Triplet Population Research Articles

FDMR of chlorophyll triplets in integrated particles and isolated reaction centres of Photosystem II. Identification of P680 triplet

Fluorescence detected magnetic resonance (FDMR) of various chlorophyll a triplets in PS II large particles was studied with the aim of identifying the nature of the different components underlying the complex line shape of the signals. The latter are deconvoluted into sums of Gaussians in different experimental conditions, and the charge recombination triplet is unequivocally identified as the source of two lines with negative signs at 722.5 and 992 MHz |D| = 0.0286 cm −1, |E| = 0.0043 cm −1). The origin of the other triplet signals from the integrated particles is discussed in terms of functional antenna components and products from alteration of the native environment of pigments. FDMR signals also arise from isolated reaction centre complexes and are all to be attributed to different triplet populations of the primary electron donor P680. The results are compared with those obtained by other authors on ADMR. The heterogeneity of P680 pigments in D 1D 2-cyt- b 559 complexes is shown to arise during preparation procedures.

FT-IR studies on the triplet state of P680 in the photosystem II reaction center: triplet equilibrium within a chlorophyll dimer.

The structure and molecular interactions of the primary donor (P680) in the reaction center (D1-D2-cytochrome b-559 complex) of photosystem II (PS II) have been investigated by detecting light-induced FT-IR difference spectra upon the formation of its triplet state (3P680). The 3P680/P680 spectrum obtained was analyzed by comparing it with difference spectra between the ground and lowest triplet states of purified chlorophyll a (Chl) in organic solvents. The negative peaks at 1669 and 1707 cm-1 accompanied by the positive peaks at 1627 and 1659 cm-1 in the 3P680/P680 spectrum were assigned to the keto C = O stretching mode, and the appearance of these two pairs of bands indicated that P680 has a dimeric structure analogous to that of the bacterial primary donor. From the band positions of the keto and carbomethoxy C = O stretches, the hydrogen-bonding properties of these two Chl molecules were found to be asymmetrical; in one Chl molecule both the keto and carbomethoxy C = O groups form hydrogen bonds, while in the other Chl molecule the keto C = O is not hydrogen-bonded whereas the carbomethoxy C = O probably is hydrogen-bonded. The temperature dependence of the intensity ratios of the keto C = O bands revealed that the triplet state is equilibrated between the two Chl molecules with an energy gap of 8.4 +/- 0.7 meV. Most of the triplet population was found to be localized on one Chl molecule (86% at 80 K), in which both of the two C = O groups are hydrogen-bonded, that is probably attached to the D1 subunit. Considering the structure of the bacterial reaction center determined by X-ray crystallography and the sequence homology between the D1 and D2 subunits of PS II and the L and M subunits of bacteria, a model of the P680 structure and its interactions with apoproteins has been proposed.

Photoinduced electron transfer between styrylanthracenes and electron donors and acceptors in acetonitrile

The decay pathways of the lowest singlet and triplet excited states (1trans* and 3trans*) of trans-n-styrylanthracenes (n-StA, where n= 1, 2 or 9 on the anthracene) have been studied in acetonitrile at room temperature. Fluorescence lifetimes (τF) and quantum yields (ϕF), as well as the yield and spectral and kinetic properties of the lowest triplet state of the three StAs, were determined by steady-state and transient techniques. The formation and the decay of the respective StA radical cations (trans˙+) were observed by laser flash photolysis; the yield of photoionization is ca. 0.07 on 353 nm excitation and is enhanced by fluorescence quenching with 1,4-dicyanobenzene. The formation and decay of the StA radical anions (trans˙–) in the presence of diethylaniline (DEA) is concluded from transient conductivity (Δκ) and optical results. DEA significantly enhances the yield of trans˙– and the initial amplitude of Δκ and correspondingly quenches ϕF. The bimolecular interaction between 1trans* and 4-bromodimethylaniline enhances the triplet population.

Positronium quenching via collisions with triplet states of photomagnetic molecules.

We have studied positronium quenching resulting from collisions with the triplet states of benzaldehyde, oxygen, benzophenone, and bromonaphthalene. Positronium pick-off decay rates are presented as functions of triplet populations via uv irradiation of benzaldehyde-ethane, benzaldehyde-helium, and oxgyen-ethane gaseous mixtures and of benzophenone and bromonaphthalene adsorbed porous silicas. Our results show that the cross sections for positronium quenching in collisions with excited triplet states are not as high as reported previously. The oxygen data suggest reactions between hot'' (nonthermal) positronium and oxygen molecules.

