Abstract

The intrinsic photophysical properties of heteroatom-doped π-conjugated systems, which are essential for optimal electronic performance, necessitate the formation of well-ordered 2D thin films with structural stability. These thin films can be achieved by self-assembly using Langmuir and Langmuir-Blodgett (LB) techniques. Manipulation of molecular design and external conditions is explored to achieve controlled film morphology desirable for electronic applications. In this context, two classes of amphiphilic heteroatom-doped π-conjugated compounds are explored: phosphole-lipids and amphiphilic tetraazaporphyrins (TAPs).
Introduction of counterions to Langmuir films of phosphole-based lipids with charged dithienophospholium backbone showed that incorporation of halide salts significantly influences the intercalation between phosphole headgroups, thereby stabilizing monolayers and promoting the formation of well-ordered structures free from 3D aggregates. However, only the counterions with their size matching the space restriction in between the headgroups enhance the film organization and facilitate the desired monolayer formation. Characterization techniques, including atomic force microscopy (AFM) and grazing incidence X-ray diffraction (GIXD) confirm the formation of condensed phase and highlight the presence of hexagonally packed chains. Exploring the effects of aromatic additives on film structure of the same phosphole-lipids demonstrates that while such additives can influence the growth patterns and organization of 2D films, challenges remain in eliminating aggregate formation.
For the amphiphilic tetraazaporphyrins featuring octaalkylthio substitution, we focused on the influence of varying alkyl chain lengths on film morphology and orientation. Notably, TAPs with shorter alkyl chains yield homogeneous monolayers with spider-like conformation where the macrocycle is oriented face-on to the substrate. GIXD confirmed that the packing is not dominated by the pentyl chains, but rather by the organization of the rings (in a molecular lattice), resulting in an ordered condensed phase. However, TAPs with longer chains exhibit complex, inhomogeneous LB films with combination of spider-like and interdigitated conformations observed by AFM, the latter attributed to the larger alkyl chains and confirmed by GIXD.
The findings of this research underscore the importance of tuning both molecular structure and external conditions to optimize the self-assembly of π-conjugated materials to enhance structural complexity and improve the functionality of the developed materials.