Unrelaxed State Research Articles

Temperature and Detection-Wavelength Dependence of the Electron Transfer Rates in Initial Stages of Photosynthesis

Unusual temperature behavior, observed in the initial electron transfer stages in the photosynthetic reaction centers of the purple bacteria, and a strong probing pulse wavelength dependence of transfer rates, determined in transient absorption spectroscopy, can easily be explained on assuming that the transfer takes place from dynamically unrelaxed states of protein environment. The transitions from the primary special pair (P) to a single bacteriochlorophyll (B) and next to a bacteriopheophytin (H) are controlled by diffusion down the energy value of underdamped vibrational modes of frequency 200 K, probably determining distances between the succeeding cofactors. The subsequent transition to the quinone A (Q) is controlled by diffusion in the position value of an overdamped conformational mode, probably corresponding to the local polarization. From the fit of available experimental data to simple theoretical formulas, the important physical conclusion arises that the very electronic transitions are fast as compared to the relaxation processes and, in the first approximation, only the latter contribute to the overall times of the initial electron transfer stages in photosynthesis.

Charge transfer in TATB and HMX under extreme conditions

Charge transfer is usually accompanied by structural changes in materials under different conditions. However, the charge transfer in energetic materials that are subjected to extreme conditions has seldom been explored by researchers. In the work described here, the charge transfer in single molecules and unit cells of the explosives TATB and HMX under high temperatures and high pressures was investigated by performing static and dynamic calculations using three DFT methods, including the PWC functional of LDA, and the BLYP and PBE functionals of GGA. The results showed that negative charge is transferred from the nitro groups of molecular or crystalline TATB and HMX when they are heated. All DFT calculations for the compressed TATB unit cell indicate that, generally, negative charge transfer occurs to its nitro groups as the compression increases. PWC and PBE calculations for crystalline HMX show that negative charge is first transferred to the nitro groups but, as the compression increases, the negative charge is transferred from the nitro groups. However, the BLYP calculations indicated that there was gradual negative charge transfer to the nitro groups of HMX, similar to the case for TATB. The unrelaxed state of the uniformly compressed TATB causes negative charge to be transferred from its nitro groups, in contrast to what is seen in the relaxed state. Charge transfer in TATB is predicted to occur much more easily than in HMX.

Dynamics of carbocation formation in the photolysis of 1,2,2,3-tetramethyl-1,2-dihydroquinoline in alcohols

Dynamics of the formation of the carbocation in the ground state as a result of photoinduced proton transfer from a solvent to the excited state of 1,2,2,3-tetramethyl-1,2-dihydroquinoline (3MDHQ) in MeOH and 2,2,2-trifluoroethanol (TFE) was registered by pump-probe laser photolysis (λpump = 310 nm) with femtosecond time resolution. The lifetimes of the excited singlet state of 3MDHQ τ = 115 and 780 ps were determined in TFE and MeOH, respectively. The transient species with absorption spectrum corre-sponding to the spectrum of the carbocation from 3MDHQ (λmax = 480 nm) is generated at time delays lower than 500 fs from the unrelaxed excited singlet state.

IDENTIFYING DYNAMICALLY YOUNG GALAXY GROUPS VIA WIDE-ANGLE TAIL GALAXIES: A CASE STUDY IN THE COSMOS FIELD ATz= 0.53

We present an analysis of a wide-angle tail (WAT) radio galaxy located in a galaxy group in the COSMOS field at a redshift of z=0.53 (hereafter CWAT-02). We find that the host galaxy of CWAT-02 is the brightest galaxy in the group, although it does not coincide with the center of mass of the system. Estimating a) the velocity of CWAT-02, relative to the intra-cluster medium (ICM), and b) the line-of-sight peculiar velocity of CWAT-02's host galaxy, relative to the average velocity of the group, we find that both values are higher than those expected for a dominant galaxy in a relaxed system. This suggests that CWAT-02's host group is dynamically young and likely in the process of an ongoing group merger. Our results are consistent with previous findings showing that the presence of a wide-angle tail galaxy in a galaxy group or cluster can be used as an indicator of dynamically young non-relaxed systems. Taking the unrelaxed state of CWAT-02's host group into account, we discuss the impact of radio-AGN heating from CWAT-02 onto its environment, in the context of the missing baryon problem in galaxy groups. Our analysis strengthens recent results suggesting that radio-AGN heating may be powerful enough to expel baryons from galaxy groups.

