Extent of charge separation in suddenly polarized twisted excited states of linear polyenes

Sudden polarization in zwitterionic excited states of organic intermediates in photochemical reactions. On a possible mechanism for bicyclo[3.1.0]hex-2-ene formation

“…The configuration \AB) again describes the lowest *D state, while µ2) and |B2) describe polar states. If the interaction between configuration µ2) and |B2> {A2\H\B2) = KAB (11) is sufficiently small10 (i.e., KAB « \JAA -JBB |) the configuration interaction, by combining µ2) and |B2) in the zwitterionic states, does not balance the excess of charge in the fragment described by the group orbital \A). Therefore, the "sudden polarization" effect occurs, giving rise to the ionic states Z\ * µ2) and Z2 » |B2) with opposite polarizations while the configuration | AB) contributes mostly to the lowest singlet state of covalent character.…”

“…In some cases (mainly if more than two atomic orbitals describe radical sites) the higher ionic state Z2 might borrow covalent character from the excited covalent states and become the lowest singlet excited state with opposite polarization from that predicted by limited (3 X 3) configuration interaction.10 The dipole moment resulting from the "sudden polarization" is certainly overestimated within this model and the polarization might be more localized at the ends of the twisted bonds. 11 We assume that the qualitative features of the "sudden polarization" effect remain independent from the details of the MO open-shell treatment with small configuration interaction. This assumption is supported by an investigation of the "sudden polarization" effect using other theoretical approaches.11 Therefore, it seems worthwhile to investigate the consequences of the "sudden polarization" effect for photochemical reactions using the Nesbet open-shell SCF method, followed by 3 X 3 Cl.…”

The role of nonadiabatic coupling and sudden polarization in the photoisomerization of olefins

“…The conclusion is that in butadiene the energy difference between Zx and Z2 is very small, as computed by other authors; indeed, the orientation of the dipole itself in the Zx state depends on the method and the basis set. 6,8 This ambiguity disappears when either C] or C2 are pyramidalized, leading to a Zj surface with two minima as in ethylene and propene.6…”

“…A large number of papers are concerned with the properties of the first electronic states of ethylene and other olefins (see ref [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] and references therein). the three lowest singlets of monoolefins at both planar and twisted geometries.…”

Diradicalar or ionic twisted excited states in the singlet cis-trans isomerization of polyenes?

“…16 This peculiar problem seems of minor interest for the cis-trans isomerization mechanism. Malrieu et al14,17 have demonstrated that the correct qualitative description of the ionic states is not given by the delocalized SCF description which does not take into account the and ir repolarization effects.…”