The Schiff base ligands benzylidene­(4-tert-butylphenyl)­amine 4-methyl ester (L1), (4-nitrobenzylidene)­(4-tert-butylphenyl)­amine (L2), and (4-cyanobenzylidene)­(4-tert-butylphenyl)­amine (L3) and the new series of cyclometalated mononuclear piano-stool complexes [(η5-C5Me5)­RhCl­(L1)] (1), [(η5-C5Me5)­RhCl­(L2)] (2), [(η5-C5Me5)­RhCl­(L3)] (3), [(η5-C5Me5)­IrCl­(L1)] (4), [(η5-C5Me5)­IrCl­(L2)] (5), and [(η5-C5Me5)­IrCl­(L3)] (6) have been synthesized. The ligands L1–L3 and complexes 1–6 have been thoroughly characterized by satisfactory elemental analyses, spectral studies (ESI-MS, IR, 1H and 13C NMR, UV–vis), and structures of 1–3 authenticated by X-ray single-crystal analyses. Efficient binding of 1–6 with calf thymus DNA (CT DNA) have been established by UV–vis and emission spectroscopic studies. Protein binding (bovine serum albumin, BSA) has been investigated by UV–vis, fluorescence, synchronous, and 3D fluorescence spectroscopy. Binding of the complexes with DNA through minor groove and hydrophobic interaction with proteins via sub domain IIA cavity has been substantiated by molecular docking studies. The complexes exhibited significant cytotoxicity against the human lung cancer cell line (A549), and 1 and 2 showed better activity than cisplatin. The cytotoxicity, morphological changes, and apoptosis have been assessed by MTT assay, Hoechst 33342/PI staining, cell cycle analysis by fluorescence-activated cell sorting (FACS), and reactive oxygen species (ROS) generation by DCFH-DA dye. The complexes 1–6 induce apoptosis in the order 2 > 1 > 4 > 3 > 5 > 6.

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With intent to fine tune the morphological and photophysical properties, three novel AIE luminogens (BQ1-BQ3) based on quinoline-BODIPY have been synthesized. A judicious choice of substituents (-H, -CH3, -OCH3) in these systems led to nanoballs in BQ1 and BQ2, while in BQ3 it led to reticulated nanofibers with diverse photophysical behaviours.

A pyrazole-appended quinoline-based 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (L1, BODIPY) has been synthesized and used as a ligand for the preparation of iridium(III) complexes [Ir(phpy)(L1)]PF (1; phpy = 2-phenylpyridine) and [(η-CMe)Ir(L1)Cl]PF (2). The ligand L1 and complexes 1 and 2 have been meticulously characterized by elemental analyses and spectral studies (IR, electrospray ionization mass spectrometry, H andC NMR, UV/vis, fluorescence) and their structures explicitly authenticated by single-crystal X-ray analyses. UV/vis, fluorescence, and circular dichroism studies showed that complexes strongly bind with calf-thymus DNA and bovine serum albumin. Molecular docking studies clearly illustrated binding through DNA minor grooves via van der Waals forces and their electrostatic interaction and occurrence in the hydrophobic cavity of protein (subdomain IIA). Cytotoxicity, morphological changes, and apoptosis have been explored by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay and Hoechst 33342 staining. IC values for complexes (1, 30 μM; 2, 50 μM) at 24 h toward the human cervical cancer cell line (HeLa) are as good as that of cisplatin (21.6 μM) under analogous conditions, and their ability to kill cancer cells lies in the order 1 > 2. Because of the inherent emissive nature of the BODIPY moiety, these are apt for intracellular visualization at low concentration and may find potential applications in cellular imaging and behave as a theranostic agent.

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A series of BODIPY-based AIEgens (QB1−QB5 and Bis-QB) containing 2-(2-hydroxyphenyl)quinazoline have been synthesized and thoroughly characterized. Photophysical properties of these compounds in solution and the aggregated state have been meticulously investigated and fine-tuned via structural modifications. These display green emission (∼530 nm) in solution and bright red emission (600−655 nm) in the aggregated/solid state with increased quantum yield. Crystal structure analyses and spectral studies revealed efficient J-type aggregation in these derivatives. Significant impact of 2-(2hydroxyphenyl)quinazoline toward modulating intermolecular interactions and facilitating J-type stacking between BODIPY units has also been established. Moreover, the essential role of excited-state intramolecular proton transfer (ESIPT) in inducing emission in the aggregated state and tuning of ESIPT emission by variation of substituents have been supported by various studies.

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