Nonadiabatic molecular dynamics simulations of correlated electrons in solution. 1. Full configuration interaction (CI) excited-state relaxation dynamics of hydrated dielectrons

The hydrated dielectron is composed of two excess electrons dissolved in liquid water that occupy a single cavity; in both its singlet and triplet spin states there is a significant exchange interaction so the two electrons cannot be considered to be independent. In this paper and the following paper,we present the results of mixed quantum/classical molecular dynamics simulations of the nonadiabatic relaxation dynamics of photoexcited hydrated dielectrons, where we use full configuration interaction (CI) to solve for the two-electron wave function at every simulation time step. To the best of our knowledge, this represents the first systematic treatment of excited-state solvation dynamics where the multiple-electron problem is solved exactly. The simulations show that the effects of exchange and correlation contribute significantly to the relaxation dynamics. For example, spin-singlet dielectrons relax to the ground state on a time scale similar to that of single electrons excited at the same energy, but spin-triplet dielectrons relax much faster. The difference in relaxation dynamics is caused by exchange and correlation: The Pauli exclusion principle imposes very different electronic structure when the electrons' spins are singlet paired than when they are triplet paired, altering the available nonadiabatic relaxation pathways. In addition, we monitor how electronic correlation changes dynamically during nonadiabatic relaxation and show that solvent dynamics cause electron correlation to evolve quite differently for singlet and triplet dielectrons. Despite such differences, our calculations show that both spin states are stable to excited-state dissociation, but that the excited-state stability has different origins for the two spin states. For singlet dielectrons, the stability depends on whether the solvent structure can rearrange to create a second cavity before the ground state is reached. For triplet dielectrons, in contrast, electronic correlation ensures that the two electrons do not dissociate, even if the dielectron is artificially kept from reaching the ground state. In addition, both singlet and triplet dielectrons change shape dramatically during relaxation, so that linear response fails to describe the solvation dynamics for either spin state. In the following paper (Larsen, R. E.; Schwartz, B. J. J. Phys. Chem. B 2006, 110, 9692), we use these simulations to calculate the pump-probe spectroscopic signal expected for photoexcited hydrated dielectrons and to predict an experiment to observe hydrated dielectrons directly.     

EPR study of electron spin polarization in the photoexcited triplet state of chlorophyll a and b

The photoexcited triplet states of chlorophyll à and b are studied by the EPR method at ≈85 K using modulated light excitation. Both compounds show anomalous EPR line intensities and transient kinetics, indicating electron spin polarization (ESP) in the photoexcited triplet state. EPR studies, using Mg-tetraphenyl porphyrin (MgTPP) dissolved in n-octane show that ESP occurs also in that solvent. It is shown that the zero field splitting (ZFS) parameters of MgTPP depend strongly on the solvent. From the analysis of the data for chlorophyll a and b we evaluate: (1) the population rate constants (k_p); (2) the ratio between the population rate constants (A_p) (p = x, y, z) and, (3) the spin lattice relaxation rate W. In both chlorophylls the in-plane component, x, is predominantly populated and depopulated. The ZFS parameters have been also determined for the above compounds.     

Coherent averaging of electron spins by rotary echoes in excited triplet states

A theoretical treatment of rotary echoes derived from average hamiltonian formalism in photoexcited triplet states is presented. From a general relaxation matrix, it is shown that relaxation fields perpendicular to the driving field H_eff are completely averaged, whereas relaxation fields parallel to H_eff are not. Rotary echoes are found to differ from other coherence techniques only in the geometry of the experiment and not in the averaging properties per se. Off-resonance driving field conditions are shown to impair the efficiency of the averaging and to cause beats in the observed echo decay. Finally, inhomogenous line broadening is shown to produce a damped echo decay with beats even when the field is on-resonance.     

Aggregation‐Induced Emission Properties of Copper Iodide Clusters

The aggregation‐induced emission (AIE) properties of two different copper iodide clusters have been studied. These two [Cu₄I₄L₄] clusters differ by their coordinated phosphine ligand and the luminescent mechanochromic properties are only displayed by one of them. The two clusters are AIE‐active luminophors that exhibit an intense emission in the visible region upon aggregation. The formed particles present luminescent thermochromism comparable to that of the bulk compounds. The observed AIE properties can be attributed to suppression of nonradiative relaxation of the excited states in a more rigid state, in relation to the large structural relaxation of the excited triplet state. The differences observed in the AIE properties of the two clusters can be related to the different ligands. A correlation between the luminescence mechanochromic properties and the AIE effect is not straightforward, but the formation of “soft” molecular solids is a common characteristic that can explain the photoactive properties of these compounds.     

