Electron hopping and magnetic field dependent spin dynamics of radical ions in solution

The role of hopping on the geminate recombination of radical ions (N,N-dimethylaniline cation and anthracene anion) in acetonitrile is studied via the nanosecond time-resolved magnetic field effect on the triplet yield and the influence of donor concentration thereon. Increasing donor concentration leads to lifetime broadening of the magnetic field dependence of the triplet yield. Responsible for this effect is the perturbation of the coherent spin motion caused by hopping of the electron spin between donor sites of different nuclear spin configuration. Comparison of experimental results with calculations based on the semiclassical theory of spin motion yields an estimate of the hopping rates. Deuteration of both radicals influences the halfwidth of the magnetic field effect: at long probing times and low donor concentrations the halfwidth measured for protonated radical ions exceeds the one for the deuterated species: at short delay times and large donor concentrations, i.e. high hopping rates, this isotopic effect is reversed.     

Use of an “ordered” electron spin state in molecular spectroscopy: Theoretical and experimental considerations of spin-ordering in excited states

The effect of the dynamics of excited states on the observed decay of an ordered electron spin state induced in an electronically-excited ensemble is presented and used to illustrate the sensitivity of this technique to experimental conditions. Considered are feeding and decay processes involving the excited state, spin—lattice relaxation between the levels that are ordered, energy transfer to both translationally equivalent and inequivalent sites, and two different types of exchange processes. The treatment shows that the observed ordered-state decay is very sensitive to exprimental conditions. In particular, experiments performed off-resonance can display severely distorted decay curves with non-decaying components. These distortions limit the information on energy exchange in the excited state that can be obtained with electron spin ordering by adiabatic demagnetization in the rotating frame (ADRF). Also presented are treatments for the production and detection of order in excited states to complete a set of basic experimental considerations that affect data quality in ADRF measurements. Experimentally observed decays are presented to illustrate the predicted effects.     

Trapping time statistics and efficiency of transport of optical excitations in dendrimers

We theoretically study the trapping time distribution and the efficiency of the excitation energy transport in dendritic systems. Trapping of excitations, created at the periphery of the dendrimer, on a trap located at its core, is used as a probe of the efficiency of the energy transport across the dendrimer. The transport process is treated as incoherent hopping of excitations between nearest-neighbor dendrimer units and is described using a rate equation. We account for radiative and nonradiative decay of the excitations while diffusing across the dendrimer. We derive exact expressions for the Laplace transform of the trapping time distribution and the efficiency of trapping, and analyze those for various realizations of the energy bias, number of dendrimer generations, and relative rates for decay and hopping. We show that the essential parameter that governs the trapping efficiency is the product of the on-site excitation decay rate and the trapping time (mean first passage time) in the absence of decay.     

Temperature dependence of small polaron population decays in iron-doped lithium niobate by Monte Carlo simulations

The population decay of light-induced small polarons in iron-doped lithium niobate is simulated by a Monte-Carlo method on the basis of Holstein's theory. The model considers random walks of both bound polarons (Nb_Li⁴⁺) and free polarons (Nb_Nb⁴⁺) ending to deep traps (Fe_Li³⁺). The thermokinetic interplay between polaron species is introduced by trapping and de-trapping rates at niobium antisites (Nb_Li). The decay of the Nb_Li⁴⁺ population proceeds by three possible channels: direct trapping at Fe_Li³⁺ sites, hopping on niobium antisites and hopping on Nb regular sites after conversion to the free state. Up to three regimes, each one reflecting the predominance of one of these processes, appear with different activation energies in the Arrhenius plots of the decay time. The influence of Fe_Li and Nb_Li concentrations on the transition temperatures is evidenced. For both polaron species, the length of the final hop (trapping length) is found much larger than the usual hopping length and decreases at rising temperature. This trap size effect is a natural consequence of Holstein's theory and may explain some unclear features of polaron-related light-induced phenomena, such as the temperature-dependent stretching exponent of light-induced absorption decays and the anomalous increase of the photoconductivity at high doping levels.     

