Time-dependent Spectral Shift Research Articles

Laplace Transform Fitting as a Tool To Uncover Distributions of Reverse Intersystem Crossing Rates in TADF Systems

Donor–acceptor (D–A) thermally activated delayed fluorescence (TADF) molecules are exquisitely sensitive to D–A dihedral angle. Although commonly simplified to an average value, these D–A angles nonetheless exist as distributions across the individual molecules embedded in films. The presence of these angle distributions translates to distributions in the rates of reverse intersystem crossing (krISC), observed as time dependent spectral shifts and multiexponential components in the emission decay, which are difficult to directly quantify. Here we apply inverse Laplace transform fitting of delayed fluorescence to directly reveal these distributions. Rather than a single average value, the crucial krISC rate is instead extracted as a density of rates. The modes and widths of these distributions vary with temperature, host environment, and intrinsic D–A torsional rigidity of different TADF molecules. This method gives new insights and deeper understanding of TADF host–guest interactions, as well as verifies future design strategies that target D–A bond rigidity.

Studies of Ultrafast Molecular Dynamics by Femtosecond Extreme Ultraviolet Absorption Spectroscopy

Transient absorption spectroscopy is one of the most widely used experimental techniques for time-resolved studies of ultrafast phenomena. Recent advances in the generation of short-wavelength pulses of ultrashort duration have made transient absorption probing of atomic core level transitions possible. In this Highlight Review, we introduce our recent work on using extreme ultraviolet (XUV) transient absorption spectroscopy to investigate the ultrafast molecular dynamics induced by intense laser fields. While investigations of intense laser-molecule interaction have primarily focused on elucidating electron dynamics, it is important to understand the vibrational dynamics that are triggered by intense laser fields because of their coupling to electron dynamics. Probing the iodine 4d core level, our XUV transient absorption studies of iodoalkanes irradiated by intense laser fields elucidate ultrafast dissociative ionization dynamics and vibrational wave packet dynamics. Time-dependent XUV spectral shifts and global analysis of time-resolved XUV transient absorption spectra can be used to reconstruct C–I dissociation dynamics and identify multiple dissociation channels. In addition, XUV transient absorption spectroscopy also unravels both small- and large-amplitude coherent vibrational motion, whose origin can be identified by analyzing the phase of the oscillatory XUV transition energies. Our studies show that XUV transient absorption spectroscopy is a powerful technique for the study of ultrafast molecular dynamics.

Residence and diffusion of a dynamically prototropic hydration probe in AOT reverse micelles

We investigated the hydration dynamics in the nanosized water pool of an Aerosol-OT (AOT) reverse micelle (RM) using a prototropic (proton-donating) fluorescent dye, N-methyl-7-hydroxyquinolinium (NM7HQ+), as the probe. NM7HQ+ is a photoacid, and the deprotonation of its excited-state cationic form (C*) gives rise to a neutral keto species (K*). In the present study, the residence sites of C* and K* in water pools of various sizes were determined by monitoring their time-resolved fluorescence anisotropy. Additionally, the evolutions of the C* and K* bands in the time-resolved fluorescence spectra were analyzed by fitting the spectra to the sum of two lognormal peak functions. The time-dependent spectral shifts of the species with different formal charges revealed the characteristic hydration behavior at the interface of the anionic AOT headgroups and the bound water region, as well as the rotational and translational diffusion of the probe within the RM.

