Showing papers on "Triplet state published in 1984"

Magnetic-field effects on geminate radical-pair recombination

Abstract: Pairs of radical ions, 2A–+2D+, generated in acetonitrile by nanosecond laser flashes have been investigated in the presence of external magnetic fields. The pairs are produced in the overall singlet state via photoinduced electron transfer between electron-donor (D, dimethylaniline) and electron-acceptor (A, pyrene) molecules and they recombine geminately by back electron transfer to form the molecular triplet state of pyrene.The results obtained with both freely diffusing systems and polymethylene-linked compounds (A—[CH2]n—D, n= 6–12) can be interpreted quantitatively on the basis of the assumption that the spin realignment in the initial radical ion pair is governed by the hyperfine interaction, ΔEhfi, between the unpaired electron spin and the nuclear spins in each radical and by the exchange interaction, J(n), of the radical spins in the pair. The latter increases with decreasing chain length, n.The differences in behaviour with respect to geminate triplet formation of the linked compounds with short (n⩽ 6), medium (6 < n < 12) and long (n 12) polymethylene chains are discussed.

Crystal structure and magnetic and EPR properties of bis(.mu.-azido)tetrakis(4-tert-butylpyridine)dicopper(II) perchlorate: a new copper(II) dinuclear complex with a large stabilization of the ground triplet state

Abstract: Cristallisation dans le groupe P2 1 /c avec a=12,819, b=13,76 et c=13,65 A, β=100°, Z=2. Coordinence 4 des atomes Cu avec un environnement plan. Variation de la susceptibilite magnetique avec la temperature de 2 a 300 K. Spectre RPE

Aromatic thioketone triplets and their quenching behaviour towards oxygen and di-t-butylnitroxy radical. A laser-flash-photolysis study

Abstract: Results obtained using nanosecond laser flash photolysis with various exitation wavelengths (337.1–600 nm) are reported for triplet-related properties of five aromatic thioketones, namely xanthione, thioxanthione, thiobenzophenone and p,p′-dimethoxy and p,p′-bis(N,N-dimethylamino) derivatives of thiobenzophenone. The triplet yields (ϕT) of these thioketones in benzene are high (0.5–1.0) and show a dependence on excitation wavelengths. The intersystem-crossing efficiency is less than unity (0.5–0.6) when laser excitation is carried out into the second excited singlet state, S2(λex= 355 and 425 nm), but approaches unity when the excitation leads to absorption into the lowest excited singlet, S1(532 and 600 nm). The intrinsic triplet lifetimes are short (0.8–1.8 µs) and the self-quenching rate constants are in the range (2.6–7.1) 109 dm3 mol–1 s–1. Quantitative data concerning triplet–triplet absorption spectra and triplet quenching by azulene, ferrocene, β-carotene and 2,5-dimethyl-2,4-hexadiene are presented. The oxygen quenching rate constants [(2.8–9.7)× 109 dm3 mol–1 s–1] increase when electron-donating groups (methoxy and N,N-dimethyl-amino) are present in the thiobenzophenones, suggesting that charge-transfer interaction is important. The efficiency of singlet-oxygen generation in the course of the oxygen quenching of the p,p′-dimethoxythiobenzophenone triplet is unity. The stable free radical, di-t-butyl-nitroxide, quenches the thioketone triplets with unusually high rate constants [(1.4–3.3)× 109 dm3 mol–1 s–1]; this behaviour appears to be a manifestation of electron-exchange-mediated intersystem crossing (S0 [graphic omitted] T1), enhanced by strong spin–orbit coupling or spin–spin interaction associated with the thiocarbonyl sulphur atom.

A quantum mechanical explanation for Hund's multiplicity rule

Abstract: Hund's multiplicity rule, according to which a high spin state has a lower energy than any other state of lower spin arising from the same configuration, was deduced from atomic spectra immediately before the advent of modern quantum mechanics. Since then, the lower energy of the state of higher multiplicity has generally been explained by assuming greater electron repulsion in the lower spin states. Numerous calculations1–13 have shown, however, that the traditional explanation of Hund's rule is invalid. In every neutral species for which the requisite calculations have been carried out, the electron–electron repulsion, Vee, is greater in the high spin state, and the lower energy of the high spin state is due to its greater electron–nucleus attraction, Ven. I show here that Hund's rule is a consequence of the Fermi correlation between electrons with parallel spin which leads to less screening of the nucleus in the high spin state.

