Effect Of Hydrophobic Alkyl Chain On Organic Interfacial Aggregates

Conventional surfactants may form surface micelles of spherical, cylindrical shapes at the liquid/solid interface, but these surface micelles are not stable enough against drying process. In order to obtain stable surface micelles, bolaform amphiphiles bearing rigid mesogenic groups and soft spacers were synthesized and then surface aggregation behaviors were studied by Scanning Probe Microscopy (SPM). The aggregation behaviors of a series of bolaform amphiphiles bearing azobenzene groups at the liquid/solid interface were investigated systematically. Azo-11, the bolaform amphiphile with longer spacers, can form stable straight stripe structures at the interface even against drying process. While, the bolaform amphiphile with shorter spacers(azo-6), presents diversified aggregate morphologies at the solution/solid interface with the changing of solution concentration. Azo-6 forms loosely packed wormlike surface micelle at lower concentration. With the increase of the solution concentration, the wormlike surface micelles gradually convert into rodlike micelles in short-range order and further into globular aggregates. Keeping azo-6 at a constant concentration(1.0×10-4 mol/L), sodium salicylate (NaSal) with different concentration was added to study the effect of NaSal on the formation of surface micelles at the solution/mica interface. AFM studies show that the aggregate structure of azo-6 has obviously been changed by the addition of NaSal. It was found that when a proper concentration of NaSal was added, the azo-6 can form ordered stripe structure which is stable enough against drying. But the aggregate morphologies of azo-11 could be less affected by the addition of NaSal than that of azo-6.To further investigate the influence of the rigid mesogenic group and soft spacer on the aggregation of bolaform amphiphiles at the interface, another series of bolaform amphiphiles bearing biphenyl mesogenic groups and alkyl chains with different length were designed and synthesized. It is observed that the surface aggregates of bolaform amphiphiles depend largely on the length of alkyl chains. BP-10 and BP-11 molecules that have slightly longer spacer, can form stable spaghetti-like micelles at the interface. In contrast, the BP-5 molecule with short alkyl chain forms spherical surface micelle at the interface. The surface aggregates of BP-15 with much longer spacer are unstable at the solution/mica interface. In conclusion, bolaform amphiphile bearing rigid mesogenic group and proper soft spacer length (e.g. 10~11 alkyl chain) are critically important to form stable surface micelle at the liquid/solid interface. Based on these principles, we can set up some models to fabricate ordered nano patterns. Quinacridone and its derivatives show high photoluminescent efficiency, and they are used as dopant emitters to fabricate high performance electroluminescent devices. Their crystal structures were usually characterized by the X-ray diffraction instrument. But for those quinacridone derivatives that are difficult to obtain single crystals, their structure could not be characterized by X-ray diffraction. Scanning tunneling microscope (STM) has some advantages, such as high resolution and its ability for local observation and could be used to characterize the 2D structure of quinacridine derivatives at the liquid/graphite interface. In chapter 4, adlayers of N-alkyl substituted quinacridone derivatives have been imaged at HOPG/solution interface by in situ STM. It was found that QA-C6 forms grain-like arrays, while QA-C22 and QA-C16 forms straight stripes with long-range order. These structural differences are attributed to collective intermolecular interactions, which are influenced by the different lengths of the substituted alkyl chains. The systematic study of the packing structures of these quinacridone derivatives enables us to understand why they are good candidates for electroluminescent materials.