Ultrafast field assisted exciton dissociation in oligofluorenes

We report field assisted pump-probe measurements on a 9,9,9Œ,9Œ,9,9-Hexa-sec-butyl-[2,2Œ,7Œ,2]terfluorene trimer. An electric field as high as 2.2 MV/cm is applied in reverse bias to produce dissociation of the photogenerated singlets without charge injection. The electric field differential transmission dynamics at early times suggest Stark shift of the lower singlet excited state S1. The magnitude of the shift provides us with a change of polarizability of 0.43 ~ 10-19 eV m2/V2 between the lower excited and ground states. Differing from poly(9,9-dioctyl fluorene), (PFO), singlet dissociation is not instantaneous but occurs during timescales not inferior to 80 ps. Beyond these timescales we observe a decrease in the signal associated to polarons followed by a repopulation of singlets.