Showing papers on "Triplet state published in 1998"

Highly efficient phosphorescent emission from organic electroluminescent devices

Abstract: The efficiency of electroluminescent organic light-emitting devices1,2 can be improved by the introduction3 of a fluorescent dye. Energy transfer from the host to the dye occurs via excitons, but only the singlet spin states induce fluorescent emission; these represent a small fraction (about 25%) of the total excited-state population (the remainder are triplet states). Phosphorescent dyes, however, offer a means of achieving improved light-emission efficiencies, as emission may result from both singlet and triplet states. Here we report high-efficiency (≳90%) energy transfer from both singlet and triplet states, in a host material doped with the phosphorescent dye 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine platinum(II) (PtOEP). Our doped electroluminescent devices generate saturated red emission with peak external and internal quantum efficiencies of 4% and 23%, respectively. The luminescent efficiencies attainable with phosphorescent dyes may lead to new applications for organic materials. Moreover, our work establishes the utility of PtOEP as a probe of triplet behaviour and energy transfer in organic solid-state systems.

Spin-triplet superconductivity in Sr2RuO4 identified by 17O knight shift

Abstract: Superconductivity — one of the best understood many-body problems in physics — has again become a challenge following the discovery of unconventional superconducting materials: these include heavy-fermion1, organic2 and the high-transition-temperature copper oxide3 superconductors In conventional superconductors, the electrons form superconducting Cooper pairs in a spin-singlet state, which has zero total spin (S = 0) In principle, Cooper pairs can also form in a spin-triplet state (S = 1), analogous to the spin-triplet ‘p-wave’ state of paired neutral fermions in superfluid 3He (ref 4) At present, the heavy-fermion compound UPt3 is the only known spin-triplet superconductor5,6, although the layered oxide superconductor Sr2RuO4 (ref 7) is believed, on theoretical grounds8, to be a promising candidate The most direct means of identifying the spin state of Cooper pairs is from measurements of their spin susceptibility, which can be determined by the Knight shift (as probed by nuclear magnetic resonance (NMR)) Here we report Knight-shift measurements of Sr2RuO2 using 17O NMR Our results show no change in spin susceptibility on passing through the superconducting transition temperature, which provides the definitive identification of Sr2RuO4 as a spin-triplet superconductor

Photoacoustic Measurements of Porphyrin Triplet-State Quantum Yields and Singlet-Oxygen Efficiencies

Abstract: Photoacoustic calorimetry was used to measure the quantum yields of singlet molecular oxygen production by the triplet states of tetraphenylporphyrin (TPP), ZnTPP and CuTPP in toluene, yielding values of 0.67 0.14, 0.68 0.19 and 0.03 0.01, respectively. We show that a novel dichlorophenyl derivative of ZnTPP is capable of singlet-oxygen production with a 0.90 0.07 quantum yield. The synthesis and characterisation of a new photostable chlorin with high absorptivity in the red that is capable of singlet-oxygen production with 0.54 0.06 quantum yield is described. Our results suggest that chlorinated chlorins may be interesting new sensitisers for photodynamic therapy.

Synthesis and Electronic Structure of Platinum-Containing Poly-ynes with Aromatic and Heteroaromatic Rings

Abstract: We have studied the dependence of intersystem crossing and the spatial extent of singlet and triplet excitons in platinum-containing poly-yne polymers with the general formula [Pt(PR3)2C⋮CLC⋮C]n (R = Et, nBu; L = pyridine, phenylene, or thiophene) as a function of electron delocalization in the spacer group L. We also report the synthesis route of those compounds. The optical absorption, photoluminescence, and photoinduced absorption of the corresponding polymers and monomers have been measured. We find that conjugation is increased but intersystem crossing is reduced by the electron-rich thiophene unit, while the opposite occurs for the electron-deficient pyridine unit as compared to the phenylene unit. For all investigated systems, we find that the singlet excited state and a higher lying Tn triplet excited state extend over more than a repeat unit while the T1 triplet state remains localized to less then one repeat unit.

