J. M. Drake

Porous vycor glass: The microstructure as probed by electron microscopy, direct energy transfer, small‐angle scattering, and molecular adsorption

We provide a comprehensive analysis of the microstructure of the porous glass, vycor. Using transmission electron microscopy, small‐angle x‐ray scattering, molecular adsorption, and the dynamic process of direct energy transfer, a consistent picture of the mass, pore, and interfacial features of this material is presented. From a transmission‐electron‐microscopy image of an ultrathin section of vycor the material appears to have a homogeneous distribution of mass with no hierarchical organization. The pore interface exhibits a roughness which is probed by both small‐angle x‐ray scattering and molecular adsorption. The roughness has an upper cutoff of <20 A which is not resolved in the transmission‐electron‐microscopy image and is shown to be unimportant to the dynamics of the direct energy transfer process. The dimensionality probed by direct energy transfer is shown to be related to interfacial geometrical crossover from two dimensional to three dimensional, which is characterized by a persistent length ...

A photoluminescence study of poly(phenylene vinylene) derivatives: The effect of intrinsic persistence length

We report the results of light scattering, absorption, excitation, and emission spectroscopy of three polyphenylene vinylene (PPV) derivatives; poly[2‐methoxy, 5‐(2’‐ethyl‐hexyloxy‐p‐phenylene‐ vinylene] (MEH‐PPV), poly[2‐butoxy, 5‐(2’‐ethyl‐hexyloxy‐p‐phenylene‐vinylene] (BEH‐PPV), and poly[2‐dicholestanoxy‐p‐phenylene‐vinylene] (BCHA‐PPV) in solution with p‐xylene. We find that increasing the size of the solubilizing side chains increases the intrinsic persistence length of the polyphenylene vinylene backbone and that this change in stiffness has dramatic effects on the photoluminescence of polyphenylene vinylene. We have determined the luminescence quantum efficiencies of the polyphenylene vinylene derivatives relative to a known standard, Rhodamine 6G, and find that the photoluminescence can be greatly enhanced by increasing the intrinsic stiffness of the polymer backbone. The stiffest polymer, poly[2‐dicholestanoxy‐p‐phenylene‐vinylene] (BCHA‐PPV), has a quantum efficiency of 0.66±0.05. The quantum e...

The solute size effect in rotational diffusion experiments: A test of microscopic friction theories

The experimentally measured rotational diffusion times of various neutral solute molecules are compared with hydrodynamic and molecular models for microscopic friction. New viscosity and temperature dependent results for the rotational time of a large neutral solute (26 A long) indicate nearly perfect agreement with stick hydrodynamic predictions in both alkane and alcohol solvents. For smaller solute molecules, the results of previous studies show an increasingly large deviation from hydrodynamic predictions as the solute size decreases. This solute size dependent effect is compared with the theoretical predictions of Zwanzig’s hydrodynamic ‘‘bumpy cylinder’’ model, Dote–Kivelson‐Schwartz’s molecular free volume model, and a new generalized hydrodynamic model for the effects of frequency dependent viscosity on molecular rotational diffusion. Although hydrodynamic predictions agree with experiment in some respects, it is found that molecular aspects of the solvation process must be introduced in order to ...

Critical evaluation of the application of direct energy transfer in probing the morphology of porous solids

The decay of an initially excited donor due to the presence of acceptor molecules embedded in porous silica gels has been studied. The time evolution of the decay process has been related to the geometrical restrictions imposed on the participating molecules by the porous structure of the silicas. Fractal and regular geometrical shape models have been confronted with experimental findings. We conclude that for silica gels simple pore geometries, i.e., spheres and cylinders, account well for the relaxation behavior. The decay patterns exhibit temporal crossovers typical of dynamics in restricted geometries.

Relaxation in restricted geometries

Abstract We study the relaxation of initially excited molecules due to energy transfer to acceptors embedded in confined spaces and due to interaction with diffusing quenchers. Both models have been recently applied to relaxation in porous systems.

Direct energy transfer in pores: Geometrical cross-overs and apparent dimensionality

Abstract We have studied direct energy transfer between donor and acceptor molecules randomly distributed on an infinite cylindrical surface. We show how geometrical cross-overs and experimental constraints permit to use an apparent dimension to fit the donor survival probability. The physical meaning of this apparent dimensionality is discussed.

Excited state dynamics in restricted geometries

Abstract We demonstrate how photophysical processes in restricted geometries such as porous oxides differed from analogous processes in regular systems. We have used forced Rayleigh scattering and direct energy transfer to probe different length scales of the internal pore network.

Relaxation of excitations in porous silica gels

Abstract The relaxation of an initially excited donor molecule due to the presence of acceptor molecules embedded in porous solids has been studied. The time evolution of the relaxation process has been related to the geometrical restrictions imposed on the participatingmolecules by the porous structures. We have focused on two limits: (a) donor and acceptors adsorbed on the solid interfaces, (b) gas phase acceptors diffusing within the pore interior towards the donor. In both cases a homologous series of silica gels has been investigated. When corroborated with structural studies, the results lead to classical pore models for the embedding systems. The mean pore size, R p , appears to be a relevant scaling parameter for both dynamical and structural observables.

Direct energy transfer from excited organic adsorbates to intrinsic defect sites in silicalite

The decay of the excited singlet state of an aromatic molecule such as naphthalene adsorbed to the surface of silica is nonexponential. This nonexponential decay is shown to result from a multipolar interaction between the excited adsorbate and the intrinsic defects of silica. We present experimental evidence for direct energy transfer between the donor molecules, naphthalene and 2‐methoxynaphthalene, and acceptor defects sites of a crystalline silica zeloite (silicalite). The principal condition for direct energy transfer,spectral overlap between the optical absorption of the defects and the fluorescence emission of the adsorbate is demonstrated for this system, and the nonexponential relaxation of the donor is shown to be described by a Forster‐type stretched exponential. Based on the spectral overlap and energy transfer rate determined, the intrinsic defect density of silicalite is calculated to be 3±2×1020 cm−3.

The nature of porous silica as probed by pyrene dynamics

Abstract The dynamics of the pyrene monomer-excimer system has been studied on silica gel surfaces with various areas and pore size distributions. A possible relation between the excimer-monomer emission ratio and the fractal nature of the surface is proposed.