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PubMed1990

Chalcogenapyrylium dyes as photochemotherapeutic agents. 2. Tumor uptake, mitochondrial targeting, and singlet-oxygen-induced inhibition of cytochrome c oxidase.

Cationic selena- and tellurapyrylium dyes 1d-g and 1i were found to inhibit cytochrome c oxidase upon irradiation of isolated mitochondrial suspensions treated with 10 microM solutions of dye. The amount of inhibition by these dyes was found to be related to oxygen concentration and inversely related to the concentration of added imidazole, a singlet-oxygen trap, suggesting that singlet oxygen is responsible, at least in part, for the inhibition of the enzyme. Dyes 1d-g and 1i, containing either selenium or tellurium, produce singlet oxygen with a quantum efficiency, phi (1O2), between 0.005 and 0.09 in methanol. Dyes 1a-c, containing the lighter chalcogens oxygen and sulfur, have values of phi (1O2) that are less than 0.0008 in methanol and do not inhibit cytochrome c oxidase in irradiated mitochondrial suspensions. Dyes 1c and 1d have nearly identical spectral and redox properties. Only the selenapyrylium dye 1d inhibits the enzyme, suggesting that neither ground-state nor excited-state electron transfer is important in inhibition of the enzyme. Electron micrographs of human U251 glioma cells, treated in vitro with 1i and light, showed pronounced morphology changes in the mitochondrial membranes relative to electron micrographs of untreated cells. Epifluorescence microscopy of the treated cells showed granular yellow-green fluorescence presumably from photooxidized dye in the mitochondria.

Adenocarcinoma↗
PubMed2001

Role of singlet diradicals in reactions of 2-carbenabicyclo[3.2.1]octa-3,6-diene.

The generation of 2-carbenabicyclo[3.2.1]octa-3,6-diene (1) results in the formation of C(8)H(8) hydrocarbons endo-6-ethynylbicyclo[3.1.0]hex-2-ene (4), semibullvalene (5), and 5-ethynyl-1,3-cyclohexadiene (6), and C(8)H(10) hydrocarbons bicyclo[3.2.1]octa-2,6-diene (7), tricyclo[3.2.1.0(4,6)]oct-2-ene (8), and tetracyclo[3.3.0.0(2,8)0(4,6)]octane (9). Focus is placed on three mechanistic pathways for the formation of the C(8)H(10) hydrocarbon fraction: (a) abstraction of hydrogen by triplet carbene 1T to produce an equilibrating set of monoradicals, (b) interconversion of triplet carbene 1T into tricyclic triplet diradical 19T and tetracyclic triplet diradical 20T, and (c) interconversion of singlet 1S with analogous singlet diradical 19S and 20S. Ab initio calculations at the (U)B3LYP/6-311+G(3df,2p)//(U)B3LYP/6-31G(d,p) and broken spin symmetry UBS B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d,p) levels rule out choices (a) and (b) and are consistent with the singlet diradical process.

Bridged Bicyclo Compounds↗
PubMed2004

Theoretical designs of singlet localized 1,3-diradicals.

Spin preference and S-T gaps of localized 1,3-diradicals were studied by an orbital phase theory and theoretical calculations. The orbital phase theory was applied to rationalize thermodynamic and kinetic stabilities of diradicals. We theoretically designed some singlet localized 1,3-diradicals, substituted trimethylenes, which are more stable than the lowest triplets. Some diradicals with the four-membered rings, 2,4-disilacyclobutane-1,3-diyls, were designed and shown to have singlet ground states and to be more stable than the sigma-bonded isomers, 2,4-disilabicyclo[1.1.0]butanes. The ab initio calculations supported predictions of the stable carbon-centered localized singlet 1,3-diradicals.

Journal Article↗
PubMed2005

Two-photon photosensitized production of singlet oxygen: sensitizers with phenylene-ethynylene-based chromophores.

[reaction: see text] Singlet molecular oxygen (a(1)Delta(g)) has been produced and optically monitored in time-resolved experiments upon nonlinear two-photon excitation of photosensitizers that contain triple bonds as an integral part of the chromophore. Both experiments and ab initio computations indicate that the photophysical properties of alkyne-containing sensitizers are similar to those in the alkene-containing analogues. Most importantly, however, in comparison to the analogue that contains double bonds, the sensitizer containing alkyne moieties is more stable against singlet-oxygen-mediated photooxygenation reactions. This increased stability can be advantageous, particularly with respect to two-photon singlet oxygen imaging experiments in which data are collected over comparatively long time periods.

Journal Article↗
PubMed2005

(Sensitized) photolysis of diazonium salts as a mild general method for the generation of aryl cations. Chemoselectivity of the singlet and triplet 4-substituted phenyl cations.

