Excited-state Kinetics Research Articles

Comprehensive understanding of multiple resonance thermally activated delayed fluorescence through quantum chemistry calculations

Molecules that exhibit multiple resonance (MR) type thermally activated delayed fluorescence (TADF) are highly efficient electroluminescent materials with narrow emission spectra. Despite their importance in various applications, the emission mechanism is still controversial. Here, a comprehensive understanding of the mechanism for a representative MR-TADF molecule (5,9-diphenyl-5,9-diaza-13b-boranaphtho[3,2,1-de]anthracene, DABNA-1) is presented. Using the equation-of-motion coupled-cluster singles and doubles method and Fermi’s golden rule, we quantitatively reproduced all rate constants relevant to the emission mechanism; prompt and delayed fluorescence, internal conversion (IC), intersystem crossing, and reverse intersystem crossing (RISC). In addition, the photoluminescence quantum yield and its prompt and delayed contributions were quantified by calculating the population kinetics of excited states and the transient photoluminescence decay curve. The calculations also revealed that TADF occurred via a stepwise process of 1) thermally activated IC from the electronically excited lowest triplet state T1 to the second-lowest triplet state T2, 2) RISC from T2 to the lowest excited singlet state S1, and 3) fluorescence from S1.

Prediction of Fluorophore Brightness in Designed Mini Fluorescence Activating Proteins

The de novo computational design of proteins with predefined three-dimensional structure is becoming much more routine due to advancements both in force fields and algorithms. However, creating designs with functions beyond folding is more challenging. In that regard, the recent design of small beta barrel proteins that activate the fluorescence of an exogenous small molecule chromophore (DFHBI) is noteworthy. These proteins, termed mini fluorescence activating proteins (mFAPs), have been shown to increase the brightness of the chromophore more than 100-fold upon binding to the designed ligand pocket. The design process created a large library of variants with different brightness levels but gave no rational explanation for why one variant was brighter than another. Here, we use quantum mechanics and molecular dynamics simulations to investigate how molecular flexibility in the ground and excited states influences brightness. We show that the ability of the protein to resist dihedral angle rotation of the chromophore is critical for predicting brightness. Our simulations suggest that the mFAP/DFHBI complex has a rough energy landscape, requiring extensive ground-state sampling to achieve converged predictions of excited-state kinetics. While computationally demanding, this roughness suggests that mFAP protein function can be enhanced by reshaping the energy landscape toward conformations that better resist DFHBI bond rotation.

Metal-Metal-to-Ligand Charge Transfer in Pt(II) Dimers Bridged by Pyridyl and Quinoline Thiols.

The investigation of two distinct species of square planar dinuclear Pt(II) dimers based on anti-[Pt(C∧N)(μ-N∧S)]2, where C∧N is either 2-phenylpyridine (ppy) or benzo(h)quinoline (bzq) and N∧S is pyridine-2-thiol (pyt), 6-methylpyridine-2-thiol (Mpyt), or 2-quinolinethiol (2QT), is presented. Each molecule was thoroughly characterized with electronic structure calculations, static UV-vis and photoluminescence (PL) spectroscopy, and cyclic voltammetry, along with transient absorbance and time-gated PL experiments. These visible absorbing chromophores feature metal-metal-to-ligand charge-transfer (MMLCT) excited states that originate from intramolecular d8-d8 metal-metal σ-interactions and are manifested in the ground- and excited-state properties of these molecules. All five molecules reported (anti-[Pt(ppy)(μ-Mpyt)]2 could not be isolated), three of which are newly conceived here, possess electronic absorptions past 500 nm and high quantum yield PL emission with spectra extending into the far red (λem > 700 nm), originating from the charge-transfer state in each instance. Each chromophore displays excited-state decay kinetics adequately modeled by single exponentials as recorded using dynamic absorption and PL experiments; each technique yields similar decay kinetics. The combined data illustrate that pyridyl and quinoline-thiolates in conjunction with select cyclometalates represent classes of MMLCT chromophores that exhibit excited-state properties suitable for promoting light-energized chemical reactions and provide a molecular platform suitable for evaluating coherence phenomena in transient metal-metal bond-forming photochemistry.