Controlling opto-electronic characteristics of ternary II–VI alloyed quantum dots: alcohol processing assay

For their ultra-wide color gamut, high efficiency, robustness, and solution processability, Cd-based alloy semiconductor quantum dots (AQDs) continue to proliferate by driving innovations in the fields of optoelectronics, photovoltaics, multiplex bio-imaging, and cancer research. Herein, non-toxic, low-cost isopropyl alcohol vapor-based oxidative treatment protocol is developed and applied to tune the light emission spectrum of crystalline core–shell CdSe1−xSx/ZnS quantum dots. As evidenced by the results of structural investigations, these AQDs when exposed to vapors produced ultrasonically from 10:1 isopropyl alcohol-to-water mix undergo an isotropic, diameter non-specific size reduction at the rate of ∼1.3 Å min−1. Nonlinear time-dependent spectral shifts, revealed experimentally, are consistent with the results of the effective-mass approximation treatment. The emission yields are seen to undergo an initial drop, yet to plateau as the etch time increases. The study opens a door to a soft, top-down monotonic tailoring of the light emission characteristics and opto-electronic response of stoichiometrically- and hierarchically-complex core–shell constructs in technologically-viable group II–VI nano-semiconductors as well as AQD-based catalytic conversion of organic compounds.

Spectral recording of gene expression history by fluorescent timer protein.

Monitoring spatio-temporal patterns of gene expression by fluorescent proteins requires longitudinal observation, which is often difficult to implement. Here, we fuse a fluorescent timer (FT) protein with an immediate early gene (IEG) promoter to track live gene expression in single cells. This results in a stimulus- and time-dependent spectral shift from blue to red for subsequent monitoring with fluorescence activated cell sorting (FACS) and live cell imaging. This spectral shift enables imputing the time point of activity post-hoc to dissociate early and late responders from a single snapshot in time. Thus, we provide a tool for tracking stimulus-driven IEG expression and demonstrate proof of concept exploiting promoter::FT fusions, adding new dimensions to experiments that require reconstructing spatio-temporal patterns of gene expression in cells, tissues or living organisms.

Application of decay- and evolution-associated spectra for molecular systems with spectral shifts or inherent inhomogeneities

Decay- and evolution-associated spectra (DAS and EAS) are among the most popular methods for global analysis of time-resolved spectroscopy data. To interpret the obtained results in terms of a physical model, the system under consideration must satisfy several assumptions—spectral and temporal independence and lack of intrinsic disorder. In this work we study possible applicability of the DAS/EAS analysis for the time-resolved fluorescence spectra of several model systems, which fail to satisfy the required assumptions. We show that time-dependent spectral shifts can be distinguished by systematic comparison of DAS/EAS with different number of compartments. Somewhat unexpectedly, we demonstrate that DAS/EAS are rather robust for small systems even when they possess inherent disorder of excitation energy transfer or relaxation rates. In the case of larger molecular aggregates, the interpretation of DAS/EAS becomes much more ambiguous as the obtained results cannot be easily related to a physical kinetic and/or structural model.

Similarity of Coomassie Dye Spectral Absorbance Dynamic of Sequentially Distant Polymeric N‐Terminal Segments of Glycine and GABA Transporters

AbstractClassical interaction of Coomassie dye with proteins stabilizes its blue dye form, resulting in 595 nm absorbance peak used in protein quantitation. We previously showed that a dynamic Coomassie interaction mode might exist in certain disordered proteins, resulting in time dependent spectral absorbance shifts, blocked by the presence of protein secondary structure. To separate unstructured GlyT1a N‐terminal peptide from transporter ordered regions and regain its sufficient affinity to Coomassie dye, we previously prepared polymeric forms of GlyT1aN peptide using special stepwise cloning technique. In this work we applied identical technique to create polymeric regions derived from the N‐termini of neurotransmitter transporters rGlyT1b, rGAT3 and rGAT1. Polymeric proteins with low secondary structure content showed very similar dynamic spectral absorbance signatures, when complexed with Coomassie dye. Diversity of their primary amino acid sequences indicates that previously not recognized microstructural similarity between them might be potentially investigated by their interaction with Coomassie Brilliant Blue G‐250.

Photoacidic and Photobasic Behavior of Transition Metal Compounds with Carboxylic Acid Group(s).

