Temperature-dependent energy gap of the primary charge separation in photosystem I: study of delayed fluorescence at 77-268 K

The dynamics of fluorescence decay and charge recombination were studied in the ether-extracted photosystem I reaction center isolated from spinach with picosecond resolution over a wide time range up to 100 ns. At all temperatures from 268 to 77 K, a slow fluorescence decay component with a 30-40 ns lifetime was detected. This component was interpreted as a delayed fluorescence emitted from the singlet excited state of the primary donor P700*, which is repopulated through charge recombination that was increased by the lack of secondary acceptor phylloquinone in the sample. Analysis of the fluorescence kinetics allowed estimation of the standard free-energy difference -DeltaG between P700* and the primary radical pair (P700(+)A0(-)) state over a wide temperature range. The values of -DeltaG were estimated to be 160/36 meV at 268/77 K, indicating its high sensitivity to temperature. A temperature-dependent -DeltaG value was also estimated in the delayed fluorescence of the isolated photosystem I in which the secondary acceptor quinone was partially prereduced by preillumination in the presence of dithionite. The results revealed that the temperature-dependent -DeltaG is a universal phenomenon common with the purple bacterial reaction centers, photosystem II and photosystem I reaction centers.     

Lifetimes of bacteriochlorophyll fluorescence in Rhodopseudomonas viridis and Heliobacterium chlorum at low temperatures

Fluorescence lifetimes of isolated membranes of Rhodopseudomonas viridis were measured in the temperature range of 77 K to 25 K. At room temperature, the main component of the fluorescence decay of bacteriochlorophyll (BChl) b had a time constant of 50 ps. In contrast to other purple bacteria, the emission at low temperature was spectrally homogeneous and showed essentially single lifetimes of 140 ps at 77 K and 180 ps at 25 K, with the primary electron donor in the oxidized state. Taking into account the relative fluorescence yields with open and closed reaction centers, we arrive at numbers of 125 ps and 215 ps, respectively, for open reaction centers. These numbers are significantly smaller than expected on the basis of measurements of the efficiency of charge separation, perhaps suggesting that the excitation decay in the absence of reaction centers is considerably faster at low temperature than at room temperature. At least four different spectral components with different lifetimes were observed at 25 K in the emission of Heliobacterium chlorum, a short-wavelength component of about 30 ps and three longer-wavelength components of about 100 ps, 300 ps, and 900 ps. This indicates a strong heterogeneity in the emitting pigment, BChl g-808. The component with the shortest lifetime does not appear to be affected by the redox state of the reaction center and might reflect energy transfer to BChl g species which are connected to the reaction center.     

Formation and thermal-induced decay of radical ions in polymer matrices

The formation of radical ions in γ-irradiated polymethyl-methacrylate (PMMA) matrices at 77°K and thermal-induced reaction of these radical ions were of studied by optical absorption spectroscopic measurements. The radical ions of stilbene and pyrene were investigated. These radical ions decay according to second-order kinetics, which means that the neutralization reaction of the cationic species and anionic species participates in the decay process. The kinetic plots consist of two straight lines; that is, fast and slow decay processes are concerned. The activation energies were estimated to be E_fast = 2.4 kcal/mol and E_slow = 6.4 kcal/mol, respectively. The probability of recombination reaction depends on the distance between cationic and anionic species.     

THE PRIMARY REACTION IN THE PHOTOREDUCTION OF PROTOCHLOROPHYLLIDE MONITORED BY NANOSECOND FLUORESCENCE MEASUREMENTS

Abstract— Time resolved fluorescence measurements, carried out on protochlorophyllide reductase enriched membranes from oat ( Avena sativa ), are described. A fast (1 ns at 293 K) decaying fluorescence component is assigned to the photoactive NADPH-protochlorophyllide-enzyme complex, while a slower (5 ns) component is ascribed to non-photoactive protochlorophyllide. The results are interpreted in terms of a new fast primary step in the light requiring step of chlorophyll synthesis. The temperature dependence of the rate of this reaction has been studied by measuring the decay time of the fast fluorescence component at various temperatures from 77 to 293 K. Complete spectra of the kinetic fluorescence components have been measured at 293, 160 and 77 K.     

