Search

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.

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 results of a laser flash photolysis investigation of a dendritic methano[60]fullerene octadeca-acid (DA) and its tert-butyl ester (DE) are reported. DE possesses photophysical properties typical of a [60]fullerene mono-adduct with a singlet oxygen quantum yield approaching unity in toluene and a triplet absorption spectrum with a maximum at 710 nm. In methanol DA also possesses properties typical of a [60]fullerene mono-adduct, but in aqueous solution its photophysical behaviour shows a degree of aggregation that is a function of pH and concentration. At pH 7.4 and higher, Coulombic repulsion between de-protonated carboxylate groups reduces the propensity to aggregation and dilute solutions (

International audience; To resolve conflicting results in the literature, we determine quantitatively the properties of the lowest triplet excited state of water soluble 2:1 host:guest complexes of C$_{60}$ embedded in $\gamma$-cyclodextrin ($\gamma$-CD/C$_{60}$) using laser flash photolysis. The results show that the triplet excited state characteristics of $\gamma$-CD/C$_{60}$ in aqueous solutions are very similar to those observed for C$_{60}$ in organic solvents. Moreover oxygen, in contrast with non-gaseous solutes, can reach the proximity of C$_{60}$ embedded in 2:1 $\gamma$-CD inclusion complexes. In addition, photodestruction quantum yields have been determined for C$_{60}$ in toluene and for C$_{60}$ in $\gamma$-CD inclusion complexes in water.

Laser flash photolysis and pulse radiolysis have been used to determine properties of the lowest triplet state of a series of water-soluble malonic acid derivatives of C60 , C60[C(COOH)2]n with n = 2–6, in water at pH 7.4. Similar studies were carried out on the corresponding ethyl esters, C60[C(COOEt)2]n with n = 1–6, in the non-polar solvents, benzene or toluene. The properties include the T–T absorption spectra, the triplet molar absorption coefficients and the quantum yields of formation, ΦT and ΦΔ of the triplet and of singlet oxygen, respectively. This study shows a general tendency of the λmax of the T–T absorption spectra, and of the ΦΔ values to decrease with increasing number of addends. For regioisomers, the spectroscopic and photophysical parameters are modulated with the pattern of addition. The ΦΔ quantum yields for the acids in aqueous solution are systematically lower than those of the ethyl esters in toluene. The lower ΦΔ values observed for the acids in aqueous solution, and to a lesser extent for the esters in benzonitrile, can be interpreted as due to the presence of solute clusters in polar solvents.

A new methanofullerene, C 61 (poly(aryl)acetylene) 2 , where the mono-adduct contains dendrimer units, has been studied with respect to its photophysical properties. The visible absorption and the emission spectra, the triplet molar extinction coefficient as well as the near unity value of the quantum yield of singlet oxygen production upon photoexcitation reveal characteristics in keeping with other methanofullerenes previously studied, where the addend species is not an electron donating group. The results indicate that, even when the methanofullerene is photoexcited, this large addend does not electronically communicate through the methano bridge neither through space. Photophysical properties of methanofullerenes can thus be preserved when they are functionalised with such addends.

The fluorescence spectra, quantum yields and lifetimes of C70 and a pseudo-dihydro derivative (C70R) have been measured in a wide range of solvents at room temperature. This information is important for the development of reverse saturable absorbers. Phosphorescence spectra and phosphorescence lifetimes were also measured at low temperature. The fluorescence is subject to quenching by halogenated compounds. The eAciency of quenching follows the order I > Br Cl. The nature of the quenching is shown to vary, with chlorinated compounds exhibiting static quenching of fullerene fluorescence, owing to nonfluorescent complex formation, whilst those compounds containing bromine and iodine exhibit dynamic quenching due to the external heavy-atom eAect, that increases the intersystem crossing rate constant in the fluorophore‐perturber complex. This constant is evaluated by an original method from the bimolecular quenching rate constants. The phosphorescence quantum yield of both fullerenes at 77 K slightly increases in the presence of iodobenzene, in spite of a strong decrease in phosphorescence lifetime. The marked increase of the intersystem crossing rate constant in concentrated solutions owing to the external heavy-atom eAect is of interest for the application of fullerenes as fast-responding optical limiters (reverse saturable absorbers) of intense laser pulses, even in cases where the triplet quantum yield is of the order of unity. ” 2001 Elsevier Science B.V. All rights reserved.

