Vectorial Photoinduced Electron Transfer in Phytochlorin−[60]Fullerene Langmuir−Blodgett Films

A novel molecular electron donor−acceptor (DA) dyad, composed of a phytochlorin donor and a [60]fullerene acceptor, was used for the preparation of solid molecular films capable of performing vectorial photoinduced electron transfer (VPET). Being mixed with octadecylamine at concentrations of 50 mol % and lower, the DA compounds form a stable monolayer, which can be transferred onto a solid substrate. Thus prepared Langmuir−Blodgett (LB) monolayer films are characterized by uniform orientation of the DA molecules and, consequently, can undergo VPET. This was confirmed by time-resolved Maxwell displacement charge (TRMDC) measurements. The rate constant for the electron transfer was ca. 109 s-1 as estimated from the fluorescence lifetime measurements. The majority of the charge transfer states of the DA molecules (>60%) recombined with a time constant of ca. 30 ns, being almost independent of the DA concentration in the concentration range from 2 to 50 mol %, as revealed from TRMDC experiments. Therefore, VPET is most probably an intramolecular reaction. The dependences of the TRMDC signal amplitude on the DA concentration and on the density of the excitation energy indicated that an energy transfer takes place prior to the electron transfer. A variation in the charge recombination rate was observed when a static bias voltage was applied across the films. An estimation of the charge displacement distance across the film gave a value of ca. 0.5 nm.