Self-assembly of long-lived cis-azobenzenes into crystalline nanoparticles

Abstract

The development of functional nanostructured materials for optoelectronic and biological applications requires practical approaches to the simple synthesis of specific structures and the long-term stability of optical properties. Here we demonstrate that simple UV light irradiation of a new type of ortho-alkylated azobenzene (1-[(E)-2-sec-butyl-4-(4′-ethoxy-3,5-diethylbiphenyl-4-yldiazenyl)phenoxy]hexadecane, 1) generates an unusually long-lived cis-azobenzene and the self-assembly of the long-lived cis-azobenzene drives the formation of strongly fluorescent and so far unknown cis-azobenzene nanocrystals of approximately 1–7 nm in size. TEM (transmission electron microscopy) experiments exhibit the characteristic lattice fringe of 2.3 Å spacing from each nanocrystal. Interestingly, because thermal cis-to-trans back isomerization of 1 possessing bulky substituents at ortho positions is significantly restrained, the self-assembled cis-azobenzene nanocrystals show highly stable absorbance and fluorescence efficiency for more than 6 months.