Recent advances in AIEgen-based chemosensors for small molecule detection, with a focus on ion sensing.

Since the aggregation-based emission (AIE) phenomenon emerged in 2001, numerous chemical designs have been built around the AIE concept, displaying its utility for diverse applications, including optics, electronics, energy, and biosciences. The present review critically evaluates the broad applicability of AIEgen-based chemical models towards sensing small analytes and the structural design strategies adjusting the mode of action reported since the last decade. Various AIEgen models have been discussed, providing qualitative and quantitative estimation of cationic metal ions and anionic species, as well as biomolecular, cellular, and organelle-specific probes. A systematic overview of the reported structural design and the underlying working mode will pave the way for designing and developing the next generation of AIEgens for specific applications.

Advancements in the development of fluorescent chemosensors based on CN bond isomerization/modulation mechanistic approaches.

The CN bond isomerization/modulation as a fluorescence signalling mechanism was explored by studying the photophysical properties of conformationally restricted molecules. From the beginning, the CN bond isomerization method has attracted the attention of researchers owing to its simplicity, high selectivity, and sensitivity in fluorescence evaluation. Continuous developments in the field of sensing using CN bond-containing compounds have been achieved via the customization of the isomerization process around the CN bond in numerous ways, and the results were obtained in the form of specific discrete photophysical changes. CN isomerization causes significant fluorescence enhancement in response to detected metal cations and other reactive species (Cys, Hys, ClO-, etc.) straightforwardly and effectively. This review sheds light on the process of CN bond isomerization/modulation as a signalling mechanism depending on fluorescence changes via conformational restriction. In addition, CN bond isomerization-based fluorescent sensors have yet to be well reviewed, although several fluorescent sensors based on this signalling mechanism have been reported. Therefore, CN-based fluorescent sensors are summarized in this review.

Anticounterfeiting Feature of a Writable and Self-Erasable Ni(II)-Metallogel Pad via Fluorescent "Turn-On" Detection of Cyanide.

A Schiff base 5-(2-hydroxy-3-methoxybenzylidieneamino)-1-H-imidazole-4-carboxamide (HL) comprising multibinding sites has been synthesized with the aim of fabricating a supramolecular gel. The gelator HL was characterized by FT-IR, 1H & 13C NMR, and ESI-MS techniques and also formed a [Ni(L)2] complex. The gelation property of HL was investigated with various metal ions, wherein Ni(II) selectively forms a mechanically and thermally stable supramolecular metallogel (MG) in the presence of a triethylamine base in DMF-MeOH media. Characterization of MG was accomplished with different spectro-analytical techniques such as FT-IR, ESI-MS, powder-XRD, SEM, rheological investigations, UV/vis, and fluorescence. The gelator HL displays moderate emission upon addition of Ni2+ and gives "turn-off" fluorescence output by forming the complex [Ni(L)2] (MG) due to the chelation-enhanced quenching of fluorescence (CHEQ). Job plot and ESI-MS data suggested a 2:1 stoichiometry between HL and Ni(II) in MG. Further, MG exhibited highly selective and ultrasensitive "turn-on" fluorescence signaling with CN- in the background presence of several cations and anions. The limit of detection (LoD) of MG was determined to be 6.9 × 10-9 M for CN- using the fluorescence technique. Notably, MG behaves as a fluorescent writable pad material explicitly with CN- under 365 nm UV light but not under ordinary light and the fluorescent text is self-erased after 15 min. Hence, MG can be used as a metallogel pad in the presence of CN- to communicate secret messages. Overall, the present work explores the fabrication of a thermo- and mechanostable Ni(II)-metallogel (MG), which selectively and ultrasensitively detects CN- both in the solution phase and in the gel form, wherein MG behaves as a writable and self-erasable pad with anticounterfeiting features for practical applications.

A multi-cation responsive Ni(II)-supramolecular metallogel mimics a molecular keypad lock via reversible fluorescence switching.

