Pounraj Thanasekaran

One-Step Orthogonal-Bonding Approach to the Self-Assembly of Neutral Rhenium-Based Metallacycles: Synthesis, Structures, Photophysics, and Sensing Applications

Self-assembled metallacycles offer structural diversity and interesting properties based on their unique frameworks and host-guest chemistry. As a result, the design and synthesis of these materials has attracted significant research interest. This Account describes our comprehensive investigations of an effective orthogonal-bonding approach for the self-assembly of neutral Re-based metallacycles. We discuss the various types of assemblies that can be created based on the nuclearity of the luminophore, including bimetallic materials, rectangles, cages, and calixarenes. This approach permits the preparation of a rectangular molecule, rather than two molecular squares, in excellent yields. We extended this strategy to the high yield synthesis of a series of Re-based metallacycles with different shapes. With the rich spectroscopic and luminescence properties, Re(I) metallacycles provide an excellent platform for studies of host-guest interactions. When possible, we also present potential applications of the luminescent Re-based metallosupramolecular assemblies. The orthogonal-bonding approach involves the simultaneous introduction of two ligands: a bis-chelating ligand to coordinate to two equatorial sites of two fac-(CO)(3)Re cores and a monotopic or ditopic nitrogen-donor ligand to the remaining orthogonal axial site. Furthermore, by the appropriate choice of the predesigned organic ligands with various backbones and connectivity information and fac-Re(CO)(3) metal centers, we could also design other novel functional metallacycles including rotors, gondolas, cages, triangles, and metallacalixarenes in high yields. The incorporation of flexible ligands into the Re(I) metallacycles allows us to introduce various conformation states and novel structures. As a result, these structures acquire new functions, such as adaptive recognition properties. For example, we assembled Re(I)-based metallacyclic rotors via a one-step process. These rotors, which contain a para-phenylene unit that rapidly rotates within the metallacycles, are prototypes of a neutral altitudinal rotor. Most of the metallacycles are luminescent. The ability to chemically modify the organic ligands offers opportunities to create structural diversity and to tune the photophysical properties of these Re(I) metallacycles efficiently. Several strategies for increasing emission quantum yields and excited-state lifetimes and tuning the colors in Re(I) metallacycles are available. The cyclometalated ligands in Re(I) metallacycles improve excited state lifetimes and quantum yields, and these C-H bond-activated metallacycles are considerably more emissive than their non-C-H bond-activated analogues. The introduction of crown-ether-like recognition sites into neutral gondola-shaped metallacycles that selectively recognize metal ions also enhanced emission. Rhenium-based rectangular boxes, synthesized via a simple one-step route, contain a large and tunable hydrophobic inner cavity, which selectively recognizes benzene molecules. Such structures were the best host for benzene reported to date. In addition, we designed and synthesized novel neutral metallacalixarenes with tunable size, cavity, color, and functionality. These structures are efficient hosts for the recognition of planar aromatic guests.

A journey in search of single-walled metal–organic nanotubes

Single-walled metal–organic nanotubes (SWMONTs) represent a family of new, structured porous materials. Metal–organic nanotubes (MONTs) offer an attractive alternative to carbon nanotubes because cationic metal ions are incorporated into the backbones of MONTs. However, efforts regarding the preparation of metal–organic nanotubes have been few in number, compared with the focus on carbon nanotubes (CNTs) and synthetic nanotubes (SNTs). In particular, the preparation of single-walled metal–organic nanotubes (SWMONTs) remains largely unexplored. The goal of this feature article is to highlight synthetic strategies, the structural characteristics of this unique class of SWMONTs materials and explore possible applications.

Aggregation-Induced Emission Enhancement in Alkoxy-Bridged Binuclear Rhenium(I) Complexes: Application as Sensor for Explosives and Interaction with Microheterogeneous Media

The aggregation-induced emission enhancement (AIEE) characteristics of the two alkoxy-bridged binuclear Re(I) complexes [{Re(CO)3(1,4-NVP)}2(μ2-OR)2] (1, R = C4H9; 2, C10H21) bearing a long alkyl chain with 4-(1-naphthylvinyl)pyridine (1,4-NVP) ligand are illustrated. These complexes in CH2Cl2 (good solvent) are weakly luminescent, but their intensity increased enormously by almost 500 times by the addition of poor solvent (CH3CN) due to aggregation. By tracking this process via UV-vis absorption and emission spectral and TEM techniques, the enhanced emission is attributed to the formation of nanoaggregates. The nanoaggregate of complex 2 is used as a sensor for nitroaromatic compounds. Furthermore, the study of the photophysical properties of these binuclear Re(I) complexes in cationic, cetyltrimethylammonium bromide (CTAB), anionic, sodium dodecyl sulfate (SDS), and nonionic, p-tert-octylphenoxypolyoxyethanol (TritonX-100, TX-100), micelles as well as in CTAB-hexane-water and AOT-isooctane-water reverse micelles using steady-state and time-resolved spectroscopy and TEM analysis reveals that the nanoaggregates became small and compact size.