TRIPLET FORMATION AND TRIPLET DECAY IN REACTION CENTERS FROM THE PHOTOSYNTHETIC BACTERIUM Rhodopseudomonas sphaeroides

Abstract: — In the reaction center of photosynthetic bacteria, with the primary ubiquinone reduced, the triplet state PR of the primary electron donor (a pair of bacteriochlorophylls named P) is PO ulated with a takes place in a few ns. We measured by flash absorption spectroscopy the influence of temperature on formation and decay kinetics of PR and 3Car in the reaction center of several strains of R. sphaeroides. The rate of triplet energy transfer, measured as the decay of PR after a flash, decreases when the temperature is lowered. Between 60 and 30 K the half-time of energy transfer becomes longer than the 3Car half-time decay (about 6 μs) and below 20 K the transfer is slower than the internal decay of PR (about 100 μs). In several cases it is clear that PR and 3Car decay independently and are not in thermal equilibrium. The singlet energy transfer from carotenoid to P occurs with a high efficiency at all temperatures. The data can be accounted for on the basis of estimated energy levels of PR and 3Car, in the context of the equilibrium 3P 3D where 3P is the localized triplet state of P-870 and 3D is another triplet state. A reasonable kinetic scheme leads us to estimate that 3D is 0.0025 ± 0.005 eV above 3P. 3D may thus be the state observed by Shuvalov and Parson (1981). We propose that both triplet and singlet energy transfer between P and the carotenoid occur via a bacteriochlorophyll, to which the carotenoid should be tightly coupled via exchange interaction.

Excited-state properties of linear polyenes studied through a nonempirical Heisenberg Hamiltonian

Abstract: Etude des energies de transition vers les etats inferieurs singulet 1 A g et triplet 3 B u en fonction du nombre d'atomes C. Geometries relaxees planes des etats excites triplet et singulet. Chemin de la photoisomerisation cis-trans

ODMR of recombination centres in crystalline quartz

Abstract: The well known 2.8 eV luminescence band in quartz has been studied using optically detected magnetic resonance (ODMR). A triplet state with a very large fine-structure splitting is found to contribute to the emission. The principle axes of the triplet are identified. The possibility that the luminescence originates in the decay of a self-trapped exciton involving a transient oxygen Frenkel pair is discussed.

Theoretical investigation of the dication of carbon monoxide (CO2

Abstract: Calculs ab initio et CI. Courbes de potentiel, energies et durees de vie pour la predissociation rotationnelle de tous les niveaux (ν, I) des etats lies et quasi lies

Optical and electron spin resonance study of cation and cation-crown ether induced dimerization of tetrakis(4-sulfonatophenyl)porphyrin

Abstract: The effect of dimerization on the optical spectra of the title compound TPPS and the ESR spectra of its photoexcited triplet state were investigated. Dimerization can be promoted by addition of cations and cation-crown ether complexes. Two distinct dimer species could be identified with ESR. Dimerization produces red shifts of the optical absorption (Q) bands and fluorescence peaks. In addition, the fluorescence intensity is strongly quenched. These results parallel finding reported for a variety of cofacial covalently linked chlorophylls and porphyrins. However, a striking difference exists between the triplet characteristics of the TPPS dimers on the one hand and covalently linked dimers. The former exhibit a pronounced dimerization-induced reduction in zero-field splitting values. The values of the latter are very similar to those of their monomer precursors. In this respect, the TPPS system mimics changes found in the triplet ESR spectra associated with the transition from chlorophyll monomers (in vitro) to chlorophyll dimers in reaction centers of

The microwave spectrum of a triplet carbene: HCCN in the X 3Σ− state

Abstract: A simple carbene, the HCCN radical, has been identified in the gas phase using a microwave spectroscopic method. The HCCN molecule was generated in a free space absorption cell by the reaction of CH3CN with the microwave discharge products of CF4. Five rotational transitions, each split into three fine structure components, were observed in the region of 110 to 198 GHz. No hyperfine structure was resolved, although some of the observed lines showed broadening. The rotational constant, the centrifugal distortion constant, the spin–spin coupling constant, and the spin‐rotation coupling constant were determined with good precision. The observed spectrum is completely consistent with that expected for a linear molecule in a 3Σ state, in agreement with an earlier matrix EPR study of Bernheim et al. [J. Chem. Phys. 43, 196 (1965)].

Picosecond excited-state absorption of alkylnitrobenzenes in solution

Abstract: Etude de ces composes par spectroscopie transitoire de l'ordre de la picoseconde: les deux bandes observees dans la region 400-650 nm sont attribuees a l'etat triplet excite n, π*. Une conversion intersysteme rapide est mise en evidence par un temps de montee ≤5 ps pour le nitrobenzene et une abstraction d'hydrogene intramoleculaire se produit a partir de l'etat triplet avec une constante de vitesse de 0,9×10 9 s −1

Intermolecular effects on intersystem crossing studied on the picosecond timescale: the solvent polarity effect on the rate of singlet-to-triplet intersystem crossing of diphenylcarbene