Synthesis of a bidentate N,O-donor Schiff base fluorescent ligand 5-(diethylamino-2-((4-(diethylamino-2-((4-(diethylamino)phenylimino)ethyl)phenol) (HL) adopting a new preparation procedure and its complexes with Ni(II) (1) and Zn(II) (2) has been illustrated. Structures of HL and 1 have been elucidated using X-ray single crystal analysis. Moreover, HL leads to the formation of a mechanically stable Ni(II)-gel (MG) upon treatment with Ni(NO3)2·6H2O in the presence of triethylamine (TEA) using THF/MeOH (1 : 1) solvents at rt. The gelator HL, complexes 1-2 and MG have been characterized by different spectroscopic and microscopic techniques including NMR (1H & 13C), FT-IR, ESI-MS, SEM, powder-XRD, rheology, UV/vis and fluorescence analysis. Rheological studies suggested good mechanical and thermal stability, whereas SEM analysis reveals a porous earth crust-like morphology of MG. Notably, 1 : 1 complexation between HL and Ni(II) forms a stable gel (MG), whereas 2 : 1 (HL : Ni2+) complexation leads to partial gelation. Formation of the Ni(II)-MG leads to slight "Turn-OFF" fluorescence relative to HL with a limit of detection (LOD) of 7.76 × 10-9 M; however, MG is considered as the "ON" state due to moderate emission. Remarkably, Ni(II)-MG further displayed reversible "ON-OFF-ON" fluorescence switching behavior through detection of Zn2+, Cu2+ and Hg2+. The emission intensity of MG is quenched with Cu2+/Hg2+ but enhances with Zn2+ in 1 : 1 (MG : M2+) stoichiometry. Therefore, MG mimics a sequence dependent molecular keypad lock for Cu2+ (C), Hg2+ (H) and Zn2+ (Z) to give the maximum output. Association and quenching constants were calculated by the Benesi-Hildebrand method, and from the Stern-Volmer plot the LOD was determined to be 4.2 × 10-6 M, 5.8 × 10-6 M and 7.8 × 10-6 M for MG with Zn(II), Cu(II) and Hg(II), respectively. To date, Ni(II) based MGs have been explored only toward electrochemical, thermal and conduction studies; however, the present work demonstrates the fluorescent reversible cation detection behavior of Ni(II)-MG to act as a molecular keypad lock for development of password protection devices.

A zinc (II) complex comprising an aminoethyl-nitropyridine-derived N,N,O-donor Schiff base ligand serves as an efficient ON-OFF probe for Cu(II).

A new fluorescent zinc (II) complex-based probe 1 encompassing a Schiff's base (E)-2-methoxy-6-((2-[5-nitropyridin-2-ylamino]ethylimino)methyl)phenol (HL) was designed, synthesized, and used for the highly selective detection of Cu2+ . Ligand HL and complex 1 were characterized using various spectroscopic techniques such as 1 H, 13 C-NMR, and FTIR spectroscopy, high-resolution mass spectronomy (HRMS), UV/visible light spectroscopy, and fluorescence studies. Ligand HL did not exhibit any considerable change in fluorescence in the presence of various cations. Notably, its Zn(II) complex 1 exhibited highly selective 'TURN-OFF' fluorescence signalling towards Cu2+ that remained uninterrupted with competing analytes. Probe 1 interacted with Cu2+ in 1:2 (1:Cu2+ ) stoichiometry as estimated through a Job's plot. Moreover, the selectivity of 1 was further confirmed through the interaction of the 1 + Cu2+ complex with some possible interfering metal ions inducing an insignificant response. Additionally, the association and quenching constant were determined to be 3.30 × 104 M-1 and 0.21 × 105 M-1 through the Benesi-Hildebrand method and Stern-Volmer plot, respectively.

Development of highly selective fluorescent ferrocenyl-iminopyridine chemosensor for biologically relevant Fe3.

