Silvia E. Braslavsky

Glossary of terms used in photocatalysis and radiation catalysis (IUPAC Recommendations 2011)

This glossary of terms covers phenomena considered under the very wide terms photocatalysis and radiation catalysis. A clear distinction is made between phenomena related to either photochemistry and photocatalysis or radiation chemistry and radiation catalysis. The term “radiation” is used here as embracing electromagnetic radiation of all wavelengths, but in general excluding fast-moving particles. Consistent definitions are given of terms in the areas mentioned above, as well as definitions of the most important parameters used for the quantitative description of the phenomena. Terms related to the up-scaling of photocatalytic processes for industrial applications have been included. This Glossary should be used together with the Glossary of terms used in photochemistry, 3rd edition, IUPAC Recommendations 2006: (doi:10.1351/pac200779030293) as well as with the IUPAC Compendium of Chemical Terminology, 2nd ed. (the “Gold Book”, 2006– doi:10.1351/goldbook) because many terms used in photocatalysis are defined in these documents.

Time-resolved thermal lensing and phosphorescence studies on photosensitized singlet molecular oxygen formation. Influence of the electronic configuration of the sensitizer on sensitization efficiency

Abstract The absolute quantum yields, Φ Δ , of photosensitized formation of singlet molecular oxygen, O 2 ( 1 Δ g ), by various sensitizers in benzene were measured by time-resolved thermal lensing (TRTL). Relative measurements with time-resolved O 2 ( 1 Δ g ) phosphorescence detection were used to confirm the absolute values from TRTL. For aromatic ππ* triplet states, S Δ = Φ Δ /Φ ISC = 0.9 ± 0.1 and the rate constant for quenching of the triplet state of O 2 , k q = (2 ± 0.5 ) × 10 −1 M 9 s −1 , is about 1/9 of the diffusional rate constant. For the more polarizable nπ* triplet states S Δ = 0.3 ± 0.05 and k q sharply increases with triplet energy.

THE PRODUCTION OF SINGLET MOLECULAR OXYGEN BY ZINC(II) PHTHALOCYANINE IN ETHANOL AND IN UNILAMELLAR VESICLES. CHEMICAL QUENCHING AND PHOSPHORESCENCE STUDIES

Abstract— Zn(II)phthalocyanine (ZnPc) generates O2(1Δg) with a quantum yield of ca. 0.4 upon photocxcitation at 354 or 600 nm in ethanolic solution as determined by time‐resolved phosphorescence studies at 1270 nm and photooxidation experiments using 1,3‐diphenylisobenzofuran (DPBF) as substrate. The quantum yield of photooxidation slightly increases upon incorporation of ZnPc into unilamellar liposomes of dipalmitoylphosphatidylcholine. Under our irradiation conditions (600 nm, 18°C, and short light exposure times), DPBF(5–50 μM) undergoes photooxidation by a pure Type II mechanism; the rate constant for the O2(1Δg) + DPBF reaction is (1.1 ±0.1) x 109 M‐1 s_1 in ethanol solution and determined to be about two orders of magnitude smaller when both ZnPc and DPBF are embedded into liposomes.

Thermal-lensing measurements of singlet molecular oxygen (1Δg): quantum yields of formation and lifetimes

Abstract Absolute quantum yields φΔ for singlet molecular oxygen (1O2(1Δg)) production were determined by time-resolved thermal-lensing (TL) using several sensitizers. The φΔ value is smaller than the quantum yield φisc for intersystem crossing for tetraphenylporphyrin, zinc tetraphenylporphyrin and anthracene is air-saturated benzene solutions. For anthracene, the values φisc = 0.78 ± 0.08 and φΔ/ = 0.78 ± 0.16 were measured using this method. The lifetimes of 1O2, determined by time-resolved TL in several solvents at concentrations similar to those detected by emission, are in good agreement with literature data for values up to about 100 μs. For longer lifetimes the values are smaller than those in the literature. Various possible reasons for the discrepancies are discussed.

Photophysical, photochemical and antibacterial photosensitizing properties of a novel octacationic Zn(II)-phthalocyanine

A novel Zn(II)-phthalocyanine (1). peripherally substituted with four bis(N,N,N-trimethyl)amino-2-propyloxy groups prepared by chemical synthesis is shown to be an efficient photodynamic sensitizer with a quantum yield of 0.6 for singlet oxygen generation in neat water, which is reduced to about 0.3 in phosphate-buffered saline. The physicochemical properties of 1 in both the ground and the electronically excited states strongly depend on the nature of the medium; in particular, aggregation of 1 was favoured by polar media of high ionic strength. Compound 1 exhibited an appreciable affinity for a typical Gram-positive bacterium (Staphylococcus aureus) and a typical Gram-negative bacterium (Escherichia coli). Both bacterial strains were extensively inactivated upon 5 min-irradiation with 675 nm light in the presence of 1 microM photosensitizer, even though the binding of 1 to the two bacterial cells appears to occur according to different pathways. In particular, E. coli cells underwent initial photodamage at the level of specific proteins in the outer wall, thus promoting the penetration of the photosensitizer to the cytoplasmic membrane where some enzymes critical for cell survival were inactivated.

THE PHOTOPHYSICAL PROPERTIES OF PORPHYCENES: POTENTIAL PHOTODYNAMIC THERAPY AGENTS*

Porphycene and a tetra‐n‐propyl derivative remained unaltered on irradiation in toluene at room temperature. Quantum yields of fluorescence, S T intersystem crossing, and singlet molecular oxygen sensitization, as well as lifetimes of the singlet and triplet excited states were measured. In view of their structural relationship to porphyrin, their high absorption above 620 nm, their stability towards photooxidation, and their high quantum yields of fluorescence and singlet oxygen sensitization, these compounds qualify as potential agents for tumor marking and photodynamic therapy.

