Suppiah Navaratnam

Singlet oxygen quenching and the redox properties of hydroxycinnamic acids

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.

S1∼>T1 intersystem crossing in π-conjugated organic polymers

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.

Measurement of the S0–T1 energy gap in poly(2-methoxy,5-(2′-ethyl-hexoxy)–p-phenylenevinylene) by triplet–triplet energy transfer

Abstract 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 S 0 –T 1 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 =1.19×10 5 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%.

Triplet state dynamics on isolated conjugated polymer chains

Triplet state behaviour has been studied with several conjugated polymers in dilute benzene solutions by flash photolysis, photoacoustic calorimetry (PAC) and pulse radiolysis/energy transfer. With polythiophenes and the ladder poly(p-phenylene) MeLPPP, singlet–triplet intersystem crossing (ISC) is relatively efficient. In contrast, it is inefficient with poly(p-phenylenevinylene)s (PPVs) and polyfluorene, while with cyano-substituted PPV, there is no evidence for any long-lived triplet state. Energy transfer from triplet biphenyl to MEH-PPV is diffusion controlled and triplet state lifetimes are typically tens or hundreds of μs. All the triplet states are quenched by molecular oxygen, leading to formation of singlet oxygen with yields which are generally close to those for triplet formation. With pulse radiolysis at high doses, it is possible to have more than one triplet state per polymer chain. This can lead to delayed fluorescence via intrachain triplet–triplet annihilation. Kinetic analysis of this shows slow movement of triplets by hopping along the chain.

Photochemistry and photoinduced chemical crosslinking activity of type I & II co-reactive photoinitiators in acrylated prepolymers

Abstract 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.

A comparative study of water soluble 5,10,15,20-tetrakis(2,6-dichloro-3-sulfophenyl)porphyrin and its metal complexes as efficient sensitizers for photodegradation of phenols

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.

Primary photophysical properties of ofloxacin.

Abstract 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 × 1010 dm3 mol−1 s−1, and the anion thus produced has two absorption bands at 410 nm (extinction coefficient = 3000 ± 300 dm 3 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 dm 3 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 and ≤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.

LASER FLASH AND STEADY-STATE PHOTOLYSIS OF BENOXAPROFEN IN AQUEOUS SOLUTION*

Abstract— On laser flash photolysis of aqueous solutions of benoxaprofen (BP), the BP triplet state is produced with a quantum yield of 0.19. One other unidentified species is also formed probably in a yield less than 0.02. Identical observations were made in alcoholic solution. The effect of potential hydrogen donors, isopropanol and histidine, on the kinetic reactivity of the BP triplet state, showed that for isopropanol the rate constant is slow (< 8 × 103 dm3 mol‐1 s‐1) if it occurs at all, whilst no reactivity with histidine could be seen. In oxygen uptake experiments, it was deduced that singlet oxygen is produced via the BP triplet state with a quantum yield of 0.18. There was no evidence for superoxide anion radical production. In other steady‐state photolysis experiments using high performance liquid chromatography, the quantum yield for loss of BP was determined to be 0.20, while the quantum yield for the only observed product, decarboxylated benoxaprofen, was estimated as 0.18. The involvement of the two major processes, i.e. singlet oxygen formation and BP decarboxylation, in BP phototoxicity is discussed.

The triplet state of the ladder-type methyl-poly(p-phenylene) as seen by pulse radiolysis-energy transfer

Abstract The lowest triplet state of a ladder poly(p-phenylene) (MeLPPP) has been produced by sensitised energy transfer following pulse radiolysis of benzene solution, and absorbs at 1.34 eV with a lifetime ⩾170 μs. An S0–T1 energy separation of 2.15 ± 0.07 eV was determined, in excellent agreement with recent phosphorescence measurements, confirming the validity of the technique for the determination of the triplet energies of conjugated polymers. At high triplet populations intrachain triplet–triplet annihilation (TTA), leading to delayed fluorescence (DF) occurs. The DF spectrum is in good agreement with the prompt fluorescence and the DF kinetics support a TTA mechanism.

Photochemistry of novel 4-alkylamino benzophenone initiators : a conventional laser flash photolysis and mass spectrometry study

Abstract 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.