Geminate Recombination Rate Research Articles

Dispersive photoluminescence decay by geminate recombination in amorphous semiconductors

The photoluminescence decay in amorphous semiconductors is described by power law $t^{-delta}$ at long times. The power-law decay of photoluminescence at long times is commonly observed but recent experiments have revealed that the exponent, $delta sim 1.2-1.3$, is smaller than the value 1.5 predicted from a geminate recombination model assuming normal diffusion. Transient currents observed in the time-of-flight experiments are highly dispersive characterized by the disorder parameter $alpha$ smaller than 1. Geminate recombination rate should be influenced by the dispersive transport of charge carriers. In this paper we derive the simple relation, $delta = 1+ alpha/2 $. Not only the exponent but also the amplitude of the decay calculated in this study is consistent with measured photoluminescence in a-Si:H.

Dynamic Regulation of the Inducible Nitric-oxide Synthase by NO: COMPARISON WITH THE ENDOTHELIAL ISOFORM

We studied by ultrafast time-resolved absorption spectroscopy the geminate recombination of NO to the oxygenase domain of the inducible NO synthase, iNOSoxy, and to mutated proteins at position Trp-457. This tryptophan interacts with the tetrahydrobiopterin cofactor BH4, and W457A/F mutations largely reduced the catalytic formation of NO. BH4 decreases the rate of NO rebinding to the ferric iNOSoxy compared with that measured in its absence. The pterin has a larger effect on W457A/F than on the WT protein by increasing NO release from the protein. Therefore, BH4 raises the energy barrier for NO recombination to the mutated proteins in contrast with our observations on eNOS (Slama-Schwok, A., Négrerie, M., Berka, V., Lambry, J.-C., Tsai, A.-L., Vos, M., and Martin, J.-L. (2002) J. Biol. Chem. 277, 7581-7586). Thus, we show a differential effect of BH4 on NO release from eNOS and iNOS. Compared with the position of this residue in the BH4-repleted enzyme, simulations of the NO dissociation dynamics point out at a swing of Trp-457 toward the missing pterin in the absence of BH4. NO geminate-rebinding data show a more efficient NO release from eNOS than from iNOS once NO is formed. Consistently, NO produced by iNOS is regulated by its ferric nitrosyl complex in contrast with eNOS. We show that the small enhancement of the NO geminate recombination rate in W457A/F compared with that in the WT enzyme cannot explain the decrease of NO yield because of the mutation; the major effect of the mutation thus arises from an uncoupled catalysis (Wang, Z. Q., Wei, C. C., Ghosh, S., Meade, A. L., Hemann, C., Hille, R., and Stuehr, D. J. (2001) Biochemistry 40, 12819-12825).

Conformational dynamics of the transcriptional regulator CooA protein studied by subpicosecond mid-infrared vibrational spectroscopy.

CooA, which is a transcriptional regulator heme protein allosterically triggered by CO, is studied by femtosecond visible-pump mid-IR-probe spectroscopy. Transient bleaching upon excitation of the heme in the Soret band is detected at approximately 1979 cm(-1), which is the absorption region of the CO bound to the heme. The bleach signal shows a nonexponential decay with time constants of 56 and 290 ps, caused by the rebinding of the CO to the heme. About 98% of dissociated CO recombines geminately. The geminate recombination rate in CooA is significantly faster than those in myoglobin and hemoglobin. The angle of the bound CO with respect to the porphyrin plane is calculated to be about 78 degrees on the basis of the anisotropy measurements. A shift of the bleached mid-IR spectrum of the bound CO is detected and has a characteristic time of 160 ps. It is suggested that the spectral shift is caused by a difference in the frequency of the bound CO in different protein conformations, particularly in an active conformation and in an intermediate one, which is on the way toward an inactive conformation. Thus, the biologically relevant conformation change in CooA was traced. Possible assignment of the observed conformation change is discussed.

