Highly transparent and birefringent chromophores for organic photorefractive materials

A series of chromophores for application in organic photorefractive (PR) ma terials is investigated by electro-optical absorption measurements (EOAM). This experimental technique yields information on the transition dipole mom ent mu(ag), the ground-stare dipole moment mu(g). and the change of the dip ole moment upon optical excitation Delta mu within the intense charge trans fer (CT) band of the dyes. It is shown that the results of the EOAM experim ent allow us to estimate the PR figures-of-merits (FOMs) of the chromophore s by either perturbational two-level equations or Kramers-Kronig transforma tion. In particular, chromophores based on the heterocyclic dihydropyran an d dihydropyridine groups in combination with dicyano and cyanocarboxy accep tor units were investigated. These donor-acceptor pairs yield chromophores close to the 'cyanine limit' that is characterized by a small dipole differ ence, but a large ground-state dipole moment and a large polarizability ani sotropy. This leads to very high PR FOMs of the new PR chromophores that ar e demonstrated to be superior to conventional second-order nonlinear optica l (NLO) chromophores in situations where the medium has a low glass transit ion. (C) 1999 Elsevier Science B.V. All rights reserved.