Nuclear Spin Relaxation in Viscous Liquids: Relaxation Stretching of Single-Particle Probes.

Spin-lattice relaxation rates R1(ω,T), probed via high-field and field-cycling nuclear magnetic resonance (NMR), are used to test the validity of frequency-temperature superposition (FTS) for the reorientation dynamics in viscous liquids. For several liquids, FTS is found to apply so that master curves can be generated. The susceptibility spectra are highly similar to those obtained from depolarized light scattering (DLS) and reveal an excess wing. Where FTS works, two approaches are suggested to access the susceptibility: (i) a plot of deuteron R1(T) vs the spin-spin relaxation rate R2(T) and (ii) a plot of R1(T) vs an independently measured reference time τref(T). Using single-frequency scans, (i) allows one to extract the relaxation stretching as well as the NMR coupling constant. Surveying 26 data sets, we find Kohlrausch functions with exponents 0.39 < ßK ≤ 0.67. Plots of the spin-spin relaxation rate R2─rescaled by the NMR coupling constant─as a function of temperature allow one to test how well site-specific NMR relaxations couple to a given reference process. Upon cooling of flexible molecule liquids, the site-specific dynamics is found to merge, suggesting that near Tg the molecules reorient essentially as a rigid entity. This presents a possible resolution for the much lower stretching parameters reported here at high temperatures that contrast with the ones that were reported to be universal in a recent DLS study close to Tg. Our analysis underlines that deuteron relaxation is a uniquely powerful tool to probe single-particle reorientation.

Single molecule probing of dynamics in supercooled polymers.

Fluorescence experiments with single BODIPY molecules embedded in a poly(methyl acrylate) matrix have been performed at various temperatures in the supercooled regime. By using pulsed excitation, fluorescence lifetime and linear dichroism time trajectories were accessible at the same time. Both observables have been analyzed without data binning. While the linear dichroism solely reflects single particle dynamics, the fluorescence lifetime observable depends on the molecular environment, so that the dynamics from the polymer host surrounding a chromophore contributes to this quantity. We observe that the lifetime correlation decays slightly faster than polarization correlation, indicating the occurrence of large angular reorientations. Additionally, dichroism time trajectories have been adducted to reveal directly the geometry of rotational dynamics. We identify small but also significantly larger rotational jumps being responsible for the overall molecular reorientation.

Statistical analysis of time resolved single molecule fluorescence data without time binning.

We depict two algorithms to calculate correlation functions from two different time resolved single molecule fluorescence experiments without the need of time binning. Our first procedure allows to calculate the reduced linear dichroism from polarization resolved fluorescence data. Since we process single photon counts instead of time binned data, considerably faster fluctuations of the dichroism can be analyzed than with conventional methods. With our second procedure time resolved fluorescence obtained with a time correlated single photon counting setup can be analyzed with respect to fluorescence lifetime fluctuations. Again this new algorithm processes single photon events making time binning of photon counts obsolete. Both methods presented are characterized by enhanced time resolution thus allowing to study fast fluctuations of either single molecular orientation or fluorescence life times, respectively.

Intramolecular electronic excitation energy transfer in donor/acceptor dyads studied by time and frequency resolved single molecule spectroscopy.

Electronic excitation energy transfer has been studied by single molecule spectroscopy in donor/acceptor dyads composed of a perylenediimide donor and a terrylenediimide acceptor linked by oligo(phenylene) bridges of two different lengths. For the shorter bridge (three phenylene units) energy is transferred almost quantitatively from the donor to the acceptor, while for the longer bridge (seven phenylene units) energy transfer is less efficient as indicated by the occurrence of donor and acceptor emission. To determine energy transfer rates and efficiencies at the single molecule level, several methods have been employed. These comprise time-correlated single photon counting techniques at room temperature and optical linewidth measurements at low temperature (1.4 K). For both types of measurement we obtain broad distributions of the rate constants of energy transfer. These distributions are simulated in the framework of Forster theory by properly taking into account static disorder and the flexibility of the dyads, as both effects can substantially contribute to the distributions of energy transfer times. The rate constants of energy transfer obtained from the calculated distributions are smaller on average than those extracted from the experimental distributions, whereby the discrepancy is larger for the shorter bridge. Furthermore, by plotting the experimentally determined transfer rates against the individual spectral overlaps, approximately linear dependencies are found being indicative of a Forster-type contribution to the energy transfer. For a given single molecule such a linear dependence could be followed by spectral diffusion induced fluctuations of the spectral overlap. The discrepancies between measured energy transfer rates and rates calculated by Forster theory are briefly discussed in light of recent results of quantum chemical calculations, which indicate that a bridge-mediated contribution is mainly responsible for the deviations from Forster theory. The availability of the inhomogeneous distributions of donor and acceptor electronic transition frequencies allows for comparing the energy transfer process at liquid helium and room temperature for the same set of molecules via simple simulations. It is found that on average the energy transfer is by a factor of approximately 3 faster at room temperature, which is due to an increase of spectral overlap.

