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K George Thomas

Publications and source records attributed to K George Thomas.

9 recordsLinked to original sources
PubMed2006

Photochemistry of ruthenium trisbipyridine functionalized on gold nanoparticles.

Design of nanohybrid systems possessing several ruthenium trisbipyridine (Ru(bpy)(3)(2+)) chromophores on the surface of gold nanoparticles, by adopting a place exchange reaction, was reported and their photophysical properties were tuned by varying the density of chromophores. The charge shift between the excited and ground-state Ru(bpy)(3)(2+) chromophores was reported for the first time, leading to the formation of Ru(bpy)(3)(+) and Ru(bpy)(3)(3+). Electron-transfer products were not observed on decreasing the concentration of Ru(bpy)(3)(2+) functionalized on Au nanoparticles or in a saturated solution of unbound chromophores. The close proximity of the chromophores on periphery of the gold core may lead to an electron transfer reaction and the products sustained for several nanoseconds before undergoing recombination, probably due to the stabilizing effect of the polar ethylene glycol moieties embedded between the chromophore groups.

Electrons↗
PubMed2006

Singlet and triplet excited-state interactions and photochemical reactivity of phenyleneethynylene oligomers.

The rigid rodlike character of phenyleneethynylenes and their ability to communicate charge/excitation energy over long distances have made them useful as molecular linkers in the light energy harvesting assemblies and molecular electronics devices. These linker molecules themselves possess rich photochemistry as evident from the relatively large yields of the excited singlet (0.5-0.66) and triplet (0.4-0.5) states of two model oligomers, 1,4-bis(phenylethynyl)-2,5-bis(hexyloxy)benzene (OPE-1) and 1,4-bis((4-phenylethynyl)phenylethynyl)-2,5-bis(hexyloxy)benzene (OPE-2). In particular, the long-lived triplet excited state is capable of undergoing deactivation by self-quenching processes such as ground-state quenching and triplet-triplet (T-T) annihilation. The T-T annihilation occurs with a nearly diffusion-controlled rate (approximately 2 x 10(9) M(-1) s(-1)), and ground-state quenching occurs with a rate constant of approximately 6 x 10(7) M(-1) s(-1). The electron transfer from the excited OPE-1 and OPE-2 to benzoquinone as characterized from the transient absorption spectroscopy illustrates the ability of these molecules to shuttle the electrons to acceptor moieties. In addition, pulse radiolysis experiments confirm the spectroscopic fingerprint of the cation radical (or "trapped hole") with absorption bands in the 500-600 nm region.

Alkynes↗
PubMed2006

Photophysical and theoretical investigations of oligo(p-phenyleneethynylene)s: effect of alkoxy substitution and alkyne-aryl bond rotations.

The unique photophysical, conformational, and electronic properties of two model phenyleneethynylene-based rigid rod molecular systems, possessing dialkoxy substitutions, are reported in comparison with an unsubstituted system. Twisting of the phenyl rings along the carbon-carbon triple bond is almost frictionless in these systems giving rise to planar as well as several twisted ground-state conformations, and this results in broad structureless absorption in the spectral region of 250-450 nm. In the case of 1,4-bis(phenylethynyl)benzene, a broad absorption band was observed due to the HOMO-LUMO transition, whereas dialkoxy-substituted compounds possess two well-separated bands. Dialkoxy substitution in the 2,5-position of the phenyl ring in phenyleneethynylenes alters its central arene pi-orbitals through the resonance interaction with oxygen lone pairs resulting in similar orbital features for HOMO and HOMO-1/HOMO-2. Electronic transition from the low-lying HOMO-1/HOMO-2 orbital to LUMO results in the high-energy band, and the red-shifted band originates from the HOMO-LUMO transition. The first excited-state transition energies at different dihedral angles, calculated by the TDDFT method, indicate that the orthogonal conformation has the highest excitation energy with an energy difference of 15 kcal/mol higher than the low-lying planar conformation. The emission of these compounds originates preferentially from the more relaxed planar conformation resulting in well-defined vibronic features. The fluorescence spectral profile and lifetimes were found to be independent of excitation wavelengths, confirming the existence of a single emitting species.

Acetates↗
PubMed2006

Gold nanorods to nanochains: mechanistic investigations on their longitudinal assembly using alpha,omega-alkanedithiols and interplasmon coupling.

