RING SUBSTITUTION POSITION AND THE MONOLAYER AND LANGMUIR-BLODGETT MULTILAYER PROPERTIES OF HENEICOSA-2,4-DIYNYL CARBOXYBENZOATES

The ortho isomer of heneicosa-2,4-diynyl carboxybenzoate exhibits very different monolayer and Langmuir-Blodgett (LB) behavior from that of the meta and para isomers. Its pi/A isotherm can be more expanded and exhibits a turnover, signaling a significant reorientation in the monolayer state. Moreover, the monolayer (and LB film) of ortho is readily polymerized by UV irradiation, even when highly expanded. Various monolayer conformations can be proposed for each isomer to explain the turnover in the isotherm for ortho, compared to the condensed isotherms given by meta and para. Lateral attractive interactions involving the aromatic ring cause domain formation in the monolayers. In the case of ortho only, packing within the domains is conducive to polymerization of the diacetylene moieties. With meta and para lateral bridging by subphase cadmium ions is likely, as is the formation of interfacial dimers and liquid crystalline phases. The monolayer behavior of each isomer is mirrored by its LB characteristics. The differences in lateral and cross-layer interactions are obviously responsible for the spontaneous formation of Z-type films for ortho and Y type films for meta and para.