Nonadiabatic excited-state dynamics of polar pi-systems and related model compounds of biological relevance

Multireference ab initio dynamics simulations have become available as a tool for the investigation of photochemical processes, mainly for those related to nonadiabatic phenomena taking place in the sub-picosecond time scale. For organic molecules, these phenomena are in many cases deeply dependent on the relaxation of the photoexcited pi-system. We review the latest contributions of our group to this subject and report new results for systems studied previously, grouping them in single pi bonds, chains and aromatic rings. The dynamics of ethylene and substituted ethylenes is discussed mainly in connection to the competition between the two available relaxation paths in the excited states and their relation to the conical intersections in large systems. The trans-cis and the cis-trans dynamics of the pentadieniminium cation is investigated as well. Finally, we discuss the photodynamics of aminopyrimidine starting in the S1 and S2 states and the conclusions, which can be drawn from this for the interpretation of the adenine dynamics.     

Interchain coupling effects on dynamics of photoexcitations in conjugated polymers

Within an extended Su-Schrieffer-Heeger model including interchain interactions and the extended Hubbard model, the dynamical relaxation of photoexcitations in two coupled conjugated polymer chains is investigated by using a nonadiabatic evolution method. Initially, one of the two chains is photoexcited and the other chain is in the dimerized ground state. Due to the interchain interactions, the electron and/or the hole can be transferred from one chain to the other chain. For weak interchain coupling, the dynamical evolution of the lattice on the photoexcited chain is similar to that found in an isolate single chain case. With interchain interactions increasing, the amplitude of the distortions on the photoexcited chain decreases, and simultaneously, that on the other chain gradually increases. Until stronger interchain coupling, the deformations of the two chains have almost the same amplitude. Besides intrachain polaron-excitons and intrachain oppositely charged polaron pairs as found in single chain case, interchain polaron-excitons and interchain separated charged polaron pairs are obtained. The results show that the yield of interchain products increases and that of intrachain products decreases with interchain coupling increasing. Totally, the yield of charged polarons (including intrachain oppositely charged polaron pairs and interchain oppositely charged polaron pairs) is about 25%, in good agreement with results from experiments.     

Chemically induced dynamic electron spin polarization-detected energy transfer. Substrate size effects and solvent dependence

Time-resolved electron paramagnetic resonance spectroscopy is used to probe energy transfer from aromatic photoexcited triplet states to azo compounds in liquid solution. The observation of chemically induced dynamic electron spin polarization in the spectra gives precise information regarding the spin physics and mechanism of the energy transfer process. The substrate size is varied by altering the chain length of alkyl chains covalently attached to the azo compounds via ester or amide linkages. The solvent dependence of the energy transfer process is also investigated. The results are discussed in terms of Dexter and F?rster mechanisms for energy transfer, the properties of the excited states, and the diffusive properties of the molecules in the solvents of interest. Decomposition rate studies and fluorescence measurements are also reported.     

Nonadiabatic molecular dynamics of photoexcited Li2(+) Ne(n) clusters

We investigate the relaxation of photoexcited Li(2)(+) chromophores solvated in Ne(n) clusters (n = 2-22) by means of molecular dynamics with surface hopping. The simplicity of the electronic structure of these ideal systems is exploited to design an accurate and computationally efficient model. These systems present two series of conical intersections between the states correlated with the Li+Li(2s) and Li+Li(2p) dissociation limits of the Li(2)(+) molecule. Frank-Condon transition from the ground state to one of the three lowest excited states, hereafter indexed by ascending energy from 1 to 3, quickly drives the system toward the first series of conical intersections, which have a tremendous influence on the issue of the dynamics. The states 1 and 2, which originate in the Frank-Condon area from the degenerated nondissociative 1(2)Π(u) states of the bare Li(2)(+) molecule, relax mainly to Li+Li(2s) with a complete atomization of the clusters in the whole range of size n investigated here. The third state, which originates in the Frank-Condon area from the dissociative 1(2)Σ(u)(+) state of the bare Li(2)(+) molecule, exhibits a richer relaxation dynamics. Contrary to intuition, excitation into state 3 leads to less molecular dissociation, though the amount of energy deposited in the cluster by the excitation process is larger than for excitation into state 1 and 2. This extra amount of energy allows the system to reach the second series of conical intersections so that approximately 20% of the clusters are stabilized in the 2(2)Σ(g)(+) state potential well for cluster sizes n larger than 6.     

