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PubMed1997

Quantification of singlet oxygen from hematoporphyrin derivative by electron spin resonance.

The mechanism of the generation and the quantitative analysis of singlet oxygen (1O2) formed by the exposure of a hematoporphyrin derivative (HpD) to light was re-evaluated by electron spin resonance (ESR) combined with 2,2,6,6,-tetramethyl-4-piperidine (TMPD). The change from TMPD to 2,2,6,6,-tetramethyl-4-piperidine-N-oxide (TAN) has been reported to depend on singlet oxygen. However, we confirmed that this reagent also react with superoxide anion (O2-) and hydroxyl radicals (OH). Therefore, the reactions between TMPD and 1O2, O2- and OH were re-examined using a kinetic approach. We found that the generation of TAN was proportional to the concentration of TMPD and HpD, as well as to the duration and strength of the illumination. The generation of TAN was not inhibited by dimethyl-sulfoxide (DMSO) or superoxide dismutase (SOD). The reaction rate between TMPD and 1O2 was determined to be 5.0 x 10(-7) M min-1. The generation of 1O2 from HpD was 2.7 x 10(-7) M min-1 under our conditions. The competitive reaction observed between 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) and TMPD for O2- or OH shows that TMPD reacts with both forms of active oxygen, but gave no ESR signal. The second-order reaction rate constant of TMPD between O2- and OH was calculated as 73 M-1 s-1 and 1.5 x 10(9) M-1 s-1, respectively. The photochemical generation of 1O2 from methylene blue, another sensitizer, was also demonstrated by this method. These results show that ESR signal of TAN can be used for the highly selective monitoring of 1O2.

Cyclic N-Oxides↗
PubMed1996

Optimization of a pulse laser spectrometer for the measurement of the kinetics of singlet oxygen O2(delta 1g) decay in solution.

A simple sensitive pulse laser spectrometer to measure the kinetics of singlet oxygen decay in solution is described. It utilizes a NdYag laser for pulse excitation of the 1O2 photosensitizers perinaphthenone and Rose Bengal or eosine at 355 nm and 532 nm, respectively. Singlet oxygen phosphorescence is detected by a germanium diode, to which collimated phosphorescence light is directed by concave and parabolic mirrors via a pair of Fresnel lenses and an infrared interference filter. The 1O2 transient decays are monitored using a Hewlett Packard digitizing oscilloscope and a PC computer. The excellent sensitivity of this spectrometer allows us to easily observe 1O2 decay (1 microsecond resolution) after a single pulse of laser excitation, which is important for quenching measurements of chemical substrates that react readily with 1O2. The small sample volume (0.1 ml) prepared in quartz tubes permits fast and inexpensive data acquisition for the determination of 1O2 quenching rate constants in most solvents.

Chemical Phenomena↗
PubMedDate not supplied

Evidence of DNA repair in organ cultures of hamster tracheal epithelium following exposure to gas phase singlet oxygen.

Autoradiographic identification of unscheduled DNA synthesis (UDS) in short-term organ culture of hamster tracheal epithelium has been used as a predictive test for mutagenic and/or carcinogenic compounds. Tracheal explants were treated for 2 h with singlet delta oxygen plus [3H]thymidine. Silver grains over the nuclei of epithelial cells from the superficial layer of the mucosa were observed, indicating UDS. Control cultures, exposed to the gas phase without singlet oxygen, failed to elicit UDS.

Animals↗
PubMed1999

Irradiation of titanium dioxide generates both singlet oxygen and superoxide anion.

Although photoexcited TiO2 has been known to initiate various chemical reactions, such as the generation of reactive oxygen species, precise mechanism and chemical nature of the generated species remain to be elucidated. The present work demonstrates the generation of singlet oxygen by irradiated TiO2 in ethanol as measured by ESR spectroscopy using 2,2,6,6-tetramethyl-4-piperidone (4-oxo-TMP) as a 1O2-sensitive trapping agent. Under identical conditions, the superoxide ion was also detected by spin trapping agent 5,5-dimethyl-pyrroline-N-oxide (DMPO). Kinetic analysis in the presence of both 4-oxo-TMP and DMPO revealed that singlet oxygen is produced directly at the irradiated TiO2 surface but not by a successive reaction involving superoxide anion. The basis for this view is the fact that DMPO added in the mixture increased the signals responsible for 4-oxo-2,2,6,6-tetramethyl-1-piperidinyloxy (4-oxo-TEMPO), a reaction product of 4-oxo-TMP and 1O2. The detailed mechanism for the generation of 1O2 and superoxide ion by irradiated TiO2 and reactions between these species and DMPO are discussed.

