Arylazopyrazole photoswitches in aqueous solution: Substituent effects, photophysical properties and host-guest chemistry.

Arylazopyrazoles (AAPs) represent a new class of photochromic azo compounds that offer high-yield E/Z isomerization in both directions upon irradiation with UV or green light, respectively. Additionally, AAPs show a light-responsive host-guest interaction with beta-cyclodextrin with comparable binding affinities to those reported for azobenzenes. Substitution of one benzene of the azobenzene for a dimethylpyrazole leads to a separation of the n-pi* and pi-pi* absorbances of the molecules to enable optimized photochromic behavior. Because it is expected that AAPs show a strong dependence of the photophysical properties on the substitution pattern of the photochromic core, herein a comprehensive study on the effects of various substituents on water- soluble AAPs is reported. Therefore, substituents with different electronic and steric effects were introduced on different positions of AAP. The E/Z isomerization, the half-life of the Z isomer, the quantum yields, and the stability against glutathione were investigated. Additionally, DFT calculations were conducted and the host-guest chemistry with bcyclodextrin was investigated.