A unified theory for charge-carrier transport in organic crystals

To characterize the crossover from bandlike transport to hopping transport in molecular crystals, we study a microscopic model that treats electron-phonon interactions explicitly. A finite-temperature variational method combining Merrifield's transformation with Bogoliubov's theorem is developed to obtain the optimal basis for an interacting electron-phonon system, which is then used to calculate the bandlike and hopping mobilities for charge carriers. Our calculations on the one dimensional (1D) Holstein model at T=0 K and finite temperatures show that the variational basis gives results that compared favorably to other analytical methods. We also study the structures of polaron states at a broad range of parameters including different temperatures. Furthermore, we calculate the bandlike and hopping mobilities of the 1D Holstein model in different parameters and show that our theory predicts universal power-law decay at low temperatures and an almost temperature independent behavior at higher temperatures, in agreement with experimental observations. In addition, we show that as the temperature increases, hopping transport can become dominant even before the polaron state changes its character. Thus, our result indicates that the self-trapping transition studied in conventional polaron theories does not necessarily correspond to the bandlike to hopping transition in the transport properties in organic molecular crystals. Finally, a comparison of our 1D results with experiments on ultrapure naphthalene crystals suggests that the theory can describe the charge-carrier mobilities quantitatively across the whole experimental temperature range.     

Nonadiabatic decay dynamics of 9H-guanine in aqueous solution

The nonadiabatic decay of the biologically relevant guanine tautomer (9H-guanine) in aqueous solution has been investigated by trajectory surface hopping simulations in a quantum mechanical-molecular mechanical (QM-MM) framework. The QM part (9H-guanine) was treated at the semiempirical OM2/MRCI level, while the MM part (water) was described by the TIP3P force field. The optimized geometries for the relevant minima and conical intersections are qualitatively similar for 9H-guanine in the gas phase and in aqueous solution, while there are notable solvent-induced shifts in the computed vertical excitation energies (up to about 0.4 eV). Overall, the results from the static OM2/MRCI-based calculations are in reasonable agreement with the available ab initio and experimental data. The dynamics simulations show ultrafast nonradiative decay for 9H-guanine in water that is even slightly faster than in the gas phase, with time constants of 20 fs and around 0.3 ps for the S(2)→ S(1) and S(1)→ S(0) internal conversions, respectively. They predict a change in the S(1)→ S(0) decay mechanism when going from the gas phase to aqueous solution: the major pathway for 9H-guanine in water involves a conical intersection with an out-of-plane distortion of the carbonyl oxygen atom, which does not play any significant role in the gas phase, where the decay mainly proceeds via two other conical intersections characterized by ring distortions and out-of-plane displacement of the amino group, respectively. Possible reasons for this change in the mechanism are analyzed.     

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.     

Surface hopping simulation of the vibrational relaxation of I2 in liquid xenon using the collective probabilities algorithm

A surface hopping simulation of the vibrational relaxation of highly excited I(2) in liquid xenon is presented. The simulation is performed by using the collective probabilities algorithm which assures the coincidence of the classical and quantum populations. The agreement between the surface hopping simulation results and the experimental measurements for the vibrational energy decay curves at different solvent densities and temperatures is shown to be good. The overlap of the decay curves when the time axis is linearly scaled is explained in terms of the perturbative theory for the rate constants. The contribution of each solvent atom to the change of the quantum populations of the solute molecule is used to analyze the mechanism of the relaxation process     

Alpha-methylstilbene and the duality of mechanism in the quenching of stilbene triplets by molecular oxygen

Spin-exchange quenching of alpha-methylstilbene triplets by molecular oxygen and by the free radical di-tert-butyl nitroxide is shown to favor the cis isomer more than does natural decay. The effect of the two quenching events is an identical 7% decrease in the fraction of perpendicular triplets that decay to the trans isomer. The conclusion that relaxed stilbene triplets reside in a shallow minimum corresponding to a geometry in which the two benzyl moieties are orthogonal was based on the observation that their quenching by O2 does not alter the trans/cis photostationary ratio. Our results with alpha-methylstilbene confirm the hypothesis that in the case of stilbene spin exchange quenching by O2 at the twisted geometry favors the cis isomer but occurs in competition with excitation transfer from transoid triplets that leads to the trans isomer and to singlet oxygen. The opposite effects of the two oxygen quenching paths on stilbene isomer composition cancel accidentally leading to an overall insensitivity of benzophenone-sensitized photostationary states to the presence of oxygen. Quenching rate constants derived on the basis of this cancellation are close to diffusion-controlled and predict singlet O2 quantum yields of 0.08 and 0.13 in the presence of air and under an O2 atmosphere, in good agreement with experimental measurements.     

