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

[Image: see text] 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 (tBRub) derivative. The increased intermolecular distance and altered molecular packing in tBRub 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 tBRub films despite differences in intermolecular coupling. Under weak intermolecular coupling as in tBRub, 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.