N. Manoj

Vacuum-ultraviolet photochemically initiated modification of polystyrene surfaces: morphological changes and mechanistic investigations.

Vacuum-ultraviolet (VUV) irradiation (lambda(exc): 172 +/- 12 nm) of polystyrene films in the presence of oxygen produced not only oxidatively functionalized surfaces, but generated also morphological changes. Whereas OH- and C=O-functionalized surfaces might be used for e.g. secondary functionalization, enhanced aggregation or printing, processes leading to morphological changes open new possibilities of microstructurization. Series of experiments made under different experimental conditions brought evidence of two different reaction pathways: introduction of OH- and C=O-groups at the polystyrene pathways is mainly due to the reaction of reactive oxygen species (hydroxyl radicals, atomic oxygen, ozone) produced in the gas phase between the VUV-radiation source and the substrate. However, oxidative fragmentation leading to morphological changes, oxidation products of low molecular weight and eventually to mineralization of the organic substrate is initiated by electronic excitation of the polymer leading to C-C-bond homolysis and to a complex oxidation manifold after trapping of the C-centred radicals by molecular oxygen. The pathways of oxidative functionalization or fragmentation could be differentiated by FTIR-ATR analysis of irradiated polystyrene surfaces before and after washing with acetonitrile and microscopic fluorescence analysis of the surfaces secondarily functionalized with the N,N,N-tridodecyl-triaza-triangulenium (TATA) cation. Ozonization of the polystyrene leads to oxidative functionalization of the polymer surface but cannot initiate the fragmentation of the polymer backbone. Oxidative fragmentation is initiated by electronic excitation of the polymer (contact-mode AFM analysis), and evidence of the generation of intermediate C-centred radicals is given e.g. by experiments in the absence of oxygen leading to cross-linking (solubility effects, optical microscopy, friction-mode AFM) and disproportionation (fluorescence).

Photophysical and electron transfer studies of a few 2,6-dimethyl-4-(alkylphenyl)pyrylium and thiopyrylium derivatives

Abstract The photoinduced electron transfer properties of pyrylium salts 1a-d and the corresponding thiopyrylium derivatives 2a-d were investigated. Electron transfer from biphenyl (BP) and 4,4′-dimethoxystilbene to the singlet and triplet excited states of the salts led to efficient formation of the separated radical pairs. Using BP as donor, we have estimated the extinction coefficients and also the yields of the pyranyl/thiopyranyl radicals. The yields obtained were higher than those observed in the cases of similar cationic sensitizers. We have tried to correlate the radical yields to the triplet quantum yields and also to the stability of the pyranyl radicals. The high radical ion yields observed in these cases suggest that these compounds can be used as efficient sensitizers for photoinduced electron transfer reactions.

Inclusion complexation of a few pyrylium salts by β-cyclodextrin studied by fluorescence, NMR and laser flash photolysis

Inclusion complex formation between pyrylium salts 1–6 and β-cyclodextrin (β-CD) was investigated. Because of the positive charge present in the molecule, simple pyrylium salts are hydrophilic and do not show any tendency for encapsulation in β-CD. However, by tuning the hydrophobicity of the substituents present in the 4-position, one can prepare pyrylium salts, which are sufficiently hydrophobic to undergo encapsulation in β-CD cavities. Thus, 2–5 undergo encapsulation as is evident from their NMR and fluorescence spectra in the presence of β-CD. Fluorescence quantum yields of 2–5 were enhanced in the presence of β-CD and these enhancements were used to calculate the association constants by a Benesi–Hildebrand treatment. Electron transfer to the encapsulated pyrylium salts from a water-soluble donor was studied by fluorescence quenching and laser flash photolysis. The pyranyl radicals formed within the β-CD cavity as a result of electron transfer was found to be long-lived and this is attributed to the reduced rate of back electron transfer reaction achieved due to the protection afforded to the radical by the hydrophobic β-CD cavity.

Photophysical and electron-transfer properties of a few 2,6-dimethyl-4-arylpyrylium derivatives

Abstract Photophysical and electron transfer properties of pyrylium salts 1–4 were investigated. The photophysical properties include absorption and fluorescence spectra, quantum yields, fluorescence lifetimes and phosphorescence spectra. Laser flash photolysis of 1–4 in degassed dichloromethane led to the formation of transients assigned to the triplets of these molecules. The quantum yields of triplet formation and the extinction coefficients of the triplet–triplet absorption were determined. Electron transfers to the excited states of 1–4 from biphenyl were studied by fluorescence quenching and laser flash photolysis. The intermediate radical ions formed in the electron transfer reactions were characterized. In the case of 1–3, only the singlet excited states were involved in electron transfer reactions, whereas, in the case of 4, the singlet as well as the triplet excited states act as electron acceptors. Involvement of the triplet in the electron transfer reactions, led to higher Φ ion and lower k bet values in the case of 4. Thus, suitable modification of the structure of pyrylium salts can lead to molecules with very good photoelectron transfer properties.

Structure-photophysics correlation in a series of 2,6-dimethyl-4-arylpyrylium derivatives

Abstract Photophysical properties of a group of closely related pyrylium cations, 1–5, are reported. Our results indicate that these cations have two excited states with similar energies and that the substituent determines which of these has the lowest energy. For cations 1 and 2, the lowest excited state has TICT character. In the case of 3–5, the photophysical properties are better explained by invoking a planar excited state.

Triphenylpyrylium-salt-sensitized electron transfer oxygenation of furan derivatives. Product isolation, fluorescence quenching and laser flash photolysis studies

Abstract The 2,4,6-triphenylpyrylium-salt-sensitized photo-oxygenation of furans 1a–1c and 2 is reported. Furans 1a–1c give the corresponding enediones 4a–4c , whereas hardly any reaction is observed for 2 . Fluorescence quenching and laser flash photolysis studies indicate that the primary process is electron transfer from the furans to the singlet or triplet excited state of the pyrylium salt. The radical cations of the furans were characterized using laser flash photolysis studies. A mechanism consistent with the experimental observations is suggested for the photo-oxygenation reactions of these furans.