2-Hydroxyazobenzenes to tailor pH pulses and oscillations with light

This paper evaluates the 2-hydroxyazobenzene platform for tailoring proton concentration pulses and oscillations with monochromatic light. The easily prepared 2-hydroxyazobenzenes exhibit large absorptions in the near-UV range. Photoisomerization was investigated by UV/Vis absorption, (1)H NMR spectroscopy, and steady-state fluorescence emission. In the whole investigated series, the trans stereoisomer of the 2-hydroxyazobenzene motif provides the corresponding cis derivative with an action cross section in the 10(3) M(-1) cm(-1) range. At the same time, photoisomerization is accompanied by a significant pK drop of the phenol group. According to the phenyl-substituent pattern, cis-to-trans thermal back-isomerization can be tuned in the 10 ms-100 s range. Up to 2 units of reversible pH drops or pH oscillations on the 10 s timescale have been obtained by appropriately tailoring single-wavelength illumination of 2-hydroxyazobenzene solutions.