Micellar control of the photooxidation pathways of 10-methyl phenothiazine: electron versus energy transfer mechanisms.

10-Methyl phenothiazine (MPS) was chosen as a model compound to investigate the effects of compartmentalisation and of charged interfaces on the primary mechanisms involved in the phototoxic reactions related to phenothiazine drugs. Two most important pathways resulting from the interaction of the triplet excited state of MPS ((3)MPS*) with molecular oxygen ((3)O2) have to be considered: (i) energy transfer producing singlet oxygen ((1)O2) and (ii) electron transfer generating the superoxide anion (O2˙(-)) and the radical cation (MPS˙(+)). The quantum yields of (1)O2 production by MPS solubilized in the dispersed pseudo-phase of aqueous micellar systems were found to be similar to those determined in solvents of various polarities, regardless of the anionic or cationic nature of the surfactant (SDS or CTAC). However, micellar compartmentalisation and surfactant charge affect considerably both the sensitized and the self-sensitized photooxidation of MPS. The formation of 10-methyl phenothiazine sulfoxide (MPSO), produced by the reaction of MPS with (1)O2, proceeds at a higher rate in SDS micelles than in neat polar solvents. This result may be explained by the protonation of the zwitterionic intermediate Z (MPS(+)OO(-)) at the micellar interface to yield the corresponding cation C (MPS(+)OOH) that is stabilized in the negatively charged micelles and reacts much faster with MPS than Z to yield MPSO. The electron transfer reaction from (3)MPS* to O2 yielding MPS˙(+) and O2˙(-) is also enhanced in SDS micelles, as back electron transfer (BET) is prevented by ejection of O2˙(-) to the aqueous bulk phase and stabilization of MPS˙(+) in the anionic micelles. The size of the SDS micelles modulates the relative contribution of each pathway (formation of MPSO or MPS˙(+)) to the overall conversion of MPS to its oxidation products. Photooxidation of MPS in cationic micelles is a very slow process, as the formation of neither C nor MPS˙(+) is favoured in positively charged micelles.

Direct grafting of long-lived luminescent indicator dyes to GaN light-emitting diodes for chemical microsensor development.

Two new methods for covalent functionalization of GaN based on plasma activation of its surface are presented. Both of them allow attachment of sulfonated luminescent ruthenium(II) indicator dyes to the p- and n-type semiconductor as well as to the surface of nonencapsulated chips of GaN light-emitting diodes (blue LEDs). X-ray photoelectron spectroscopy analysis of the functionalized semiconductor confirms the formation of covalent bonds between the GaN surface and the dye. Confocal fluorescence microscopy with single-photon-timing (SPT) detection has been used for characterization of the functionalized surfaces and LED chips. While the ruthenium complex attached to p-GaN under an oxygen-free atmosphere gives significantly long mean emission lifetimes for the indicator dye (ca. 2000 ns), the n-GaN-functionalized surfaces display surprisingly low values (600 ns), suggesting the occurrence of a quenching process. A photoinduced electron injection from the dye to the semiconductor conduction band, followed by a fast back electron transfer, is proposed to be responsible for the excited ruthenium dye deactivation. This process invalidates the use of the n-GaN/dye system for sensing applications. However, for p-GaN/dye materials, the luminescence decay accelerates in the presence of O(2). The moderate sensitivity is attributed to the fact that only a monolayer of indicator dye is anchored to the semiconductor surface but serves as a demonstrator device. Moreover, the luminescence decays of the functionalized LED chip measured with excitation of either an external (laser) source or the underlying LED emission (from p-GaN/InGaN quantum wells) yield the same mean luminescence lifetime. These results pave the way for using advanced LEDs to develop integrateable optochemical microsensors for gas analysis.

Microsensors based on GaN semiconductors covalently functionalized with luminescent Ru(II) complexes.

