Direct evidence for singlet-triplet exciton annihilation in pi-conjugated polymers

A blend of two conjugated polymers with different optical band gaps and different triplet exciton (TE) magnetic resonance features was studied by means of photoinduced absorption (PA) and photoluminescence detected magnetic resonance. From the latter we find that a reduction of TE's on either of the two polymers enhances the radiative singlet exciton (SE) decay on both polymers nearly identically. The PA results rule out other possible mechanisms to yield this signal, except one: a long-range SE-TE annihilation due to a dipole-dipole transfer mechanism. Based on this finding, we calculate the transfer radius and quantify the annihilation process for this system and for other conjugated polymers.