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

Abstract

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.

Enhancing NIR-to-visible photon upconversion in cast solid by introducing bulky substituents in rubrene and by suppressing back energy transfer

“…The better condition for efficient TTA-UC can be found by the introduction of bulky groups, which is consistent with the previous works. 21,51 We have demonstrated this by obtaining the highest-class values of η UC with the rubrene derivatives. This was achieved by lowering the sensitizer concentration and thus by suppressing the BET, which masked the improvements.…”

Molecular stacking controlling coherent and incoherent singlet fission in polymorph rubrene single crystals

“…[41] The possible interferences from polaron or oxygen-related defects which induces broad PIA band between 550 and 800 nm or emission peak at 650 nm, respectively, are also negligible (Figure S4 and Figure 1C). [31,33] In SF process, triplet species could involve different electronic configurations at different stages, including initial correlated triplet pairs (TT), subsequential spatially separated but spin−entangled triplet pairs (T⋅⋅⋅T) and ultimate free triplets (2×T). [9] In general, it is hard to distinguish them on TA spectroscopy due to similar spectral features, and the detailed assignments could be diverse across different literatures.…”

“…In this study, we choose rubrene, a tetracene backbone with four phenyl side groups, to tackle this problem. [31][32][33] Fortunately, by controlling crystallization process, rubrene molecules can form single crystals with different phases thus stacking geometries, including orthorhombic (Orth. ), triclinic (Tri.…”

Origin of the Delayed Fluorescence by Triplet–Triplet Annihilation in Solids with a Power Law Exponent of between −1/2 and −1

“…For example, if photoemission occurs by geminate recombination, where the recombination occurs at the reaction radius with a finite recombination rate constant and the initial distance of the pairs is equal to the reaction radius, then the power-law decay 1/ t a with a = 1 + α/2 is obtained in three dimensions . α = k B T / E 0 , and the normal diffusion corresponds to α = 1 and a = 3/2. − In this respect, we comment on the delayed fluorescence arising from triplet–triplet annihilation (TTA) following the singlet exciton fission (SEF). ,,− The power-law decay with a = 3/2 is obtained when the delayed fluorescence originates from the TTA following the SEF in one and three dimensions, where the triplet exciton pairs are generated with a specific distance by SEF. ,,, We have shown that the power-law decay with a ≤ 1 can be obtained for delayed fluorescence arising from TTA following triplet sensitization; when delayed fluorescence originates from the TTA following SEF, a > 1 has been theoretically predicted. ,,, …”