PHOTOCHEMISTRY OF MEROCYANINE 540

The photophysical properties of merocyanine 540 have been determined in methanol solution over a modest temperature range. Triplet state population is inefficient (the limiting triplet quantum yield being 0.25) due to rapid isomerization of the central double bond from the first excited singlet state. Activation energies have been measured for isomerization from the excited singlet state (20 kJ mol-1) and for conversion of the resultant cis-isomer back to the original trans-form (63 kJ mol-1), both processes involving formation of a twisted species. The dye is easily oxidized to give an unstable adduct which decomposes on the sub-ms timescale. Reversible redox chemistry occurs upon excitation in the presence of electron acceptors. These various observations are discussed in terms of the known chemotherapeutic activity of MC540 and it is concluded that the most probable mechanisms for cytotoxicity involve either local thermal disruption of cell membranes or in situ photogeneration of toxins derived from breakdown of the dye.

Triplet selectivity in surface ejection of electrons by laser-excited metastables of aniline

We utilize the phenomenon of surface electron ejection, induced by metastables, to measure the triplet-state lifetimes of aniline in molecular beams. The triplet population originates from optically excited S 1 vibronic states via intersystem crossing. The good correspondence of the measured lifetimes to previously reported values supports our notion that only excess electronic energy induces the electron ejection. Vibrationally excited ground state molecules with the same energy content do not induce the process.

Physical conditions in the cool parts of prominences. II - The MG triplet lines

The statistical equilibrium of Mg under prominence conditions is considered. It is shown that the principal lines of the triplet system can serve as an equivalent to the Na D lines for spectral diagnostic purposes. Like the Na D lines, these Mg lines are relatively easy to measure in prominences and exhibit small optical depths. It is demonstrated that potential complications due to possible radiative transfer effects in the Mg resonance transition (3 1S0 - 3 1P1) have little consequence for the triplet populations. The results are used to repeat the spectral analysis of Landman (1983) using the Mg multiplets in place of the Na ones. The results of the two analyses are in general agreement. The finding of a low degree of H ionization in the cool prominence regions and a correspondingly high gas pressure is confirmed.

Photoprocesses in photosystem I model systems

Photoprocesses in covalently linked bis(pyrochlorophyllide a) ethylene glycol diester molecules have been investigated by laser absorption and fluorescence techniques. Solvent interactions significantly alter the photochemical properties that resemble those of the special pair chlorophyll (P700) of photosystem I of green plants. Photoexcitation of the model system results in the formation of an S/sub 1/ state. This excited state returns to the ground state primarily by fluorescence and internal conversion. Under appropriate excitation conditions stimulated (laser) emission is observed. Long (30 ns) excitation pulses produce significant triplet populations. The configuration and possible reactions involving this triplet are discussed. In contrast, the folded pairs in methylene chloride/ethanol solution exhibit an unusually short fluorescence lifetime and a correspondingly low quantum yield. Some of the anomalous behavior of the folded pairs photoexcited in methylene chloride are shown to result from differences in ground-state structure and composition. Rapid internal conversion processes prevent the build-up of a significant excited-state population in this solvent system. The internal conversion process responsible for the lifetime shortening involves the formation of charge-transfer states. 53 references, 9 figures, 3 tables.

Time-resolved resonance raman spectra and decay kinetics of triplet anthracene in fluid media

Reported here are the time-resolved resonance Raman spectra and decay kinetics of the lowest triplet state ( 3B 2u +) of anthracene- h 10 and anthracene- d 10 molecules in fluid media at room temperature. The triplet population (≈3 × 10 −5 M) is observed to decay at microsecond times by triplet—triplet annihilation. Vibrational assignments for the observed Raman bands are proposed.

Room-temperature kinetics of the photoexcited triplet state of acridine in fluorene crystals as obtained from electron spin echo studies

It is shown that electron spin echo results allow an analysis of the complete kinetics of excited triplet spin states at room temperature, not only with respect to all relevant kinetic parameters of the triplet decay and spin relaxation but also the spin selectivity of the triplet population and decay processes.