Abstract: Generation du diphenylcarbene par photo-excitation dans l'UV du diphenyldiazomethane. Vitesse k 5 T formation de 3 DPC suivant la reaction 1 *Ph 2 C=N 2 → −Nd 2 1 Ph 2 C→ 3 Ph 2 C

Time-resolved luminescence measurements of triplet-sensitized singlet-oxygen production: variation in energy-transfer efficiencies

Abstract: L'acridine, l'aceto-2 naphtone et la benzophenone sont soumis a une excitation par laser a impulsion a 355 nm dans le benzene; dans chaque cas, le rendement du sensibilisateur triplet et de l'oxygene singulet, ainsi que l'intensite IR sont controles respectivement par spectroscopie d'absorption cinetique et d'emission cinetique

The generation of triplet methoxy cations

Abstract: The triplet methoxy cation has been generated in the gas phase by the collision induced charge reversal of methoxy anions. Its heat of formation was estimated to be 1034±20 kJ mol−1, in reasonable agreement with ab initio calculations. 3[H3CO]+ and its isomer [H2COH]+ fragment unimolecularly by loss of H2 via a common transition state and some 60% of the reactant's excess energy appears as translational energy of the products. Large isotope effects are involved in the former's fragmentation. The barrier for the isomerization reaction 3[H3CO]+[H2COH]+ is small, <20 kJ mol−1. The collision induced dissociation 3[H3CO]+[H3C]++O proceeds via two reaction channels.

Nonvertical triplet excitation transfer to cis- and trans-stilbene

Abstract: Etude experimentale de la variation thermique des constantes de vitesse pour le transfert d'excitation triplet endothermique de l'anthracene ou du dichloro-9,10 anthracene vers les stilbenes, en solution dans le toluene

Structures and properties of excited states of benzene and some monosubstituted benzenes

Abstract: Calculs CI a l'approximation SINDO 1 pour l'optimisation de la structure des etats excites inferieurs singulets et triples de benzene, fluorobenzene, phenol, aniline, toluene, nitrobenzene. Comparaison des geometries d'equilibre, energies d'excitation adiabatiques, charges moments dipolaires et degres d'aromaticite pour les divers etats excites

Reaction of diphenylcarbene with methanol

Abstract: La reaction du diphenylmethylene triplet avec le methanol s'effectue certainement via une excitation thermique en l'etat singulet suivie d'une reaction de cet etat avec l'alcool pour former l'ether

Coexistence of hydrogen atom transfer reactions through and not through triplet ion pair between p-chloranil and durene.

Abstract: Mechanisms of hydrogen atom abstraction reactions by triplet state p-chloranil (3CA) from durene (DH) were studied by picosecond and nanosecond laser photolysis and transient photoconductivity measurements. 3CA was quenched by DH through diffusional encounter to form a triplet ion pair (IP) between CA and DH, p-chloranil semiquinone radical (CAH·), and 2,4,5-trimethylbenzyl radical (D·). Ionic dissociation of IP was observed in 1,2-dichloroethane (DCE) as well as in acetonitrile. However, no transient species was observed by direct excitation of a charge-transfer (CT) band of the electron donor-acceptor (EDA) complex between CA and DH. The H-atom transfer leading to production of CAH· was found to proceed through two distinct mechanisms; H-atom transfer via IP (Mechanism I) and a more rapid transfer competing with IP formation (Mechanism II). The quantum yields of CAH· produced by Mechanisms I and II and the first-order rate constants for proton transfer, ionic dissociation, and intersystem crossing compe...

Vibrational and electronic excitation in p-benzoquinone by electron impact

Abstract: Vibrational and electronic excitation by electron impact in p-benzoquinone was studied using a trochoidal electron spectrometer. Two distinct patterns of vibrational excitation were observed. First, low quanta of a few selected vibrations are specifically excited at incident electron energies corresponding to shape resonances. Some resonances excite mainly the CO stretch, others the CH stretch vibration, and this selectivity is used in the discussion of the assignment of the resonances. A second pattern is an unspecific excitation of a quasi-continuum where no structure due to individual vibrational levels can be discerned. This feature peaks at threshold, large amounts of vibrational energy can be deposited in the molecule, and the excitation also proceeds via shape resonances. Electronic excitation spectra in the valence and Rydberg regions are also presented and discussed. A band observed at 4.37 eV with low residual energies has been tentatively assigned to the second π — π* triplet state 3B3g.

