Electrospun fibers were produced using a variety of solvents to investigate the influence of polymer/solvent properties on the fiber surface morphology. Electrospinning is a novel processing technique for the production of fibers with diameters in the range of a few nanometers to tens of micrometers. We have been able to produce polymeric fibers with a high surface area through the introduction of a micro- and nanostructured surface structure, which we refer to as a “porous” morphology. These features could be introduced in several different polymeric fibers increasing their range of application significantly. The pores vary from densely packed, well-formed nanopores with diameters in the range 20−350 nm to larger flat pores of about 1 μm. The increased surface area of polymeric fibers was correlated with high volatility solvents used in the electrospinning process. The effect of processing parameters on the fiber surface morphology was also investigated using optical microscopy, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM).

The orientation of the S 1 r S 0 π,π* transition dipole moments of oxonine (Ox + ), pyronine (Py + ), and POPOP (5,5′-diphenyl-2,2′-p-phenylenebis(oxazole)) in the channels of zeolite L crystals was investigated by means of fluorescence microscopy and single-crystal imaging. Qualitative observations led to the result that the transition moment of POPOP is aligned along the c-axis of the hexagonal crystals whereas the fluorescence of Ox + and Py + is not. More detailed investigations on Ox + showed a cone-shaped distribution of the transition moments with a half-cone angle of 72°. The orientation of the transition dipole moment for all of these molecules is parallel to the molecules' long axis. We found by means of space-filling arguments that POPOP, the van der Waals length of which is about 21 Å, can only be aligned along the channel axis. This is in full agreement with the observed fluorescence anisotropy. For Ox + and Py + , geometrical arguments based on the zeolite L structure give room for only two possible arrangements of the molecules' long axis: a half cone angle of up to 40°for Ox + and up to 30°for Py + , and an angle of about 90°for both of them with respect to the c-axis of zeolite L. The surprising discrepancy between geometrical considerations and the results of the fluorescence measurements can be explained by assuming that Ox + and Py + are exposed to a considerable anisotropic electrical field in the zeolite channels.

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The stacking of pyronine and oxonine in the channels of zeolite L microcrystals is possibly due to their high affinity for entering the channels and to the narrowness of inside the channels, which prevents the dyes from gliding past each other. This allowed us to invent experiments for observing energy migration in pyronine-loaded zeolite L microcrystals of cylinder morphology. Organic dyes have the tendency to form aggregates at relatively low concentrations which cause fast thermal relaxation of electronic excitation energy. The role of the zeolite is to prevent this aggregation even at very high concentrations and to superimpose a specific organization. Light is absorbed by a pyronine molecule located somewhere in one of the zeolite channels. The excitation energy migrates preferentially in both directions along the axis of the cylinder because of the pronounced anisotropy of the system. It is eventually trapped by an oxonine located at the front or at the back of the microcrystal. This process is called front−back trapping. The electronically excited oxonine then emits the excitation with a quantum yield of approximately one. The pronounced anisotropy of the electronic transition moments of both pyronine and oxonine can be observed in an optical fluorescence microscope by means of a polarizer. Maximum luminescence appears parallel to the longitudinal axis of the cylindrical microcrystals, extinction appears perpendicular to it, and their base always appears dark. We report experimental results for the front−back trapping efficiency of pyronine-loaded zeolite L microcrystals of different average lengths, namely 700, 1100, and 1500 nm, different pyronine occupation probability, ranging from 0.03 to 0.48; and modification at the base with oxonine as luminescent traps. Extremely fast electronic excitation energy migration along the axis of cylindrical crystals has been observed, supported by the increase of the effective excitation lifetime caused by self-absorption and re-emission of the pyronine vertical to the cylinder axis. Effective energy migration lengths of up to 166 nm upon pyronine excitation have been observed, which thus leads to the remarkable properties of this material.

The degree of exchange of methyl viologen (MV2+) within the channels of zeolite L microcrystals was determined as a function of the amount of MV2+ added to an aqueous zeolite L suspension. Care was taken to remove molecules that have been adsorbed on the outer zeolite surface. Thus, the obtained values for the maximum occupation probability per unit cell are 0.78 for the commercial and 0.85 MV2+ for the self-synthesized potassium zeolite L for which the equilibrium constant was found to be in the order of 104 at room temperature. Adsorption isotherms and BET results show that the self-synthesized zeolite has a larger specific surface area as well as a larger and, thus, more accessible pore volume for the MV2+ than the commercial sample. IR spectra of very thin layers on ZnSe in high vacuum and Raman spectra at ambient conditions of MV2+−L zeolite at different loading levels are presented. They are compared with a MVCl2−KBr pellet and MV2+−Y zeolite spectra. The MV2+−L zeolite spectra indicate weak interactions between the MV2+ and the zeolite framework. They also indicate that the two pyridyl rings of the intercalated MV2+ are twisted. It was found that framework vibrations of the zeolite can be used as an internal standard for fast and nondestructive determinations of the MV2+ loading. Raman spectra are better suited for this purpose than IR. The reason for this is that the IR intensities of the zeolite framework vibrations at about 1050 cm-1 are much higher than those for all MV2+ modes, while the only strong zeolite framework Raman band at about 500 cm-1 is narrow and well-isolated and of similar intensity to the relevant MV2+ signals at high loading. On the basis of Rietveld refinement of X-ray data and molecular modeling, a model of the MV2+ location in the channels of zeolite L is proposed. The MV2+ lies along the channel wall, and the angle between the main MV2+ axis and the c-axis of the zeolite is 27°.

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Fine tuning of the size of zeolite L crystals in a large range is possible by changing the composition of the starting gel for otherwise constant reaction conditions. We describe a convenient way to prepare different crystalline materials in the size range of 30 nm up to 3000 nm. Representative data on the morphology, the pore volume, the size distribution and the optical antenna system behavior for light harvesting and transport are reported. We have extended the investigations on energy migration in pyronine-loaded zeolite L crystals as donor molecules, modified with oxonine as luminescent acceptors (traps) at the crystal ends. The preparation procedure reported and the extended zeolite materials now available lead to a large improvement of the energy migration efficiency.

Raman spectra of as-spun fibers produced through electrospinning have shown that high S/N data can be obtained on 50-μm diameter fibers in relatively short collection times (25 s). Using this same instrumental approach, “real time” Raman spectra of the electrospinning liquid fiber jet at the origin of the jet and 1 cm downstream have been obtained. The results show that “on-line” analysis of the solvent/polymer ratio and spectroscopic measurements of polymer orientation are possible and will lead to a more quantitative understanding of the development of the polymer microstructure during the electrospinning process.