Thermal, glass-forming, nonlinear optical and holographic properties of "push-pull" type azochromophores with triphenyl moieties containing isophorene and pyranylidene fragments

Abstract

Molecular organic compounds with electron donating fragment bounded through π-conjugated system with electron acceptor fragment, as well as with incorporated triphenyl groups in their molecules show potential for creating cheap and simple solution processable materials with nonlinear optical properties. Additional insertion of azobenzene fragment in their structures makes them also possible to form holographic volume and surface relief gratings (SRG) after exposure to laser radiation, which could be useful for holographic data storage. For these purposes polymers are generally used. However, their application is complicated and challenging task as in every attempt to obtain the same polymer it will have different physical properties. On the other hand, the synthetic procedure of molecular glasses is more simple as their structure and physical properties are strongly defined. Unfortunately, there is still no clear relation between compound organic structures and their thermal, glass-forming and optical properties. In order to investigate the above mentioned regularities, we have synthesized and investigated ten molecular glassy organic compounds with three different fragments as main backbones of the molecules: indene-1,3-dione (WE-1, WE-2, WE-3), isophorene (IWK-1D, IWK-2M, IWK-2D) and pyranylidene (DWK-2TB, ZWK-2TB, JWK-2TB, ZWK-3AZO). Compounds containing isophorene fragment in their molecules had the highest NLO efficiencies (d 33 up to 125.7 pm/V for IWK-2D) and also were the most effective holographic data storage compounds with holographic self diffraction efficiency 13% and holographic diffraction efficiency 20%, also for IWK-2D, but their thermal stability (T d from 288°C to 295°C) and glass transition (T g from 90°C to 105°C) values were just average. Pyranylidene type compounds had the highest thermal stability and highest glass transition (T g from 115°C to 180°C). But their ability to form and maintain amorphous structure were low and they had average NLO efficiencies (d 33 up to 66.2 pm/V for ZWK-2TB) and average holographic self diffraction efficiency 2% and holographic diffraction efficiency 8% for ZWK-3AZO. The molecules with just azobenzene fragment and indene-1,3-dione as electron acceptor has the lowest thermal (T d from 250°C to 282°C, T g from 70°C to 98°C) and also the lowest holographic properties with holographic diffraction and self diffraction efficiencies at 4% for WE-1 and lower for other compounds. Nevertheless, some of the investigated molecular glasses show potential as multifunctional optical materials.