Photochemical properties of multi-azobenzene compoundsElectronic supplementary information (ESI) available: Chemical syntheses of BPAPA and TPAPA, X-ray diffraction structure of TPAPA (cif file), tabulated results of excited-state calculations, 1H and 2D-HSQC NMR data for TPAPA. CCDC 897971. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c2pp25291k

• Main Authors: Bahrenburg, Julia, Sievers, Claudia M, Schönborn, Jan Boyke, Hartke, Bernd, Renth, Falk, Temps, Friedrich, Näther, Christian, Sönnichsen, Frank D
• Format: Article
• Language: English
• ISSN: 1474-905X; 1474-9092
• DOI: 10.1039/c2pp25291k

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. The photochemical properties of a bis- and a tris-azobenzene derivative were investigated by UV/VIS and NMR methods, the nature of the excited states was explored using quantum chemical calculations.