The Doubly Excited State in Singlet Fission

Abstract

In recent years, a variety of studies have investigated in great depth the details of singlet fission mechanisms in terms of electronic energies and couplings. The nature and properties of the triplet‐pair intermediate state or the role of charge separated configurations (CT states) have been extensively explored with the help of theoretical models, ab initio calculations, and experimental characterization. On the other hand, much less is known about the involvement of the localized double electron excitation, that is the D state. In organic conjugated molecules, the D state can be typically described as the HOMO‐to‐LUMO promotion of two electrons, which makes it inactive to photoabsorption (dark state). In the present study, we tackle the possible roles of the D state by means of electronic structure calculations and the simulation of singlet fission dynamics. Our results suggest that, indeed, the D state can have a significant impact on singlet fission. On one hand, if it lies energetically below the optical exciton it is able to drive the system to singlet fission through direct coupling or via a CT‐mediated path. The D state can also act as a mediator in the formation of the triplet‐pair state from initial photoactivation. These results should motivate researchers in the field to include this state in their analysis, and help to clarify electronic mechanisms in singlet fission materials.

High-Lying 31Ag Dark-State-Mediated Singlet Fission

“…Finally, Sandoval-Salinas and Casanova recently proposed a dark-state-mediated SF mechanism for a simple ethylene-dimer model in which the locally doubly excited HOMO-to-LUMO singlet state has a significant impact on SF dynamics at the eclipsed conformation. 48 However, this dark state is the lowest excited singlet state of an intermolecular SF system, which is different from our proposed higher-lying dark-state-mediated SF mechanism for intramolecular SF systems.…”

“…These free triplets have long-lived lifetimes at the μs time scale. Besides 1 (TT), the high-spin triplet-pair 5 (TT) state is also involved in intermolecular SF dynamics. − ,− For instance, Guldi and co-workers recently explored the SF dynamics of carbene-based diradical dimers and found both 1 (TT) and 5 (TT) are close to each other in energy because of the weak binding energy of intertriplets . This is also consistent with the recent theoretical work by the Scholes group .…”

Distinguishing between Triplet-Pair State and Excimer Emission in Singlet Fission Chromophores Using Mixed Thin Films

“…However, one complication that arises in the investigation of SF materials by PL spectroscopy is the fact that the intermediate 1 (TT) state is a dark state that becomes only emissive via Herzberg–Teller coupling. ,,− The resulting red shift of the corresponding emission compared to the energy of the 1 (TT) state leads to an overlap of its spectral signature with other contributions at low photon energies such as excimers ,,,,− or trap-state emission. ,− This poses the experimental challenge to disentangle the various contributions leading to emission features at low photon energies (in the following referred to as red-shifted luminescence features (RSL)) and to clarify whether the related states mediate SF ,,,,− or constitute a loss channel. ,,,,,,− This question directly relates to an ongoing debate in the literature, which concerns the role of excimer formation in the SF process. ,,,− ,, Excimers as a competing channel for SF have been observed experimentally in tetracene (TET) ,,, and antradithiophene (ADT) derivatives, , but have also been proposed as an intermediate state for SF. ,,, …”

Singlet fission in covalent dimers of methylene-locked 1,3-diphenyl-isobenzofuran: semiclassical simulations of nonadiabatic dynamics

“…Doubly excited states are not uncommon in conjugate p systems and may play a role in singlet ssion, depending on their energetics. 39 In the present case, however, we shall see in Section 3.2 that the S 2 S 0 and S 0 S 2 states are not particularly active in the singlet ssion dynamics.…”