Showing papers on "Triplet state published in 1977"

Mechanism of ethidium bromide fluorescence enhancement on binding to nucleic acids

Abstract: The mechanism of the enhancement of the fluorescence of ethidium bromide on binding to double helical RNA and DNA has been investigated. From an examination of the effect of different solvents on the fluorescence lifetime, quenching of fluorescence by proton acceptors, and the substantial lengthening of lifetime observed upon deuteration of the amino protons, regardless of the medium, we conclude that proton transfer from the excited singlet state is the process primarily responsible for the approximately equal to 3.5-fold increase in the lifetime of free ethidium bromide in going from H2O to D2O; the fact that addition of small amounts of water to nonaqueous solvents decreases the fluorescence whereas addition of small amounts of D2O enhances the fluorescence; and the enhancement of the ethidium bromide triplet state yield on binding to DNA. Other proposed mechanisms are shown to be inconsistent with our findings.

The Photoenolization of 2-Methylacetophenone and Related Compounds

Abstract: A reinvestigation of 2-methylacetophenone (1) by ns flash photolysis has provided detailed evidence for the reaction sequence of photoenolization. The triplet reaction proceeds adiabatically from the lowest excited triplet state of the ketone, 3K (1), to the enol excited triplet state, 3E (1), which decays both to enol and ketone ground state. The Z- and E-isomers of the photoenol, Z-E (1) and E-E (1) are formed in about equal yield by the triplet pathway, while direct enolization from the lowest excited singlet state of 1 yields (predominantly) the Z-isomer. Intramolecular reketonization from Z-E (1) to 1 proceeds at a rate of ca. 108s−1 in cyclohexane, but can be retarded to ca. 104s−1 in hydrogen-bond-acceptor solvents. The proposed mechanism is summarized in Scheme 1 and rationalized on the basis of a state correlation diagram, Scheme 2. 3,3,6,8-Tetramethyl-1-tetralone (2) was used as a reference compound with fixed conformational position of the carbonyl group, and some results from a brief investigation of 2,4-dimethylbenzophenone (3) are also reported.

Structure and energetics of simple carbenes methylene, fluoromethylene, chloromethylene, bromomethylene, difluoromethylene, and dichloromethylene

Abstract: There is very little experimental data concerning the singlet--triplet separations of carbenes. In many cases, it is not even possible to form estimates of this separation from experimental results. CH/sub 2/ is a distinct exception; here substantial, although contradictory, data are available. The equilibrium geometries of the lowest singlet states of several carbenes have been determined experimentally. However, the geometries of many states are not known and the triplet state geometries are also generally unknown. In order to obtain theoretical information about these quantities for a series of halogenated carbenes, we have studied CH/sub 2/, CHF, CHCl, CHBr, CF/sub 2/, and CCl/sub 2/ using a consistent level of ab initio electronic structure theory. One configuration (triplet state) and two configuration (singlet state) wave functions have been computed. Double zeta basis sets augmented by a d function on the carbon atom, optimized separately for both the singlet and triplet states of each molecule, were used. The theoretical equilibrium geometries agree well with the available experimental data. The trends of the singlet-triplet separations are discussed and correlated with the carbon atom Mulliken gross populations for the singlet and triplet states.

Laser photolysis studies of duroquinone triplet state electron transfer reactions

Abstract: Mit Methoden der Laser-Photolyse wird die Elektronenubertragung durch Donatoren wie z.B. Fe", Fe(CN 2′, Diphenylamin und Carbonationen auf Triplett-Durochinon in Wasser-Athanol-Gemischen untersucht.

Magnetic properties of oxyhemoglobin.

Abstract: When the magnetic susceptibility of frozen aqueous solutions of human oxyhemoglobin was measured in the range between 25 and 250 K, it showed a temperature-dependent behavior typical of a thermal equilibrium between a ground singlet state and an excited triplet state for two electrons per heme, the energy separation being [2J] = 146 cm-1. By contrast, within the same temperature range, carboxyhemoglobin was found to be diamagnetic, as already reported.

Misleading sigmas and neglected geometries: the effects of methyl substitution on rates of triplet state hydrogen abstraction by the benzoyl group

Abstract: The effects of ring methyl and polymethyl substitution on rate constants for γ-hydrogen abstraction in triplet phenyl alkyl ketones have been determined. The effect of meta and para methyls is to increase the energetic separation between the reactive n, π * triplet and the lowest π, π * triplet and to thus lower the equilibrium concentration of the former. Ortho-methyl groups also introduce a competing enolization reaction which is very rapid in the syn conformer but is limited by the rate of anti → syn rotation in the anti conformer.

The photochemistry of riboflavin--VI. The photophysical properties of isoalloxazines.

