A cationic organoiridium(iii) complex-based AIEgen for selective light-up detection of rRNA and nucleolar staining

A green emissive cationic organoiridium(iii) complex, 2[PF6], with a benzimidazole-substituted 1,2,3-triazole-pyridine (BiPT) ligand has been synthesized for target-specific cellular imaging and selective detection of ribosomal RNA (rRNA) over other competitive biomolecules in aqueous buffer solution at physiological pH. Complex 2 shows aggregation-induced emission enhancement (AIEE) properties and forms nano-aggregates in the presence of poor solvents. DFT and TD-DFT-based quantum mechanical calculations were performed to substantiate some photophysical features and to establish the intermolecular π-π interactions which detain the vibrational as well as rotational motions to form the aggregates, resulting in enhanced photoluminescence (PL). To corroborate the formation of nano-aggregates and to understand the morphology of the aggregated particles, dynamic light scattering (DLS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM) measurements were performed. 2[PF6] showed low cytotoxicity and good biocompatibility and was successfully employed in organelle-specific intracellular imaging. The in vivo and in vitro photoluminescence investigations affirmed that the probe stains cell nucleoli and selectively binds rRNA. It is assumed that the supramolecular π-π interactions between the benzimidazole of the BiPT ligand and the secondary structures of rRNA may facilitate aggregation and enable PL enhancement.