Formation and stability of N-heterocyclic carbenes in water: the carbon acid pKa of imidazolium cations in aqueous solution

We report second-order rate constants kDO (M-1 s-1) for exchange for deuterium of the C(2)-proton of a series of simple imidazolium cations to give the corresponding singlet imidazol-2-yl carbenes in D2O at 25 degrees C and I = 1.0 (KCl). Evidence is presented that the reverse protonation of imidazol-2-yl carbenes by solvent water is limited by solvent reorganization and occurs with a rate constant of kHOH = kreorg = 10(11) s-1. The data were used to calculate reliable carbon acid pK(a)s for ionization of imidazolium cations at C(2) to give the corresponding singlet imidazol-2-yl carbenes in water: pKa = 23.8 for the imidazolium cation, pKa = 23.0 for the 1,3-dimethylimidazolium cation, pKa = 21.6 for the 1,3-dimethylbenzimidazolium cation, and pKa = 21.2 for the 1,3-bis-((S)-1-phenylethyl)benzimidazolium cation. The data also provide the thermodynamic driving force for a 1,2-hydrogen shift at a singlet carbene: K12 = 5 x 10(16) for rearrangement of the parent imidazol-2-yl carbene to give neutral imidazole in water at 298 K, which corresponds to a favorable Gibbs free energy change of 23 kcal/mol. We present a simple rationale for the observed substituent effects on the thermodynamic stability of N-heterocyclic carbenes relative to a variety of neutral and cationic derivatives that emphasizes the importance of the choice of reference reaction when assessing the stability of N-heterocyclic carbenes.     

NMR and ESR studies of the solid-state polymerization of vinyl monomers. II. Acrylic acid

The post-polymerization of acrylic acid, γ-irradiated at 77°K, has been studied by broad-line NMR and ESR between 240°K and 286°K (mp). The changes in the structure of NMR spectra during the polymerization has been related to the conversion yield checked by gravimetry of the polymer formed. The very fast reaction occurring above ?270°K has been followed simultaneously by NMR and thermal analysis. This last method indicates an activation energy of chain propagation of 18.6 ± 2 kcal/mole in satisfactory agreement with the value given by the initial slope of the conversion—time curves. ESR experiments show that, while the average radical concentration decays by a factor of about ten between 77°K and 240°K, the local concentration of radicals persisting after prolonged annealing at T > 240°K, remains the same as at 77°K, i.e., 1.4 × 10¹⁹ spins/g. A kinetic scheme, assuming an exponential decay of propagation and termination rate constants with chain length, has been proposed to explain the shape of conversion—time curves as well as the almost constancy of the local concentration of growing polymer chains.     

Triplet diphenylcarbenes protected by o-Aryl groups

Diphenyldiazomethanes (2a-N(2)) having phenyl groups at the ortho positions were prepared and the corresponding diphenylcarbenes (2a) photolytically generated from them were characterized not only by the traditional product analysis method but also by ESR and UV/vis spectroscopy in a rigid organic matrix at low temperature and in laser flash photolysis in solution at room temperature. Product analysis indicated that fluorenes 4a are formed almost exclusively. Fluorene is most likely produced by the attack of singlet carbene (1)2a on an ortho carbon of the phenyl substituent to generate isofluorene, followed by 1,5-H shift. Irradiation of 2a-N(2) in a 2-methyltetrahydrofuran (2-MTHF) matrix at 77 K gave ESR signals ascribable to triplet carbenes ((3)2a). UV/vis spectra of (3)2a were obtained by irradiating 2a-N(2) under identical conditions. However, laser flash photolysis (LFP) of 2a-N(2) in degassed benzene at room temperature showed transient absorption bands completely different from those observed in photolysis in the 2-MTHF matrix at 77 K. The transient band decayed in first-order with a rate constant of 7.6 x 10(4) s(-1), which was not retarded by deuterium substitution of o-phenyl hydrogens. Essentially the same transient band was observed in LFP of fluorene 4a. The LFP of 2a-N(2) in nondegassed benzene gave transient absorption bands ascribable to triplet carbene (3)2a and the corresponding carbonyl oxide. The quenching rate constant of (3)2a by oxygen and the lifetime of (3)2a in the absence of oxygen were estimated by plotting the observed pseudo-first-order rate constant of the formation of the carbonyl oxide against the concentration of oxygen. They were 1.9 x 10(7) M(-1)s(-1) and 16 micros, respectively. Similar studies with diphenyldiazomethanes having a series of substituents (4-Ph, 3,5-(CF(3))(2), and 2-F) on the ortho-phenyl rings gave essentially identical results, indicating that those substituents exhibit little effect on overall reaction pathway. From those studies, it is suggested that triplet carbenes (3)2 are also trapped by the ortho-phenyl ring to give eventually fluorenes.     

