Operando XAS and UV-Vis Characterization of the Photodynamic Spiropyran-Zinc Complexes

Thermal and photoinduced processes accompanying complexation of photochromic spiropyrans (SPP) with Zn ions in acetone solution were characterized by means of UV-vis and X-ray absorption spectroscopies in operando regime. SPP ligands are usually transparent at λ > 350 nm, but after mixing with Zn ions, they produce a stable colored (ε = 32 000-38 000 M^-1 cm^-1) complex with maximum absorption at 525 nm, which makes them a powerful tool for monitoring metal-ion concentration in solution. The complex revealed fluorescence and photochromic behavior under static irradiation with visible light with constant photoreaction quantum yield across its characteristic absorption band. Zn K-edge X-ray absorption spectra show prominent changes in Zn local atomic structure between free Zn ions and Zn complex. The pump-flow-probe scheme was adapted to measure operando changes in Zn coordination upon light irradiation. Within experimental time resolution, we have determined that 20 μs after light irradiation, Zn ion is released out of the complex. This is the first example of the direct spectroscopic probe of the Zn photorelease from the spiropyran complex.