Symmetry breaking in charge-transfer compounds. The effects of electric fields and substituents on the properties of bipyrazine cations

The effects of an electric field and of various substituents on the symmetry breaking of degenerate near-midgap orbitals and on different properties in bi-N,N-pyrazine-1,6-hexatriene dications ([C₄N₂H₄—(CH)₆—C₄N₂H₄]²⁺) are investigated by means of semiempirical PM3 and INDO CI methods. The electric field is simulated by applying positive/negative point charges at varying distances from the end-points, and the substitutions are done with single chlorine atoms or with CN, OH or CH₃ groups, at various positions along the chain or on one of the pyrazine rings. The results are compared with calculations on the unsubstituted, field-free system. It is found that an electric field (e.g., as applied over a membrane) leads to significant symmetry breaking and also polarizes the HOMO and LUMO, such that electron transfer between these orbitals generates large dipole-moment shifts and non-negligible oscillator strengths. With substituents, no major symmetry breaking is observed for the ground state. Instead, strong modifications of the orbital picture are observed, in particular when using the stronger electron-withdrawing substituents. Placing the substituent in a ring position does, furthermore, lead to the possibility of large charge transfer.     

Nonlinear optical properties of some substituted biphenyls

A series of donor-acceptor substituted biphenyls is examined using semiempirical SCF-MO methods. The geometries, dipole moments and first hyperpolarizabilities are obtained from AM1, MNDO and PM3 calculations. The finite-field method is used to calculate the polarizabilities. The properties of the substituents and the torsion angle between the phenyl rings are of fundamental importance for the hyperpolarizabilities. Among the three methods, AM1 leads to torsion angles closest to the experimental data. For each molecule, the HOMO is found to be localized on the donor part to some extent, whereas the LUMO is more concentrated on the acceptor part. The spectroscopic properties of the molecules are investigated using the ZINDO model. For all the molecules considered, the transition from the HOMO to the LUMO is found to be the most important for charge transfer, and this transition has been used in a two-state model to compute the first hyperpolarizability.