Covalent tethering of a Ru(II) dye to gallium nitride surfaces has been accomplished as a key step in the development of innovative sensing devices in which the indicator support (semiconductor) plays the role of both support and excitation source. Luminescence emission decays and time-resolved emission spectra confirm the presence of the dye on the semiconductor surfaces, while X-ray photoelectron spectroscopy proves its covalent bonding. The O(2) sensitivity of the new device is comparable to those of other ruthenium-based sensor systems. This achievement paves the way to a new generation of integrable ultracompact microsensors that combine semiconductor emitter-probe assemblies.

Relationship between the microscopic and macroscopic world in optical oxygen sensing: a luminescence lifetime microscopy study.

An investigation based on confocal fluorescence lifetime imaging microscopy (FLIM) of silica-loaded silicone films doped with a molecular oxygen-sensitive ruthenium(II) polyazaheterocyclic complex is presented. The effect of the silica type (hydrophilic/hydrophobic), particle size and amount of silica filler on the luminescence decay of the immobilized indicator dye has thoroughly been studied. A higher amount of hydrophilic silica leads to both a higher solubility of molecular oxygen into the silicone film and to higher levels of the metal indicator dye. Thus, incorporation of 10% (by wt) pyrogenic silica into silicone shortens the mean luminescence lifetime from 1.4 to 0.9 micros. However, an excess of filler may lead to overloading of the dye into the film producing new phenomena such as triplet-triplet annihilation and excitation energy homotransfer, as observed from their influence on the emission lifetime of the metal complex. Those phenomena do not take place when trimethylated silica (hydrophobic filler) is used. In this case, no increase on the oxygen or dye concentration is observed after addition of the filler and no significant reduction of the luminescence lifetime is measured. Both the addition of silica and the possible precipitation of dye crystals lead to the appearance of microdomains where the molecular probe exhibits widely different excited state lifetimes. For the first time, such microdomains within the oxygen sensing layer are visualized and analyzed by means of FLIM, showing the potential of this technique and the usefulness of our conclusions to the future design and development of novel luminescent oxygen sensor films for environmental and process analysis.

The spin chemistry and magnetic resonance of H2@C60. From the Pauli principle to trapping a long lived nuclear excited spin state inside a buckyball.

One of the early triumphs of quantum mechanics was Heisenberg's prediction, based on the Pauli principle and wave function symmetry arguments, that the simplest molecule, H(2), should exist as two distinct species-allotropes of elemental hydrogen. One allotrope, termed para-H(2) (pH(2)), was predicted to be a lower energy species that could be visualized as rotating like a sphere and possessing antiparallel ( upward arrow downward arrow) nuclear spins; the other allotrope, termed ortho-H(2) (oH(2)), was predicted to be a higher energy state that could be visualized as rotating like a cartwheel and possessing parallel ( upward arrow upward arrow) nuclear spins. This remarkable prediction was confirmed by the early 1930s, and pH(2) and oH(2) were not only separated and characterized but were also found to be stable almost indefinitely in the absence of paramagnetic "spin catalysts", such as molecular oxygen, or traces of paramagnetic impurities, such as metal ions. The two allotropes of elemental hydrogen, pH(2) and oH(2), may be quantitatively incarcerated in C(60) to form endofullerene guest@host complexes, symbolized as pH(2)@C(60) and oH(2)@C(60), respectively. How does the subtle difference in nuclear spin manifest itself when hydrogen allotropes are incarcerated in a buckyball? Can the incarcerated "guests" communicate with the outside world and vice versa? Can a paramagnetic spin catalyst in the outside world cause the interconversion of the allotropes and thereby effect a chemical transformation inside a buckyball? How close are the measurable properties of H(2)@C(60) to those computed for the "quantum particle in a spherical box"? Are there any potential practical applications of this fascinating marriage of the simplest molecule, H(2), with one of the most beautiful of all molecules, C(60)? How can one address such questions theoretically and experimentally? A goal of our studies is to produce an understanding of how the H(2) guest molecules incarcerated in the host C(60) can "communicate" with the chemical world surrounding it. This world includes both the "walls" of the incarcerating host (the carbon atom "bricks" that compose the wall) and the "outside" world beyond the atoms of the host walls, namely, the solvent molecules and selected paramagnetic molecules added to the solvent that will have special spin interactions with the H(2) inside the complex. In this Account, we describe the temperature dependence of the equilibrium of the interconversion of oH(2)@C(60) and pH(2)@C(60) and show how elemental dioxygen, O(2), a ground-state triplet, is an excellent paramagnetic spin catalyst for this interconversion. We then describe an exploration of the spin spectroscopy and spin chemistry of H(2)@C(60). We find that H(2)@C(60) and its isotopic analogs, HD@C(60) and D(2)@C(60), provide a rich and fascinating platform on which to investigate spin spectroscopy and spin chemistry. Finally, we consider the potential extension of spin chemistry to another molecule with spin isomers, H(2)O, and the potential applications of the use of pH(2)@C(60) as a source of latent massive nuclear polarization.

