Synthesis, Characterization, and Photophysical Properties of Water-Soluble AIE Chromophores Based on Alkynyl-Styrene Pt(II) Pincer Complexes Conjugated with Polyvinylpyrrolidone Using RAFT Polymerization

A series of five Pt(II) complexes Pt1-Pt5 containing (N∧N∧C) pincer ligands and alkynyl-styrene were synthesized and characterized using mass spectrometry, NMR spectroscopy, and X-ray crystallography. The complexes exhibit phosphorescence in dilute dichloromethane solution from the excited state of mixed LLCT and MLCT character, which for all but one complex may be ascribed to emissions from isolated chromophore molecules. In contrast, in the solid state, emission occurs from the aggregated chromophores and displays a strong bathochromic shift into the NIR area, which is characteristic for aggregation-induced emission (AIE). The complexes readily take part in the RAFT polymerization reaction with polyvinylpyrrolidone to yield water-soluble amphiphilic block copolymers containing 4-6 complex molecules in the hydrophobic block. The copolymers are luminescent and display a solvent-dependent photophysical behavior. In organic solvents, their photophysical characteristics are nearly identical to those observed for the complexes in DCM, which is indicative of the absence of intramolecular aggregation in the polymeric chain. In aqueous media, the copolymer molecules form nanospecies of the micellar type, which exhibit AIE behavior due to intermolecular aggregation of the mononuclear chromophores. DFT and TD DFT calculations yield the emission parameters, which fit well with the suggested models of platinum chromophore behavior in solutions, solid state, and in the chains of block copolymers.