Singlet fission is an exciton multiplication mechanism in organic materials whereby high energy singlet excitons can be converted into two triplet excitons with near unity quantum yields. As new singlet fission sensitizers are developed with properties tailored to specific applications, there is an increasing need for design rules to understand how the molecular structure and crystal packing arrangements influence the rate and yield with which spin-correlated intermediates known as correlated triplet pairs can be successfully separateda prerequisite for harvesting the multiplied triplets. Toward this end, we identify new electronic transitions in the mid-infrared spectral range that are distinct for both initially excited singlet states and correlated triplet pair intermediate states using ultrafast mid-infrared transient absorption spectroscopy of crystalline films of 6,13-bis­(triisopropylsilylethynyl) pentacene (TIPS-Pn). We show that the dissociation dynamics of the intermediates can be measured through the time evolution of the mid-infrared transitions. Combining the mid-infrared with visible transient absorption and photoluminescence methods, we track the dynamics of the relevant electronic states through their unique electronic signatures and find that complete dissociation of the intermediate states to form independent triplet excitons occurs on time scales ranging from 100 ps to 1 ns. Our findings reveal that relaxation processes competing with triplet harvesting or charge transfer may need to be controlled on time scales that are orders of magnitude longer than previously believed even in systems like TIPS-Pn where the primary singlet fission events occur on the sub-picosecond time scale.

Singlet fission is a spin-allowed process of exciton multiplication that has the potential to enhance the efficiency of photovoltaic devices. The majority of studies to date have emphasized understanding the first step of singlet fission, where the correlated triplet pair is produced. Here, we examine separation of correlated triplet pairs. We conducted temperature-dependent transient absorption on 6,3-bis(tri isopropylsilylethynyl)pentacene (TIPS-Pn) films, where singlet fission is exothermic. We evaluated time constants to show that their temperature dependence is inconsistent with an exclusively thermally activated process. Instead, we found that the trends can be modeled by a triplet-triplet energy transfer. The fitted reorganization energy and electronic coupling agree closely with values calculated using density matrix renormalization group quantum-chemical theory. We conclude that dissociation of the correlated triplet pair to separated (but spin-entangled) triplet excitons in TIPS-Pn occurs by triplet-triplet energy transfer with a hopping time constant of approximately 3.5 ps at room temperature.

The ability to undergo spin-allowed exciton multiplication makes singlet fission materials promising for photovoltaic applications. Here, we examine the separation of correlated triplet pairs, 1(T…T), in polycrystalline pentacene films via temperature-dependent transient absorption spectroscopy. Single wavelength analysis reveals a profound delay in 1(T…T) dynamics. Moreover, the dynamics of 1(T…T) exhibit temperature dependence, whereas other features show no discernable temperature dependence. Previous literatures have suggested that correlated triplet separation is mediated by a thermally activated hopping process. Surprisingly, we found that the time constants governing triplet pair separation display two distinct temperature-dependent regimes of triplet transport. The high temperature regime follows a thermally activated hopping mechanism. The experimentally derived reorganization energy and electronic coupling is verified by density matrix renormalization group quantum chemical calculations. In addition, we evaluated the low temperature regime and show that the trend can be modelled by a Miller–Abrahams-type model that incorporates the effects of energetic disorder. We conclude that the correlated triplet pair separation is mediated by thermally activated hopping or a disorder driven Miller–Abrahams-type mechanism at high and low temperature, respectively. We observe that crossover between two regimes occurs ∼226 K. We find the time constant for triplet–triplet energy transfer to be 1.8 ps at ambient temperature and 21 ps at 77 K.

We have provided a model for understanding two isoelectronic matryoshka clusters, [Sn@Cu12@Sn20](12-) and [As@Ni12@As20](3-). By dividing each of the clusters in a layer-by-layer manner and allowing each layer to follow a simple electron-filling rule, we can formulate a consistent model to explain experimental and computed properties of both matryoshka clusters that cannot be adequately explained by existing models. By analysing these clusters in a way analogous to peeling an onion, we can not only have an understanding of the structure and bonding of the two matryoshka clusters under study, but also have a generalizable model to handle certain p/d-block@d-block endohedral clusters.

A long excited state lifetime is a desirable quality of photocatalysts because it enables a higher probability of energy or electron transfer from the photocatalyst to a substrate.