Energy landscapes of ion clusters in isotropic quadrupolar and octupolar traps

The energy landscapes of ion clouds confined in isotropic quadrupolar and octupolar traps are characterized for several representative cluster sizes. All clusters exhibit stable multishell structures that belong to separate funnels. Quadrupolar confinement leads to more homogeneous clusters and denser distributions of isomers than octupolar confinement. Statistical analysis of the transition states indicates that the barriers associated with intrashell motion are lower but more asymmetric and more cooperative compared to intershell motion. The relaxation between low-energy funnels with different arrangements of shells mostly exhibits Arrhenius kinetics, with a weak variation of the activation energy at higher temperatures.     

Wavelength dependence of electronic relaxation in isolated adenine using UV femtosecond time-resolved photoelectron spectroscopy

Electronic relaxation pathways in photoexcited nucleobases have received much theoretical and experimental attention due to their underlying importance to the UV photostability of these biomolecules. Multiple mechanisms with different energetic onsets have been proposed by ab initio calculations yet the majority of experiments to date have only probed the photophysics at a few selected excitation energies. We present femtosecond time-resolved photoelectron spectra (TRPES) of the DNA base adenine in a molecular beam at multiple excitation energies between 4.7-6.2 eV. The two-dimensional TRPES data is fit globally to extract lifetimes and decay associated spectra for unambiguous identification of states participating in the relaxation. Furthermore, the corresponding amplitude ratios are indicative of the relative importance of competing pathways. We adopt the following mechanism for the electronic relaxation of isolated adenine; initially the S(2)(ππ*) state is populated by all excitation wavelengths and decays quickly within 100 fs. For excitation energies below ～5.2 eV, the S(2)(ππ*)→S(1)(nπ*)→S(0) pathway dominates the deactivation process. The S(1)(nπ*)→S(0) lifetime (1032-700 fs) displays a trend toward shorter time constants with increasing excitation energy. On the basis of relative amplitude ratios, an additional relaxation channel is identified at excitation energies above 5.2 eV.     

Theory of spin-peierls transitions in chains of exchange clusters

A theoretical approach to the analysis of magnetostructural phase transitions of chain polymeric heterospin complexes is suggested. The approach is based on a model of the spin-Peierls transition in chains of exchange clusters. The chain elasticity parameter is found to be a main factor determining the order of phase transition.     

Pi topology and spin alignment in unique photoexcited triplet and quintet states arising from four unpaired electrons of an organic spin system

Syntheses, electronic structures in the ground state, unique photoexcited states, and spin alignment are reported for novel biradical 1, which was designed as an ideal model compound to investigate photoinduced spin alignment in the excited state. Electron spin resonance (ESR), time-resolved ESR (TRESR), and laser-excitation pulsed ESR experiments were carried out. The magnetic properties were examined with a SQUID magnetometer. In the electronic ground state, two radical moieties interact very weakly (almost no interaction) with each other through the closed-shell diphenylanthracene spin coupler. On photoirradiation, a novel lowest photoexcited state with the intermediate spin (S = 1) arising from four unpaired electrons with low-lying quintet (S = 2) photoexcited state was detected. The unique triplet state has an interesting electronic structure, the D value of which is reduced by antiferromagnetic spin alignment between two radical spins through the excited triplet spin coupler. The general theoretical predictions of the spin alignment and the reduction of the fine-structure splitting of the triplet bis(radical) systems are presented. The fine-structure splitting of the unique photoexcited triplet state of 1, as well as the existence of the low-lying quintet state, is interpreted well on the basis of theoretical predictions. Details of the spin alignment in the photoexcited states are discussed.     

Collapse kinetics of vibrated granular chains

The kinetics of the collapse of the coil state into condensed states is studied with vibrated granular chain composed of N metal beads partially immersed in water. The radius of gyration of the chain, R(g) is measured. For short chains (N < 140), disk-like condensed state is formed and R(g) decreases with time such that the function ΔR(g)(2) (≡ R(g)(2) - R(g)(2)(∞)) = A e(-t/τ), where the relaxation time τ follows a power-law dependence on the chain length N with an exponent γ = 1.9 ± 0.2. For the chains with length N ≥ 300, rod-like clusters are observed during the initial stage of collapse and R(g)(2) = R(g)(2)(0) - Bt(β), with β = 0.6 ± 0.1. In the coarsening stage, the exponential dependence of ΔR(g)(2) on time still holds, however, the relaxation time τ fluctuates and has no simple dependence on N. Furthermore, the time dependence of the averaged radius of gyration of the individual clusters, R(g,cl) can be described by the theory of Lifshitz and Slyozov. A peak in the structure function of long chains is observed in the initial stage of the collapse transition. The collapse transition in the bead chains is a first order phase transition. However, features of the spinodal decomposition are also observed.     

