Photoinduced electron transfer (charge separation) and back electron transfer (charge recombination) were investigated for the following hetero-Langmuir-Blodgett film device: ITO//insulator layers, 185 angstrom/two layers of steroidal tetracyano-p-quinodimethane as an acceptor (A), 36 A/two layers of a Cu-phtbalocyanine derivative as a sensitizer (S), 28 angstrom/two layers of steroidal p-phenylenediamine as a donor (D), 48 angstrom/insulator layers, 380 angstrom//Hg or Al. A transient electric current was observed by the selective photoexcitation of S and was ascribed to the net electron transfer from D to A. The resulting charge separation state in the ASD device survived for a few minutes. Applying electric biases and/or lowering temperatures caused the charge-separated state to last much longer. The charge recombination process observed for the ASD structure could not be described by the usual first-order reaction kinetics. We have elucidated the charge recombination mechanism on the basis of the Marcus theory by taking account of some distribution effects of both the activation energies and distances between the dye layers. The results of the lowering of the temperature (290 --> 110 K) indicated a wide distribution of the activation energy (D(E)), suggesting a distribution of free energy change during the electron transfer. The free energy change distribution (D(G)) was found to be nearly in accordance with the product of the Gaussian distribution of the energy levels for charge-separated states and the Fermi distribution of the electric charges. Insertion of insulator layers between A and S also brought a distance distribution between A and S, which was probably caused by some structural defects in the insulator layers. The charge separation processes are also discussed.
