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During the oxidative degradation of 2,4-dimethylaniline (2,4-xylidine) by means of the H2O2/UV method, a series of hydroxylated aromatic amines are formed, this result confirming the role of the hydroxyl radical as an initiator of the oxidative chain reaction. Thermal or photochemically enhanced Fenton reactions in the presence of 2,4-dimethylaniline (2,4-xylidine) yield primarily 2,4-dimethylphenol as an intermediate product, the genesis of which may only be explained by an electron transfer mechanism. Experimental evidence for such a mechanism is presented, and values for the quantum yields of the photochemically enhanced reduction of iron(III) to iron(II) in aqueous solutions of 2,4-xylidine are given.

(1 5. I. 96) In the context of our studies on ruthenium(I1) complexes containing polyazaheterocyclic ligands, we have determined the rate constants of quenching by molecular oxygen (k,) of the metal-to-ligand charge-transfer-excited state of a series of homoleptic [RuL,] complexes (where L stands for 2,2'-bipyridine (bpy), I,l0-phenanthroline (phen), 2,2'-bipyrazine (bpz), 4,7-diphenyI-l,lO-phenanthroline (dip), diphenyl-l,IO-phenanthroline-4,7-disulfonate (dpds), and l,l0-phenanthroline-5-octadecanamide (poda)) in H20 and in MeOH. These compounds are singlet-oxygen ( 0 2 ( ' A g ) ) sensitizers, and quantum yields of singlet-oxygen production (Gd) in both solvents are also Ag)), the lowest electronically excited state of the oxygen molecule, is a highly reactive species and a key intermediate in chemical and biological processes, such as photooxidation reactions, DNA damage, and photodynamic therapy of cancer [l-51. Most often, singlet oxygen is produced by photosensitization involving electronic-energy transfer from the triplet excited state of a sensitizer (generally a dye) to ground-state molecular oxygen. Investigations on sensitizers able to produce efficiently singlet oxygen, as well as studies aimed at elucidating the role of the solvent or the surrounding medium, are of great interest because of the importance of singlet oxygen in both photochemical and photobiological applications.Coordination compounds of Ru" with polyazaheteroaromatic chelating ligdnds have attracted considerable attention during the last fifteen years. This is due to the unique combination of their spectroscopic, redox, and photochemical features, together with the possibility of 'fine tuning' their properties by a judicious choice of the type, number, and substituents of the heteroaromatic chelating ligands of the metal coordination sphere [6]. The excitation of Ru" chelate complexes in the 180-550-nm spectral region is followed by a fast intersystem crossing leading to the formation of the metal-to-ligand charge-transfer

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Steady-state and time-resolved studies have been performed on four compounds of the pterin family (pterin, 6-carboxypterin, 6-formylpterin and folic acid) in aqueous solution, using the single photon counting technique. The fluorescence characteristics (spectra, quantum yields, lifetimes) of these compounds and their dependence on the pH have been investigated. Most pterins can exist in two acid-base forms over the pH range between 3 and 13. Emission spectra and excitation spectra were obtained for both forms of each compound studied. Fluorescence quantum yields (phi(F)) in acidic and basic media were measured. The phi(F) of folic acid (< 0.005 in both media) is very low compared to those of pterin (0.27 in basic media and 0.33 in acidic media), 6-carboxypterin (0.18 in basic media and 0.28 in acidic media) and 6-formylpterin (0.07 in basic media and 0.12 in acidic media). The variation in integrated fluorescence intensity and fluorescence lifetimes (tau(F)) was analysed as a function of pH. Dynamic quenching by OH- was observed and the corresponding bimolecular rate constants for quenching of fluorescence (k(q)) were calculated. The reported values for k(q) (M(-1) s(-1)) are 3.6 x 10(9), 1.9 x 10(9) and 1.1 x 10(10) M(-1) s(-1) for pterin, 6-carboxypterin and 6-formylpterin, respectively.

The efficiency of aromatic ketones as singlet-oxygen ('02('Ag)) sensitizers can vary considerably with the electronic configuration of their lowest triplet state and the solvent used. Near-infrared measurements of the luminescence of singlet oxygen have shown that the quantum yield of singlet-oxygen production (Gd) by 1H-phenalen-I-one (1) is close to unity in both polar ( @, , = 0.97zk0.03 in methanol) and non-polar solvents ( @ A = 0.93+0.04 in benzene). Analysis of the absorption spectra of the ground state and phosphorescence measurements show that the lowest singlet and triplet states have dominant x , A * electronic configurations. The quantum yield of intersystem crossing (cDISc) of 1, determined by laser flash photolysis (partial-saturation method), is equal to unity. In comparison with other aromatic ketones, these parameters are important for the discussion of the surprisingly high GIsc of 1 and the efficient energy transfer from its triplet state to molecular oxygen. The Iff-phenalen-1-one (l), being one of the most efficient singlet-oxygen sensitizers in both polar and non-polar media, could be used as a reference sensitizer, in particular in the area of relatively high energies of excitation.Introduction. -Because of the relatively small energy difference between the ground state (' Z;) and the ('A,) excited state of molecular oxygen (94.2 kJ .mol-'), singlet oxygen ('02('Ag)) should be easily produced by energy transfer from the lowest triplet state of most sensitizers to molecular oxygen (Eqn. I). However, only few sensitizers are of practical use for the generation of singlet oxygen, an activated oxidizing species which plays an important role in photo-oxidation reactions (type-I1 photo-oxidation) [ 1-71.