Triplet Transport to and Trapping by Acceptor End Groups on Conjugated Polyfluorene Chains

Triplet excited states created in polyfluorene (pF) molecules having average lengths up to 170 repeat units were transported to and captured by trap groups at the ends in less ∼40 ns. Almost all of the triplets attached to the chains reached the trap groups, ruling out the presence of substantial numbers of defects that prevent transport. The transport yields a diffusion coefficient D of at least 3 × 10–4 cm2 s–1, which is 30 times typical molecular diffusion and close to a value for triplet transport reported by Keller (J. Am. Chem. Soc. 2011, 133, 11289–11298). The triplet states were created in solution by pulse radiolysis; time resolution was limited by the rate of attachment of triplets to the pF chains. Naphthylimide (NI) or anthraquinone (AQ) groups attached to the ends of the chains acted as traps for the triplets, although AQ would not have been expected to serve as a trap on the basis of triplet energies of the separate molecules. The depths of the NI and AQ triplet traps were determined by intermolecular triplet transfer equilibria and temperature dependence. The trap depths are shallow, just a few times thermal energy for both, so a small fraction of the triplets reside in the pF chains in equilibrium with the end-trapped triplets. Trapping by AQ appears to arise from charge transfer interactions between the pF chains and the electron-accepting AQ groups. Absorption bands of the end-trapped triplet states are similar in peak wavelength (760 nm) and shape to the 760 nm bands of triplets in the pF chains but have reduced intensities. When an electron donor, N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD), is added to the solution, it reacts with the end-trapped triplets to remove the 760 nm bands and to make the trapping irreversible. New bands created upon reaction with TMPD may be due to charge transfer states.