DETERMINATION OF THE KINETICS OF THE BACK REACTION OF PHOTOSYSTEM II IN THE PRESENCE OF 3–(3,4-DICHLORPHENYL)-1,1-DIMETHYLUREA FROM LUMINESCENCE MEASUREMENTS

Abstract— An alternative method to that used by Mar and Roy (1974) for the determination of the kinetics of the back reaction of photosystem II from the luminescence decay curve in the presence of 3–(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) has been suggested. The new theory relies upon two hypotheses: the well-known recombination hypothesis of luminescence and the assumption that the luminescence yield in the seconds region is given by the variable part of the live fluorescence yield. The second hypothesis was introduced since assuming a constant luminescence yield results in kinetic data that are not consistent with measurements of the kinetics of the back reaction by the restoration of the area over the fluorescence rise curve. The dependence of the live fluorescence yield from the concentration of closed PS II traps was assumed to be represented by Delosme's expression originally derived for the rise curve of the fluorescence yield in the presence of DCMU.
The theory is based on the fact that then the partial and total light sums of luminescence are simple functions of the concentration of the primary electron acceptor Q^- of PS II. Thus, after integrating the luminescence decay curve the theory permits a convenient evaluation of the kinetics of the back reaction [Q^-]( t ) in terms of the partial and total light sum.
This method was applied in order to determine the kinetics of the back reaction in Chlorella fusca in the presence of DCMU. It is shown that the kinetics of deactivation of the S, state can be described using the expression for the kinetics of the back reaction derived by Mar and Roy. As an alternative explanation, a biphasic first order decay of S₂ is proposed.     

Determination of temperatures within acoustically generated bubbles in aqueous solutions at different ultrasound frequencies

Mean acoustic bubble temperatures have been measured using a methyl radical recombination (MRR) method, at three ultrasound frequencies (20, 355, and 1056 kHz) in aqueous tert-butyl alcohol solutions (0-0.5 M). The method is based on yield measurements of some of the hydrocarbon products formed from the recombination of methyl radicals that are thermally generated within collapsing bubbles containing tert-butyl alcohol vapor. The mean bubble temperatures were found to decrease substantially with increasing tert-butyl alcohol concentration at 355 and 1056 kHz but only to a small extent at 20 kHz. Extrapolating the mean temperatures measured to zero concentration of tert-butyl alcohol, at a bulk solution temperature of 20 degrees C, gave the order 355 kHz (4300 +/- 200 K) > 1056 kHz (3700 +/- 200 K) > 20 kHz (3400 +/- 200 K). It is also concluded that the temperature derived from the MRR method is a useful diagnostic parameter for sensing the thermal conditions within an active acoustic bubble. However, attention must be given to the fact that the temperature derived from the MRR method is not theoretically well defined.     

Ultrafast spectroscopy of the solvent dependence of electron transfer in a perylenebisimide dimer

We investigate the photoinduced intramolecular electron-transfer (IET) behavior of a perylenebisimide dimer in a variety of solvents using femtosecond transient absorption spectroscopy. Overlapping photoinduced absorptions and stimulated emission give rise to complicated traces, but they are well fit with a simple kinetic model. IET rates were found to depend heavily on solvent dielectric constant. Good quantitative agreement with rates derived from fluorescence quantum yield and time-resolved fluorescence measurements was found for forward electron transfer and charge recombination rates.     

Optically stimulated recombination fluorescence in polar media at 77 K: Quantum yield and kinetics of its isothermal evolution

The isothermal evolution of the visible-light stimulated recombination fluorescence (VSF) of tryptophan and 1-methyltryptophan in ethyleneglycol—water and KOH solutions at 77 K is studied in parallel with the isothermal recombination fluorescence. A theoretical model is derived which makes it possible to explain most of the VSF properties. It is shown that the VSF and the ITF, in a more pronounced manner, originate from a population of trapped electrons (e_t) having a distance distribution function maximum closer to the photoionized molecules than the one characterized by the e_t optical absorption spectrum. The study of the VSF quantum yield as a function of the stimulation wavelength indicates that e_t mobilization may occur above the photomobilization threshold i.e.: 2.1 eV, by tunneling after optical activation.     

