Abstract
Salicylideneaniline (SA) sorbed in cation-exchanged M-ZSM-5 (M = H+, Li+, Na+, K+, Rb+, Cs+ and Zn2+) zeolites was studied by spectroscopic techniques assisted by quantum-chemical calculations. The nature of extra-framework cations present in the zeolite void was found to affect the spectral signature of the sorbed SA molecule that points to the shift of tautomeric equilibrium between the enol and keto forms. Small size cations, such as H+ and Li+, stabilize a cis-keto SA tautomer along with a enol one in the zeolite structure. The calculations indicate that the sorbed cis-keto tautomer may have the dipole large enough to be considered as a zwitterion. New features appearing in the spectra with the increase of the cation size were attributed to the presence of trans-keto SA tautomer, which up to now has been observed only in time-resolved spectroscopic experiments. A strong interaction of the molecule with cations in Zn-ZSM-5 zeolite results in the chelation of enol SA with the divalent Zn2+ ions. The results of the study suggest that the tautomeric equilibrium of molecules belonging to the Schiff base family can be tuned by the confinement in the nanoporous materials via a choice of topology of zeolite framework and the nature of extra-framework cations.
Highlights
The Diffuse Reflectance UV-Visible (DRUVv) spectra of the SA@M-ZSM-5 systems are shown in Figure 2, where they are compared to the spectrum of SA in silicalite-1 and to that of the molecule in a solid state
Salicylideneaniline sorbed in alkali-metal exchanged ZSM-5 zeolites was studied by means of electronic and vibrational spectroscopies
The results obtained for the SA@M-ZSM-5 zeolites with small size cations, such as H+ and Li+, point to the stabilization of a cis-keto SA tautomer that co-exists with a enol one in the zeolite void
Summary
Among molecules belonging to the Schiff base family, N-salicylideneaniline (SA) is a model compound largely studied in both the solid state [1,2,3,4,5] and solutions with various solvents [3,5,6,7,8,9,10,11,12,13].Due to its photochromic, thermochromic and solvatochromic properties, this molecule is of significant interest for potential applications in various domains from optical devices [14,15,16] and molecular switches [17,18,19] to medicine [20,21,22]. Among molecules belonging to the Schiff base family, N-salicylideneaniline (SA) is a model compound largely studied in both the solid state [1,2,3,4,5] and solutions with various solvents [3,5,6,7,8,9,10,11,12,13]. SA is well known to adopt an enol form in the ground state. This form returns to the ground state enol SA conformer via either (cis-keto)∗ → cis-keto deactivation or (cis-keto)∗ → trans-keto isomerization followed by a conversion to the cis-keto tautomer in the ground state [5,25,26,27,28]. Results of quantum-chemical calculations generally confirm this scenario [25,29,30]
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