Diffusion Limited Oxygen Quenching of Photo‐Induced Electrochemical Reactions in Sandwich Systems with Molecular Crystals

Abstract

Photoelectrochemical reactions in sandwich systems with the dye 3,3′‐distearyl‐oxacarbocyanine chlorate adsorbed in monolayer assemblies on p‐chloranil single crystals have been used to demonstrate the quenching action of paramagnetic oxygen on sensitized as well as on triplet exciton induced photocurrents. Simultaneously, the presented measurements are a method to study the lateral diffusion of oxygen into monolayer assemblies between the crystal surface and an evaporated aluminum electrode by photoelectrical detection. From the functional time dependence of a reciprocal square‐root law and together with the discussion of a kinetic scheme the rate constants for the quenching of excited dye molecules and of n‐π*‐triplet excitons at the phase boundary could be evaluated to be kq = 2.5 · 1012 M−1 s−1 and 3kq = 1012 M−1 s−1, respectively, on the basis of an estimated upper limit for the diffusion coefficient D = 4 · 10−6 cm2 s−1 for oxygen diffusion into the slab with a bilayer structure. – The method has been extended to sandwich systems with rhodamine B on the surface of anthracene and p‐chloranil crystals that showed no effect due to oxygen, whereas the systems cyanine dye/anthracene revealed less marked photocurrent enhancements as well as decreasings. These results are discussed in terms of hindered oxygen access and of an extended reaction scheme that involves the well‐known oxidizing action of oxygen in sensitization.

A thin absorbing layer at the center of a Fabry-Pérot interferometer

“…The pK-values of benzoic acid is around 4.2 [10], therefore it is very likely that the dye molecule exhibits a positive net charge. Some space filling models suggest a thickness of 0.6-0.7 nm and an area of about 1.25 nm2 [12]. Actually, in a preliminary force run we measured a layer thickness of m 0.6 nm, the repulsive force law extended 3 nm, which agrees reasonably well with a Debye-length of 3 nm, expected in such a solution.…”