Excited state dynamics in alternating polyfluorene copolymers

We present the excited state dynamics of the low band gap alternating polyfluorene copolymer, DiO-PFDTBT. The copolymer structure is an alternating sequence of donor and acceptor units consisting of 9,9-dioctyl-9H-fluorene and 4,7-di-thiophen-2-yl-benzo[1,2,5]thiadiazolyl units. Calculations have predicted that the first excited state has charge transfer character, while the second strongly allowed excited state is a delocalized π-conjugated state. We show that the excited state dynamics involves internal conversion (IC) from the second strongly allowed excited state to the first excited state. Furthermore, excitation energy transfer (EET) on the ∼1-10 ps time scale is observed as well as a subsequent formation of a stabilized long-lived emissive state, which has a decay time of 3.25 ns. The origin of this stabilized state is discussed in relation to the intrinsic charge transfer nature of the DiO-PFDTBT.