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

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 S-n formed by the singlet-singlet annihilation (singlet fusion followed by singlet fission). The singlet fission from the S-n state was about 2 times more efficient in amorphous PFO films than in ordered beta-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 beta-PFO film. As a result, the overall triplet generation yield was comparable in these films. The decrease in the fission efficiency in the beta-PFO is discussed in terms of the presence of another relaxation pathway from the S-n state. The efficiency of triplet pair dissociation is discussed in terms of the triplet exciton diffusion.