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Anthraquinones have been the subject of numerous photochemical studies and their photopolymerization activities have been examined under various conditions to improve more efficient photochemical systems. This article involves further detailed investigations into the photophysical, photochemistry and photopolymerisation properties of 4 commercial derivatives of 2-substituted anthraquinone, namely, 2-Bromomethylanthraquinone (2BA), 2 Chloromethylanthraquinone (2CA), 2 Ethylanthraquinone (2EA), 2 Hydroxymethylanthraquinone (2HA) and one novel synthesized anthraquinone, 2 Acryloxymethylanthraquinone (2AA). 2AA is synthesized from 2HA. The results from both spectroscopic and analysis studies proved the 2AA to having the ester link. Absorption spectroscopy and solvent shift data are used to characterise their spectral activity. Luminescence studies involving fluorescence and phosphorescence analysis indicates efficient intersystem crossing to triplet state and n-π* nature of the lowest excited triplet state. The polymerisation activity was studied using methyl methacrylate (MMA) and analysis of the cure rate was measured using the gravimetric method. All the compounds are shown to be highly dependent on the structure. However, the rate of polymerisation (Rp) was reduced in the presence of amine. This is consistent with other results, proving the behaviour of derivatives with n-π* configuration. Hardness tests for all compounds took place using a different formula of acrylated resin/monomer systems. The excited state characteristics of the methyl derivatives have also been examined using micro and nanosecond flash photolysis. Triplet absorption spectra of all the anthraquinone derivatives show a significant red shift in the region of 340–370 nm with increasing solvent polarity due to stabilisation of the lowest triplet state by solvent reorganization. Hydrogen atom abstraction takes place in 2 propanol, forming a semiquinone radical. In the presence of the tertiary amine, triethylamine, all anthraquinone derivatives show the formation of intermediary species related to either the exciplex or the radical ion pair. Under aerobic conditions, the first decay rate for all anthraquinone derivatives increases and showed oxygen to be a good quencher with a bimolecular rate constant of around 2 × 10 mol. dm s. Relative to benzophenone, the molar absorption coefficient, ε, and quantum yield of intersystem crossing, Φisc were calculated, and it is summarised that the value for Φisc for all compound is less than 1.00 and controls to a major extent their photochemical activities., The authors thank the University of Sains Malaysia for funding Hazira Mazah.

Time-resolved pulse radiolysis investigations reported herein show that the carotenoids β-carotene, lycopene, zeaxanthin and astaxanthin (the last two are xanthophylls – oxygen containing carotenoids) are capable of both reducing oxidized guanosine as well as minimizing its formation. The reaction of the carotenoid with the oxidized guanosine produces the radical cation of the carotenoid. This behavior contrasts with the reactions between the amino acids and dietary carotenoids where the carotenoid radical cations oxidized the amino acids (tryptophan, cysteine and tyrosine) at physiological pH.

Time-dependent UV/visible absorption changes are observed on photolysis of aerated solutions of monodisperse, vinyl end-capped 2,5-diheptyloxyl substituted PV-oligomers (OPV, n = 1, 2, 3). In all cases, photolysis occurs in at least two distinct stages. Evidence for the intermediacy of singlet oxygen comes from pulse radiolysis and time-resolved luminescence studies, which confirm formation of the OPV triplet state and its quenching by molecular oxygen to produce singlet oxygen. The subsequent reaction with oligomers is observed directly by time-resolved phosphorescence and indirectly through product identification. The results strongly suggest that the reaction between singlet oxygen and OPV proceeds via cycloaddition, leading to bond scission and aldehyde group formation. The reactivity of vinylene linkages is greater than of vinyl end groups. Consequently, the photostability decreases as the chain length is extended. Implications on the oxidative degradation of poly(p-phenylenevinylene)s in optoelectronic devices, and of competing singlet oxygen and superoxide radical anion degradation pathways, are discussed.

The photophysical properties of the fluoroquinolone antibiotic sarafloxacin (SFX) were investigated in aqueous media. SFX in water, at pH 7.4, shows intense absorption with peaks at 272, 322 and 335 nm, (epsilon=36800 and 17000 dm3 mol(-1) cm(-1), respectively). Both the absorption and emission properties of SFX are pH-dependent; pKa values for the protonation equilibria of both the ground (5.8 and 9.1) and excited singlet states (5.7 and 9.0) of SFX were determined spectroscopically. SFX fluoresces weakly, the quantum yield for fluorescence emission being maximum (0.07) at pH 8. Laser flash photolysis and pulse radiolysis studies have been carried out in order to characterize the transient species of SFX in aqueous solution. Triplet-triplet absorption has a maximum at 610 nm with a molar absorption coefficient of 17,000+/-1000 dm3 mol(-1) cm(-1). The quantum yield of triplet formation has been determined to be 0.35+/-0.05. In the presence of oxygen, the triplet reacts to form excited singlet oxygen with quantum yield of 0.10. The initial triplet (3A*) was found to react with phosphate buffer to form triplet 3B* with lower energy and longer lifetime and having an absorption band centered at 700 nm. SFX triplet was also found to oxidize tryptophan to its radical with concomitant formation of the anion radical of SFX. Hence the photosensitivity of SFX could be initiated by the oxygen radicals and/or by SFX radicals acting as haptens.

Free radicals from one-electron oxidation of the antimalarial drug pyronaridine have been studied by pulse radiolysis. The results show that pyronaridine is readily oxidised to an intermediate semi-iminoquine radical by inorganic and organic free radicals, including those derived from tryptophan and acetaminophen. The pyronaridine radical is rapidly reduced by both ascorbate and caffeic acid. The results indicate that the one-electron reduction potential of the pyronaridine radical at neutral pH lies between those of acetaminophen (707 mV) and caffeic acid (534 mV). The pyronaridine radical decays to produce the iminoquinone, detected by electrospray mass spectrometry, in a second-order process that density functional theory (DFT) calculations (UB3LYP/6-31+G*) suggest is a disproportionation reaction. Important calculated dimensions of pyronaridine, its phenoxyl and aminyl radical, as well as the iminoquinone, are presented.

Spectroscopic and chemical properties of γ-CD radicals, resulting from the abstraction by HO radicals of hydrogen atoms, have been investigated using pulse radiolysis. The reactions of γ-CD radicals with C60 in 2:1 γ-CD/C60 inclusion complexes have been studied in aqueous solutions. It has been demonstrated that the γ-CD radicals are reducing species producing C60- monoanion radicals, as well as doubly reduced C602-, well characterised by their absorption spectra in the near IR. The oxidation potential of γ-CD radical is estimated to be more negative than −390 mV vs. NHE. The kinetics of the C60 reduction by γ-CD radicals have been determined and compared with kinetics by other reducing species including the solvated electron (eaq-) and CO2- radicals. It was observed that the method of preparation of the 2:1 γ-CD/C60 inclusion complexes modifies the C60 reduction mechanism.

