Monolayer Formation Properties of Cholesterol-Based Azobenzene Amphiphiles with the Natural and the Inverted C3 Configuration

Abstract

The monolayer formation properties of cholesterol-based azobenzene amphiphiles with the natural (S)-C3 configuration (Is) and the inverted (R)-C3 configuration (1R) were examined at the air-water interface. The computational study reveals that Is adopts an extended conformation whereas 1R adopts an L-shaped bent conformation. Is gave an expanded phase with A 0 (limiting area) = 0.60 nm 2 molecule -1 and A 1 (lift-off area) = 0.64 nm 2 molecule -1 whereas 1R gave a condensed phase with A 0 = 0.49 nm 2 molecule -1 and A 1 = 0.54 nm 2 molecule -1 . Examination using reflectance spectroscopy established that Is forms a monolayer with a J-aggregation mode (λ max 407 nm) and with an increase in the compressibility it changes to an H-aggregation mode (λ max 336 nm) whereas 1R forms a monolayer having a λ max (362 nm) comparable with the monomeric absorption maximum, indicating that the electronic interaction among the azobenzene moieties is absent. The morphological changes in the monolayers were directly observed by an optical microscope and reasonably correlated with the spectroscopic changes. The results indicate that the absolute configuration at C3 has a crucial influence on the aggregation properties of cholesterol-based amphiphiles in a monolayer system.