Spiroconjugation-enhanced intramolecular charge-transfer state formation in a polyspirobifluorene homopolymer

In this work we demonstrate the complex excited-state behavior of polyspirobifluorene in the solid state, which, due to the interaction of the spiroconjugated side group, readily forms a charge-transfer (CT) excited state of lower energy than the singlet exciton. The polymer is compared to standard polyfluorene materials using pump-probe and field-assisted pump-probe spectroscopy. It has been found that the small energy barrier between the singlet exciton and the CT state allows for population of the emissive singlet state at room temperature giving rise to a long tail in the singlet lifetime compared to an apparent single-exponential lifetime at low temperature. It is proposed that in devices the CT state is populated on charge recombination before the singlet states are excited by electron transfer from the CT state.