The essential thiol of the enzyme papain has been caged by linking to an aromatic thiol. The resulting caged protein is inactive but enzymatic activity is fully restored upon chemical cleavage of the protective disulfide bond. We have exploited the chemistry of this disulfide bond to uncage papain by pulse radiolysis. We have shown that up to 10% of the enzyme activity can be restored by reductive pulse radiolysis. This approach has been tested on a small-molecule model system, and experiments on this model compound show that pulse radiolysis of the mixed cysteine-aromatic disulfide results in selective reduction of the disulfide bond to generate a thiol in 10-20% yield, consistent with the radiolytically restored activity of the caged papain quantified by the biochemical assay.

The photophysical properties of the fluoroquinolone antibiotic moxifloxacin (MOX) were investigated in aqueous media. MOX in water, at pH 7.4, shows two intense absorption bands at 287 and 338 nm (epsilon = 44,000 and 17,000 dm(3) mol(-1) cm(-1), respectively). The absorption and emission properties of MOX are pH-dependent, pK(a) values for the protonation equilibria of both the ground (6.1 and 9.6) and excited singlet states (6.8 and 9.1) of MOX were determined spectroscopically. MOX fluoresces weakly, the quantum yield for fluorescence emission being maximum (0.07) at pH 8. Phosphorescence from the excited triplet state in frozen ethanol solution has a quantum yield of 0.046. Laser flash photolysis and pulse radiolysis studies have been carried out to characterize the transient species of MOX in aqueous solution. On laser excitation, MOX undergoes monophotonic photoionization with a quantum yield of 0.14. This leads to the formation of a long-lived cation radical whose absorption is maximum at 470 nm (epsilon(470) = 3400 dm(3) mol(-1) cm(-1)). The photoionization process releases hydrated electron which rapidly reacts (k = 2.8 x 10(10) dm(3) mol(-1) s(-1)) with ground state MOX, yielding a long-lived anion radical with maximum absorption at 390 nm (epsilon(390) = 2400 dm(3) mol(-1) cm(-1)). The cation radical of MOX is able to oxidize protein components tryptophan and tyrosine. The bimolecular rate constants for these reactions are 2.3 x 10(8) dm(3) mol(-1) s(-1) and 1.3 x 10(8) dm(3) mol(-1) s(-1), respectively. Singlet oxygen sensitized by the MOX triplet state was also detected only in oxygen-saturated D(2)O solutions, with a quantum yield of 0.075.

The H 2 O 2 /persulphate systems are of enormous environmental and commercial importance with the sulphate radical (SO 4 − ) being assumed as the oxidizing/bleaching species. We show that under normal conditions (air-saturated) no SO 4 − is produced and, most likely, a much longer-lived species, the adduct of O 2 and the persulphate radical, is formed.

Steady-state fluorescence has been used to study the excited singlet state of ofloxacin (OFLX) in aqueous solutions. Fluorescence emission was found to be pH dependent, with a maximum quantum yield of 0.17 at pH 7. Two pKa*s of around 2 and 8.5 were obtained for the excited singlet state. Laser flash photolysis and pulse radiolysis have been used to study the excited states and free radicals of OFLX in aqueous solutions. OFLX undergoes monophotonic photoionization from the excited singlet state with a quantum yield of 0.2. The cation radical so produced absorbs maximally at 770 nm with an extinction coefficient of 5000 +/- 500 dm3 mol-1 cm-1. This is confirmed by one-electron oxidation in the pulse radiolysis experiments. The hydrated electron produced in the photoionization process reacts with ground state OFLX with a rate constant of 2.0 +/- 0.2 x 10(10) dm3 mol-1 s-1, and the anion thus produced has two absorption bands at 410 nm (extinction coefficient = 3000 +/- 300 dm3 mol-1 cm-1) and at 530 nm. Triplet-triplet absorption has a maximum at 610 nm with an extinction coefficient of 11,000 +/- 1500 dm3 mol-1 cm-1. The quantum yield of triplet formation has been determined to be 0.33 +/- 0.05. In the presence of oxygen, the triplet reacts to form both excited singlet oxygen and superoxide anion with quantum yields of 0.13 andor = 0.2, respectively. Moreover, superoxide anion is also formed by the reaction of oxygen with the hydrated electron from photoionization. Hence the photosensitivity due to OFLX could be initiated by the oxygen radicals and/or by OFLX radicals acting as haptens.

In this study, we report the protonation reactions for astaxanthin and canthaxanthin radical anions in methanol, alkaline methanol, and aqueous 2% Triton X-100 at different pH values. The pKa values for the corresponding alpha-hydroxy radical derivatives of astaxanthin, canthaxanthin, and beta-apo-8'-carotenal were estimated in 2% Triton X-100. Also, the effects of the microenvironment and the structure of the carotenoids on the protonation rate constant are discussed.

We have synthesised nine monomeric azaheterofullerene (AZA) derivatives, RC(59)N, with a wide variety of different side chains R and investigated their spectroscopic and photophysical properties in toluene and o-dichlorobenzene (ODCB). Measurements include their ground-state absorption spectra, molar absorption coefficient (epsilon(G)), fluorescence spectra, fluorescence quantum yields (Phi(F)), singlet-state lifetimes (tau(F)), triplet-state absorption spectra, triplet molar absorption coefficients (epsilon(T)), singlet oxygen (Phi(Delta)), and triplet state (Phi(T)) quantum yields. The replacement of a carbon by a nitrogen atom in the C(60) sphere strongly affects most of the spectroscopic and photophysical properties. The chemical nature of the R moiety has definite effects on these properties in contrast with minor effects on the chemical nature of the addends in [6,6]-ring bridged monoadduct methano[60]fullerene derivatives. These effects concern properties of the ground state, singlet excited state, and triplet states of our nine RC(59)N derivatives and in particular the values of photophysical parameters epsilon(G), epsilon(T), Phi(Delta), and Phi(T), which are significantly lower than those of analogous monoadduct [6,6]-ring bridged methano[60]fullerene derivatives.

In search of a simple internal complexation of C 60 in cyclodextrins, six 2:1 cyclodextrin:[60]fullerene conjugates involving different types of linkers have been synthesised. Using spectrophotometry, laser flash photolysis and pulse radiolysis, we have investigated spectroscopic and photophysical properties of these 2:1 cyclodextrin [60]fullerene monoadduct conjugates ( CDF ), including ground and triplet state absorption spectra, triplet molar absorption coefficients ( e T ), quantum yields of triplet ( Φ T ) and singlet oxygen formation ( Φ Δ ) in water and toluene, in order to probe the possibility of 2:1 inclusion complexes of C 60 between two cyclodextrins. In water, the CDF spectroscopic and photophysical properties indicate hydrophobically driven self-assemblies of the amphiphilic conjugates into spherical micelles with no evidence for the presence of a 2:1 inclusion complex, while in toluene they show no sign of aggregation. It is observed that formation of aggregates in water does not prevent scavenging of HO radical by the C 60 moiety of the CD–C 60 conjugates.

The influence of side-chain structure on the mode of reaction of ortho-quinone amines has been investigated with a view, ultimately, to developing potential methods of therapeutic intervention by manipulating the early stages of melanogenesis. Four N-substituted dopamine derivatives have been prepared and quinone formation studied using pulse radiolysis and tyrosinase-oximetry. Ortho-quinones with an amide or urea side chain were relatively stable, although evidence for slow formation of isomeric para-quinomethanes was observed. A thiourea derivative cyclized fairly rapidly (k = 1.7/s) to a product containing a seven-membered ring, whereas a related amidine gave more rapidly (k approximately 2.5 x 10(2)/s) a stable spirocyclic product. The results suggest that cyclization of amides, ureas and carbamates (NHCO-X; X = R, NHR or OR) does not occur and is not, therefore, a viable approach to the formation of tyrosinase-activated antimelanoma prodrugs. It is also concluded that for N-acetyldopamine spontaneous ortho-quinone to para-quinomethane isomerization is slow.