Analysis of the exponential character of single molecule rotational correlation functions for large and small fluorescence collection angles.

Coarse-grained molecular dynamics simulations and single molecule fluorescence microscopy experiments have been performed in order to investigate the influence of the numerical aperture (NA) of the microscope objective on the exponential character of the rotational correlation functions of probes embedded in complex matrices. The results obtained by using either a dry lens (NA=0.95) or an oil objective (NA=1.4) show that, in the moderately (simulations) and deeply (experiment) supercooled melts, the rotational (linear dichroism) correlation functions of the single molecules (SMs) exhibit a nonexponential character. Furthermore, by fitting Kohlrausch-Williams-Watt functions to the correlation curves, the stretching parameters turn out to be very similar for both types of objectives. Our results demonstrate that the nonexponentiality is intrinsic to the complex rotational dynamics of the SM in the supercooled solid and point to the validity of the use of a high NA dry lens to perform such experiments.

Correlation of primary relaxations and high-frequency modes in supercooled liquids. II. Evidence from spin-lattice relaxation weighted stimulated-echo spectroscopy.

Using spin-lattice relaxation weighted stimulated-echo spectroscopy, we report evidence for a correlation of the primary and secondary relaxation times. The experiments are performed using deuteron nuclear magnetic resonance somewhat above the calorimetric glass-transition of ortho-terphenyl, D-sorbitol, and cresolphthalein-dimethylether. The data analysis is based on the procedure outlined in the accompanying theoretical paper [B. Geil, G. Diezemann, and R. Böhmer, Phys. Rev. E 74, 041504 (2006)]. Direct experimental evidence for a modified spin-lattice relaxation is obtained from measurements on a methyl deuterated acetyl salicylic acid glass. The limitations of the present experimental method are discussed.

Correlation of primary and secondary relaxations in a supercooled liquid.

The widespread assumption that primary and secondary relaxations in glass-forming materials are independent processes is scrutinized using spin-lattice relaxation weighted stimulated-echo spectroscopy. This nuclear magnetic resonance (NMR) technique is simultaneously sensitive to the dynamics on well-separated time scales. For the deeply supercooled liquid sorbitol, which exhibits a strong secondary relaxation, the primary relaxation (that is observable using NMR) can be modified by suppressing the contributions of those subensembles which are characterized by relatively slow secondary relaxations. This is clear evidence for a correlation between primary and secondary relaxation times. In the disordered crystal orthocarborane high-frequency processes are absent and consequently no such modifications could be achieved.

A comparison of relaxation processes in structurally related van der Waals glass formers: the role of internal degrees of freedom.

Depolarized dynamic light scattering (DLS), dielectric relaxation (DS), and deuterium NMR studies of fragile van der Waals glass forming liquids phenylphthalein-dimethylether (PDE) and cresolphthalein-dimethylether (KDE) are presented. In PDE a new dielectric loss process was found, which can be attributed to the 180 degrees flip of the phenyl rings. The previous finding that the distribution of the structural relaxation times measured for PDE and KDE by DS is substantially narrower than that measured by DLS is explained by partial decoupling of the dynamics of the dipole moment from the structural relaxation of the sample. The dynamics of PDE and KDE is compared with the previous studies of two other structurally similar liquids: 1,1'-di(4-methoxy-5-methylphenyl)cyclohexane (BMMPC) and 1,1'-bis(p-methoxyphenyl)cyclohexane (BMPC) in order to relate dynamical features with the chemical structure of the material. The evidence for the intramolecular character of the secondary relaxations observed in BMPC and PDE is presented.

Time-resolved measurements of intramolecular energy transfer in single donor/acceptor dyads.

