Influence of Excited-State Delocalization on Singlet Fission: Tuning Triplet-Pair-State Emission in Thin Films

The coherent distribution of an electronic excitation over multiple organic molecules in the solid state, namely excited-state delocalization, plays an important role in photophysical processes such as singlet fission. However, experimental studies of the influence of excited-state delocalization on singlet fission have been challenging mainly for two reasons. First, there is no easy way of measuring the excited-state delocalization, and second, tracking the resulting changes for singlet fission is demanding due to the triplet-pair state, which is a crucial intermediate in singlet fission, being an optically dark state and hence hard to access experimentally. Binary systems offer a way to adapt the growth conditions of a singlet fission material, which enables tuning of the excited-state delocalization, possibly due to the impact of structural disorder on exciton localization. By varying the growth conditions, we demonstrate that emission from the triplet-pair state via Herzberg-Teller coupling is detectable in films with low growth rates of the singlet fission material, while the triplet-pair state shows no luminescence in the other cases due to triplet dissociation outcompeting the luminescent decay. With this we find that triplet-pair state luminescence correlates with higher excited-state delocalization.