Gamma-radiolysis of some crown ethers and their analogues. Electron spin resonance specra of free radicals

The method of low temperature ESR spectroscopy was used to study the free radicals generated by -irradiation of crown ethers: 12-crown-4 /1/; 15-crown-5 /2/; dicyclohexane-24-crown-8 /3/; and their analogues: tetrahydrofurane /4/ and 1,4-dioxane /5/. ESR spectra of radicals 4 and 5 taken at 77 K represent a simple singlet; ESR spectra of radicals generated from 1, 2 and 3 have a complex, multiplet structure. The kinetics of thermal decay of free radicals 4 in dependence on temperature starting from 103 K was investigated. The radicals 4 decay very fast at 253 K.     

Recoil spectrometer for the detection of single atoms

Kinetic decay of paramagnetic species formed in the radiolysis of tetracycline hydrochloride in powder form, at 77 K, shows that most of the H· is not able to migrate. Geminate recombination between H· and its partner is the main reaction: H·+TC· TCH, although a few H· succeed in diffusing slowly. But when H· acquires enough kinetic energy to migrate, it reacts preferentially with species different from its partner. The e^– is not observed by ESR, at 77 K, although tetracycline hydrochloride is a chemical trap for e^– in methanol, benzyl alcohol and alkaline aqueous solutions. In the radiolysis of tetracycline hydrochloride in powder form, the dimethylammonium group blocks H· abstraction reaction at 77 K.From a thesis submitted by S. M. L. G. to the University of São Paulo in partial fulfillment of the requirements for a Doctor of Science in Nuclear Technology.     

Proton transfer reaction of a new orthohydroxy Schiff base in some protic and aprotic solvents at room temperature and 77 K

The ground and excited state proton transfer reactions of a new orthohydroxy Schiff base, salicylidine-3,4,7-methyl amine (SMA) has been studied by means of absorption, emission and time resolved fluorescence spectroscopy in some polar protic and aprotic solvents at room temperature and 77K. The spectral behavior of SMA has been investigated both in neutral and basic conditions. The intramolecularly hydrogen bonded enol and zwitterion have been detected in pure methanol and ethanol, the anion is detected in basic condition. At 77K, SMA shows phosphorescence in pure methanol and ethanol. From nanosecond measurements and quantum yields of fluorescence, we have estimated the decay rates of proton transfer reaction in methanol and ethanol.     

Preparation of bis(diazo) compounds incorporated into butadiyne and thiophene units and generation and characterization of their photoproducts

(2,6-dimethyl-4-tert-butylphenyl)(2,4,6-tribromophenyl)diazomethane(-N(2)) was found to be stable enough to survive under Sonogashira coupling reaction conditions, and aryldiazomethyl substituents were introduced at the 1,4-positions of butadiyne (4-2N(2)) and the 2,5-positions of thiophene(5-2N(2)). Irradiation of those bis(diazo) compounds generated bis(carbenes), which were characterized by using ESR and UV/vis spectroscopic techniques in a matrix at low temperature as well as time-resolved UV/vis spectroscopy in solution at room temperature. These studies revealed that both of the bis(carbenes), 4 and 5, have singlet quinoidal diradical ground states with a very small singlet-triplet energy gap of less than 1 kcal mol(-1). A remarkable increase in the lifetime of bis(carbenes), as opposed to that of the monocarbene (2), was noted and was interpreted to indicate that bis(carbenes) are thermodynamically stabilized as a result of delocalization of unpaired electrons throughout the pi net framework. In spite of the stability, both bis(carbenes) are readily trapped by molecular oxygen to afford bis(ketones). Presumably, the reaction of the upper-lying localized quintet states with oxygen is much faster than that for lower-lying states.     