EPR Investigation of Photoinduced Radical Pair Formation and Decay to a Triplet State in a Carotene−Porphyrin−Fullerene Triad

Abstract: The photochemistry of a molecular triad consisting of a porphyrin (P) covalently linked to a carotenoid polyene (C) and a fullerene derivative (C60) has been studied at 20 K by time-resolved EPR sp...

Molecular orbital study of H2 and CH4 activation on small metal clusters. I. Pt, Pd, Pt2, and Pd2

Abstract: The electronic structure of Pd/Pt dimer and the detailed reaction mechanism of H2 and CH4 activation on these clusters have been studied with density functional (B3LYP) and complete active space second-order perturbation (CASPT2) theories It was found that B3LYP calculations gave reliable results on the electronic structures of the Pd/Pt dimers, in comparison with our CASPT2 calculations and data from previous theoretical investigations Full geometry optimization has been carried out in the current study in contrast to previous work where only limited potential energy scans have been carried out, which led to dramatically different reaction mechanisms In the case of Pt2+H2/CH4, H–H/C–H activation preferentially takes place at first on one metal atom via structures far from planar, then one of the H atoms migrates to the other Pt atom with negligible barrier On both the singlet and the triplet state, H–H activation is barrierless, while C–H activation has a distinct barrier on the singlet state for rea

Mechanism of Pyrene Photochemical Oxidation in Aqueous and Surfactant Solutions

Abstract: The photolysis of pyrene has been studied in water and in Brij 35 micellar media. Photolysis in both media lead to the formation of 1,6- and 1,8-pyrenequinones as stable products. The first step in the photochemical oxidation is proposed to involve an electron transfer from the excited singlet state of pyrene to molecular oxygen in a contact charge-transfer pair. The effects of oxygen and HgCl2 on the photolysis are interpreted as ruling out mechanistic involvement of a triplet state of pyrene. 1-Hydroxypyrene is identified as a product of the initial photochemical oxidation. In the optically dilute aqueous solutions, 1-hydroxypyrene undergoes further photochemical oxidation to produce 1,6- and 1,8-pyrenequinones. Pyrene photolysis quantum yields are decreased by a factor of approximately 2 in the micellar media relative to the quantum yields determined in water. Fluorescence data suggest that pyrene resides in the micelles among the polyoxyethylene portion of the surfactant and not within the core of the...

Ethylenedione: An Intrinsically Short‐Lived Molecule

Abstract: Ethylenedione C2O2 is one of the elusive small molecules which have remained undetected even after numerous attempts with different experimental techniques, This is surprising, since theoretical studies predicted the triplet state of C2O2 to be stable towards spin-allowed dissociation and hence long-lived. Here we report a comprehensive study of charged and neutral ethylenedione by means of charge reversal and neutralization -reionization mass spectrometry. These experimental results, in conjunction with theoretical calculations, suggest that neutral ethylenedione is intrinsically short-lived rather than being elusive, Both the singlet and triplet states of C2O2 are predicted to dissociate rapidly into two ground-state CO molecules, and for the triplet species, this dissociation involves facile curve-crossing to the singlet surface within a few nanoseconds.

Photochemical Rearrangement of Enediynes: Is a "Photo-Bergman" Cyclization a Possibility?

Abstract: The enediynes 1 and 2 were photolyzed in 2-propanol to yield products akin to those analogous to a thermal Bergman cyclization mechanism. In addition, products resulting from photoreduction of one of the triple bonds of the enediyne functionality are also formed. The product distributions and yields were found to be dependent on the substituents on the triple bonds and on the nature of the double bond; in particular, phenyl substituents on the triple bonds eliminated the photoreduction products, and overall yields were higher when the double bond of the enediyne was of aromatic character. Triplet sensitization studies and laser flash photolysis experiments point toward radical mechanisms taking place during formation of both classes of products, with the photoreduction products forming from the excited triplet state and the cyclized naphthyl products forming from either the singlet or the triplet states. For the cyclization reaction, a substituted 1,4-dehydronaphthalene biradical species, structurally ide...

