Persistent, Well-Defined, Monodisperse, π-Conjugated Organic Nanoparticles via G-Quadruplex Self-Assembly

Several oligo(p-phenylene-vinylene) oligomers capped with a guanosine or a guanine moiety have been prepared via a palladium-catalyzed cross-coupling reaction. Their self-assembly, in both the absence and presence of alkaline salts, has been studied by means of different techniques in solution (NMR, MS, UV-vis, CD, fluorescence), solid state (X-ray diffraction), and on surfaces (STM, AFM). When no salt is added, these pi-conjugated molecules self-associate in a mixture of hydrogen-bonded oligomers, among which the G-quartet structure may be predominant if the steric hindrance around the guanine base becomes important. In contrast, in the presence of sodium or potassium salts, well-defined assemblies of eight functional molecules (8mers) can be formed selectively and quantitatively. In these assemblies, the pi-conjugated oligomers are maintained in a chirally tilted (J-type) stacking arrangement, which is manifested by negative Cotton effects, small bathochromic absorption and emission shifts, and fluorescence enhancements. Furthermore, these self-assembled organic nanostructures, similar to 1.5-2.0 nm high and 8.5 nm wide, exhibit an extraordinary stability to temperature or concentration changes in apolar media, and they can be transferred and imaged over solid substrates as individual nanoparticles, showing no significant dissociation or further aggregation.