Mechanistic investigations on the end-to-end assembly of Au nanorods to nanochains, in the presence of alpha,omega-alkanedithiols, were reported. A decrease in the longitudinal plasmon absorption was observed along with a concomitant formation of a new red-shifted band above a critical concentration of dithiol, which is attributed to the interplasmon coupling in assembled nanorods. However, no noticeable spectral changes were observed below the critical concentration, and the TEM studies indicate that the nanorods remain isolated and randomly distributed. This step is ascribed as an incubation step wherein one of the thiol groups of alpha,omega-alkanedithiol preferentially binds onto the edges of the nanorods, leaving the other thiol group free. Above the critical concentration, a chain up process proceeds through the interlocking of nanorods, initially to dimers and subsequently to oligomers, which results in longitudinal interplasmon coupling. The dimerization step follows second-order kinetics which deviates with time due to oligomerization. The rate constants for dimerization of nanorods possessing various dithiols and their energy of activation were determined. The large activation energy for the dimerization further confirms that the process is not diffusion but activation controlled.

Journal Article↗
PubMed2004

Self-assembled linear bundles of single wall carbon nanotubes and their alignment and deposition as a film in a dc field.

A one-step process of solubilization of single wall carbon nanotubes (SWCNT) in an organic solvent has enabled us to polarize them asymmetrically in a dc electric field. Quaternary ammonium ion-capped SWCNTs readily suspend in organic solvents; under the influence of a dc electric field, they assemble as stretched bundles anchored on the positive electrode. At low dc applied field (approximately 40 V), all of the SWCNTs from the suspension are deposited on the electrode, thus providing a simple methodology to design robust SWCNT films. At higher applied voltages (>100 V), the SWCNT bundles stretch out into the solution and orient themselves perpendicular to the electrode surface. The alignment of these bundles is responsive to the ON-OFF cycles of the applied electric field. The possibility of modulating the alignment of SWCNT in an electric field opens new ways to achieve electrical contacts in nano- to micro-devices.

Journal Article↗
PubMed2003

Dynamics of photoinduced electron-transfer processes in fullerene-based dyads: effects of varying the donor strength.

Two classes of fullerene-based donor-bridge-acceptor (D-B-A) systems containing donors of varying oxidation potentials have been synthesized. These systems include fullerenes linked to heteroaromatic donor groups (phenothiazine/phenoxazine) as well as substituted anilines (p-anisidine/p-toluidine). In contrast to the model compound, an efficient intramolecular electron transfer is observed from the fullerene singlet excited state in polar solvents. An increase in the rate constant and quantum yield of charge separation (kcs and phi cs) has been observed for both classes of dyads, with decrease in the oxidation potentials of the donor groups. This observation indicates that the rates of the forward electron transfer fall in the normal region of the Marcus curve. The long-lived charge separation enabled the characterization of electron transfer products, namely, the radical cation of the donor and radical anion of the pyrrolidinofullerene, by using nanosecond transient absorption spectroscopy. The small reorganization energy (lambda) of C60 coupled with large negative free energy changes (-delta G degree) for the back electron transfer places the back electron process in the inverted region of Marcus curve, thereby stabilizing the electron transfer products.

Journal Article↗
PubMed2003

Light-induced modulation of self-assembly on spiropyran-capped gold nanoparticles: a potential system for the controlled release of amino acid derivatives.

A photoswitchable double-shell structure on Au nanoparticles, consisting of photochromic spiropyran as the first shell, which regulates the assembly and release of an outer shell of amino acid derivatives upon irradiation, is being reported for the first time. The light-regulated changes in the topographic properties of spiropyran-capped Au nanoparticles (i.e., interconversion between the zwitterionic and neutral forms) are exploited for the assembly and release of amino acid-based therapeutic agents such as l-DOPA.

Amino Acids↗
PubMed2003

Chromophore-functionalized gold nanoparticles.

We report in this Account the design of chromophore-functionalized metal nanoparticles and the detailed investigation of the ground- and excited-state interactions between the metal nanoparticles and the photoactive molecules. The methodologies adopted for organizing chromophore-functionalized gold nanoparticles on two-dimensional surfaces and the mechanistic details of various deactivation channels of the photoexcited chromophores, such as energy and electron transfer to the metal nanoparticle, are presented. The possible applications of such chromophore-functionalized gold nanoparticles in photovoltaics, light-mediated binding and release of biologically important molecules such as amino acid derivatives, and fluorescent display devices are described.

Binding Sites↗