Giant metal–organic frameworks with bulky scaffolds: from microporous to mesoporous functional materials

New concepts on the design and synthesis of crystalline metal-organic frameworks (MOFs) have made them a subject of considerable interest in the growing field of materials science. By creating larger cavity sizes by a nearly infinite combination of metal nodes and organic linkers, many innovative characteristics of microporous MOFs have been revealed. The primary goal of this perspective article is to highlight the frontiers in the development of giant MOFs that are deliberately constructed from metallated or metal-free bulky scaffolds. Incorporating these types of distinct bulky ligands into giant MOFs may lead to MOFs with a large cavity size, intriguing properties and new framework topology. Emerging applications of these materials in catalysis, adsorption, and sensors are also discussed.

Highly emissive cyclometalated rhenium metallacycles: structure-luminescence relationship.

We report on a series of new self-assembled cyclometalated dirhenium(I) metallacyclic complexes via an unprecedented rhenium-mediated C-H bond activation and the relationship between their structures and luminescence properties.

Photoswitchable alkoxy-bridged binuclear rhenium(I) complexes – a potential probe for biomolecules and optical cell imaging

We describe the solvothermal synthesis, structural characterization, photophysics and potential applications as probes of two alkoxy-bridged binuclear Re(I) complexes, [{Re(CO)3(1,4-NVP)}2(μ2-OR)2] (1, R = C4H9; 2, R = C10H21; 1,4-NVP = 4-(1-naphthylvinyl)pyridine). Irradiation of 1 and 2 at 365 nm leads to an interesting trans-cis photoisomerization process, which was examined by 1H NMR, UV-vis, emission and time-resolved techniques. Compounds 1 and 2 exhibit photoswitchable luminescence enhancement arising from photoinduced intramolecular energy transfer from the 3MLCT state of the Re(I) chromophore to the triplet excited state of the ligand 1,4-NVP. In addition, these Re(I) complexes serve as excellent probes for the ultrasensitive detection of biological molecules, including bovine serum albumin (BSA) and a platelet derived growth factor (PDGF) binding aptamer. Our results also suggest that, since these Re(I) complexes have low cytotoxicity and fluorescence properties under physiological conditions, they could be a useful probe for optical imaging of cancer cells by confocal laser scanning microscopy.

Steric effects in the photoinduced electron transfer reactions of ruthenium(II)-polypyridine complexes with 2,6-disubstituted phenolate ions

The rate constants (kq) for the photoinduced electron transfer reactions of Ru(II)-polypyridyl complexes (Ru(NN)32+) with 2,6-disubstituted phenolate ions in aqueous acetonitrile are highly sensitive to change in the bulkiness of the ligand in Ru(NN)32+ as well as the phenolate ion. The decrease in kq value with the increase in the size of the ligand in Ru(NN)32+ and the phenolate ion is ascribed to the decrease in the electronic coupling matrix element, ∣HDA ∣, between the donor and acceptor with the increase in the electron transfer distance. The hydrophobic interaction or possible π–π stacking between the pyridine rings of Ru(NN)32+ and the aryl moiety of ArO− leads to less steric effect.

Alkoxy bridged binuclear rhenium (I) complexes as a potential sensor for β-amyloid aggregation

Alkoxy bridged binuclear rhenium(I) complexes are used as a probe for the selective and sensitive detection of aggregation of β-amyloid fibrils that are consorted with Alzheimers disease (AD). The strong binding of the complexes is affirmed by the fluorescence enhancement and calculated binding constant value in the order of 10(5)M(-1) is obtained from the Scatchard plots. The binding of β-amyloid can be attributed to π-π stacking interaction of naphthalene moiety present in rhenium(I) complexes, and it is supported by docking studies. The selectivity is quite high towards other proteins and the formation of fibrils can be observed in the range of 30-40 nm through the AFM and TEM techniques.

Electron transfer reactions of iron (III) - polypyridyl complexes with organic sulphides

Abstract The redox reactions of FeL 3 3+ (where L = 2,2 ′ -bipyridine, 4,4 ′ -dimethyl-2,2 ′ -bipyridine and 1,10-phenanthroline) with alkyl aryl sulphides in aqueous methanol have been studied by the spectrophotometric technique. The comparison of the experimental rate constant data with the values estimated from Marcus cross-reaction relationship led to the conclusion that the single electron transfer from sulphide to FeL 3 3+ is the rate controlling step. The kinetic data yield 1.0 × 10 7 M −1 s −1 for the ArSR/ArS + . R self-exchange rate constant at 25°C. The reactions of iron(III)-polypyridyl complexes with alkyl aryl sulphides in aqueous methanol proceed through electron transfer mechanism and form sulphoxides as products.

Micellar catalysis on the electron transfer reactions of iron(III)-polypyridyl complexes with organic sulfides—importance of hydrophobic interactions

The oxidation of organic sulfides with iron(III)-polypyridyl complexes [Fe(NN)3]3+ proceeds through an electron transfer mechanism and an increase in the methanol content in the methanol-water mixture favors the reaction. The reaction is catalyzed by both the anionic surfactant, sodium dodecyl sulfate (SDS) and the cationic surfactant, cetyltrimethylammonium bromide (CTAB). The micellar catalysis in the presence of SDS is accounted for in terms of strong binding of the cationic oxidant with the anionic surfactant and the development of positive charge on sulfur center of substrate in the transition state. The micellar catalysis observed on the reaction involving a trication, [Fe(NN)3]3+, in the presence of CTAB indicates the importance of hydrophobic interaction between the micelle and hydrophobic ligand of [Fe(NN)3]3+. The micellar catalysis is explained in terms of a pseudophase ion exchange model.