Tunable One-, Two-, and Three-Dimensional Self-Assemblies from an Acceptor-Donor Fullerene-N,N-Dimethylaminoazobenzene Dyad: Interfacial Geometry and Temporal Evolution

Controllable fabrication of spontaneously ordered and varied geometry fullerene C-60 based molecular architecture was achieved upon hierarchical self-assembly of the fullerene-N,N-dimethylaminoazobenzene acceptor-donor hybrid (DPNME). Simple preparation techniques, such as Langmuir-Blodgett (LB), solution-cast, and immersion at the liquid-air and solid-air interfaces, were used without templates as a function of DPNME concentration, media pH, time, and supporting substrate characteristics. The resulting structures depending upon the preparation methods were investigated with field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), and molecular modeling, which revealed a delicate role of intermolecular donor-acceptor, pi-pi, and van der Waals interactions between the electron deficient fullerene core and the N,N-dimethylaminoazobenzene electron donor under neutral conditions. Upon protonation, the electrostatics associated with the charged DPNME moiety and the dominant intermolecular fullerene-fullerene interactions guided the self-assembly process. Increased time scales led the molecular subunits to grow by maximizing the most favored orientations and yielded one-dimensional (1D) and two-dimensional (2D) structures in neutral and acidic conditions, respectively, which upon solvent evaporation formed the final multipods or stacked squares upon oriented attachment. For the protonated DPNME, 2D lamellar sheets formed from the bilayers gained cohesive energy, forming ultimately rectangular sheets. Interestingly, the Si(100) supported multilayer DPNME Langmuir films as a function of surface pressure and pH yielded a uniform and directional structure pattern in comparison with the geometry obtained from drop casting methods. This controllable structure architecture of the fullerene-azobenzene hybrid opens up a new alley in fullerene C-60 based self-assembly.