Time-resolved Polarization Spectroscopy Research Articles

Time-resolved ultrafast transient polarization spectroscopy to investigate nonlinear processes and dynamics in electronically excited molecules on the femtosecond time scale

We report a novel experimental technique to investigate ultrafast dynamics in photoexcited molecules by probing the 3rd-order nonlinear optical susceptibility. A non-collinear 3-pulse scheme is developed to probe the ultrafast dynamics of excited electronic states using the optical Kerr effect. Optical homodyne and optical heterodyne detections are demonstrated to measure the 3rd-order nonlinear optical response for the S1 excited state of liquid nitrobenzene, which is populated by 2-photon absorption of a 780 nm 40 fs excitation pulse.

Cavity effect on a biexciton in a CuCl microcavity

We studied the population and coherence dynamics of a biexciton in a planar CuCl microcavity. We developed a time-resolved two-photon polarization spectroscopy technique, by which we extracted the population dynamics of the biexciton even when the biexciton was spectrally overlapped with the lower cavity polariton. The observed lifetime of the cavity biexciton in the weak-coupling regime was much shorter than that of a bare CuCl thin film without a cavity, which directly demonstrates cavity enhancement of the radiative decay rate of the biexciton. The cavity enhancement originates from the hybridization of the unbound two-cavity-polariton state to the biexciton wave function by the biexciton-cavity coupling. By comparing the population and coherence decay curves of the biexciton, we found that the coherence of the cavity biexciton was limited by the radiative population decay.

Decoding Protein Plasticity from Single Molecules to Large Complexes

Intrinsically disordered and phenylalanine-glycine rich nucleoporins (FG-Nups) form a selective permeability barrier inside the nuclear pore complex (NPC): Large molecules can only cross the central channel of the NPC when piggybacked by nuclear transport receptors (NTRs) that specifically interact with FG-Nups. These FG-Nups, however, display complex and non-random amino acid architecture and possess repeatedly occurring FG-motifs flanked by distinct amino acid stretches. How such heterogeneous sequence composition relates to function and how homotypic interactions between such disordered stretches, and transient heterotypic interactions with folded transport receptors could give rise to a transport mechanism is still unclear. To address this challenge we developed a combined fluorescence correlation and time resolved polarization spectroscopy approach to study the binding properties of the IDP nucleoporin153 (Nup153) to NTRs. The detection of segmental backbone mobility of Nup153 within the unperturbed complex provides a readout of local, region specific, binding properties that are usually masked in measurements of the whole IDP. Binding affinity of functionally and structurally diverse NTRs to distinct regions of Nup153 differed by orders of magnitudes - a result with implications for the diversity of transport routes in nucleocytoplasmic transport. Furthermore, synergistic molecular dynamics simulations permitted visualization of previously unknown steps and binding modes during Nup•NTR interactions at atomic resolution. These results have molecular implications for the diversity of transport routes within nucleocytoplasmic transport and on how nuclear transport can pursue specifically and very fast inside the nuclear pore complex.

Mapping multivalency and differential affinities within large intrinsically disordered protein complexes with segmental motion analysis.

Intrinsically disordered proteins (IDPs) can bind to multiple interaction partners. Numerous binding regions in the IDP that act in concert through complex cooperative effects facilitate such interactions, but complicate studying IDP complexes. To address this challenge we developed a combined fluorescence correlation and time-resolved polarization spectroscopy approach to study the binding properties of the IDP nucleoporin153 (Nup153) to nuclear transport receptors (NTRs). The detection of segmental backbone mobility of Nup153 within the unperturbed complex provided a readout of local, region-specific binding properties that are usually masked in measurements of the whole IDP. The binding affinities of functionally and structurally diverse NTRs to distinct regions of Nup153 can differ by orders of magnitudes-a result with implications for the diversity of transport routes in nucleocytoplasmic transport.

Two-photon-excitation fluorescence depolarization in solutions and nano-scale-organized (macro)molecular media: Application of the wide-angular detection-aperture technique

We here discuss a problem of time-resolved fluorescence polarization spectroscopy at two-photon excitations of macroscopically isotropic molecular media, that can be organized locally on a nano-scale. The simplifying assumptions on the hydrodynamical shape of fluorophores and on the form of the two-photon absorption tensor, are considered and displayed on the corresponding examples of the emission anisotropy synthetic decays. These assumptions reduce the number of the model parameters making the theoretical description of the discussed spectroscopic technique applicable in the experimental practice. Because the two-photon excitations of fluorescence require the application of the exciting light of appropriately high intensity, and that may have a negative influence on the quality of the experimental data collected, we discuss the application of the wide-angular detection-aperture technique in such experiments. Furthermore, we consider the application of the symmetry adapted callibration (SAC) method enabling one to accurately analyze the experimental data even if the collecting optics employed is not ideal and is affected by several unexpected technical imperfections that are hard to be accounted for analytically.

