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-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[PF], and its perchlorate analogues of the 4,7-dichloro phenanthroline ligand. 1[PF] 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.

A Ru­(II) complex (Ru-1) of a substituted pyridyl-1,2,3-triazole ligand (BtPT) for highly selective “light-up” detection of hypochlorous acid is presented. An unusual anti-Markovnikov HOCl addition to the CC bond of 1,2,3-triazole and a highly specific C­(sp2)–H hydroxylation over epoxidation made Ru-1 a highly selective luminescent HOCl probe. The abnormal regio- and stereoselective HOCl addition and subsequent hydroxylation mechanism in detail is supported by the combination of ESI-MS, 1H/13C NMR spectroscopy, and 1H NMR titration. The hydroxylation at the C5 center in 1,2,3-triazole increases the electron density and makes BtPT a better σ-donor as well as π-donor, which in turn increases the 3MC–3MLCT energy gap and inhibits the nonradiative decay from the excited state of Ru-1 and is the key reason for luminescence light-up. Most importantly, the exogenous and endogenous HOCl imaging in the living HEK293T cells is also demonstrated. The probe showed low cytotoxicity and efficiently permeated the cell membrane. The cell-imaging experiments revealed rapid staining of the extranuclear region of HEK293T cells which clearly indicates the presence of cytoplasmic HOCl. The endogenous HOCl generation and imaging, stimulated by lipopolysaccharides (LPS) and paraquat in the HEK293T cells, is also demonstrated.

A 4-methylbenzothiazole linked maleimide-based single molecular bifunctional probe 1 has been synthesized for the colorimetric and fluorometric detection of highly competitive HS and cyanide ion in aqueous DMSO media. The probe 1 selectively detected CN under the UV-vis spectroscopy through the rapid appearance of deep pink color. The bright pink color developed due to ICT in the moderately stable cyano substituted enolate intermediate. The absorbance titration of 1 with CN revealed a new band at 540 nm and the nonlinear curve fitting analysis showed good fit with 1:1 model. In fluorescence channel, 1 was found to be highly selective to HS in 50% aqueous buffer (pH 7). It exhibited ∼16-fold fluorescence intensity enhancement at 435 nm after reaction with 1 equiv of HS due to the inhibition of PET. The 1-SH adduct showed TICT phenomenon and behaved like molecular rotor. It further displayed aggregation behavior at higher concentration and excitation wavelength dependent multicolor emission properties. Most interestingly, the spontaneous resolution of chiral S-isomer of the 1-SH adduct occurred during crystallization. The cell imaging study revealed the staining of the cell and multicolor emission in the presence of HS.

A bis-heteroleptic Ru(II) complex, 1[PF 6 ] 2 , of substituted pyridyl-1,2,3-triazole ligand (L1) has been developed for the photoluminescence(PL) detection of phosphate (HP 2 O 7 3À /PPi) and Ag + ion in distilled CH 3 CN and mixed aqueous buffer (carbonate-bicarbonate buffer/CH 3 CN; v/v, 9.5 : 0.5; 0.1 M; pH 10.6) at two different wavelengths, respectively. The multianalyte probe, 1[PF 6 ] 2 showed significant PL enhancement in the presence of H 2 PO 4 À , PPi, and Ag + with low detection limits of 0.48 μM, 0.43 μM, and 0.46 μM, respectively. The PL enhancement with phosphate ions is due to the triazole CÀ H•••anion H-bonding interaction that has been supported by 1 H NMR titration (for PPi) and single-crystal X-ray structure (for H 2 PO 4 À ). A unique Ag(I)-triazolide complex formation is responsible for the PL enhancement in Ag + ion detection. Triplet state TDDFT calculations were carried out to explain the PL enhancement of 1 • PPi and 1-Ag adducts. In 1-Ag, the L1 ligand becomes a better σ-donor as well as π-donor. Consequently, the 3 MLCT-3 MC energy gap is increased and the 1-Ag emits efficiently from the 3 MLCT state. The probe also shows low cytotoxicity against Hela cells and is used for intracellular Ag + imaging.

A bis-heteroleptic ruthenium­(II) complex, Ru-1, of 4,7-bis­(2-aminoethylamino)-1,10-phenanthroline for selective “turn-on” detection of highly toxic chemical warfare agent phosgene is presented. Probe Ru-1 exhibits aggregation-induced emission (AIE), and the restricted intramolecular motion is responsible for the AIE activity. In a CHCl3/CH3CN [95:5 (v/v)] solvent mixture, a unique self-assembled vesicular structure was formed after aggregation, which was supported by transmission electron microscopy, field emission scanning electron microscopy, and atmoic force microscopy studies. Probe Ru-1 showed a rapid and highly selective luminescence turn-on response for phosgene over other competitive chemical warfare agents with a low detection limit (13.9 nM) in CH3CN. The 2-aminoethylamino groups in Ru-1 act as a reacting site for nucleophilic addition to the carbonyl center of phosgene and undergo intramolecular cyclization. The final product of the phosgene-mediated reaction, Ru-1-Phos, contains 2-imidazolidinone groups, which has been confirmed by electrospray ionization mass spectometry and 1H nuclear magnetic resonance (NMR) spectroscopy. 1H NMR titration of Ru-1 with phosgene supported the reaction mechanism and also pointed to the simultaneous reaction of phosgene at two 2-aminoethylamino sites. For the first time, the crystal structure of the phosgene reaction product, Ru-1-Phos, containing the cyclized 2-imidazolidinone group was confirmed by single-crystal X-ray diffraction, which indubitably validates the reaction mechanism. Triplet state time-dependent density functional theory calculations showed that the weak luminescence of Ru-1 was mostly due to the population of the non-emissive 3MC state. The cyclization reaction with phosgene and the corresponding 2-imidazolidinone product formation populated the emissive 3MLCT state in Ru-1-Phos and is the key reason for the enhanced luminescence. Furthermore, a low-cost portable test paper strip has been fabricated with Ru-1 for the real-time selective monitoring of phosgene gas at the nanomolar level.