The hydrogen-terminated surface of porous silicon (PS) is sufficiently reactive for the uncatalyzed hydrosilation of alkenes and alkynes. These modifications produce dramatic changes to both the physical and chemical properties of the PS.

Solvent effects on the spectroscopic properties of lumichromes and lumiflavins are presented. Fluorescence yields for lumiflavins are an order of magnitude larger than those for lumichromes, due to their lower nonradiative rate constants. Solvent effects on the absorption and emission band positions are explained on the basis of hydrogen-bonding interactions. TD-DFT calculations predicted that the lowest energy states are n,π* in the case of lumichromes, but π,π* in the case of the lumiflavins. The overall consistency of the predicted singlet-singlet and triplet-triplet transitions obtained for the various compounds studied, and the good correspondence between the predicted and measured transitions, indicate that the techniques applied provide an accurate description of the spectral properties of lumiflavins and lumichromes. The measured singlet oxygen yields have shown the lumichromes to be efficient singlet oxygen sensitizers.

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The production and decay of the anthracene radical cation on silica gel has been studied using nanosecond time-resolved diffuse reflectance laser flash photolysis. The production of the radical cation has been shown to be via a multiphoton process both by a laser dose study and by millisecond flashlamp experiments. The decay kinetics of the radical cation conform well to an analysis based on geminate recombination at loadings of less than 2 µmol g -1 . At higher loadings, deviations from these kinetics are observed caused by bulk electron diffusion competing efficiently with geminate recombination. Addition of electron donors such as triphenylamine, N,N,N′,N′-tetramethyl-1,4-phenylenediamine, N,N-dimethylaniline, and azulene greatly accelerate the rate of radical ion decay via an electron transfer mechanism. Kinetic analysis reveals that the observed decay can be described by either the dispersive kinetic model of Albery et al. or a fractal dimensional rate constant model of the type which has been used to describe triplet-triplet annihilation on surfaces. The bimolecular rate constants vary considerably and do not show a simple dependence on the free energy for electron transfer. This can be explained either on the basis of bulk diffusion of electron donors being slow relative to the electron transfer process, or by the presence of a Marcus "inverted region" at relatively modest negative free energy values.

Leicestershire, U K LEZZ 3TU Central Laser Facility, Rutherford Appleton Laboratory, Didcot, Oxfordshire, U K O X 1 2 OQX The photochemical formation of the open merocyanine forms of several naphthoxazine-spiro-indolines in different solvents have been studied using both picosecond transient absorption (PTA) and picosecond time-resolved resonance Raman (PTR3) methods.The PTA studies have established the presence of several metastable species in the photochemical formation of the coloured merocyanine form of these photochromic compounds. The primary photochemical step occurs on the sub-ps timescale and is followed by the formation of a cisoid intermediate over the next 6-30 ps. This cisoid species then isomerises to the equilibrated distribution of transoid isomers of the merocyanine form with a lifetime that is dependent upon both solvent viscosity and polarity as well as the nature of the substituents on the naphthalene part of the molecule. However, the rate of this cis --+ trans isomerisation is unaffected on changing the N-alkyl group on the indoline part of the molecule from a methyl to an isobutyl group. The PTR3 studies have demonstrated that, in butan-1-01, there are at least three different transient species with characteristic vibrational spectra which evolve with different lifetimes to give the final isomeric distribution over the first few ns of the reaction. In cyclohexane it is likely that the formation of a single species is being probed, which is fully developed after the first 200 ps of the reaction. This single species has a spectrum which is the same as the equilibrated steady-state resonance Raman merocyanine spectrum. It is likely that the evolution in the PTR3 spectra obtained here, in butan-1-01, results from an equilibration of initially formed transoid merocyanine isomers to give a more stable distribution in this polar hydrogen-bonded solvent.Such an equilibration appears to be unnecessary in a non-polar solvent such as cyclohexane and it is suggested that this is because the transoid isomer, initially formed, is already in its most stable form.Naphthoxazine-spiro-indoline (NOSI) compounds that comprise two heterocyclic moieties connected by a tetrahedral spiro linkage represent an important class of photochromic compounds which have greatly improved resistance to prolonged UV irradiation compared with, for example, the structurally related spiro-pyrans. 'L' Both classes of compounds undergo a photoisomerisation reaction following irradiation in the UV region of the spectrum, which causes a breaking of the carbon-oxygen bond at the spiro linkage. Subsequent rotation about the carbon-carbon and carbon-nitrogen bonds, which remain joining together the two parts of the molecule, forms a transoid, more planar-type, structure which has a structure resembling that of a merocyanine dye, and absorbs in the visible region of the spectrum.2 Quinoid, zwitterion and even biradical structures have been proposed for the open merocyanine forms. There are several near-planar isomers arising from rotatio...

Photophysical properties for a number ruthenium(II) and osmium(II) bipyridyl complexes are reported in dilute acetonitrile solution. The lifetimes of the excited metal to ligand charge transfer states (MLCT) of the osmium complexes are shorter than for the ruthenium complexes. Rate constants, kq, for quenching of the lowest excited metal to ligand charge transfer states by molecular oxygen are found to be in the range (1.1-7.7) x 10(9) dm3 mol(-1) s(-1). Efficiencies of singlet oxygen production, fDeltaT, following oxygen quenching of the lowest excited states of these ruthenium and osmium complexes are in the range of 0.10-0.72, lower values being associated with those compounds having lower oxidation potentials. The rate constants for quenching of the excited MLCT states, kq, are found to be generally higher for osmium complexes than for ruthenium complexes. Overall quenching rate constants, kq were found to give an inverse correlation with the energy of the excited state being quenched, and also to correlate with the oxidation potentials of the complexes. However, when the contribution of quenching due exclusively to energy transfer to produce singlet oxygen, kq1, is considered, its dependence on the energy of the excited states is more complex. Rate constants for quenching due to energy dissipation of the excited MLCT states without energy transfer, kq3, were found to show a clear correlation with the oxidation potential of the complexes. Factors affecting both the mechanism of oxygen quenching of the excited states and the efficiency of singlet oxygen generation following this quenching are discussed. These factors include the oxidation potential, the energy of the lowest excited state of the complexes and spin-orbit coupling constant of the central metal.

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