Nanoindentation-induced phase transformation in relaxed and unrelaxed ion-implanted amorphous germanium

We have investigated nanoindentation-induced plastic deformation in amorphous germanium (a-Ge) prepared by high-energy self-ion implantation. Using cross-sectional transmission electron microscopy, micro-Raman spectroscopy, and force-displacement curve analysis, we find strong evidence for a pressure-induced metallic phase transformation during indentation. Crystalline diamond-cubic Ge-I is observed in residual indents. Relaxed and unrelaxed structural states of a-Ge exhibit similar behavior on loading, but transform at different pressures on unloading. Both forms are markedly softer mechanically than crystalline Ge. These results assist in furthering the understanding of the intriguing phenomenon known as “explosive crystallization.”

A GIANT RADIO HALO IN THE MASSIVE AND MERGING CLUSTER ABELL 1351

We report on the detection of diffuse radio emission in the X-ray luminous and massive galaxy cluster A1351 (z=0.322) using archival Very Large Array data at 1.4 GHz. Given its central location, morphology, and Mpc-scale extent, we classify the diffuse source as a giant radio halo. X-ray and weak lensing studies show A1351 to be a system undergoing a major merger. The halo is associated with the most massive substructure. The presence of this source is explained assuming that merger-driven turbulence may re-accelerate high-energy particles in the intracluster medium and generate diffuse radio emission on the cluster scale. The position of A1351 in the logP$_{1.4 GHz}$ - logL$_{X}$ plane is consistent with that of all other radio-halo clusters known to date, supporting a causal connection between the unrelaxed dynamical state of massive ($>10^{15} M_{\odot}$) clusters and the presence of giant radio halos.

Variation of Excitation Energy Influences the Product Distribution of a Two-Step Electron Transfer: S2 vs S1 Electron Transfer in a Zn(II)porphyrin−Viologen Complex

We have, for the first time for molecular systems in solution, shown a case where the variation of excitation energy influences the product distribution of a two-step electron transfer. The photoexcitation was to the porphyrin-localized S(2) state or either of the S(1)(v = 1) or S(1)(v = 0) states of an aqueous Zn(II)-meso-tetrasulphonatophenyl-porphyrin-methylviologen (ZnTPPS(4-)/MV(2+)) complex. Both forward and back electron transfer occur on a subpicosecond time scale (tau(FET) approximately 0.2, tau(BET) = 0.7 ps). The excess energy of the higher excitations partially survives both electron transfer steps. This is seen by different distributions of unrelaxed ground states, which are generated by the back electron transfer and has unique UV-vis spectroscopic signatures. State selective electron transfer opens interesting possibilities for reaction control, and the results represent initial steps in that direction.

