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In the quest to develop artificial light-harvesting systems (LHSs) with high energy transfer efficiency, hydrazone ligands L1–L2 and their –BF2 complexes (BODIHYs; B1 and B2) have been synthesized. Ligands L1 and L2 and the BODIHYs have been thoroughly characterized by various techniques (1H, 13C, 11B, and 19F NMR, ESI-MS, UV-vis and fluorescence). Further, the BODIHYs (B1 and B2) have been utilized as a new platform for artificial LHSs. It has been categorically shown that efficient artificial LHSs can be achieved by combining AIEgens B1 and B2 with rhodamine B (RhB) under aqueous conditions. In these systems B1 and B2 serve as a donor while RhB as an acceptor. The calculated energy transfer efficiency and antenna effect for the BODIHYs in combination with RhB came out to be appreciably high (B1 = 68%, 28.7; B2 = 80%, 35.5) at a donor/acceptor ratio of 25 : 1.

The synthesis of Zn(II) complexes (C1–C2) based on azo-Schiff base ligands 2-((E)-(phenyl-imino)methyl)-4-((E)-(2-(phenylthio)phenyl)diazenyl)phenol (L1) and 4-((E)-(2-(phenyl-thio)phenyl)diazenyl)-2-((E)-(p-tolylimino)methyl)phenol (L2) has been described. These have been thoroughly characterized by spectroscopic studies (IR, 1H, 13C, ESI-MS, electronic absorption, emission) and the structures of C1 and C2 have been determined by X-ray single crystal analyses. The ligands L1 and L2 exhibit reversible acid/base induced “ON–OFF–ON” switching in solution and solid state. Upon exposure to UV light (λ, 365 nm) C1 and C2 display cis–trans photoisomerisation and after removal of light they transform to more stable trans-form. Electronic absorption and 1H NMR studies on C1 and C2 revealed rather rapid (1.27 × 10−1 s−1) photoisomerization for C2 relative to C1 (2.7 × 10−2 s−1) which has also been supported by theoretical studies (DFT). The rather fast photoisomerization for C2 compared to C1 may be related to a small energy gap between HOMO and LUMO levels for the respective isomers.

The strategic design, synthesis and thorough characterization of four novel hydroxyl-substituted tetraphenylimidazole (HPI) based boron dipyrromethene (BODIPY) fluorophores (HPIB1-HPIB4) have been reported. Single crystal X-ray structure determination unveiled non-planar twisted orientations for these molecules. The non-planar orientations entirely restrict detrimental π-π interactions and avoid the non-radiative relaxation pathway for excited states in the solid/aggregated state and make them AIE active. The AIE characteristics of these compounds have been related to fine J-aggregation (evident from their crystal structures) along with restricted intra-molecular rotations (RIRs). These compounds display significant sensitivity toward viscosity and can serve as fluorescent molecular rotors due to multiple phenyl groups around the imidazole ring, which has been confirmed by measuring fluorescence quantum yields and lifetimes.

The fluorescence chemosensing behavior of Zn(ii), Cu(ii), and Cd(ii) based complexes toward cations has been described. Cation detection via conventional mechanisms, metal-metal exchange and chemodosimetric approaches along with the importance of metal ions and the scope, significance, and challenges with regard to the detection of cations by metal complex based probes will be discussed in detail. The fundamentals of photophysical behavior and mechanisms involved in the fluorescence detection of analytes will also be described. This article provides a detailed overview of Zn(ii), Cu(ii), and Cd(ii) based complexes as fluorescent probes for cations, together with essential discussions pertaining to detection mechanisms.

Fluorescent azo-phenol BODIPYs (1–3) have been obtained by the substituent (–OCH3/–CH3) directed synthesis of ortho (L1) and para (L2–L3) azo-phenol aldehydes. These display aggregation caused quenching (ACQ, 1) and aggregation induced emission enhancement (AIEE, 2 and 3) depending on the position of azo relative to the phenolic hydroxyl group. An intriguing time dependent morphological transition from nanospheres to ordered nanorods and subsequent emission changes in AIEE active azo-phenol BODIPYs have been successfully realized by time dependent fluorescence, scanning electron (SEM), transmission electron (TEM) and fluorescence optical microscopy (FOM) studies. The existence of one-dimensional (1D) nanorods as ultimate species in these compounds (2–3) has been supported by crystal packing patterns. Diverse aggregated forms and hierarchical nanostructures have been related to variable extents of fluorescence enhancement. The plausible charge transfer process and its role in AIEE have been supported by DFT studies.

