Aggregation-Induced Emission-Active Ruthenium(II) Complex of 4,7-Dichloro Phenanthroline for Selective Luminescent Detection and Ribosomal RNA Imaging

The development of red emissive aggregation-induced emission (ME) active probes for organelle-specific imaging is of great importance. Construction of metal complex-based AIE-active materials with metal-to-ligand charge transfer (MLCT), ligand-to-metal charge transfer (LMCT) emission together with the ligand-centered and intraligand (LC/ILCT) emission is a challenging task. We developed a red emissive ruthenium(II) complex, 1[PF6](2), and its perchlorate analogues of the 4,7-dichloro phenanthroline ligand. 1[PF6](2) has been characterized by spectroscopic and single-crystal Xray diffraction. Complex 1 showed ME enhancement in water, highly dense polyethylene glycol media, and also in the solid state. The possible reason behind the ME property may be the weak supramolecular pi center dot center dot center dot pi, C-H center dot center dot center dot pi, and C-Cl center dot center dot center dot H interactions between neighboring phen ligands as well as C-Cl center dot center dot center dot O halogen bonding (XB). The crystal structures of the two perchlorate analogues revealed C-Cl center dot center dot center dot O distances shorter than the sum of the van der Waals radii, which confirmed the XB interaction. The ME property was supported by scanning electron microscopy, transmission electron microscopy, dynamic light scattering, and atomic force microscopy studies. Most importantly, the probe was found to be low cytotoxicity and to efficiently permeate the cell membrane. The cell-imaging experiments revealed rapid staining of the nucleolus in HeLa cells via the interaction with nucleolar ribosomal ribonucleic acid (rRNA). It is expected that the supramolecular interactions as well as C-Cl center dot center dot center dot O XB interaction with rRNA is the origin of aggregation and possible photoluminescence enhancement. To the best of our knowledge, this is the first report of red emissive ruthenium(II) complex-based probes with ME characteristics for selective rRNA detection and nucleolar imaging.