Singlet/Triplet Reversal in Strongly-Coupled GaSe Nanoparticle Aggregates

GaSe nanoparticles have been synthesized in the absence of tightly binding edge ligands. These particles are ligated primarily with TOP and TOPO ligands that are easily displaced by far less bulky alkyl aldehydes. Ligand replacement results in nanoparticle aggregates in which the lowest energy optical transitions are strongly coupled, resulting in a large red shift of the absorption spectrum (about 2600 cm−1) and a reversal of the singlet and triplet states. The reversal of the spin states results in changes in the polarization spectroscopy and a dramatic decrease in the radiative lifetime. Specifically, the exciton singlet states are linear oscillators, and time-resolved fluorescence polarization spectroscopy gives an initial anisotropy very close to the linear oscillator limit of 0.4. The radiative lifetime of this fluorescence is about 12 ns, compared to about 82 ns for the monomers. Upon aggregation, the fluorescence quantum yield increases from 4.7 to 61%.

Investigation of Cu-doped Li2B4O7 single crystals by electron paramagnetic resonance and time-resolved optical spectroscopy

A low-temperature study of the thermoluminescent dosimeter material, lithium tetraborate(Li2B4O7) doped by Cu, has been carried out by the methods of electron paramagnetic resonance(EPR) and time-resolved polarization spectroscopy using 4–20 eV synchrotron radiation and1 µs Xe flash lamp pulses in the region 3–6 eV. The observed EPR spectra of an unpaired holewith strong d-character and characteristic hyperfine splittings can be ascribed toCu2+ substituted at a Li lattice site and displaced due to relaxation. The results on theCu+-related luminescence strongly support the conclusion about a low-symmetry position of copperimpurity ions in the lithium tetraborate lattice. The temperature dependence of the decay kinetics ofthe Cu+-related 3.35 eV emission indicates a triplet nature for the relaxed excited state of theCu+ centres. An off-centreposition of the Cu+ ion in the relaxed excited state is suggested.

Spatial Organization of GaSe Quantum Dots: Organic/Semiconductor Liquid Crystals

Mixed liquid crystals consisting of the smectic-A phase of 8CB and 8 nm GaSe nanoparticles have been formed and studied using static and time-resolved optical polarization spectroscopy. The mixture...

Anomalous magnetic field dependence of theT1spin lifetime in a lightly doped GaAs sample

The ${T}_{1}$ spin lifetime of a lightly doped $n$-type GaAs sample has been measured via time-resolved polarization spectroscopy under a number of temperature and magnetic field conditions. Lifetimes up to $19\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s}$ have been measured. The magnetic field dependence of ${T}_{1}$ shows a nonmonotonic behavior, where the spin lifetime first increases, then decreases, then increases again with field. The initial increase in ${T}_{1}$ is understood to be due to correlation between electrons localized on donors. The decrease in ${T}_{1}$ is likely due to phonon-related spin-orbit relaxation. The final increase in ${T}_{1}$ with $B$ indicates a suppression of the spin-orbit relaxation that may involve a level-crossing related cusp in the Rashba or Dresselhaus contributions to relaxation, or may arise from an unknown source.

Melittin as model system for probing interactions between proteins and cyclodextrins.

Cylcodextrin sugars are cyclic sugars that have a hydrophilic exterior and a hydrophobic center. This enables cyclodextrins to solubilize hydrophobic molecules in aqueous media. Cyclodextrins may inhibit aggregation by intercalating surface aromatic residues and competing with interprotein aromatic clusters (pi-pi interactions). In order to investigate this concept, the interaction of hydroxypropyl-beta-cyclodextrin (HPBCD) with melittin is studied with steady-state and time-resolved fluorescence, fluorescence polarization, circular dichroism, and IR spectroscopy. HPBCD inhibits the aggregation of melittin. This inhibition and the spectroscopic results are consistent with the lone aromatic tryptophan of the peptide being intercalated within HPBCD.

