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 π-conjugated system with an intense and narrow absorption band (εmax = 140 000 L mol-1 cm-1), high polarizability anisotropy (δα0 = 55 × 10-40 C V-1 m2), 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 (Tg). 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 Δn ≈ 0.007 and a gain coefficient of Γ ≈ 180 cm-1 at a moderate electrical field strength of E = 35 V μm-1. Even larger effects were observed with thin amorphous films consisting of the pure glass-forming dye ATOP-4 (Tg = 16 °C) and 1 wt % of the photosensitizer 2,4,7-trinitro-9-fluorenylidene-malononitrile (TNFM). This material showed complete internal diffraction at a field strength of only E = 10 V μm-1 and Δn reached 0.01 at only E = 22 V μm-1 without addition of any specific photoconductor.