Using pulse radiolysis and laser flash photolysis, we have investigated the reactions of the deleterious species, e(-)(aq), HOz, O(2)(*)(-) and O(2)((1)Delta(g)) with 10 water-soluble cyclopropyl-fused C(60) derivatives including a mono-adduct dendro[60]fullerene (d) and C(60) derivatives based on C(60)[C(COOH)(2)](n=2-6), some of which are known to be neuroprotective in vivo. The rate constants for reactions of e(-)(aq) and HOz lie in the range 0.5-3.3 x 10(10) M(-1) s(-1). The d and bis-adduct monoanion radicals display sharp absorption peaks around 1000 nm (epsilon = 7 000-11 500 M(-1) cm(-1)); the anions of the tris-, tetra-, and penta-adduct derivatives have broader, weaker absorptions. The monohydroxylated radicals have their most intense absorption maxima around 390-440 nm (epsilon = 1000-3000 M(-1) cm(-1)). The anion and hydroxylated radical absorption spectra display a blue-shift as the number of addends increases. The radical anions react with oxygen (k approximately 10(7)-10(9) M(-1) s(-1)). The reaction of O(2)(*)(-) with the C(60) derivatives does not occur via an electron transfer. The rate constants for singlet oxygen reaction with the dendrofullerene and eee-derivative in D(2)O at pH 7.4 are k approximately 7 x 10(7) and approximately 2 x 10(7) M(-1) s(-1) respectively, in contrast to approximately 1.2 x 10(5) M(-1) s(-1) for the reaction with C(60) in C(6)D(6). The large acceleration of the rates for electron reduction and singlet oxygen reactions in water is due to a solvophobic process.

The ground-state absorption spectrum of C 76 was measured in toluene solution over the interval 300–1100 nm. Pulse radiolysis and laser flash photolysis have been used to determine properties of the lowest triplet-state of C 76 in benzene, including the triplet difference spectrum between 310 and 1100 nm, the difference and absolute molar absorption coefficients at 840 nm (e=4460 and 5730 M −1 cm −1 , respectively), the quantum yield of triplet formation ( Φ T =0.03±0.01) and the quantum yield for C 76 -sensitized formation of singlet oxygen in aerated benzene ( Φ Δ =0.04±0.01). Our results suggest that the S 1 –T 1 interval is smaller than in C 60 . The strongly lipophilic nature of C 76 , coupled with its low triplet and sensitized singlet oxygen yield, suggests its use as a carrier for delivery of highly polar drugs to hydrophobic sites and tissues without phototoxic sensitization.

Ground and triplet state spectroscopic and photophysical properties of C60H18 and C60H36 have been studied using the complementary techniques of nanosecond pulse radiolysis and laser flash photolysis to determine triplet-triplet (T-T) absorption spectra, triplet lifetimes, T-T molar absorption coefficients and triplet quantum yields, in particular for C60H18. The spectroscopic and photophysical propperties of C60H18 are similar to those of an sp2 network species such as C60. whereas those of C60H36 resemble benzene. No T-T absorption was observed for C60H36 at λ > 300 nm, so that its triplet state parameters could not be measured. The most probable assignment of the C60H36 samples studied, on the basis of vibrational and electronic spectra and structure, is to an S6 symmetry isomer, but the lowest energy T and D3d isomers are not entirely excluded. Our partially hydrogenated fullerene samples were prepared by a transfer hydrogenation method. Other preparation techniques may have produced other isomers of these species, thus accounting for some differences in reported spectroscopic results.

Redox reactions of endogenous and exogenous sulphur-containing compounds are involved in protection against oxidative damage arising from the incidence and/or treatment of many diseases, including cancer. We have investigated, via pulse radiolysis, the one-electron oxidation of ergothioneine, a molecule with antioxidant properties which is detected at millimolar concentrations in certain tissues and fluids subject to oxidative stress, including erythrocytes and plasma. The spectrum of the transient species, assigned to the product of one-electron oxidation, observed after reaction of ergothioneine with the oxidizing radicals OH•, N3• and CCl3O2• has a maximum absorption at 520 nm and is very similar to that obtained by oxidation of analogous molecules such as 2-mercaptoimidazole, 1-methyl-2-mercaptoimidazole, S-methyl- and S,N-dimethyl-ergothioneine. In the presence of vitamin C, the oxidized form of ergothioneine is repaired by a rapid reduction (k = 6.3×108 M-1·s-1) producing ascorbyl radicals. This co-operative interaction between ergothionine and ascorbate, similar to that previously observed between vitamin E and ascorbate, may contribute to essential biological redox protection.

