Photoisomerization of Azobenzene Units Controls the Reversible Dispersion and Reorganization of Fibrous Self-Assembled Systems

Abstract

N-(L-valyl-L-valyl-L-valyl)azobenzene-4-carboxamide [Azo(LVal)(3)] is a low molecular weight gelator that forms a photofunctional fibrous assembled system; this assembly undergoes dispersion/reorganization upon trans-to-cis photoisomerization, which, as a result of the breaking and reforming of hydrogen bonds, induces reversible sol-gel transitions. In this paper, we describe the mechanism by which azobenzene isomerization induces the breaking and reorganization of these assembled systems. We applied Fourier transform infrared spectroscopy to investigate the effect of the irradiation time on the change in absorption intensity in the amide I region. The lifetime of the cis isomer influences the photoinduced breaking and reforming of hydrogen bonds between trivalyl units. Because the cis isomer of Azo(LVal)(3) had a long lifetime, its assemblies underwent reversible phototriggered dispersion and organization. In contrast, the lifetime of the cis isomer of 4'-dimethylamino-N-(L-valyl-L-valyl-L-valyl)azobenzene-4-carboxamide [pMR(LVal)(3)] was too short to disrupt the hydrogen bonds in its fibrous self-assembled system.