Naturally Derived Organic Dyes for LED Lightings of High Color Rendering and Fidelity Index

AbstractLight‐emitting diodes (LEDs) are a lighting technology with a huge and ascending market. Typically, LED backlights are often paired with inorganic phosphors made from rare‐earth elements (REEs) to tune the emission lineshapes for different applications. However, REE production is a resource‐intensive process with many negative environmental impacts. Herein organic hybrid LEDs are developed using organic dyes synthesized from an abundant and non‐toxic natural product (theobromine) to replace REE phosphors. The resulted hybrid LED generates continuous emission from 400–740 nm, resulting in a high color rendering index (the current industry standard) of 90 and a color fidelity index (the most advanced and comprehensive standard) of 92, challenging commercial LEDs based on REE phosphors. In addition, the light‐converting composite is made from 99 wt% SBS, an inexpensive industrial polymer, and 1 wt% theobromine dyes, reducing the cost of the light converter to ¢1.30 for a 1 W LED, compared to approximately ¢ 19.2 of commercial products. The light converting efficiency of the dye‐SBS composite is 82%. Excited state kinetics experiments are also conducted to provide guidance to further increase the light‐converting efficiency of the theobromine dyes while maintaining excellent color rendering and fidelity.

Kinetic-Model-Free Analysis of Transient Absorption Spectra Enabled by 2D Correlation Analysis.

Here, we present, to the best of our knowledge for the first time, a systematic study of utilizing 2D correlation analysis in the field of femtosecond transient absorption (fs-TA) spectroscopy. We present that the application of 2D correlation spectroscopy (2DCOS) to fs-TA spectroscopy enables a model-free means to analyze excited state kinetics, which is demonstrated on the model system [(tbbpy)2Ru(dppz)]2+ in different solvents. We show that TA-2DCOS is able to determine the number of processes contributing to the time-resolved spectral changes in fs-TA data sets, as well as extract the spectral response of these components. Overall, the results show that TA-2DCOS leads to the same results as obtained with methods relying on global lifetime analysis or multivariate curve resolution but without the need to specify a predetermined kinetic model. The work presented therefore highlights the potential of TA-2DCOS as a model-free approach for analyzing fs-TA spectral data sets.

Excited-state structural dynamics of nickel complexes probed by optical and X-ray transient absorption spectroscopies: insights and implications.

Excited states of nickel complexes undergo a variety of photochemical processes, such as charge transfer, ligation/deligation, and redox reactions, relevant to solar energy conversion and photocatalysis. The efficiencies of the aforementioned processes are closely coupled to the molecular structures in the ground and excited states. The conventional optical transient absorption spectroscopy has revealed important excited-state pathways and kinetics, but information regarding the metal center, in particular transient structural and electronic properties, remains limited. These deficiencies are addressed by X-ray transient absorption (XTA) spectroscopy, a detailed probe of 3d orbital occupancy, oxidation state and coordination geometry. The examples of excited-state structural dynamics of nickel porphyrin and nickel phthalocyanine have been described from our previous studies with highlights on the unique structural information obtained by XTA spectroscopy. We close by surveying prospective applications of XTA spectroscopy to active areas of Ni-based photocatalysis based on the knowledge gained from our previous studies.

Ultrafast and long-time excited state kinetics of an NIR-emissive vanadium(iii) complex I: synthesis, spectroscopy and static quantum chemistry

In spite of intense, recent research efforts, luminescent transition metal complexes with Earth-abundant metals are still very rare owing to the small ligand field splitting of 3d transition metal complexes and the resulting non-emissive low-energy metal-centered states. Low-energy excited states decay efficiently non-radiatively, so that near-infrared emissive transition metal complexes with 3d transition metals are even more challenging. We report that the heteroleptic pseudo-octahedral d2-vanadium(iii) complex VCl3(ddpd) (ddpd = N,N′-dimethyl-N,N′-dipyridine-2-yl-pyridine-2,6-diamine) shows near-infrared singlet → triplet spin–flip phosphorescence maxima at 1102, 1219 and 1256 nm with a lifetime of 0.5 μs at room temperature. Band splitting, ligand deuteration, excitation energy and temperature effects on the excited state dynamics will be discussed on slow and fast timescales using Raman, static and time-resolved photoluminescence, step-scan FTIR and fs-UV pump-vis probe spectroscopy as well as photolysis experiments in combination with static quantum chemical calculations. These results inform future design strategies for molecular materials of Earth-abundant metal ions exhibiting spin–flip luminescence and photoinduced metal–ligand bond homolysis.