Polarized photoluminescence from surface-passivated lead sulfide nanocrystals

Effective surface passivation of lead sulfide (PbS) nanocrystals (NCs) in an aqueous colloidalsolution has been achieved following treatment with CdS precursors. The resultantphotoluminescent emission displays two distinct components, one originating from theabsorption band edge and the other from above the absorption band edge. We show thatboth of these components are strongly polarized but display distinctly different behaviours.The polarization arising from the band edge shows little dependence on the excitationenergy while the polarization of the above-band-edge component is strongly dependent onthe excitation energy. In addition, time-resolved polarization spectroscopy reveals that theabove-band-edge polarization is restricted to the first couple of nanoseconds, whilethe band edge polarization is nearly constant over hundreds of nanoseconds. Werecognize an incompatibility between the two different polarization behaviours,which enables us to identify two distinct types of surface-passivated PbS NC.

Electron Injection Dynamics of RuII(dcbpy)2(SCN)2on Zirconia

The electron injection dynamics of RuII(4,4‘-dicarboxy-2,2‘-bipyridine)2(SCN)2 (the so-called N3-dye) adsorbed on colloidal ZrO2 particles in room-temperature solution have been studied using time-resolved absorption polarization spectroscopy. The solvents used are acetonitrile and methanol. Electron injection is evidenced by the transient absorption anisotropy and/or the wavelength-dependent unpolarized absorption intensity. The results indicate that 630 nm excitation results in no N3* to ZrO2 electron transfer in both acetonitrile and methanol solvents, 515 nm excitation results in electron injection in acetonitrile, and 495 nm excitation results in electron injection in both acetonitrile and methanol. These results may be understood in terms of simple energetic considerations: 630 nm excitation produces N3* with insufficient energy for electron injection while 495 nm excitation produces N3* with an energy just sufficient for electron injection. In all cases where electron injection is energetically fa...

Fluorescence Polarization Measurements of the Local Viscosity of Hydroxypropyl Guar in Solution

The local viscosity experienced by the chain backbone of fluorescein-labeled hydroxypropylguar (HPG) in aqueous solution has been studied using time-resolved and steady-state fluorescence polarization spectroscopy. These measurements have been undertaken over a range of HPG concentrations and in a series of glycerol/water solutions of varying Newtonian viscosity. The results indicate that despite the bulk viscosities varying over several orders of magnitude as a function of HPG concentration, the rotational dynamics of the fluorophore attached to the backbone are only measurably affected at concentrations of ∼10 wt %. Samples of the HPG at concentrations as high as 10 wt % therefore contain regions with viscosity similar to the aqueous phase as sensed by the probe molecule. Intermolecular interaction between the chains introduces a slow rotational mode at concentrations approaching 10 wt %. The rotational correlation time of the probe attached to the HPG backbone shows a square root dependence on the solvent viscosity in the glycerol/water mixtures. This may be attributed to the restricted motion of the probe molecule around the central axis in two dimensions.

Photophysical properties of meso, meso-linked porphyrin arrays: steady-state and time-resolved fluorescence polarization

The photophysical properties of a series of directly meso ,meso-linked porphyrin arrays (Zn(II) 5,15-di(3,5-di-tert-butylphenyl) porphyrin (Z1), its dimer (Z2), trimer (Z3), and tetramer (Z4), were investigated by using the steady-state and time-resolved fluorescence polarization spectroscopy. While the Soret band of Z1 exhibits the absorption maximum at 415 nm, those of Z2, Z3, and Z4 are split into two bands because of the strong exciton coupling. The fluorescence band is red-shifted and its lifetime decreases as the number of porphyrin moieties increases. In contrast to Z1, the initial anisotropy values ( r 0) of Z2, Z3 and Z4 exhibit the negative sign with photoexcitation at 410 nm, indicating a large difference between absorption and emission dipole orientations. However, the positive values are obtained in all compounds with the visible excitation. The emission anisotropy decay appeared to be monoexponential at all excitation wavelengths with a decrease of its rate as the number of porphyrin moieities increases. In addition, the rotational decay is observed to depend on excitation wavelength, which is probably due to the local heating of the solvent by excess vibrational energy. The anisotropy values calculated from the time-resolved anisotropy decay are in a good agreement with those obtained from the steady-state polarized excitation spectra.

Hot luminescence from F centers in KCl studied by time-resolved polarization spectroscopy

A dynamical de-excitation process for optically excited electrons in F centers in KCl is studied by using time-resolved polarization spectroscopy. The ordinary luminescence (OL) and the hot luminescence (HL) in the OL region are separated from a resonant secondary radiation, and the HL is decomposed into two components with different behaviors. They are the HL in a short time after excitation and an emission from 2p-like states after a long time from excitation. The dynamics of the optical excitation is interpreted phenomenologically in terms of a configuration coordinate model.

