Scanning tunneling microscopy investigation of the ordered structures of dialkylamino hydroxylated squaraines adsorbed on highly oriented pyrolytic graphite

Squaraine dyes form aggregates in solution and in the solid state. We have found that squaraines form two-dimensional (2D) ordered layers when adsorbe d onto HOPG from phenyloctane and from liquid crystalline solvents. We inve stigated with scanning tunneling microscopy the 2D structures of the adsorb ed phases of five bis(4-alkylamino-2-hydroxyphenyl) squaraines (both symmet ric and asymmetric) and mixtures of these squaraines. Differences in the st ability of the 2D structures and molecular packing are observed when the al kyl tail length and symmetry are varied. A transition in the 2D structure f rom a herringbone packing to a lamellar packing occurs between the tail len gths of 4 carbons and 8 carbons. Many of the compounds form a number of 2D polytypes on HOPG. Multilayers of squaraine molecules were observed for mos t of the studied molecules. Squaraines with tail lengths of 12 carbons exhi bited a tendency toward registry with the HOPG substrate whereas all other squaraine compounds investigated showed a lack of molecule- substrate regis try. Domain sizes of the investigated molecules varied from tens of nanomet ers to a micron.