The Origin of Magnetic Field Dependent Recombination in Alkylcobalamin Radical Pairs

Magnetic field effect studies of alkylcobalamin photolysis provide evidence for the formation of a reactive radical pair that is born in the singlet spin state. The radical pair recombination process that is responsible for the magnetic field dependence of the continuous-wave (CW) quantum yield is limited to the diffusive radical pair. Although the geminate radical pair of adenosylcob(III)alamin also undergoes magnetic field dependent recombination (A. M. Chagovetz and C. B. Grissom, J. Am. Chem. Soc. 115, 12152–12157, 1993), this process does not account for the magnetic field dependence of the CW quantum yield that is only observed in viscous solvents. Glycerol and ethylene glycol increase the microviscosity of the solution and thereby increase the lifetime of the spin-correlated diffusive radical pair. This enables magnetic field dependent recombination among spin-correlated diffusive radical pairs in the solvent cage. Magnetic field dependent recombination is not observed in the presence of nonviscosigenic alcohols such as isopropanol, thereby indicating the importance of the increased microviscosity of the medium. Paramagnetic radical scavengers that trap alkyl radicals that escape the solvent cage do not diminish the magnetic field effect on the CW quantum yield, thereby ruling out radical pair recombination among randomly diffusing radical pairs, as well as excluding the involvement of solvent-derived radicals. Magnetic field dependent recombination among alkylcobalamin radical pairs has been simulated by a semiclassical model of radical pair dynamics and recombination. These calculations support the existence of a singlet radical pair precursor.     

Intermolecular hydrogen bonding in chlorine dioxide photochemistry: A time-resolved resonance Raman study

The geminate-recombination and vibrational-relaxation dynamics of chlorine dioxide (OClO) dissolved in ethanol and 2,2,2-trifluoroethanol (TFE) are investigated using time-resolved resonance Raman spectroscopy. Stokes spectra are measured as a function of time following photoexcitation using degenerate pump and probe wavelengths of 398 nm. For OClO dissolved in ethanol, subpicosecond geminate recombination occurs resulting in the reformation of ground-state OClO with a quantum yield of 0.5±0.1. Following recombination, intermolecular-vibrational relaxation of OClO occurs with a time constant of 31±10 ps. For OClO dissolved in TFE, recombination occurs with a time constant of 1.8±0.8 ps and a quantum yield of only 0.3±0.1. The intermolecular-vibrational-relaxation time constant of OClO in TFE is 79±27 ps. The reduced geminate-recombination quantum yield, delayed recombination, and slower vibrational relaxation for OClO in TFE is interpreted in terms of greater self-association of the solvent. Degenerate pump–probe experiments are also presented that demonstrate decay of the Cl-solvent charge-transfer complex on the ∼1-ns time scale in ethanol and TFE. This time is significantly longer than the abstraction times observed for other systems demonstrating that Cl hydrogen abstraction from alcohols occurs in the presence of a significant energy barrier.     

Yield of excited CO molecules from dissociative recombination of HCO+ and HOC+ ions with electrons

The authors have investigated CO band emissions arising from the dissociative recombination of HCO(+) and HOC(+) ions with thermal electrons in a flowing afterglow plasma. The quantitative analysis of the band intensities showed that HCO(+) recombination forms the long-lived CO(a (3)Pi) state with a yield of 0.23+/-0.12, while HOC(+) recombination favors formation of CO(a' (3)Sigma(+)) and CO(d (3)Delta) with a combined yield of greater than 0.4. The observed vibrational distribution for the CO(a) state reproduces theoretical predictions quite well. The vibrational distributions for CO(a') and CO(d) are, in part, inverted, presumably as a consequence of a change in CO equilibrium bond length during recombination. The observations are compatible with current knowledge of the potential surfaces of states of HCO and HCO(+).     

Hot recombination of photogenerated ion pairs

The recombination dynamics of ion pairs generated upon electron transfer quenching of perylene in the first singlet excited state by tetracyanoethylene in acetonitrile is quantitatively described by the extended unified theory of photoionization/recombination. The extension incorporates the hot recombination of the ion pair passing through the level-crossing point during its diffusive motion along the reaction coordinate down to the equilibrium state. The ultrafast hot recombination vastly reduces the yield of equilibrated ion pairs subjected to subsequent thermal charge recombination and separation into free ions. The relatively successful fit of the theory to the experimentally measured kinetics of ion accumulation/recombination and free ion yield represents a firm justification of hot recombination of about 90% of primary generated ion pairs.     