The ground state and lowest triplet state properties of three hydrofullerenes, C60H2, C60H4 and [1-methylsuccinate-4-methyl-cyclohexadiene-2,3]buckminsterfullerene, have been investigated in toluene at room temperature. Their visible ground state absorption spectra are compared to those of methanofullerenes and methanolfulleroids. The triplet properties, including the absorption spectrum between 300 and 800 nm, molar absorption coefficient, quantum yield of formation and quantum yield for the photosensitized production of O 2 ( 1 Δ g ) , are compared to those of other C60 derivatives.

Pulse radiolysis and laser flash photolysis have been used to determine properties of the lowest triplet state of C70 in benzene at room temperature, including its absorption spectrum between 300 and 1620 nm, its molar absorption coefficients (ϵT=3800 dm3 mol−1 cm−1 at 970 nm) and its quantum yield of formation in benzene T=0.76±0.15. Measurements were also made of the quantum yield Δ=0.82±0.15 for the C70 sensitised formation of O2(1Δg). Transition assignments, oscillator strengths, and optical limiting properties of C70 are discussed.

Pulse radiolysis and, to a lesser extent, laser flash photolysis have been used to determine properties of the lowest triplet of C 60 in benzene at room temperature, including its absorption spectrum between 300 and 1600 nm, its molar absorption coefficient (ϵ T = 20200 ± 2000 dm 3 mol −1 cm −1 at 750 nm) and its quantum yield of formation (Φ T = 0.88 ± 0.15). The features of the triplet spectrum are assigned according to the triplet transition energies based on molecular orbital calculations previously reported; the strong band at 750 nm is attributed to the 2 3 G u -1 3 T 2g transition. Implications of the results are discussed, including those related to C 60 as an optical limiting material.

Transient electronic spectra of mono-hydroxylated fullerene radicals, initially derived from water soluble malonic acid derivatives (C60[C(COOH)2]n, n = 2–6) reacting with hydroxyl radical at a diffusion-controlled rate, show a unimolecular growth in absorption on a timescale of 0.1–1 ms. It is suggested that the reason for this slower growth is migration of the hydroxy group to a more stable position. Semi-empirical molecular orbital (AM1 and PM3) calculations on the symmetry-distinct C60[C(COOH)2]3OH− anions derived from two most abundant tris(malonate)s demonstrate a relation between Coulson’s free valence index for radical attack at a particular position and the stability of the corresponding product. They also suggest that after attack of the hydroxyl radical at the first encountered double bond, migration subsequently takes place to the position of highest free valence.

A variety of molecular triads and dyads consisting of covalently linked carotenoid (C) andor porphyrin (P) moieties have been prepared and studied with transient absorption and time-resolved fluorescence techniques. Diporphyrins of the type P A -P B and C-P A -P B triads demonstrate interporphyrin singlet-singlet energy transfer with rate constants ranging from 8.1×10 8 to 2.3×10 10 s −1 . Interporphyrin photoinitiated electron transfer is observed in molecules possessing sufficient thermodynamic driving force to produce P A •+ -P B •− and C-P A •+ -P B •− charge-separated states. The electron-transfer rate constant increases with increasing reaction free energy change for the molecules studied, and rate constants up to 3.5×10 9 s −1 and quantum yields up to 0.68 were measured. The carotenodiporphyrin triad systems undergo a subsequent electron-transfer step to give final C •+ -P A -P B •− states

The quantum yield of triplet formation, phi T, and that of the photosensitized formation of singlet molecular oxygen, phi delta, were determined for a rare nucleoside, 4-thiouridine (4t-Urd), in water and in acetonitrile, using singlet molecular oxygen phosphorescence, laser-induced optoacoustics and time-resolved thermal lensing. These yields, phi T and phi delta, the latter in aerated solutions, were found to be, respectively, in water: 0.67 +/- 0.17 and 0.18 +/- 0.04 and in acetonitrile: 0.61 +/- 0.15 and 0.50 +/- 0.20. The fraction of the 4t-Urd triplet molecules quenched by oxygen leading to singlet molecular oxygen, S delta, was calculated to be between 0.7 and unity in both solvents, this value being indicative of a pi pi*character for the lowest triplet state of 4t-Urd.

