ATOP dyes.: Optimization of a multifunctional merocyanine chromophore for high refractive index modulation in photorefractive materials

This paper reports synthesis, characterization and structural optimization of amino-thienyl-dioxocyano-pyridine (ATOP) chromophores toward a multifunctional amorphous material with unprecedented photorefractive performance. The structural (dynamic NMR, XRD) and electronic UV/vis, electrooptical absorption, Kerr effect measurements) characterization of the ATOP chromophore revealed a cyanine-type, pi -conjugated system with an intense and narrow absorption band (epsilon (max) = 140 000 L mol(-1) cm(-1)), high polarizability anisotropy (delta alpha (0) = 55 x 10(-40) C V-1 m(2)), and a large dipole moment (13 D). This combination of molecular electronic properties is a prerequisite for strong electrooptical response in photorefractive materials with low glass-transition temperature (T-g). Other important materials-related properties such as compatibility with the photoconducting poly(N-vinylcarbazole) (PVK) host matrix, low melting point, low Tg, and film-forming capabilities were optimized by variation of four different alkyl substituents attached to the ATOP core. A morphologically stable PVK-based composite containing 40 wt % of ATOP-3 showed an excellent photorefractive response characterized by a refractive index modulation of Deltan approximate to 0.007 and a gain coefficient of Gamma approximate to 180 cm(-1) at a moderate electrical field strength of E = 35 V mum(-1). Even larger effects were observed with thin amorphous films consisting of the pure glass-forming dye ATOP-4 (T-g = 16 degreesC) and 1 wt % of the photosensitizer 2,3,7-trinitro-9-fluorenylidene-malononitrile (TNFM), This material showed complete internal diffraction at a field strength of only E = 10 V mum(-1) and Deltan reached 0.01 at only E = 22 V mum(-1) without addition of any specific photoconductor.