Electron Spin Resonance Spectroscopy↗
PubMed1976

Study of triple-singlet energy transfer in an enzyme-dye complex using optical detection of magnetic resonance.

We have made optical detection of magnetic resonance (ODMR) measurements on the enzyme alpha-chymotrypsin, as well as on its complex with the dye, proflavin. Evidence that triplet-singlet energy transfer occurs in the complex is provided by the observation of characteristic tryptophan ODMR signals while monitoring the delayed fluorescence of the dye. The luminescence decay kinetics of the complex indicates that nontrivial triplet-singlet transfer originates from several (at least three) tryptophan residues of the enzyme. ODMR sensitivity can be enhanced by coupling the sublevels of a weakly radiative triplet state to a fluorescent dye which satisfies Förster's (Förster, T. (1948), Ann. Phys. (Leipzig) 2, 55; (1965), in Modern Quantum Chemistry, Istanbul Lectures, Part III, Sinanoglu, O., Ed., New York, N.Y., Academic Press, p 93) conditions for energy transfer.

Acridines↗
PubMed2000

Pulsed EPR studies of doublet signal and singlet-like signal in oriented Ca2+-depleted PS II membranes: location of the doublet signal center in PS II.

Doublet signal and singlet-like signal induced in Ca(2+)-depleted PS II were investigated by pulsed EPR in one-dimensionally oriented photosystem (PS) II membranes. The doublet signal showed marked angular dependent change in its spectrum in term of the applied magnetic field, indicating that the magnetic dipole-dipole interaction is mainly responsible for the doublet signal. The singlet-like signal also showed angular dependence, which was less pronounced than that of the doublet signal. The parameters of dipole and exchange interactions used to simulate the doublet signal indicate that the signal arises from a magnetically coupled organic radical pair. Angular dependence of the doublet signal indicates that the radius vector of the radical pair (r) and the normal of the thylakoid membrane is at an angle of 65 degrees. Pulsed ELDOR studies in the oriented membranes indicate that the vector (R) connecting the doublet-signal center with the Y(D)(*) radical and the plane of the thylakoid membrane are at an angle of 8 degrees. Furthermore, the angle between the projections of the R and r vectors on the plane of the thylakoid membrane was determined to be 64 degrees. The location of the doublet-signal species in PS II is discussed.

Anisotropy↗
PubMed2003

Crystal structure and magnetic properties of an ionic C60 complex with decamethylcobaltocene: (Cp*2Co)2C60(C6H4Cl2, C6H5CN)2. Singlet-triplet transitions in the C60(2-) anion.

The C(60) complex with decamethylcobaltocene, (Cp(2)Co)(2)C(60)(C(6)H(4)Cl(2), C(6)H(5)CN)(2) (1) (C(6)H(4)Cl(2) = 1,2-dichlorobenzene; C(6)H(5)CN = benzonitrile), has been obtained as single crystals by the diffusion method. The IR and UV-vis-NIR spectra show the presence of the C(60)(2)(-) and the Cp(2)Co(+) ions, which form a three-dimensional framework with channels accommodating solvent molecules. EPR and SQUID measurements show that C(60)(2)(-) has a diamagnetic singlet (S = 0) state in the 2-140 K range. The appearance of a broad EPR signal and the increase in magnetic susceptibility of 1 above 140 K are assigned to a thermal population of a close lying triplet (S = 1) state. The singlet-triplet energy gap for C(60)(2)(-) in solid 1 is estimated to be 730+/-10 cm(-)(1).

Journal Article↗
PubMed2001

Rational design of fluorescein-based fluorescence probes. Mechanism-based design of a maximum fluorescence probe for singlet oxygen.