Photoinduced recombination reaction of trapped electrons and macroradicals in radiolyzed cellulose

Light-induced reaction of photorecombination of trapped electrons and free radicals (mainly, the C₁-centered radical) has been detected in cellulose γ-irradiated at 77 K. The dark process of annealing of trapped electrons in γ-irradiated cellulose occurs in the range of 77–170 K without participation of alkyl radicals and includes only the recombination of electrons with cations. The action of light on the radiolyzed cellulose increases the yield of carbon dioxide (upon heating up photo-bleached samples) by a factor of 2–2.5 as compared to the dark process. It is supposed that the reaction of photorecombination of trapped electrons and free radicals follows the chain mechanism: at doses of preliminary irradiation up to 100 kGy, the decay of one electron accompanied by disappearance of 4 to 20 alkyl radicals and evolution of up to 25 CO₂ molecules.     

Fluorescence reabsorption in anthracene single crystals: Lifetime variations with emission wavelength and temperature

Fluorescence spectra and lifetimes of anthracene melt-grown single crystals and sublimation flakes have been examined at 298 and 77°K, using a mono-photon counting technique for the lifetime measurements. The observed emission decay times were nearly independent of the excitation wavelength, though a small dependence of the fluorescence spectrum on the excitation wavelength was noted. By contrast, large variations of fluorescence lifetimes in thick crystals were found as a function of emission wavelength. For thick melt-grown single crystals at 298°K the lifetime was found to increase from 9.8 nsec at 405 nm to 20.4 nsec at 445 nm. For sublimation flakes at 77°K and at 298°K and for thick melt-grown crystals at 77°K, the lifetimes were less than 10 nsec and were nearly independent of emission wavelength. Despite these relatively large variations in lifetimes, the decay rates at each separate wavelength remained exponential, within experimental error. Theoretical calculations were made of emission lifetimes based on a model with one reabsorbing state. The calculations are in substantial agreement with the experimental results.     

Direct measurements of intersystem crossing rates and triplet decays of luminescent conjugated oligomers in solutions

Photothermal calorimetry and fluorescence spectroscopy were used to determine the relaxations of the photoexcited singlet state of two PPV and polyfluorene oligomers, (E,E)-1,4-bis[(2-benzyloxy)styryl]benzene (PVDOP) and ter(9,9'-spirobifluorene) (TSBF). The decay rates of different S1 relaxation channels, which include intersystem crossing (ISC), radiative, and nonradiative decay can be determined by the combination of photoacoustic calorimetry (PAC) and the time-correlated single photon counting (TCSPC) technique. The triplet state energy level is determined by the phosphorescence (Ph) spectra recorded at 77 K. The ISC yields are approximately 3% and 6% for PVDOP and TSBF, respectively. The T1 to S0 transition decay rate is acquired by PAC and photothermal beam deflection (PBD) measurements. The triplet state decay rate is 17 and 21 ms(-1) at room temperature. The Ph intensity decay at 77 K shows that the triplet state lifetime increases by 4 orders of magnitude, as compared to room temperature.     

Proton transfer reaction of a new orthohydroxy Schiff base at room temperature and 77 K

One new orthohydroxy Schiff base, 2-(N-benzyl-alpha-iminoethyl)naphthol (BEIN) has been synthesized. The proton transfer reaction of BEIN has been investigated by means of absorption, steady state and time resolved fluorescence spectroscopy in different solvents at room temperature and 77K. The behavior of BEIN in ethanol and water, has been studied in neutral, acidic and basic conditions. Excited state intramolecular proton transfer (ESIPT) is evidenced by a large Stokes shifted ( approximately 11,000 cm-1) fluorescence in solid crystalline media at room temperature. We present the observation of phosphorescence both in non-polar and protic solvents at 77K. The observed decay dynamics of the phosphorescence and delayed fluorescence indicates that the triplet state can be attributed to the cis-keto form. The molecular structures are determined by B3LYP/6-31G** calculation. From theoretical study it is suggested that the strengthening of hydrogen bond result from the steric repulsion of the phenyl ring. The presence of benzene ring increases the proton transfer barrier in case of BEIN compared to previously studied 7-ethylsalicylidenebenzylamine (ESBA).     

