Overlap-Driven Splitting of Triplet Pairs in Singlet Fission

Abstract

We analyze correlated-triplet-pair (TT) singletfission intermediates toward two-triplet separation (T...T) using spin-state-averaged density matrix renormalization group electronic-structure calculations. Specifically, we compare the triplet− triplet exchange (J) for tetracene dimers, bipentacene, a subunit of the benzodithiophene−thiophene dioxide polymer, and a carotenoid (neurosporene). Exchange-split energy gaps of J and 3J separate a singlet from a triplet and a singlet from a quintet, respectively. We draw two new insights: (a) the canonical tetracene singlet-fission unit cell supports precisely three lowlying TT intermediates with order-of-magnitude differences in J, and (b) the separable TT intermediate in carotenoids emanates from a pair of excitations to the second triplet state. Therefore, unlike with tetracenes, carotenoid fission requires above-gap excitations. In all cases, the distinguishability of the molecular triplets that is, the extent of orbital overlapdetermines the splitting within the spin manifold of TT states. Consequently, J represents a spectroscopic observable that distnguishes the resemblance between TT intermediates and the T...T product.

Nuclear dynamics of singlet exciton fission in pentacene single crystals

“…. T ) ( 46 , 47 ). The 1-THz motion modulates J , hence the splitting between the n ( TT ) states, and allows for admixture of higher n ( TT ) states such as 5 ( TT ).…”

Spin Statistics for Triplet–Triplet Annihilation Upconversion: Exchange Coupling, Intermolecular Orientation, and Reverse Intersystem Crossing

“… 46 , 49 When J ≪ D , however, the eigenstates of Ĥ become degenerate and possess very different spin character. 49 In the case of parallel molecules (related by a translation operation only), there are three pure triplets | T x , y , z ⟩ and three pure quintets | Q x , y , z ⟩. The remaining three, | xx ⟩, | yy ⟩, and | zz ⟩, can be written as mixtures of | S ⟩, | Q a ⟩ and | Q b ⟩.…”

Mutually exclusive hole and electron transfer coupling in cross stacked acenes

“…Several experimental and theoretical investigations on the impact of specific arrangement of chromophores and the consequent electronic coupling on the emergent optical and charge transfer properties have been carried out in the recent years. [15][16][17][18][19] Conventional H-aggregates with co-facial alignment of transition dipoles (chromophores in most cases) have been utilized for their high charge carrier mobility characteristics while J-aggregates possessing slip-stacked alignment of transition dipoles are renowned for high fluorescence efficiency with low to considerable charge carrier property. [20][21][22] Distinct crystalline molecular arrangements with cross-dipole 23,24 (rotational angle,  = 70°) and magic angle dipole stacking 25 (slip angle,  = 54.7°) have been observed to exhibit exceptional fluorescence properties owing to Coulombic and charge transfer mediated coupling, respectively.…”