Photosensitized Reduction Research Articles

Effects of hydrophobic association and interfacial surface potential on the photosensitized reduction of anthraquinone sulphonate in positively charged micelles and vesicles

Photoinduced reductions of anthraquinone sulophonates in positively charged micelles and vesicles are affected by the interfacial surface potential and the nature of the counter ion of the electron acceptor; with a surfactant counter ion, close association of the electron acceptor with the charged interface is achieved, effective control of the photosensitized electron transfer process is accomplished, and the photoreduction of the quinone proceeds with a quantum yield of ϕ= 2.6 × 10–1.

Photosensitized electron transfer processes in SiO 2 colloids and sodium lauryl sulfate micellar systems: Correlation of quantum yields with interfacial surface potentials

The effectiveness of negatively charged colloidal SiO(2) particles in controlling photosensitized electron transfer reactions has been studied and compared with that of the negatively charged sodium lauryl sulfate (NaLauSO(4)) micellar system. In particular, the photosensitized reduction of the zwitterionic electron acceptor propylviologen sulfonate (PVS(0)) with tris(2,2'-bipyridinium)ruthenium(II) [Ru(bipy)(3) (2+)] as the sensitizer and triethanolamine as the electron donor is found to have a quantum yield of 0.033 for formation of the radical anion (PVS([unk])) in the SiO(2) colloid compared with 0.005 in the homogeneous system and 0.0086 in a NaLauSO(4) micellar solution. The higher quantum yields obtained with the SiO(2) colloidal system are attributed to substantial stabilization against back reaction of the intermediate photoproducts-i.e., Ru(bipy)(3) (3+) and PVS([unk])-by electrostatic repulsion of the reduced electron acceptor from the negatively charged particle surface. The binding properties of the SiO(2) particles and NaLauSO(4) micelles were investigated by flow dialysis. The results show that the sensitizer binds to both interfaces and that the SiO(2) interface is characterized by a much higher surface potential than the micellar interface ( approximately -170 mV vs. -85 mV). The effect of ionic strength on the surface potential was estimated from the Gouy-Chapman theory, and the measured quantum yields of photosensitized electron transfer were correlated with surface potential at different ionic strengths. This correlation shows that the quantum yield is not affected by surface potentials smaller than approximately -40 mV. At larger potentials, the quantum yield increases rapidly. The quantum yield obtained in the micellar system at different strengths fits nicely on the correlation curve for the colloid SiO(2) system. These results indicate that the surface potential is the dominant factor in the quantum yield improvement for PVS(0) reduction.

Photosensitized electron transport across lipid vesicle walls: Enhancement of quantum yield by ionophores and transmembrane potentials

The photosensitized reduction of heptylviologen in the bulk aqueous phase of phosphatidylcholine vesicles containing EDTA inside and a membrane-bound tris(2,2'-bipyridine)ruthenium(2+) derivative is enhanced by a factor of 6.5 by the addition of valinomycin in the presence of K(+). A 3-fold stimulation by gramicidin and carbonyl cyanide m-chlorophenylhydrazone is observed. The results suggest that, under these conditions, the rate of photoinduced electron transfer across vesicle walls in the absence of ion carriers is limited by cotransport of cations. The rate of electron transfer across vesicle walls could be influenced further by generating transmembrane potentials with K(+) gradients in the presence of valinomycin. When vesicles are made with transmembrane potentials, interior more negative, the quantum yield of heptylviologen reduction is doubled, and, conversely, when vesicles are made with transmembrane potentials, interior more positive, the quantum yield is decreased and approaches the value found in the absence of valinomycin.

Transfusion Therapy in a Patient With Erythropoietic Protoporphyria

An 8-year-old boy with erythropoietic protoporphyria had a marked decrease in photosensitivity symptoms after several transfusions with washed, packed RBCs. The reduction in photosensitivity was associated with a decline in free erythrocyte protoporphyrin levels.

Photosensitized electron transfer reactions in organized interfacial systems

The separation of photoproducts formed in photosensitized electron transfer reactions is essential for efficient energy conversion and storage. The organization of the components involved in the photoinduced process in interfacial systems leads to efficient compartmentalization of the products. Several interfacial systems, e.g. lipid bilayer membranes (vesicles), water-in-oil microemulsions and a solid SiO 2 colloidal interface, were designed to accomplish this goal. An electron transfer across a lipid bilayer membrane leading to the separation of the photoproducts at opposite sides of the membrane is facilitated by establishing a transmembrane potential and organizing the cotransport of cations with specific carriers. Colloidal SiO 2 particles provide a charged interface that interacts with charged photoproducts. By designing a system that results in oppositely charged photoproducts, a retardation of recombination by the charged interface can be produced. The photosensitized reduction of a neutral acceptor by positively charged sensitizers is described. The reactions are substantially enhanced in the SiO 2 colloid compared with in the homogeneous phase. The effect of the SiO 2 interface is attributed to a high surface potential that results in the separation of the intermediate photoproducts. The quantum yields of the photosensitized reactions are correlated with the interfacial surface potential and the electrical effects of other charged interfaces such as micelles are compared with those of SiO 2. The possible utilization of the energy stored in the stabilized photoproducts in further chemical reactions is discussed. Special attention is given to the photodecomposition of water.