The photolysis of a series of 4-X-benzenediazonium tetrafluoroborates is studied in MeCN. Loss of nitrogen occurs from the singlet excited state with X=H, t-Bu, and NMe2 and leads to the singlet aryl cation. This adds to the solvent yielding the corresponding acetanilides. With other substituents, ISC competes with (X=Br, CN) or overcomes (X=COMe, NO2) fragmentation and the aryl cation is formed in part or completely in the triplet state. In neat MeCN, this either abstracts hydrogen from the solvent (in most cases inefficiently) or undergoes intersystem crossing to the more stable singlet that reacts as above. In the presence of pi nucleophiles (allyltrimethylsilane or benzene), the triplet aryl cation is efficiently trapped giving substituted allylbenzenes and biphenyls, respectively. By triplet sensitization by xanthone, the triplet cation and the products from it are obtained from the whole series considered. The direct or sensitized photodecomposition of diazonium fluoroborates, substituted with both electron-donating and -withdrawing substituents, in the presence of alkenes and arenes offers an access to an alternative arylation procedure.

Journal Article↗
PubMed1996

Antenna-Initiated Photochemistry in Polyfunctional Steroids. Intramolecular Singlet and Triplet Energy Transfer between Aryl, Ketone, and Alkene Groups in 6beta-(Dimethylphenylsiloxy)-5beta-androstanes(1).

Steroids have been prepared that bear a dimethylphenylsiloxy (DPSO) group and additional C3 and/or C17 ketone functionalities. The DPSO group has been used to harvest 266 nm photons and then activate the ketone functionalities through intramolecular singlet-singlet energy transfer (intra-SSET). Thus, the monoketones 3,3-(ethylenedioxy)-6beta-(DPSO)-5beta-androstan-17-one (6) and 6beta-(DPSO)-5beta-androstan-3-one (8) both exhibit DPSO-initiated photochemistry at the carbonyl groups. Irradiation of the diketone, 6beta-(DPSO)-5beta-androstane-3,17-dione (5), gives two ring D-derived photoproducts, an epimer (19) and an enal (18), both coming from the C17 ketone excited singlet state. Here Phi(intra-SSET) from the aryl antenna to the carbonyl groups is ca. 88% efficients and occurs with a rate of ca. 6.5 x 10(9) s(-)(1), with the chemistry indicative of facile intra-SSET between the C3 and C17 ketones. The alkylidene group at C3 (i.e., as in 6beta-(DPSO)-3(E)-ethylidene-5beta-androstan-17-one (33) and its Z isomer (34)) has no effect on the rate or efficiency of aryl activation of the C17 ketone.

Journal Article↗
PubMed1997

Photochemical Reactions of Mesityl Azide with Tetracyanoethylene: Competitive Trapping of Singlet Nitrene and Didehydroazepine.

Irradiation of the title azide 4 in the presence of TCNE gives a mixture of two stable adducts. One of them is identified as the azomethine ylide 5, the structure of which is strictly determined by X-ray crystallography. The other is spectroscopically assigned to the spiroazepine 6. The effect of wavelength of the light employed in the photolysis reveals that the TCNE-4 charge-transfer complex (lambda(max) 454 and 550 nm in dichloromethane) does not participate in the adduct formation. The ratio of the adducts obtained in the photolysis is dependent linearly upon the initial concentration of TCNE, which strongly suggests that the adducts 5 and 6 are produced by competitive trapping of singlet mesitylnitrene (8S) and trimethyldidehydroazepine (9), respectively. The rate constant for the reaction of 8S with TCNE is estimated to be on the order of 10(9) M(-)(1) s(-)(1) or greater. The PM3 calculation indicates that the azomethine ylide 5 is thermodynamically more stable than the aziridine 7, which is thought to be initially formed by the reaction of 8S with TCNE. Thus, we propose that these findings make the first example of competitive trapping of singlet arylnitrene and its ring-expanded isomer with an alkene, which definitely reveals the intervention of singlet nitrene in the photolysis of an aryl azide.