Design, synthesis, characterization, and ion detection studies of two ferrocene-appended Schiff bases namely N-(2-[ferrocenylamino]ethyl)-5-nitropyridin-2-amine (1) and ferrocenylamino-1H-imidazole-4-carboxamide (2) been reported. Both the chemosensors have been thoroughly characterized using Fourier transfer infrared, 1 H and 13 C nuclear magnetic resonance, high resolution mass spectrometry, and ultraviolet/visible (UV/visible) and fluorescence spectral techniques. Probes 1 and 2 were designed with the aim of appending the ferrocenyl group with pyridine ring having an amine substitution (for 1) and imidazole ring with an amide substitution (for 2). Interaction of these probes with a series of cations and anions was examined through UV/vis and fluorescence spectral techniques. Probe 2 exhibited an insignificant response towards anions and loss of selectivity for cations, whereas 1 displayed highly selective detection towards biologically important Fe3+ in 2:1 (probe:cation) stoichiometry. Notably, none of the cations and anions could interfere the selectivity of Fe3+ ensured by 1 in aqueous medium. The limit of detection for Fe3+ detection using 1 was determined to be 0.2 ppm. The results strongly suggest that 1 could find promising future application as a chemosensor for Fe3+ in biological systems for quantification and qualitative analysis.

Synthesis, characterization, and electrocatalytic behaviour of hydrothermally grown nanostructured La2 O3 and La2 O3 /K-complex.

Rare earth metals play a conspicuous role in magnetic resonance imaging (MRI) for detecting cancerous cells. The alkali metal potassium is a neurotransmitter in the sodium-potassium pump in biomedical sciences. This unique property of rare earth metals and potassium drew our attention to carry forward this study. Therefore, in this work, previously synthesized potassium (K) complexes formed by the reflux of 4-N,N-dimethylaminobenzoic acid (DBA) and potassium hydroxide in methanol, and named [(µ2-4-N,N-dimethylaminobenzoate-κO)(µ2-4-N,N-dimethylaminobenzoic acid-κO)(4-N,N-dimethylaminobenzoic acid-κO) potassium(I) coordination polymer)] were treated hydrothermally with La2 O3 nanomaterials to obtain a nanohybrid La2 O3 /K-complex. After that, the K-complex was analyzed using single-crystal X-ray diffraction and 1 H and 13 C NMR spectroscopy. In addition, the structural and morphological properties of the as-prepared nanostructured La2 O3 /K-complex were also characterized, which involved an investigation using X-ray diffraction (XRD)spectroscopy, Fourier transform infrared (FTIR) spectroscopy, atomic force spectroscopy (AFM), transmission electron microscopy (TEM), and energy dispersive X-ray (EDX) analysis. After this, the electrochemical redox behaviour of the synthesized nanohybrid material was studied using cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Therefore, the results from these studies revealed that the as-prepared material was a La2 O3 /K-complex that has a promising future role in sensing various analytes, as it showed effective electrocatalytic behaviour.

Catalytic C-H Bond Activation and Knoevenagel Condensation Using Pyridine-2,3-Dicarboxylate-Based Metal-Organic Frameworks.

Three 1D coordination polymers (CPs) [M(pdca)(H2O)2] n (M = Zn, Cd, and Co; 1-3), and a 3D coordination framework {[(CH3)2NH2][CuK(2,3-pdca)(pa)(NO3)2]} n (4) (2,3-pdca = pyridine-2,3-dicarboxylate and pa = picolinic acid), have been synthesized adopting a solvothermal reaction strategy. The CPs have been thoroughly characterized using various spectral techniques, that is, elemental analyses, FT-IR, TGA, DSC, UV/vis, and luminescence. Structural information on 1-4 was obtained by PXRD and X-ray single-crystal analyses, whereas morphological insights were attained through FESEM, AFM, EDX, HRTEM, and BET surface area analyses. Roughness parameters were calculated from AFM analysis, whereas dimensions of small domains and interplanar spacing were defined with the aid of HRTEM. CPs 1-3 are 1D isostructural networks, whereas 4 is a 3D framework. Moreover, 1-4 display moderate luminescence at rt. In addition, 1-4 have been applied as economic and efficient porous catalysts for the Knoevenagel condensation reaction and C-H bond activation under mild conditions with good yields (95-98 and 97-99%), respectively. Notably, 1-3 can be reused up to seven cycles, whereas 4 can be reused up to five catalytic cycles with retained catalytic efficiency. Relative catalytic efficacy toward the Knoevenagel condensation reaction follows in the order 2 > 1 > 3 > 4, whereas 2 > 4 > 1 > 3 for C-H activation. The present result demonstrates synthetic, structural, optical, morphological, and catalytic aspects of 1-4.

