Singlet fission in pentacene dimers

Singlet fission (SF) has the potential to supersede the traditional solar energy conversion scheme by means of boosting the photon-to-current conversion efficiencies beyond the 30% Shockley–Queisser limit. Here, we show unambiguous and compelling evidence for unprecedented intramolecular SF within regioisomeric pentacene dimers in room-temperature solutions, with observed triplet quantum yields reaching as high as 156 ± 5%. Whereas previous studies have shown that the collision of a photoexcited chromophore with a ground-state chromophore can give rise to SF, here we demonstrate that the proximity and sufficient coupling through bond or space in pentacene dimers is enough to induce intramolecular SF where two triplets are generated on one molecule.Singlet fission (SF) is a spin-allowed process to convert one singlet excited state into two triplet excited states, namely a correlated triplet pair (1). The ability to effectively implement SF processes in solar cells could allow for more efficient harvesting of high-energy photons from the solar spectrum and allow for the design of solar cells to circumvent the Shockley–Queisser performance limit (2). Indeed, several recent studies have demonstrated remarkably efficient solar cell devices based on SF (3–6).One requirement that needs to be met to achieve SF is that the photoexcited chromophore in its singlet excited state must share its energy with a neighboring ground-state chromophore. As such, the potential of coupled chromophores to afford two triplet excited states via SF has been elucidated in, for example, a tetracene dimer with an SF yield of around 3% (3, 7). Additionally, past experiments in single-crystal, polycrystalline, and amorphous solids of pentacene have documented that the efficiency of SF relates to the electronic coupling between these two chromophores (8, 9). Hence, molecular ordering in terms of crystal packing, that is, proximity, distances, orbital overlap, etc., is decisive with respect to gaining full control over and to fine-tuning interchromophoric interactions in the solid state (10, 11). Of equal importance are the thermodynamic requirements, namely that the energy of the lowest-lying singlet absorbing state must match or exceed the energy of two triplet excited states (S₁ ≥ 2T₁) (11). In light of both aspects, hydrocarbons such as acenes––tetracene, pentacene, hexacene––and their derivatives are at the forefront of investigations toward a sound understanding and development of molecular building blocks for SF. In tetracenes, the singlet- and triplet-pair energy levels are nearly degenerate (S₁ = 2T₁), leaving no or little standard enthalpy of reaction for SF (12). In solution, the latter is, however, offset by sizable entropy rendering the process rather slow and, thus, inefficient (13). In addition, the low SF yield relates to the dimer geometry. Its nature hinders electronic coupling through space, leaving only through-bond coupling effective. The latter is, however, insufficient to enhance the SF rate (7, 14). In stark contrast, the relaxed triplet excited state in pentacenes has significantly less than half the energy of the singlet excited state. In turn, the thermodynamic SF requirement, that is (S₁ ≥ 2T₁), is fulfilled for pentacenes rendering this process exothermic and unidirectional (13). Finally, from SF in hexacenes two triplet excited states plus a certain amount of phonons are derived as the main relaxation products. In other words, the dissipated heat leads to a decrease in SF yields and rates (15).In terms of exploiting SF for improving device performances, e.g., hybrid solar cells, it is necessary to efficiently dissociate correlated triplet pairs as they are formed, to overcome triplet–triplet annihilation (5, 16). Rapid injection of electrons into fullerenes, perylene diimides, colloidal nanocrystals, semiconductor substrates, etc. suggests a viable strategy. If successful, two charge carriers might be produced per absorbed photon and the photocurrents of the device can reach external quantum efficiencies of more than 100% (5, 17, 18).A provocative debate has been ignited about the mechanism of SF at the molecular level (19, 20). Controversy exists around the electronic states that are involved in the process, the coupling among them, and the effective nuclear dynamics (14, 21–27). Two contrasting SF mechanisms have been traditionally postulated––the direct and the two-step mechanism. These mechanisms differ in the number and nature of the electronic states that are involved in the SF process. For the direct mechanism, the nonradiative relaxation of the initially populated bright state proceeds via a correlated triplet pair state of singlet character––sometimes called multiexcitonic (ME) state––which then dissociates into two separated triplet excited states (28). For the two-step mechanism, the relaxation of the bright state occurs via an intermediate charge transfer (CT) state to the ME state. As in the direct mechanism, the ME state eventually allows the two triplet excited states involved to separate and undergo separate spin relaxation (11, 21–23). Recent works have, however, challenged these traditional viewpoints on the SF mechanism and several new models have been proposed. In particular, it has been suggested (13, 29, 30) that the initial excitation produces a coherent superposition of the lowest-lying absorbing state and the ME state, with the latter splitting into two separated triplet states after decoherence. On the other hand, recent theoretical works (14, 27) have proposed a model for SF in dimers, in which the ME state is formed from the absorbing state via a superexchange mechanism involving CT states, although the kinetics of the process is one-step–like. All of the above suggests that the traditional classification of SF mechanisms as direct or two-step is presumably too simple to describe the complexity of the process.Until recently, most studies regarding SF have been carried out in the solid state (11). However, in a groundbreaking report, Friend and coworkers showed that SF could be observed in solution at room temperature for a pentacene derivative (31). Key to this discovery was the formation of an intermediate state via the collision of a singlet excited-state pentacene and a second pentacene that was in the ground state. This study broke for the first time, to our knowledge, the dogma of molecular order as a mandate for SF and demonstrated that the order and packing might not be as crucial as believed, leaving SF as an intrinsic property even in a state of disorder.In this present contribution, we report on intramolecular SF within a set of three different pentacene dimers (Fig. 1), reaching triplet quantum yields as high as 156 ± 5%, as established by means of pump–probe experiments in solution. Experimental results are complemented by theoretical calculations using high-level ab initio multireference perturbation theory methods. In the series of pentacene dimers, the two pentacenes are linked via a phenylene spacer in an ortho-, meta-, and para-arrangement to impose geometrical control, which influences through-space and through-bond couplings in the ground- and excited state. The electronic coupling element, which governs SF and triplet–triplet annihilation, is mediated through-bond in the linearly and cross-conjugated para- and meta-isomers, respectively. In the ortho-isomer, through-space coupling should dominate due to the unique spatial proximity of the pentacenes (32, 33). Through experiments, we establish a sound picture of the SF mechanism in covalently linked pentacene dimers, showing the involvement of CT states.Open in a separate windowFig. 1.(Upper) Synthesis of pentacene dimers o-2, m-2, and p-2. (Lower) SF mechanism.