Hydrogen Bonding Optimizes Singlet Fission in Carboxylic Acid Functionalized Anthradithiophene Films

The rate of singlet fission, the process of generating two triplet excitons with photoexcitation of one singlet exciton, depends on a combination of singlet/triplet energy balance and intermolecular coupling. Here, we perform carboxylic acid functionalization of anthradithiophene (ADT) derivatives that results in hydrogen bonds that drive molecular orientation and strong electronic coupling of polycrystalline ADT thin films, leading to ultrafast singlet fission without significant enthalpic driving force. ADT with a single carboxylic acid group exhibits weak intermolecular coupling and slow and inefficient singlet fission, much like the parent ADT, and substitution of different alkylsilyl solubilizing groups has little effect. However, the addition of two carboxylic acid groups on either end of the long axis favors significant coupling and crystallinity in as-deposited thin films that increase the effective singlet fission rate by roughly three orders of magnitude. The properties of the triplet pair, particularly its propensity to form long-lived independent triplets, are also influenced by the degree of long-range intermolecular coupling. The enhancement of intermolecular coupling specific to singlet fission using the ubiquitous cyclic hydrogen bonding motif could impact triplet pair utilization schemes in a variety of contexts.