Coherent effects in energy transport in model dendritic structures investigated by ultrafast fluorescence anisotropy spectroscopy

Measurements of ultrafast fluorescence anisotropy decay in model branched dendritic molecules of different symmetry are reported. These molecules contain the fundamental branching center units of larger dendrimer macromolecules with either three (C(3))- or four (T(d), tetrahedral)-fold symmetry. The anisotropy for a tetrahedral system is found to decay on a subpicosecond time scale (880 fs). This decay can be qualitatively explained by F?rster-type incoherent energy migration between chromophores. Alternatively, for a nitrogen-centered trimer system, the fluorescence anisotropy decay time (35 fs) is found to be much shorter than that of the tetramers, and the decay cannot be attributed to an incoherent hopping mechanism. In this case, a coherent interchromophore energy transport mechanism should be considered. The mechanism of the ultrafast energy migration process in the branched systems is interpreted by use of a phenomenological quantum mechanical model, which examines the two extreme cases of incoherent and coherent interactions.     

Restricted orientational motion of nitroxides in molecular glasses: direct estimation of the motional time scale basing on the comparative study of primary and stimulated electron spin echo decays

A comparative study of anisotropic relaxation in two-pulse primary and three-pulse stimulated electron spin echo decays provides a direct way to distinguish fast (correlation time tau(c)<10(-6) s) and slow (tau(c)>10(-6) s) motions. Anisotropic relaxation is detected as a difference of the decay rates for different resonance field positions in anisotropic electron paramagnetic resonance spectra. For fast motion anisotropic relaxation influences the primary echo decay and does not influence the stimulated echo decay. For slow motion it is seen in both two-pulse echo and three-pulse stimulated echo decays. For nitroxide spin probes dissolved in glassy glycerol only fast motion was found below 200 K. Increase of temperature above 200 K results in the appearance of slow motion. Its amplitude increases rapidly with temperature increase. While in glycerol glass slow motion appears above glass transition temperature T(g), in ethanol glass it is observable below T(g). The scenario of motional dynamics in glasses is proposed which involves the broadening of the correlation time distribution with increasing temperature.     

Computational Insights into the Charge Relaying Properties of β‐Turn Peptides in Protein Charge Transfers

Density functional theory calculations suggest that β‐turn peptide segments can act as a novel dual‐relay elements to facilitate long‐range charge hopping transport in proteins, with the N terminus relaying electron hopping transfer and the C terminus relaying hole hopping migration. The electron‐ or hole‐binding ability of such a β‐turn is subject to the conformations of oligopeptides and lengths of its linking strands. On the one hand, strand extension at the C‐terminal end of a β‐turn considerably enhances the electron‐binding of the β‐turn N terminus, due to its unique electropositivity in the macro‐dipole, but does not enhance hole‐forming of the β‐turn C terminus because of competition from other sites within the β‐strand. On the other hand, strand extension at the N terminal end of the β‐turn greatly enhances hole‐binding of the β‐turn C terminus, due to its distinct electronegativity in the macro‐dipole, but does not considerably enhance electron‐binding ability of the N terminus because of the shared responsibility of other sites in the β‐strand. Thus, in the β‐hairpin structures, electron‐ or hole‐binding abilities of both termini of the β‐turn motif degenerate compared with those of the two hook structures, due to the decreased macro‐dipole polarity caused by the extending the two terminal strands. In general, the high polarity of a macro‐dipole always plays a principal role in determining charge‐relay properties through modifying the components and energies of the highest occupied and lowest unoccupied molecular orbitals of the β‐turn motif, whereas local dipoles with low polarity only play a cooperative assisting role. Further exploration is needed to identify other factors that influence relay properties in these protein motifs.     

