Controlling Singlet Fission by Molecular Contortion Felisa S. Conrad-Burton, †,# Taifeng Liu,* ,†,‡,# Florian Geyer, † Roberto Costantini, †,∥,⊥ Andrew P. Schlaus, † Michael S. Spencer, † Jue Wang, † Raul Herna ́ ndez Sa ́ nchez, † Boyuan Zhang, † Qizhi Xu, †,§ Michael L. Steigerwald, † Shengxiong Xiao, ‡ Hexing Li, ‡ Colin P. Nuckolls,* ,†,‡ and Xiaoyang Zhu* ,† † Department of Chemistry, Columbia University, New York, New York 10027, United States ‡ Department of Chemistry, Shanghai Normal University, Shanghai, China § Department of Chemistry, Wuhan University of Science and Technology, Wuhan, China ∥ CNR-IOM, AREA Science Park, Basovizza, 34149 Trieste, Italy ⊥ Physics Department, University of Trieste, Via Valerio 2, 34127 Trieste, Italy * S Supporting Information ABSTRACT: 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. Many studies on singlet fission 1,2 utilize acenes and oligo- acenes due to their high singlet fission yields, 3−11 but acenes are not sufficiently stable for practical applications. Past attempts at expanding the molecular library for singlet fission has been limited by the very small number of known chromophores 12−17 that satisfy the energetic requirement of a first excited singlet energy greater than or equal to twice the triplet energy, E(S 1 ) ≥ 2xE(T 1 ). This limitation has confined much of the chemical design for singlet fission to controlling the linkage chemistry between chromophores or controlling the molecular packing in solids. Here, we present a new strategy, to create practically applicable chromophores for singlet fission by contorting aromatic structures through intramolecular strain to tune their excited state energies. Singlet and triplet energies are determined not only by the molecular structure but also by the degree of contortion, such as bowing and twisting, present in that molecular structure. 18 Here we explore this strategy for tuning the energetics of chromophores from unfavorable to favorable for singlet fission. To demonstrate the strategy of using molecular contortion to control singlet fission we employed perylene diimide (PDI, Figure 1a) as our scaffold. PDI and its derivatives are useful in this context: it is exceedingly stable to harsh environmental conditions, it is the basis for highly efficient organic solar cells, and it is known to undergo singlet fission in the solid state with low rates due to unfavorable energetics leading to endoergic singlet fission. 16,19,20 Here we introduce contortion to create bowing of the PDI by adding two terphenyl groups (see Figure 1a for molecular structure and Figure 1b for crystalline packing). Importantly, this contortion results in a lowering of the singlet and the triplet energies, and a larger singlet−triplet gap due to an increase in exchange energy. DFT calculations (SI8) on this structure indicate the energies are similarly lowered by ∼100−200 meV (Figure 1c). Thus, in the longitudinally bowed structure, PDI-B, the S 1 → 2T 1 process goes from endoergic in planar PDI to approximately isoergic. To test this strategy, we study crystalline films of PDI-B and find that singlet fission occurs in 2.5 ps, which is 2−3 orders of magnitude faster than corresponding processes in films formed from planar PDI systems. 16,19,20 ■ RESULTS AND DISCUSSION Figure 1a displays the molecule we designed to test whether the longitudinal contortion induces efficient singlet fission. Details for its synthesis and characterization are contained in the Supporting Information. The bottom part of Figure 1a displays the molecular structure from single crystal X-ray diffraction. Repulsion between the middle phenyl of the terphenyl bridges and the PDI bay position, as well as strong repulsion between the outer phenyl of the terphenyl and the carbonyl of the imide, together bend the PDI along its long axis. Figure 1b illustrates the crystal packing structure of PDI- B. In this system, there is only pi-pi interaction along the b-axis Received: May 19, 2019 Published: July 29, 2019 Article pubs.acs.org/JACS Cite This: J. Am. Chem. Soc. 2019, 141, 13143-13147 © 2019 American Chemical Society 13143 DOI: 10.1021/jacs.9b05357 J. Am. Chem. Soc. 2019, 141, 13143−13147 Downloaded via COLUMBIA UNIV on July 23, 2020 at 21:42:25 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.