[4] Time-resolved singlet oxygen detection

Publisher Summary Techniques for the detection of singlet oxygen [O 2 ( 1 Δ g )] are increasingly implemented in many laboratories, reflecting the growing awareness that O 2 ( 1 Δ g ) plays crucial roles in several photoexcited systems. Time-resolved detection techniques have been used for (1) the identification of O 2 ( 1 Δ g ), (2) the measurement of quantum yields of O 2 ( 1 Δ g ) production in photosensitized processes, Φ Δ , and (3) the determination of rate constant for the interaction of O 2 ( 1 Δ g ) with substrates. This chapter describes the methods used most commonly to attain the just-described goals associated with the technique of time-resolved near-infrared detection of O 2 ( 1 Δ g ) phosphorescence at 1.27μm (TRNIR). Focus is placed on laboratory procedures rather than on techniques. Only methods for homogeneous solutions are discussed. Clues are given for extending them to more complex systems, but the number of possible situations precludes general statements. It is restricted to systems in which O 2 ( 1 Δ g ) is produced by photosensitization, thus excluding chemical or enzymatic production.

THE PHOTOPHYSICS OF MEROCYANINE 540. A COMPARATIVE STUDY IN ETHANOL AND IN LIPOSOMES

Abstract— Absorption and fluorescence emission spectra, fluorescence lifetimes, fluorescence quantum yields, photoisomerization quantum yields and triplet quantum yields were measured for Merocyanine 540 (MC540) in ethanol and in large unilamellar dimyristoyl phosphatidylcholine vesicles. The major differences in the photophysics between the two media are the increase of the fluorescence quantum yield from 0.15 in ethanol to 0.6 in vesicles at 25° C, and the appearance of a second fluorescence decay with a lifetime of 1.87 ns in the latter medium. Upper and lower limits for the photoisomerization quantum yields were determined by combining the data from laser flash photolysis and optoacoustic spectroscopy. The decrease in photoisomerization quantum yield upon incorporation of the dye into the lipid bilayer by a factor 2 suggests that this process competes directly with fluorescence. The temperature dependence of the fluorescence and photoisomerization quantum yields in solution supports this model. In both media MC540 has a very low triplet quantum yield with values 0.002 > (> øT > 0.02 in ethanol and 0.01 > øT‐ > 0.09 in liposomes Our data are consistent with the model whereby the dye is incorporated into the lipid bilayer as a monomer with two different orientations and this model is adopted on the basis of the biexponential behaviour of the fluorescence and photoisomer decay.

THE KINETICS OF THE EARLY STAGES OF THE PHYTOCHROME PHOTOTRANSFORMATION Pr→ Pfr. A COMPARATIVE STUDY OF SMALL (60 kDalton) and NATIVE (124 kDalton) PHYTOCHROMES FROM OAT

A comparative study of the decay kinetics of photogenerated transients from small (60 kDalton) and native (124 kDalton) oat phytochrome in the red‐absorbing form (Pr) in phosphate buffer containing 5 mM ethylenediamine tetraacetic acid, pH 7.8, (PB) and in PB containing 20% ethylene glycol, has been carried out in the temperature range 275–298 K. The analysis confirmed that at least two primary photoproducts, intermediates Ii700s and Ii7oo are formed from Pr. The kinetic parameters, as observed in PB at 695 nm and 275 K, are similar for the I700 intermediates of both small and native phytochrome. Namely, the lifetimes are about 21 μs (component percentages 38%) for the I Ii700s and about 200 μ.s (62%) for the Ii700S‐ Arrhenius preexponential factors (A) of about 1016 and 1015 s‐1and activation energies of about 61 and 56 kJ/mol were measured for the absorbance decays of the I700S of small and native phytochrome, respectively. The kinetic data favour parallel paths for the formation of the Ii700s from Pr, and the activation parameters indicate that the primary photoreactions of the transformation from Pr to the far‐red‐absorbing form are restricted to the chromophore within the protein. Moreover, the relatively modest temperature dependence of the lifetimes of the Ii700S from small and native Pr supports the working hypothesis that the ground state reactions to the Iibl, intermediates–although somewhat influenced by the polypeptide fragment that is removed upon degradation of native to small Pr–are localized to the chromophore, as is most probably the case also for the primary photoreactions. The effect of the addition of 20% ethylene glycol on the pre‐exponential factors of the time‐dependent decay functions is discussed in similar terms of the early stages of the phototransformation.

CARBOXYLATED ZINC‐PHTHALOCYANINES–II* DIMERIZATION AND SINGLET MOLECULAR OXYGEN SENSITIZATION IN HEXADECYLTRIMETHYLAMMONIUM BROMIDE MICELLES

The dimerization of the diamide of zinc‐tetracarboxyphthalocyanine was studied spectro‐scopically in hexadecyltrimethylammonium bromide (CTAB) micelles at surfactant concentrations from 0.026 to 0.1 M and dye concentrations between 0.1 and 10 μM. The apparent dimerization constant in CTAB 0.1 M is 8.6 × 105M−1, while the intramicellar dimerization constant is 1.8 × 103. The dimer absorption spectrum was also obtained. Singlet molecular oxygen sensitization was studied by steady state photolysis using 1,3‐diphenylisobenzofurane as scavenger in 0.1 M CTAB. The usual sensitization mechanism is extended to include dimer reactions. Singlet molecular oxygen sensitization yields for monomer and dimer in the micelles are 0.7 and 0.1, respectively. With the reported values it is possible to calculate the average yield of singlet molecular oxygen production at any surfactant and dye concentrations.