Time evolution of a single spin inhomogeneously coupled to an interacting spin environment

We study the time evolution of a single spin coupled by exchange interaction to an environment of interacting spin bath modeled by the XY Hamiltonian. By evaluating the spin correlator of the single spin, we observed that the decay rate of the spin oscillations strongly depends on the relative magnitude of the exchange coupling between the single spin and its nearest neighbor J(') and coupling among the spins in the environment J. The decoherence time varies significantly based on the relative coupling magnitudes of J and J('). The decay rate law has a Gaussian profile when the two exchange couplings are of the same order J(') approximately J but converts to exponential and then a power law as we move to the regimes of J(')>J and J(')相似文献   

Orientational dynamics of water in the nafion polymer electrolyte membrane and its relationship to proton transport

Orientational anisotropies are calculated from molecular dynamics simulations of bulk water and the Na(+) and H(+) forms of hydrated Nafion and then compared with corresponding experimental values. The extended jump model of Laage and Hynes is applied to water reorientations for each system, and the anisotropies are explored as a product of hydrogen bond restricted "wobble-in-a-cone" reorientations and that due to the discrete jumps of hydrogen bond reorganization. Additionally, the timescales of hydrogen bond switching and proton transport are presented for bulk water and the H(+) form of hydrated Nafion. The short time scale of proton hopping is found to be independent of Nafion water loading, suggesting the short time dynamics of proton hopping are relatively insensitive to the level of hydration. Furthermore, the long time decay for the forward rate of hydrogen bond switching is shown to be identical to the long time decay in the forward rate of proton hopping, for bulk water and all water loadings of Nafion investigated, suggesting a unified process.     

Role of the guanine N1 imino proton in the migration and reaction of radical cations in DNA oligomers

Oxidation of a guanine nucleobase to its radical cation in DNA oligomers causes an increase in the acidity of the N1 imino proton that may lead to its spontaneous transfer to N3 of the paired cytosine. This proton transfer is suspected of playing an important role in long-distance radical cation hopping in DNA and the decisive product-determining role in the reaction of the radical cation with H2O or O2. We prepared and investigated DNA oligomers in which certain deoxycytidines are replaced by 5-fluoro-2'-deoxycytidines (F5dC). The pKa of F5C was determined to be 1.7 units below that of dC, which causes proton transfer from the guanine radical cation to be thermodynamically unfavorable. Photoinitiated one-electron oxidation of the DNA by UV irradiation of a covalently attached anthraquinone derivative introduces a radical cation that hops throughout the oligomer and is trapped selectively at GG steps. The introduction of F5dC does not affect the efficiency of charge hopping, but it significantly reduces the amount of reaction at the GG sites, as revealed by subsequent reaction with formamidopyrimidine glycosylase. These findings suggest that transfer of the guanine radical cation N1 proton to cytosine does not play a significant role in long-range charge transfer, but this process does influence the reactions with H2O and/or O2.     

ESR and electrical conduction in pyrolyzed polyimides

Electrical conductivity and electron spin resonance (ESR) of pyrolyzed polyimides change drastically in air after passage of a characteristic time, which depends on the temperature and time of pyrolysis. Samples must be kept from air throughout the measurements. ESR and resistivity measurements in vacuum show that there are two types of magnetic species: localized and delocalized. A. variable-range hopping model is proposed as a possible mechanism for conduction.     

The decay mechanism of photoexcited guanine - a nonadiabatic dynamics study

Ab initio surface hopping dynamics calculations were performed for the biologically relevant tautomer of guanine in gas phase excited into the first ππ? state. The results show that the complete population of UV-excited molecules returns to the ground state following an exponential decay within ～220 fs. This value is in good agreement with the experimentally obtained decay times of 148 and 360 fs. No fraction of the population remains trapped in the excited states. The internal conversion occurs in the ππ? state at two related types of conical intersections strongly puckered at the C2 atom. Only a small population of about 5% following an alternative pathway via a nπ? state was found in the dynamics.