Excited state proton transfer studies of six Ru polypyridyl compounds with carboxylic acid/carboxylate group(s) revealed that some were photoacids and some were photobases. The compounds [Ru(II)(btfmb)2(LL)](2+), [Ru(II)(dtb)2(LL)](2+), and [Ru(II)(bpy)2(LL)](2+), where bpy is 2,2'-bipyridine, btfmb is 4,4'-(CF3)2-bpy, and dtb is 4,4'-((CH3)3C)2-bpy, and LL is either dcb = 4,4'-(CO2H)2-bpy or mcb = 4-(CO2H),4'-(CO2Et)-2,2'-bpy, were synthesized and characterized. The compounds exhibited intense metal-to-ligand charge-transfer (MLCT) absorption bands in the visible region and room temperature photoluminescence (PL) with long τ > 100 ns excited state lifetimes. The mcb compounds had very similar ground state pKa's of 2.31 ± 0.07, and their characterization enabled accurate determination of the two pKa values for the commonly utilized dcb ligand, pKa1 = 2.1 ± 0.1 and pKa2 = 3.0 ± 0.2. Compounds with the btfmb ligand were photoacidic, and the other compounds were photobasic. Transient absorption spectra indicated that btfmb compounds displayed a [Ru(III)(btfmb(-))L2](2+)* localized excited state and a [Ru(III)(dcb(-))L2](2+)* formulation for all the other excited states. Time dependent PL spectral shifts provided the first kinetic data for excited state proton transfer in a transition metal compound. PL titrations, thermochemical cycles, and kinetic analysis (for the mcb compounds) provided self-consistent pKa* values. The ability to make a single ionizable group photobasic or photoacidic through ligand design was unprecedented and was understood based on the orientation of the lowest-lying MLCT excited state dipole relative to the ligand that contained the carboxylic acid group(s).

Monitoring Ultrafast Chemical Dynamics by Time-Domain X-ray Photo- and Auger-Electron Spectroscopy.

The directed flow of charge and energy is at the heart of all chemical processes. Extraordinary efforts are underway to monitor and understand the concerted motion of electrons and nuclei with ever increasing spatial and temporal sensitivity. The element specificity, chemical sensitivity, and temporal resolution of ultrafast X-ray spectroscopy techniques hold great promise to provide new insight into the fundamental interactions underlying chemical dynamics in systems ranging from isolated molecules to application-like devices. Here, we focus on the potential of ultrafast X-ray spectroscopy techniques based on the detection of photo- and Auger electrons to provide new fundamental insight into photochemical processes of systems with various degrees of complexity. Isolated nucleobases provide an excellent testing ground for our most fundamental understanding of intramolecular coupling between electrons and nuclei beyond the traditionally applied Born-Oppenheimer approximation. Ultrafast electronic relaxation dynamics enabled by the breakdown of this approximation is the major component of the nucleobase photoprotection mechanisms. Transient X-ray induced Auger electron spectroscopy on photoexcited thymine molecules provides atomic-site specific details of the extremely efficient coupling that converts potentially bond changing ultraviolet photon energy into benign heat. In particular, the time-dependent spectral shift of a specific Auger band is sensitive to the length of a single bond within the molecule. The X-ray induced Auger transients show evidence for an electronic transition out of the initially excited state within only ∼200 fs in contrast to theoretically predicted picosecond population trapping behind a reaction barrier. Photoinduced charge transfer dynamics between transition metal complexes and semiconductor nanostructures are of central importance for many emerging energy and climate relevant technologies. Numerous demonstrations of photovoltaic and photocatalytic activity have been performed based on the combination of strong light absorption in dye molecules with charge separation and transport in adjacent semiconductor nanostructures. However, a fundamental understanding of the enabling and limiting dynamics on critical atomic length- and time scales is often still lacking. Femtosecond time-resolved X-ray photoelectron spectroscopy is employed to gain a better understanding of a short-lived intermediate that may be linked to the unexpectedly limited performance of ZnO based dye-sensitized solar cells by delaying the generation of free charge carriers. The transient spectra strongly suggest that photoexcited dye molecules attached to ZnO nanocrystals inject their charges into the substrate within less than 1 ps but the electrons are then temporarily trapped at the surface of the semiconductor in direct vicinity of the injecting molecules. The experiments are extended to monitor the electronic response of the semiconductor substrate to the collective injection from a monolayer of dye molecules and the subsequent electron-ion recombination dynamics. The results indicate some qualitative similarities but quantitative differences between the recombination dynamics at molecule-semiconductor interfaces and previously studied bulk-surface electron-hole recombination dynamics in photoexcited semiconductors.