Evidence for triplet interchain polaron pairs and their transformations in polyphenylenevinylene

Photoluminescence of films of poly-$(p$-phenylenevinylene) (PPV), and changes of its intensity under conditions of electron spin resonance as a function of temperature and light intensity were studied. The experimental technique was based on the modulation of the spin state of paramagnetic species by resonant microwave transitions between Zeeman sublevels of dynamically spin-polarized pairs. Three types of resonant signals were found in the magnetic resonance spectrum: (i) a narrow (1.7 mT width at the half height), (ii) a broad (140 mT) enhancement signal at $g$=2, and (iii) the signal at $g$=4. The results permitted one to conclude that Coulomb bound polaron pairs are produced in PPV with high yield under 488-nm photoexcitation. The narrow signal is assumed to appear due to microwave-induced resonant transitions in triplet polaron pairs. This implies that the resonant transitions change the rate of geminate recombination of the pairs that leads to the formation of triplet intrachain excitons. Those excitons annihilate in the second-order reaction and show themselves in the intensity of the photoluminescence. The annihilation rate was found to be influenced by resonant transitions in triplet exciton pairs as well and resulted in broad and $g$=4 signals. The lifetime of triplet intrachain excitons was estimated from microwave modulation frequency dependence of resonant signal intensities. The results showed that the energy level of the lowest polaron pair state situated below that of singlet intrachain exciton can act as a sink of the excitation energy influencing the quantum yields of the photoluminescence, electroluminescence, and photoconductivity.

Evidence for triplet inter-chain polaron pairs in poly(phenylene vinylene) and their transformations

Photoluminescence of films of poly(phenylene vinylene) (PPV), and changes of its intensity under conditions of electron spin-resonance as a function of temperature and light intensity were studied. Three types of resonant signals were found in magnetic resonance spectrum:(i) a narrow (17mT), (ii) a broad (130mT) enhancement signals at g=2, and (iii) the signal at g=4. The results permitted to conclude that Coulomb bound polaron pairs are produced in PPV under photoexcitation. The narrow signal was shown, to appear due to a microwave induced resonant transition in triplet polaron pairs. The resonant transitions changed the rate of geminate recombination of the pairs that was revealed to lead to formation of triplet intra-chain excitons. Those excitons annihilated in the second order reactions showing themselves as delayed fluorescence. The annihilation rate was found to be influenced by resonant transitions in triplet exciton pairs as well. The lifetime of triplet intra-chain excitons was estimated from microwave modulation frequency dependence. The results showed that energy level of the lowest polaron pair state situated below that of singlet intra-chain exciton can act as a sink of the excitation energy influencing the quantum yields of the photoluminescence, electroluminescence and photoconductivity

Alkyl Radical Geometry Controls Geminate Cage Recombination in Alkylcobalamins

The radical pair that results from photolysis of adenosylcob(II1)alamin (AdoCbl') undergoes primary geminate recombination with a first-order rate constant of 1 x lo9 s-l. In contrast, methylcob(II1)alamin (CH3Cbl') and aristeromicylcob(II1)alamin (AriCblII', the carbocyclic analogue of AdoCbl' in which the ribofuranose ring oxygen has been replaced with a methylene group) does not undergo primary geminate recombination. The ribofwanose group enables a high rate of geminate recombination in the (Ado' Cbl') radical pair. This may be due to a stereoelectronic (p-anomeric) effect that maintains a pyramidal geometry at the 5'-carbon of the 5'-deoxyadenosyl radical, or it may be due to hindered rotation about the C4t-C5, bond such that /?-elimination to the olefin is prevented. Recombination in the geminate singlet radical pair is in competition with diffusive escape to form a solvent-separated radical pair. Hyperfine coupling from Co promotes intersystem crossing to the triplet radical pair (Chagovetz, A. M.; Grissom, C. B. J. Am. Chem. SOC. 1993, 115, 12152). Recombination of the (CH3' Cbl) radical pair is not prevented by a lack of intersystem crossing, as neither unlabeled or I3C-labeled CH3Cbl' undergoes geminate recombination. There is only a small difference in the rate of diffusive recombination in the solvent cage for AdoCbl', AriCbl', and CH3Cbl' following photolysis: 2.01 x 10 s-l, 2.20 x lo4 s-l, and 1.16 x lo4 s-l. The rate of diffusive recombination is limited by productive collisions and not by radical geometry or intersystem crossing. The CF3' radical that results from photolysis of (trifluoromethyl)cob(III)alamin (CF3Cbl') maintains its pyramidal geometry and undergoes faster diffusive recombination in the solvent cage at 51 x lo4 s-l. The C-Co bond dissociation enthalpy in AriCbl' is 37 f 1.4 kcaymol. The profound difference in geminate recombination rates for AdoCbl' and CH3Cbl' is consistent with their different biological roles as enzymatic cofactors: AdoCbl' is an initiator of radical chain chemistry in the active site, whereas CH3Cbl' is a methyl group donor in an S~2-type process.

