Susan Elaine Hartman

Selected aspects of photochemistry in polymer media

Abstract A number of selected aspects of the reciprocal interactions of polymers with excited solutes or polymer-bound chromophores are given. The role of free volume, glass-transition temperature (Tg), microscopic viscosity, and polarity or environments to which excited molecules are exposed is emphasized in order to illustrate possible manners in which the photophysical, photochemical, and subsequent chemistries can be influenced. Many of these factors can affect several monomolecular and bimolecular processes encountered in the sensitization of photocrosslinkable polymers: the photophysics of the sensitizer, energy transfer to the reactive sites on the polymer, and the formation of crosslinks. In fact, the triplet yields of aroylnaphthothiazole derivatives, a widely used class of sensitizers, are considerably higher in the more viscous polymeric matrices. These 0. in the polymer approach, however, a value of only ca. 0.7. As a result a search for more efficient triplet sensitizers led to a new class of compounds: 5- and 7-substituted 3-ketocoumarins. As a model for study of the bimolecular processes in polymers, we chose exciplex and excimer probes. The following conclusions were drawn: 1. Several exciplex and excimer emissions in polymeric media are considerably shifted to shorter wavelengths as compared with the maxima measured in fluid media, indicating that interactions are impaired in polymeric matrices. 2. Emissions from the polymer matrix above the glass-transition temperature are similar in wavelength and temperature dependence thereof to those observed in fluid solutions. 3. Improper orientation of the reactants in polymers is responsible for the shift observed in excimer emission and for a part or all of the shift in exciplex emissions. 4. Little if any difference is observed between exciplex emissions in polymers of low and moderate macroscopic polarities. This may be attributed to two causes: a) Due to improper orientation the dipole moment of the exciplex in the polymer is expected to be smaller and, therefore, less solvation energy can be gained. b) The segmental motion of the polymer required to properly solvate the complex is probably too slow at room temperature compared with the lifetime of the exciplex. 5. Bichromophoric molecules, which form exciplexes in fluid media, fail to reach an exciplex configuration when dissolved in polymers.

Hydrophobic Interactions of Aromatic Hydrocarbons Induced by Surfactants and Polyelectrolytes

The microenvironment of charged molecules can be significantly altered in the presence of ionic molecules such as surfactants, lipids, and polyelectrolytes. Such a change in the microenvironment of aromatic hydrocarbon chromophores can be conveniently studied spectroscopically.