Singlet Fission (SF) is a bichromophoric process whereby a singlet exciton is converted to two triplet excitons through an overall spin conserved process. The last decade has seen an upsurge in SF research driven by its potential application in photovoltaics to exceed the power conversion efficiency of solar cells beyond the Shockley-Quiesser (S-Q) limit along with the fact that it provides an interesting model system to investigate many-body interactions. The spatial separation of triplets to produce long-living free triplets has been a major challenge in intramolecular singlet fission (iSF) particularly due to the enhanced triplet-triplet annihilation (TTA) rates from the triplet pair states. In this study, we investigate the role of interchromophore twist in assisting free triplet generation through the precise control of bridge torsion along the molecular axis in an array of planar and twisted pentacene dimers. Transient Absorption measurements show that iSF can be broadly divided into three categories with planar dimers showing fast iSF (τiSF &lt; 10ps) and short triplet lifetimes while twisted dimers showing slow iSF (τiSF &gt; 500ps) and longer triplet lifetimes. A third group consisting of flexible dimers that were free to adopt a planar geometry exhibited intermediate rates (10ps &lt; τiSF &lt; 100ps). We demonstrate systematically that bridge torsion along the molecular axis can be an effective synthetic handle to modulate iSF dynamics and improve triplet lifetimes.

Investigating the role of interchromophore torsion : A molecular approach to tune the electronic coupling for Intramolecular singlet fission

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