Self quenching of 1-naphthol. Connection between time-resolved and steady-state measurements

The photochemistry of 1-naphthol following its electronic excitation in aqueous solution was investigated by a single-photon counting apparatus. It was found that following the proton dissociation the excited naphtholate anion decays non-exponentially with a t − sol1 2 dependence over time. This we attribute to a self-quenching reaction of the naphtholate anion by its geminate proton. The quenching rate on contact is 3.5 × 10 10 s −1. We also found that some of the protons interact adiabatically with a geminate recombination rate in D 2O fo 8 × 10 9 s −1. The time-dependent escape probability of the naphtholate iopn-par was sam;ed by carrying out the experiment in presence of a proton scavenger. We show that the leading term in the ultimate scavenging probability is given by the time integral of the homogeneous scavenging rate of the isolated pair at relative long times following its creation.

Distal pocket residues affect picosecond ligand recombination in myoglobin. An experimental and molecular dynamics study of position 29 mutants.

Time courses for intramolecular NO and O2 recombination to native and three position 29 mutants of sperm whale myoglobins were measured after laser photolysis on picosecond and nanosecond time scales. The rates for the first phase of NO recombination were 1.8, 2.5, 29, and > or = 100 ns-1 for Ala29, Val29, Leu29 (native), and Phe29 myoglobin, respectively, at room temperature. This order is not correlated with the overall association rate constants for NO binding which were all in the range 20-50 x 10(6) M-1 s-1 and is the opposite of that observed for the rate constants for the overall thermal dissociation of NO which were 5.0, 2.8, 0.98, and 0.21 x 10(-4) s-1 for Ala29, Val29, Leu29 (native), and Phe29 myoglobin, respectively, at 20 degrees C. This inverse correlation suggests that photo- and thermally dissociated ligand molecules experience similar kinetic and equilibrium barriers to rebinding. The larger side chains of Leu29 and Phe29 inhibit rapid movement of the ligand away from the iron atom facilitating geminate recombination. The smaller side chains of Val29 and Ala29 increase the space available to the ligand, decreasing the rate of geminate recombination and enhancing complete dissociation. Diffusion of NO in the distal pocket of myoglobin was simulated using a variant of the molecular dynamics program CHARMM that includes the locally enhanced sampling protocol (Elber, R., and Karplus, M. (1991) J. Am. Chem. Soc. 112, 9161-9175; Roitberg, A., and Elber, R. (1991) J. Chem. Phys. 95, 9277-9287) and the x-ray structures of Carver et al. (Carver, T. E., Brantley, R. E., Jr., Singleton, E. W., Arduini, R. M., Quillin, M. L., Phillips, G. N., Jr., and Olson, J. S. (1992) J. Biol. Chem. 267, 14443-14450). Both accelerated (5,000 K) and room temperature ligands were used, and comparisons were made between simulations with a complete hydration shell surrounding the protein and those with only eight water molecules near the distal histidine. Photodissociated ligands initially move away from the heme plane, past Leu29, and toward Leu32, Phe33, Ile107, and Ile111. These theoretical results confirm that a complete description of picosecond ligand recombination must include the dynamics of ligand movement in the distal portion of the heme pocket.

