Multiresponsive Photo‑, Solvato‑, Acido‑, and Ionochromic Schiff Base Probe

Nonclassical protomeric tautomerism in Schiff bases have the advantage of controlling and differentiating specific interactions in the −CN– linkage since its interactions are not governed by keto–enol tautomerism. Here, we report about the optical properties of a Schiff base probe (P1). X-ray structure analysis evidenced the existence of an intramolecular hydrogen bond which is responsible of a photochromic-fluorescent behavior. The properties of P1 were investigated by UV–vis and fluorescence spectroscopy in solution and solid state. A positive solvatochromism resulting from specific interactions taking place in P1 was studied by three different solvent scales, namely Lippert–Mataga, Kamlet–Taft, and Catalán, finding consistent results. Moreover, a strong acidochromic behavior was detected and the pK_a and pK_a* values were determined, finding a photobasic character. Further, an ionochromic behavior was stablished. P1 exhibits a λ-ratiometric fluorescence response toward Sn­(IV) giving a luminescence color change from blue to green, displaying also a chromogenic response. Finally, theoretical calculations were conducted to analyze the probe mechanism in terms of natural transition orbitals (NTOs) and spatial extent of charge transfer excitations. The present contribution focused on the factors determining the ability of a single Schiff base probe to present photo-, solvato-, acido-, and ionochromism.