Rational Design of Crown Ether‐Based Fluorescent Sensors for Group II Cations: Insights From Vibrational Spectroscopy and Computation

Rational design of fluorescent sensors requires knowledge of the molecular structure of the sensor–analyte complex. We use infrared spectroscopy on isolated sensor‐cation complexes allied with density functional theory (DFT) calculations to reveal, for the first time, solution phase structures of crown ether‐based fluorescent sensors for group II cations. We show that similar fluorescent responses can arise from different ion binding modes thereby rationalizing the observed differences in UV absorption spectra in benchmark sensors. Finally, we are able to outline a structural explanation for the dramatic water sensitivity of the fluorescent response of these sensors to Ca ²⁺ .