Rare Crystal Structure of Open Spirolactam Ring along with the Closed-Ring Form of a Rhodamine Derivative: Sensing of Cu²⁺ Ions from Spinach

Crystal structures of a rhodamine derivative in its closed and open spirolactam ring forms were developed, which allows selective and sensitive detection of Cu²⁺ ions at a micromolar range in neutral medium. The chemosensing properties of the probe through a pentacoordinate Cu²⁺ ions were proven by spectroscopic and theoretical analysis. The spirolactam ring opening as the Cu²⁺ selective sensor was applied to spinach (Spinacia oleracea) to estimate the accumulation of copper as copper­(II) in the plant.