Journal Article↗
PubMed2000

Paterno-Buchi photocyclization of 2-siloxyfurans and carbonyl compounds. Notable substituent and carbonyl (Aldehyde vs ketone and singlet- vs triplet-excited state) effects on the regioselectivity (Double-bond selection) in the formation of bicyclic exo-oxetanes

Paterno-Buchi coupling, photochemical [2 + 2] cycloaddition, of carbonyl compounds 2a-f with 2-siloxyfurans 1a-d has been investigated in detail. The stereoselective formations of exo-oxetanes 3 and 4 were observed in high yields. The regioselectivity (double-bond selection, 3 vs 4) was found to be largely dependent upon the carbonyls, the substituents at the furan ring, and the excited state of the carbonyls (singlet vs triplet). The photoreaction of aldehydes 2a-c gave bicyclic exo-oxetanes 3 and 4 at regio-random, independent upon their excited states and the substituents at furan ring. However, the photoreaction of the triplet state of ketones 2d-f was found to give regioselectively exo-oxetanes 4, except for the 4-methyl-2-siloxyfurane 1d case. The singlet-excited state of acetone 2f gave both oxetanes 3 and 4 at regio-random. For the singlet-state photochemistry, the approach direction of the electrophilic oxygen of the excited carbonyls to the furan ring is proposed to be an important factor for the exo-stereoselection. The Griesbeck model can rationalize the regio- and exo-selective formation of oxetanes in the triplet-state photoreaction.

Journal Article↗
PubMed2005

Time-resolved spectroscopy of the excited singlet states of tirapazamine and desoxytirapazamine.

Laser flash photolysis (LFP, 400 nm excitation) of the anti-cancer drug tirapazamine (TPZ) in acetonitrile produces the singlet excited-state S1 with lambda(max) = 544 nm. The lifetime of this state is 130 ps, in good agreement with the reported fluorescence lifetime. The excited state is reduced to the corresponding radical anion by KSCN or KI. The spectrum of the radical anion is in good agreement with previously reported pulse radiolysis studies and time-dependent density functional theory (TD-DFT) calculations. LFP of desoxytirapazamine (dTPZ) also produces the first excited singlet state, S1. The fluorescence quantum yield and lifetime (5.4 ns) of the dTPZ singlet excited state are both much greater than the corresponding values of TPZ. This is explained by DFT calculations that predict that cyclization of TPZ to form an oxaziridine is thermodynamically facile but that cyclization of dTPZ to form an oxadiaziridine is not. Thus, the S1 state of TPZ has a short lifetime and low fluorescence quantum yield due to ready cyclization whereas the cyclization of the S1 state of dTPZ is unimportant and does not limit either the fluorescence quantum yield or the fluorescence lifetime. This conclusion is confirmed by studies of dTPZ', an isomer of dTPZ containing the C=N-O moiety which has a low quantum yield and short fluorescence lifetime similar to that of TPZ.

Journal Article↗
PubMed2005

Second hyperpolarizability (gamma) of singlet diradical system: dependence of gamma on the diradical character.

The dependence of the second hyperpolarizability (gamma) on the diradical character (y) for singlet diradical systems is investigated using a model compound, the p-quinodimethane (PQM) molecule with different both-end carbon-carbon (C-C) bond lengths, by several ab initio molecular orbital and density functional theory methods. The diradical character based on UHF calculations indicates that at equilibrium geometry PQM is in a singlet ground state and primarily exhibits a quinoid structure, whereas the diradical character increases when increasing both-end C-C bond lengths. At the highest level of approximation, that is, using the UCCSD(T) method with the 6-31G+diffuse p (zeta = 0.0523) basis set, the longitudinal static gamma of PQM presents a maximum value for intermediate diradical character (y approximately 0.5) while the gamma values are larger for intermediate and large diradical character (y approximately 0.5-0.7) than for small diradical character (y < 0.2). This feature suggests that the gamma values of singlet diradical systems in the intermediate and somewhat strong correlation regimes are significantly enhanced as compared to those in the weak correlation regime. These results are substantiated by a complementary study of the variation in gamma upon twisted ethylene.

Journal Article↗
PubMed2005

Singlet and triplet products of the geminate recombination of a radical pair with a single magnetic nucleus (I = 1/2).

The double-channel recombination and separation of the photochemically created singlet radical pair is investigated, taking into account the spin conversion in a zero magnetic field and the arbitrary initial distance between the radicals. The quantum yields of the singlet and triplet products and the free radicals production are found analytically, assuming that the recombination of the diffusing radicals occurs at contact. All the yields are related to the singlet and triplet populations of the recombining radical pair, subjected to spin conversion and contact exchange interaction. The general analytical expressions for the quantum yields are specified for the particular limits of the weak and strong exchange. They are greatly simplified in the case of polar solvents, especially at the contact start. A close similarity is obtained with the results of a previously developed incoherent model of spin conversion, provided that the conversion rate is appropriately related to the hyperfine coupling constant.

Journal Article↗
PubMed2006

Singlet energy migration along an alternating block copolymer of oligothiophene and oligosilylene in solution.

