Karical R. Gopidas

Cu(II) Mediated Generation and Spectroscopic Study of the Tris(4-anisyl)amine Radical Cation and Dication. Unusually Shielded Chemical Shifts in the Dication

The reaction of tris(4-anisyl)amine (TAA) with Cu(2+) ion leading to formation of the TAA radical cation and dication is described. Spectroscopic studies confirm the formation of the radical cation and dication. (1)H and (13)C NMR spectral studies reveal interesting structural features of the dication.

Cu(II)-Mediated Generation of Triarylamine Radical Cations and Their Dimerization. An Easy Route to Tetraarylbenzidines

Triphenylamine (TPA) derivatives react with Cu2+ in acetonitrile to give TPA radical cations which undergo dimerization and deprotonation reactions to yield tetraphenylbenzidines (TPB). Synthetic utility of this reaction is demonstrated using several triphenylamine derivatives, and yields in excess of 80% are obtained in most cases. Involvement of the amine radical cations in these reactions was confirmed by ESR and absorption spectroscopic studies. A mechanism consistent with all observations is proposed. This study also revealed a very good correlation between the free energy change for radical cation formation and product yields.

Photocatalytic degradation of wastewater pollutants. Titanium-dioxide-mediated oxidation of polynuclear aromatic hydrocarbons☆

Polynuclear aromatic hydrocarbons, such as acenaphthene, anthracene, fluorene and naphthalene, were found to undergo efficient photocatalytic oxidation in aqueous suspensions of TiO2 on irradiation with artificial light as well as sunlight. The rate of photo-oxidation was monitored by following the loss of fluorescence and oxygen uptake in these systems. 2-Formyl-3-hydroxycinnamaldehyde (7) and 5,8-dihydroxy-2,4-naphthaquinone (13) from naphthalene and anthraquinone (17) from anthracene were isolated during photo-oxidation. A mechanism involving hydroxyl and superoxide radicals has been proposed for the formation of the different intermediates in these cases.

Synthesis and Characterization of Gold-Nanoparticle-Cored Dendrimers Stabilized by Metal–Carbon Bonds

Synthesis and characterization of gold-nanoparticle-cored dendrimers (NCDs), in which the dendrons are attached to the gold core through gold-carbon bonds, are described. Synthesis of these materials involved the simultaneous reduction of HAuCl(4) and a Fréchet-type dendron with a diazonium group at the focal point, all in an organic solvent such as toluene. These materials possess a nanometer-sized gold core surrounded by a shell of polyaryl ether dendrons, which are connected radially to the core. The NCDs were characterized by TEM, thermogravimetric analysis (TGA), and IR, UV, and NMR spectroscopic techniques. Average particle diameter of the NCDs ranged from 4.7 to 5.5 nm for the different generations. All NCDs exhibit the characteristic plasmon absorption of gold nanoparticles at 520 nm. Average numbers of dendrons per NCD in AuG(n) were calculated using results from TGA and TEM studies. Multiple layering of the dendrons is proposed as a possible reason for the high dendron/NCD value.

Photoinduced electron transfer in azatriangulenium salts

Abstract Fluorescence quenching and laser flash photolysis studies of stable azatriangulenium ions 2⊕–4⊕ are reported. The fluorescence quenching rate constants were correlated to ΔG0 values. The results suggest that 2⊕ is an excited state electron acceptor and 4⊕ is an excited state electron donor. 3⊕, on the other hand, can act as an acceptor or donor in photoinduced electron transfer (PET) reactions. Laser flash photolysis studies also supported these observations. These cations thus present an interesting case, where replacement of oxygen atoms by N-alkyl groups leads to a gradual shift of photoinduced electron transfer property from an acceptor to that of a donor.