Novel binuclear Zn(ii) complexes (1-2) derived from bis-chelating salen type ligands (H2L(1) and H2L(2)) possessing N,N-diethylamine moieties on the periphery of the molecules have been synthesized and thoroughly characterized by satisfactory elemental analyses and spectral (FT-IR, (1)H, (13)C NMR, UV-vis, fluorescence and ESI-MS) studies. The structures of H2L(1) and 1 have been authenticated by single crystal X-ray diffraction analyses. Complexes 1 and 2 strongly fluoresce and act as highly selective and sensitive chemosensors for picric acid in different organic as well as aqueous media. Both 1 and 2 showed strong potential to detect traces of PA in vapour/solid phase through contact mode analysis. Spectral and theoretical (DFT) studies suggested that the observed fluorescence quenching may be associated with ground state (GS) charge transfer as well as electrostatic interactions between 1/2 and PA. The fluorescence lifetime for the representative complex 1 displayed a double exponential curve and unaltered lifetime (τav, 0.63 nm) in the absence and presence of PA and strongly suggested that quenching follows a static mechanism. Further, DFT calculations on 1 and 2 strongly supported the static mechanism through GS charge transfer between complexes and PA. In addition, (1)H NMR spectral studies on 1-2 in the presence of PA firmly advocated strong hydrogen bonding and π-π stacking between the phenolic rings of 1-2 and the aromatic ring of PA. These complexes are capable of detecting PA either individually or in a competitive environment of other nitro- explosives. Florescence spectral studies on the model complex M lacking N,N-diethylamine groups revealed moderate selectivity and sensitivity towards PA and supported the key role of N,N-diethylamine moieties in the selectivity and sensitivity of complexes.

Four organometallic complexes [(η(6)-C6H6)RuCl(pmpzdpm)], 1; [(η(6)-C6H6)RuCl(pypzdpm)], 2; [(η(6)-C10H14)RuCl(pmpzdpm)], 3 and [(η(6)-C10H14)RuCl(pypzdpm)], 4 containing 5-(2-pyrimidyl-piperazine)phenyldipyrromethene (pmpzdpm) and 5-(2-pyridylpiperazine)phenyldipyrromethene (pypzdpm) have been designed and synthesized. The complexes 1-4 have been fully characterized by elemental analyses and spectroscopic studies (ESI-MS, IR, (1)H, (13)C NMR, UV-vis). Their electrostatic/intercalative interaction with CT DNA has been investigated by UV-vis and competitive ethidium bromide displacement studies while their protein binding affinity toward bovine serum albumin (BSA) was realized by UV-vis, fluorescence, synchronous and three dimensional (3D) fluorescence studies. The interaction with DNA and protein has further been validated by in silico studies. Cellular uptake, in vitro cytotoxicity and flow cytometric analyses have been performed to determine the mode of cell death against the kidney cancer cell line ACHN. Cell cycle analysis suggested that the complexes cause cell cycle arrest in the subG1 phase and overall results indicated that the in vitro antitumor activity of 1-4 lies in the order of 3412 (IC50, 7.0 1; 8.0 2; 2.0 3; 4.0 μM,4 ).

An expeditious synthesis of new click reaction-based BODIPY–piperazine conjugates separated by alkyl spacers (4–7) has been described. The compounds under investigation have been thoroughly characterized by various physicochemical techniques viz., elemental analyses, IR, HRMS, NMR (1H, 13C, 11B and 19F), electronic absorption, emission and theoretical studies. The comparative sensing abilities of 4–7 toward a range of metal ions have been investigated by various methods. Among these compounds, only 7 exhibited appreciable selectivity towards Hg2+, while the others remained inactive in the presence of various metal ions. Binding constant and Job's plot analysis indicated 1 : 1 stoichiometry between 7 and Hg2+. HRMS data and theoretical studies undoubtedly indicated the formation of a 7·Hg2+ complex and interaction of Hg2+ with the probe via nitrogen atoms in the piperazine and triazole units.

