Deciphering pressure-induced nanoarchitectonics in a monolayer of heterocoronene-based discotics at air-water and air-solid interfaces.

Abstract

Understanding and control of molecular alignment at the nanoscale in self-assembled supramolecular structures is a prerequisite for the subsequent exploitation of molecules in functional devices. Here, we have clarified the surface-pressure induced molecular nanoarchitectures in a monolayer of a heterocoronene-based discotic liquid crystal (DLC) at air-water and air-solid interfaces using surface manometry, real-time Brewster angle microscopy, and real-space atomic force microscopy (AFM). Chloroform-spread DLCs at a concentration of ∼108 μM exhibit floating domains at the air-water interface comprising small aggregates of edge-on stacked molecules interacting <i>via</i> peripheral alkyl chains. Detailed analysis of surface manometry and relaxation measurements reveal that, upon compression, these domains coalesce to form a coherent monolayer which then undergoes irreversible structural transformations <i>via</i> mechanisms such as monolayer loss due to desorption and localized nucleation of defects. AFM images of the films transferred on a hydrophilic substrate reveal that with increasing surface-pressure, the nanoscale structure of the monolayer transforms from randomly oriented nanowires to tightly-packed nanowire domains, and finally to fragmented wire segments which diffuse locally above the film. These results provide a facile method for the preparation of compact, two-dimensional films of ambipolar DLC molecules with a tunable nanoarchitecture which will be crucial for their applications in nanoscale electronic devices.