Pulse radiolysis has been used to study the positively charged state of isolated chains of poly[9,9-di(ethylhexyl)fluorene] in chloroform solution. This is found to have an absorption maximum at 560 nm and a weaker band around 420 nm. Intermolecular electron transfer is observed from the aromatic amine tri(p-tolylamine) to this species. With the corresponding polyfluorene derivative end-capped by an arylamine, intramolecular transfer of electrons from the triphenylamine end-cap to the positively charged polyfluorene chain is found on timescales faster than the detection time of the system (ca. 500 ns). http://www.sciencedirect.com/science/article/B6TFN-4BFVRCF-5/1/96f5575d22e4c0ef6aa08c7e4cfe88a2

Quantum yields for triplet formation have been determined for seven common π-conjugated polymers in benzene solution using time-resolved photoacoustic calorimetry (PAC) in conjunction with fluorescence quantum yields, singlet and triplet energies. The polymers studied include three poly(thiophenes), poly(2-methoxy,5-(2′-ethylhexyloxy)-p-phenylenevinylene) (MEH-PPV), a cyano derivative of MEH-PPV, a ladder type poly(p-phenylene) (MeLPPP), and a poly(fluorene). Yields of singlet oxygen formation have also been determined for these polymers in benzene by time-resolved phosphorimetry, and are in reasonable agreement with triplet yields obtained by PAC. Polythiophenes show the highest intersystem crossing yields, which are suggested to result from extensive spin-orbit coupling. Where singlet oxygen yields are less than triplet yields, it is suggested that interaction of molecular oxygen with the ground state of the polymers may be involved.

Pulse radiolysis has been used to generate high triplet populations on isolated conjugated polymer chains in benzene solutions. This allows direct observation of delayed fluorescence (DF) arising from intramolecular triplet–triplet annihilation (TTA). In poly(2-methoxy,5-(2 ′ -ethyl-hexyloxy)- p -phenylenevinylene) the triplet lifetime at low triplet concentration is 90 μs which decreases to 15 μs as the number of triplets saturates at ca. 30 triplets per chain. Analysis of the DF kinetics gives an upper limit of 1.5% for the number of triplets that contribute to TTA. We also observed DF in a range of other conjugated polymers.

The formation and reactivity of the triplet state and free radicals of mefloquine hydrochloride (MQ) have been investigated by pulse radiolysis and flash photolysis. The excited triplet, cation radical and anion radical have been produced and their absorption characteristics determined. The triplet-triplet absorption spectrum of MQ showed a maximum at 430 nm, with a molar absorption coefficient of 3600 M(-1) cm(-1) and the quantum yield for intersystem crossing was determined to be close to unity. Deactivation of the triplet, in the absence of oxygen, led to the formation of MQ cation and/or anion radicals. The molar absorption coefficient of the cation radical at 330 nm was determined to be 2300 M(-1) cm(-1), whilst that for the anion radical was 2400 M(-1) cm(-1) at 620 nm and 3600 M(-1) cm(-1) at 350 nm. The molar absorption coefficients of the proposed neutral radical at 320 nm and 520 nm were 4000 M(-1) cm(-1) and 1300 M(-1) cm(-1) respectively. The quantum yield for the formation of singlet oxygen, sensitized by MQ triplet, was determined to be close to unity. Aqueous solutions of MQ were found to photoionize to yield hydrated electron and cation radical of MQ in a biphotonic process. The influences of pH, buffer concentration, oxygen concentration and addition of sodium azide on the formation and reactivity of the transients were evaluated. The reactions between MQ and solvated electrons and superoxide anion were also studied.

The charged states of the conjugated polymers poly(2-methoxy,5-(2′-ethylhexyloxy)-p- phenylenevinylene) (MEH-PPV) and poly(2,5-pyridinediyl) (PPY) have been studied by pulse radiolysis. Following pulse radiolysis of argon-saturated solutions of MEH-PPV in chloroform, a new absorption is seen to grow in over a few hundred microseconds. This has a principal absorption at 1.43 eV and a weaker, low energy band (⩽0.80 eV), and is assigned to the positive one-electron charge state (positive polaron) of MEH-PPV. The slow absorption decay is unaffected by oxygen. Negative charge states (negative polarons), with absorptions around 1.4 eV, are produced upon pulse radiolysis of MEH-PPV in argon-saturated solutions in tetrahydrofuran (THF) or benzonitrile. A small solvatochromic shift is observed. In contrast to the behavior of the positive polaron, the MEH-PPV negative charge carriers decay fairly rapidly, and are readily quenched by molecular oxygen. Previous results on chemically produced positive and negative cha...

The photoreactivity of Type I and II photointiators, namely, 4-(2-hydroxyethoxy)-phenyl-(2-hydroxy2-methylpropyl) ketone (Irgacure 2959, Ciba-Geigy) and 4-hydroxybenzophenone, respectively, have been examined in the free and bound form in melamine and urethane acrylate prepolymers. Co-reaction in the prepolymers was undertaken by partial replacement of the hydroxyacrylate components with the photoinitiators in the reaction with the isocyanate component. Both the photoinitiators were also converted into their corresponding acrylate derivatives for study. The free, bound and acrylated derivatives were then analysed by various spectroscopic techniques to evaluate the relationship between their photophysical properties and photoinitiation activity for photocrosslinking of commercial acrylated monomers and prepolymers. Photocuring studies using Hg sources (conveyor and RTIR) indicated that whilst binding the Type I photointiator reduced its efficiency in cure, the Type II photoinitiator was enhanced. The presence of an amine co-synergist, however, generally eliminated these differentials. Acrylation of the Irgacure 2959 also reduced photoactivity. Absorption spectroscopy indicated the presence of a strongly absorbing nπ * transition in the far UV region for both chromophore types which undergoes a blue shift on binding to the prepolymer. Phosphorescence spectra, lifetimes and quantum yields also indicate the presence of low lying triplet nπ * states for both types of chromophore. Binding appears to enhance the triplet lifetime and reduce the quantum yield of emission due to increased electron donation into the chromophore by the resin component and may in part account for their lower photoactivity. Microsecond flash photolysis identified the formation of benzoyl radicals in the case of the Type I system with a small enhancement in radical formation on binding. The Type II system gave ketyl radicals formed by hydrogen atom abstraction which were enhanced in the bound resin system. The latter would account for increased photoactivity for the bound Type II system. Nanosecond laser flash photolysis experiments identified the triplet–triplet absorption in the case of the Type II initiator. In the presence of a tertiary amine (triethylamine) the transient absorption and its lifetime were significantly enhanced due to exciplex formation. However, binding the benzophenone initiator to the resin significantly quenched the transient absorption and significantly reduced its lifetime. In the case of the Type I initiator the benzoyl radical was observed directly, confirming the microsecond flash photolysis data. Binding of the initiator to the melamine acrylate resin also reduced benzoyl radical formation and reduced the radical lifetime. This may account for the reduced photoactivity of the Type I bound initiator. Radical formation was also reduced when bound to the urethane acrylate but the lifetime was enhanced. For the initiator bound resin systems the presence of an amine co-synergist enhanced benzoyl radical formation. The implications of initiator co-reactions are discussed in terms of both the photochemical and commercial benefits.