THE DYNAMICAL STATE OF BRIGHTEST CLUSTER GALAXIES AND THE FORMATION OF CLUSTERS

A large sample of Abell clusters of galaxies, selected for the likely presence of a dominant galaxy, is used to study the dynamical properties of brightest cluster members (BCMs). From visual inspection of Digitized Sky Survey images combined with redshift data we identify 1426 candidate BCMs in 1221 redshift components in 1169 different Abell clusters, the largest such sample published so far. By our own morphological classification we find ~92% of these BCMs to be early-type galaxies, and 48% of cD type. We confirm previous findings based on much smaller samples, namely that a large fraction of BCMs have significant peculiar velocities. For a subsample of 452 clusters with at least 10 measured radial velocities, we find a median BCM peculiar velocity of 32% of their host clusters' radial velocity dispersion. This suggests that most BCMs are not at rest in the potential well of their clusters, and that the phenomenon is thus not a special trait of clusters hosting cD galaxies. We show that the peculiar velocity of the BCM is independent of cluster richness and only slightly dependent on the Bautz-Morgan type. We also find a weak trend for the peculiar velocity to rise with the cluster velocity dispersion. The strongest dependence is with the morphological type of the BCM: cD galaxies tend to have lower relative peculiar velocities than elliptical galaxies. This result points to a connection between the formation of the BCMs and that of their clusters. Our data are qualitatively consistent with the merging-groups scenario, where BCMs in clusters formed first in smaller subsystems comparable to compact groups of galaxies. In this scenario, clusters would have formed recently from the mergers of many such groups and would still be in a dynamically unrelaxed state.

Nanoscale diffusion in amorphousFe75Zr25films

Self-diffusion of Fe in amorphous ${\text{Fe}}_{75}{\text{Zr}}_{5}$ films has been studied over a wide temperature range by combining secondary-ion-mass spectrometry and nuclear-resonance reflectivity measurements. Subnanometer accuracy of nuclear-resonance reflectivity in diffusion length measurement allows quantitative determination of time-dependent diffusivity of Fe during structural relaxation. A clear correlation between diffusivity and different types of structural relaxations is observed. It is found that in both structurally relaxed and unrelaxed states, diffusive jumps occur via a collective motion of a group of atoms. However, the presence of excess free volume in unrelaxed amorphous films causes the activation energy as well as the diffusion entropy to decrease, suggesting that the average number of atoms participating in a diffusive jump is significantly less as compared to that in the fully relaxed state. The typical diffusion length involved in annihilation of free volume is 0.7 nm, which agrees with the length scale of structural fluctuations as seen in neutron- and x-ray-scattering experiments.

Push–pull hyperbranched molecules. A theoretical study

AbstractThe electronic properties of the ground state, unrelaxed and relaxed first excited states of push–pull hyperbranched molecules bearing amino and nitro terminal groups have been studied at BB1K/cc‐pvdz//HF/6‐31g(d), TD‐BB1K/cc‐pvdz//HF/6‐31g(d) and TD‐BB1K/cc‐pvdz//CIS/6‐31g(d) levels of theory, respectively. It was demonstrated that dendritic architecture of push–pull molecules favours the charge transfer in the excited state compared to linear molecules. The possibility of adopting a plane conformation is an important condition for the charge transfer in an excited state. According to the calculations 1:1 ratio of donor and acceptor groups is another important precondition for the manifestation of strong charge separation in the excited state. In case of excess of nitro groups over the amino, some of the excitations participating in the S0 → S1 transition favour the charge transfer in the excited state in the opposite directions, thus decreasing the charge separation. Copyright © 2008 John Wiley & Sons, Ltd.

Steric repulsions, rotation barriers, and stereoelectronic effects: A real space perspective

Widely used chemical concepts like Pauli repulsion or hyperconjugation, and their role in determining rotation barriers or stereoelectronic effects, are analyzed from the real space perspective of the interacting quantum atoms approach (IQA). IQA emerges from the quantum theory of atoms in molecules (QTAIM), but is free from the equilibrium geometry constraint of the former. A framework with both electronically unrelaxed and relaxed wavefunctions is presented that leads to an approximate correspondence between the IQA concepts and those used in the EDA (energy decomposition analysis) or NBO (natural bond orbital) procedures. We show that no net force acts upon the electrons in an electronically relaxed system, so that any reasonable definition of Pauli repulsion must involve unrelaxed state functions. Using antisymmetrized fragments clarifies that Pauli repulsions are energetically connected to the IQA deformation energies, leaving footprints in the finally relaxed states. Similarly, EDA or NBO hyperconjugative stabilizations are found to be naturally related to the IQA electron delocalization patterns. Applications to the rotation barrier of ethane and other simple systems are presented, and the very often forgotten role of electrostatic contributions in determining preferred conformations is highlighted.