Zinc(ii), copper(ii) and cadmium(ii) complexes as fluorescent chemosensors for cations.

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.

Development of a fused imidazo[1,2-a]pyridine based fluorescent probe for Fe3+ and Hg2+ in aqueous media and HeLa cells.

A new fluorescent sensor 5 based on a fused imidazopyridine scaffold has been designed and synthesized via cascade cyclization. The reaction features the formation of three different C-N bonds in sequence. Imidazopyridine based fluorescent probe 5 exhibits highly sensitive and selective fluorescent sensing for Fe3+('turn-on') and Hg2+('turn-off'). The excellent selectivity of imidazopyridine for Fe3+/Hg2+ was not hampered in the presence of any of the competing cations. The limit of detection (LOD) of 5 toward Fe3+ and Hg2+ has been estimated to be 4.0 ppb and 1.0 ppb, respectively, with a good linear relationship (R 2 = 0.99). Notably, 5 selectively detects Fe3+/Hg2+ through fluorescence enhancement signalling both in vitro and in HeLa cells.

A Dual-Signaling Ferrocene-Pyrene Dyad: Triple-Mode Recognition of the Cu(II) Ions in Aqueous Medium.

We report a structure of ferrocene-pyrene conjugate (1) comprising electro and photo-active dual-signaling units. In particular, 1 upon interaction with Cu(II), displays selectively one-photon fluorescence quenching, but it shows two-photon absorption (TPA) cross-section 1230 GM (at 780 nm). Further, 1 displayed two irreversible oxidative waves at 0.39 V and 0.80 V (vs Ag/AgCl), in the electrochemical analysis which upon addition of Cu2+, led to the negative potential shift in both the oxidative waves to appear at 0.25 V and 0.68 V. The triple mode changes in presence of Cu(II) suggesting the possible application of 1 for the detection of Cu(II) in aqueous media. Graphical Abstract.

Self-assembled copper(II) metallacycles derived from asymmetric Schiff base ligands: efficient hosts for ADP/ATP in phosphate buffer.

Novel asymmetric Schiff base ligands 2-{[3-(3-hydroxy-1-methyl-but-2-enylideneamino)-2,4,6-trimethylphenylimino]-methyl}-phenol (H2L(1)) and 1-{[3-(3-hydroxy-1-methyl-but-2-enylideneamino)-2,4,6-trimethylphenylimino]-methyl}-naphthalen-2-ol (H2L(2)) possessing dissimilar N,O-chelating sites and copper(ii) metallacycles (CuL(1))4 (1) and (CuL(2))4 (2) based on these ligands have been described. The ligands and complexes have been thoroughly characterized by satisfactory elemental analyses, and spectral (IR, (1)H, (13)C NMR, ESI-MS, UV/vis) and electrochemical studies. Structures of H2L(2) and 1 have been unambiguously determined by X-ray single crystal analyses. The crystal structure of H2L(2) revealed the presence of two distinct N,O-chelating sites on dissimilar cores (naphthalene and ß-ketoaminato groups) offering a diverse coordination environment. Metallacycles 1 and 2 having a cavity created by four Cu(ii) centres coordinated in a homo- and heteroleptic fashion with respective ligands act as efficient hosts for adenosine-5'-diphosphate (ADP) and adenosine-5'-triphosphate (ATP) respectively, over other nucleoside polyphosphates (NPPs). The disparate sensitivity of these metallacycles toward ADP and ATP has been attributed to the size of the ligands assuming diverse dimensions and spatial orientations. These are attuned for π-π stacking and electrostatic interactions suitable for different guest molecules under analogous conditions, metallacycle 1 offers better orientation for ADP, while 2 for ATP. The mechanism of the host-guest interaction has been investigated by spectral and electrochemical studies and supported by molecular docking studies.

Novel tetranuclear copper |2 + 4| cubanes resulting from unprecedented C-O bond formation cum dearomatization.

