Photochemical properties of multi-azobenzene compounds

A systematic study is reported of the photochemical properties of the multi-azobenzene compounds bis[4-(phenylazo)phenyl]amine (BPAPA) and tris[4-(phenylazo)phenyl]amine (TPAPA) compared to the parent molecule 4-aminoazobenzene (AAB). The bis- and tris-azobenzenes were synthesised by a variant of the Ullmann reaction and exist in their stable all-E forms at room temperature. Striking changes in the spectral positions and intensities of their first ππ* absorption bands compared to AAB reveal strong electronic coupling between the AB units. The nature of the excited states was explored by quantum chemical calculations at the approximate coupled-cluster (CC2) level. Upon UV/VIS irradiation, the molecules isomerise to the Z-isomer (AAB), ZE- and ZZ-isomers (BPAPA), and ZEE-, ZZE- and ZZZ-isomers (TPAPA), respectively. The photoswitching behaviours were investigated by UV/VIS and NMR spectroscopies. All individual isomers were detected by one-dimensional (1D) (1)H NMR spectroscopy (BPAPA) and two-dimensional (2D) HSQC NMR spectroscopy (TPAPA). A kinetic analysis provided the isomer-specific thermal lifetimes. The variance of the thermal lifetimes demonstrates a dependence of the Z-E isomerisation on the chromophore size and number of AB units.