Tetraarylporphyrins substituted with nitro groups at beta-pyrrolic positions are potential candidates for electron-accepting pigments in model systems for photosynthesis. The photophysics of 2-nitro-5,10,15,20-tetra-p-tolylporphyrin and its zinc analog have been studied in order to evaluate this potential. The ground state absorption spectrum, the triplet-triplet absorption spectrum, the fluorescence emission spectrum, and associated photophysical parameters have been determined. The molecules have short singlet lifetimes and anomalous temperature- and solvent-dependent emission spectra which are consistent with the formation of an intramolecular charge transfer state of the type P+.-NO2-. in which the nitro group is twisted about its bond to the porphyrin, relative to the ground state conformation.

This review describes recent advances in the development of DNA- photocleavage agents. Major mechanisms of photosensitized DNA photocleavage are presented and the most popular categories of compounds are considered, which include metal complexes and many organic functional derivatives. DNA-targeted conjugates of photosensitizers are also described and discussed

Aggregation of C60 and of three C60 derivatives in phosphatidylcholine liposome model membranes was studied via photophysical investigations. The triplet properties of these fullerenes, including the absorption spectrum between 400 and 900 nm, molar absorption coefficient, quantum yield of formation and/or quantum yield for the photosensitised production of 1O2 were determined in benzene solution and in liposomes. One of the derivatives was a cyclopropyl-fused hexa-addended C60 containing as addend C(CO2C18H37)2 (1). The other two derivatives were cyclohexyl-fused C60 with leucine (2) or hydrophilic acidic (3) mono-addend functionalisation. The synthesis of derivative 2, by thermal extrusion of sulfur dioxide from the corresponding sulfone in the presence of C60, is described. [60]Fullerene and derivative 1 show no T–T absorption in liposomes, although they do so in benzene solution (with a low triplet quantum yield for hexa-addended 1). The absence of triplet–triplet absorption in liposomes is indicative of fullerene aggregation. By contrast, the amphiphilic mono-addend derivatives 2 and 3 present appreciable T–T absorption in liposomes, although less marked than in benzene solution. Mono-addend functionalisation therefore appears to be an interesting way to prevent or diminish fullerene aggregation. Moreover, in this type of derivatisation most of the photophysical properties of pristine [60]fullerene are retained.

The three [6,6]-ring bridged methanofullerenes 1, 2, and 3 were synthesized; their ground-state absorption spectra in toluene and cyclohexane solutions are presented and possible assignments discussed. Although these three molecules are very different, they have very similar triplet–triplet spectra and near-unity values for the quantum yield for O2(1Δg) production by energy transfer from the triplet states, implying that the quantum yield of triplet production is approximately 1.

Due to its hydrophobic character, the hydrocarbon core of ultrathin phospholipid bilayer membranes lends itself to the insertion of fullerenes. When doped with C 60 these membranes are photoconducting, the photoconductivity being affected by the presence of quenchers of the excited triplet state such as oxygen. The photoconductivity involves trans-membrane electron transfer; its dependence on C 60 concentration and on the excitation wavelength suggests the participation of small aggregates in the membrane. As derived from time-resolved triplet absorption and fluorescence quenching measurements, the suggested mechanism makes more precise (i) the role of the aggregates which might act as a collecting antenna for the benefit of the monomers, and (ii) the role of the monomers which have a much higher yield of photoinduced triplet state formation than the aggregates.

The bimolecular rate constants (k) of quenching of molecular singlet oxygen 1O2 (1 delta g) by organo-selenium compounds exhibiting glutathione peroxidase activity and by sulfur analogs have been determined by time resolved phosphorescence detection of 1O2 in CD3OD and C6D6, with no solvent effect. The rate constants of quenching by the Se-containing compounds were found to be approximately one order of magnitude higher than those of the S-containing homologs. A linear correlation was observed between log k and the Hammett constant omega ortho with p = -0.89, the rate constant being higher for molecules with an electron-donating substituent and lower for those with an electron-withdrawing substituent. This observation is consistent with the involvement of a charge transfer complex in the deactivation of singlet oxygen.

