Energy-Transfer Pathways and Triplet Lifetime Manipulation in a Zinc Porphyrin/F8BT Hybrid Polymer

Triplet states are ubiquitous in organic electronics and their properties are increasingly being exploited to enhance device efficiencies. The difficulty in accurately probing triplet states dictates that more fundamental understanding is required of their properties. In this work, a hybrid co-polymer of poly­(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) with 10% by weight zinc porphyrin was synthesized and a transient absorption spectroscopy study performed. It was observed that a dual energy-transfer mechanism was active, whereby the ultimate fate of each photogenerated F8BT singlet exciton depended upon its distance to a porphyrin unit. F8BT excitons generated within the bulk of the F8BT polymer showed typical F8BT photophysics, with the small proportion of F8BT triplets created able to diffuse to and undergo triplet energy transfer to the porphyrin units. In contrast, F8BT singlet excitons formed within their diffusion length to a porphyrin unit displayed singlet energy transfer, followed by intersystem crossing to create the lower energy porphyrin triplet. Intriguingly, the F8BT–HAPAPP triplets generated have a lifetime intermediate between the two pristine materials. Density functional theory calculations suggest that this is due to orbital mixing between energetically close benzothiadiazole- and porphyrin-localized molecular orbitals, creating a mixed F8BT/porphyrin triplet state