Fluorescein is one of the best available fluorophores for biological applications, but the factors that control its fluorescence properties are not fully established. Thus, we initiated a study aimed at providing a strategy for rational design of functional fluorescence probes bearing fluorescein structure. We have synthesized various kinds of fluorescein derivatives and examined the relationship between their fluorescence properties and the highest occupied molecular orbital (HOMO) levels of their benzoic acid moieties obtained by semiempirical PM3 calculations. It was concluded that the fluorescence properties of fluorescein derivatives are controlled by a photoinduced electron transfer (PET) process from the benzoic acid moiety to the xanthene ring and that the threshold of fluorescence OFF/ON switching lies around -8.9 eV for the HOMO level of the benzoic acid moiety. This information provides the basis for a practical strategy for rational design of functional fluorescence probes to detect certain biomolecules. We used this approach to design and synthesize 9-[2-(3-carboxy-9,10-dimethyl)anthryl]-6-hydroxy-3H-xanthen-3-one (DMAX) as a singlet oxygen probe and confirmed that it is the most sensitive probe currently known for (1)O(2). This novel fluorescence probe has a 9,10-dimethylanthracene moiety as an extremely fast chemical trap of (1)O(2). As was expected from PM3 calculations, DMAX scarcely fluoresces, while DMAX endoperoxide (DMAX-EP) is strongly fluorescent. Further, DMAX reacts with (1)O(2) more rapidly, and its sensitivity is 53-fold higher than that of 9-[2-(3-carboxy-9,10-diphenyl)anthryl]-6-hydroxy-3H-xanthen-3-ones (DPAXs), which are a series of fluorescence probes for singlet oxygen that we recently developed. DMAX should be useful as a fluorescence probe for detecting (1)O(2) in a variety of biological systems.

Fluorescein↗
PubMed2002

Carbene formation in its lower singlet state from photoexcited 3H-diazirine or diazomethane. A combined CASPT2 and ab initio direct dynamics trajectory study.

The potential energy surfaces of the ground and valence excited states of both 3H-diazirine and diazomethane have been studied computationally by mean of the CASSCF method in conjunction with the cc-pVTZ basis set. The energies of the critical points found on such surfaces have been recomputed at the CASPT2/cc-pVTZ level. Additionally, ab initio direct dynamic trajectory calculations have been carried out on the S(1) and S(2) surfaces, starting each trajectory run at the region dominated by the conformational molecular rearrangement of diazomethane. It is found that both isomers are interconnected along a C(s)() reaction coordinate on each potential surface. Radiationless deactivation of the corresponding S(1) state of each isomer occurs through the same point on the surface, an S(1)/S(0) conical intersection. Thereafter, the system has enough energy to surmount the barrier which leads to dissociation products (CH(2) + N(2)) on S(0) state. Therefore, photoexcitation to S(1) state of either diazirine of diazomethane produces methylene in its lower singlet state on a very short time scale (ca. 100 fs). Furthermore, both isomers can generate excited singlet carbene when they are excited onto the S(2) surface; in this case, they lose the activation energy passing through another common S(2)/S(1) conical intersection and then proceed to dissociation into carbene and N(2) on the S(1) surface. For the special case of methylene, it rapidly experiences deexcitation to S(0) state.

Journal Article↗
PubMed2002

A new experimental protocol for intrazeolite photooxidations. The first product-based estimate of an upper limit for the intrazeolite singlet oxygen lifetime.

The intrazeolite photooxidations of several alkenes have been examined in hexane and in perfluorohexane slurries. The ability of perfluorohexane to increase the affinity of alkenes for the interior of the zeolite was documented. The nearly identical reaction rates in the two slurry solvents are attributed to a leveling of the singlet oxygen lifetime by the zeolite framework. A consideration of the rates of the various intrazeolite processes was used to estimate an upper limit for the intrazeolite lifetime of singlet oxygen.

Journal Article↗
PubMed2002

Photophysics of monodisperse platinum-acetylide oligomers: delocalization in the singlet and triplet excited states.

A series of monodisperse Pt-acetylide polymers that contain the [-CC-(p-C6H4)-CC-(t-Pt(PBu3)2)-]n repeat unit has been prepared for n = 1, 2, 3, 4, 5, and 7. The photophysical properties of the series provide information concerning the relationship between the oligomer length and delocalization in the singlet and triplet excited states of the pi-conjugated electron system. The results imply that the singlet excited state is delocalized over approximately 6 repeat units; however, the triplet state is considerably more localized. The triplet energy is almost invariant with oligomer length, but the phosphorescence spectra and triplet nonradiative decay rates indicate that the electron-vibrational coupling in the triplet state decreases with increasing oligomer length.