Ultrafast and long-time excited state kinetics of an NIR-emissive vanadium(iii) complex II. Elucidating triplet-to-singlet excited-state dynamics.

We report the non-adiabatic dynamics of VIIICl3(ddpd), a complex based on the Earth-abundant first-row transition metal vanadium with a d2 electronic configuration which is able to emit phosphorescence in solution in the near-infrared spectral region. Trajectory surface-hopping dynamics based on linear vibronic coupling potentials obtained with CASSCF provide molecular-level insights into the intersystem crossing from triplet to singlet metal-centered states. While the majority of the singlet population undergoes back-intersystem crossing to the triplet manifold, 1–2% remains stable during the 10 ps simulation time, enabling the phosphorescence described in Dorn et al. Chem. Sci., 2021, DOI: 10.1039/D1SC02137K. Competing with intersystem crossing, two different relaxation channels via internal conversion through the triplet manifold occur. The nuclear motion that drives the dynamics through the different electronic states corresponds mainly to the increase of all metal–ligand bond distances as well as the decrease of the angles of trans-coordinated ligand atoms. Both motions lead to a decrease in the ligand-field splitting, which stabilizes the interconfigurational excited states populated during the dynamics. Analysis of the electronic character of the states reveals that increasing and stabilizing the singlet population, which in turn can result in enhanced phosphorescence, could be accomplished by further increasing the ligand-field strength.

A Theoretical Framework for Ratiometric Single Ion Luminescent Thermometers—Thermodynamic and Kinetic Guidelines for Optimized Performance

AbstractLuminescence (nano)thermometry is an increasingly important field for remote temperature sensing with high spatial resolution. Most typically, ratiometric sensing of the luminescence emission intensities of two thermally coupled emissive states based on a Boltzmann equilibrium is used to detect the local temperature. Dependent on the temperature range and preferred spectral window, various choices for potential candidates appear possible. Despite extensive experimental research in the field, a universal theory covering the basics of luminescence thermometry is virtually nonexistent. In this manuscript, a general theoretical framework of single ion luminescent thermometers is presented that offers simple, user‐friendly guidelines for both the choice of an appropriate emitter and respective embedding host material for optimum temperature sensing. The results show that the optimum performance (thermal response and sensitivity) around T0 is realized for an energy gap ∆E21 between thermally coupled levels between 2kBT0 and 3.41kBT0. Analysis of the temperature‐dependent excited state kinetics shows that host lattices in which ∆E21 can be bridged by one or two phonons are preferred over hosts in which higher order phonon processes are required. Such a framework is relevant for both a fundamental understanding of luminescent thermometers but also the targeted design of novel and superior luminescent (nano)thermometers.

Investigation of ultrafast multi-photon absorption and optical limiting response in a pyrazine-based N-heteroacene

The ultrafast optical nonlinearity of a pyrazine-based N-heteroacene (Py-0-TQ) is systematically investigated via Z-scan, optical limiting and transient absorption spectrum. Reverse saturable absorption at visible wavelengths is observed. Further study on excited-state kinetics indicates that the strong nonlinear optical (NLO) absorption originates from the cooperating effect between different NLO mechanisms including excited-state absorption (ESA) and two-photon absorption (TPA). Structure-property relation is analyzed, result indicates that the central π-system in Py-0-TQ plays a key role in the cooperating effect via providing a broadband, long-lived ESA and an enhanced TPA as trigger. The experimental and theoretical studies suggest that pyrazine-based N-heteroacene molecules with proper design may have great potential in NLO applications.

Revisiting the Luminescence Decay Kinetics of Energy Transfer Upconversion.

Energy transfer upconversion (ETU) can efficiently upconvert near-infrared photons into higher-energy photons. Although a comprehensive understanding of ETU is fundamental to the design of ETU materials, the basic excited-state decay kinetics of ETU remains a complicated problem. Here we unravel the mechanism underlying ETU decay in benchmark β-NaYF4:Er3+ and β-NaYF4:Ln3+/Yb3+ (Ln = Er, Ho, Tm) ETU microcrystals by combining rate equation analyses with ETU decay measurements. The results show that all of the excited states of one ETU system decay concordantly, with the ETU decay of the emitting state determined by only its intrinsic decay and the product of the ETU decays of the two intermediate states directly responsible for the emitting-state photon upconversion. This general mechanism may serve as a basic rule for excited-state kinetics in upconversion microparticles and nanoparticles, which could provide detailed insight into ETU processes and guide the design of efficient ETU materials.