Abstract: — Many of the photophysical properties of riboflavin and several other N-10 substituted isoal-loxazines have been measured and these include: quantum yields of fluorescence at 77 K and 298 K, φ, quantum yields of phosphorescence at 77 K, φP, lifetimes of the triplet state by electron spin resonance and phosphorescence at 77 K, φp,. and the quantum yield of intersystem crossing. φisc. For riboflavin in an alcoholic matrix at 77 K the limiting values were: φJ= 0.32, φP= 0.007, φp= 0.20s and φisc= 0.7. At 298 K, φf for riboflavin in water and alcohol were 0.25 and 0.32, respectively. The results for the photophysical processes are compared with several photochemical processes known to involve the triplet state of riboflavin in aqueous solution. It is concluded that the φisc decreases from 0.7 in alcohol to 0.6 in water at 298 K.

Electron spin resonance of the lowest triplet state of palladiumporphin in a n-octane crystal at 1·3 K

Abstract: E.S.R. experiments performed at 1·3 K by optical detection are reported for the photo-excited triplet state of palladiumporphin in a single crystal of n-octane, and the observation of a level anticrossing signal is described. In the crystal the orbital degeneracy of the 3 E u state of the free molecule is lifted by the crystal field and in n-octane the energy difference between the two orbital components |x> and |y> is found to be 58 ± 2 cm-1. The spinorbit coupling (SOC) and the orbital Zeeman interaction couple the triplet manifolds of |x> and |y>, and for a proper understanding of the magnetic properties of these states it is necessary to work in the basis of the six spin-orbit functions deriving from the 3 E u state of the free molecule. It is shown that either of the two triplet states can be described by an effective spin hamiltonian of the common form and expressions for the zero-field parameters D and E and the principal values of the g tensor are given. The experimental values of the parameters i...

Singlet → triplet intersystem crossing quantum yields of photosynthetic and related polyenes

Abstract: The extinction coefficients for the triplet–triplet maxima of all-trans-β-carotene, 15,15′-cis-β-carotene, all-trans-lycopene, 7,7′-dihydro-β-carotene, all-trans-retinol, all-trans-retinal, 13-cis-retinal and all-trans-retinylidene-N-butylamine have been determined via pulse-radiolysis using a refinement of the energy transfer technique introduced previously. Together with Wolff and Witt's data, the coefficient found for β-carotene suggests that ∼10 % of the total carotene present in photosynthesising spinach chloroplasts is available as a protective valve against overillumination.These extinction coefficients were employed to estimate, via laser flash photolysis, the corresponding singlet → triplet intersystem crossing quantum yields using 265 and 353 nm excitation. The very low yields ( < 0.001) found for β-carotene and derivatives mean that carotenoid triplets formed in chloroplasts during the course of the valve function could not have been formed directly, but only via triplet–triplet energy transfer sensitation via another triplet (chlorophyllT?), or via1O2 quenching. The low cross-over efficiencies also mean that in their accessory pigment role no carotenoid excited singlets are wasted in crossing over to the triplet state.

Photoreduction of 1,10-phenanthroline

Abstract: The effects of solvent polarity on the photophysical properties of 1,10-phenanthroline indicate the close proximity of (n, π*) and (π, π*) excited singlet states, whilst the lowest triplet state is (π, π*) in all solvents. In hydrocarbon solvent, the first excited singlet state is of (n, π*) character but in water a (π, π*) is situated ∼ 700 cm–1 below this state. Even in water, the (n, π*) state is capable of abstracting a hydrogen atom from the solvent forming an intermediate semidiaza radical. The radical can be detected by flash photolysis and e.s.r. techniques and, in organic solvents, it decays with second order kinetics due to a disproportionation reaction. The major product of reaction is dihydrophenanthroline which, in hydrocarbon solvents, is formed with a quantum efficiency of ∼ 25 %. In aqueous solvent, the yield and rate of decay of the semidiaza radical are pH dependent, and the pK for protonation of the radical is 7.3. Thus, irradiation in water at pH 5 results in formation of the stable diprotonated radical cation.

An optically active cyclopropene as a mechanistic probe in cyclopropene photochemistry

Abstract: The photochemistry of methyl 1-methyl-2-phenylcyclopropene-3-carboxylate has been examined in both the racemic and optically active forms. In the excited singlet the cyclopropene is converted to 2-methoxy-5-methyl-4-phenylfuran whereas the triplet state dimerizes to a tricyclohexane derivative. Experiments with optically active cyclopropene ester demonstrate that photochemical racemization occurs about 2.5 times as fast as conversion to the furan product indicating that there is an intermediate vinyl carbene on the singlet surface. No racemization is observed in the triplet state.

Studies of self-trapped exciton luminescence in KCl

Abstract: Measurements have been made of the luminescence intensity, lifetime, emission spectra, and linear polarization of self-trapped exciton luminescence in KCl (2.31 eV) at temperatures of ${7}^{\mathrm{o}}$K and above. Data are interpreted in terms of an exciton energy-level structure composed of a singlet state approximately 15 meV above a triplet state. The singlet state is populated only by thermal excitation from the triplet. The observed polarization data are consistent with a reorientation of the self-trapped exciton in a state higher than those responsible for the characteristic 2.31-eV luminescence.