Non-equilibrium reaction and relaxation dynamics in a strongly interacting explicit solvent: F + CD3CN treated with a parallel multi-state EVB model.

We describe a parallelized linear-scaling computational framework developed to implement arbitrarily large multi-state empirical valence bond (MS-EVB) calculations within CHARMM and TINKER. Forces are obtained using the Hellmann-Feynman relationship, giving continuous gradients, and good energy conservation. Utilizing multi-dimensional Gaussian coupling elements fit to explicitly correlated coupled cluster theory, we built a 64-state MS-EVB model designed to study the F + CD3CN → DF + CD2CN reaction in CD3CN solvent (recently reported in Dunning et al. [Science 347(6221), 530 (2015)]). This approach allows us to build a reactive potential energy surface whose balanced accuracy and efficiency considerably surpass what we could achieve otherwise. We ran molecular dynamics simulations to examine a range of observables which follow in the wake of the reactive event: energy deposition in the nascent reaction products, vibrational relaxation rates of excited DF in CD3CN solvent, equilibrium power spectra of DF in CD3CN, and time dependent spectral shifts associated with relaxation of the nascent DF. Many of our results are in good agreement with time-resolved experimental observations, providing evidence for the accuracy of our MS-EVB framework in treating both the solute and solute/solvent interactions. The simulations provide additional insight into the dynamics at sub-picosecond time scales that are difficult to resolve experimentally. In particular, the simulations show that (immediately following deuterium abstraction) the nascent DF finds itself in a non-equilibrium regime in two different respects: (1) it is highly vibrationally excited, with ∼23 kcal mol(-1) localized in the stretch and (2) its post-reaction solvation environment, in which it is not yet hydrogen-bonded to CD3CN solvent molecules, is intermediate between the non-interacting gas-phase limit and the solution-phase equilibrium limit. Vibrational relaxation of the nascent DF results in a spectral blue shift, while relaxation of the post-reaction solvation environment results in a red shift. These two competing effects mean that the post-reaction relaxation profile is distinct from what is observed when Franck-Condon vibrational excitation of DF occurs within a microsolvation environment initially at equilibrium. Our conclusions, along with the theoretical and parallel software framework presented in this paper, should be more broadly applicable to a range of complex reactive systems.

On the interpretation of decay associated spectra in the presence of time dependent spectral shifts

Time dependent spectral shifts caused by energetic relaxation processes frequently contribute to time resolved spectral data. They make a global analysis of transient spectra challenging, since the basic assumption of separability of time and frequency dependence does not hold. We compare the global analysis results of synthetic transient spectra to a Taylor expansion of their spectral shape at infinite time. Thereby we can elucidate the correlation of time constants and decay associated spectra resulting from global analysis with the underlying shift dynamics. The obtained insights are applied to transient absorption spectra of Coumarin 152 and its solvation dynamics.

