Singlet fission of hot excitons in π-conjugated polymers

We used steady-state photoinduced absorption (PA), excitation dependence (EXPA(omega)) spectrum of the triplet exciton PA band, and its magneto-PA (MPA(B)) response to investigate singlet fission (SF) of hot excitons into two separated triplet excitons, in two luminescent and non-luminescent pi-conjugated polymers. From the high energy step in the triplet EXPA(omega) spectrum of the luminescent polymer poly(dioctyloxy)phenylenevinylene (DOO-PPV) films, we identified a hot-exciton SF (HE-SF) process having threshold energy at E approximate to 2E(T) (= 2.8 eV, where E-T is the energy of the lowest lying triplet exciton), which is about 0.8 eV above the lowest singlet exciton energy. The HE-SF process was confirmed by the triplet MPA(B) response for excitation at E > 2E(T), which shows typical SF response. This process is missing in DOO-PPV solution, showing that it is predominantly interchain in nature. By contrast, the triplet EXPA(omega) spectrum in the non-luminescent polymer polydiacetylene (PDA) is flat with an onset at E = E-g (approximate to 2.25 eV). From this, we infer that intrachain SF that involves a triplet-triplet pair state, also known as the 'dark' 2A(g) exciton, dominates the triplet photogeneration in PDA polymer as E-g > 2E(T). The intrachain SF process was also identified from the MPA(B) response of the triplet PA band in PDA. Our work shows that the SF process in pi-conjugated polymers is a much more general process than thought previously.