Andreev Reflection in Unitary and Non-Unitary Triplet States

Abstract: The quasiparticle reflection and transmission properties at normal conductor-superconductor interfaces are examined for unitary and non-unitary spin triplet pairing states recently discussed in connection with Sr 2 RuO 4 . We find resonance peaks in the Andreev reflection amplitude, which are related to surface bound states in the superconductor. They lead to conductance peak features below the quasiparticle gap in the superconductor. The symmetry of the pairing state determines the specific dependence of the peak on the angle of incidence. Based on this observation we propose a possible experiment which allows to distinguish between different superconducting states.

Spin uncoupling in surface chemisorption of unsaturated hydrocarbons

Abstract: Unsaturated hydrocarbons, such as acetylene and ethylene, show strong geometrical distortions when coordinated to transition metals or to surfaces; the bonding is normally analysed in terms of a π-donation—π*-backdonation process. In the present work we use chemisorption of the unsaturated hydrocarbons (ethylene, acetylene, and benzene) on cluster models of the copper (100), (110), and (111) surfaces to demonstrate the importance of considering the available excited states of the free molecule in analyzing the bonding scheme of the adsorbate at the surface. By comparison to the structures of the triplet excited states in the gas phase we demonstrate that these must be considered as the states actually involved in the bonding. This implies a spin-uncoupling in both adsorbate and substrate as part of the chemisorption process or bond formation. In particular, for benzene we identify the quinoid gas phase triplet state as the specific state that is most strongly bound to the Cu(110) substrate; the structure ...

Exchange Interaction in Radical-Triplet Pairs: Evidences for CIDEP Generation by Level Crossings in Triplet-Doublet Interactions

Abstract: Chemically induced dynamic electron polarization (CIDEP) generated through interaction of the excited triplet state of 1-chloronaphthalene, benzophenone, benzil, and Buckminsterfullerene (C60) with 2,2,6,6,-tetramethyl-1-piperidinyloxyl (TEMPO) radical was investigated by using time-resolved ESR spectroscopy. We carefully examined what factors affect the CIDEP intensities. By comparing CIDEP intensities of TEMPO in the 1-chloronaphthalene, benzophenone, and benzil systems with that obtained in the C60−TEMPO system, the absolute magnitude of net emissive polarization was determined to be −2.2, −6.9, and −8.0, respectively, in the units of Boltzmann polarization. In the 1-chloronaphthalene−TEMPO system, the viscosity effect on the magnitude of net polarization was studied by changing the temperature (226−275 K) in 2-propanol. The emissive polarization was concluded to result from the state mixing between quartet and doublet manifolds in a radical−triplet pair induced by the zero-field splitting interaction ...

Laser Flash Photolysis Studies on the Monohydroxy Derivatives of Benzophenone

Abstract: Time-resolved studies in the pico- and nanosecond time domain have been performed to characterize the triplet states of monohydroxy-substituted benzophenones, namely, para- (p-), meta- (m-), and ortho- (o-) hydroxybenzophenones (HOBP). Due to a very fast intersystem crossing (ISC) process, only the triplet states have been detected in the subnanosecond time domain. Spectral characteristics and lifetimes of the triplet states of HOBP have been seen to be extremely sensitive to the position of the OH group in the phenyl ring as well as the solvent characteristics. In case of m-HOBP and p-HOBP, the excited triplet state in non-hydrogen-bond-forming solvents has an nπ* configuration and is capable of abstracting a hydrogen atom from another unexcited molecule to form ketyl and phenoxy type radicals. But in hydrogen-bond-forming solvents, the triplet state, which is strongly associated with the solvent molecules as a hydrogen-bonded complex, is very short-lived due to fast nonradiative relaxation via hydrogen-...

Photophysical studies on the interaction of two water-soluble porphyrins with bovine serum albumin. Effects upon the porphyrin triplet state characteristics

