Singlet fission of hot excitons in π -conjugated polymers

We used steady-state photoinduced absorption (PA), excitation dependence (EXPA(ω )) 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-luminescentπ -conjugated polymers. From the high energy step in the triplet EXPA(ω ) spectrum of the luminescent polymer poly(dioctyloxy)phenylenevinylene (DOO-PPV) films, we identified a hot-exciton SF (HE-SF) process having threshold energy atE ≈2E T (=2.8 eV, whereE 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 atE >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(ω ) spectrum in the non-luminescent polymer polydiacetylene (PDA) is flat with an onset atE =E g (≈2.25 eV). From this, we infer that intrachain SF that involves a triplet–triplet pair state, also known as the ‘dark’ 2Ag exciton, dominates the triplet photogeneration in PDA polymer asE 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 inπ -conjugated polymers is a much more general process than thought previously.