On the nature of the ’’sudden polarization’’ effect in low lying singlet excited states

Abstract

The PPP‐like models with full CI of the twisted s‐cis, s‐trans diallyl are used to analyze the nature and properties of the large polarity in the singlet excited states of twisted conjugated molecules. Small deviations from the common value of Coulombic integrals or introduction of small hopping parameters between nonnearest neighbors imply the ’’sudden polarization’’ effect in low lying singlet excited states. It is shown that these two different ways of violating alternancy which give rise to the ’’sudden polarization’’ proceed through entirely different mechanisms yielding, therefore, also different characteristics of the highly polarized states. The properties of the sudden polarization effect obtained by the ab initio calculations are qualitatively the same as in the PPP‐like model with different Coulombic integrals. Moreover, the analysis of this model yields explicitly a charge excess in the trans allylic triad of the twisted diallyl in the lowest singlet excited state. According to our analysis it is highly improbable that the correlation effects can remove the sudden polarization.