Abstract: The interaction of two metal-free water soluble porphyrins (PPh): meso -tetrakis ( p -sulfo-natophenyl)porphyrin (TPPS 4 ) and meso -tetrakis(4- N -methyl-pyridiniumyl)porphyrin (TMPyP), with bovine serum albumin (BSA) was investigated in the pH range from 4.0 to 8.5 using the flash-photolysis technique in comparison with results obtained by optical absorption, fluorescence and resonance light scattering. It was found that in the presence of BSA, TPPS 4 can exist in aqueous solutions as free monomers, aggregates and/or monomers bound to a single BSA molecule and monomers inside the BSA aggregates, while TMPyP does not form aggregates at binding. Binding to BSA transforms the profile of the triplet decay curve from monoexponential to biexponential form with the component lifetimes and relative amplitudes depending on binding characteristics. The triplet lifetime of a bound porphyrin monomer is longer than that of a free one. The aggregation of TPPS 4 observed at [TPPS 4 ]/[BSA] > I reduces the T-T absorption since the lifetimes of the aggregate excited states are very short and/or quantum yield of the aggregate triplet state is very low. The porphyrin binding to BSA reduces the quenching constants of the porphyrin triplet states by molecular oxygen due to obstacles produced by binding. This effect is especially pronounced for the porphyrin molecules located inside BSA aggregates formed around the porphyrin molecules in excess BSA.

Novel approach to description of spin-liquid phases in low-dimensional quantum antiferromagnets

Abstract: We consider quantum spin systems with dimerization, which at strong coupling have singlet ground states. To account for strong correlations, the $S\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1$ elementary excitations are described as a dilute Bose gas with infinite on-site repulsion. This approach is applied to the two-layer Heisenberg antiferromagnet at $T\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$ with general couplings. Our analytic results for the triplet gap, the excitation spectrum, and the location of the quantum critical point are in excellent agreement with numerical results obtained by dimer series expansions.

Efficiency of singlet oxygen generation by alloxazines and isoalloxazines

Abstract: Some spectroscopic properties of alloxazines and the lifetimes of their singlet and triplet states in acetonitrile are reported. In addition, the efficiencies of singlet oxygen production by energy transfer from the excited states of a range of substituted alloxazines and lumiflavine have been determined in aerated acetonitrile solution. Triplet state quantum yields have also been measured and it has been shown that the efficiency of singlet oxygen production from the triplet state is equal to unity within experimental error in all cases.

Photophysics of the cationic 5, 10, 15,20-tetrakis (4-N-methylpyridyl) porphyrin bound to DNA, [poly(dA-dT)]2 and [poly(dG-dC)]2: interaction with molecular oxygen studied by porphyrin triplet—triplet absorption and singlet oxygen luminescence

Abstract: Interaction between molecular oxygen and the cationic free-base 5,10,15,20-tetrakis(4-N-methylpyridyl)porphyrin (H2TMpyP4+) complexed with [poly(dA-dT)]2, [poly(dG-dC)]2 and calf thymus DNA, has been monitored in air-saturated heavy water solutions through porphyrin triplet—triplet absorption and singlet oxygen luminescence. Three different rate constants of porphyrin triplet state quenching have been found which correspond to different accessibilities of molecular oxygen to porphyrins embedded in the duplexes. The longest triplet state lifetime (30 μs), found for porphyrin bound to [poly(dG-dC)]2, corresponds to molecules well protected from oxygen. This supports the hypothesis of an intercalative binding mode of the porphyrin between GC base-pairs (‘type A’ sites). The fraction iTδ of the porphyrin triplet states quenched by molecular oxygen with singlet oxygen generation, is unity. In [poly(dA-dT)]2-porphyrin complexes, two sites (‘type B’ and ‘C’ sites of interaction) are involved, yielding very different triplet state lifetimes (5.5 μs and 20.5 μs) and and efficiencies of singlet oxygen generation (iTδ = 0.50 and 0.82). The iTδ decreases can likely be explained in terms of competition between energy and electron transfer from the porphyrin excited triplet state to molecular oxygen. All three types (A, B and C) of interaction sites can be expected in porphyrin-DNA complexes.

DFT and ab Initio Study of the Unimolecular Decomposition of the Lowest Singlet and Triplet States of Nitromethane

Abstract: The fully optimized potential energy curves for the unimolecular decomposition of the lowest singlet and triplet states of nitromethane through the C−NO2 bond dissociation pathway are calculated using various DFT and high-level ab initio electronic structure methods. We perform gradient corrected density functional theory (DFT) and multiconfiguration self-consistent field (MCSCF) to conclusively demonstrate that the triplet state of nitromethane is bound. The adiabatic curve of this state exhibits a 33 kcal/mol energy barrier as determined at the MCSCF level. DFT methods locate this barrier at a shorter C−N bond distance with 12−16 kcal/mol lower energy than does MCSCF. In addition to MCSCF and DFT, quadratic configuration interactions with single and double substitutions (QCISD) calculations are also performed for the singlet curve. The potential energy profiles of this state predicted by DFT methods based on Becke's 1988 exchange functional differ by as much as 17 kcal/mol from the predictions of MCSCF ...

