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 (AIE) 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-meal 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
X-ray diffraction. Complex 1 showed AIE enhancement in
water, highly dense polyethylene glycol media, and also in the solid
state. The possible reason behind the AIE property may be the weak
supramolecular π···π, C–H···π,
and C–Cl···H interactions between neighboring
phen ligands as well as C–Cl···O halogen bonding
(XB). The crystal structures of the two perchlorate analogues revealed
C–Cl···O distances shorter than the sum of the
van der Waals radii, which confirmed the XB interaction. The AIE 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···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 AIE characteristics for selective rRNA detection and nucleolar
imaging.