We have elucidated the lowest triplet state in the soluble luminescent conjugated polymer poly(2-methoxy,5-(2′-ethyl-hexoxy)–p-phenylenevinylene) (MEH–PPV) in benzene. Using a range of triplet sensitisers we determine that the S0–T1 energy separation is 1.27±0.07 eV. Triplet–triplet absorption is observed at 1.5 eV. In solution, the triplet state has a lifetime ⩾100 μs. The molar absorption coefficient at 1.5 eV is concentration dependent; at 200 mg/l, e=1.19×105 M−1 cm−1. Up to 50 mg/l, the triplet decay rate showed no change, indicating self-quenching of triplets is not significant. Oxygen quenches the MEH–PPV triplet, leading to singlet oxygen formation with a quantum yield of 2.5±1%.

The excited state characteristics of 13 novel 1-chloro-4-oxy/acylox derivatives ofthioxanthone were determined using micro-, nano- and picosecond flash photolysis techniques. Triplet energy levels have also been determined using phosphorescence analysis while photoinitiated polymerisation activities were measured using photocalorimetry. All the initiators exhibit high photopolymerisation activity except the 4-hydroxy model and 2-methyl-4-n-propoxy derivatives. The triplet energies are found to be fairly insensitive to solvent polarity with an observed spectral broadening from non-polar to polar solvents. This is consistent with close lying mixed triplet states of 3ππ* and 3nπ* character. Both the least active 4-hydroxy and 2-methyl-4-propoxy derivatives exhibit lower triplet energies suggesting the presence of a less active lower triplet 3ππ* state. Triplet-triplet absorption spectra are obtained with all the thioxanthones in the range 600-680 nm with a marked blue shift from non-polar to polar solvents owing to stabilisation of the lowest triplet state by solvent reorganisation. In photo-reductive solvents such as methyl alcohol and 2-propanol a longer lived species is observed absorbing in the region 400-500 nm associated with the formation of the ketyl radical. This observation is highly solvent dependent and further supported by microsecond flash spectroscopy in 2-propanol. The 4-hydroxy derivative gave only weak transient absorption and is consistent with its much lower initiation activity. In the presence of a tertiary amine no ketyl radical is observed. Triplet lifetimes increase with solvent polarity confirming the presence of mixed 3ππ* and 3nπ* states where vibronic coupling influences the rate of intersystem crossing to the ground So state. Bimolecular triplet quenching rate constants indicate all the thioxanthones, except the 2-methyl-4-n-propoxy and 4-hydroxy, interact strongly with a range of tertiary amines, DABCO, triethylamine and methyldiethanolamine. The lower triplet quenching constants for the 2-methyl4-n-propoxy and 2-hydroxy derivatives (an order of magnitude) indicates weaker interaction by the amine and is consistent with their lower photoinitiation activities. Low triplet quenching rates are also observed in the presence of monomer (methyl methacrylate). Bimolecular triplet quenching rates are also measured with naphthalene and are similar to those for benzophenone except the 2-methyl-4-n-propoxy and 4-hydroxy, derivatives which are an order of magnitude less. From this data triplet molar extinction coefficients are determined and found to be higher than that for benzophenone owing to the presence of mixed states. Again, the 2-methyl-4-n-propoxy and 4-hydroxy derivatives exhibited lower values as did the 4-benzoyloxy derivative. Relatively high quantum yields of intersystem crossing are observed ( < 0.6) but are all lower than that of benzophenone with the side chain substituents having no significant effect on the rate. The growth rate of the triplet state could be measured and triplet maxima accurately determined via picosecond flash photolysis. Within the growth time of the triplet state (6-18 ps) there is a concurrent formation of the ketyl radical except for the 4-acetyloxy and 4-acryloxy derivatives. The importance of this technique in gaining valuable information on the relative rates of concurrent excited state reactions for thioxanthones is discussed.

The singlet oxygen quenching rate constants (kq) for a range of hydroxycinnamic acids in acetonitrile and D2O solutions were measured using time resolved near infrared phosphorescence in order to establish their antioxidant activity. The magnitude of kq observed depends on both the nature of the substituent groups and solvent polarity. The variations in kq depend on the energy of the hydroxycinnamic acid/molecular oxygen charge transfer states, (O2delta- ...HCAdelta+). In D2O the values of kq range from 4x10(7) M(-1) s(-1) to 4x10(6) M(-1) s(-1) for caffeic acid and o-coumaric acid respectively. In acetonitrile, the charge transfer energy levels are raised and this is reflected in lower singlet oxygen quenching rate constants with a kq value of 5x10(6) M(-1) s(-1) for caffeic acid. The phenoxyl radical spectra derived from the hydroxycinnamic acids were determined using pulse radiolysis of aqueous solutions and the reduction potentials were found to range from 534 to 596 mV. A linear correlation is observed between reduction potential, and hence free energy for electron transfer, and log kq. These correlations suggest a charge transfer mechanism for the quenching of singlet oxygen by the hydroxycinnamic acids.

Spectral and redox properties of the phenoxyl radicals from hydroxycinnamic acid derivatives and one selected component of phenylpropanoid glycosides, verbascoside, were studied using pulse radiolysis techniques. On the basis of the pH dependence of phenoxyl radical absorptions, the p K a values for deprotonation of sinapic acid radical and ferulic acid radical are 4.9 and 5.2. The rate constants of one electron oxidation of those antioxidants by azide radical and bromide radical ion were determined at pH 7. The redox potentials of those antioxidants were determined as 0.59–0.71 V vs NHE at pH 7 with reference standard 4-methoxyphenol and resorcinol.

The photophysical, photochemical and photopolymerisation properties have been studied for four derivatives of 2-substituted anthraquinone. Photopolymerisation activity is highly dependent on the configuration of the lowest triplet excited state. Fluorescence and phosphorescence analysis indicates efficient intersystem crossing to the triplet state and the π-π∗ or n-π∗ nature of the lowest excited triplet state. The four derivative possess either an amide or an ester type substituent. The amide derivatives possess an excited triplet state with π-π∗ configuration, whereas the ester derivatives exhibit an n-π∗ excited triplet state. Photoinduced polymerisation of lauryl acrylate monomer has been recorded by differential scanning photocalorimetry. Without an amine cosynergist, absolute quantum yields of photopolymerisation show that the anthraquinones with an ester type substituent and possessing an n-π∗ configuration are more active. In the presence of a tertiary amine cosynergist the anthraquinone derivates with an amide substituent possessing a longer wavelength π-π∗ triplet state show enhanced activity. Nanosecond and microsecond flash photolysis studies and absolute quantum yields of photoreduction have been related to their photopolymerisation activity. The results indicate the presence of semianthraquinone and radical anion intermediates in the photoreduction step. The type of photoreduction reaction occurring is dependent on the nature of the excited state. The two types of derivative react predominantly via either direct hydrogen atom abstraction or electron transfer followed by proton transfer. The predominant method by which each molecule undergoes photoreduction has been demonstrated by steady state irradiations and flash photolysis studies.