The Benzophenone S1(n,π*) → T1(n,π*) States Intersystem Crossing Reinvestigated by Ultrafast Absorption Spectroscopy and Multivariate Curve Resolution

The well-known benzophenone intersystem crossing from S(1)(n,pi*) to T(1)(n,pi*) states, for which direct transition is forbidden by El-Sayed rules, is reinvestigated by subpicosecond time-resolved absorption spectroscopy and effective data analysis for various excitation wavelengths and solvents. Multivariate curve resolution alternating least-squares analysis is used to perform bilinear decomposition of the time-resolved spectra into pure spectra of overlapping transient species and their associated time-dependent concentrations. The results suggest the implication of an intermediate (IS) in the relaxation process of the S(1) state. Therefore, a two step kinetic model, S(1) --> IS --> T(1), is successfully implemented as an additional constraint in the soft-modeling algorithm. Although this intermediate, which has a spectrum similar to the one of T(1)(n,pi*) state, could be artificially induced by vibrational relaxation, it is tentatively assigned to a hot T(1)(n,pi*) triplet state. Two characteristic times are reported for the transition S(1) --> IS and IS --> T(1), approximately 6.5 ps and approximately 10 ps respectively, without any influence of the solvent. Moreover, an excitation wavelength effect is discovered suggesting the participation of unrelaxed singlet states in the overall process. To go further discussing the spectroscopic relevancy of IS and to rationalize the expected involvement of the T(2)(pi,pi*) state, we also investigate 4-methoxybenzophenone. For this neighboring molecule, triplet energy level is tunable through solvent polarity and a clear correlation is established between the intermediate resolved by multivariate data analysis and the presence of a T(2)(pi,pi*) above the T(1)(n,pi*) triplet. It is therefore proposed that the benzophenone intermediate species is a T(1)(n,pi*) high vibrational level in interaction with T(2)(pi,pi*) state.

Direct Observation of Nanostructural Fluctuation during Radiation-Induced Amorphization

An atomistic study of radiation-induced amorphization in the NiTi intermetallic compound was performed by using in-situ high-resolution high-voltage electron microscopy and molecular dynamics in conjunction with image simulations. Both theoretical and experimental results show that metastable nanometer-size inherent atomic clusters form and disappear during irradiation, so that a spatiotemporal fluctuation under amorphization is induced. The random formation and annihilation of such inherent nanoclusters are believed to be responsible for these fluctuations, which appear to be related to transitions between the ideal glass state and metastable, unrelaxed states in an energy-dissipative system under irradiation.

Temperature-Dependent Mechanistic Transition for Photoinduced Electron Transfer Modulated by Excited-State Vibrational Relaxation Dynamics