Measurement of the absolute yield of CO(a 3Π)+O products in the dissociative recombination of CO2+ ions with electrons

Abstract: A flowing-afterglow technique is described for measuring the absolute yield of a radiative product state from ion–electron recombination. The technique is applied to CO2++e− dissociative recombination. The measured yield of CO(a 3Π)+O(3P) is 0.29±0.10. This includes cascade from higher triplet states of CO. The vibrational distribution in CO(a 3Π,v=0–3) is approximately Boltzmann, with an effective temperature of 4200±300 K. The measured rate constant for quenching of CO(a) by CO2 is (1.0±0.2)×10−11 cm3 s−1, somewhat lower than previous measurements.

Singlet molecular oxygen-mediated photo-oxidation of tetracyclines: kinetics, mechanism and microbiological implications

Abstract: Members of the biologically active series tetracyclines (TCs) suffer visible light-promoted photodynamic degradation to different extents, depending on their respective chemical structures and reaction conditions (solvent polarity and pH). The photo-oxidation is accompanied by a partial loss of the antimicrobial power. The photodamage is very fast in the alkaline pH range and less aggressive. although not negligible in kinetic terms, in the physiological pH region. Photo-oxidation quantum efficiencies, evaluated for eight TC derivatives, through singlet molecular oxygen [O2(1Delta(g))] phosphorescence detection, spectrophotometric and polarographic methods, range from 0.12 to 0.65 as upper limits in alkaline medium. The photo-oxidation essentially proceeds via a O2(1Delta(g)) mediated process, with rose bengal or eosine as dye-sensitizers, Nevertheless, as a minor reactive pathway,the excited triplet state of the dye sensitizers interacts with TCS in a competitive process with O2(1Delta(g) generation. The O2(1Delta(g)-mediated photo-oxidation of TCs appears to be a plausible mechanism to account for their phototransformations in biological media, in the presence of visible-absorbing pigments. In both highly and moderately polar media, the quenching of the excited oxygen species is mainly represented by a reactive interaction. It is exerted by the TC molecule through a cooperative effect from the different contributions of several nuclear and extranuclear O2(1Delta(g)-sensitive substituents, as discussed in detail in this paper. The TC lower than 0.03 in the most favourable cases. Nevertheless, the TC photoproduct, formed through direct irradiation, efficiently generates O2(1Delta(g) with Phi(Delta)=0.24. This important finding constitutes the first direct evidence of Type II sensitization by TC photoproducts, and could contribute to the elucidation of the mechanism of TC phototoxicity.

Quantum Chemical Analysis of para-Substitution Effects on the Electronic Structure of Phenylnitrenium Ions in the Gas Phase and Aqueous Solution

Abstract: Ab initio calculations for para-substituted phenylnitrenium ions predict larger singlet−triplet splittings, shorter singlet C−N+ bond lengths, and higher singlet aromatic ring stretching frequencies for substituents with greater electron-donating character. Trends in these properties correlate linearly with para-substituent constants σR+, indicating that phenylnitrenium ions closely resemble other electron-deficient aromatic systems where resonance interactions with substituents are dominant. Sensitivity to substitution is large as judged by the slope of the correlating line for aqueous singlet−triplet splittings, ρ = 6.4. For 13 of 15 substituted cases, aqueous solvation preferentially stabilizes the singlet state by 0.9 to 4.4 kcal/mol; for the p-CO2H and p-CF3 cases, the triplet state is better solvated by less than 1 kcal/mol.

