Generation of long-lived triplet-triplet multiexciton in Pentacene-(Tetracene)2-Pentacene intramolecular singlet fission compound - the theoretical perspective

Singlet fission (SF) is a spin-allowed conversion of the optical singlet exciton of an organic semiconductor to the optically dark triplet-triplet ¹(T₁T₁). In chromophores with small triplet-triplet binding energy ¹(T₁T₁) can separate into two free triplets T₁, each of which can donate an electron to an acceptor, thereby doubling the photoconductivity of an organic solar cell. Successful implementation of SF requires both ultrafast generation of the ¹(T₁T₁) and rapid dissociation into free triplets, which is a challenge, as the former requires strong coupling between the triplets, which in turn implies strong ¹(T₁T₁) binding energy. Pun et al. synthesized a series of oligomers Pentacene-(Tetracene)n-Pentacene, PTnP, in which rapid generation of pentacene-tetracene triplet-triplet ¹(T_1[P]T_1[T]) is followed by rapid triplet separation to long-lived pentacene-pentacene triplet-triplet ¹(T_1[P]T_1[P]) [1]. We present the results of many-body investigations of the electronic structures of the optical singlet and all low-energy triplet-triplets in PT2P that find distinct ¹(T_1[P]T_1[T]) and ¹(T_1[P]T_1[P]). We also report the ground and excited state absorptions that clarify triplet-triplet generation and triplet separation mechanisms. The mechanism of triplet separation is different from the simplest triplet migration that is assumed in reference [1], and that occurs in PT2P [2].

[1] A. B. Pun et al., Nat. Chem. 11, 821 (2019).

[2] R. Chesler et al., Phys. Rev. B 110, L121104 (2024).