Singlet Fission in Poly(9,9 '-di-n-octylfluorene) Films

Abstract

The dynamics of singlet fission in poly(9, 9′-di-n-octylfluorene) (PFO) films was studied by transient absorption spectroscopy. Under a high excitation intensity, triplet excitons were rapidly interconverted from singlet excitons on a picosecond time stage. From the excitation intensity dependence of the generation yield, the rapid triplet exciton formation is attributed to the singlet fission from a higher singlet excited state Sn formed by the singlet–singlet annihilation (singlet fusion followed by singlet fission). The singlet fission from the Sn state was about 2 times more efficient in amorphous PFO films than in ordered β-PFO films, in which the conjugated main chains are planarly extended and aligned. On the other hand, about a half of triplet pairs were deactivated in the amorphous PFO film through the triplet–triplet annihilation with a time constant of 1.7 ns, while no triplet pair decay was observed in the β-PFO film. As a result, the overall triplet generation yield was comparable in these films. The decrease in the fission efficiency in the β-PFO is discussed in terms of the presence of another relaxation pathway from the Sn state. The efficiency of triplet pair dissociation is discussed in terms of the triplet exciton diffusion.