The inhibitory effects of synthetic phenolic compounds on benzo(a)pyrene-induced neoplasia of the mouse forestomach have been measured by Wattenberg et al6 The efficiency of this inhibition has been estimated for each phenol, using R, the ratio of the number of tumors per mouse in the protected group over the number of tumours per mouse in the control group. We have observed a linear correlation between the chemoprotection efficiency R and the logarithm of the rate of quenching of singlet oxygen. k. by this family of phenols, log k being itself correlated with the one-electron oxidation potential of the phenols. These correlations suggest a charge transfer mechanism for the inhibition of neoplasia induced hy benzo(a)py-rene. The correlations described provide a theoretical basis for scaling the inhibitors of mutagenicity induced by polycyclic aromatic compounds in terms of their oxidation potentials

Thermodynamic and kinetic aspects of intramolecular electron-transfer reactions in carotenoporphyrin dyads and carotenoid-porphyrin-quinone triads have been studied by using pulse radiolysis and cyclic voltammetry. Rapid (

Nanosecond flash photolysis of rhodopsin with 530 or 353 nm light produces an initial transient absorption spectrum with peaks at ˜57O and ˜420nm, and a subsequent transient species with a maximum absorption at 480 nm. These results are interpreted as the initial formation of prelumi-rhodopsin (570 nm) followed by its conversion to lumirhodopsin (470 nm). The peak at 420 nm in the first transient may be due to either hypsorhodopsin or isorhodopsin.

—The triplet excited states of four derivatives of ubiquinone-30, in which various ring substituents are progressively altered, have been studied by laser flash photolysis (265 nm) and pulse radiolysis (9–12 MeV electrons). Triplet absorption spectrum, extinction coefficient, lifetime, energy level and quantum efficiency of formation were determined. By comparison with previous studies with ubiquinone-30, it is deduced that the low triplet energy and quantum efficiency of formation of triplet ubiquinone-30 is caused by the presence of the two adjacent methoxy substituents, rather than to the isoprenoid side chain. The low quantum efficiency of triplet formation, although consistent with in vivo ubiquinone photomodification occurring via the triplet, suggests that little of the ubisemiquinone observed in bacterial photosynthesis is formed via excited ubiquinone.

Carotenoporphyrin-quinone triad molecules have previously been found to successfully mimic many aspects of photosynthetic electron and energy transfer. These molecules generate long-lived, energetic charge-separated states via a biomimetic multistep electron transfer scheme. In many of these molecules, the overall quantum yield for charge separation has been limited by an unfavorable partitioning of an intermediate charge-separated state between charge recombination and further reaction to yield the desired species. One strategy for overcoming this limitation is based on the fact that, in general, electron transfer rate constants do not depend linearly on reaction free energy change. In the triads, raising the energy of the intermediate charge-separated state would be expected to speed up the desired electron transfer (“normal” Marcus behavior), but slow down charge recombination (which formally lies in the “inverted” region). Laser flash photolysis and transient fluorescence measurements on two new triad molecules reveal enhanced quantum yields of long-lived charge-separated states which are consistent with this expectation.

— A new molecule consisting of a carotenoid polyene covalently linked to a pyropheophorbide has been prepared. This molecule exists in solution in an extended conformation with the carotenoid moiety relatively far from the tetrapyrrole π-electron system. The compound shows negligible antenna function (singlet energy transfer from carotenoid to pyropheophorbide), but substantial photoprotection from singlet oxygen production (due to rapid triplet energy transfer from pyropheophorbide to carotenoid). The photochemical properties of this molecule parallel those of conformationally similar carotenoporphyrins. Thus, carotenoporphyrins appear to be valid synthetic systems for modeling in vivo carotenoid-chlorophyll interactions.

— The absorption spectra, lifetimes, extinction coefficients and intersystem crossing quantum yields of the lowest triplet T1 of 4-thiouridine have been determined both in acetonitrile and in water. An ordering of 1,3(n,π)* and 1,3(π,π)* states is suggested. Triplet quenching rate constants with various pyrimidine bases or amino acids are reported.