We have investigated electronic excitation energy transfer in a specifically designed bichromophoric donor/acceptor dyad in which the donor (perylenediimide) and acceptor (terrylenediimide) are linked by a rigid heptaphenyl-spacer. Because of the choice of the bridge, which defines the distance and orientation of the two chromophores, donor as well as acceptor emission is observed. The significantly smaller photostability of the donor allows for time-resolved measurements of the acceptor emission at the single-molecule level with and without energy transfer from the donor. By analyzing the differences of the rise/decay profiles for both pathways, we could determine time constants of energy transfer with high accuracy for single dyads. The results show that the experimental approach presented here works even for situations in which the energy transfer times are smaller than the temporal resolution of the detection system.

Rotational correlation functions of single molecules.

Single molecule rotational correlation functions are analyzed for several reorientation geometries. Even for the simplest model of isotropic rotational diffusion our findings predict nonexponential correlation functions to be observed by polarization sensitive single molecule fluorescence microscopy. This may have a deep impact on interpreting the results of molecular reorientation measurements in heterogeneous environments.

From strong to fragile glass formers: secondary relaxation in polyalcohols.

We have studied details of the molecular origin of slow secondary relaxation near T(g) in a series of neat polyalcohols by means of dielectric spectroscopy and (2)H NMR. From glycerol to threitol, xylitol, and sorbitol the appearance of the secondary relaxation changes gradually from a wing-type scenario to a pronounced beta peak. It is found that in sorbitol the dynamics of the whole molecule contributes equally to the beta process, while in glycerol the hydrogen bond forming OH groups remain rather rigid compared to the hydrogens bonded to the carbon skeleton.

Surface-induced order and diffusion in 5CB liquid crystal confined to porous glass.

Liquid crystals confined into small cavities are known to have a weak orientational order even above the nematic-isotropic transition temperature. The surface-induced order and molecular dynamics in this temperature range are studied with the aid of deuteron NMR spectra, spin relaxation times T(1) and T(2,) proton dipolar-correlation effect, and direct measurements of the effective diffusion coefficient for the liquid crystal 5CB confined to controlled-pore glasses. Our results show that an arrangement of molecules parallel to the wall is induced by local molecular interactions between the liquid crystal and solid, resulting in a weak and temperature independent surface order parameter, S(0) approximately 0.02 +/- 0.01. There is no indication of a significant slowing-down of molecular diffusion at the wall, neither rotational nor translational. In cavities of nanometer size, where the nematic order evolves gradually upon cooling, a broadening of the NMR linewidths due to dynamic effects should be taken into account.

Orientational dynamics in supercooled liquids near T(c) and comparison with ideal mode-coupling theory

Orientational dynamics in supercooled salol and ortho-terphenyl were measured near their critical temperatures, T(c), with optical Kerr effect experiments spanning a very broad range of times. Above T(c), the decays are shown to be in excellent agreement with the master curve predicted by ideal mode-coupling theory when higher order terms are included. Between the critical decay and the von Schweidler power laws, the intermediate time range of the data can be modeled by a power law. This intermediate power law, located at 2

Experimental Determination of Four-Time Stimulated Echoes in Liquids, Colloidal Suspensions, and Crystals

Four-time stimulated echo experiments have been used to monitor the temporal evolution of the reorientation rates of deuterated molecules or colloidal suspensions. We present extended phase cycles for this seven-pulse experiment. In order to test its performance three vastly different materials are chosen. These include a crystal in which the molecules carry out well-defined 180 degrees flips and a supercooled liquid characterized by a distribution of jump angles centered around 10 degrees. As an example for rotational diffusion, data on a concentrated suspension of polystyrene spheres in a viscous medium are presented. Copyright 1998 Academic Press.

[Growth and nutrition: long term follow-up in early treated phenylketonuric and hyperphenylalaninemic children (author's transl)]. / Wachstum und Ernährung: Langzeitbeobachtungen bei frühbehandelten Kindern mit Phenylketonurie- und Hyperphenylalaninämie-Varianten.

In a retrospective longitudinal study physical growth was related to nutritional data in 35 (23 male, 12 female) early treated patients with phenylketonuria and hyperphenylalaninemic variants. In males growth was slightly retarded only in the first year of life, weight gain was normal in both sexes as was growth in females. Bone changes were noted in 10/33 patients. Protein was prescribed as phenylalanine free protein hydrolysate or phenylalanine free aminoacid mixture and as natural protein according to individual phenylalanine tolerance. In 34/35 patients total serum protein was normal whereas hemoglobin was near the lower normal limit. Serum phenylalanine increased continuously with age.