Theoretical study of the valence π→π* excited states of polyacenes: Benzene and naphthalene

Abstract

Multireference perturbation theory with complete active space self-consistent field ͑CASSCF͒ reference functions was applied to the study of the valence →* excited states of benzene and naphthalene. The eigenvectors and eigenvalues of CASSCF with valence active orbitals satisfy pairing properties for the alternant hydrocarbons to a good approximation. The excited states of polyacenes are classified into the covalent minus states and ionic plus states with the use of the alternancy symmetry. The present theory satisfactorily describes the ordering of low-lying valence →* excited states. The overall accuracy of the present approach is surprisingly high. We were able to predict the valence excitation energies with an accuracy of 0.27 eV for singlet u states and of 0.52 eV or better for singlet g states of naphthalene. Our predicted triplet states spectrum provides a consistent assignment of the triplet-triplet absorption spectrum of naphthalene. For benzene we were able to predict the valence excitation energy with an accuracy of about 0.29 eV. The covalent minus states and ionic plus states exhibit different behavior as far as the electron correlation is concerned. The ionic plus states are dominated by the single excitations but covalent minus states include a large fraction of doubly excited configurations. The covalent minus states always give lower energy than the corresponding ionic plus states. This is true for triplet states. The dynamicpolarization effects introduced by perturbation theory are significant for the ionic plus states while those on covalent excited states are usually of the same order as in the covalent ground state. The enlargement of the active space of the reference functions represents a great improvement of the description of the ionic states. The present approach with the pairing properties has proved to be of great value in understanding and predicting the experimental data of the alternant hydrocarbons.

A study of electron scattering from benzene: Excitation of the 1B1u, 3E2g, and 1E1u electronic states

“…Similarly, different versions of the additivity rule have been applied in calculating total cross sections [33][34][35] for this scattering system. Theoretical optical oscillator strengths for the 1 E 1u electronic state have also been reported by several groups [36][37][38][39][40][41] with significant differences found between those results. As the theoretical OOS is an indication for the quality of the wavefunction used in a calculation, those differences therefore simply reflect the differing accuracy in the various wavefunctions used in those computations.…”

“…As the theoretical OOS is an indication for the quality of the wavefunction used in a calculation, those differences therefore simply reflect the differing accuracy in the various wavefunctions used in those computations. [36][37][38][39][40][41] In Sec. II of this paper we discuss our experimental details and analysis techniques.…”

Large-scale correlated study of excited state absorptions in naphthalene and anthracene

“…Our results also agree well with the CNDO/S-CI calculations of Swiderek et al, 21 who computed 1B 3g − , 2A g − , and 3A g − states at 5.37, 5.61, and 5.76 eV, respectively. Upon comparing our results with the MRMP and CASSCF calculations of Hashimoto et al, 16 we found that their CASSCF energies are much higher than ours, as well as compared to experimental energies, while MRMP results are comparable. Similarly, SCI results of Pariser 18 predict much higher energy differences as compared to our values.…”

Theory and computations of two-photon absorbing photochromic chromophores

“…Since the ground state is 1Ag, one-photon transitions to Bu states are dipoleallowed and to Ag states are dipole-forbidden in the one-photon regime, while two-photon transitions are forbidden to Bu states and allowed to Ag states. Besides the spatial symmetry, π-states can be classified according to the so-called alternacy, or particle-hole symmetry [93]. It is exact only with some model Hamiltonians (Huckel or Parriser-Parr-Pople), and becomes approximate after σ-π and second-neighbor interactions are included.…”