Novel tetranuclear copper |2 + 4| cubanes 1 and 2 have been synthesized from the reaction of hydrated copper salts/precursors with N,O chelating ß-ketoaminato ligands. Creation of 1 and 2 occurs by in situ generation of the tridentate chelating species HL1O(2-) via copper mediated C-O bond formation and dearomatization of H3L1, while anhydrous salts afforded mononuclear complex 3.

Structural and mechanistic insights into an Fe³âº-triggered quinazoline based molecular rotor.

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.

A Schiff base and its copper(II) complex as a highly selective chemodosimeter for mercury(II) involving preferential hydrolysis of aldimine over an ester group.

The syntheses of a new Schiff base, diethyl-5-(2-hydroxybenzylidene)aminoisophthalate (HL), and a copper complex, [Cu(L2)] (1), imparting L(-), have been described. Both the ligand HL and complex 1 have been thoroughly characterized by elemental analyses, electrospray ionization mass spectrometry, FT-IR, NMR ((1)H and (13)C), electronic absorption, and emission spectral studies and their structures determined by X-ray single-crystal analyses. Distinctive chemodosimetric behavior of HL and 1 toward Hg(2+) has been established by UV/vis, emission, and mass spectral studies. Comparative studies further revealed that the chemodosimetric response solely originates from selective hydrolysis of the aldimine moiety over the ester group and 1 exhibited greater selectivity toward Hg(2+) relative to HL while the sensitivity order is reversed. Further, these followed different hydrolytic pathways but ended up with the same product analyzed for diethyl-5-aminoisophthalate (DEA). Hg(2+)-induced displacement of Cu(2+) and subsequent hydrolysis of the -HC═N- moiety in 1 affirmed the identity of the actual species undergoing hydrolysis as HL. The occurrence of Cu(2+) displacement and Hg(2+) detection via hydrolytic transformation has been supported by various physicochemical studies.

Swift photoswitching in a binuclear Zn(II) metallacycle relative to a salen-type ligand.

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.

DNA/protein binding, molecular docking, and in vitro anticancer activity of some thioether-dipyrrinato complexes.

Syntheses and characterizations of the arene ruthenium [(η(6)-C6H6)RuCl(4-mtdpm)] (1), [(η(6)-p-MeC6H4Pr(i))RuCl(4-mtdpm)] (2), and structurally analogous rhodium/iridium complexes [(η(5)-C5Me5)RhCl(4-mtdpm)] (3) and [(η(5)-C5Me5)IrCl(4-mtdpm)] (4) [4-mtdpm = 5-(4-methylthiophenyl)dipyrromethene] have been reported. Their identities have been established by satisfactory elemental analyses, electrospray ionization-mass spectrometry (ESI-MS), FT-IR, NMR ((1)H, (13)C), UV/vis, emission spectral, and electrochemical studies. Structure of the representative complex 3 has been authenticated by X-ray single crystal analyses. The complexes 1-4 effectively bind with calf thymus DNA (CT DNA) through intercalative/electrostatic interactions. In addition, these exhibit significant cytotoxicity toward Dalton lymphoma (DL) cell line and cause static quenching of the bovine serum albumin (BSA) fluorophore. The antiproliferative activity, morphological changes, and apoptosis have been evaluated by MTT assay, acridine orange/ethidium bromide (AO/EtBr) fluorescence staining, and DNA ladder assay. Mode of interaction of the complexes with DNA/protein has also been supported by molecular docking. Various studies revealed remarkable decrease in the in vitro DL cell proliferation and induction of the apoptosis by 1-4, which lies in the order 2 > 1 > 4 > 3.

DNA binding and anti-cancer activity of redox-active heteroleptic piano-stool Ru(II), Rh(III), and Ir(III) complexes containing 4-(2-methoxypyridyl)phenyldipyrromethene.