Photophysics and photochemistry of phenosafranin dye in aqueous and acetonitrile solutions

Abstract The singlet and triplet excited state properties of phenosafranin dye were characterized in aqueous and acetonitrile solutions. The singlet excited state exhibited an absorption maximum at ∼ 430 nm and a lifetime of 930 ps in aqueous medium. The transient absorption spectra of the protonated and unprotonated forms of excited triplet were investigated by the laser flash photolysis technique. The protonated phenosafranin triplet had absorption maxima at 380 nm (ϵ = 11 000 M −1 cm −1 ), 615 nm (ϵ = 9700 M −1 cm −1 ) and 665 nm (ϵ = 10 800 M −1 ) and the unprotonated phenosafranin triplet had absorption maxima at 410 nm (ϵ = 8800 M −1 cm −1 ), 700 nm (ϵ = 6900 M −1 cm −1 ) and 790 nm (ϵ = 11 600 M −1 cm −1 ) in aqueous medium. The p K a of the protonated triplet phenosafranin was found to be 7.25. Diffusion-controlled quenching of the excited phenosafranin triplet by amines led to the reduction of the dye. The absorption spectra of the semireduced phenosafranin, which was characterized in acetonitrile, exhibited absorption maxima at 440 nm (ϵ = 29 000 M −1 cm −1 ) and 870 nm (ϵ = 19 000 M −1 cm −1 ).

Photoelectrochemistry in particulate systems. Photosensitized charge injection into opaque TiO2 semiconductor powder as probed by time-resolved diffuse reflectance laser flash photolysis☆

Abstract Fluorescence emission of phenosafranin dye was quenched when this dye was adsorbed on TiO 2 particles. Diffuse reflectance laser flash photolysis measurements have shown transient absorption corresponding to the cation radical of phenosafranin when the dye adsorbed on TiO 2 was excited with a 532 nm laser pulse. The process of charge injection into TiO 2 particles was further confirmed by comparing the results of the diffuse reflectance and emission experiments with those from the dye-coated silica particles. Dependence of relative efficiency of charge injection on the surface coverage of the dye has also been elucidated.

Photophysical and electron transfer studies of a stable carbocation

Photophysical and electron transfer properties of the stable trioxatriangulenium carbocation (1) are reported. Photophysical studies include absorption, fluorescence and phosphorescence spectra, singlet and triplet state quantum yields and lifetimes. Both the singlet and triplet excited states of 1 can accept an electron from donor molecules leading to the formation of the donor radical cation and the radical of 1. In aqueous solution, 1 can photo-oxidize DNA nucleosides such as guanosine and adenosine indicating that 1 may have potential use as a DNA cleaving agent.

Generation of Triarylamine Radical Cations through Reaction of Triarylamines with Cu(II) in Acetonitrile. A Kinetic Investigation of the Electron-Transfer Reaction

Triphenylamine derivatives react with Cu(2+) in acetonitrile to give radical cations, which subsequently undergo dimerization to provide tetraphenylbenzidine derivatives. Kinetic aspects of radical cation formation were examined by stopped-flow spectrophotometry. A broad range of triphenylamine derivatives were studied, and the driving force for the electron-transfer reaction ranged from +3.67 to -8.56 kcal M(-1) with rate constants varying from 1.09 x 10(2) to 2.15 x 10(5) M(-1) s(-1) for these systems. Reorganization energy for the electron-transfer reaction was estimated using experimentally determined activation parameters. Fitting of the rate data to the Marcus equation using different values of the electronic coupling matrix element H(el) provided a good fit with H(el) = 100 cm(-1) suggesting that electron transfer in the TPA/Cu(2+) system conforms to the Marcus-type electron transfer. Furthermore, the high reorganization obtained from these studies is consistent with significant bond cleavage in the transition state, and a mechanism consistent with the experimental data is proposed.

Unusual self-assembly of a hydrophilic β-cyclodextrin inclusion complex into vesicles capable of drug encapsulation and release

Formation of a hydrophilic β-cyclodextrin bis-inclusion complex and its self-assembly into supramolecular vesicles are described. The vesicles can be loaded with the anti-cancer drug doxorubicin and the loaded drug can be released upon addition of a competitive inclusion binder such as adamantane carboxylate.