Tuning Electrostatics\nto Promote Ordered Monolayers\nof Phosphole-Lipids

Heteroatom-doped π-conjugated\nsystems have been\nexploited\nfor electronic device applications. Phosphole-containing backbones\nare particularly intriguing with regard to electron delocalization\nand ultimately control over the photophysical, redox, and charge transfer\ncharacteristics. However, practical application of these materials\ncommonly relies on well-structured, thin film architectures. Self-assembly\noffers a route to generate ordered 2D structures deposited on solid\nsubstrates. The orientation of these deposited films can be manipulated\nby the introduction of alkyl chains of varying length (to form phosphole-based\nlipids) and chemical modification of the phosphole-derived headgroup.\nExternal controls can also be considered to tune these films’\nproperties, e.g., the electrostatic interactions within the film can\nbe controlled by varying the environment with the addition of simple\nsalt counterions. A series of lipids with phosphole-based π-conjugated\nheadgroups have been designed and exhibit intramolecular conformational\nchanges in response to external conditions. Herein the 2D film structure\nin Langmuir and Langmuir–Blodgett films is reported in the\npresence and absence of halide salts. The film morphology obtained\nfrom Brewster angle and atomic force microscopy shows the formation\nof a condensed phase but also 3D aggregates, and grazing incidence\nX-ray diffraction confirmed the presence of untilted, hexagonally\npacked chains. The size of the counterion influences its ability to\nintercalate between the phosphole headgroups, which ultimately provides\na means to induce the formation of a well-ordered, single monolayer\nfilm without aggregates that can be transferred to the solid substrate.