Molecular electronics requires materials of molecular size exhibiting switching or memory properties. The Langmuir-Blodgett method (L.B.) and its variants provide controlled fabrication of molecular films one molecule thick and their transfer onto solid supports. In these 2D monolayers, molecules are highly organized, so that complex organizates aimed at specific properties can be built. One of these, a semi-amphiphilic complex made of two NCS and two amphiphilic phenantrolines in a cis position, exhibits a spin transition around - 20 °C. It switches from high spin (S = 2) at room temperature to low spin (S = 0) at low temperature. The transition does not follow the expected thermal Boltzman statistics, but is rather steep, pointing out molecular cooperation. The L.B. film exhibits a hysteresis about 4 °C in width, while the parent 3D compound exhibits a hysteresis of less than 0.2 °C, but a much sharper transition. This apparently contradictory behaviour is ascribed to the high impurity content and the multicristallinity of the L.B. film in association with high intermolecular coupling. Because of its intrisic properties this new material is a breakthrough on the way to magnetic memories at the molecular scale for molecular electronics.