Laser flash photolysis spectra of chromophore extracts from Blepharisma japonicum cells have been measured. The variation of the triplet state spectra with pH is reported and the related pK a , values of 2–3 for deprotonation of the excited triplet state was determined. The anionic Blepharismin triplet state does not appear to readily undergo one-electron reduction by mild reducing agents, but undergoes oxidation by benzoquinone. Pulse radiolysis has been used to independently determine the spectra of the one-electron reduced and one-electron-oxidised forms of Blepharismin.

The absorption spectra of charge-transfer excited states and one-electron-reduced states of a number of mono- and binuclear ruthenium complexes in water have been obtained using laser flash photolysis and pulse radiolysis techniques. For Ru(dhp)[sub 3][sup +*], the absorption bands are assigned by comparison with Ru(bpy)[sub 3][sup +*]. The spectra obtained for Ru(dpp)[sub 3][sup +*] and Ru(dpq)[sub 3][sup +*] are similar to their respective ground-state absorption spectra. Bimolecular rate constants for the reaction of e[sub aq] with the above complexes as well as with the mixed-ligand complexes Ru(bpy)[sub 2]dpp[sup 2+] and Ru(bpy)[sub 2]dhp[sup 2+], were also determined. From their characteristic absorption spectra and the bimolecular rate constants, it was concluded that the added electron is localized on one of the ligands. The excited-state absorption spectra of the two mixed-ligand complexes showed bands at 360 and 450 nm attributed to [pi]-[pi]* transitions of the ligand anion radical. The lifetimes of the excited states of the complexes decreased on changing the solvent from acetonitrile to water. It was also observed that the lifetimes of the excited states of the binuclear complexes were shorter than those of the corresponding mononuclear complexes.

One-electron oxidation of the ferrous tris -PQ complex, a model for lipoxygenase, was attempted using oxidants such as •OH, N • 3 , Br •- 2 , Tl 2+ and TlOH + . •OH was found to react with the complex with a bimolecular rate constant of (3.9±0.6)x10 9 dm 3 mol -1 s -1 , a rate which is not very dissimilar to that for the reaction with the ligand PQ. However the product of the reaction was found to be a OH-adduct rather than a cation radical. No reaction was found to occur with N • 3 or Br •- 2 . Both Tl 2+ and TlOH + reacted with the complex to form its oxidised species with rate constants of (7.0±1)x10 8 dm 3 mol -1 s -1 and (4.0±0.8)x10 8 dm 3 mol -1 s -1 , respectively. From a comparison of the rate constants and the transient spectra it was concluded that the centre of oxidation is the ligand rather than the metal.

Pulse radiolysis experiments were carried out with aqueous solutions of thionine dye methylene blue (MB) and toluidine blue in the presence of polyvinyl alcohol. The transient spectra obtained show maxima at 400 and 880 nm and are assigned to the respective semiquinone radical anions. The semiquinone radical decayed by a second order process. Furthermore, MB in PVA films were bleached by γ-radiolysis and the change in absorption was found to be linear with the dose.

The photochemistry of benoxaprofen(BP) and four of its analogues in acetonitrile has been investigated using both steady-state and laser-flash photolysis. On steady-state photolysis, all compounds decarboxylated, albeit in small quantum yields. The fluorescence yields were found to be in the range 0.4–0.6. The triplet-triplet absorption spectra of all the compounds studied show absorption maxima at 410 nm with extinction coefficients of around 25 000 dm3 mol−1 cm−1. Furthermore, all the compounds underwent intersystem crossing with efficiencies of around 0.4. Direct measurements of the time-resolved luminescence from singlet oxygen (1Δg) using benzophenone as a triplet-state sensitizer in aerated solutions of benoxaprofen and its analogues, indicated that the efficiency of singlet oxygen formation (SΔ) was close to unity. These results imply that the singlet-oxygen quantum yields are close to 0.4.

The photochemical and photopolymerisation properties of 1-fluoro-4-propoxythioxanthone (FPTX), 1-chloro-4-propoxythioxanthone (CPTX) and 1-bromo-4-propoxythioxanthone (BPTX) have been determined and compared with the corresponding activity of 2-propoxythioxanthone (ITX). Both CPTX and BPTX exhibit high photoinitiation activity when used alone and in conjunction with a tertiary amine cosynergist and benzophenone. The ITX and FPTX are active only when used with a tertiary amine cosynergist and/or benzophenone. Absorption and luminescence spectra indicate that the lowest excited states possess mixed ππ*/nπ* character with the BPTX exhibiting the highest phosphorescence quantum yield probably due to heavy-atom spin–orbit coupling by the Br atom. Photolysis/photoreduction quantum yields are higher in a reductive solvent such as propan-2-ol than in non-reductive acetonitrile for the three halogenated derivatives. They are enhanced further in the presence of a tertiary amine. These data contrast with that for ITX where photolysis/photoreduction is observed only in reductive propan-2-ol, in the absence and presence of a tertiary amine. Ketyl radical formation, with a wavelength maximum at 390 nm on microsecond flash photolysis, is similar for all the thioxanthones and is virtually insensitive to oxygen except for the BPTX derivative. The three halogenated derivatives give strong triplet–triplet absorptions on nanosecond laser flash photolysis in propan-2-ol and acetonitrile, similar to those for ITX. Quenching of the triplet state by a tertiary amine is found to increase with increasing amine concentration, with ITX being the most sensitive initiator. This quenching is consistent with their behaviour as photopolymerisation initiators. Bimolecular triplet quenching by naphthalene gives rate constants for ITX and FTPX which are similar to that of benzophenone while that for CPTX is an order of magnitude less. From these data we have determined that the molar absorption coefficients of the triplet–triplet absorptions for FPTX and CPTX are half that of ITX with all three being greater than that of benzophenone. The FPTX is also found to exhibit a significantly lower yield of intersystem crossing compared with CPTX and ITX. The latter two initiators exhibit rates of intersystem crossing which are almost comparable with that of benzophenone. Values of molar absorption coefficients and rates of intersystem crossing for BPTX could not be measured since the laser flash studies indicated the formation of a charge-transfer complex with triplet quenchers such as naphthalene. Solution photolysis studies indicated an additional reaction involving subsequent scission of the halogen–thioxanthone bond in the case of BPTX and CPTX but the FPTX gave no such reaction. Various mechanisms are discussed to account for the effective photoinitiation activity of BPTX and CPTX including their potential role as hybrid initiators.