Time resolved studies of pentacene triplets by electron spin echo spectroscopy

The triplet kinetics of pentacene imbedded in p-terphenyl and benzoic acid crystals have been studied by electron spin echo spectroscopy in the temperature range of 77–300 K. Population inversions have been observed for both H0‖‖x and H0‖‖z in the benzoic acid crystal but only for H0‖‖z in the p-terphenyl crystal at room temperature. Detailed analyses showed that the triplet population and decay of the pentacene are very much host and temperature dependent. Dimer formation and tautomerization (proton transfer of the benzoic acid dimer) may be responsible for the peculiar triplet kinetics of the pentacene in the benzoic acid crystal.

Nonradiative relaxation of pyridine vapor: Transient absorption studies of triplet state formation, decay, quenching, and structure

Relaxation processes in pyridine vapor have been studied using high-performance transient absorption spectrometry following excitation low in the S1 manifold. The ultraviolet spectrum of the lowest triplet state has been identified and measured, and its dependence on delay time and background pressure has been investigated for a variety of collision partners. It is concluded that formation of the triplet through S1 → T1 intersystem crossing proceeds in the statistical limit. Subsequent radiationless decay of the triplet population was found to show unusual pressure-dependent kinetics which apparently reflects collisional interconversion between two forms having very different intrinsic lifetimes. A simple model is proposed to explain the nonradiative behavior, collisional quenching, and spectra of the lowest triplet in terms of strong pseudo-Jahn–Teller vibronic coupling between nearly degenerate 3ππ* and 3nπ* states that leads to a double minimum in the T1 potential surface along the out-of-plane coupling coordinate. It is suggested that vibrationally relaxed T1 pyridine is nonplanar in structure whereas the vibrationally activated form is quasiplanar, and that the nonradiative T1 → S0 decay rates of these two forms are <105 s−1 and ∼5×106 s−1, respectively. Quenching of the triplet by ground state molecular oxygen was found to follow a sequential kinetic mechanism in which a transient intermediate was spectroscopically intercepted. This species is thought to be a weak complex formed between triplet pyridine and oxygen.

ChemInform Abstract: AN ELECTRON SPIN ECHO STUDY OF THE PHOTOEXCITED TRIPLET STATE OF TETRACENE IN P‐TERPHENYL CRYSTALS AT ROOM TEMPERATURE

Room temperature magnetic resonance studies of the photoexcited triplet state of tetracene in p‐terphenyl crystals have been carried out by the electron spin echo technique. The studies yield the following structural and dynamical information: (1) zero‐field splittings; D/hc=±0.0551(1) and E/hc=∓0.0042(1) cm−1; (2) spin densities: ρ1(α)=0.055, ρ2(β)=0.016, ρ5(γ)=0.217, ρ13=−0.025, and ρ15=−0.027; and (3) relative triplet population rates: px≊py≳pz.

An electron spin echo study of the photoexcited triplet state of tetracene in p-terphenyl crystals at room temperature

Room temperature magnetic resonance studies of the photoexcited triplet state of tetracene in p-terphenyl crystals have been carried out by the electron spin echo technique. The studies yield the following structural and dynamical information: (1) zero-field splittings; D/hc=±0.0551(1) and E/hc=∓0.0042(1) cm−1; (2) spin densities: ρ1(α)=0.055, ρ2(β)=0.016, ρ5(γ)=0.217, ρ13=−0.025, and ρ15=−0.027; and (3) relative triplet population rates: px≊py≳pz.

ChemInform Abstract: QUANTITATIVE TRIPLET PHOTOPHYSICS BY PICOSECOND PHOTOMETRY

Abstract The absolute triplet yields of organic compounds are usually probed photochemically by an indirect method, since flash techniques present difficulties in estimating the concentration of triplets generated. Repetitive picosecond photometry provides a novel approach to resolve this problem, since singlet and triplet populations are probed simultaneously. A further refinement is feasible when the role of laser-induced gratings generated in the picosecond interferometer at zero time delay is understood. For the first time, the transient-transmission plot indicates whether there is appreciable internal conversion apart from the deactivation path via the triplet. An investigation of the heavy-atom effect in Uranine and its derivatives is also presented.