Effects of geminate recombination in measurements of rate constants in perturbation/relaxation and NMR line-shape experiments on fast bimolecular reactions in solution

Geminate recombination plays an important role in the kinetics of a reversible bimolecular association A+B⇄C in solution. Since geminate pain usually have mean lifetimes of 10 -10 s or less, a pseudo-steady flux through geminate pairs is closely approached within about 10 -9 s, after which the rate of geminate recombination is first order in C. Thus, geminate pairs behave as if they were sparsely populated subspecies of C, not independent molecules A and B

Ultrafast investigation of condensed phase chemical reaction dynamics using transient vibrational spectroscopy: Geminate recombination, vibrational energy relaxation, and electronic decay of the iodine A′ excited state

The reaction dynamics of iodine geminate recombination on the excited A′ 3Π2u state are investigated using picosecond transient Raman spectroscopy. Vibrational energy relaxation and the dynamics of geminate recombination are directly observed in the transient Raman spectrum. Geminate recombination rates are significantly slower for the excited electronic state compared to the ground electronic state. This observation implies that a fundamentally different geminate recombination mechanism is responsible for the formation of the A′ 3Π2u state. Evidence is reported which suggests the possible role of iodine atom–solvent complexes in the reaction step leading from photodissociated atoms to formation of the excited state. The solvent dependence of vibrational energy relaxation suggests that the rate is dominated by V–T (and possibly V–R) energy relaxation which is also found to be significantly slower in the excited electronic state compared to the ground state. In n-hexane the vibrational relaxation rate appears to be enhanced by V–V energy transfer. Electronic relaxation is investigated and is found to be largely the result of the formation and decay of I2⋅solvent exciplexes.

Monte Carlo modelling of radical-ion recombination in multiparticle tracks

In the framework of the isolated cluster model for radical-ion recombination, the survival probability, recombination rate, escape probability and the ratio of the geminate recombination rate to the total recombination rate have been calculated using the Monte Carlo method. This ratio is important for interpreting the magnitude of magnetic field effects in recombination fluorescence. The results are compared with experiment.

Photodissociation of CO and O2 from alpha and beta hemoglobin chains studied by using picosecond absorption spectroscopy

The picosecond photodissociation of the CO and O2 forms of alpha and beta chains of hemoglobin were studied by following pi pi Soret absorption changes using a Nd3+ phosphate-glass laser, 531-nm pump pulse, 8 ps full width half maximum, and a pump-probe double-beam absorption apparatus. Three intermediates were observed within the first 50 ps after photon absorption. The most notable differences between the two monomers are the extent and rate of geminate recombination with the two ligands. We attribute this result to differences between the tertiary protein structure of the alpha and beta forms of Hb, both distal and proximal.

Protoheme-carbon monoxide geminate kinetics.

Recombination kinetics of CO to protoheme after laser photolysis have been measured vs. temperature and viscosity. A 25-ns laser pulse was focused on the sample to produce an excitation rate of 10(9)/s per heme. This temporarily populates the heme-CO state of dissociated pairs which either separate or recombine on a picosecond time scale in viscous glycerol-water solutions. From the equilibrium amplitude of the fraction dissociated during the laser pulse, the geminate recombination rate constant is calculated to be 3 X 10(9)/s. This rate coefficient is only weakly dependent on temperature or viscosity. As previously observed the fraction that escapes depends on the solvent viscosity [Marden, M. C. (1983) Ph.D. Thesis, University of Illinois-Urbana]. A model consisting of a single barrier plus diffusive escape is used to simulate the kinetics during and just after the flash.

Geminate recombination of molecular iodine. The role of A and A′ states

Picosecond absorption spectroscopy has been used to study the geminate recombination of I2 in room temperature solution. In all solvents studied (CCl4, CH2Cl2, CHCl3, hydrocarbons, and ethylene glycol) a transient absorption band in the 575–800 nm region was observed, and assigned to transitions originating from A(3Π1) and/or A′(3Π2) states. No spectral shift of this band was observed throughout its lifetime, making an assignment to a vibrationally unrelaxed ground state unlikely. Branching ratios for ground state vs A and A′ state recombinations were found to depend upon solvent viscosity. These results are explained in terms of the internuclear separation obtained upon dissociation. The geminate recombination rates into A or A′ states were found to be >(30 ps)−1 and ∼(90–140 ps)−1 into the ground state. At present we can not determine whether surface hopping to the ground state or subsequent diffusion is the rate limiting step in direct ground state recombination. Intersystem crossing rates from the A, A′ states to the ground state were found to vary from ∼(100 ps)−1 in hydrocarbons to (2.7 ns)−1 in CCl4.

