Photoinduced electron transfer in nanostructures of ultrathin polyimide films containing porphyrin moieties

Photoinduced electron transfer between porphyrin moieties and pyromellitimide fragments has been investigated in multi-layered structures of ultrathin polyimide films prepared by the Langmuir–Blodgett (LB) technique. The LB films were composed of three kinds of polyimides, which contained zinc tetraphenylporphyrin (ZnTPP) unit as an electron donor (D-layer), no chromophoric groups (S-layer), and pyromellitimide fragments as an electron acceptor (A-layer). The layered structure and orientational distribution of porphyrin moieties in the LB films were evaluated by surface plasmon measurement and absorption dichroism measurement, respectively. The thickness of monolayer was estimated to be 0.9 nm for the polyamic acid films and 0.4 nm for the polyimide films. The molecular plane of porphyrin moieties was oriented in the direction parallel to the substrate plane. In the multi-layered structures of polyimide LB films, the efficiencies of photoinduced electron transfer from porphyrin moieties to pyromellitimide fragments varied sharply with the number of spacing layers, indicating that the short-range interactions such as electron transfer could be controlled by the fabric of ultrathin films. The rate of electron transfer observed by the fluorescence quenching measurements was numerically simulated for the nanostructure using the Monte Carlo method.