Vectorial photoinduced electron transfer in phytochlorin-[60]fullerene Langmuir-Blodgett films

A novel molecular electron donor-acceptor (DA) dyad, composed of a phytochl orin donor and a [60]fullerene acceptor, was used for the preparation of so lid molecular films capable of performing vectorial photoinduced electron t ransfer (VPET). Being mixed with octadecylamine at concentrations of 50 mol % and lower, the DA compounds form a stable monolayer, which can be transf erred onto a solid substrate. Thus prepared Langmuir-Blodgett (LB) monolaye r films are characterized by uniform orientation of the DA molecules and, c onsequently, can undergo VPET. This was confirmed by time-resolved Maxwell displacement charge (TRMDC) measurements. The rate constant for the electro n transfer was ca. 10(9) s(-1) as estimated from the fluorescence lifetime measurements. The majority of the charge transfer states of the DA molecule s (> 60%) recombined with a time constant of ca. 30 ns, being almost indepe ndent of the DA concentration in the concentration range from 2 to 50 mol % , as revealed from TRMDC experiments. Therefore, VPET is most probably an i ntramolecular reaction. The dependences of the TRMDC signal amplitude on th e DA concentration and on the density of the excitation energy indicated th at an energy transfer takes place prior to the electron transfer. A variati on in the charge recombination rate was observed when a static bias voltage was applied across the films. An estimation of the charge displacement dis tance across the film gave a value of ca. 0.5 nm.