— 3-Carbethoxypsoralen (3-CPs) has been tested in the photochemotherapy of psoriasis. It only forms monoadducts with DNA and is being considered as a non-carcinogenic alternative to 8-MOP which itself forms DNA crosslinks that arc difficult to repair. Using laser flash photolysis or pulse radiolysis, the triplet state of 3-CPs, a possible intermediate in photosensitization, has been generated in several solvents: ethanol, water and benzene. The triplet lifetime, extinction coefficient and quantum efficiency of formation have been measured. Triplet reactivities towards (i) the solvents used, (ii) 3-CPs, (iii) oxygen, (iv) tryptophan and (v) tyrosine, leading, respectively, to photoadditions with water, ethanol and 3-CPs, to 1O2, semioxidized tryptophan and semioxidized tyrosine, (vi) thymine and (vii) uracil have been investigated. The dark binding of 3-CPs to DNA has been studied by comparing the reactivity of eaq- with free 3-CPs, free DNA and the 3-CPs DNA complex. Some photophysical and photochemical properties of 4′,5′di-hydro-3-carbethoxypsoralen(DH–3-CPs), model of the main fluorescent photo-product of 3-CPs, have also been investigated. Biological consequences of the photochemical properties of 3-CPs andDH–3-CPs have been studied in a cellular system (haploid yeast).

Triplet absorption spectra, extinction coefficients (ɛT), decay rates (K1), oxygen quenching rates (kq) and intersystem crossing yields (φT) for 3-carbethoxypsoralen (3-CPs). 8-methoxypsoralcn (8-MOP) and 5-methoxypsoralen (5-MOP) in methanol are reported. For 8-MOP and 3-CPs corresponding values are also reported with water as the solvent. Some photophysical data are also reported for 5-MOP in water, but ɛT and φT were not obtained. The phosphorescence spectra for these furocoumarin derivatives in ethanol at 77 K are reported together with the corresponding lowest triplet energy and lifetime. The values of the various photophysical properties obtained are compared with values reported by previous workers.

— Photophysical and photodynamic properties of a ehlorin type molecule derived from hydroxyethylvinyldeuteroporphyrin are presented. It photosensitizes singlet oxygen production as efficiently as mesotetraphenylporphin. The high absorptions of both its ground and triplet states in the red (660 nm) make it a potent photosensitizer which might act not only by photo-oxidation via singlet oxygen but also by radicals produced via sequential biphotonic absorption.

The photophysical properties of the lowest excited triplet states of pyrido[3,4-c]psoralen (PyPs) and 7-methylpyrido[3,4-c]psoralen (MePyPs) have been investigated by laser flash photolysis, including energy transfer from these triplets to oxygen-generating singlet oxygen. A parallel study of the photosensitization of yeast in vivo by these compounds in the presence and absence of oxygen is also reported. The low triplet and hence singlet oxygen yields, reflected in the lack of an oxygen effect in yeast, suggest that photoadditions to DNA are likely to be the main source of the photosensitized lethal effects induced by these pyridopsoralens in vivo.

Carotenoid-porphyrin-quinone triad molecules undergo a photodriven two-step electron-transfer reaction which results in the generation of a high-energy charge-separated state with a lifetime on the microsecond time scale at ambient temperatures in fluid solution. These systems mimic the initial charge separation steps of photosynthesis. A series of these tripartite molecules which differ systematically in the nature of the linkages joining the porphyrin to the quinone and carotenoid moieties has been synthesized in order to investigate the effect of structure on the yield and lifetime of the charge-separated state. The time-averaged solution conformations of these molecules have been determined from porphyrin ring current induced shifts in the /sup 1/H NMR resonances of the carotenoid and quinone moieties. Studies of the triads and related molecules in dichloromethane solution using time-correlated single photon counting fluorescence lifetime techniques have yielded the rate constant for the first of the photoinitiated electron-transfer steps as a function of the linkage joining the porphyrin and the quinone. The rate constants range from 1.5 x 10/sup 8/ to 9.7 x 10/sup 9/ s/sup -1/. For most members of the series, the results are consistent with an exponential dependence of the electron-transfer rate on the experimentally determined donor-acceptor separation, with themore » exponential factor ..cap alpha.. = 0.6 A/sup -1/.« less

Four transient species have been detected in the time scale 10−9 to 10 seconds following pulse radiolysis of dimethyl sulphoxide.

