Energy Transfer in Competition to Charge Transfer as a Method to Determine the Rate Constant for Charge Carrier Injection in the Sandwich System: Cyanine Dye Monolayer/p-Chloranil Crystal

A new method is described to calculate the rate constant for charge transfer (CT) from an excited dye to a molecular crystal by taking into account competing energy transfer to a metal electrode. For the system oxacarbocyanine dye/p-chloranil single crystal in a sandwich arrangement with additional arachidic acid monolayers and evaporated aluminium electrodes, based on photocurrent and fluorescence measurements kCT - (1.5 ± 0.7) 109 sec-1 has been calculated.This overall rate constant is discussed in terms of a detailed but simple reaction scheme for charge carrier injection. It turned out to be equal to the dissociation rate constant of the excited dye times the quantum efficiency ΦCT for dissociation of an intermediate “charge transfer” state. Dissociation occurs from a vibrationally relaxed excited state of the dye.Finally, comparison is made with a straightforward calculation using the well-known unipolar current density. Nearly quantitative agreement of estimations emphasizes the use of the mono-layer technique to build "dry" electrochemical cells in order to study the principles of photo-electrochemical reactions.