Biomedical subjects

P Thanasekaran

Publications and source records attributed to P Thanasekaran.

8 recordsLinked to original sources

PubMed2006

Gondola-shaped luminescent tetrarhenium metallacycles with crown-ether-like multiple recognition sites.

The self-assembly of gondola-shaped tetrarhenium metallacyclophanes was achieved in near quantitative yield from Re(CO)3 corners, a ditopic heterocyclic clip, and a bischelating-bridging unit using an orthogonal-bonding approach. The highly luminescent metallacycles contain crown-ether-like recognition sites, which are capable of selectively recognizing metal ions and planar aromatic molecules.

Binding Sites↗

PubMed2006

CH...pi interaction for rhenium-based rectangles: an interaction that is rarely designed into a host-guest pair.

Alkoxy- and thiolato-bridged Re(I) molecular rectangles [{(CO)3Re(mu-ER)2Re(CO)3}2(mu-bpy)2] (ER = SC4H9, 1a; SC8H17, 1b; OC4H9, 2a; OC12H25, 2b; bpy = 4,4'-bipyridine) exhibit strong interactions with several planar aromatic molecules. The nature of their binding was studied by spectral techniques and verified by X-ray diffraction analysis. Standard absorption and fluorescence titrations showed that a relatively strong 1:1 interaction occurs between aromatic guests such as pyrene and these rectangles. The results of a single-crystal X-ray diffraction analysis show that the recognition of 1 with a pyrene molecule is mainly due to CH...pi interactions and the face of the guest pyrene is located over the edges of the bpy linkers of 1. This is a fairly novel example of an interaction that is rarely designed into a host-guest pair. Furthermore, the interaction of 1 with Ag+ results in the self-organization of supramolecular arrays, as revealed by solid-state data.

Crystallography, X-Ray↗

PubMed2006

Photoluminescence electron-transfer quenching of rhenium(I) rectangles with amines.

Electron-transfer (ET) reactions from aromatic amines to excited states of rhenium(I)-based molecular rectangles [{Re(CO)3(mu-bpy)Br}{Re(CO)3(mu-L)Br}]2 (bpy = 4,4'-bipyridine, L = 4,4'-dipyridylacetylene (dpa), I; L = 4,4'-dipyridylbutadiyne (dpb), II; and L = 1,4-bis(4'-pyridylethynyl)benzene (bpeb), III) were investigated in a dichloromethane solution using luminescence quenching techniques. Direct evidence for the ET reaction was obtained from the detection of the amine cation radical in this system using time-resolved transient absorption spectroscopy. The values of the luminescence quenching rate constants, kq, of the 3MLCT excited state of Re(I) rectangles with amines were found to be higher than those for the monomeric Re(I) complexes and other Re(I)-based metallacyclophanes. The observed kq values were correlated well with the driving force (Delta G degrees) for the ET reactions. In addition, a semiclassical theory of ET was successfully applied to the photoluminescence quenching of Re(I) rectangles with amines.

Journal Article↗

PubMed2006

Self-assembly, reorganization, and photophysical properties of silver(I)-Schiff-base molecular rectangle and polymeric array species.

A self-assembly of AgClO(4) with a Schiff-base ligand N,N'-bis(pyridin-2-ylmethylene)benzene-1,4-diamine (1) gave a 1D zigzag polymeric array [[Ag(2)(C(18)H(14)N(4))(2)](ClO(4))(2)(CH(3)CN)](n) (3), while the self-assembly of AgClO(4) with 3,3'-dimethyl-N,N'-bis(pyridin-2-ylmethylene)biphenyl-4,4'-diamine (2) afforded the molecular rectangle [[Ag(2)(C(26)H(22)N(4))(2)](ClO(4))(2)] (4). The structures of 3 and 4 were characterized by single-crystal X-ray diffraction analysis. Structural data for 3 indicate that the Ag(I) ion is coordinated by two ligands of 1 in a distorted tetrahedral fashion thereby leading to a 1D zigzag polymeric array. The zigzag chains are interdigitated with weak pi-pi stacking interactions. The structure of 4 consists of a discrete molecular rectangle where the silver atom has a distorted square-planar coordination with the pyridyl ligands and azomethine nitrogen atoms of 2. An intramolecular pi-pi interaction between the phenyl rings of adjacent Schiff-base 2 functions to stabilize the rectangular architecture. The Ag(I)-Schiff-base coordination polymer 3 is not stable in solution. The degradation and reorganization of 3 to form a [2 x 2] grid architecture [[Ag(4)(C(26)H(22)N(4))(4)](ClO(4))(4)] (3g) was supported in a FAB-MS study. The rectangular structure of 4 remains intact in solution at ambient temperature. The complexes 3g and 4 exhibit unusual luminescence behavior in solution at room temperature with significantly red-shifted emission in the visible region.

Journal Article↗

PubMed2005

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.

Journal Article↗

PubMed2003

Synthesis and photophysical properties of neutral luminescent rhenium-based molecular rectangles.

A series of neutral luminescent molecular rectangles [[Re(CO)(3)(mu-bpy)Br][Re(CO)(3)(mu-L)Br]](2) (1-4) having fac-Re(CO)(3)Br as corners and 4,4'-bipyridine (bpy) as the bridging ligand on one side and other bipyridyl ligands of varying length (L) on the other side have been synthesized and characterized. The crystal structure of 1 shows a rectangular cavity with the dimensions of 11.44 x 7.21 A. When the cavity size is tuned from 1 to 4, a dimension of 11.4 x 20.8 A could be achieved, as revealed by the molecular modeling. These rectangles exhibit luminescence in solution at room temperature. In particular, compound 4 containing 1,4-bis(4'-pyridylethynyl)benzene (bpeb) as bridging ligand shows the excited-state lifetime of 495 ns. Fine-tuning of the cavity size of the rectangles improves their excited-state properties. These properties facilitate the study of excited-state electron-transfer reactions with electron acceptors and donors and host-guest binding. Crystallographic information: 1.6CH(3)COCH(3) is monoclinic, P2(1)/c, with a = 12.0890(2), b = 24.2982(2), and c = 12.8721(2) A, beta = 107.923(1) degrees, and Z = 2.

Journal Article↗

PubMed2003

Self-assembly of octarhenium-based neutral luminescent rectangular prisms.

Self-assembly of rhenium-based nanoscale rectangular prismatic boxes has been achieved in quantitative conversion. The fac-rhenium corner provided three mutually perpendicular coordination sites and served as a good candidate for the construction of 3-D boxes. These are the first Re-based, neutral, luminescent prisms of M(8)L(2)L'(8) type that has been characterized crystallographically. Their luminescent properties and molecular recognition capabilities make these molecular prisms interesting supramolecules.

Journal Article↗

PubMed2002

Luminescence enhancement induced by aggregation of alkoxy-bridged rhenium(I) molecular rectangles.

Self-assembly of rhenium(I)-based molecular rectangles containing long alkyl chains has been achieved in one-pot synthesis by solvothermal methods. An enormous enhancement in the emission intensity, quantum yield, and lifetime of the rectangles has been observed when the solvent medium is changed from organic to aqueous. Addition of water favors the aggregation of Re(I) molecular rectangle resulting in the luminescence enhancement, and this phenomenon has been traced out using light scattering techniques.

Journal Article↗