Metal complex-based multi-stimuli responsive amphiphiles: development into monolayers and giant vesicles

Abstract

Stimuli-responsive soft materials have been extensively studied for numerous applications and fundamental understanding of protocells. In this work, multi-stimuli-responsive ruthenium complexes (p-1 and d-1) were incorporated into monolayers and giant multilamellar vesicles. The complex p-1 with two long alkyl chains has a cylinder-like shape, while the photoisomerized complex d-1 has a truncated cone shape. At the air–water interface, p-1 acted as a photoresponsive monolayer due to photoisomerization to d-1. The experimentally obtained molecular area of p-1 (0.79 nm² molecule⁻¹) showed good agreement with the calculated size of the hydrophilic moiety in p-1 (0.76 nm² molecule⁻¹) according to π-A isotherm curves. The molecular area increased to 1.00 nm² molecule⁻¹ upon light irradiation, indicating photoisomerization to d-1 at the air–water interface. Spherical giant vesicles incorporating phospholipids and p-1 displayed distortion under visible light irradiation due to photoisomerization to cone-shaped d-1. Upon heating in the dark, the shape of the vesicle changed owing to thermal back isomerization to p-1. The vesicles showing a slight distortion due to light illumination displayed recovery to the original spherical shape, where the degree of distortion correlated with the time of visible light irradiation and heating. In contrast, those with more distortion due to light irradiation showed further distortion or division. The morphological changes in the vesicles were highly correlated with the composition of phospholipids of the vesicles, indicating that membrane fluidity is a key factor for the muti-stimuli-responsivity of the giant multilamellar vesicles.