Environmental effects on cellular photosensitization: correlation of phototoxicity mechanism with transient absorption spectroscopy measurements

Abstract: Little is directly known about the influence of the local environment experienced by a photosensitizer in a biological system on its photophysics and photochemistry. In this paper, we have addressed this issue by correlating mechanistic studies using laser flash photolysis with cellular phototoxicity data, obtained under the same experimental conditions. In particular, we have focused on the interaction between local concentrations of photosensitizer (deuteroporphyrin) and oxygen in determining the mechanism of phototoxicity in L1210 cells. In cells, as well as in models such as liposomes and red blood cell ghosts, hypochromicity and a reduction in fluorescence and intersystem crossing yields are observed on increasing the photosensitizer concentration between 0.5 and 20 microM, which illustrates the onset of a self-association. In aerated cellular preparations, the phototoxicity is predominantly type II (singlet oxygen) for all concentrations studied but an oxygen-independent mechanism occurs at the higher concentrations in deaerated samples. These observations are readily explained by consideration of triplet state kinetics as a function of oxygen and photosensitizer concentrations in cells. The rate constant for quenching of the photosensitizer triplet state by oxygen in cells was measured as 6.6 x 10(8) M-1 s-1 and by photosensitizer ground state as approximately 10(6) M-1 s-1 (in terms of local concentration). The latter reaction gave rise to a long-lived species that is presumably responsible for the oxygen-independent phototoxicity observed at the higher photosensitizer concentrations used. This self-quenching of the triplet state is postulated to arise from electron transfer resulting in radical ion formation. Under conditions where no self-quenching contributes, the phototoxicity measured as a function of oxygen concentration correlates well with a model based on the determined kinetic parameters, thus, unambiguously proving the intermediacy of singlet oxygen. These effects should be borne in mind when interpreting phototoxicity mechanisms from in vitro cell studies. The excellent correlation achieved between laser flash photolysis data and measured phototoxicity gives credence to the direct use of photophysical techniques to elucidate photochemical mechanisms in biological media.

The unimolecular dissociation of H2CO on the lowest triplet potential-energy surface

Abstract: The unimolecular dissociation reaction of H2CO on the triplet potential-energy surface has been studied via ab initio electronic structure theory. The stationary point geometries for the equilibrium and transition state are determined employing the configuration interaction with single and double excitations (CISD), coupled cluster with single and double excitations (CCSD), and CCSD with perturbative triple excitations [CCSD(T)] levels of theory with large basis sets up to the correlation consistent (cc)-pVQZ basis. With the best method, cc-pVQZ CCSD(T), the first excited triplet (a 3A″) state lies 72.2 kcal/mol (25 260 cm−1) above the ground (X 1A1) state of H2CO, which is in excellent agreement with the experimental observation of 72.03 kcal/mol (25 194 cm−1). The dissociation limit (H⋅+HCO⋅) is located at 86.3 kcal/mol (30 170 cm−1) above the ground state of H2CO, which is again in good agreement with the two experimentally determined values of 86.57 kcal/mol (30 280 cm−1) and 86.71 kcal/mol (30 328.5 cm−1). With the same method the triplet dissociation transition state lies 92.4 kcal/mol (32 300 cm−1) above the ground state. Consequently, the activation energy for the dissociation reaction of H2CO on the triplet surface is determined ab initio to be 18.9–20.1 kcal/mol (6620–7040 cm−1) (including an estimated error bar of 1.2 kcal/mol or 420 cm−1). The zero-point vibrationally corrected exit barrier height is predicted to be 4.9–6.1 kcal/mol (1710–2130 cm−1). These newly predicted energies are consistent with the recent experimental observations by the Moore group at University of California-Berkeley (1987) and by the Wittig group at University of Southern California (1997).

Energy spectrum of centres in spherical quantum dots

Abstract: The properties of negatively charged donor centres have been studied for semiconductor quantum dots with the finite spherically symmetric confinement potential. The energy levels of the ground state and the excited states of both the spin-singlet and spin-triplet configurations have been calculated by variational means. It has been shown that the excited states of the centre in quantum dots are bound for sufficiently strong confinement potential. The conditions of binding for the excited states have been determined as functions of the potential-well depth and quantum-dot radius. The formation of the bound excited states of the centre is a new property, which results from the confinement of electrons in the quantum dot. A possible application of the present results to the ion trapped in a microcavity is discussed.