Strain-dependent dynamic properties of filled rubber network systems

A model for strain-dependent dynamic properties of filler loaded rubber systems has been derived based on the Links-Nodes-Blobs (L-N-B) model of percolation theory. It is the first time that a L-N-B model is applied in the study of dynamic properties of filled rubbers. The density distribution function of the number of singly connected bonds f_1a(ϵ) and the apparent yield strain amplitude ϵ_app that corresponds to the on-set point of corruption of the filler network are introduced in the model. Simulation results indicate that both f_1a(ϵ) and ϵ_app control the break-down and recombination of the filler network. Two recombination mechanism are adopted in this study. Results of simulations from the extreme ends recombination mechanism match the experimental data better than those from the zero strain recombination mechanism. Also, via the proposed model, the strain-dependent storage modulus correlates well with the peak loss modulus at a low strain range of around 0.1% to 100%. Moreover, a universal plot of the normalized storage modulus (Z_L-N-B) as a function of the normalized Log strain amplitude (ϵ₀/ϵ_app) for different rubber systems is obtained. The loss moduli of systems are also simulated by the L-N-B model.     

BIPHOTONIC PROCESSES IN PROTEINS: COMPARATIVE STUDIES OF DELAYED FLUORESCENCE AND ISOTHERMAL OR OPTICALLY STIMULATED RECOMBINATION LUMINESCENCE

Abstract —The properties of delayed fluorescence (DF) of proteins are studied in parallel with their properties of recombination luminescence, produced after ultraviolet excitation and photoionization of chromophoric residues at low temperature, in water-ethylene glycol glasses. It is shown that the DF emission of proteins, originating from tryptophan residues, is produced via triplet-triplet absorption processes, photoionization of tryptophan residues, and recombination of the charged species. Electron scavengers that reduce the DF yield act differently upon the DF emission and the recombination luminescence as a consequence of the protein structure. Influence of energy transfer on the protein DF yield is also emphasized.     

Inhomogeneous electrochemiluminescence. II Markovian encounter theory of the phenomenon

The free energy dependence of the electro-chemiluminescence quantum yield is specified, with the Markovian encounter theory accounting for the reversibility of triplet production competing with the irreversible recombination to the ground state. It is shown that diffusional ion recombination is highly inhomogeneous in space. It proceeds at either large positive ionization free energy (mainly to the triplet product) or at large negative free energy when recombination to the ground state dominates. On the contrary at medium free energies, the quasi-resonant generation of triplets is under kinetic control and therefore much more homogeneous. In this case, both recombination products are generated in comparable amounts.The multiple reversible ionization is shown to act as an independent quenching mechanism previously unknown. The role of the triplet quenching at the electrode is also specified. These effects reduce noticeably the luminescence quantum yield but only at larger triplet life times and in different free energy regions.     

Light activated recombination

Many genes elicit their actions through their expression in precise spatial patterns in tissues. Photoregulated expression systems offer a means to remotely pattern gene expression in tissues. Using currently available photopatterning methods, gene expression is only transient. Herein is described a general method to permanently alter a cell's genome under the control of light. The photocaged estrogen receptor (ER) antagonists, nitroveratryl-hydroxytamoxifen (Nv-HTam) and nitroveratryl-hydroxytamoxifen aziridine (Nv-HTaz), mediate exposure-dependent recombination in cells expressing the Cre-ER, a fusion of the site-specific recombinase Cre and ER. Both Nv-HTam and Nv-HTaz only activate recombination by Cre-ER after exposure to light. When released only intracellularly, the covalent-modifying Taz can mediate significant amounts of recombination in an exposure-dependent manner. Nv-HTaz and Cre-ER represent perhaps the first compound that can be used to photopattern gene expression through recombination.     

