Highly transparent and birefringent chromophores for organic photorefractive materials Original Research Article

A series of chromophores for application in organic photorefractive (PR) materials is investigated by electro-optical absorption measurements (EOAM). This experimental technique yields information on the transition dipole moment μag, the ground-state dipole moment μg, and the change of the dipole moment upon optical excitation Δμ within the intense charge transfer (CT) band of the dyes. It is shown that the results of the EOAM experiment allow us to estimate the PR figures-of-merits (FOMs) of the chromophores by either perturbational two-level equations or Kramers–Kronig transformation. In particular, chromophores based on the heterocyclic dihydropyran and dihydropyridine groups in combination with dicyano and cyanocarboxy acceptor units were investigated. These donor–acceptor pairs yield chromophores close to the `cyanine limitʹ that is characterized by a small dipole difference, but a large ground-state dipole moment and a large polarizability anisotropy. This leads to very high PR FOMs of the new PR chromophores that are demonstrated to be superior to conventional second-order nonlinear optical (NLO) chromophores in situations where the medium has a low glass transition.