Sudden polarization in push-pull polyenes: a model exact study

Abstract

In substituted polyenes, it has been conjectured that a large charge separation occurs when a rotation of 90° is carried out about a double bond. In this paper, using a model exact solution of the Pariser-Parr-Pople (PPP) model for the low-lying states of push-pull polyenes, we show that this is indeed the case. Among these states, a state derived from the lowest one-photon state and another derived from a higher two-photon state of the unsubstituted polyene exhibit sudden polarization. The exact position of these states in the eigenvalue spectrum depends upon the push-pull strength and the length of the chain. A large change in the dipole moment at a rotational angle of 90° is observed in these states even for very small push-pull strengths and is independent of the actual position of the substituents provided the substitution breaks both the electron-hole symmetry and the inversion symmetry of the polyene.