Ultrafast dynamics of carotenoid excited States-from solution to natural and artificial systems.

Cluster expansion of the wavefunction. Excited states

Cluster expansion of the wavefunction. Electron correlations in ground and excited states by SAC (symmetry-adapted-cluster) and SAC CI theories

We present a detailed investigation of the acene series using high-level wave function theory. Our ab initio density matrix renormalization group algorithm has enabled us to carry out complete active space calculations on the acenes from napthalene to dodecacene correlating the full π-valence space. While we find that the ground state is a singlet for all chain lengths, examination of several measures of radical character, including the natural orbitals, effective number of unpaired electrons, and various correlation functions, suggests that the longer acene ground states are polyradical in nature.

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The radical character of the acenes: A density matrix renormalization group study

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.

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Singlet exciton fission in solution

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.

Correlation effects in the spectra of polyenes

Extensive configuration interaction (CI) is needed to achieve satisfactory descriptions of the optical spectra and photochemical properties of the π‐electron systems of polyenes. Although a basis of single and double excitations with respect to the SCF ground state yields a qualitatively correct energy level scheme, such a treatment introduces an imbalance in the ground state correlation (well described) relative to that of the excited states (poorly described). The result is a divergence in the excitation energies with increasing size of the π‐electron system. A renormalized configuration interaction method is developed to account correctly for the excited state correlation. The method is based on the finding that the main contribution to the correlation energy in the excited states is from the electrons not directly involved in the excitation, so that the correlation correction closely resembles that in the ground state. A detailed analysis of the excited state energy permits one to isolate the dominant...

I. Ohmine

Papers

Correlation effects in the spectra of polyenes

Renormalized configuration interaction method for electron correlation in the excited states of polyenes