The synthesis of four novel heteroleptic dipyrrinato complexes [(η(6)-arene)RuCl(2-pcdpm)] (η(6)-arene = C6H6, 1; C10H14, 2) and [(η(5)-C5Me5)MCl(2-pcdpm)] (M = Rh, 3; Ir, 4) containing a new chelating ligand 4-(2-methoxypyridyl)-phenyldipyrromethene (2-pcdpm) have been described. The complexes 1-4 have been fully characterized by various physicochemical techniques, namely, elemental analyses, spectral (ESI-MS, IR, (1)H, (13)C NMR, UV/vis) and electrochemical studies (cyclic voltammetry (CV) and differential pulse voltammetry (DPV)). Structures of 3 and 4 have been determined crystallographically. In vitro antiproliferative and cytotoxic activity of these complexes has been evaluated by trypan blue exclusion assay, cell morphology, apoptosis, acridine orange/ethidium bromide (AO/EtBr) fluorescence staining, and DNA fragmentation assay in Dalton lymphoma (DL) cell lines. Interaction of 1-4 with calf thymus DNA (CT DNA) has also been supported by absorption titration and electrochemical studies. Our results suggest that in vitro antitumor activity of 1-4 lies in the order 2 > 1 > 4 > 3.

Luminescent N,O-chelated chroman-BF2 complexes: structural variants of BODIPY.

The synthesis and characterization of 5-(chromen-4-one)-dipyrromethane (1), 5-(6-methyl-chromen-4-one)-dipyrromethane (2), 5-(6-isopropyl-chromen-4-one)-dipyrromethane (3) and the respective chromans, 7-[2-pyrrolo]-pyrrole[1,2-a]12H pyrrolino[2,3-b]chroman-4-one (4), 4-methyl-7-[2-pyrrolo]-pyrrole[1,2-a]12H pyrrolino[2,3-b]-chroman-4-one (5) and 4-isopropyl-7-[2-pyrrolo]-pyrrole[1,2-a]12H pyrrolino[2,3-b]-chroman-4-one (6) have been described. Chroman derivatives 4-6 have been used in the synthesis of highly stable, fluorescent, borondifluoride complexes (7-9). All the compounds have been fully characterised by various physicochemical techniques viz., elemental analyses, IR, HRMS, NMR ((1)H, (13)C), electronic absorption, emission (solution and solid state), electrochemical and thermal studies. Crystal structures of dipyrromethanes 1 and 2, chroman derivative 6 and boron complexes 7-9 have been determined by X-ray single crystal analyses. Structural studies revealed that the formation of 4-6 takes place by DDQ mediated C-N coupling. The higher stability of 4-6 relative to their respective dipyrrins has been supported by theoretical studies.

Heteroleptic dipyrrinato complexes containing 5-ferrocenyldipyrromethene and dithiocarbamates as coligands: selective chromogenic and redox probes.

Six heteroleptic dipyrrinato complexes [Ni(fcdpm)(dedtc)] (1), [Ni(fcdpm)(dipdtc)] (2), [Ni(fcdpm)(dbdtc)] (3), [Pd(fcdpm)(dedtc)] (4), [Pd(fcdpm)(dipdtc)] (5), and [Pd(fcdpm)(dbdtc)] (6) (fcdpm = 5-ferrocenyldipyrromethene; dedtc = diethyldithiocarbamate; dipdtc = diisopropyldithiocarbamate; dbdtc = dibutyldithiocarbamate) have been synthesized and characterized by elemental analyses and spectral (ESI-MS, IR, (1)H, (13)C NMR, UV-vis) and electrochemical studies. Crystal structures of 1, 2, 4, and 5 have been authenticated by X-ray single-crystal analyses. Nickel-based complexes 1-3 display selective chromogenic and redox sensing for Hg(2+) and Pb(2+) ions, while palladium complexes 4-6 display selective chromogenic and redox sensing only for Hg(2+). Electronic absorption, ESI-MS, and electrochemical studies indicated that sensing arises from interaction between 1-3 and Hg(2+)/Pb(2+) through sulfur of the coordinated dithiocarbamates, while it arises from the pyrrolic nitrogen of fcdpm and dithiocarbamate sulfur from 4-6 and Hg(2+). Different modes of binding between Ni and Pd complexes have further been supported by theoretical studies. The receptor-cation binding constants (K(a)) and stoichiometry between probes and Hg(2+)/Pb(2+) have been estimated by the Benesi-Hildebrand method and Job's plot analysis. Detection limits for 1-3 toward Hg(2+)/Pb(2+) and 4-6 for Hg(2+) have been found to be reasonably high.