Optimizing through-space interaction for singlet fission by using macrocyclic structures

Abstract

There is great interest in the exploitation of singlet fission (SF) materials to improve the power conversion efficiency (PCE) of solar cells. Usually, ultrafast SF is achieved as an intermolecular process (xSF). However, it isn’t easy to precisely tune crystal packing in the experiment. In contrast, electronic structural properties can be exactly tuned in intramolecular SF (iSF) materials by changing different linkers. Nevertheless, few designs can achieve ultrafast SF in iSF materials. In this work, we use macrocyclic structures to maintain the π–π packing between two pentacenes and optimize through-space interaction in iSF. First, we conduct a detailed discussion on the experimentally discovered bipentacene macrocycle (BPc) and phenylene-linked bipentacene (BP1) by performing high-level electronic structure calculations and molecular dynamics (MD) sampling. The calculated iSF rates are in good agreement with experimental measurements. More importantly, the macrocyclic scaffold in BPc only plays a role in restricting the relative position of two pentacenes and does not affect the electronic coupling between excited states. Accordingly, 19 optimal structures are screened out from 97 initial candidates, and those 19 systems exhibit remarkably efficient iSF features with unprecedented ultrafast time constants (tens of femtoseconds).