Journal Article↗
PubMed2003

Singlet hydrocarbon carbenes with high barriers toward isomerization: a computational investigation.

A prerequisite for a stable singlet hydrocarbon carbene is the existence of high barriers toward isomerization. Four derivatives of cyclopentylidene (1-4) with rigid and varying carbon cages are examined computationally at the B3LYP/6-311+G(d,p) level of theory. Singlet ground states are predicted for carbenes 1-4, with DeltaE(ST)'s = 7-22 kcal/mol. The rearrangement paths considered are 1,3-hydrogen shift, 1,2-carbon shift and beta-CC bond-cleavage. Carbenes 3 and 4 lie in relatively shallow potential-energy wells (around 4 and 6 kcal/mol, respectively) and are expected to rearrange via 1,3-hydrogen shifts to cyclopropane derivatives. For 1 and 2, the lowest energy rearrangement path is beta-CC bond-cleavage requiring about 12 and 20 kcal/mol, respectively, placing 2 in the deepest potential-energy well among the four carbenes.

Journal Article↗
PubMed2003

Activated decay pathways for planar vs twisted singlet phenylalkenes.

The ground state conformation, spectroscopy, and photochemical behavior of styrene and eight of its vinyl- and ring-methylated derivatives have been investigated. Introduction of methyl groups at the alpha-position of the vinyl group or the ortho positions of the phenyl results in increased phenyl-vinyl dihedral angles. Styrenes possessing both alpha-methyl and ortho-methyl groups adopt orthogonal geometries. Decreased planarity results in a progressive blue-shift in the lowest energy allowed pi,pi transition and a decrease in the singlet lifetime. Kinetic modeling of the temperature-dependent singlet lifetimes provides activation parameters for the activated decay pathway, which is assigned to C=C torsion for styrenes with phenyl-vinyl dihedral angles, phi, < 60 degrees. Planar styrenes have large torsional barriers (7 +/- 1 kcal/mol) and decay predominantly via intersystem crossing and fluorescence at room temperature. Styrenes with values of 30 degrees < phi < 60 degrees have smaller torsional barriers and decay predominantly via C=C torsion at room temperature. Highly nonplanar styrenes decay predominantly via relatively rapid, weakly activated intersystem crossing.

Journal Article↗
PubMed2004

Energy transfer between singlet (1Delta(g)) and triplet (3Sigma(g)-) molecular oxygen in aqueous solution.

We clearly demonstrate the occurrence of energy transfer between 18O2 (1Deltag) and 16O2 in the ground state (3Sigmag-) with subsequent conversion of the latter species into its singlet excited state (1Deltag) in aqueous solution. This was inferred from the results of incubation experiments involving DHPN18O2 as a chemical generator of 18O2 (1Deltag) and the water-soluble disodium salt of anthracene (EAS) used as a chemical trap of singlet oxygen. The products of the reaction were accurately analyzed by HPLC-ESI-MS.

Journal Article↗
PubMed2005

Ab initio studies on the radiationless decay mechanisms of the lowest excited singlet states of 9H-adenine.

The mechanisms that are responsible for the rapid deactivation of the (1)npi and( 1)pipi excited singlet states of the 9H isomer of adenine have been investigated with multireference ab initio methods (complete-active-space self-consistent-field (CASSCF) method and second-order perturbation theory based on the CASSCF reference (CASPT2)). Two novel photochemical pathways, which lead to conical intersections of the S(1) excited potential-energy surface with the electronic ground-state surface, have been identified. They involve out-of-plane deformations of the six-membered aromatic ring via the twisting of the N(3)C(2) and N(1)C(6) bonds. These low-lying conical intersections are separated from the minimum energy of the lowest ((1)npi) excited state in the Franck-Condon region by very low energy barriers (of the order of 0.1 eV). These properties of the S(1) and S(0) potential-energy surfaces explain the unusual laser-induced fluorescence spectrum of jet-cooled 9H-adenine, showing sharp structures only in a narrow energy interval near the origin, as well as the extreme excess-energy dependence of the lifetime of the singlet excited states. It is suggested that internal-conversion processes via conical intersections, which are accessed by out-of-plane deformation of the six-membered ring, dominate the photophysics of the lowest vibronic levels of adenine in the gas phase, while hydrogen-abstraction photochemistry driven by repulsive (1)pisigma states may become competitive at higher excitation energies. These ultrafast excited-state deactivation processes provide adenine with a high degree of intrinsic photostability.