On the stability of Langmuir films of pyramidic mesogens

Pyramidic mesogens forming thermotropic liquid crystal bulk phases were spread in an air-water interface. Pressure surface measurements and polarizing microscopy on the Langmuir films were used to characterize the various states of these pyramidic-like molecules. For two compounds bearing short lateral aliphatic chains, the surface pressure isotherms exhibit a large plateau region corresponding to a metastable monolayer in which the molecules may adopt an 'edge-on' arrangement. The coexistence of multilayered, anisotropic, slowly growing domains with the monolayer in the plateau region has been observed at long time scale. The film area relaxation kinetics at constant surface pressure show the existence of two nucleation mechanisms for the formation of these domains.     

Dynamics of Bond‐Fluctuation Model Chains in Good and Theta Solvents

The dynamic Monte Carlo algorithm is employed to explore the dynamics of flexible linear chains. The chains are represented by the bond‐fluctuation model with and without attractions between non‐bonded units placed at close distances. This mimics the behavior of real chains in the good and poorer solvents. We obtain the chain sizes, diffusion coefficients, Rouse modes, and their relaxation times. We also evaluate the time correlation function of the end‐to‐end vector at different concentrations. Subsequently, we compare the dependence of the simulation results on chain length, solvent quality, concentration, and mode order with the corresponding theoretical predictions. We observe a retardation of diffusion for non‐dilute systems close to the theta state. This retardation is too high to be exclusively attributed to the increase of global friction and can be caused by temporary adherence of the chains to transient clusters.     

Nanoparticle Dispersion and Glass Transition Behavior of Polyimide-grafted Silica Nanocomposites

How to control the spatial distribution of nanoparticles to meet different performance requirements is a constant challenge in the field of polymer nanocomposites.Current studies have been focused on the flexible polymer chain systems.In this study,the rigid polyimide(PI) chain grafted silica particles with different grafting chain lengths and grafting densities were prepared by "grafting to" method,and the influence of polymerization degree of grafted chains(N),matrix chains(P),and grafting density(a) on the spatial distribution of nanoparticles in the PI matrix was explored.The glass transition temperature(Tg) of PI composites was systematically investigated as well.The results show that silica particles are well dispersed in polyamic acid composite systems,while aggregation and small clusters appear in PI nanocomposites after thermal imidization.Besides,the particle size has no impact on the spatial distribution of nanoparticles.When σ·N0.5<<(N/P)2,the grafted and matrix chains interpenetrate,and the frictional resistance of the segment increases,resulting in restricted relaxation kinetics and Tg increase of the PI composite system.In addition,smaller particle size and longer grafted chains are beneficial to improving Tg of composites These results are all propitious to complete the microstructure control theory of nanocomposites and make a theoretical foundation for the high performance and multi-function of PI nanocomposites.     

Further insight into the relaxation dynamics of photoexcited I-(H2O)n clusters

First-principles molecular dynamics simulations of the excited-state dynamics of I-(H2O)3 have been performed to gain some insight into the general features of the relaxation process of photoexcited I-(H2O)n clusters. The relaxation of excited I-(H2O)3 is characterized by rapid motion of water molecules and slow recoil motion of the iodine atom. Both solvent reorganization and iodine atom motion appear to be important for interpreting the existing femtosecond photoelectron spectroscopy experimental results.     

Influence of thermal fluctuations on interfacial electron transfer in functionalized TiO2 semiconductors

The influence of thermal fluctuations on the dynamics of interfacial electron transfer in sensitized TiO2-anatase semiconductors is investigated by combining ab initio DFT molecular dynamics simulations and quantum dynamics propagation of transient electronic excitations. It is shown that thermal nuclear fluctuations speed up the underlying interfacial electron transfer dynamics by introducing nonadiabatic transitions between electron acceptor states, localized in the vicinity of the photoexcited adsorbate, and delocalized states extended throughout the semiconductor material, creating additional relaxation pathways for carrier diffusion. Furthermore, it is shown that room-temperature thermal fluctuations reduce the anisotropic character of charge diffusion along different directions in the anatase crystal and make similar the rates for electron injection from adsorbate states of different character. The reported results are particularly relevant to the understanding of temperature effects on surface charge separation mechanisms in molecular-based photo-optic devices.