Selective Turn-On Luminescent Recognition of Perchlorate Ion Using Pyridyl-Benzimidazole-Based Probe

Anions play a crucial role in various environmental, chemical, and biological processes. Among various anions, the production of perchlorate (ClO4 -) ion is expected to rise in upcoming years, and thus, an efficient method for the detection of perchlorate ion is highly desirable. In this effort, a pyridyl-benzimidazole-based luminescent probe (RSB1) containing two N-H donor sites has been synthesized for selective detection of perchlorate ion. Different spectral techniques such as FT-IR, NMR, ESI-mass, UV-Vis, and fluorescence analyses have been used to characterize this probe. High selectivity of RSB1 for ClO4 - was realized even in presence of strongly interfering species in aqueous-acetonitrile (CH3CN-H2O; 4:1, v/v) solution. Notably, RSB1 served as a "turn-on" perchlorate-responsive probe and exhibited an emission enhancement at 363 nm when excited at 300 nm. The detection limit (LoD) and the binding constant (Kb) values were depicted to be 0.121 μM and 2.6 × 105 M-1, respectively, while the binding mechanism for RSB1-ClO4 - was validated via Job's plot, NMR, and DFT analyses. Furthermore, this probe was successfully employed to trace perchlorate in real samples such as tap water, distilled water, and soil samples with good to excellent recovery values.

A macrocycle-based new organometallic nano-vessel towards sustainable C2-selective arylation of free indole in water

C2-selectivity of unsubstituted indole over facile C3-substitution is attempted by utilizing the π-cavity of a nano-vessel made up of a palladium complex of an amino-ether heteroditopic macrocycle. Functional group tolerance (cyano, nitro, halo, ester, etc.), a broad substrate scope and outstanding selectivities with excellent yields (80-93%) of the desired products have been achieved in 12 h by maintaining all sustainable conditions like aqueous medium, recyclable catalyst, one-pot reaction, no external additives, mild temperature, etc. Interestingly, we observed that electron-deficient indole derivatives underwent the present transformation with marginally superior reactivity in comparison with electron-rich indole derivatives. This approach establishes a green pathway for selective C-C coupling employing a π-cavitand as a nano-reactor.

From construction to application of a new generation of interlocked molecules composed of heteroditopic wheels

Over the last few decades, research on mechanically interlocked molecules has significantly evolved owing to their unique structural features and interesting properties. A substantial percentage of the reported works have focused on the synthetic strategies, leading to the preparation of functional MIMs for their applications in the chemical, materials, and biomedical sciences. Importantly, various macrocyclic wheels with specific heteroditopicity (including phenanthroline, amide, amine, oxy-ether, isophthalamide, calixarene and triazole) and threading axles (bipyridine, phenanthroline, pyridinium, triazolium, etc.) have been designed to synthesize targeted multifunctional mononuclear/multinuclear pseudorotaxanes, rotaxanes and catenanes. The structural uniqueness of these interlocked systems is advantageous owing to the presence of mechanical bonds with specific three-dimensional cavities. Furthermore, their multi-functionalities and preorganised structural entities exhibit a high potential for versatile applications, like switching, shuttling, dynamic properties, recognition and sensing. In this feature article, we describe some of the most recent advances in the construction and chemical behaviour of a new generation of interlocked molecules, primarily focusing on heteroditopic wheels and their applications in different directions of the modern research area. Furthermore, we outline the future prospects and significant perspectives of the new generation heteroditopic wheel based interlocked molecules in different emerging areas of science.

Development of fluorophoric [2]pseudorotaxanes and [2]rotaxane: selective sensing of Zn(II)

Fluorophoric [2]pseudorotaxanes {NiPR1(ClO4)2-NiPR3(ClO4)2} are synthesized by utilizing newly designed fluorophoric bidentate ligands (L1-L3) and a heteroditopic naphthalene containing macrocycle (NaphMC) with high yields via Ni(II) templation and π-π stacking interactions. Subsequently, a fluorophoric [2]rotaxane (NAPRTX) is established through a Cu(I) catalysed click reaction between an azide terminated pseudorotaxane, {NiPR4(ClO4)2}, which contains the newly designed fluorophoric ligand L4, and alkyne terminated bulky stopper units. All these fluorophoric [2]pseudorotaxanes and the [2]rotaxane were characterized using numerous techniques such as mass spectrometry, NMR, UV/Vis, PL, and elemental analysis, wherever applicable. Furthermore, to investigate the effect of the fluorophoric moieties, the coordinating ability of chelating units, and size and shape of the three dimensional cavity generated by the mechanical bond in the interlocked [2]rotaxane (NAPRTX), we have performed a sensing study of various metal ions. Thus, the interlocked [2]rotaxane is found to have potential as a selective fluorescent sensor for Zn(II) metal ions over other transition, alkali and alkaline earth metal ions, where the 2,2'-bipyridyl arylvinylene moiety of the axle acts as a fluorescence signalling unit.

