Charge Transfer in Single Chains of a Donor–Acceptor Conjugated Tri-Block Copolymer

The photophysics of a conjugated triblock copolymer comprising poly­(9,9-dioctyl­fluorene-co-bis-N,N′-(4-methyl­phenyl)-bis-N,N′-phenyl-1,4-phenylene­diamine) (PFM) electron donor and poly­(4-(9,9-dioctyl-9H-fluoren-2-yl)­benzo­[c]­[1,2,5]-thiadiazole) (F8BT) electron acceptor blocks has been studied in solution, in films, and as single chains. While an additional long-wavelength emission apparent in neat films of the copolymer is attributed to interchain exciplex formation, no such long-wavelength emission is apparent in solution or from single molecules. However, in these cases, time-resolved fluorescence measurements indicate the presence of a delayed fluorescence. The kinetics of the delayed emission can be interpreted in terms of an equilibrium between a locally excited and a charge-transfer state at the interface of the copolymer block components. Rate constants and thermodynamic quantities associated with these processes have been evaluated. The single-molecule results allow the assignment of an intramolecular charge-transfer state in an isolated conjugated block copolymer chain.