Influence of subdiffusive motion on spin relaxation and spin effects in radical pairs

Specific features of spin relaxation and the kinetics of spin effect generation in radical pairs (RPs) undergoing subdiffusive relative motion are studied in detail. Two types of processes are analyzed: (1) spin relaxation in biradicals, resulting from anomalously slow subdiffuisive reorientation (with the correlation function P(t) approximately (wt)(-alpha), where 0 < alpha < 1) and (2) spin effect generation in subdiffusion-assisted RP recombination. Analysis is made with the use of the non-Markovian stochastic Liouville equation (SLE) derived within the continuous time random walk approach. The SLE predicts anomalous (very slow and nonexponential) spin relaxation in biradicals which results in some peculiarities of the spectrum of the system. In RP recombination, the subdiffusive relative motion shows itself in slow dependence of the reaction yield Y(r)() on reactivity and parameters of the RP spin Hamiltonian and anomalous electron spin polarization of escaped radicals. The spectrum of the reaction yield detected magnetic resonance, that is, the Y(r)() dependence on the frequency omega of microwave field, is found to be strongly non-Lorenzian with the width determined by the field strength omega(1) and very broad wings depending on alpha. Analysis shows that the majority of interesting, specific features of the observables in both systems are controlled only by the parameter alpha.     

Carrier generation process on photoconductive polymer films as studied by magnetic field effects on the charge-transfer fluorescence and photocurrent

We have studied the magnetic field effects (MFEs) on the charge-transfer fluorescence and transient photocurrent of a 1,2,4,5-tetracyanobenzene-doped poly(N-vinylcarbazole) film, which reflect the recombination and escape yields of the carriers, respectively. The recombination yield dependence of the external magnetic field (B) clearly shows two types of the MFEs, growth with increasing B due to the hyperfine mechanism (HFM) and a negative dip due to the level-crossing mechanism (LCM). On the other hand, the escape yield indicates complementary MFEs with a sharp decrease in yield with increasing B and then a positive dip. Simultaneous observation of the HFM- and LCM-MFEs proves the stepwise hole-hopping mechanism rather the long-range hole-jumping one. The quantitative analysis of the recombination and escape MFEs is performed using the stochastic Liouville equations (SLE) for a one-dimensional lattice model in which the stepwise hole hops take place between the nearest neighbor carbazole units with spin conservation. The SLE analysis provides the recombination and hole transfer rate constants of 7.0 x 10(7) and 4.5 x 10(8) s(-1), respectively. The boundary site number for the ion pairs in the one-dimensional model is estimated by the best fit to the experimental results. The interionic distance of the boundary ion pair in the one-dimensional model including eight sites agrees with the thermalization distance in the Onsager model. Hence, it is concluded that the elementary processes in the Onsager model applied to molecular amorphous solids are the stepwise hole hops rather than a long-range hole jump.     

Reactivity between biphenyl and precursor of solvated electrons in tetrahydrofuran measured by picosecond pulse radiolysis in near-ultraviolet, visible, and infrared

The initial decrease of solvated electrons in tetrahydrofuran (THF) upon addition of biphenyl was investigated by picosecond pulse radiolysis. Transient absorption spectra derived from the biphenyl radical anion (centered at 408 and 655 nm) and solvated electrons of THF (infrared) were successfully measured in the wavelength region from 400 to 900 nm by the extension of a femtosecond continuum probe light to near-ultraviolet using a second harmonic generation of Ti:sapphire laser and a CaF2 plate. From the analysis of kinetic traces at 1300 nm considering the overlap of primary solvated electrons and partial biphenyl radical anion, C37, which is defined by the solute concentration to reduce the initial yield of solvated electrons to 1/e, was found to be 87 +/- 3 mM. The rate constant of solvated electrons with biphenyl was determined as 5.8 +/- 0.3 x 10(10) M(-1) s(-1). We demonstrate that the kinetic traces at both 408 nm mainly due to biphenyl radical anion and 1300 nm mainly due to solvated electrons are reproduced with high accuracy and consistency by a simple kinetic analysis. Much higher concentrations of biphenyl (up to 2 M) were examined, showing further increase of the initial yield of biphenyl radical anion accompanying a fast decay component. This observation is discussed in terms of geminate ion recombination, scavenging, delayed geminate ion recombination, and direct ionization of biphenyl at high concentration.     

The residence probability: single molecule fluorescence correlation spectroscopy and reversible geminate recombination

The residence probability of a freely diffusing particle within an open d-dimensional ball is calculated as a function of time, for an initial distribution that is either a spherical delta function or uniform within the sphere. The latter is equivalent to the autocorrelation function (ACF) of fluorescence correlation spectroscopy (FCS) when utilizing near-field scanning optical microscopy (NSOM) probes. Starting from the general equation for the Laplace transform of the residence probability, we solve it in Laplace space for any dimensionality, inverting it into the time domain in one- and three-dimensions. The short- and long-time asymptotic behaviors of the residence probability are derived and compared with the exact results. Approximations for the two-dimensional ACF are discussed, and a new approximation is derived for the NSOM-FCS ACF. Also of interest is an analytic expression for a three-dimensional ACF, which could be useful for two-photon FCS. Analogy with the binding probability for reversible geminate recombination suggests that more information could be extracted from the long-time tails in FCS experiments.     