A time resolved fluorescence and quantum chemical study of the solar cell sensitizer D149

The steady-state spectroscopic properties and ultrafast relaxation dynamics of the very promising indoline-based sensitizer dye D149 are reported in various organic solvents. For the study of relaxation dynamics of both S2 and S1 states, time resolved femtosecond upconversion spectroscopy was used. Quantum chemical calculations of excitation energies and dipole moments at the DFT level as well as using high-level correlated second-order approximate coupled-cluster model with singles and doubles (CC2) have been also performed. The difference between the first excited and ground state dipole moments was found experimentally to be ∼13D in agreement with calculations. According to our time resolved results, the population of the S2 state has shown a rapid internal conversion to S1, taking place within 0.15–0.45 ps, depending on the solvent. This is followed by a slower decay component indicating that some S2 species relax on a long timescale. As the detection is shifted to the transition from the S1 state, the overall decay dynamics is retarded, while a slow fluorescence rise is observed at long wavelengths, indicating a time-dependent spectral shift. The lifetime of S1 was found to depend on the solvent used, being 630 ps in toluene, 720 ps in chloroform, 670 ps in THF, 700 ps in dichloromethane, 220 ps in acetonitrile and 190 ps in EtOH. The time resolved emission spectra as well as the solvent response function, C(t), obtained in the aprotic solvents, have shown that as the solvent polarity increases a more rapid spectral shift is observed. An exception is found in the case of the protic solvent EtOH due to specific H-bonding effects. The time resolved emission spectra in all solvents experience no significant change of shape and width with time, except from THF, where the initially formed Intramolecular Charge Transfer state is observed at early times.

Existence of a New Emitting Singlet State of Proflavine: Femtosecond Dynamics of the Excited State Processes and Quantum Chemical Studies in Different Solvents

Proflavine (3,6-diaminoacridine) shows fluorescence emission with lifetime, 4.6 ± 0.2 ns, in all the solvents irrespective of the solvent polarity. To understand this unusual photophysical property, investigations were carried out using steady state and time-resolved fluorescence spectroscopy in the pico- and femtosecond time domain. Molecular geometries in the ground and low-lying excited states of proflavine were examined by complete structural optimization using ab initio quantum chemical computations at HF/6-311++G** and CIS/6-311++G** levels. Time dependent density functional theory (TDDFT) calculations were performed to study the excitation energies in the low-lying excited states. The steady state absorption and emission spectral details of proflavine are found to be influenced by solvents. The femtosecond fluorescence decay of the proflavine in all the solvents follows triexponential function with two ultrafast decay components (τ(1) and τ(2)) in addition to the nanosecond component. The ultrafast decay component, τ(1), is attributed to the solvation dynamics of the particular solvent used. The second ultrafast decay component, τ(2), is found to vary from 50 to 215 ps depending upon the solvent. The amplitudes of the ultrafast decay components vary with the wavelength and show time dependent spectral shift in the emission maximum. The observation is interpreted that the time dependent spectral shift is not only due to solvation dynamics but also due to the existence of more than one emitting state of proflavine in the solvent used. Time resolved area normalized emission spectral (TRANES) analysis shows an isoemissive point, indicating the presence of two emitting states in homogeneous solution. Detailed femtosecond fluorescence decay analysis allows us to isolate the two independent emitting components of the close lying singlet states. The CIS and TDDFT calculations also support the existence of the close lying emitting states. The near constant lifetime observed for proflavine in different solvents is suggested to be due to the similar dipole moments of the ground and the evolved emitting singlet state of the dye from the Franck-Condon excited state.

