Self-assembly of phosphole-lipids in 2D films: the influence of π-interactions and steric constraints†

Promising photophysical properties of π-conjugated phosphole-based materials make them appealing building blocks for electronic and optoelectronic devices. In practical terms, a well-ordered 2D film organization is re4quired that can be obtained by deposition and/or self-assembly of thin films on a solid substrate. Manipulation of the existing noncovalent interactions within the films, via altering the chemical structure or environmental conditions to modify the molecular arrangements, is one approach to control the electronic properties of these thin films. The inter- and intramolecular π–π interactions influencing the 2D film structure in Langmuir and Langmuir–Blodgett films of a series of lipids with phosphole-based, π-conjugated headgroups is explored in the presence and absence of aromatic additives. Brewster angle and atomic force microscopy demonstrated the simultaneous formation of 3D aggregates and a condensed phase. GIXD measurements confirmed that the 3D material formation should be hindered to promote the formation of an ordered 2D film. Different approaches were considered to manipulate the π–π interactions in the film: addition of small-molecule aromatics, mixtures of phosphole-lipids with phenolic surfactants, as well as conjugation extension of phosphole-lipid headgroup. Such π–π interactions can modify the directional growth of domains within 2D film, however, it is not strong enough to completely eliminate the 3D aggregate formation.