Model systems for photosynthesis. VII. Chlorophyll a photosensitized reduction of methyl viologen by hydroquinones

The use of hydroquinones as electron donors for in vitro models of Photosystem I has been investigated. Excited chlorophyll a (Chi*) was first reacted with methyl viologen (MV 2+ ) to give ChI + and MV t . The subsequent reaction of ChI t with hydroquinones was slower than the comparable reaction with alternative electron donors such as cysteine or ascorbic acid, and an empirical relation was observed between the redox potential of the hydroquinone and the rate constant for its reaction with ChI t . This sequence of reactions stores 40 % of the energy available in the chlorophyll a triplet state, but no permanent storage of energy was achieved since MV t back-reacts with quinone to give a cyclic process.

Model of photofuel element on the base of chlorophyll sensitized reactions

Photogalvanic effect has been studied in ethanol-aqueous solution containing: 1 × 10 −3 M cysteine, 5 × 10 −5 M chlorophyll ‘A’ and 1 × 10 −4 M methylviologen. During photosensitized reduction of methylviologen open-circuit photovoltage was −650 mV and short-circuit photocurrent was 100 μA/cm 2. The maximum power output from the photogalvanic element was 2 × 10 −5 W at load resistance 6 kΩ. Principal differences and advantages of investigated triple photochemical system have been discussed as compared with double photochemical systems consisting of pigment and electron donor. Since the power output from the element consists of absorbed light power and power of electron donor oxidation, such electricity source may be called ‘photofuel element’. The possibility of application of the investigated photochemical system for creation of high efficiency photoelectroconverters of solar energy has been suggested.

Effects of photodecomposition on the electrophotographic properties of amorphous As2Se3

Selenium-arsenic alloys undergo a photochemical reaction when exposed to radiation of band-gap energy or greater. The influence of this photoinduced reaction on the electrophotographic photosensitivity of amorphous arsenic triselenide films was studied. A progressive change in the short-wavelength photoresponse was seen in films which were exposed to intense light. The characteristics of the change depend upon the substrate temperature during the vacuum deposition of the film. Films prepared at substrate temperatures below Tg initially showed an increase in sensitivity, possibly due to structural relaxation or molecular rearrangement within the film. The initially measured spectral sensitivity of films prepared below Tg was always lower than that for films prepared near Tg. All films eventually showed a reduction in short-wavelength sensitivity. The rate of this change increased at elevated temperatures. The photolytic reaction itself does not affect the photosensitivity. The reduction in photosensitivity is a result of the oxidation and subsequent depletion of arsenic at the surface of the photoconductor which is enhanced by the prior photodissociation of the arsenic triselenide.

Semiconductor electrodes. Photo-induced electrogenerated chemiluminescence and up-conversion at semiconductor electrodes

A photosensitized reduction process, leading to electrogenerated chemiluminescence (ECL) at a p-type silicon electrode in which neither the energy of the light used for sensitization nor the electrical energy input reach the threshold energy for the production of the emitted photon if applied separately, is described and discussed. The main experiment consisted of irradiating a p-type Si electrode, immersed in an acetonitrile solution containing 9,10-dichloro-9,10-dihydro-9,10-diphenyl-anthracene (DPACl 2) and held at a potential where DPACl 2 reduction does not occur, with light of energy less than 1.7 eV. The photosensitized reduction of DPACl 2 leads to the production of excited singlet 9, 10-diphenylanthracene and the emission of 3.0 eV light.

Behavioural and electrographic effects of L-5-hydroxytryptophan and D,L-parachlorophenyl-alanine on epileptic senegalese baboon ( Papio papio)

In the Senegalese baboon, Papio papio, the behavioural and electrographic effects of the serotonin precursor, L-5-hydroxytryptophan (5-HTP) and a serotonin depletor, D,L-para-chlorophenylalanine (pCPA) were examined with special attention to the pre-existing photosensitivity and epileptiform electrographic activity. 1. 1. L-5-HTP (10–35 mg/kg) depressed motor and EEG responses to photic stimulation for 1–4 h after injection. 2. 2. 48 h after D,L-pCPA (200–300 mg/kg), myoclonic responses to photic stimulation were enhanced. Consistent reduction of photosensitivity was observed 4–7 days after pCPA. 3. 3. Both drugs activated spontaneous epileptiform electrographic abnormalities. These changes in background activity were often chronologically independent of the changes in photically induced responses. 4. 4. It is suggested that serotonergic systems can play a role in the photomyoclonic syndrome in Papio papio.

Sensitization Mechanism in the Fluorescein Photosensitized Reduction of Ferric Ion by EDTA.

Sensitization Mechanism in the Fluorescein Photosensitized Reduction of Ferric Ion by EDTA.

A reported photosynthesis in vitro.

NUMEROUS attempts to achieve a photo-sensitized reduction of carbon dioxide by water in vitro are on record1. Various degrees of success have been claimed; but in almost every case the repetition of such experiments in other laboratories has failed and, in any event, the yields originally reported were extremely small. A significant advance, however, appeared to have been made in 1943 by Baur and his co-workers2. After many years of experimentation, Baur developed a two-phase system consisting of glycerol, geraniol, chlorophyll and methylene blue, which on illumination in a carbon dioxide atmosphere was stated to produce formaldehyde and oxygen in a photochemical yield amounting to about 5 per cent of that of natural photosynthesis. Although it is now known that formaldehyde is not an intermediate in natural photosynthesis, the realization of a photosensitized hydrogen-transfer reaction between carbon dioxide and water outside the living cell would be a result of great significance, and a confirmation of Baur's results, which have been widely quoted, therefore appeared desirable.