Correlation effects in the spectra of polyenes
01 Jun 1976-Journal of Chemical Physics (American Institute of Physics)-Vol. 64, Iss: 11, pp 4422-4441
TL;DR: In this paper, PPPP-SCF-CI calculations were carried out to describe the electron correlation in the excited singlet π,π* states of the polyacenes [C4n+2H2n+4], and the results led to the following: (1) new excited states entailing the promotion of two electrons from the ground state (some of them predicted previously by other authors), and (2) a partial reordering of those (well-known) excited states already accounted for by a S•CI representation.
Abstract: In order to describe the electron correlation in the excited singlet π,π* states of the polyacenes [C4n+2H2n+4], we have carried out PPP–SCF–CI calculations including all single and double excitations in the CI expansion up to n=5, including all triple excitations up to n=3, and all quadruple excitations up to n=2. Compared to previous CI descriptions which included single excitations only, e.g., the classic work of Pariser [J. Chem. Phys. 24, 250 (1956)], our calculations lead us to predict the following: (1) ’’new’’ excited states entailing the promotion of two electrons from the ground state (some of them predicted previously by other authors), and (2) a partial reordering of those (well‐known) excited states already accounted for by a S‐CI representation. Single and double excitations in a CI expansion (D‐CI) satisfactorily describe the ordering of all excited states up to 7 eV; the effect of higher excitations is to correct the excitation energies overestimated by the D‐CI description. Our predicted spectra provide a consistent assignment of all one‐ and two‐photon spectral data but do not yield a quantitative agreement.
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TL;DR: A triplet yield of 200% and triplet formation rates approaching the diffusion limit in solutions of bis(triisopropylsilylethynyl (TIPS) pentacene are reported, and a transient bound excimer intermediate is observed, formed by the collision of one photoexcited and one ground-state TIPS-pentacene molecule.
Abstract: Solution-based studies of singlet exciton fission have provided valuable insight to this spin-allowed process in organic chromophores, whereby a photogenerated spin-singlet exciton splits into two spin-triplet excitons on separate molecules. Here we review the most significant experimental contributions made regarding fission in solution, in both intra- and intermolecular systems. Intramolecular fission allows a clearer examination of the molecular excited states involved in triplet formation, and the ability to control inter-chromophore structure offers a route to directly investigate the role of molecular coupling. In diffusional, intermolecular systems the conformational freedom and slower timescales of fission reveal the nature of intermediate states.
431 citations
01 Jan 1982
345 citations
TL;DR: In this article, a multireference double excitation configuration interaction method (MRD•CI) was presented for polyenes with up to 16 π electrons. But the complexity of such calculations is high.
Abstract: A correct description of the electronic excitations in polyenes demands that electron correlation is accounted for correctly. Very large expansions are necessary including many‐electron configurations with at least one, two, three, and four electrons promoted from the Hartree–Fock ground state. The enormous size of such expansions had prohibited accurate computations of the spectra for polyenes with more than ten π electrons. We present a multireference double excitation configuration interaction method (MRD‐CI) which allows such computations for polyenes with up to 16 π electrons. We employ a Pariser–Parr–Pople (PPP) model Hamiltonian. For short polyenes with up to ten π electrons our calculations reproduce the excitation energies resulting from full‐CI calculations. We extend our calculations to study the low‐lying electronic excitations of the longer polyenes, in particular, the gap between the first optically forbidden and the first optically allowed excited singlet state. The size of this gap is shown to depend strongly on the degree of bond alternation and on the dielectric shielding of the Coulomb repulsion between the π electrons.
314 citations
TL;DR: The time-dependent local-density approximation (TDLDA) is applied to the optical response of conjugated carbon molecules in the energy range of 0--30 eV, with calculations given for carbon chains, polyenes, retinal, benzene, and C{sub 60}.
Abstract: The time-dependent local-density approximation (TDLDA) is applied to the optical response of conjugated carbon molecules in the energy range of 0--30 eV, with calculations given for carbon chains, polyenes, retinal, benzene, and C{sub 60}. The major feature of the spectra, the collective {pi}-{pi}{sup *} transition, is seen at energies ranging from below 2 to 7 eV and is reproduced by the theory to a few tenths of an electron volt with a good account of systematic trends. However, there is some indication that TDLDA predicts too much fragmentation of the strength function in large molecules. Transition strengths are reproduced with a typical accuracy of 20%. The theory also predicts a broad absorption peak in the range of 15--25 eV, and this feature agrees with experiment in the one case where quantitative data is available (benzene).
303 citations
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TL;DR: In this paper, the classification of π-orbitals in a cata-condensed aromatic system is like that of the orbitals of a free electron traveling in a one-dimensional loop of constant potential around the perimeter.
Abstract: The classification of π‐orbitals in a cata‐condensed aromatic system is like that of the orbitals of a free electron traveling in a one‐dimensional loop of constant potential around the perimeter. To take into account electron interaction, certain quantities corresponding to angular momenta may be added or subtracted. Introduction of the cross‐links in the molecule removes the degeneracy. The first excited configuration in such systems gives two low frequency singlet weak absorption bands and two higher singlet strong dipole absorption bands. Selection and polarization rules are given. The levels are identified from the spectra and some of their properties are determined. An explanation is given of the regularities found by Klevens and Platt. A systematic nomenclature is given. The results agree qualitatively with LCAO theory, can be applied easily to unsymmetrical molecules, and can possibly be extended to other types of ring systems.
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TL;DR: In this paper, a previously given theory of the electronic spectra and structure of complex unsaturated molecules is further elucidated by listing specific formulas for the charge density, bond order, transition moment, and configuration interaction matrix elements.
Abstract: A previously given theory of the electronic spectra and structure of complex unsaturated molecules is further elucidated by listing specific formulas for the charge density, bond order, transition moment, and configuration interaction matrix elements.The theory is then applied to give a number of simple rules for alternant hydrocarbons. These refer to the energies of the singly excited states and the effects of configuration interaction, the degeneracy between certain singlet and triplet states, the prohibition of certain electronic transitions which would be allowed from a group theoretical point of view, and the charge density distribution of the ground and singly excited states. A mechanism is proposed to explain some of the anomalously rapid singlet‐triplet radiationless transitions which are observed in connection with studies of the lower triplet states of complex molecules.Quantitative application of the theory is made to the spectra of the polyacenes, benzene through pentacene, and to the bond ord...
964 citations