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Electronic excitation energy transfer in a specifically designed bichromophoric donor/acceptor dyad in which the donor and acceptor are linked by a rigid heptaphenyl-spacer is investigated. The results reveal that the experimental approach works even for situations in which the energy transfer times are smaller than the temporal resolution of the detection system.

Orientation relaxation of supercooled o-terphenyl (OTP) is evaluated over a wide range of temperatures and time scales by using optical heterodyne detected optical Kerr effect techniques. The dynamics exhibited by the supercooled glass-forming liquid OTP in the relaxation curves present features in agreement with the predictions of mode-coupling theory (MCT).

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 Förster 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 Förster-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 Förster 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 Förster 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 ∼3 faster at room temperature, which is due to an increase of spectral overlap. [ABSTRACT FROM AUTHOR]

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° 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. [ABSTRACT FROM AUTHOR]

The small-molecule glass former methyl tetrahydrofuran (MTHF) was investigated using dielectric spectroscopy, spin-lattice relaxometry, multidimensional stimulated-echo nuclear magnetic resonance techniques, and field gradient diffusometry. We show experimentally that MTHF nicely fits into the pattern of related small-molecule glass-forming liquids, including the existence of a high-frequency contribution to the dielectric loss, the appearance of a pronounced translational enhancement, the dominance of small average rotational jump angles, and the existence of short-lived dynamical heterogeneity. [ABSTRACT FROM AUTHOR]

We have studied details of the molecular dynamics in a series of pure polyalcohols by means of dielectric spectroscopy and [SUP2]H nuclear magnetic resonance (NMR). From glycerol to threitol, xylitol and sorbitol a systematic change in the dynamics of the primary and secondary relaxation is found. With increasing molecular weight and fragility an increase in the width of the α-peak is observed. Details of the molecular reorientation process responsible for the α-relaxation were exploited by two-dimensional NMR experiments. It is found that in the same sequence of polyalcohols the appearance of the secondary relaxation changes gradually from a wing type scenario to a pronounced β-peak. From NMR experiments using selectively deuterated samples the molecular origin of the secondary relaxation could be elucidated in more detail. [ABSTRACT FROM AUTHOR]

Time-resolved transient grating experiments with various polarizations are used to separate different responses and measure their dynamics in supercooled liquid salol. A contribution to signal from orientational alignment induced by flow that arises from thermal expansion is demonstrated. This contribution is distinct from that due to orientational alignment induced directly by the excitation light through the molecular polarizability anisotropy (i.e., through the optical Kerr effect). It is also distinct from signal contributions due to density modulations induced by thermal expansion. The results offer additional insight into salol dynamics and into time-dependent transient grating measurements of this class. Depending on the light polarizations used, any of the signal contributions can be eliminated or highlighted. [ABSTRACT FROM AUTHOR]

Presents a time domain technique for studying molecular orientational relaxation in viscous liquids. Cause of molecular alignment through translational-rotational coupling; Monitoring of molecular orientational relaxation using an optical heterodyne detection method.

The effect of spin diffusion on the decay of longitudinal magnetization in deuterated glassy systems is investigated. The decay of longitudinal magnetization was measured for different dilutions of amorphous deuterated toluene in protonated toluene. For increasing dilution the magnetization was found to decay more slowly, indicating the importance of spin diffusion also in deuteron spin systems. A strong correlation between the apparent width of the distribution of spin lattice relaxation times and the absolute value of the average spin lattice relaxation time was found. This correlation can be explained in terms of spin diffusion. A simple model is able to reproduce the main features of the observed phenomena. The initial decay rates are shown to be independent of spin diffusion. Conclusions concerning the frequency dependencies of observed average spin lattice relaxation times and width parameters are drawn. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

2H-NMR spin-lattice relaxation times T1 of phenyl ring deuterated toluene have been measured as a function of temperature (75–290 K) at Larmor frequencies between 13.8 and 55.8 MHz. The results are interpreted by assuming a Cole–Davidson rotational correlation time distribution for the α process dominating T1 at temperatures down to about 20 K above the glass transition (Tg=117 K) and one further process at lower temperatures. The latter is analyzed below Tg using a model with temperature dependent librational angles Δ[uppercase_phi_synonym]<13° and mean correlation times τ>=10-9 s. Self-diffusion coefficients are determined at temperatures down to 136 K (D=1.5×10-14 m2 s-1) using a static field gradient 1H-NMR method. The product of D and the mean rotational correlation time shows a tendency to increase at the lowest temperatures. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Investigates the reorientational mechanism in the selectively deuterated glass-former glycerol. Use of the method of stimulated echoes; Reference of the reorientation process as isotropic; Determination of the decay constant of the quadrupolar spin order in the regime of ultraslow motion; Identification of the rotational correlation function.