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.

We present a generic theory of primary photoexcitations in low band gap donor-acceptor conjugated copolymers. Because of the combined effects of strong electron correlations and broken symmetry, there is considerable mixing between a charge-transfer exciton and an energetically proximate triplet-triplet state with an overall spin singlet. The triplet-triplet state, optically forbidden in homopolymers, is allowed in donor-acceptor copolymers. For an intermediate difference in electron affinities of the donor and the acceptor, the triplet-triplet state can have stronger oscillator strength than the charge-transfer exciton. We discuss the possibility of intramolecular singlet fission from the triplet-triplet state, and how such fission can be detected experimentally.PACS numbers: 78.66. Qn, 71.20.Rv, 71.35.Cc,The primary photophysical process in polymer solar cells is photoinduced charge transfer, whereby optical excitation at the junction between a donor conjugated polymer and acceptor molecules creates a charge transfer (CT) exciton whose dissociation leads to charge carriers. The donor polymeric materials used to be homopolymers such as polythiophene which absorb in the visible range of the solar spectrum [1]. Homopolymers have recently been replaced by block copolymers whose repeat units consist of alternating donor (D) and acceptor (A) moieties [2][3][4][5][6][7][8][9][10][11]. This architecture reduces the optical gap drastically, and the DA copolymers absorb in the near infrared, where the largest fraction of the photons emitted by the Sun lie. The power conversion efficiencies (PCEs) of organic solar cells with DA copolymers as donor materials have exceeded 10% [11], and there is strong interest in the development of structure-property correlations that will facilitate further enhancement of the PCE. Clearly, this requires precise understanding of the nature of the primary photoexcitations of DA copolymers.Existing electronic structure calculations of DA copolymers are primarily based on the density functional theory (DFT) approach or its time-dependent version (TD-DFT) [12][13][14][15][16][17][18]. The motivations behind these calculations have largely been to understand the localized versus delocalized character of the excited state reached by ground state absorption. Experimentally, DA copolymers exhibit a broad low energy (LE) absorption band at ∼ 700 − 800 nm and a higher energy (HE) absorption band at ∼ 400 − 450 nm [2][3][4]. There is agreement between the computational studies that the LE band is due to CT from D to A, and the HE band is a higher π-π * excitation.Recent optical studies indicate that the above simple characterization of the LE band might be incomplete, and as in the homopolymers [19], electron correlations play a stronger role in the photophysics of the DA copolymers than envisaged within DFT approaches. Grancini et al. determined from ultrafast dynamics studies that the broad LE band in PCPDT-BT (the Supplemental Material [20] for the structures of this and other DA copolymers) is c...

Strong electron correlation effects in the photophysics of quasi-one-dimensional π-conjugated organic systems such as polyenes, polyacetylenes, polydiacetylenes, etc., have been extensively studied. Far less is known on correlation effects in two-dimensional π-conjugated systems. Here we present theoretical and experimental evidence for moderate repulsive electron−electron interactions in a number of finite polycyclic aromatic hydrocarbon molecules with D 6h symmetry. We show that the excited state orderings in these molecules are reversed relative to that expected within oneelectron and mean-field theories. Our results reflect similarities as well as differences in the role and magnitude of electron correlation effects in these two-dimensional molecules compared to those in polyenes.

We employ dynamical mean field theory (DMFT) with a Quantum Monte Carlo (QMC) atomic solver to investigate the finite temperature Mott transition in the Hubbard model with the nearest neighbor hopping on a triangular lattice at half-filling. We estimate the value of the critical interaction to be Uc = 12.0 ± 0.5 in units of the hopping amplitude t through the evolution of the magnetic moment, spectral function, internal energy and specific heat as the interaction U and temperature T are varied. This work also presents a comparison between DMFT and finite size determinant Quantum Monte Carlo (DQMC) and a discussion of the advantages and limitations of both methods.PACS numbers: I. MOTIVATIONSystems with triangular lattice structure have been a source of attention mostly due to the frustration effects resulting from their non-bipartite structure. As a result of the competition between the frustration and strong electron correlations, these systems exhibit a wide range of exotic phases. Recent studies of the metalinsulator transition, superconductivity, and antiferromagnetism in the organic compounds κ-(BEDT-TTF) 2 X with X as an anion 1,2 , discovery of superconductivity in Na x CoO 2 .yH 2 O, 3 and the recent discovery of the Mott transitions in 0.33 monolayers of Sn on Ge(111) at 30 K, 4 as a few examples, underline the importance of these systems and their physics.Theoretical work has been dedicated to the discovery of the magnetically ordered phases in the ground state of the Hubbard and t − J models on triangular lattices as a function of the on-site electron-electron Coulomb interaction U at different fillings. 5,6,7,8,9,10 Unlike the square lattice at half-filling, which is a Mott insulator with antiferromagnetic order at arbitrarily small values of U/t, the ground state of a triangular lattice has a variety of magnetically ordered and disordered phases. This is due to the lack of perfect nesting in the non-interacting Fermi surface of a triangular lattice at half-filling.For triangular lattices, according to the Hartree-Fock calculations of Krishnamurthy and co-workers, 5,6 the Mott transition occurs from a paramagnetic metal to a paramagnetic insulator at half-filling for values of U larger than the band width W = 9t. A variety of the physical properties of the triangular lattices including tendencies towards superconductivity within the small to intermediate U regime have been studied at finite temperature using correlated electron approaches which go beyond mean field theory, such as the fluctuation exchange approximation (FLEX) 11 or one-loop renormalization-group. 12 However, when U/W ≥ 1, a more powerful cluster solving technique such as the Quantum Monte Carlo (QMC) is required to accurately describe the phase transition. Finite size lattice determi-nant Quantum Monte Carlo (DQMC) has already been employed for triangular and kagomé lattices. 13,14 Unfortunately, the finite size lattice QMC method incurs sign problems at both low temperatures and away from halffilling. In addition, due to the finite s...

We present numerical studies of one- and two-photon excited states ordering in a number of polycyclic aromatic hydrocarbon molecules: coronene, hexa-peri-hexabenzocoronene, and circumcoronene, all possessing D(6h) point group symmetry versus ovalene with D(2h) symmetry, within the Pariser-Parr-Pople model of interacting π-electrons. The calculated energies of the two-photon states as well as their relative two-photon absorption cross-sections within the interacting model are qualitatively different from single-particle descriptions. More remarkably, a peculiar role of molecular geometry is found. The consequence of electron correlations is far stronger for ovalene, where the lowest spin-singlet two-photon state is a quantum superposition of pairs of lowest spin triplet states, as in the linear polyenes. The same is not true for D(6h) group hydrocarbons. Our work indicates significant covalent character, in valence bond language, of the ground state, the lowest spin triplet state and a few of the lowest two-photon states in D(2h) ovalene but not in those with D(6h) symmetry.