Influence of Structural Isomerism on the Photophysical Properties of a Series of Donor-Acceptor 1-Naphthalenecarbonitrile Derivatives Possessing Amine Substituents.

In an effort to probe the influence of structural isomerism on the excited-state properties of a naphthalene-based donor-acceptor (D-A) system, four 1-naphthalenecarbonitrile compounds with amine substituents in the 2-, 3-, and 4-positions were synthesized and their photophysical properties were examined. Specifically, the molecules 2-dimethylamino-1-naphthalenecarbonitrile (2DA), 2-(1-piperidinyl)-1-naphthalenecarbonitrile (2P), 3-dimethylamino-1-naphthalenecarbonitrile (3DA), and 4-(1-piperidinyl)-1-naphthalenecarbonitrile (4P) were studied. The substitution position of the amine donor has a significant impact on both the ground-state absorption and excited-state properties of the complexes in toluene solution. The energy, band shape, and extinction coefficient of the ground-state absorption spectra are highly dependent on the substitution position of the amine donor. All of the derivatives exhibit fluorescence at room temperature. The fluorescence observed from 2DA, 2P, and 3DA demonstrates a vibronic structure with all three molecules possessing Stokes shifts on the order of 40 nm, whereas the fluorescence observed from 4P is broad and has a Stokes shift 2 times greater than the other derivatives. The fluorescence lifetimes, fluorescence quantum yields, and intersystem crossing quantum yields vary greatly with the substitution position of the amine donor. 2DA and 2P display intermediate fluorescence lifetimes (2.7 ns) and fluorescence quantum yields (0.20) while possessing the greatest intersystem quantum yield (0.80). 3DA has a much greater fluorescence lifetime (16.9 ns) and fluorescence quantum yield (0.82) at the expense of the intersystem crossing quantum yield (0.12). 4P has the shortest lifetime (0.53 ns), with the lowest fluorescence and intersystem crossing quantum yields (<0.05). The singlet-triplet energy gaps are nearly identical for 2DA, 2P, and 3DA with values on the order of 0.70 eV. This singlet-triplet gap is larger in 4P, with a calculated value of 0.94 eV. The triplet-triplet absorption spectra of 2DA, 2P, and 3DA are similar. Broad peaks in the UV and visible regions with maxima around 330 and 500 nm characterize all three spectra. The triplet excited-state extinction coefficient values for 3DA were found to be 1.5 times larger than those in 2DA and 2P. The triplet-triplet absorption spectrum of 4P is markedly different from the triplet-triplet absorption spectra of the other derivatives. The spectrum is broad, with the four local maxima observed at 374, 445, 624, and 774 nm. All four molecules display delayed fluorescence and laser-power-dependent triplet excited-state decay kinetics, indicating the involvement of triplet-triplet annihilation in the deactivation of the triplet excited states. Both the intrinsic triplet lifetimes and triplet-triplet annihilation rate constants were determined. These values are similar for all of the derivatives with triplet lifetimes on the order of 100 μs and diffusion-controlled rates of triplet-triplet annihilation.

Self-consistent electron energy distribution functions, vibrational distributions, electronic excited state kinetics in reacting microwave CO2 plasma: An advanced model

An advanced model for the calculation of electron energy distribution functions (eedfs), vibrational distributions, and electronic excited state densities of reacting CO2 in microwave (MW) discharges has been developed for clarifying: (1) the role of electronic states of the relevant neutral species in affecting the eedf and (2) the contribution to the CO2 dissociation of the electron impact and heavy particle dissociation mechanisms. To model the discharge, the power density typical of MW discharges is used as a parameter. Different case studies including optically thick and thin plasmas and the dependence of the CO2 dissociation rates on the gas temperature are investigated. The results show that at a low gas temperature, i.e., 300 K, the heavy-particle dissociation mechanism, also called the pure vibrational mechanism, prevails on the electron impact dissociation one, while at a high gas temperature, i.e., 2000 K, the two mechanisms become competitive and the global behavior strongly depends on the choice of electron impact dissociation cross sections. Large differences appear in the eedf, especially in the post-discharge regime, when considering thick and thin plasmas. In the thick case, a well-structured eedf appears as a result of superelastic collisions mainly involving the electronic states of the relevant neutral species. In the thin plasma, many peaks disappear because the concentration of the excited states strongly decreases. Finally, our model gives the results of conversion and energy efficiency as well as vibrational distributions in satisfactory agreement with the corresponding results calculated by the Antwerp group.