Study of hydrogen atom abstraction reactions of triplet diphenylcarbene in solution

Abstract: Par photolyse laser eclair du diphenyldiazomethane, formation du diphenylcarbene (DPC) triplet (spectre d'absorption a 314 nm). Dans le cyclohexane DPC, par transfert d'atome H conduit au radical Ph 2 CH. Cinetique. Constantes de vitesse de formation du radical dans divers solvants. Effets isotopiques cinetiques

Fluorenylidene: kinetics and mechanisms

Abstract: Irradiation laser du diazo-9 fluorene en solution conduisant a une espece transitoire, un carbene triplet en equilibre thermique avec l'etat singulet plus reactif. Insertion de l'etat singulet dans les liaisons C-H. Etude des diverses reactions de l'etat singulet. Caracterisation du radical fluorenyle-9

Photoisomerization mechanism and conformational equilibria of styrylnaphthalenes. A study based on photophysical properties and molecular-orbital calculations

Abstract: The photophysical properties of trans-1- and trans-2-styrylnaphthalene (1-StN and 2-StN) have been studied. The conformational isomers of 2-StN have been reinvestigated by measuring the solvent and temperature effects and by m.o. calculations (CNDO/S) on the nature of the lowest excited states. Fluorescence quantum yields and lifetimes and kinetic parameters of decay have been obtained for the two conformers in n-hexane and ethanol. The energy difference between the two conformers and the activation energy for their trans→cis photoisomerization have been obtained from the temperature effect in both solvents. Evidence has been obtained for two photoisomerization mechanisms, a triplet mechanism predominant below room temperature and a singlet mechanism favoured in ethanol and/or at higher temperatures.

Scattering length and effective range in two dimensions: application to adsorbed hydrogen atoms

Abstract: The concepts of scattering length (a) and effective range (re) are introduced in two dimensions for an isotropic finite-range potential. Application to mutual scattering of pairs of hydrogen atoms in the triplet state adsorbed at a liquid helium surface shows the usefulness of a description in terms of two parameters. The values derived for this case are a=2.468a0 and re=11.62a0.

Complete determination of 14N hyperfine and quadrupole interactions in the metastable triplet state of free‐base porphin via electron spin echo envelope modulation

Abstract: Through the use of the electron spin echo envelope modulation method, we have accomplished the complete determination of the 14N hyperfine and quadrupole interaction tensors in the lowest photo‐excited triplet state of free‐base porphin in an n‐octane single crystal at ∼14 K The results give insight into the molecular structure of the porphin in the excited state, the relationship between the nitrogen spin density and observed hyperfine couplings, and the effect of hydrogen bonding interactions on the quadrupole coupling constants and principal axes

A Variational Study of the Ground State of Frustrated Quantum Spin Models

Abstract: A variational study of the ground state of the antiferromagnetic spin 1/2 X Y and Heisenberg models is presented The present approach to the quantum spin system is complementary to what has done by Anderson and Fazekas In the present approach the obtained ground state wave functions show a kind of sublattice order A consideration on properties of the true ground state of the Heisenberg system is also presented

(all‐E)‐1,3,5,7‐Octatetraene: Electron‐Energy‐Loss and Electron‐Transmission Spectra

Abstract: Electron-energy-loss and electron-transmission spectra of (all-E)-1,3,5,7-octatetraene were recorded with a trochoidal electron spectrometer. The energy-loss spectra reveal two triplet states at 1.73 and 3.25 eV (0,0-transitions), the UV-active 11Bu state at 4.40 eV, and a higher-lying singlet state at 6.04 eV. The 21Ag state recently reported to be the lowest excited singlet state in this polyene, could not be observed. This failure is probably de to a small excitation cross section under the scattering conditions used and the presence of the second triplet sate in the pertinent energy region. The electron-transmission spectrum revealed three resonances (i.e. short-lived anion states) at 1.5, 2.5, and 4.15 eV.

Intramolecular quenching of carbonyl triplets by .beta.-phenyl rings

Abstract: The presence of a phenyl ring in a position ..beta.. to the carbonyl of a phenyl alkyl ketone results in greatly reduced ketone triplet lifetimes. For example, in deaerated benzene solutions, triplet lifetimes were 2700 and 0.89 ns for propiophenone (P) and ..beta..-phenylpropiophenone (PP) and 3500 and 50 ns for 4'-methoxypropiophenone (MeOP) and ..beta..-phenyl-4'-methoxypropiophenone, respectively. Triplet energy transfer from the ketones to 1-methylnapthalene (MeN) proceeded at nearly diffusion-controlled rates. The rate of electron transfer from the triplet carbonyls to methylviologen was also found to be very efficient, occurring with rate constants of 8.8 X 10/sup 9/ and 8.3 X 10/sup 9/ M/sup -1/ s/sup -1/ for MeOP and PMeOP, respectively. From the triplet decays and the shapes of the triplet-triplet absorbance spectra, a quenching mechanism involving intramolecuular hydrogen abstraction was ruled out. A process involving an exciplex intermediate, characterized by a degree of charge transfer (CT), was proposed as the likely mechanism for the intramolecular quenching of triplet lifetimes in PP and PMeOP. 6 figures, 2 tables