1. 1. Semiquinone anion minus quinone (Q − -Q) and neutral semiquinone minus quinone (QH-Q) difference spectra have been measured for plastoquinone-9, ubiquinone-10 and ubiquinone-0, using pulse radiolysis in methanolic solution. The difference spectra were used to determine the absolute spectra of each semiquinone. 2. 2. The (Q − -Q) difference spectrum for plastoquinone-9 is similar to the difference spectrum detected at a time less than 20 ns after flash excitation of the photosynthetic light reaction Photosystem II in spinach chloroplasts [Stiehl, H. H. and Witt, H. T., (1968) Z. Naturforsch. 23b, 220–224; Witt, H. T. (1971) Q. Rev. Biophys. 4, 365–477]. 3. 3. The ubiquinone Q − -Q spectra are similar to difference spectra found on illumination of reaction centre preparations from Rhodopseudomonas spheroides and Rhodospirillum rubrum [Slooten, L. (1972) Biochim. Biophys. Acta 275, 208–218; Clayton, R. K. and Straley, S. C. (1972) Biophys , J. 12, 1221–1234]. Ubiquinone has been suggested to be closely associated with the primary electron acceptor of bacterial photosynthesis. 4. 4. Rate constants were measured for formation of semiquinones by reaction of e staggered− staggeredCH 3 OH , ĊH 2 OH and ĊH 2 O − with quinone, and for the decays of the neutral semiquinones. The semiquinone anions were stable for seconds. The ubisemiquinone-6 anion reacts only slowly with oxygen, k ≈ 10 3 M −1 · s −1 . 5. 5. The effect of substituents upon semiquinone physicochemical properties is discussed.

Nanosecond flash photolysis has been carried out on benzophenone solutions in benzene and acetonitrile in presence of oxygen- and sulfur-containing reductants such as anisole, thioanisole, dibutyl sulfide and methyl 2-octyl ether. The quantum yield of ketyl formation is compared to previous results on the quantum yields of the final benzophenone disappearance. This comparison is also related to previous data on benzophenone reduction by nitrogen-containing reductants such as amines and amides and suggests a general mechanism for benzophenone photoreduction by heteroatom-containing donors.

The triplet—triplet absorption spectra, quantum efficiencies of intersystem crossing, triplet extinction coefficients, and triplet energy levels of 1,6-diphenyl-1,3,5-hexatriene, and 1,8-diphenyl-1,3,5,7-octatetraene, in ethanol and benzene have been obtained. The quantum efficiencies of intersystems crossing are found to be less than 0.03 for both molecules in both solvents, indicating that the excited singlet is efficiently deactivated by internal conversion and flourescence emission. The triplet—triplet absorption spectra and the triplet energy level of 1,6-diphenyl-1,3,5,hexatriene are compared with previous results in other solvent systems.

THIS note presents results from a neodymium laser flash spectroscopy study of bovine rod outer segments (ROS), the only chromophore of which is rhodopsin. Previously unreported transients are interpreted in terms of a charge-transfer excited state which gives rise to radicals and/or triplets.

Photochemical transmembrane electron transfer processes are an integral part of natural photosynthetic solar energy conversion and are also central to the design of biomimetic energy conversion schemes1–6. Here we report the synthesis and membrane-associated photoelectrochemical properties of carotenoporphyrin–quinone triad (I), a compound containing a photochemically active porphyrin and electron donor and acceptor moieties, and with the molecular architecture necessary to span a phospholipid bilayer. On excitation of compound 1 by visible light, charge is separated across a planar phospholipid bilayer membrane (BLM) in an intramolecular step; in the presence of suitable electron donor and acceptor species in the aqueous phases, a steady-state photo-current is observed in an external circuit bridging the BLM. Artificial membranes containing I thus mimic key features of the photodriven transmembrane electron transfer processes characteristic of photosynthetic organisms.

