Chain-length dependence of the optical properties of a series of push-pull polyenes: application to doped photorefractive polymers

Push-pull molecules are of considerable interest for their large quadratic hyperpolarizabilities. However, incorporated as doping chromophores in low glass-transition temperature photorefractive polymers, their Pockels contribution to the modulation of the refractive index is generally weak or even negligible. We present an experimental and analytical study of a series of push-pull polyenes with a number of double bonds in the chain increasing from 1 to 5. The molecules were dissolved in chloroform and the two contributions to the total birefringence, i.e. the orientational birefringence and the Pockels effect, were measured using a specific ellipsometric technique and analyzed in a two-form two-state formalism. For the longest polyene studied, a giant efficiency of the refractive index modulation, arising equally from the orientational contribution and from the Pockels effect, can be expected. (C) 1999 Elsevier Science B.V. All rights reserved.