Adenine↗
PubMed2005

Ground-state singlet L3Fe-(mu-N)-FeL3 and L3Fe(NR) complexes featuring pseudotetrahedral Fe(II) centers.

Pseudotetrahedral iron(II) coordination complexes that contain bridged nitride and terminal imide linkages, and exhibit singlet ground-state electronic configurations, are described. Sodium amalgam reduction of the ferromagnetically coupled dimer, {[PhBP(3)]Fe(mu-1,3-N(3))}(2) (2) ([PhBP(3)] = [PhB(CH(2)PPh(2))(3)](-)), yields the diamagnetic bridging nitride species [{[PhBP(3)]Fe}(2)(mu-N)][Na(THF)(5)] (3). The Fe-N-Fe linkage featured in the anion of 3 exhibits an unusually bent angle of approximately 135 degrees , and the short Fe-N bond distances (Fe-N(av) approximately equal to 1.70 A) suggest substantial Fe-N multiple bond character. The diamagnetic imide complex {[PhBP(3)]Fe(II)(triple bond)N(1-Ad)}{(n)()Bu(4)N} (4) has been prepared by sodium amalgam reduction of its low-spin iron(III) precursor, [PhBP(3)]Fe(III)(triple bond)N(1-Ad) (5). Complexes 4 and 5 have been structurally characterized, and their respective electronic structures are discussed in the context of a supporting DFT calculation. Diamagnetic 4 provides a bona fide example of a pseudotetrahedral iron(II) center in a low-spin ground-state configuration. Comparative optical data strongly suggest that dinuclear 3 is best described as containing two high-spin iron(II) centers that are strongly antiferromagnetically coupled to give rise to a singlet ground-state at room temperature.

Journal Article↗
PubMed2004

Singlet vinylcarbenes: spectroscopy and photochemistry.

The first direct spectroscopic characterization of singlet vinylcarbenes is reported. Several vinyl amidines were converted to the corresponding vinylchlorodiazirines through straightforward Graham oxidation. Irradiation of the diazirines at 8 K in N2 matrices gave the corresponding singlet vinylchlorocarbenes, which could be characterized by IR, UV/vis, and calculational modeling. Hence, vinylchlorocarbene, 1-methylvinylchlorocarbene, and 1-cyclopentenylchlorocarbene were all generated and investigated. The spectra are consistent with localized carbene structures compared to the very delocalized triplet vinylcarbenes. In all cases, the carbenes readily cyclize to the corresponding cyclopropenes on visible irradiation, together with H-shift in the parent vinylchlorocarbene. In the cyclopentenyl system, cyclization leads to a highly strained bicyclo[3.1.0]hexene.

Journal Article↗
PubMed2004

Activation energies for the singlet excited state processes of substituted benzenes: para, meta, and ortho isomers of methylbenzonitrile and methylanisole in acetonitrile.

The rate constants of decay of the excited singlet states of the methylbenzonitriles (1-3) and the methylanisoles (4-6) have been determined by the measurement of fluorescence lifetimes over a broad range of temperatures (-45 to +65 degrees C) in acetonitrile. By fitting this data to a nonlinear expression that includes the Arrhenius equation, rate constants for the activated process (reaction) and the unactivated ones (fluorescence and intersystem crossing) can be reliably obtained. Available literature data for benzene, toluene, and ortho-xylene were also analyzed. The results indicate that the excited singlet state of substituted benzenes is quite reactive and forms a prefulvene biradical intermediate efficiently (quantum yield = 0.69 for benzene itself) by an activated route. In contrast, the efficiency of isolable product formation is quite low because the dominant process for this intermediate is returned to starting material. These observations explain why Ermolaev's rule does not apply to benzene derivatives.

Journal Article↗