The singlet excited-state properties of the block copolymers of oligothiophene and oligosilylene in solution were investigated with several fast spectroscopic methods. Time-resolved fluorescence measurements at room temperature and in a glassy matrix revealed that the singlet excited states of the block copolymers are deactivated accompanying structural changes of the polymer. It became clear from the transient absorption spectroscopy that the absorption peak of the singlet excited state shifted to the longer wavelength side compared to that of the corresponding oligothiophenes because of the sigma-pi conjugation of the oligothiophene and oligosilylene. The intersystem crossing process generating the triplet excited state was also revealed by the transient absorption spectroscopy. Energy migration along the polymer chain was revealed by the fluorescence anisotropy measurements. The time constant for the energy migration became faster as the size of the oligothiophene in the polymer repeating unit became shorter. From comparison with the Förster theory, the energy migration process was attributed to an incoherent hopping mechanism.

Journal Article↗
PubMed2006

The effect of solvent polarity on the balance between charge transfer and non-charge transfer pathways in the sensitization of singlet oxygen by pipi triplet states.

A large set of literature kinetic data on triplet (T(1)) sensitization of singlet oxygen by two series of biphenyl and naphthalene sensitizers in solvents of strongly different polarity has been analyzed. The rate constants and the efficiencies of singlet oxygen formation are quantitatively reproduced by a model that assumes the competition of a non-charge transfer (nCT) and a CT deactivation channel. nCT deactivation occurs from a fully established spin-statistical equilibrium of (1)(T(1)(3)Sigma) and (3)(T(1)(3)Sigma) encounter complexes by internal conversion (IC) to lower excited complexes that dissociate to yield O(2)((1)Sigma(g)(+)), O(2)((1)Delta(g)), and O(2)((3)Sigma(g)(-)). IC of (1,3)(T(1)(3)Sigma) encounter complexes is controlled by an energy gap law that is generally valid for the transfer of electronic energy to and from O(2). (1,3)(T(1)(3)Sigma) nCT complexes form in competition to IC (1)(T(1)(3)Sigma) and (3)(T(1)(3)Sigma) exciplexes if CT interactions between T(1) and O(2) are important. The rate constants of exciplex formation depend via a Marcus type parabolic model on the corresponding free energy change DeltaG(CT), which varies with sensitizer triplet energy, oxidation potential, and solvent polarity. O(2)((1)Sigma(g)(+)), O(2)((1)Delta(g)), and O(2)((3)Sigma(g)(-)) are formed in the product ratio (1/6):(1/12):(3/4) in the CT deactivation channel. The balance between nCT and CT deactivation is described by the relative contribution p(CT) of CT induced deactivation calculated for a sensitizer of known triplet energy from its quenching rate constant. It is shown how the change of p(CT) influences the quenching rate constant and the efficiency of singlet oxygen formation in both series of sensitizers. p(CT) is sensitive to differences of solvent polarity and varies for the biphenyls and the naphthalenes as sigmoidal with DeltaG(CT). This quantitative model represents a realistic and general mechanism for the quenching of pipi triplet states by O(2), surpassing previous advanced models.

Journal Article↗
PubMed2001

Singlet oxygen imaging in Arabidopsis thaliana leaves under photoinhibition by excess photosynthetically active radiation.

Arabidopsis thaliana leaves were infiltrated with DanePy (3-(N-diethylaminoethyl)-N-dansyl)aminomethyl-2,5-dihydro-2,2,5,5-tetramethyl-1H-pyrrole), a double, fluorescent and spin sensor of singlet oxygen. DanePy fluorescence was imaged by laser scanning microscopy. We found that DanePy penetrated into chloroplasts but did not alter the functioning of the photosynthetic electron transport as assessed by chlorophyll fluorescence induction. In imaging, DanePy fluorescence was well distinct from chlorophyll fluorescence. Photoinhibition by excess photosynthetically active radiation caused quenching of DanePy fluorescence in the chloroplasts but not in other cell compartments. When leaves were infiltrated with dansyl, the fluorescent group in DanePy, there was no fluorescence quenching during photoinhibition. This shows that the fluorescence quenching of DanePy is caused by the conversion of its pyrrol group into nitroxide, i.e. it was caused by the reaction of singlet oxygen with the double sensor and not by artifacts. These data provide direct experimental evidence for the localization of singlet oxygen production to chloroplasts in vivo.

Journal Article↗
PubMed2002

The photophysics of 1-hydroxypyrene, the acidity of its singlet excited state, and the nature of its photoionization process in polar media.