Influence of the rate of radiation energy on the charge-carrier kinetics application of all-inorganic CsPbBr3 perovskite nanocrystals.

In the field of optoelectronics, all-inorganic CsPbBr3 perovskite nanocrystals (PNCs) have gained significant interest on account of their superb processability and ultra-high stability among all the counterparts. In this study, we conducted an in-depth analysis of CsPbBr3 PNCs using joint transient optical spectroscopies (time-resolved photoluminescence and ultrafast transient absorption) in a very comprehensive manner. In order to understand the in-depth analysis of excited-state kinetics, the transient absorption spectroscopy has been performed. The structure of interest of CsPbBr3 PNCs was subjected to the rates of the radiation energy of 0.10 mW (κr/κnr = ∼0.62) and 0.30 mW (κr/κnr = ∼0.64). With the rate of radiation energy 0.30 mW, it was observed that there was a significant increase in hot carrier relaxation together with high radiative recombination, resulting in a decrease in charge trappings. Herein, we demonstrate that the tuning of the rate of radiation energies helps to understand the charge-carrier kinetics of CsPbBr3 PNCs, which would thus improve the manufacturing of efficient photovoltaic devices.

NON-THERMAL PLASMA AT ATMOSPHERIC PRESSURE AND ITS OPPORTUNITIES FOR APPLICATIONS

The subject of this review is the low-temperature (or "cold") weakly-ionized but strongly non-equilibrium plasma created at atmospheric pressure in gaseous mixtures or directly in atmospheric air. Cold plasma is rather new, but very perspective object. The strong non-equilibrium of the weakly-ionized plasma leads to that energetic electrons despite their small quantity very effectively excite and dissociate the neutral particles which are contained in surrounding gas, for example, of a molecule of oxygen and water. The pointed above property of cold plasma is valuable from the practical point of view because it allows creating in plasma-forming gas rather intensive ultra-violet radiation and high concentration of physically and biochemically reactive species (metastable atoms and molecules, radicals, ozone, and others) with rather small specific energy consumption. Now the usage of cold plasma at atmospheric pressure gives the opportunity to solve many practical problems which were earlier seeming unsolvable. It is possible even to claim that the approaches based on the use of cold plasma in dense gases define modern progress in many fields of science, biomedicine and, in particular, in the field of chemical technology. The review of modern experimental methods of creation of the cold plasma at atmospheric pressure is given. Physical and chemical features of cold plasma in dense gases have been considered. Special attention is paid to the kinetics of the charged particles in non-equilibrium plasma and the vibrationally excited molecules as well. Additionally, the kinetics of the electronic excited and metastable states is taken into account because they also influence a biochemical activity of low-temperature plasma. A lot of places is given to concrete examples of the modern practical use of such plasma in ecology for the destruction of the low-concentrated harmful organic and inorganic impurities in the exhausted airflows at atmospheric pressure.

Optical spectroscopy and luminescence properties of a Tm3+-doped LiKB4O7 glass

Spectroscopic properties of the Tm-doped borate glass with LiKB4O7 (0.5Li2O–0.5K2O–2B2O3) basic composition are investigated and analysed using optical absorption and photoluminescence (excitation, emission, decay kinetics) experimental techniques as well as Judd–Ofelt analysis. Optical absorption spectra exhibit absorption bands related with transitions of Tm3+ ions. The theoretical and experimental oscillator strengths for observed absorption transitions as well as Judd–Ofelt intensity parameters (Ω2, Ω4, Ω6) have been calculated. Photoluminescence spectra reveal a lot of emission bands in the visible and infrared spectral ranges related with 4f – 4f transitions of Tm3+ ions. Luminescence decay kinetics of several excited states was detailed investigated and analysed. Radiative properties such as radiative transition probabilities (Arad), luminescence branching ratios (β), radiative lifetimes (τrad), quantum efficiency (η), and stimulated emission cross-section (σem) have been estimated. The obtained results show that the investigated LiKB4O7:Tm glass belongs to perspective luminescent and laser materials in the visible and infrared spectral ranges.