Comparison of radiolysis of cyclopentane, 2,3-dimethylbutane,and neopentane at 4.2 K with that at 77 K as studied by analysis of dimer products

The γ-radiolysis of cyclopentane, 2,3-dimethylbutane, and neopentane at 4.2 K was compared with that at 77 K by analysis of dimer products with capillary gas chromatography. In the radiolysis of cyclopentane, the yield of bicyclopentyl dimer at 4.2 K is lower than that at 77 K. The difference of the yields at two temperatures was explained in terms of disproportionation reaction of cyclopentyl radicals at very low temperature. In the radiolysis of 2,3-dimethylbutane, the distribution of dimer products at 4.2 K is different from that at 77 K. The results is due to the effect of phase change between two temperatures. In the radiolysis of 2,3-dimethylbutane, the fraction of unsaturated dimers in all dimer products at 77 K is much larger than that at 4.2 K. The olefinic dimer-products at 77 K is related to favorable formation of olefinic cations at 77 K. In the radiolysis of solid neopentane, addition of helium gas promotes the formation of 2,2,3,3-tetramethylbutane by a non-radical process.     

Stabilizing the Hammick intermediate

The possible stabilization of pyridine-2-ylidene has been investigated at B3LYP/6-31+G* and B3LYP/6-311+G** levels of theory. The aromaticity of this compound is a significant contributor to its stability (ISE(c) = 24-27 kcal mol(-1), Bird index = 77, NICS(0) = -6.8 ppm, NICS(1) = -9.1 ppm; all about 80% of those for the H-shift isomer pyridine). The stabilization energy of the carbene (measured by an appropriate isodesmic reaction) can be influenced by the substitution at nitrogen with bulky groups, and at the carbon atom by amino groups. Considering possible competitive deprotonation reactions of the most likely synthetic route, 1j, 1k, and 1L are suggested as synthesizeable new stable carbenes.     

Paramagnetic Centers in Irradiated Potassium Picrate

The formation of paramagnetic centers in potassium picrate in a radiation field was found using EPR spectroscopy. Stable paramagnetic centers were identified as 2,6-dinitro-para-quinone, iminoxyl, and ^– radicals. The kinetics of buildup of stable paramagnetic centers was studied over a wide dose range (0.14–5 MGy), and the temperature stability was studied at 77–550 K. It was found that the decay of paramagnetic centers in the temperature range 373–523 K obeyed the laws of polychronous kinetics. A reaction scheme for the radiation-thermal decomposition of sodium picrate with the participation of radical radiolysis products was proposed.     

Temperature dependence of trapped electrons in crystalline D2O ice from 77 to 6 k

The temperature dependence of the abundance of trapped electrons which absorb in the visible (e^?_vis) and infrared (e^?_IR) in crystalline D₂O ice has been studied by pulse radiolysis between 77 and 6 k. The yield (G) and decay of e^?_vis show little dependence on temperature or doping with NH₄F. At 6 K G(e^?_vis) is 0.54 and the electron decays by half within 5 μs. These observations are consistent with e^?_vis being mainly located in spurs. The yield and decay of e^?_IR, on the other hand, show a more marked dependence on temperature. In the pure crystal G(e^?_IR) increases more than tenfold from ≈ 0.1 at 77 K to 1.3 at 6 K and its decay rate is greatly decreased at the lower temperature. Doping with NH₄F increases G(e^?_IR) to 0.85 to 77 K and to 1.8 at 6 K and some decay is observed at 6 K but not at 77 K. These results are interpreted on the basis that geminate recombination between electrons and holes is very fast at 77 K but becomes sufficiently slow for the electrons to be observed at 6 K. It is also inferred that the hole is more mobile than e^?_IR. The mechanisms causing the decays of e^?_vis and e^?_IR are discussed.     

On the decay of H and D atoms in aqueous sulphuric acid glasses. Isotope effects

Glassy samples of a mixture of 9.16 M H₂SO₄ in H₂O and 9.16 M D₂SO₄ in D₂O were X-irradiated at 77 K. ESR-measurements showed that the decay of D atoms was considerably faster than the decay of atoms at 118 K. This isotope effect was even more pronounced when the decay of H atoms in 9.16 M H₂SO₄ and D atoms in 9.16 M D₂SO₄ were measured separately. At 77 K no isotope effects were found, neither with, nor without 1 M 2-propanol as an H-atom scavenger.