F. W. Deeg

Solvation dynamics of nile blue in ethanol confined in porous sol–gel glasses

We report on solvation dynamics measurements of the chromophore nile blue in ethanol confined to sol–gel glasses with 50 A and 75 A average pore size and compare them with the dynamics of the respective bulk solution. Both the amplitude of the dynamic Stokes shift as well as the dynamics of the solvation process are drastically changed upon confinement. In both confined solutions the dynamic Stokes shift is reduced by a factor of about 2. As the large majority of the chromophores is adsorbed at the inner pore surfaces the solute molecules interact with only a “half-space” of solvent molecules. In a first approximation this decreases the stabilization energy by a factor of 2. The solvation dynamics in the confined solutions show nonexponential behavior comparable to the bulk. However, the whole solvation process slows down and the single decay time constants characterizing it increase with decreasing pore size of the sol–gel glass. We have introduced two phenomenological models to rationalize this behavior...

Influence of surface interaction on the reorientational dynamics of pentylcyanobiphenyl confined in a porous glass

Abstract Time-resolved transient grating optical Kerr effect experiments were employed to investigate the reorientational dynamics of pentylcyanobiphenyl confined in the nanometer lengthscale pores of silica glasses. Temperature and pore-size-dependent measurements show the drastic influence of geometrical restriction on the cooperative reorientational motion in the nematogenic liquid. At temperatures far above the phase transition, a bulk phase and a distinct surface layer with different dynamic properties are observed. With pore size reduction,to 25 A, a decrease of the surface layer thickness and an increase of the surface layer relaxation time are found. This is attributed to the modification of the surface layer structure by geometrical factors, e.g., the pore curvature.

Spectral diffusion in solvent-filled AlPO4-5 molecular sieves: modification of amorphous phase properties by spatial restrictions

Abstract We report on persistent spectral hole-burning studies of thiazine and oxazine dyes encapsulated in an AlPO 4 -5 aluminophosphate molecular sieve. A non-photochemical hole-burning mechanism is provided by small additional solvent molecules, e.g. glassy ethanol or amorphous water, in the pore system. Hole-burning efficiency and spontaneous hole-filling are in the typical range of glassy bulk systems. However, spectral diffusion is reduced drastically in the solvent-filled molecular sieve compared to the corresponding bulk glass. Based on previous work by the Fayer group [J. Chem. Phys. 92 (1990) 4125; Chem. Phys. Lett. 168 (1990) 371] this finding is discussed as an example of spatial restrictions to the solvation shell around the dye molecules.

Spectral hole-burning studies of organic chromophores in geometrically restricted amorphous phases

Abstract We present optical spectroscopy and persistent spectral hole-burning data of oxazine-1 embedded in the nanometer lengthscale pores of inorganic hosts, which are filled with solvents like ethanol and 3-methyl-pentane as coadsorbate. Absorption spectra, hole burning efficiency and hole filling allow to determine the location of the chromophore within the pores and give information about the structure of the local solvation shell. Spectral diffusion reflects the modified spatial distribution of the two-level systems of the amorphous phase and allows an estimation of the interaction length between chromophore and two-level system.

Characteristics of Solvation Dynamics of Rhodamine 700 in Cyanobiphenyls

Abstract The results of solvation dynamics measurements of rhodamine 700 in the isotropic phase of two different liquid crystalline substances, 8CB and 4CB, are presented. It is shown that solvation takes place on a length scale shorter than the orientational correlation length of the pseudonematic domains. Two processes contribute to the solvation. The slower process can be described by dielectric relaxation. The faster component is interpreted in terms of intermo-lecular interactions specific to liquid crystalline samples.

Solvation dynamics of ionic dyes in the isotropic phase of liquid crystals

Abstract We present a solvation dynamics study of Rhodamine 700 in the isotropic phase of octylcyanobiphenyl approaching the nematic-isotropic phase transition. From our experiments we detect two kinds of processes. A fast process, with decay time constants of tens of ps, shows no clear temperature dependence and is attributed to local reorientational motion of the solvent in the proximity of the chromophore. A slow process, with time constants in the hundreds of ps, shows a temperature activated behavior. This process does not seem to be influenced by the molecular order displayed by nematogenic substances and resembles the solvation dynamics behavior observed in simple polar liquids and predicted by dielectric theories.

Holographische Methoden in der Festkörperspektroskopie

Holographie wird im allgemeinen als eine Aufzeichnungsmethode angesehen, mit der ein dreidimensionales Bild von einem Objekt erhalten werden kann [11], [13], [26]. Dies ist sicherlich ein spektakularer Aspekt. Holographie kann jedoch auch als eine besonders leistungsfahige Methode zur Untersuchung einer Vielfalt von photochemischen und photophysikalischen Prozessen angewendet werden [7], [8]. Dabei stutzen sich die experimentellen Techniken auf die Tatsache, das das Hologramm, charakterisiert durch kleine raumliche Modulationen der optischen Eigenschaften (Brechungsindex und Absorptionskoeffizient) des Probenmaterials, einen einfallenden Lichtstrahl abzulenken vermag. Indem man die Zu- oder Abnahme der Intensitat des abgelenkten Strahls beobachtet, lassen sich die Photoprozesse verfolgen, die die Anderungen der optischen Eigenschaften in der Probe hervorrufen. Wird zur Erzeugung des Hologramms ein kontinuierlicher Laser benutzt, so kann man photochemische Prozesse in Festkorpern untersuchen. Wird ein gepulster Laser verwendet, so konnen zeitabhangige Prozesse wie Energietransfer, Diffusion, Rotationsrelaxation, Ladungstransport etc. erfast werden. Gegenuber konventionellen spektroskopischen Methoden weisen die holographischen Techniken mehrere Vorteile auf. So sind sie z. B. sehr empfindliche Nullmethoden und ermoglichen die weitgehend freie Wahl von Wellenlange sowie Intensitat des Detektionsstrahls.

Spectral Hole-Burning Investigations in the Restricted Geometry of Molecular Sieves

Abstract Low-temperature spectroscopy and persistent spectral hole-burning have been used to characterize several aspects of the dynamics of physiadsorbed chromophores and amorphous phases confined to the A-lengthscale voids of molecular sieves. It is shown that subtle differences of geometric factors, i.e. the relationship between molecular size and form and the confining void play a decisive role in the mobility and dynamics of thiazine dyes encapsulated in solvent-free faujasite structures. Additional solvent in the pores forms an amorphous phase which allows the formation of persistent spectral holes. Spectral diffusion in some of the solvent/molecular sieve systems is largely reduced due to the effect of the restricted geometry on the structure of the amorphous phase and/or the spatial distribution of two-level systems. For a hydrated AIPO4-5 host with phthalocyanine-zinc derivates efficient hole formation has been demonstrated up to 80 K, the highest temperature for which stable non-photochemical ho...

A dispersion step in the ultrasonic region of a nematic liquid single crystal elastomer

Abstract Temperature- and orientation-dependent propagation of ultrasonic waves in a liquid crystalline side-chain polysiloxane elastomer was measured by impulsive stimulated thermal scattering for acoustic frequencies in the range of 100 MHz–1 GHz. A strong anisotropy of the viscoelastic constants in the nematic phase with a small residual anisotropy above the nematic to isotropic phase transition was found. The frequency dependence reveals a dispersion step for the speed of sound and a strong anisotropy for the acoustic attenuation constant in the investigated frequency range consistent with a recently proposed hydrodynamic model of a coupling of the elasticity to the nematic order parameter in this class of substances.