Ashoka G. Samuelson

Schiff base linked ferrocenyl complexes for second-order nonlinear optics

Abstract A series of substituted ferrocenyl compounds where one of the cyclopentadienyl rings is linked to an aromatic Schiff base, have been synthesized and analyzed for their second-order nonlinearity (β). Two photon fluorescence corrected β, of these complexes correlates well with the electron withdrawing nature of the substituted benzene ring. The well-known two-state model has been invoked to rationalize the observed values of the first hyperpolarizability, β, of these complexes. The metal to ligand charge transfer (MLCT) transition dominates their second-order response. These compounds form charge transfer (CT) complexes with acceptors such as iodine, p-chloranil (CA), 2, 3-dichloro-5, 6-dicyano-1, 4-benzoquinone (DDQ), tetracyanoethylene (TCNE), and 7, 7, 8, 8-tetracyanoquinodimethane (TCNQ). The CT complexes exhibit much higher second-order response. A series of bisferrocenyl complexes where two ferrocene moieties are linked through the same aromatic Schiff base spacer has also been synthesized and characterized. The β values of the bisferrocenyl complexes and their CT counterparts are much higher than the corresponding monoferrocene complexes. In all these compounds there is a strong resonant contribution to β due to the MLCT transition around 532 nm. The dispersion free hyperpolarizability, β0 of these complexes have also been calculated using the two-state model.

Cytotoxicity of half sandwich ruthenium(II) complexes with strong hydrogen bond acceptor ligands and their mechanism of action

Neutral and cationic organometallic ruthenium(II) piano stool complexes of the type [(eta(6)-cymene)RuCl(X)(Y)] (complexes R1-R8) has been synthesized and characterized. In cationic complexes, X, Y is either a eta(2) phosphorus ligand such as 1,1-bis(diphenylphosphino)methane (DPPM) and 1,2-bis(diphenylphosphino)ethane (DPPE) or partially oxidized ligands such as 1,2-bis(diphenylphosphino)methane monooxide (DPPMO) and 1,2-bis(diphenylphosphino)ethane monooxide (DPPEO) which are strong hydrogen bond acceptors. In neutral complexes, X is chloride and Y is a monodentate phosphorous donor. Complexes with DPPM and DPPMO ligands ([(eta(6)-cymene)Ru(eta(2)-DPPM)Cl]PF(6) (R2), [(eta(6)-cymene)Ru(eta(2)-DPPMO)Cl]PF(6) (R3), [(eta(6)-cymene)Ru(eta(1)-DPPM)Cl(2)] (R5) and [(eta(6)-cymene)Ru(eta(1)-DPPMO)Cl(2)] (R6) show good cytotoxicity. Growth inhibition study of several human cancer cell lines by these complexes has been carried out. Mechanistic studies for R5 and R6 show that inhibition of cancer cell growth involves both cell cycle arrest and apoptosis induction. Using an apoptosis PCR array, we identified the sets of anti-apoptotic genes that were down regulated and pro-apoptotic genes that were up regulated. These complexes were also found to be potent metastasis inhibitors as they prevented cell invasion through matrigel. The complexes were shown to bind DNA in a non intercalative fashion and cause unwinding of plasmid DNA in cell-free medium by competitive ethidium bromide binding, viscosity measurements, thermal denaturation and gel mobility shift assays.

Quadratic nonlinearity of one- and two-electron oxidized metalloporphyrins and their switching in solution.

We report the quadratic nonlinearity of one- and two-electron oxidation products of the first series of transition metal complexes of meso-tetraphenylporphyrin (TPP). Among many MTPP complexes, only CuTPP and ZnTPP show reversible oxidation/reduction cycles as seen from cyclic voltammetry experiments. While centrosymmetric neutral metalloporphyrins have zero first hyperpolarizability, beta, as expected, the cation radicals and dications of CuTPP and ZnTPP have very high beta values. The one- and two-electron oxidation of the MTPPs leads to symmetry-breaking of the metal-porphyrin core, resulting in a large beta value that is perhaps aided in part by contributions from the two-photon resonance enhancement. The calculated static first hyperpolarizabilities, beta0, which are evaluated in the framework of density functional theory by a coupled perturbed Hartree-Fock method, support the experimental trend. The switching of optical nonlinearity has been achieved between the neutral and the one-electron oxidation products but not between the one- and the two-electron oxidation products since dications that are electrochemically reversible are unstable due to the formation of stable isoporphyrins in the presence of nucleophiles such as halides.