Microsecond and nanosecond laser flash photolysis studies have been carried out on six 2-substituted anthraquinone compounds, namely the acrylamido, acryloxy, acetoylamino, benzoylamino, hydroxy and amino derivatives, and the data related to their photo-cross- linking activity in nylon 6,6 polymer. All the compounds give rise to triplet—triplet absorptions, with the benzoyl and acetoylamino derivatives showing the strongest absorptions followed by the acrylamido and acryloxy derivatives. Transient half-lives are found to be 7.0, 1.7, 0.7 and 0.63 μs for the acrylamido, acryloxy, benzoyl and acetoylamino derivatives respectively, which follow the order of their ability to induce photo-cross-linking of nylon 6,6 polymer, long triplet lifetimes being the more active. In the reductive solvent 2-propanol the triplet state of the 2-arylamido derivative is rapidly quenched owing to hydrogen atom abstraction. For both the acryloxy and acrylamido derivatives the first-order decay rates of the triplet state are found to be independent of the concentration of the ground state, suggesting that some degree of intramolecular interaction is involved between the side chain and the quinone carbonyl groups. In nylon 6,6 polymer, on the other hand, the acrylamido derivative gives a long-lived triplet—triplet absorption which decays to give a long-lived residual absorption with a maximum at 500 nm. The latter is associated with the formation of a radical anion species formed from a triplet exciplex involving the terminal amine end groups. On microsecond flash photolysis in 2-propanol the initiators give both semiquinone and radical anion absorptions formed by hydrogen atom and electron abstraction reactions, with the former being stronger for the acryloxy derivative.

The structural factors underlying the peculiar optical properties and visible chromophore of eumelanin biopolymers are largely uncharted. It is known that synthetic eumelanins from 5,6-dihydroxyindole are black and display a featureless UV-visible absorption spectrum, whereas those from 5,6-dihydroxyindole-2-carboxylic acid (1) are lighter in color and exhibit a distinct band around 310 nm, but the origin of this difference has never been addressed in detail. Recently, we showed that 5,6-dihydroxyindole dimers generate on pulse radiolysis oxidation strongly absorbing transients with intense maxima in the 500-600 nm region, which have been attributed to planar extended quinone methide species. We now report the unexpectedly different behavior of three oligomers from 1, namely, the 4,4'-biindolyl 2, the 4,7'-biindolyl 3, and the 4,7':4',7''-terindolyl 4. Pulse radiolysis oxidation of 2-4 led initially to semiquinone intermediates exhibiting similar absorption maxima at 360-380 nm. Semiquinone absorption decay followed second-order kinetics (2k = 1.4 x 10(8), 3.2 x 10(8), and 1.4 x 10(8) M(-1) s(-1) for 2, 3, and 4, respectively) but did not lead to significant chromophore development in the visible region. Similar absorption traces were obtained from monomer 1. DFT calculations predicted 5,6-dihydroxyindolyl-5,6-indolequinone structures with significant dihedral twists across the interunit single bonds for the most stable two-electron oxidation products of 2 and 3. The computed absorption spectra consistently featured strong bands around 310 nm but little or no absorption in the visible region. It is suggested that the effective conjugation length in oligomeric/polymeric eumelanin components from 1 may be controlled by hindered rotation around inter-ring bonds preventing planarization of the continuous array of indole units. This may provide an explanation for the difference in the absorption properties of polymers from the two key eumelanin monomers.

The spectroscopic properties and photopolymerization activities of a mono- and tetraperester derivative of benzophenone are examined and compared with those of benzophenone. Their photopolymerization activity in methyl methacrylate (MMA) and an ethoxylated bis-phenol-A diacrylate have been studied using a combination of gel permeation chromatography (GPC), and real time Fourier transform infrared spectroscopy (RTFTIR) as well as the commercial pendulum hardness test. Using GPC analysis on the poly(methyl methacrylate) samples the tetra--t-butyl perester derivative of benzophenone is found to give a higher weight average (Mw) and number average (Mn) molecular weight polymer than that produced using the mono perester. For the two peresters of benzophenone photopolymerization efficiency using real time FTIR increases with increasing initiator concentration, and no self-termination is observed up to 0.75% w/w concentration with the mono perester derivative being the more efficient initiator. Similar results were obtained using the commercial pendulum hardness tester with a triacrylate/epoxyurethane acrylate resin with benzophenone exhibiting the lowest activity. Photopolymerization activities of the initiators correlate well with their spectroscopic properties. The phosphorescence quantum yield is higher for the monoperester than the tetraperester derivative and is consistent with a shorter lifetime and lower photolysis quantum yields in 2-propanol. Compared with benzophenone, phosphorescence analysis indicates that the perester groups impart a degree of charge-transfer content to the molecule which is consistent with the degree of substitution. Ketyl radical formation on microsecond flash photolysis follows the order benzophenone > mono- > tetraperester derivative and is consistent with the phosphorescence quantum yields. On nanosecond laser flash photolysis in nitrogen-saturated acetonitrile, triplettriplet absorption is extremely weak for both the perester derivatives, being stronger for the monoperester.

The formation and reactivity of excited states and free radicals from primaquine, a drug used in the treatment of malaria, was studied in order to evaluate the primary photochemical reaction mechanisms. The excited primaquine triplet was not detected, but is likely to be formed with a short lifetime (50 ns) and with a triplet energy250 kJ/mol as the drug is an efficient quencher of the fenbufen triplet and the biphenyl triplet, and forms (1)O(2) by laser flash photolysis ((PQ)Phi(Delta)=0.025). Primaquine (PQ) exists as the monocation (PQH(+)) in aqueous solution at physiological pH. PQH(+) photoionises by a biphotonic process and also forms the monoprotonated cation radical (PQH(2+)*) by one electron oxidation by HO* (k(q)=6.6 x 10(9) M(-1) s(-1)) and Br*(2)(-) (k(q)=4.7 x 10(9) M(-1) s(-1)) at physiological pH, detected as a long-lived transient decaying essentially by a second order process (k(2)=7.4 x 10(8) M(-1) s(-1)). PQH(2+)* is scavenged by O(2), although at a limited rate (k(q)=1.0 x 10(6) M(-1) s(-1)). The reduction potential (E degrees) of PQH(2+)*/PQH(+) is+1015 mV, as measured versus tryptophan (TRP*/TRPH). Primaquine also forms PQH(2+)* at pH 2.4, by one electron oxidation by Br*(2)(-) and proton loss (k(q)=2.7 x 10(9) M(-1) s(-1)). The non-protonated cation radical (PQ(+)*) is formed during one electron oxidation with Br*(2)(-) at alkaline conditions (k(q)=4.2 x 10(9) M(-1) s(-1) at pH 10.8). The estimated pK(a)-value of PQH(2+)*/PQ(+)* is pK(a) approximately 7-8. Primaquine is not a scavenger of O*(2)(-) at physiological pH. Thus self-sensitization by O*(2)(-) is eliminated as a degradation pathway in the photochemical reactions. Impurities in the raw material and photochemical degradation products initiate photosensitized degradation of primaquine in deuterium oxide, prevented by addition of the (1)O(2) quencher sodium azide. Photosensitized degradation by formation of (1)O(2) is thus important for the initial photochemical decomposition of primaquine, which also proceeds by free radical reactions. Formation of PQH(2+)* is expected to play an essential part in the photochemical degradation process in a neutral, aqueous medium.

We report the direct observation of the quenching of the weakly absorbing transient due to the amino radical by oxygen and, hence determine, by a totally direct method, the corresponding rate constant (k = (1.1 +/- 0.1) x 10(9) dm3 mol(-1) s(-1)). We also report the rate constants for the reactions of the amino radical with several amino acids and models of black eumelanin and blond/red phaeomelanin. These reactions lead to a mechanism, based on free radicals, that can explain why ammonia is useful in commercial hair (melanin) bleaching, avoiding excessive amino acid (hair protein) damage.

