title: Photochemical properties of multi-azobenzene compounds.
abstract: 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.
authors.affiliation: Institut für Physikalische Chemie, Christian-Albrechts-Universität zu Kiel, Olshausenstr. 40, D-24098 Kiel, Germany.
authors.forename: Julia
authors.initials: J
authors.lastname: Bahrenburg
authors.forename: Claudia M
authors.initials: CM
authors.lastname: Sievers
authors.forename: Jan Boyke
authors.initials: JB
authors.lastname: Schönborn
authors.forename: Bernd
authors.initials: B
authors.lastname: Hartke
authors.forename: Falk
authors.initials: F
authors.lastname: Renth
authors.forename: Friedrich
authors.initials: F
authors.lastname: Temps
authors.forename: Christian
authors.initials: C
authors.lastname: Näther
authors.forename: Frank D
authors.initials: FD
authors.lastname: Sönnichsen