micellar-type nanoparticles with a hydrophilic p(VP) corona and a hydrophobic Pt-containing core, in which strong intra- and intermolecular Pt···Pt and/or π···π interactions result in a significant red shift of absorption and emission down to 600 and 816 nm, respectively. Despite of emission shift into NIR area where emission is commonly quenched by nonradiative vibrational relaxation, an increase in the emission quantum yield occurs in complete agreement with the typical aggregation-induced emission (AIE) emitters’ behavior. Quantum mechanics/molecular mechanics simulations of aggregation processes also confirm the trends in the relationship between aggregation mode and photophysical behavior, particularly, in the variations of energy gaps between the ground state of the AIEgens and their excited singlet and triplet states.