Photosynthesis converts light to chemical potential energy in the form of long-lived charge separation across a bilayer membrane. Recombination of the charge-separated state is prevented by the large distance between the final electron donor and acceptor. Because the quantum yield of a single long-range electron transfer across the bilayer would be vanishingly small, reaction centers employ a series of electron-transfer steps, each of which occurs over a short distance with high yield. Synthetic carotenoid-porphyrin-quinone triad molecules and other molecular systems which begin to model this approach have recently been reported. In photosynthetic bacteria, the charge separation sequence includes electron donation from a bacteriopheophytin to a quinone which then transfers an electron to a second quinone. An elegant molecule that models such electron transfer has been prepared, although it did not reproduce the long-lived charge separation characteristic of the natural system. The authors now report the synthesis of a tetrachromophoric molecule 1 consisting of a porphyrin (P) covalently linked to both a carotenoid polyene (C) and a rigid diquinone moiety (Q/sub A/-Q/sub B/), excitation of which produces a long-lived (460 ns in dichloromethane, 4 ..mu..s in acetonitrile charge-separated state C/sup .+/-P-Q/sub A/-Q/sub B//sup 0 -/ with a quantum yield of 0.23more » at ambient temperatures and 0.50 at 240 K.« less

A new carotenoporphyrin has been prepared in which a synthetic carotenoid is joined to a tetraarylporphyrin through a flexible trimethylene linkage. This molecule exists primarily in an extended conformation with the carotenoid chromophore far from the porphyrin π- electron system. In benzene solution, where large-amplitude molecular motions are rapid, the molecule can momentarily assume less stable conformations which favor triplet energy transfer, and quenching of the porphyrin triplet by the carotenoid is fast. In a polystyrene matrix or frozen glass such motions are slow, and energy transfer cannot compete with other pathways for depopulating the triplet state. These observations help establish the requirements for biological photoprotection .

The key steps in the photosynthetic conversion of light to chemical potential energy include not only photodriven charge separation, but also prevention of the back-reaction (charge recombination). Although the first of these steps has been achieved in several biomimetic solar energy conversion systems, retarding the back-reaction has proved more difficult. This may be accomplished by rapidly moving the electron, the hole, or both away from the site of excitation to more stabilizing environments. In photosynthetic membranes, the electron is transferred sequentially over several closely coupled molecules, including tetrapyrroles and quinones1–3. In semiconductor/liquid interfacial systems both the electron and the hole migrate following excitation4,5. We now report that substantial slowing of the back-reaction has been achieved with a tripartite molecule in which a long-lived photodriven charge-separated state of relatively high potential is formed from an excited singlet state in accordance with the above principles. This molecular triad (compound I) consists of a tetraarylporphyrin covalently linked to both a carotenoid and a quinone. In solution, excitation of the porphyrin moiety by visible light results in the rapid (

Pulse radiolysis of o- m-, and p-xylene mesitylene and and pseudocumene give rise to three main absorption spectra at about 600,400 and 320 nm. Those transients might be correlated to an excimer state (600nm) a biradical (320 nm) a triplet state (400 nm).

Molecular triads consisting of an amphipathic porphyrin derivative linked to a quinone and to a carotenoid polyene have been synthesized in order to model the transmembrane charge separation observed in reaction centers of photosynthetic organisms. Electron transfer to biradical triad species in solution has been demonstrated. Amphipathic triads have been shown to incorporate into bilayer lipid membranes and upon excitation to form biradical species which undergo electron transfer reactions with aqueous phase donors. This interfacial electron transfer reaction is catalyzed by cytochrome c. When incorporated into planar BLM systems these triads demonstrate transmembrane photoconduction which is remarkably similar to that observed when reaction centers are incorporated into BLMs.

— Absorption spectra of triplets, radical anions and radical cations of four furocoumarins, psoralen (Ps), 8-methoxypsoralen (8MOP), 5-methoxypsoralen (5MOP) and 3-carbethoxypsoralen (3CPs), have been determined by laser flash photolysis and pulse radiolysis. The triplet spectra of 8MOP, 5MOP and 3CPs are strongly modified when going from an H-bonding solvent such as water to a non H-bonding solvent as benzene or acetonitrile while the triplet spectrum of Ps is solvent independent. Theoretical considerations using a CNDO/S method are able to explain the existence of these two different triplets. For 8MOP, 5MOP and 3CPs in water the triplets might be considered as triplet exciplexes 3(FCδ-. H2Oδ+) consistent with these triplet spectra being similar to the spectra of the corresponding radical anions.