Conventional microsecond and nanosecond laser flash photolysis studies have been carried out on 15 novel 4-alkylamino-substituted benzophenone photoinitiators. Using the microsecond technique, ketyl radical formation is observed from all 15 photoinitiators with a half-life longer than that for the ketyl radical of benzophenone. The formation of the radical anion through an intramolecular exciplex is considered to be an important precursor to this reaction. Triplet-triplet absorption decays by a first-order process; this decay is independent of the ground state concentration of the initiator when intramolecular hydrogen atom abstraction is structurally feasible. The solvent is also seen to influence intramolecular hydrogen atom abstraction and it dominates in acetonitrile where pseudo-first-order hydrogen atom abstraction does not occur. Photolysed product analysis using mass spectrometry complies with the conclusions from laser flash photolysis.

The photochemistry of three novel t-butylperester derivatives of fluorenone was examined and compared with unsubstituted fluorenone and a mono-t-butylperester of benzophenone using both conventional microsecond and nanosecond laser flash photolysis. On conventional microsecond flash photolysis in 2-propanol, all four fluorenone compounds gave transient absorption in the region 300–400 nm due to a ketyl radical formed from the abstraction of a hydrogen atom from the solvent by the upper excited triplet n—π* state of the fluorenone chromophore. This assignment was confirmed by a pH-dependent study on the transient absorption spectra. The nitro-t-butylperester derivative of fluorenone gave additional absorption above 400 nm due to species associated with the nitro group. No evidence for benzoyloxy radical formation could be found in non-hydrogen-atom-donating solvents with microsecond flash photolysis which is associated with homolysis of the perester groups. On nanosecond laser flash photolysis of the fluorenone compounds at 355 nm excitation in acetonitrile and hexa-fluorobenzene, transient absorptions were observed in the region 320–640 nm due to the corresponding triplet states. All the t-butylperester derivatives showed residual absorbances at longer time delays which were tentatively assigned to the corresponding benzoyloxy radicals produced by homolysis of the perester groups. In contrast, the mono-t-butylperester of benzophenone, included for comparison only, showed very weak transient absorption in the region 320–640 nm compared with that of the strong triplet of benzophenone under the same excitation conditions. The triplet absorptions and lifetimes of the fluorenone compounds were correlated with their photopolymerization activities in bulk methylmethacrylate monomer. In oxygenated solutions, the triplet absorptions of fluorenone and benzophenone were effectively quenched; however, long-lived transient growths were observed for all the t-butylperester derivatives. The intensities of these novel transient absorptions appear to correlate with the total number of t-butylperester groups in the fluorenone molecule and tentative assignments are discussed.

The photochemistries of the monomer 2-acryloxythioxanthone and its copolymers with methyl methacrylate have been examined and compared with that of a model 2-acetoxythioxanthone using both conventional microsecond and nanosecond laser flash photolysis in a variety of solvent systems. On microsecond flash photolysis in tetrahydrofuran, transient absorption spectra are produced with absorption maxima at 300 and 420 nm (assigned to the ketyl radical) together with a weak, broad absorption tail extending to longer wavelengths due to the radical-anion. Transient absorbances are reduced in non-hydrogen atom donating acetonitrile apart from that of the copolymer due to intramolecular hydrogen atom abstraction. Transient intensities follow the order monomer > model > 8% > 21% copolymer. Intramolecular self-quenching is believed to be an important process here with the effect of a tertiary amine (diethylethanolamine) being variable. On nanosecond laser flash photolysis in acetonitrile, transient absorption maxima are produced at 330, 440 and 630 nm and assigned to a triplet-triplet absorption and follow the order monomer > model > copolymer (21%). The triplet is strongly reduced in the copolymer due to intramolecular self-quenching. The presence of a tertiary amine (diethylethanolamine) enhances the triplet absorptions at 330 and 630 nm for the monomer and model thioxanthones due to triplet exciplex formation but is quenched by the addition of methyl methacrylate monomer. The effect is more pronounced for the model than for the monomer but again strong quenching is observed for the copolymer (21%). At 440 nm the transient absorption is significantly increased for both the monomer and model because of electron transfer within the exciplex to give the radical-anion. In the case of the copolymer, only a slight increase in the transient absorption in this region is observed because intramolecular coiling of the polymer chain sterically impairs exciplex formation with the tertiary amine. Concentration studies on transient decays show that whilst intermolecular interactions are important for the model, intramolecular self-quenching is important to some degree for the monomer. On the other hand, in the case of the copolymer (21%), the rate constant is observed to decrease with increasing concentration of copolymer in solution because of cage effects enhancing the triplet lifetime. The effect of various tertiary amines on transient decays for the model show a linear correlation between the first-order quenching rate constants and the ionization potential of the amine, confirming the importance of electron transfer via triplet exciplex formation. The importance of these results in photo-initiated polymerization is discussed.

Three potential routes to generation of reactive oxygen species from a tocopherolquinone have been identified. The quinone of the water-soluble vitamin E analogue Trolox C (Trol-Q) is reduced by hydrated electron and isopropanol a hydroxyalkyl radical, and the resulting semiquinone reacts with molecular oxygen to form superoxide with a second order rate constant of 1.3 x 108 dm3 mol-1 s-1, illustrating the potential for redox cycling. Illumination (UV-A, 355 nm) of the quinone of 2,2,5,7,8-pentamethyl-6-hydroxychromanol (PMHC-Q) leads to a reactive short-lived (ca 10-6 s) triplet state, able to oxidise tryptophan with a second order rate constant greater than 109 dm3 mol-1 s-1. The triplet states of these quinones sensitize singlet oxygen formation with quantum yields of about 0.8. Such potentially damaging reactions of a tocopherolquinone may in part account for the recent findings that high levels of dietary vitamin E supplementation lack any beneficial effect and may lead to slightly enhanced levels of overall mortality.

The synthesis and photophysical study of (C/\N)Pt(II)Q complexes, where C/\N is a bidentate cyclometalating ligand and Q is 8-hydroxyquinoline or quinoline-8-thiol, are presented. The compounds were obtained as a single isomer with N atoms of the C/\N and Q ligands trans-coordinated to the Pt(II) center as shown by X-ray crystallography. These chromophores absorb intensely in the visible region and emit in the deep-red spectral region from a quinolate-centered triplet intraligand charge-transfer excited state. The emission maxima are in the range 675-740 nm, with the quantum yields and lifetimes of up to 0.82% and 5.3 mus, respectively, in deoxygenated organic solvents at room temperature. These complexes are efficient photosensitizers of singlet oxygen in air-saturated solutions, with yields up to 90%.

