Analysis of the electrostatics of charge carrier injection into molecular crystals during nonequilibrium electron transfer reactions demonstrates the unique advantage of aqueous or similar electrodes in measuring limiting currents which yield the rate constant of the injection process. At the phase boundary molecular crystal/aqueous electrolyte image forces are negligible due to the slow orientation polarization of water molecules with respect to the hopping frequency of injected charge carriers. Coulomb forces arising from slowly mobile or localized counter charges are shown to be effectively screened by water as a consequence of its relatively higher static dielectric constant as compared to that of the crystal.
Recombination of the bacteriochlorophyll dimer cation and the bacteriopheophytin anion depends on the spin multiplicity of this radical ion pair and can be studied by optical spectroscopy in an external static magnetic field in the absence (MARY) and presence of microwaves (RYDMR). Model calculations of MARY- and RYDMR-spectra are discussed.
The dependence of the sensitized injection current and of the crystal's sensitized delayed fluorescence on the electric and magnetic field is utilized to examine the process of charge separa-tion at the phase boundary crystal/water electrode. Hole currents, sensitized by electron transfer interaction between rhodamine B molecules in their lowest excited singlet state and the anthracene crystal, can be shown to originate from a thermal injection process. This conclusion is further supported by the changed phenomenology of the injection current and the delayed fluorescence in the presence of molecular oxygen in the aqueous electrode. In this case an alternative injection reaction predominates due to the oxygen enhancement of the intersystem crossing rate and thus of the sensitized population of triplet excitons in the crystal, the latter ones injecting holes through electron transfer to oxygen present at the surface.
Neglecting image-and Coulomb forces at the interface crystal/electrolyte, an analytical ex-pression for the current-voltage relation can be given which includes the influence of the space charge. This expression does well agree with the experimental current-voltage characteristics ob-served for different surface recombination rates of charge carriers.
The light guide properties of planar lipid bilayers are discussed and experimentally verified. The interaction between the light and the constituents of the bilayer is substantially increased since the light path is as large as the diameter of the film, e.g. of the order of several millimeters. The influence of electric fields on the bilayer and on the bilayer-torus transition region has been investigated. Field induced generation of scattering centers is detected in solvent containing bilayers.