Nonradiative deactivation of singlet oxygen ((1)O2) by cubane and its derivatives.

The bimolecular quenching rate constants of singlet oxygen ((1)O2) by cubane and cubane derivatives were determined and found to be in the order of 10(3)-10(4) M(-1) s(-1). These values represent larger values than expected for aliphatic alkanes as a model for C-H vibrational deactivation. This result is explained by the occurrence of two different deactivation mechanisms: energy transfer to cubane C-H vibrational modes and the formation of a charge-transfer complex between (1)O2 and cubane ((1)O2(*-)...cubane(*+)).

Can H2 inside C60 communicate with the outside world?

The quenching rate constants of singlet oxygen by C60, H2@C60, D2@C6o, H2, and D2 in solution were measured. The presence of a hydrogen (H2@C60) or deuterium (D2@C60) molecule inside the fullerene did not produce any observable effect based on triplet lifetime or EPR measurements. However, a remarkable effect was found for the 1O2 quenching by C60, H2@C60, D2@C6o, H2, and D2. Singlet oxygen was generated by photosensitization or by thermal decomposition of naphthalene endoperoxide derivatives. Comparison of the rate constants for quenching of 1O2 by H2@C60 and D2@C60 demonstrates a significant vibrational interaction between oxygen and H2 inside the fullerene. The quenching rate constant for H2 is 1 order of magnitude higher than that of D2, in agreement with the results observed for the quenching of 1O2 with H2@C60 or D2@C60.

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).

Vacuum-ultraviolet photochemically initiated modification of polystyrene surfaces: chemical changes.

Fourier-Transform infrared (FTIR) spectroscopy and surface energy analysis (contact angle measurements) have been performed as a means of identification and quantification of the functionalization of polystyrene surfaces upon vacuum ultraviolet- (VUV-) photochemically initiated oxidation. Photochemical oxidation was performed in the presence of water vapor and molecular oxygen using a pulsed Xe2-excimer radiation source (lambda(exc): 172 nm). Surface oxidation was studied as a function of two parameters: irradiation time and distance between sample and radiation source. During the first 1-2 min of irradiation, an increase of the concentrations of hydroxyl (OH) and carbonyl (C=O) groups on the surface was observed, both reaching limiting values. As expected, the rate of oxidation diminished exponentially with increasing distance between the radiation source and the surface of the polystyrene film. Changes in the surface energy due to the introduction of these polar (i.e. OH and C=O) groups were also determined. The densities of the functional groups decreased upon washing with acetonitrile, and analysis of the washing solution by means of gas chromatography-mass spectrometry (GC-MS) revealed the presence of a large number of products. The application of pulsed Xe2-excimer radiation sources as a valuable alternative to conventional means (i.e. laser and plasma) for the photochemical oxidation and surface modification of polystyrene is discussed.