Orientational Dynamics of β-Cyclodextrin Inclusion Complexes

The structure and dynamics of organic dye molecules included in β-cyclodextrin are studied using spectroscopic measurements and theoretical models. The effect of the charge and the size of the guest molecule on the properties of the host−guest complex is probed by comparing complexes formed with three different chromophores: resorufin (anion), oxazine-118 (cation), and oxazine-725 (cation) in aqueous solutions of cyclodextrin. The binding is characterized using absorption and calorimetry titrations. The structure of the complexes is analyzed by molecular modeling using empirical force field and semiempirical quantum theory calculations. Time-resolved polarization spectroscopy is used to investigate the rotational dynamics of different chromophores bound to β-cyclodextrin. Internal motion of the guest and overall rotational tumbling of the complex are observed for resorufin and oxazine-118. Modeling the internal motion of the chromophore as diffusion in a cone provides the mean square diffusion angle inside the cavity. It is found that the relative host−guest size determines the character of intermolecular host−guest dynamics.

Measurement and theoretical modeling of quantum beats in picosecond time-resolved degenerate four-wave mixing and polarization spectroscopy of OH in atmospheric pressure flames

Using tunable ultraviolet picosecond laser pulses pump-probe degenerate four-wave mixing (DFWM) and polarization spectroscopy experiments were conducted in atmospheric pressure flames to investigate the temporal signal behavior in selected rotational transitions of the OH $A{}^{2}\ensuremath{\Sigma}--X{}^{2}\ensuremath{\Pi} (0,0)$ electronic band. The relaxation behavior of simultaneously excited main and satellite transitions in the Q and P branches was studied in premixed stoichiometric methane-air and hydrogen-oxygen flames. Experimental signal traces are compared with expressions from a detailed theoretical treatment of the signal generation process using perturbation calculations. The theoretical approach consists in calculating the energy density in the signal field mode taking into account the frequency spread of the pump and probe beam radiation, collisional relaxation effects, and the polarization configuration of the incident beams. Relaxation times for population and orientation deduced from the fitting algorithm are in good agreement with DFWM line-shape studies [S. Williams et al., J. Chem. Phys. 104, 3947 (1996)]. It is shown that quantitative agreement with experimental data obtained for different polarization configurations of pump, probe, and signal photons can be achieved when appropriate time correlated interactions of pump and probe photons are taken into account. In addition, it is shown that due to the frequency spread of the employed laser pulses the different frequency components in the signal beam contribute with different amplitude to the oscillating and nonoscillating parts in the temporal development of the signal intensity depending on the relative strength of the simultaneously excited transitions.

Experimental Measurements of Low-Frequency Intermolecular Host−Guest Dynamics

A direct temporal measurement of the internal motion of a host−guest bimolecular complex is reported. Time-resolved polarization spectroscopy is used to monitor the motion of a guest chromophore, anthracene-1,3-dicarboxylate, bound to a bis(guanidinium) host molecule through a combination of hydrogen bonding and electrostatic forces. The dynamics of the bound chromophore is well described by an overdamped oscillator model. Through simple modeling, it is possible to estimate the frequency of the host−guest potential and the range of the guest chromophore's angular displacement.

Quantum beat spectroscopy of OH radicals in atmospheric pressure flames

Using time-resolved polarization spectroscopy we have observed quantum beats of coherently excited states in the ( A 2Σ +− X 2П ) vibronic (0, 0) band system of OH in atmospheric-pressure flames. With a spectral bandwidth of 1 cm −1 of the tunable coherent light source employed, line splittings were deduced from the temporal beat structure. Depending on the rotational quantum number of the probed transition and the collisional environment the relaxation times of anisotropic orientational distributions in which the quantum beat oscillations were observed ranged between 120 ps ( J″ = 5) and 368 ps ( J″ = 10).

Off-centre effects in the triplet relaxed excited state of impurity ns2 ions in alkali halides

An off-centre displacement of an excited ns 2 ion from a crystal lattice site is studied for Ga + , In + and Tl + centres in alkali halides at 0.09-5 K by the time-resolved polarization spectroscopy method. A theory is developed, which considers the off-centre effects as a result of the mixing of the impurity 3 nsnp and 3 npnp excited states by T 1u vibrations.