F. Wurthner et al., ATOP dyes. Optimization of a multifunctional merocyanine chromophore for high refractive index modulation in photorefractive materials, J AM CHEM S, 123(12), 2001, pp. 2810-2824
This paper reports synthesis, characterization and structural optimization
of amino-thienyl-dioxocyano-pyridine (ATOP) chromophores toward a multifunc
tional amorphous material with unprecedented photorefractive performance. T
he structural (dynamic NMR, XRD) and electronic UV/vis, electrooptical abso
rption, Kerr effect measurements) characterization of the ATOP chromophore
revealed a cyanine-type, pi -conjugated system with an intense and narrow a
bsorption band (epsilon (max) = 140 000 L mol(-1) cm(-1)), high polarizabil
ity anisotropy (delta alpha (0) = 55 x 10(-40) C V-1 m(2)), and a large dip
ole moment (13 D). This combination of molecular electronic properties is a
prerequisite for strong electrooptical response in photorefractive materia
ls with low glass-transition temperature (T-g). Other important materials-r
elated properties such as compatibility with the photoconducting poly(N-vin
ylcarbazole) (PVK) host matrix, low melting point, low Tg, and film-forming
capabilities were optimized by variation of four different alkyl substitue
nts 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 0.0
07 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 obs
erved with thin amorphous films consisting of the pure glass-forming dye AT
OP-4 (T-g = 16 degreesC) and 1 wt % of the photosensitizer 2,3,7-trinitro-9
-fluorenylidene-malononitrile (TNFM), This material showed complete interna
l diffraction at a field strength of only E = 10 V mum(-1) and Deltan reach
ed 0.01 at only E = 22 V mum(-1) without addition of any specific photocond
uctor.