(Invited) Tuning Molecular Singlet Fission – Strong Versus Weak Electronic Coupling

Abstract

Innovative strategies are necessary to overcome the Shockley-Queisser limit, which places the maximum solar conversion efficiency for a single p-n junction to slightly more than 30%. One such strategy, singlet fission (SF), allows high energy, singlet excited state photons (S1) to be down-converted into up to twice as many low-energy, triplet excited state photons (T1). As such, SF is a fascinating many-electron problem from a physical chemistry perspective. In an optimal SF scenario, a chromophore in a singlet excited state (S1) interacts with a nearby ground state (S0) chromophore to yield up to two, non-interacting T1s. Doubling the number of excited states, thus, goes hand in hand with sacrificing half or more of the overall energy per excited state. The energetic requirement for SF is E(S1) ≥ 2(E(T1)), and the free reaction enthalpy for the S1 ® 2(T1) conversion governs the energy loss in SF. The SF rate depends on the free reaction energy and the electronic coupling matrix element between one chromophore in its S1 and another in its S0 ground state. A strict definition of the SF rate implies the formation rate of the two individual triplets (T1 + T1), not that of the correlated triplet pair, 1(T1T1). In (T1 + T1), the two triplets should have lost electronic coherence owing to interactions with the environment, but can retain spin coherence on much longer timescales. In a nutshell, electronic coupling between the chromophores should be sufficient to drive SF, while at the same time keeping the coupling sufficiently weak to allow spin decoherence leading to two independent triplet states. The systematic variation / comprehensive understanding of electronic coupling between the individual pentacenes constitutes one of the grand challenges in the field of molecular SF, especially when experiments are conducted in solution. By means of tailoring the synthesis of pentacene dimers, trimers, and tetramers we will span our investigations from a weakly coupled, charge transfer to a strongly coupled, charge separated regime. In turn, we are confident to gain control over the rates and yields of intramolecular SF and, subsequent, inter- and intramolecular charge extraction. In particular, we plan to investigate pentacene materials, in which the energy of the charge transfer state and that of the charge separated state is tuned in the range around twice of the triplet excited state, and to explore the impact on the formation of pairs of independent triplets (T1 + T1).