Synthesis of L1-L3, their thorough characterization by spectral as well as structural studies and use in selective photochemical detection of Fe(3+) and Hg(2+) at ppb level have been described. Notably, these exhibit bifunctional behaviour toward Fe(3+) (CHEF) and Hg(2+) (chemodosimetric) which has been unequivocally established by various studies.

The synthesis is described of a new bis-benzimidazole appended Schiff base ligand [2-(3,5-bis(1H-benz-imidazol-2-yl)-phenyliminomethyl)phenol] (H3L) followed by its thorough characterization by elemental analyses, spectroscopic studies (FT-IR, 1H, 13C NMR, ESI-MS, UV/vis, fluorescence) and X-ray single crystal analyses. The excellent selectivity of H3L towards Cu2+ and Al3+ via distinct responses in mixed aqueous conditions was established by spectroscopic studies. The selective detection of Cu2+ due to the creation of complex 1 in which Cu2+ occupied only the salen-type N2O2 coordination site and the strong binding with Al3+ through both benzimidazole and salen moieties to yield complex 2 was confirmed by various studies. The disparate detection of these cations by H3L was substantiated by comparative studies, performed under analogous conditions, of the precursor compound 3,5-bis(1H-benzimidazol-2-yl)-aniline (BBA). The distinct interaction of H3L with Cu2+ and Al3+ and the sensing mechanisms were investigated in detail by spectroscopic studies and were supported by theoretical studies (Density Functional Theory). It has been clearly shown that the use of appropriate substituents may facilitate the design and development of probes suitable for the real time detection of more than one analyte.

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.

Here, we report a self-assembled homochiral metal–organic framework [Cu1.5(H2LL-leu)(Ac)H2O]n·3H2O (1) obtained from an L-leucine-derived ligand (H4LL-leu) and Cu(Ac)2·H2O in a 1:1 ratio. Coordination-induced conformational change in the ligand has been monitored by circular dichroism which has been further attested by synthesizing a D-leucine-containing enantiomer H4LD-leu and its Cu(II) complex [Cu1.5(H2LD-leu)H2O]n·10H2O (2). Structure determination revealed entirely different structures for the homochiral MOFs (1 and 2) obtained from the L/D-leucine-derived enantiomer ligands under analogous reaction conditions. Further, structural dissimilarity in these MOFs has been judicially supported by proton conductance studies. MOF 1 shows higher proton (10−5 S cm−1) conductance in comparison to 2 (10−6 S cm−1) due to dissimilar alignment of the hydrogen-bonded water molecules in the hydrophilic pocket as well as crystal packing.

A highly fluorescent and efficient ratiometric ‘turn on’ probe for Hg2+ and Al3+ based on quinazoline (1) has been designed and synthesized. Limit of detection (LOD) and association constants revealed its superior binding affinity and sensitivity toward Hg2+ over Al3+. Interference studies signified displacement of the complexed Al3+ by Hg2+ which has been supported by various studies.

Biological properties of the arene–ruthenium complexes have attracted substantial current interest. Their activity is appreciably defined and controlled by the arene moieties, organic ligands and the metal center. In this review, we discuss the interaction of arene–ruthenium complexes with significant biomolecular targets (DNA and enzymes). Principally, active complexes may interact with the biomolecular targets DNA or nucleobases either by direct coordination facilitated by aquation of the complex or by intercalation/stacking of the pendant planar part of the complex, usually from the planar ancillary ligands or arenes with extended rings, between the DNA base pairs. On the other hand, kinetically inert metal complexes can also provide a potential tool (as enzyme inhibitors) for the targeting of important biomolecules (other than DNA), such as protein kinases. At the same time, coordination with a metal facilitates the outreach of the organic molecules in the intracellular region. This review also highlights the photodriven activation of arene–ruthenium complexes, important metal–drug interactions and the potential of multinuclear scaffolds as important drug candidates (e.g., metallodendrimers) and drug carriers (e.g. metallacages) for targeted delivery and activity.