The electron transfer (ET) dynamics of an unusually rigid pi-stacked (porphinato)zinc(II)-spacer-quinone (PZn-Q) system, [5-[8'-(4' '-[8' ''-(2' '' ',5' '' '-benzoquinonyl)-1' ''-naphthyl]-1' '-phenyl)-1'-naphthyl]-10,20-diphenylporphinato]zinc(II) (2a-Zn), in which sub-van der Waals interplanar distances separate juxtaposed porphyryl, aromatic bridge, and quinonyl components of this assembly, have been measured by ultrafast pump-probe transient absorption spectroscopy over a 80-320 K temperature range in 2-methyl tetrahydrofuran (2-MTHF) solvent. Analyses of the photoinduced charge-separation (CS) rate data are presented within the context of several different theoretical frameworks. Experiments show that at higher temperatures the initially prepared 2a-Zn vibronically excited S1 state relaxes on an ultrafast time scale, and ET is observed exclusively from the equilibrated lowest-energy S1 state (CS1). As the temperature decreases, production of the photoinduced charge-separated state directly from the vibrationally unrelaxed S1 state (CS2) becomes competitive with the vibrational relaxation time scale. At the lowest experimentally interrogated temperature ( approximately 80 K), CS2 defines the dominant ET pathway. ET from the vibrationally unrelaxed S1 state is temperature-independent and manifests a subpicosecond time constant; in contrast, the CS1 rate constant is temperature-dependent, exhibiting time constants ranging from 4x10(10) s(-1) to 4x10(11) s(-1) and is correlated strongly with the temperature-dependent solvent dielectric relaxation time scale over a significant temperature domain. Respective electronic coupling matrix elements for each of these photoinduced CS1 and CS2 pathways were determined to be approximately 50 and approximately 100 cm-1. This work not only documents a rare, if not unique, example of a system where temperature-dependent photoinduced charge-separation (CS) dynamics from vibrationally relaxed and unrelaxed S1 states can be differentiated, but also demonstrates a temperature-dependent mechanistic transition of photoinduced CS from the nonadiabatic to the solvent-controlled adiabatic regime, followed by a second temperature-dependent mechanistic evolution where CS becomes decoupled from solvent dynamics and is determined by the extent to which the vibrationally unrelaxed S1 state is populated.

Excess polarizabilities upon excitation from the ground state to the first dipole‐allowed excited state of diphenylpolyenes

AbstractThis paper discusses the excess polarizabilities upon excitation from the ground state to the first dipole‐allowed excited state (S1) of diphenylpolyenes by using the time‐dependent density functional theory. Two hybrid exchange‐correlation (xc) potentials Becke‐3 Lee‐Yang‐Parr (B3LYP) and Perdew‐Burke‐Ernzerhof (PBE1PBE) were employed. Our calculations indicate that the magnitude of the excess polarizability will decrease while the molecule evolves from the unrelaxed S1 state to the relaxed S1 state. This decreasing trend is found to be independent of substituents, though substituents can change the value of the excess polarizability. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007

Anisotropy analysis of the surface stress in Ag stepped surfaces with the modified embedded atom method

AbstractThe surface stresses in Ag stepped surfaces (910), (710), (510), (410), (310), (210), (320), (430) and (540) have been calculated by using the modified embedded atom method (MEAM). The surface stresses in the surface plane τxx (along the step edge) and τyy (normal to the step edge) have similar orders of magnitude as the surface energy. For surfaces having the (100) and (110) terraces, the change of τxx and τyy is very small and the variation of the surface energy is smooth. The stress τzz (normal to the surface plane) is always tensile in the unrelaxed state. The linear variation of the change in surface energy per unit change in elastic strain ${\partial \gamma\over \partial \varepsilon_{xx}}$, ${\partial \gamma\over \partial \varepsilon_{yy}}$ and ${\partial \gamma\over \partial \varepsilon_{zz}}$ (i.e. τxx − γ, τyy − γ and τzz) with the angle α between the (hk0) and (100) planes has a turning point corresponding to the (210) surface. The anisotropic ratio in the stepped surface having the (110) terrace is larger than that having the (100) terrace, and the wider the (110) terrace or the narrower the (100) terrace, the larger the ratio τyy/τxx. Copyright © 2006 John Wiley & Sons, Ltd.