Estimation of the volume neutralization effect on the ion decay in irradiated liquids

In the framework of a recently suggested recombination model it is shown that an increase of the initial ion concentration caused by higher irradiation doses leads to an increase of the fraction of ions recombining in the volume. This effect is represented for two different rate functions R/t/ assumed to describe the recombination rate for single ion pairs. Finally consequences of the volume recombination effect on the product yield of a competitive ion-scavenger reaction and for the determination of the real survival probability of the isolated ion pairs are discussed.     

Driving force and electronic coupling effects on photoinduced electron transfer in a fullerene-based molecular triad

Tuning thermodynamic driving force and electronic coupling through structural modifications of a carotene (C) porphyrin (P) fullerene (C60) molecular triad has permitted control of five electron and energy transfer rate constants and two excited state lifetimes in order to prepare a high-energy charge-separated state by photoinduced electron transfer with a quantum yield of essentially unity (> or = 96%). Excitation of the porphyrin moiety of C-P-C60 is followed by a combination of photoinduced electron transfer to give C-P(.+)-C60.- and singlet-singlet energy transfer to yield C-P-1C60. The fullerene excited state accepts an electron from the porphyrin to also generate C-P(.+)-C60.-. Overall, this initial state is formed with a quantum yield of 0.97. Charge shift from the carotenoid to yield C(.+)-P-C60.- is at least 60 times faster than recombination of C-P(.+)-C60.-, leading to the overall quantum yield near unity for the final state. Formation of a similar charge-separate species from the zinc analog of the triad with a yield of 40% is also observed. Charge recombination of C(.+)-P-C60.- in 2-methyltetrahydrofuran yields the carotenoid triplet state, rather than the ground state. Comparison of the results for this triad with those for related triads with different structural features provides information concerning the effects of driving force and electronic coupling on each of the electron transfer steps.     

The effect of a magnetic field on the singlet/triplet ratio in geminate ion recombination

Excited state formation by ion recombination in solutions of fluorene in squalene has been studied by pulse radiolysis at the center of a large electromagnet. The products of the slower part of the ion recombination are affected by an applied magnetic field. The fluorescence yield increases by a factor of about 1.5 whereas the triplet yield decreases.     

Using spin dynamics of covalently linked radical ion pairs to probe the impact of structural and energetic changes on charge recombination

We have synthesized a series of structurally related, covalently linked electron donor-acceptor triads having highly restricted conformations to study the effects of radical ion pair (RP) structure, energetics, and solvation on charge recombination. The chromophoric electron acceptor in these triads is a 4-aminonaphthalene-1,8-dicarboximide (6ANI), in which the 4-amine nitrogen atom is part of a piperazine ring. The second nitrogen atom of the piperazine ring is part of a para-substituted aniline donor, where the para substituents are X = H, OMe, and NMe(2). The imide group of 6ANI is linked to a naphthalene-1,8:4,5-bis(dicarboximide) (NI) electron acceptor across a phenyl spacer in a meta relationship. The triads undergo two-step photoinduced electron transfer to yield their respective XAn(*)(+)-6ANI-Ph-NI(*)(-) RP states, which undergo radical pair intersystem crossing followed by charge recombination to yield (3)NI. Time-resolved electron paramagnetic resonance experiments on the spin-polarized RPs and triplet states carried out in toluene and in E-7, a mixture of nematic liquid crystals (LCs), show that for all three triads, the XAn(*)(+)-6ANI-Ph-NI(*)(-) RPs are correlated radical pairs and directly yield values of the spin-spin exchange interaction, J, and the dipolar interaction, D. The values of J are all about -1 mT and show that the LC environment most likely enforces the chair conformation at the piperazine ring, for which the RP distance is larger than that for the corresponding boat conformation. The values of D yield effective RP distances that agree well with those calculated earlier from the spin distributions of the radical ions. Within the LC, changing the temperature shows that the CR mechanism can be changed significantly as the energy levels of the RPs change relative to that of the recombination triplet.