Synthesis, characterization and antiproliferative activity of a new benzimidazole based Schiff base 2-(1-methyl-1-H-benzimidazol-2-yl)phenyl)imino)methyl)phenol (HL) and dicopper(II) complex [{Cu(L)NO3}2] (CuL) containing L− has been described. Both HL and CuL have been meticulously characterized by satisfactory elemental analyses, FT-IR, NMR, ESI-MS, electronic absorption and emission spectroscopy, and their structures unambiguously determined by X-ray single crystal analyses. Titration studies (absorption and emission) revealed interaction of the ligand and its dicopper(II) complex with DNA/BSA and stronger affinity of the CuL relative to HL. Binding of the HL and CuL with DNA/BSA have been validated by in silico studies and their cytotoxic effect on human breast cancer cell lines (MCF-7) by MTT assay. IC50 values (458 μM and 22 μM for HL, CuL) clearly suggested substantial cytotoxicity of the complex CuL toward MCF-7 compared to the ligand HL. Greater antiproliferative efficacy of the CuL in contrast to HL has been evidenced by fluorescence activated cell sorting (FACS) and AO/EB fluorescence staining. The possible mode of the apoptotic pathway for CuL has further been affirmed by reactive oxygen species (ROS) generation studies.

A novel chiral coordination polymeric gel based on an amino acid derived symmetrical ligand, specific base, and Zn(2+) has been described. Zn(2+) induced conformational reorganization in the ligand leads to gelation via J-aggregated helical fibres. The vital role played by π-π stacking in gelation has been monitored by fluorescence and NMR spectroscopy.

The synthesis, characterization and photoswitching behavior of a new salen type Schiff base N,N'-bis(2-hydroxy-5-phenylazobenzilidene)-2,4,6-trimethylbenzene-1,3-diamine (H₂L) and a binuclear zinc(ii) metallacycle [{Zn(L)}2·2H2O] (1) have been described. Both H₂L and 1 have been characterized by satisfactory elemental analyses, spectral (FT-IR, (1)H, (13)C NMR, ESI-MS, electronic absorption, emission) and electrochemical studies. Crystal structures of both H₂L and 1 have been authenticated by single crystal X-ray diffraction analyses. These exhibit trans-cis photoisomerization upon exposure to UV light (365 nm) and get back to a more stable trans-form after withdrawal of the light. Electronic absorption, emission, (1)H NMR and cyclic voltammetric studies revealed that trans-cis isomerization in metallacycle 1 is rather rapid (~5.0 s) relative to H₂L (~25 s) which has been supported by theoretical studies (DFT). Relatively fast photoisomerization in 1 compared to H₂L is facilitated by a small energy gap between HOMO levels of the trans- and cis-isomers. The percentage trans-cis conversion ratio for both H₂L and 1 has been evaluated (55-45, H₂L; 60-40%, 1) by (1)H NMR studies.

Highly fluorescent, multifunctional and thermoreversible conformational switching (1) has been designed and developed by embedding two imidazo[1,2-c]quinazoline (IQ) units in the pyridyl scaffold. The origin of the conformational and optical switching of 1 to 1′ has been established by various studies and by developing a model compound.

The first examples of homo- and heteroleptic complexes containing 5-ferrocenyldipyrromethene (fcdpm) [Cu(fcdpm)2] (1) [Cu(fcdpm)(acac)] (2), [Ni(fcdpm)(acac)] (3) and [Zn(fcdpm)2] (4) (acac = acetylacetonato) are described. Formation of the asymmetric metal complexes has been followed by electronic absorption and infrared spectroscopy. The complexes have been characterised by elemental analyses and spectral studies. Structures of [Cu(fcdpm)2] (1), [Cu(fcdpm)(acac)] (2) and [Ni(fcdpm)(acac)] (3) have been determined crystallographically. Structural data revealed that the metal centre in the copper complexes [Cu(fcdpm)2] (1), [Cu(fcdpm)(acac)] (2) and the nickel complex [Ni(fcdpm)(acac)] (3) adopts a distorted square planar geometry.