Theory of Singlet Fission in Polyenes, Acene Crystals, and Covalently Linked Acene Dimers

Abstract

We report quadruple configuration interaction calculations within the extended Pariser−Parr−Pople Hamiltonian on the excited states of aggregates of polyenes, crystalline acenes and covalently linked dimers of acene molecules. We determine the precise energy orderings and analyze the cluster wavefunctions in order to arrive at a comprehensive physical understanding of singlet fission in these diverse families of materials. Our computational approach allows us to retain a very large number of basis states, and thereby obtain the correct relative energy orderings of one electron-one hole Frenkel and charge-transfer excitons versus intra-and intermolecular two electron-two hole triplet−triplet excited states. We show that from the energy orderings it is possible to understand the occurrence of singlet fission in polyene and acene crystals, as well as its near total absence in the covalently linked acene dimers. As in the acene crystals, singlet fission in the polyenes is a multichromophoric phenomenon, with the well known 2 1 A − g playing no direct role. Intermolecular charge-transfer is essential for singlet fission in both acenes and polyenes, but because of subtle differences in the natures and orderings of the aggregate excited states, the mechanisms of singlet fission are slightly different in the two classes. We are thus able to give qualitative physical reasoning for the slower singlet fission in the polyenes, relative to that in crystalline pentacene. Our work also gives new insight to the complex exciton dynamics in tetracene crystals, which has been difficult to understand theoretically. Our large-scale many-body calculations provide us with the ability to understand the qualitative differences in the singlet fission yields and rates between different classes of π-conjugated materials.

Manipulating molecules with strong coupling: harvesting triplet excitons in organic exciton microcavities

“…For example, in singlet fission the nominal S1 state contains both CT and TT character, in some cases such as pentacene films to a surprisingly high degree (circa 50% CT) 47 . By the same token, the 1 (TT) formed by singlet fission carries admixture of S1 and CT, enabling fast singlet fission 42,44 , efficient equilibration between S1 and 1 (TT) 44 , direct 1 (TT) emission 42,43 and triplet-pair binding [42][43][44]48,49 . Similar mixing likely enables the conversion from triplets to S1 via triplettriplet annihilation.…”

Theory of Primary Photoexcitations in Donor-Acceptor Copolymers

“…This is because iSF proceeds via a highly correlated two electron-two hole (2e-2h) triplettriplet (TT) state, which is not captured by TD-DFT [30,31] [32][33][34][35]. Unfortunately, the large and complex repeat units of the DA copolymers [20] preclude quadruple configuration interaction (QCI) calculations and manybody techniques such as the density matrix renormalization group.…”

Theoretical Investigation of the Intermolecular Charge‐Transfer Interactions of Poly‐p‐Phenylene Oligomers

“…43 This model has been used extensively in the studies of π-conjugated systems and proven to give reliable results for spectroscopic and other linear or nonlinear optical properties with respect to the experimental results and computationally highly extensive ab initio calculations. [44][45][46][47][48] The Pariser-Parr-Pople model has been shown to well reproduce the excitation energies as well as excited-state properties compared to the all-valence INDO/S model 33,39,40 which provides reliable data in comparison with the UV-vis and UPS spectra of many conjugated molecules. 8,49 To obtain the contribution of the CT characters to the lowest two excitation energies, four truncated models were used to compare with the full CM Hamiltonian (CTE(f ) model).…”

“…The excited state wave functions of the monomers and dimers of PPP n were obtained by the single configuration‐interaction (SCI) scheme with all occupied and unoccupied π‐levels included within the Pariser–Parr–Pople Hamiltonian . This model has been used extensively in the studies of π‐conjugated systems and proven to give reliable results for spectroscopic and other linear or nonlinear optical properties with respect to the experimental results and computationally highly extensive ab initio calculations . The Pariser–Parr–Pople model has been shown to well reproduce the excitation energies as well as excited‐state properties compared to the all‐valence INDO/S model which provides reliable data in comparison with the UV–vis and UPS spectra of many conjugated molecules .…”