Mina R. Han

A Suzuki coupling reaction produced the title compound, C₄₄H₆₆N₂O₂, which consists of a photoisomerizable azobenzene unit coupled to a biphenyl group and a long alkyl chain. The molecule has a trans configuration with respect to the azo (–N=N–) group, and the long alkyl chain adopts a well ordered zigzag arrangement.

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.

We describe here the highly fluorescent self-assembled spherical aggregates of an azobenzene molecule without a specific ionic component in organic solution under UV light illumination. The first stage of trans-to-cis photoisomerization by UV light at 365 nm was followed by a significant enhancement, up to about 1000 times, of the emission from an azobenzene molecule (CN2Azo) with a long alkyl chain, which is due to the spontaneous formation of spherical organic aggregates. Fluorescence emission was further enhanced in the dark, and the quantum yield increased to about 0.3. We also report the significant size and structural changes of the aggregates, from nanometer-scale micelle-like aggregates to micrometer-scale vesicular aggregates, obtained only from the variation in the concentration of an azobenzene derivative. The light-driven azobenzene aggregates show the size and structure dependences of emission wavelength from violet-blue to green-yellow.