Protein fibrillogenesis model tracked by its intrinsic time-resolved emission spectra

The excited-state kinetics of the fluorescence of tyrosine in a de novo protein fibrillogenesis model was investigated as a potential tool for monitoring protein fibre formation and complexation with glucose (glycation). In stark contrast to insulin the time-resolved emission spectra (TRES) recorded over the period of 700 hours in buffered solutions of the model with and without glucose revealed no apparent changes in Tyr fluorescence responses. This indicates the stability of the model and provides a measurement-supported basis for its use as a reference material in fluorescence studies of protein aggregation.

Structure and Function of Tryptophan-Tyrosine Dyads in Biomimetic β Hairpins.

Tyrosine–tryptophan (YW) dyads are ubiquitous structural motifs in enzymes and play roles in proton-coupled electron transfer (PCET) and, possibly, protection from oxidative stress. Here, we describe the function of YW dyads in de novo designed 18-mer, β hairpins. In Peptide M, a YW dyad is formed between W14 and Y5. A UV hypochromic effect and an excitonic Cotton signal are observed, in addition to singlet, excited state (W*) and fluorescence emission spectral shifts. In a second Peptide, Peptide MW, a Y5–W13 dyad is formed diagonally across the strand and distorts the backbone. On a picosecond timescale, the W* excited-state decay kinetics are similar in all peptides but are accelerated relative to amino acids in solution. In Peptide MW, the W* spectrum is consistent with increased conformational flexibility. In Peptide M and MW, the electron paramagnetic resonance spectra obtained after UV photolysis are characteristic of tyrosine and tryptophan radicals at 160 K. Notably, at pH 9, the radical photolysis yield is decreased in Peptide M and MW, compared to that in a tyrosine and tryptophan mixture. This protective effect is not observed at pH 11 and is not observed in peptides containing a tryptophan–histidine dyad or tryptophan alone. The YW dyad protective effect is attributed to an increase in the radical recombination rate. This increase in rate can be facilitated by hydrogen-bonding interactions, which lower the barrier for the PCET reaction at pH 9. These results suggest that the YW dyad structural motif promotes radical quenching under conditions of reactive oxygen stress.

Dynamics of flavin containing radical pairs in SDS micellar media probed by static and pulse magnetic field effect and pulse ADMR

ABSTRACTThe dynamics of the radical pairs generated from the electron transfer reaction from indole derivatives to flavin derivatives are studied by three techniques, time resolved magnetic field effect (TR-MARY) and pulse absorption detected magnetic resonance (ADMR), and absorption detected switching of external magnetic field techniques (AD-SEMF). The three techniques complementarily work for the precise analysis of the radical pair kinetics. The results by all three techniques reflect the difference of the hydrophobic nature of the radicals in the lifetime of RP in a micelle. Overcoming the difficulty of the determination of the short RP lifetime under the nearly quasi steady state due to slow and inhomogeneous decay kinetics of the precursor triplet excited state, AD-SEMF analysis enabled us to determine the kinetic parameters, which is consistent with the qualitatively observed by the other techniques.

On the origin of spatially dependent electronic excited-state dynamics in mixed hybrid perovskite thin films

The fundamental photophysics underlying the remarkable performance of organic-inorganic hybrid perovskites in optoelectronic device applications has been increasingly studied using complementary spectroscopic techniques. However, the spatially heterogeneous polycrystalline morphology of the solution-processed thin films is often overlooked in conventional ensemble measurements and therefore the reported results are averaged over hundreds or even thousands of nano- and micro-crystalline grains. Here, we apply femtosecond transient absorption microscopy to spatially and temporally probe ultrafast electronic excited-state dynamics in chloride containing mixed lead halide perovskite (CH3NH3PbI3–xClx) thin films. We found that the electronic excited-state relaxation kinetics are extremely sensitive to the spatial location probed, which was manifested by position-dependent transient absorption signal amplitude and decay behaviour, along with an obvious rise component at some positions. The analysis of transient absorption kinetics acquired at several distinct spatial positions enabled us to identify Auger recombination as the dominant mechanism underlying the initial portions of the spatially dependent dynamics with variable rate constants. The different rates observed suggest occurrence of distinct local electronic structures and variable contributions from impurities/defects and phonons in this nonlinear dynamical process.