Ferrocenyl donor-organic acceptor complexes for second order nonlinear optics

Abstract First hyperpolarizability ( β ) of a set of donor–acceptor complexes derived by functionalizing two organic acceptors with a donor such as ferrocene or an aromatic ring through a –CN– linkage has been measured by hyper-Rayleigh scattering in solution. Spectral analysis of the second harmonic scattered light reveals that there is a significant two-photon fluorescence (TPF) contribution to the total signal in some compounds. In fact, the TPF contribution was as high as ∼25% in two compounds. The measured β values after correcting for TPF indicate that these molecules possess high second order nonlinearity, much larger than similar organic compounds or charge transfer complexes formed by ferrocenyl donors and organic acceptors. The dispersion free β values of these complexes have been calculated using two different methods depending on the nature of the complex. The conventional nonresonant two-state model which gives a good estimate of the intrinsic second order polarizability away from resonance, has been applied to compounds with no absorption around the second harmonic wavelength. Another model in which a damping parameter related to the inhomogeneous line width of the absorption band near the second harmonic frequency is used for obtaining reliable values of β 0 , has been employed for the other compounds. In conjugation with anthraquinone as acceptor, ferrocene as a donor appears to be a better choice for quadratic nonlinear optics applications than conventional organic donors.

Non-bonding interactions of anions with nitrogen heterocycles and phenyl rings: a critical Cambridge Structural Database analysis

A study of supramolecular interactions of nitrogen heterocyclics: pyridine, bipyridine, phenanthroline and terpyridine with anions: NO3−, ClO4−, BF4−and PF6− has been carried out using the Cambridge Structural Database (CSD). A significant number of short contacts (within the sum of Van der Waals radii of the interacting atoms) is shown to exist in heterocyclic ring systems. The results are compared with interactions of anions with a phenyl ring in order to evaluate the effect of a heteroatom. The most electron deficient center is the α-carbon atom and shows the maximum number of short contacts. In general, most contacts are weak C–H⋯O and C–H⋯F hydrogen bonds, but there are also some observations of O/F⋯π interactions, which are virtually absent for simple phenyl groups.

Anticancer Activity of Hydrogen‐Bond‐Stabilized Half‐Sandwich RuII Complexes with Heterocycles

Neutral half-sandwich organometallic ruthenium(II) complexes of the type [(η(6)-cymene)RuCl(2)(L)] (H1-H10), where L represents a heterocyclic ligand, have been synthesized and characterized spectroscopically. The structures of five complexes were also established by single-crystal X-ray diffraction confirming a piano-stool geometry with η(6) coordination of the arene ligand. Hydrogen bonding between the N-H group of the heterocycle and a chlorine atom attached to Ru stabilizes the metal-ligand interaction. Complexes coordinated to a mercaptobenzothiazole framework (H1) or mercaptobenzoxazole (H6) showed high cytotoxicity against several cancer cells but not against normal cells. In vitro studies have shown that the inhibition of cancer cell growth involves primarily G1-phase arrest as well as the generation of reactive oxygen species (ROS). The complexes are found to bind DNA in a non-intercalative fashion and cause unwinding of plasmid DNA in a cell-free medium. Surprisingly, the cytotoxic complexes H1 and H6 differ in their interaction with DNA, as observed by biophysical studies, they either cause a biphasic melting of the DNA or the inhibition of topoisomerase IIα activity, respectively. Substitution of the aromatic ring of the heterocycle or adding a second hydrogen-bond donor on the heterocycle reduces the cytotoxicity.

Solid state structural and solution studies on the formation of a flexible cavity for anions by copper(I) and 1,2-bis(diphenylphosphino)ethane

Copper(l) complexes of 1,2-bis(diphenylphosphino)ethane (dppe) with a stoichiometry Cu-2(dppe)(3)(X)(2) [X- = CN- (1), SCN- (2), NO3- (3)] are obtained from direct reactions of CuX and dppe. The complexes are structurally and spectroscopically (NMR and IR) characterized. The structure of the [Cu-2(dPPe)(3)](2+) dication is similar to the structural motif observed in many other complexes with a chelating dppe and a bridging dppe connecting two copper centers. In complexes 1 -3, the anions are confined to the cavity formed by the phosphines which force a monodentate coordination mode despite the predominant bidentate/bridging character of the anions. The coordination angles rather than the thermochemical radii dictate the steric requirement of anions. While the solution behavior of 3, with nitrate, is similar to complexes studied earlier, complexes with pseudohalides exhibit new solution behavior

Thiocarbonyl to carbonyl group transformation using CuCl and NaOH

A simple and efficient procedure for the rapid and mild conversion of thiocarbonyls to carbonyls is described.

Allylic amination promoted by copper

A mild method for the synthesis of allylic amines by using a mixture of copper(II)perchlorate and copper metal is described.

The first bis(phosphine) monoxide (BPMO) complexes of copper(I)

Copper(I) complexes of bis(phosphine) monoxide ligands, bis(diphenylphosphino)ethane monoxide (dppeo) and bis(diphenylphosphino)methane monoxide (dppmo) have been prepared and characterized. One of the complexes with dppeo was characterized by X-ray crystal structure analysis confirming Cu(I) coordination to hard and soft donors. The stability of these complexes in solution was probed via spectroscopic and electrochemical studies. Copper(I) is more readily oxidized in the presence of the hard O donor ligands. In solution, they readily exchange the hard donor O, for soft ligands. Although copper(I) prefers soft ligands and is more stable towards oxidation in their presence, it coordinates to hard donors when there is electrostatic or an entropy driven advantage.