The photophysics of 1-hydroxypyrene, Py-OH, in various media has been examined by pulsed laser and steady state methods. In alkaline polar media the photophysics of the ionized form, Py-O(-), the pyrenolate ion, is reported. From the data the pK(a) is calculated to be 8.93 for the ground state and 4.5 for the excited state. However, the excited singlet state lifetime of Py-OH is too short to lead to significant dissociation into excited Py-O(-*) and H+, and no equilibrium between the species exists. Excitation of Py-OH leads to the excited singlet, the triplet, electrons (trapped), and a short-lived radical cation. The radical cation rapidly dissociates to give the pyrenyloxyl radical. Excitation of Py-O(-) gives the excited singlet state, the triplet state, solvated electrons and the pyrenyloxyl radical. No evidence of a radical cation of Py-O(-), i.e.,(+.)Py-O(-) is forthcoming, and only the neutral pyrenyloxyl radical is observed. Low temperature experiments in glasses are used to identify the various species produced.

Journal Article↗
PubMed2002

Excited singlet state and photoionization of 8-methoxypsoralen. Picosecond transient absorption study.

Time-resolved pico- and nanosecond transient absorption measurements of 8-methoxypsoralen (8-MOP) were performed in different solvents at room temperature. The excited singlet state has two strong absorption bands with maxima at 430 and 630-690 nm. The lifetime of the first excited singlet state, tau(s), ranges from <10 ps up to 2.7 ns, being shorter in low-polar solvents (toluene, THF) and longer in highly polar water, TFE and HFIP. The rate of the singlet depopulation decreases exponentially with increasing solvent polarity as expressed by the Dimroth-Reichardt (ET(N)) or Gutmann solvent acceptor number (AN). In aqueous ethanol solution (H2O: EtOH 10:1), addition of the synthetic nucleobase 2-aminopurine slightly decreased the lifetime tau(s). The long-time (t > 5 ns) absorption spectrum remaining after the complete decay of the S1 state is due to the hydrated electron e(aq) and the radical cation 8-MOPF.+. Ionization is due to both, one and two-quantum excitation. In the intensity range I = 10(6)-10(9) W cm(-2) the quantum yield of electron ejection (ionization) phi(i) does not depend on the intensity of radiation, therefore the ionization is monophotonic, phi(i) = 0.03. At intensities 1> 5 x 10(9) W cm(-2) phi1 becomes larger due to two-quantum excitation followed by ionization.

2-Aminopurine↗
PubMed2003

C70 vs. C60 in zinc porphyrin-fullerene dyads: prolonged charge separation and ultrafast energy transfer from the second excited singlet state of porphyrin.

The second excited singlet (S2) state of porphyrin was efficiently quenched by the attached fullerene C70 moiety in a zinc porphyrin-C70 dyad. The quenching is largely explained by energy transfer to C70, but the possibility of additional reactions involving the S2 state of porphyrin is discussed. Singlet energy transfer was found to be an important decay pathway also for the first excited singlet (S1) state of porphyrin. In the polar solvent benzonitrile a charge-separated state was formed, and its lifetime was 890 ps, 50% longer than in the analogous porphyrin-C60 dyad.

Journal Article↗
PubMed2003

Photosensitized generation of singlet oxygen from ruthenium(II) and osmium(II) bipyridyl complexes.

Photophysical properties for a number ruthenium(II) and osmium(II) bipyridyl complexes are reported in dilute acetonitrile solution. The lifetimes of the excited metal to ligand charge transfer states (MLCT) of the osmium complexes are shorter than for the ruthenium complexes. Rate constants, kq, for quenching of the lowest excited metal to ligand charge transfer states by molecular oxygen are found to be in the range (1.1-7.7) x 10(9) dm3 mol(-1) s(-1). Efficiencies of singlet oxygen production, fDeltaT, following oxygen quenching of the lowest excited states of these ruthenium and osmium complexes are in the range of 0.10-0.72, lower values being associated with those compounds having lower oxidation potentials. The rate constants for quenching of the excited MLCT states, kq, are found to be generally higher for osmium complexes than for ruthenium complexes. Overall quenching rate constants, kq were found to give an inverse correlation with the energy of the excited state being quenched, and also to correlate with the oxidation potentials of the complexes. However, when the contribution of quenching due exclusively to energy transfer to produce singlet oxygen, kq1, is considered, its dependence on the energy of the excited states is more complex. Rate constants for quenching due to energy dissipation of the excited MLCT states without energy transfer, kq3, were found to show a clear correlation with the oxidation potential of the complexes. Factors affecting both the mechanism of oxygen quenching of the excited states and the efficiency of singlet oxygen generation following this quenching are discussed. These factors include the oxidation potential, the energy of the lowest excited state of the complexes and spin-orbit coupling constant of the central metal.

Journal Article↗