TDSS in Trp Fluorescence Reveals Multiple Protein and Solvent Relaxation Modes

The Time-Dependent Spectral Shifts (TDSS) in the fluorescence of solvatochromic dyes in polar solvents report solvent relaxation dynamics, which in water occurs on the femtosecond timescale. The TDSS in the emission of tryptophan and other solvatochomic fluorophores in proteins span a range of timescales from femtoseconds to nanoseconds. MD simulations of the GB1 protein in TIP3P water made it possible to separate five relaxation modes and to explain their physical origins. Two of these relaxation modes also contribute to the TDSS of dyes in polar solvents. The ultrafast relaxation mode (τ∼35fs) is due in part to the librational relaxation of water molecules and in part to the small adjustments in the local protein structure. This mode is responsible for about half of the total TDSS amplitude. Two collective rotational relaxation modes of water molecules are known. The longitudinal (τL∼550fs) mode contributes to the TDSS of both dyes in water and tryptophan residues in proteins. The transverse (τD∼8.3ps) relaxation cannot contribute to the TDSS of dyes in water, but it contributes to the TDSS in proteins having internal water channels or pockets. This can be used to study internal water. In MD simulations using TIP3P water both τL and τD are ∼30% shorter than the experimental values. A small (<0.25Å) adjustment of the GB1 tertiary structure occurs in MD simulations on the time scale of 130ps and results in a 140cm−1 contribution to the TDSS. A shift in the sidechain conformation of Glu-42 in close proximity to the fluorophore (Trp-43) is the main contributor to the slow TDSS amplitude (470cm−1). This conformational change takes 2.6ns in MD simulations and only 80ps in the experiment, which reveals that in CHARMM22 the potential barriers separating sidechain conformations are too high.

Picosecond Protein Dynamics: The Origin of the Time-Dependent Spectral Shift in the Fluorescence of the Single Trp in the Protein GB1

How a biological system responds to a charge shift is a challenging question directly relevant to biological function. Time-resolved fluorescence of a tryptophan residue reflects protein and solvent response to the difference in pi-electron density between the excited and the ground state. In this study we use molecular dynamics to calculate the time-dependent spectral shift (TDSS) in the fluorescence of Trp-43 in GB1 protein. A new computational method for separating solvent, protein, and fluorophore contributions to TDSS is applied to 100 nonequilibrium trajectories for GB1 in TIP3P water. The results support several nontrivial conclusions. Both longitudinal and transverse relaxation modes of bulk solvent contribute to the TDSS in proteins. All relaxation components slower than the transverse relaxation of bulk solvent have significant contributions from both protein and solvent, with a negative correlation between them. Five exponential terms in the TDSS of GB1 are well separated by their relaxation times. A 0.036 ps term is due to both solvent (60%) and protein (40%). Two exponential terms represent longitudinal (tau(L) approximately = 0.4 ps) and transverse (tau(D) approximately = 5.6 ps) relaxation modes of TIP3P water. A 131 ps term is attributable to a small change in the tertiary structure, with the alpha-helix moving 0.2 A away from the beta-strand containing Trp-43. A 2580 ps term is due to the change in the conformation of the Glu-42 side chain that brings its carboxyl group close to the positively charged end of the excited fluorophore. Interestingly, water cancels 60% of the TDSS resulting from this conformational change.

Dynamics of polarized photoluminescence in m-plane InGaN/GaN quantum wells

Spectrally-, polarization-, and time-resolved photoluminescence (PL) experiments have been performed on 2.5 nm thick m-plane single InGaN quantum wells. It has been found that PL decay is mainly determined by nonradiative recombination through several types of recombination centers, while PL rise is largely affected by exciton transfer into localization minima. Prolonged PL rise times and time-dependent spectral shift were used to study exciton transfer into the localization centers. Characteristic time of the exciton transfer is 80–100 ps at lower temperatures and about 50 ps at room temperature, which corresponds to the exciton diffusion length of 200–500 nm. Degree of PL linear polarization was found to decrease at a similar rate. Decreased PL polarization for the localized excitons suggests that the localization centers are related to areas with relaxed strain.

Internal Dynamics and Energy Transfer in Dansylated POPAM Dendrimers and Their Eosin Complexes