Photoreduction of benzo-1,4-quinone sensitised by metalloporphyrins

Benzo-1,4-quinone quenches the triplet excited state of many metalloporphyrins in ethanol solution and the overall quenching rate constants are a simple function of the thermodynamic driving force for electron transfer. The resultant ion-pair can dissociate to separated ions or recombine to ground-state reactants. The latter process is spin forbidden but the presence of central metal ions with large spin–orbital coupling constants enhances the rate of geminate recombination. This enhancement is even more pronounced when the central metal ion is paramagnetic and no redox ions are observed.

Ultrafast relaxation in picosecond photolysis of nitrosylhemoglobin

We report the successful observation of a picosecond transient difference spectrum in human nitrosylhemoglobin. The sample (23 °C) is excited with an ~8 ps, ~ 10 μJ pulse at 353 nm that generates a prompt transient having a two-component decay: the first is approximately exponential with τ = 17 ± 4 ps, whereas the second is much weaker with an approximate τ = 100 ps. At slightly lower temperature (4 °C), the spectrum and time dependence are essentially unchanged. In contrast to our previous observations on carboxyhemoglobin and oxyhemoglobin, we find no longlived photoproduct in nitrosylhemoglobin. We tentatively attribute the 17 ± 4 ps decay to geminate recombination. These results, in conjunction with our previous work in HbO 2 † † Abbreviations used: Hb, hemoglobin; HbO( 2, oxyhemoglobin; HbCO, carboxyhemoglobin; HbNO, nitrosylhemoglobin; Mb, myoglobin; MbCO, carboxymyoglobin; MbO 2, oxymyoglobin. and HbCO, show that the rate of geminate recombination for 5 ns > gt >5 ps increases through the series HbCO < HbO 2 < HbNO. We note that trends are also seen for microsecond recombination rates HbCO < HbO 2 ≈ HbNO and for the kinetic co-operativity ratio HbCO > HbO 2 > HbNO. A “critical onbarrier” model is presented that provides a consistent representation of these results. We suggest that spin-orbit effects could be a major contribution to the different recombination characteristics exhibited by the three ligands.

The long time behavior of the rate of recombination

Simple closed form expressions for the geminate and homogeneous recombination rates are given. These relations are valid for three-dimensional, spherically symmetric systems with arbitrary potential interaction, space dependent diffusion coefficient, and reactivity; they contain the previously derived expressions as special cases. For extremely long times the familiar t−3/2 (respectively, t−1/2) behavior is obtained, but the relations are valid also for shorter times where deviations from the simple behavior are important. Also included are closed form expressions for the partially diffusion controlled recombination probability. All expressions are derived from one fundamental quantity: the diffusion controlled recombination probability, which is given as an asymptotic series.

Solution of the time-dependent Onsager problem

We obtain the Green’s function solution of the Smoluchowski equation with a Coulomb potential and an electric field, corresponding to a general boundary condition at the origin. In the low-field limit the solution exhibits diffusive behavior for long times, but above a critical field the long-time decay becomes purely exponential. We derive expressions for the time evolution of quantities such as the rate of geminate recombination and the survival probability, and for t→∞, a→0 (where a is the radius of a perfectly absorbing sphere at the origin) we recover Onsager’s results. We consider a variety of initial conditions, such as a δ function, isotropic and constant distributions, as well as a more complicated distribution applicable to fluorescence quenching in the presence of an electric field. For small values of the electric field and the Laplace transform variable we obtain an expression for the probability of a scavenging reaction taking place in competition with geminate recombination. Using the prescribed diffusion approximation, we also obtain a simple expression for the time dependence of the survival probability, valid for an isotropic initial distribution, and we discuss the results of the numerical evaluation of various quantities of physical interest. Throughout the paper we have obtained parallel results for both Coulomb attraction and repulsion between the pair of charges.

The mechanism of the Stevens rearrangement

The influence of solvent viscosity and reaction temperature upon the stereoselectivity and intramolecularity of the Stevens [1,2] rearrangement of the ylide (1) has been examined; the results are compatible with two possible mechanisms: (i) a radical pair mechanism with an average geminate recombination rate which is exceptionally fast, or (ii) dual pathways involving concurrent radical pair and concerted processes.