Study of the Triplet State Properties of Tyrosines and Tryptophan in Azurins Using Optically Detected Magnetic Resonance

Abstract: Optically detected magnetic resonance (ODMR) signals and phosphorescence spectra were seen of tyrosine in the P. aeruginosa and tryptophanless P. fluorescens azurins and of tryptophan in the former. This confirmed a conclusion from other experiments that the tryptophan of P. aeruginosa cannot effectively quench the singlet energy of both tyrosines. The ODMR signals were all very narrow, additional evidence that the chromophores are buried in the interior of the protein. Accurate values of the zero-field coupling constants D and E lead to a tentative correlation of D values with the red shift of the 0 leads to 0 peak of the phosphorescence spectrum. The environment of tryptophan in P. aeruginosa is the most hydrocarbon like of any tryptophan so far observed. The experiments raise a number of unanswered questions concerning rate processes. The intensities of the 2E transition of tyrosine and the phosphorescence of both tyrosine and tryptophan are substantially reduced when the copper is oxidized. Nevertheless the phsphorescence lifetimes are unaffected. A hole cannot be burned in the ODMR resonances. The homogeneously broadened lines may conceivably be a result of low-temperature proton tunnelling.

FLUORESCENCE SPECTRA AND ZERO‐FIELD MAGNETIC RESONANCE OF CHLOROPHYLL a‐WATER COMPLEXES

Abstract: . –Fluorescence detected magnetic resonance (FDMR) and fluorimetry have been used to study chlorophyll-water complexes at T= 4.2 K. By combining these methods, zero-field splitting (ZFS)-parameters can be assigned to the triplet state of the various species. It is found that these parameters decrease upon aggregation and/or complexation. These results can be rationalized by using a simple free-electron model for chlorophyll including excitonic interaction. Evidence is presented favouring a structure recently proposed by Shipman and Katz for a chlorophyll-dimer involving hydrogen-bonded water, present in non-polar solutions at low temperature with spectral properties similar to those of chlorophyll in vivo.

Ab initio SCF and CI study of the electronic spectrum of pyridine N-oxide

Abstract: Configuration interaction (CI) studies of ground, n → π*, π * π* electronically excited states are reported for pyridine N-oxide. The transition energy to the lowest π → π* excited 1 B 2 state is calculated at 4.35 eV, compared to the experimental spectrum range of 3.67–4.0 eV. This state lies below the lowest n → π* excited 1 A 2 state calculated at 4.81 eV above the ground state. The only experimentally reported triplet state at 2.92 eV above the ground state is predicted to be the 3 A 1 (ππ*) state. The calculated energy lies at 3.27 eV. Numerous other high-lying singlet states as well as the triplet states have also been calculated. The intramolecular charge transfer character of the ground and the excited states have been studied in terms of the calculated dipole moment and other physical properties.

Triplet state quenching involving charge transfer interactions between N-methyl indole and aromatic carbonyl compounds in benzene solution

Abstract: In laser photolysis experiments N-methyl indole quenches the triplet states of several aromatic carbonyl compounds including some with lower energy triplet states with rate constants in the range (0.02–10)× 109 dm3 mol–1 s–1. Quenching by N-methyl indole is shown to be either exclusively or partially due to electronic energy transfer from either triplet acetophenone or triplet xanthone respectively, and the sensitized spectrum assigned to the triplet state of N-methyl indole is given. Quenching of the other triplet carbonyls by N-methyl indole is shown to depend on the energy predicted for the charge transfer state of each interacting pair of compounds with N-methyl indole as the potential electron donor. In ethanol solution quenching of the triplet state of xanthone by N-methyl indole is demonstrated to occur via electron transfer leading to the formation of separate ions.Pulse radiolysis of indole, 3-methyl indole and N-methyl indole in benzene solution gives short lived triplet states of these compounds, the spectra of which have been determined. Quenching of triplet N-methyl indole by various compounds with higher and lower triplet states was investigated following pulse radiolysis in benzene solution. The rate constants for quenching of triplet N-methyl indole were also shown to depend on charge transfer interactions and evidence for energy transfer was obtained only in cases where the predicted charge transfer states lie at much higher energies than the locally excited triplet states. The reasons why charge transfer interactions cause rapid relaxation of triplet exciplexes are discussed.

The phosphorescent triplet state of p-xylene in an isotopically mixed crystal at 1·2K: an investigation by E.S.R. and MIDP

Abstract: The phosphorescence spectrum of p-xylene-h 10 in a p-xylene-d 10 host is first presented. Secondly we report E.S.R. experiments performed on p-xylene-α,α′-d 6 in p-xylene-d 10; from an analysis of the orientations of the principal axes in combination with the crystal structure and of the hyperfine splittings it is concluded that the in-plane principal axes of the zfs tensor make an angle of 68° with the molecular axes. Thirdly we discuss Microwave-Induced Delayed Phosphorescence (MIDP) experiments on p-xylene-h 10 in p-xylene-d 10 in which the relative radiative rate constants for decay from the zero-field spin components of the triplet state to single vibronic bands of the ground state have been obtained. The results of the MIDP and E.S.R. measurements are shown to be mutually consistent in terms of a model in which the effect of the crystal field and the methyl substituents on the pseudo-Jahn-Teller unstable electronic structure of the triplet state of benzene is described.