This account aims at providing an understanding of singlet fission, i.e., the photophysical process of a singlet state ( S1 ) splitting into two triplet states (2 × T1 ) in molecular chromophores. Since its discovery 50 years ago, the field of singlet fission has enjoyed rapid expansion in the past 8 years. However, there have been lingering confusion and debates on the nature of the all-important triplet pair intermediate states and the definition of singlet fission rates. Here we clarify the confusion from both theoretical and experimental perspectives. We distinguish the triplet pair state that maintains electronic coherence between the two constituent triplets, 1(TT) , from one which does not, 1(T···T) . Only the rate of formation of 1(T···T) is defined as that of singlet fission. We present distinct experimental evidence for 1(TT) , whose formation may occur via incoherent and/or vibronic coherent mechanisms. We discuss the challenges in treating singlet fission beyond the dimer approximation, in understanding the often neglected roles of delocalization on singlet fission rates, and in realizing the much lauded goal of increasing solar energy conversion efficiencies with singlet fission chromophores.

Singlet fission, the generation of two triplet excited states from the absorption of a single photon, may potentially increase solar energy conversion efficiency. A major roadblock in realizing this potential is the limited number of molecules available with high singlet fission yields and sufficient chemical stability. Here, we demonstrate a strategy for developing singlet fission materials in which we start with a stable molecular platform and use strain to tune the singlet and triplet energies. Using perylene diimide as a model system, we tune the singlet fission energetics from endoergic to exoergic or iso-energetic by straining the molecular backbone. The result is an increase in the singlet fission rate by 2 orders of magnitude. This demonstration opens a door to greatly expanding the molecular toolbox for singlet fission.

This study explores a new mode of contortion in perylene diimides where the molecule is bent, like a bow, along its long axis. These bowed PDIs were synthesized through a facile fourfold Suzuki macrocyclization with aromatic linkers and a tetraborylated perylene diimide that introduces strain and results in a bowed structure. By altering the strings of the bow, the degree of bending can be controlled from flat to highly bent. Through spectroscopy and quantum chemical calculations, it is demonstrated that the energy of the lowest unoccupied orbital can be controlled by the degree of bending in the structures and that the energy of the highest occupied orbital can be controlled to a large extent by the constitution of the aromatic linkers. The important finding is that the bowing results not only in red‐shifted absorptions but also more facile reductions.

Singlet fission occurs only in a limited number of molecules, and expanding the molecular toolbox is necessary for progress. Here, we apply the molecular contortion strategy to tune singlet and triplet energies and observe changes in the excited-state dynamics that are consistent with singlet fission playing a role in thin films of contorted perylene. Perylene is a prototypical molecular chromophore, which does not undergo singlet fission in its planar form from its S1 state. The tuning of the energetics that control singlet fission through molecular contortion can be applied to a large repertoire of established molecular chromophores.

This study explores a new mode of contortion in perylene diimides where the molecule is bent, like a bow, along its long axis. These bowed PDIs were synthesized through a facile fourfold Suzuki macrocyclization with aromatic linkers and a tetraborylated perylene diimide that introduces strain and results in a bowed structure. By altering the strings of the bow, the degree of bending can be controlled from flat to highly bent. Through spectroscopy and quantum chemical calculations, it is demonstrated that the energy of the lowest unoccupied orbital can be controlled by the degree of bending in the structures and that the energy of the highest occupied orbital can be controlled to a large extent by the constitution of the aromatic linkers. The important finding is that the bowing results not only in red‐shifted absorptions but also more facile reductions.