Phosphorescence spectroscopy and its application to the study of colloidal dynamics

Abstract

The technique of Time-resolved phosphorescence anisotropy (TPA) has been frequently used to study rotational motions of particles on the micro- to millisecond time-scale. The interpretation of the observed TPA signals is, however, not straightforward. The theoretical description of the physical processes often requires more parameters than can be determined experimentally. This is due to complex interaction within and between the particles, but also to the incomplete knowledge of the photophysics of the dye molecules. In chapter 2 to 4 four dye molecules (erythrosine, eosin, 2-iodo- and 2-bromo-fluorescein) were scanned for their suitability for TPA experiments and their polarization properties were identified. The phosphorescence quantum yield, lifetime, and zero-time anisotropy were determined for dye molecules immobilized in a polymer matrix. Tha absolute orientation of the absorption and fluorescence dipole moments in the molecular were recovered from angle-resolved fluorescence depolarization measurements on dye molecules, macroscopically aligned in stretched polymer films. These results, in combination with the phosphorescence anisotropes at two excitation wavelenghts, were used to determine the orientations of the phosphorescence dipole moments relative to the absorption and fluorescence dipole moments. We have formulated a quantitative measure to select dye molecules for particular phosphorescence depolarization experiments. In addition, we hace delineated the optimal concentration range of dye molecules for these experiments. We found that the Z number of the substituted heavy atoms primarily influences the luminescent properties of the dyes, while the number of substituted heavy atoms appears to play a sunsidiary role. The effect of the spin-orbital coupling is to pull the phosphorescence dipole moments towards the fluorescence dipole moment and not so much towards the molecular plane. In chapter 5 a novel, phosphorescent, colloidal silica sphere tagged with eosin-5-isothyocyanate was introduced as a tracer for the study of rotational diffusion with TPA. The suitability of the tracer was tested by a combination of spectral and time-resolved fluorescence and phosphorescence measurements. The silica environment of the eosin proved to be useful: quenching processes are reduced markedly and the internal motion of the dye is restricted to a small cone. The hydrodynamic radius of the tracer found from the TPA measurements coincides with the value obtained from dynamic light scattering. This shows that eosin has a suitable phosphorescence lifetime and confirms the colloidal stability of the tracers. The rotational diffusion rates of the phosphorescent tracer spheres in packings and dispersions of host spheres were studied in chapter 6. The measurements on tracers in disperions agree fairly well ith calculations for hard spheres. An important advantage of TPA technique is that it can be used for experiments on slightly scattering media. This feature was exploited for assessing the rotational diffusion of the tracers in occasionally turbid packings. We found that the fraction of immobilized tracers can be estimated accurately by the pore size distribution extracted from the Voronoi construction of a simulated random close packing. We found that the dependence of the rotational diffusion coefficient on the average distance between tracer and medium spheres is stronger for packings than for dispersions.