Cationic Effects on the Structural Dynamics of the Metal Ion-Crown Ether Complexes Investigated by Ultrafast Infrared Spectroscopy

It is usually believed that the binding affinity and selectivity of an alkali metal ion with crown ether are defined by the size matching model. However, the underlying mechanism of the specific host-guest interactions and the structural dynamics of the metal ions confined in the cavity of the crown ethers in the solutions are still not dear. In this report, a series of alkali thiocyanate salts (XSCN; X = Li, Na, K, and Cs) complexed with 18-crown-6 (a typical crown ether) in the chloroform solutions were studied by the polarization-selective infrared pump-probe spectroscopy and the ultrafast two-dimensional infrared (2D IR) spectroscopy. The SCN- counteranions were employed as the local vibrational probe to reveal the specific host-guest interactions in the crown ether complexes. The rotational dynamics and spectral diffusion of SCN- vibration were both measured by ultrafast IR spectroscopy, and it was found that the metal cations hosted by the crown ethers can have a pronounced effect on the rotational dynamics of the counteranions. The reorientational time constants of the SCN- vibration in the complexation follow the order Li+ > Na+ > K+ similar or equal to Cs+. More importantly, the spectral diffusion dynamics of SCN-, which quantifies the decay of the correlation of the frequency fluctuations in the complexation, was also affected by the metal ions but showed a different order of cationic effect. A detailed analysis of the 2D IR data showed that the spectral diffusion of SCN- counteranion clearly decayed with two different time scales in the complex of 18-crown-6 with K+. The 3-4-fold slowdown in spectral diffusion indicated that the fluctuation of SCN- vibrational transition frequency was strongly affected by the K+ cation due to the geometric constraint imposed by the crown ether. The results should help the researchers to unravel the specific host-guest interactions and further reveal the origination of the binding selectivity of crown ether for metal cations in the condensed phases from the perspective of structural dynamics.