NDI-integrated rotaxane/catenane and their interactions with anions

Complexation of alkali and alkaline earth metal ions with the heteroditopic Phen-ester oxy-ether macrocyclic wheel (PhenMC) is established for the synthesis of interlocked molecular systems. The single crystal X-ray structure of Na-bound PhenMC confirms the hexacoordinated geometry around the Na ion in the macrocycle. Further, Ca-ion-bound PhenMC (Ca-PhenMC) is explored with a fluorophoric azide-terminated NDI (naphthalene diimide) axle (NDIAz) for the synthesis of fluorophoric [2]rotaxane (NDIROT) and [2]catenane (NDICAT) via Cu(I)-catalyzed cycloaddition reaction. Characterizations of these two new interlocked molecular systems are performed by ESI-MS, NMR, UV-vis and PL spectroscopic studies wherever applicable. Moreover, the new molecular systems are explored towards anion sensing applications via colorimetric, UV-vis-NIR, PL and other spectroscopic studies.

Heteroditopic Macrobicyclic Molecular Vessels for Single Step Aerial Oxidative Transformation of Primary Alcohol Appended Cross Azobenzenes

A series of oxy-ether tris-amino heteroditopic macrobicycles (L1-L4) with various cavity dimensions have been synthesized and explored for their Cu(II) catalyzed selective single step aerial oxidative cross-coupling of primary alcohol based anilines with several aromatic amines toward the formation of primary alcohol appended cross azobenzenes (POCABs). The beauty of this transformation is that the easily oxidizable benzyl/primary alcohol group remains unhampered during the course of this oxidation due to the protective oxy-ether pocket of this series of macrobicyclic vessels. Various dimensionalities of the molecular vessels have shown specific size complementary selection for substrates toward efficient syntheses of regioselective POCAB products. To establish the requirement of the three-dimensional cavity based additives, a particular catalytic reaction has been examined in the presence of macrobicycles (L2 and L3) versus macrocycles (MC1 and MC2) and tripodal acyclic (AC1 and AC2) analogous components, respectively. Subsequently, L1-L4 have been extensively utilized toward the syntheses of as many as 44 POCABs and are characterized by different spectroscopic techniques and single crystal X-ray diffraction studies.

A Cd(ii) and Zn(ii) selective naphthyl based [2]rotaxane acts as an exclusive Zn(ii) sensor upon further functionalization with pyrene

A new multi-functional [2]rotaxane, ROTX, has been synthesized via a Cu(i) catalysed azide-alkyne cycloaddition reaction between Ni(ii) templated azide terminated pseudorotaxane composed of a naphthalene based heteroditopic wheel, NaphMC, and an alkyne terminated stopper. Subsequently, ROTX has been functionalized with pyrene moieties to develop a bifluorophoric [2]rotaxane, PYROTX, having naphthalene and pyrene moieties. Detailed characterization of these two rotaxanes is performed by utilizing several techniques such as ESI-MS, (1D and 2D) NMR, UV/Vis and PL studies. Comparative metal ion sensing studies of NaphMC (a fluorophoric cyclic receptor), ROTX ([2]rotaxane with a naphthyl fluorophore) and PYROTX ([2]rotaxane having naphthyl and pyrene fluorophores) have been performed to determine the effect of dimensionality/functionalization on the metal ion selectivity. Although NaphMC fails to discriminate between metal ions, ROTX serves as a selective sensor for Zn(ii) and Cd(ii). Importantly, PYROTX shows exclusive selectivity towards Zn(ii) over various transition, alkali and alkaline earth metal ions including Cd(ii).

Inhibitory Effect of Azamacrocyclic Ligands on Polyphenol Oxidase in Model and Food Systems

Enzymatic browning is one of the main problems faced by the food industry due to the enzyme polyphenol oxidase (PPO) provoking an undesirable color change in the presence of oxygen. Here, we report the evaluation of 10 different azamacrocyclic compounds with diverse morphologies as potential inhibitors against the activity of PPO, both in model and real systems. An initial screening of 10 ligands shows that all azamacrocyclic compounds inhibit to some extent the enzymatic browning, but the molecular structure plays a crucial role on the power of inhibition. Kinetic studies of the most active ligand (L2) reveal a S-parabolic I-parabolic noncompetitive inhibition mechanism and a remarkable inhibition at micromolar concentration (IC₅₀ = 10 μM). Furthermore, L2 action has been proven on apple juice to significantly reduce the enzymatic browning.