The spectral and photophysical properties of indigo derivatives with di-, tetra-, and hexa-substitution in their neutral (keto) form are investigated in solution. The study comprises absorption and emission spectra, together with quantitative measurements of quantum yields of fluorescence (phi(F)) and singlet oxygen formation (phi(Delta)) and fluorescence lifetimes. The energy difference between the HOMO and LUMO orbitals is dependent on the degree (number of groups) and relative position of substitution. The phi(F) and phi(Delta) values were found to be very lowor=10(-3). Because of the absence of transient triplet-triplet signal, the intersystem crossing yields (phi(T)) were estimated by assuming that all the triplet states formed give rise to singlet oxygen formation, that is, phi(Delta) approximately phi(T) . It was then possible from phi(IC)=1-phi(F)-phi(T) to estimate the S(1) approximately approximately --S(0) internal conversion yields and thus, with the other data, to determine the rate constants for all decay processes. From these, several conclusions are drawn. Firstly, the radiationless rate constants, k(NR) , clearly dominate over the radiative rate constants, k(F) , (and processes). Secondly, the main deactivation channel for the compounds in their keto form is the radiationless S(1) approximately approximately --S(0) internal conversion process. Finally, although the changes are relatively small, internal conversion yield seems to be independent of the overall pattern of substitution. A more detailed investigation of the decay profiles with collection at the blue and red emission of the fluorescence band of indigo and one di-substituted indigo reveals the decays to be bi-exponential and that at longer emission wavelengths these appear to be associated with both rise and decay times indicating that two excited species exist, which is consistent with a keto-excited form giving rise (by fast proton transfer) to the enol-form of indigo. Evidence is presented which supports the idea that intramolecular (and possibly some intermolecular) proton transfer can explain the high efficiency of internal conversion in indigo.

A pulse radiolytic investigation has been conducted to establish whether a redox reaction takes place between dopaquinone and 5,6-dihydroxyindole (DHI) and its 2-carboxylic acid (DHICA) and to measure the rate constants of the interactions. To obviate possible confounding reactions, such as nucleophilic addition, the method employed to generate dopaquinone used the dibromide radical anion acting on dopa to form the semiquinone which rapidly disproportionates to dopaquinone. In the presence of DHI the corresponding indole-5,6-quinone (and/or tautomers) was also formed directly but, by judicious selection of suitable relative concentrations of initial reactants, we were able to detect the formation of additional indolequinone from the redox exchange reaction of DHI with dopaquinone which exhibited a linear dependency on the concentration of DHI. Computer simulation of the experimental time profiles of the absorption changes showed that, under the conditions chosen, redox exchange does proceed but not quite to completion, a forward rate constant of 1.4 x 10(6)/M/s being obtained. This is in the same range as the rate constants previously established for reactions of dopaquinone with cyclodopa and cysteinyldopa. In similar experiments carried out with DHICA, the reaction more obviously does not go to completion and is much slower, k (forward) =1.6 x 10(5)/M/s. We conclude that, in the eumelanogenic pathway, DHI oxidation may take place by redox exchange with dopaquinone, although such a reaction is likely to be less efficient for DHICA.

The relative reduction potentials of a variety of carotenoids have been established by monitoring the reaction of carotenoid radical anion (CAR1(*-)) with another carotenoid (CAR2) in hexane and benzene. This order is consistent with the reactivities of the carotenoid radical anions with porphyrins and oxygen in hexane. In addition, investigation of the reactions of carotenoids with reducing radicals in aqueous 2% Triton-X 100, such as carbon dioxide radical anion (CO2(*-)), acetone ketyl radical (AC(*-)) and the corresponding neutral radical (ACH(*)), reveals that the reduction potentials for beta-carotene and zeaxanthin lie in the range -1950 to -2100 mV and those for astaxanthin, canthaxanthin and beta-apo-8'-carotenal are more positive than -1450 mV. This illustrates that the presence of a carbonyl group causes the reducing ability to decrease. The radical cations have been previously shown to be strong oxidising agents and we now show that the radical anions are very strong reducing agents.

5,10,15,20-Tetrakis(2,6-dichloro-3-chlorosulfophenyl)porphyrin and its tin and zinc complexes were synthesized with high yields and fully characterized. The corresponding water-soluble 5,10,15,20-tetrakis(2,6-dichloro-3-sulfophenyl)porphyrins were obtained by hydrolysis with water. An extensive photophysical study of the new water soluble porphyrinic compounds was carried out including absorption and fluorescence spectra, fluorescence quantum yields, triplet absorption spectra, triplet lifetimes, triplet and singlet oxygen quantum yields. These sensitizers were successfully used in the photodegradation of 4-chlorophenol and 2,6-dimethylphenol. A comparison is made of their efficiencies, and some mechanistic considerations are highlighted. FCT; FEDER; POCTI; Universidad de Burgos; GRICES–CNRS; European Commission (HPRI-CT-2002-00183).

A pyridinium bisretinoid (A2E) is the only identified blue-absorbing chromophore of retinal lipofuscin that has been linked to its aerobic photoreactivity and phototoxicity. Pulse radiolysis has been used to study both the one-electron oxidation and the one-electron reduction of A2E in aqueous micellar solutions. The reduction to the semireduced A2E (lambda(max) broad and between 500 and 540 nm) was achieved with formate radicals and the subsequent decay of A2E* was slow (over hundreds of milliseconds) via complex kinetics. The long lifetime of the A2E* should facilitate its reactions with other biomolecules. For example, with oxygen, the A2E* produced the superoxide radical anion with a rate constant of 3 x 10(8) M(-1) s(-1). The A2E was also reduced by the NAD radical, the corresponding rate constant being 2.3 x 10(8) M(-1) s(-1). Other experiments showed that the one-electron reduction potential of A2E lies in the range -640 to -940 mV. The semioxidized form of A2E (lambda(max) 590 nm) was formed via oxidation with the Br2*- radical and had a much shorter lifetime than the semireduced form. With strongly oxidizing peroxyl radicals (CCl3O2*) our kinetic data suggest the formation of a radical adduct followed by dissociation to the semioxidized A2E. With milder oxidizing peroxyl radicals such as that from methanol, our results were inconclusive. In benzene we observed an efficient oxidation of zeaxanthin to its radical cation by the A2E radical cation; this may be relevant to a detrimental effect of A2E in vision.

Solutions of glucose are used as diluents for drugs in various drug infusions. When sterilized by heat small amounts of the substance 5-hydroxymethyl-2-furaldehyde (5-HMF) is produced from glucose. At a hospital ward such infusions may be exposed to irradiation; including UV-light. The photoreactivity of the furaldehyde is investigated. It is shown to photodestabilize the catecholamine isoprenaline. It is shown to be a producer, but also a consumer, of singlet oxygen. The excited triplet, cation and anion radical have been produced by pulse radiolysis and flash photolysis and their absorbance characteristics have been determined. The triplet absorption spectrum showed absorption bands at 320 and 430 nm with molar absorption coefficients of 4700 and 2600 M-1 cm-1, respectively. The anion radical showed absorption bands at 330 and 420 nm with molar absorption coefficients of 2000 and 300 M-1 cm-1, respectively. The cation radical had an absorption band at 320 nm with a molar absorption coefficient of 5000 M-1 cm-1. The quantum yield for the production of singlet oxygen, sensitized by the 5-HMF triplet, was determined to be 0.6, whilst the quantum yield for the triplet formation was 1.0. Aqueous solutions of 5-HMF were found to photoionize to yield the hydrated electron and the cation radical of 5-HMF in a biphotonic process. The influences of pH, buffer and glucose on the formation of transients were evaluated. The reactions between 5-HMF and the solvated electron, the hydroxyl radical and the superoxide were also studied.