Sweet Spot of Intermolecular Coupling in Crystalline Rubrene: Intermolecular Separation to Minimize Singlet Fission and Retain Triplet-Triplet Annihilation.

Singlet fission is detrimental to NIR-to-vis photon upconversion in the solid rubrene (Rub) films, as it diminishes photoluminescence efficiency. Previous studies have shown that thermally activated triplet energy transport drives singlet fission with nearly 100% efficiency in closely packed Rub crystals. Here, we examine triplet separation and recombination as a function of intermolecular distance in the crystalline films of Rub and the t -butyl substituted rubrene ( t BRub) derivative. The increased intermolecular distance and altered molecular packing in t BRub films cause suppressed singlet dissociation into free triplets due to slower triplet energy transport. It was found that the formation of correlated triplet pairs 1 (TT) and partial triplet separation 1 (T···T) occurs in both Rub and t BRub films despite differences in intermolecular coupling. Under weak intermolecular coupling as in t BRub, geminate triplet annihilation of 1 (T···T) outcompetes dissociation into free triplets, resulting in emission from the 1 (TT) state. Essentially, increasing intermolecular distance up to a certain point (a sweet spot) is a good strategy for suppressing singlet fission and retaining triplet-triplet annihilation properties.