Donald P. Specht

Preparation and Characterization of Polymerized Liposomes

Each of these classes of polymerizable lipids offers useful approaches to the preparation of polymerized liposomes. The lipid diacetylenes can be very photosensitive and yield highly colored rigid photopolymers. The reaction is sensitive to lipid structure, membrane packing, and temperature; however, the reaction is difficult to drive to completion. High polymer yields (greater than 90%) and a high degree of polymerization (500) are realized with the monofunctional methacryloyl lipids 4 and 5. These liposomes are characterized by multiple polymer chains per liposome and a moderate reduction in membrane permeability to glucose. Further decreases in membrane permeability can be achieved with dienoyl lipids, e.g., 6, which can be cross-linked as well as highly polymerized (greater than 95%). Progress in synthesis of polymerizable lipids has been rapid, and the initial characterization of some of the reported materials provides guidance for future synthetic endeavors.

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.

The triplet state of methyl 1,2-diphenylcyclopropene-3-carboxylate

Abstract Methyl 1,2-diphenylcyclopropene-3-carboxylate (CP), which is of special interest because of its role as a monomer in photo-cross-linkable polymers, was selected as a representative example of a substituted cyclopropene for investigation of its triplet state. Flash photolysis was used to view the CP triplet directly and to measure the rate constants for energy transfer to CP from a series of triplet sensitizers. Because the intersystem crossing quantum yield of CP is only 5 × 10−3, the triplet state can be populated efficiently only by energy transfer from an appropriate sensitizer. In benzene solution the triplet lifetime of CP is 360 μs, and the rate constant for reaction with ground state CP to form dimers is 3.8 × 107 M−1 s−1. The 0−0 energy of the triplet state is 51.0 kcal mol−1 (17.8 kilokaysers), determined from a plot of the energy transfer data according to the Balzani equation. The profile of this plot shows that the ground and triplet state geometries of CP are different, probably because of partial relief in the triplet state of the considerable ground state strain energy.

Photochemical deamination of phenylenediamines in acidic solutions

Photolysis of alkylated o- and p-phenylene-diamine derivatives in acidic methanol leads, via a homo-lytic bond cleavage, to the corresponding aniline.