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

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-4yldiazenyl)phenoxy]hexadecane, 1) generates an unusually long-lived cis-azobenzene and the selfassembly 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 A 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. Here we show that UV light irradiation of a new type of ortho-alkylated azobenzene generates highly stable cis-azobenzene at ambient temperature and that the self-assembly of the long-lived cis-azobenzene drives the formation of unprecedented, strongly fluorescent cis-azobenzene nanocrystals for the first time (Fig. 1). The cis-azobenzenes closely packed in the crystalline states are unusually long-lived in solution as well as even in the solid state for more than I month, which results in highly stable optical properties. We anticipate that the ease with which azobenzene- based organic nanocrystals can be produced, and the advantage of maintaining stable optical properties and nanostructures over a long period, make photochromic azobenzene molecules more attractive for new optical and optoelectronic materials.