CHLOROPHYLL PHOTOSENSITIZED ELECTRON TRANSFER IN PHOSPHOLIPID VESICLE BILAYERS: INSIDE VS OUTSIDE ASYMMETRY*

Abstract— We have determined triplet quenching efficiencies. radical yields and radical recombination kinetics in mixed chlorophyll (Chl)‐egg phosphatidylcholine vesicle suspensions in the presence of electrically‐charged electron acceptors located either in the external. continuous aqueous phase or within the internal aqueous volume of the vesicles. There was a marked asymmetry between these processes as to whether they occurred at the outer or inner bilayer‐water interfaces.With methyl viologen (MV2+) as acceptor, 52 ± 4% of the total Chl triplet could be quenched from the inside. whereas only 16 ± 2% was quenchable from the outside. Approximately 35% of the triplet population was inacccssible to quenching by MV2+ from either inside or outside. Ouenching rate constants were higher from the outside than from the inside (2 × 106M−lS‐Ivs 1 × 106M−Is−1). A similar pattern was obtained when anthraquinone disulfonate or ferricyanide were used as acceptors.Radical yields and recombination kinetics also displayed asymmetric behaviour. From the inside. only 4 ± 2% of the quenched triplets gave rise to separated radicals using MV2+ as acceptor, whereas from the outside the conversion yield was 32 ± 2%. The halftime for the Chl+ MV+ reaction was approximately 100 times longer at the outer surface than at the inner surface.We conclude the following: (a) Chl is distributed asymmetrically within the bilayer such that more triplet Chl is located within quenching distance of the interface at the inner surface than at the outer surface. Furthermore, an appreciable fraction of the triplet Chl is located sufficiently far from either interface so that quenching is not possible. (b) The mobility of Chl and quencher molecules is greater at the outer surface of the vesicles than at the inner surface.

Quantitative triplet photophysics by picosecond photometry

The absolute triplet yields of organic compounds are usually probed photochemically by an indirect method, since flash techniques present difficulties in estimating the concentration of triplets generated. Repetitive picosecond photometry provides a novel approach to resolve this problem, since singlet and triplet populations are probed simultaneously. A further refinement is feasible when the role of laser-induced gratings generated in the picosecond interferometer at zero time delay is understood. For the first time, the transient-transmission plot indicates whether there is appreciable internal conversion apart from the deactivation path via the triplet. An investigation of the heavy-atom effect in Uranine and its derivatives is also presented.

Laser system and data analysis techniques for absorbing and non-absorbing excited transients

A comprehensive system for the study of nanosecond kinetics of transient absorbing species is described. It incorporates optical fibre cables facilitating 1.8-180 ns delay between the pump and probe beams. To test the system data were obtained for aerated solutions of anthracene in ethanol and cyclohexane. The build-up of the excited triplet population over approximately 10 ns was observed directly. The possibility of detecting short-lived intermediate states which neither absorb nor fluoresce is discussed.

Proton Spin-Lattice Relaxation in Solid Würster's Blue Perchlorate

Temperature dependence of the proton spin-lattice relaxation time in powdered Wurster's blue perchlorate was measured between 56 and 300 K by using the 90°–τ–90° pulse NMR method at the frequencies 8.0, 12.0 and 23.4 MHz. In the paramagnetic state above the phase transition temperature of 190 K, the relaxation rate was independent of temperature and of frequency and could be explained by the hyperfine interaction modulated by an exchange interaction. Below T c the relaxation time increased with decreasing temperature and depended on frequency in accordance with BPP theory. The relaxation rate is proportional to the product of the triplet population and the effective exchange frequency both of which vary with temperature. Between 70 and 56 K, the relaxation rate is governed by paramagnetic impurities or monomer radicals.

Quenching of enzyme-generated acetone phosphorescence by indole compounds: stereospecific effects of D- and L-tryptophan. Photochemical-like effects.

Indole compounds efficiently quench the acetone phosphorescence observed during the horseradish peroxidase catalyzed aerobic oxidation of isobutyraldehyde. Different types of Stern-Volmer plots are observed: linear, linear with two slopes, and downward- and upward-curved plots. The complexity probably stems from the operation of both dynamic and static quenching, coupled with different efficiencies of quenching of various acetone triplet populations. Binding to the enzyme may also occur, especially in the case of L-tryptophan. The different Stern-Volmer behavior of D- and L-tryptophan fully confirms that the acetone triplet is generated within the enzyme and not free in solution. This chiral discrimination toward an enzymically generated electronically excited species is novel. It is tentatively postulated that a long-range triplet-triplet exciton transfer occurs; the excited triplet indole then undergoes photochemical-like alterations.