Yuki Shibano, Hiroshi Imahori, Paiboon Sreearunothai, Andrew R. Cook and John R. Miller
J. Phys. Chem. Lett. 1, 14921496 (2010). [Find paper at ACS Publications]Abstract:
We have synthesized new conjugated, rigid rod oligomers of fluorene, Fn(C60)2, n = 4, 8, 12, and 16. These pure compounds have Fn chains up to 140 Å long. The C60 groups covalently attached at both ends serve as traps for excitons created in the Fn chains. Excitons created in the chains by photoexcitation reacted rapidly with the C60 groups with decays described well by the sum of two exponentials. Mean reaction times were 2.3, 5.5, and 10.4 ps for n = 8, 12, and 16. In F16(C60)2, the 10.4 ps reaction time was 40 times faster than that found in earlier reports on molecules of slightly longer length. The simplest possible model, that of one-dimensional diffusion of excitonic polarons that react whenever they encounter the end of a chain, fits the results to obtain diffusion coefficients. Deviations of those fits from the data may point to the need for alternative pictures or may just indicate that diffusion is not ideal. The definite lengths of these molecules enable a stringent test for theories. These results reveal that exciton transport can be much faster than previously believed, a finding that could, along with appropriate nanoassembly, enable new kinds of high-efficiency organic photovoltaics.