pH Tuning of Water-Soluble Arylazopyrazole Photoswitches.

: Arylazopyrazoles are an emerging class of photoswitches with red-shifted switching wavelength, high photostationary state, long thermal half-lives and facile synthetic access. Understanding pathways for a simple modulation of the thermal half-lives, while keeping other parameters of interest constant, is an important aspect for out-of-equilibrium systems design and application. Here, we demonstrate that the thermal half-life of a water-soluble, PEG-tethered arylazo-bis(o-methylated)pyrazole (AAP) can be tuned by more than five orders of magnitude using simple pH adjustment, which is beyond the tunability of azobenzenes. The mechanism of thermal relaxation is investigated by thorough spectroscopic analyses and density function theory (DFT) calculations. Finally, we transfer the concepts of a tunable half-life from the molecular scale to the material scale. Based on the photochromic characteristics of E- and Z-AAP, we showcase transient information storage in form of light-written patterns inside films cast from different pH, which in turn leads to different times of storage. With respect to prospective precisely tunable materials and time-programmed out-of-equilibrium systems, an externally tunable half-life is likely advantageous over changing the entire system by the replacement of the photoswitch.