Chain length-dependent photoinduced formation of azobenzene aggregates

We describe photoinduced formation of blue fluorescent aggregates of azobenzene derivatives, whose isolated molecules in Solution are not fluorescent at ambient temperature, with different alkyl chain lengths. In contrast to nonfluorescent C0 (4-hydroxyazobenzene), C10 and C24 with long alkyl chains started being blue fluorescent upon UV light irradiation for 60 min and the fluorescence intensities were more enhanced with increasing UV light irradiation time. Such a striking fluorescence enhancement by UV light irradiation is closely associated with the unique formation of self-assembled nanoaggregates (50-70 nm and 30-60 nm in diameter for C10 and C24, respectively) of cis-azobenzene derivatives with long alkyl chains as a result Of intermolecular interaction of azobenzene moieties with long alkyl chains. Moreover, photoinduced blue fluorescence kept stable even after sufficient thermal cis-to-trans isomerization.