Internal dynamics of dansylated poly(propyleneamine) dendrimers (POPAM, G1-G4) in solution and excitation energy transfer from dansyls to eosin in POPAM-eosin complexes have been studied by time-resolved fluorescence spectroscopy and molecular dynamics (MD) simulations. Combining the results from fluorescence anisotropy and the MD simulation studies suggests three time domains for the internal dynamics of the G3 and G4 generations, about 60 ps for motions of the outer-sphere dansyls, 500-1000 ps for restricted motions of back-folded dansyls, and 1500-2600 ps for the overall rotation. For the smaller generations, the contribution from the restricted motions was not entirely evident. Eosin binding hinders fast rotation of the dansyl fragments in the largest G4 dendrimer, but the motion of back-folded dansyls is more hindered in the pure dendrimer. Both fluorescence anisotropy and MD results for the G4 dendrimer support the "soft" dendrimer picture with almost free mobility and substantial back-folding of the dansyls of the dendrimers in solution. Analysis of time-dependent spectral shifts of fluorescence reveals 20-30 ps excited-state solvation relaxation around a single dansyl of a dendrimer. Dendrimer-independent excitation energy transfer from 4 to 8 ps from dansyls to eosins in POPAM-eosin complexes G2-G4 was observed.

Fluorescence Decay Characteristics of Indole Compounds Revealed by Time-Resolved Area-Normalized Emission Spectroscopy

Time-resolved fluorescence spectroscopy of tryptophan residue has been extensively applied to the studies on structure-function relationships of protein. Regardless of this, the fluorescence decay mechanism and kinetics of tryptophan residue in many proteins still remains unclear. Previous studies have demonstrated that conformational heterogeneity and relaxation dynamics are both involved in the peculiar multiexponential decay kinetics in subnanosecond resolution. In the present study, we characterized the fluorescence decay property of six indole compounds in glycerol by resolving the contribution of conformational heterogeneity and relaxation dynamics. We applied the time-resolved area-normalized fluorescence emission spectrum (TRANES) method for the fluorescence decay analysis. The results of TRANES, time-dependent shift of fluorescence spectral center of gravity, and fluorescence decay simulation demonstrated that the dielectric relaxation process independent of intrinsic rotamer/conformer and the individual fluorescence lifetime gives the peculiarity to the fluorescence decay of indole compounds. These results confirmed that TRANES and time-dependent spectral shift analysis are potent methods to resolve the origin of multiexponential decay kinetics of tryptophyl fluorescence in protein.

Direct observation of time-dependent photoluminescence spectral shift in CdS nanoparticles synthesized in polymer solutions

Direct observation of time-resolved emission spectra (TRESs) of cadmium sulfide nanoparticles in polymer solutions was carried out with picosecond resolution using a streak camera. The TRESs were found to undergo a pronounced time-dependent Stokes shift, eventually coinciding with the steady-state photoluminescence spectra within an approximately 40 ns delay time. Moreover, approximately 90% of the shift was complete within the first 1 ns after excitation, in contrast to the fact that overall photoluminescence involves very long time constants of 10-100 ns. The observed Stokes shift dynamics was very similar in CdS nanoparticles stabilized in two very different types of polymer solutions. Thus the solvent and/or polymeric stabilizer appeared to have a minimal effect on the shift. We propose that the relaxation proceeds through an internal mechanism involving the fast decay of high-energy traps into relatively slow-decaying low-energy traps. Time-dependent photoluminescence anisotropy experiments also revealed an approximately 1 ns decay component appearing only in the higher-energy end of the photoluminescence spectrum. Because this time constant is too short to represent rotational diffusion of the nanometer-sized particles, it was associated with the rapid relaxation of the high-energy trap states.

Ultrafast Energy Transfer and Structural Dynamics in DNA

Ultrafast structural dynamics concomitant to excitation energy transfer in DNA has been studied using a pair of pyrene-labeled DNA bases. The temporal evolution of the femtosecond pump-probe spectra reveals the existence of two electronic coupling pathways, through-base stack and through-space, which lead to excitation energy transfer and excimer formation even when the labeled DNA bases are separated by one AT base pair. The electronic coupling which mediates through-base stack energy transfer is so strong that a new absorption band arises in the excited-state absorption spectrum within 300 fs. From the analysis of time-dependent spectral shifts due to through-space excimer formation, the local structural dynamics and flexibility of DNA are characterized on the picosecond and nanosecond time scale.