Exciton luminescence of boron nitride nanotubes and nano‐arches

AbstractWe report photoluminescence (PL) and PL‐excitation spectroscopy of BN nanotubes (nt‐BN) mixed with some residual hexagonal crystalline (h‐BN) starting material, and of pure h‐BN microcrystalline powder. The nanotube phase exhibits a broad‐band PL near 380 nm, in agreement with a published report of cathodoluminescence from a sample comprising >90% nanotubes. This emission is almost 3 eV lower in energy than unrelaxed exciton states found in recent all‐electron theories of nt‐BN and h‐BN and about 1.4 eV lower than the lowest (perturbed dark?) exciton seen in absorption of nt‐BN. This may suggest that excitons in nt‐BN vibrationally relax to self‐trapped states before emitting, a path found in many wide‐gap solids, especially in quasi‐1‐dimensional forms. Exciton emission from bulk single‐crystal h‐BN has been shown to occur from vibrationally unrelaxed (free‐exciton) states. We suggest a hypothesis in which known nano‐arch reconstructions on the surface of h‐BN may provide the low‐dimensional environment to make exciton self‐trapping on the surfaces of h‐BN likely. This allows consistent interpretaton of the surface‐related 380 nm emission from h‐BN powder within a half‐nanotube self‐trapped exciton hypo‐ thesis. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Trapped conformational states of semiquinone (D+*QB-*) formed by B-branch electron transfer at low temperature in Rhodobacter sphaeroides reaction centers.

The reaction center (RC) from Rhodobacter sphaeroides captures light energy by electron transfer between quinones QA and QB, involving a conformational gating step. In this work, conformational states of D+*QB-* were trapped (80 K) and studied using EPR spectroscopy in native and mutant RCs that lack QA in which QB was reduced by the bacteriopheophytin along the B-branch. In mutant RCs frozen in the dark, a light induced EPR signal due to D+*QB-* formed in 30% of the sample with low quantum yield (0.2%-20%) and decayed in 6 s. A small signal with similar characteristics was also observed in native RCs. In contrast, the EPR signal due to D+*QB-* in mutant RCs illuminated while freezing formed in approximately 95% of the sample did not decay (tau >107 s) at 80 K (also observed in the native RC). In all samples, the observed g-values were the same (g = 2.0026), indicating that all active QB-*'s were located in a proximal conformation coupled with the nonheme Fe2+. We propose that before electron transfer at 80 K, the majority (approximately 70%) of QB, structurally located in the distal site, was not stably reducible, whereas the minority (approximately 30%) of active configurations was in the proximal site. The large difference in the lifetimes of the unrelaxed and relaxed D+*QB-* states is attributed to the relaxation of protein residues and internal water molecules that stabilize D+*QB-*. These results demonstrate energetically significant conformational changes involved in stabilizing the D+*QB-* state. The unrelaxed and relaxed states can be considered to be the initial and final states along the reaction coordinate for conformationally gated electron transfer.

Postcollision relaxation of small atomic clusters

Molecular-dynamics simulations are performed to investigate the effects caused by the lack of internal equilibration on the dynamics and properties of atomic clusters. The studied systems consist of Lennard-Jones clusters of five to ten atoms and a colliding vapor monomer. Cluster radius and potential energy are shown to reach a time-independent value within 30 ps after a collision with a vapor monomer. The relaxation in terms of rotational energy takes at least 200 ps. During the first couple of picoseconds after the collision time-dependent cluster decay rates are observed. The unrelaxed cluster states are expected to have minimal effect on gas-liquid nucleation rates.

Primary Photophysical and Photochemical Processes in the Photolysis of 1,2-Dihydroquinolines in Different Solvents

The fluorescence lifetimes (τfl) of alkyl and alkoxy substituted dihydroquinolines (DHQ) were measured in hexane, isopropanol, methanol, and water. It was shown that τfl was determined by the solvent nature and weakly depended on substituents on the DHQ aromatic ring and heterocycle, with τfl being substantially lower in methanol than in the other solvents. The decrease in τfl in MeOH is caused by an increase in the rate constants of the photochemical reaction and nonradiative transitions in this solvent. The quantum yield of fluorescence in H2O and MeOH, in which the photoinduced addition of the solvent molecules occurs, decreases with a decrease in the excitation wavelength within the limits of long-wavelength absorption band, thus providing strong evidence that the photochemical reaction proceeds from the unrelaxed excited state.