Fluorophoric [2]rotaxanes: post-synthetic functionalization, conformational fluxionality and metal ion chelation

Fluorophoric [2]rotaxanes form an exciplex upon interpenetration and the exciplex signals are used to monitor the chelation properties of the interlocked systems.

Cu(ii) templated formation of [n]pseudorotaxanes (n = 2, 3, 4) using a tris-amino ether macrocyclic wheel and multidentate axles

The systematic development of mono-, bi- and tri-nuclear [n]pseudorotaxanes (n = 2, 3, 4) via Cu(ii) templation and π–π stacking interactions.

Naphthalene containing amino-ether macrocycle based Cu(ii) templated [2]pseudorotaxanes and OFF/ON fluorescence switching via axle substitution

A new naphthalene containing macrocycle, NaphMC, and a new fluorophoric bidentate linear axle derivative of 5,5'-dimethyl-2,2'-bipyridine (L3) along with two other ligands 1,10-phenanthroline (L1) and 5,5'-dimethyl-2,2'-bipyridine (L2) are explored towards the synthesis of Cu(ii) templated [2]pseudorotaxanes. All ternary complexes are well characterized by ESI-MS, UV/Vis, EPR spectroscopy, elemental analysis and emission spectroscopic studies. Single crystal X-ray diffraction studies confirm the geometry around the Cu(ii) center as a distorted trigonal bipyramid via the contribution of [3 + 2] orthogonal motifs of the wheel (NaphMC) and the bidentate chelating ligands L1 and L2 in the cases of pseudorotaxanes, CuPR1 and CuPR2, respectively. Furthermore, the fluorescence "OFF" state of the fluorophoric axle L3 is achieved via threading it to the Cu(ii) complex of NaphMC, whereas fluorescence switching "ON" is demonstrated by the substitution of L3 of CuPR3 with a stronger chelating ligand L1.

Syntheses of metallo-pseudorotaxanes, rotaxane and post-synthetically functionalized rotaxane: a comprehensive spectroscopic study and dynamic properties

Herein, a bis-amido tris-amine macrocycle and five bipyridine-based bidentate chelating ligands were investigated towards various divalent transition metal ion (Ni^II, Co^II, Cu^II, and Zn^II)-templated syntheses of metallo [2]pseudorotaxanes. The formation of these ternary complexes was elucidated via different spectroscopic techniques such as ESI-MS, absorption spectroscopy, EPR spectroscopy, and single-crystal X-ray diffraction studies wherever possible. Azide-terminated Ni^II, Co^II, Cu^II, Zn^II-templated [2]pseudorotaxanes were explored to generate [2]rotaxane, ROT, via reaction with an alkyne-terminated triphenylene unit as a stopper under the mild reaction condition of the Cu^I-catalyzed azide-alkyne cycloaddition reaction. Ni^II-templated [2]pseudorotaxane was found to be the best precursor towards the high-yield synthesis of ROT. The interpenetrative nature of the center piece in metal-free rotaxane was also established through various spectroscopic techniques such as ESI-MS and 1D and 2D (COSY, NOESY, ROESY, and DOSY) NMR spectroscopy. Furthermore, ROT was functionalized via tri-acetylation as AcROT to incorporate three tertiary amides at the tris-amine centers; this AcROT exhibited rotamer-induced molecular motions in an interpenetrated system via the formation of multiple conformers/co-conformers. Additionally, the existence of multiple rotamers was established via variable-temperature NMR spectroscopic studies. Li⁺ and 12-crown-4 were found to be suitable for the reversible conformation/co-conformation fixation of tri-acetylated bis-amido tris-amine macrocyclic wheel-based rotaxane.

Threading of various ‘U’ shaped bidentate axles into a heteroditopic macrocyclic wheel via NiII/CuII templation

The Ni^II/Cu^II templated threading of various terminal group embedded ‘U’ shaped axles into an amido–amine macrocyclic wheel towards the development of a new generation of [2]pseudorotaxanes via [3 + 2] coordination assisted by other non-covalent interactions.

Copper(ii)-directed synthesis of neutral heteroditopic [2]rotaxane ion-pair host systems incorporating hydrogen and halogen bonding anion binding cavities

Neutral heteroditopic [2]rotaxane ion-pair host systems were synthesised via a copper(ii)-directed metal template strategy and shown to undergo cooperative anion recognition with a co-bound zinc(ii) cation.

Structural diversities in Ag(i) complexes of xylyl platform based isomeric bis-NHC ligands: effects of pyridine wingtip substituents

In an infinite ID polymeric structure, Ag(i) atoms are arranged linearly in a xylyl platform based tetra-dentate bis-NHC ligand.