Hydroxycoumarin encapsulated sulfonatothiacalix[4]arene: 1H NMR, steady state fluorescence and theory

Abstract

Encapsulation of 4-hydroxy coumarin (4HC) within the p-sulfonatothiacalix[4]arene has been investigated employing the steady state fluorescence, 1H NMR titration experiments combined with ωB97x-D based density function theory. Theoretical calculations have shown that in the 4HC⊂TSCX4 complex the guest encapsulates partially within the cavity of the macrocycle via CH⋅⋅⋅π interactions while the lactone ring excludes the cavity binds to sulfonate portals of the host by hydrogen bonding interactions. Natural transition orbital analyses were used to assign electronic transitions in 4HC⊂TSCX4 complex and its 7HC analogue. The first allowed vertical excitation at 266 nm in the complex arises from a transfer of electron density on the lactone ring of 4HC to aromatic ring of the macrocycle. Further the C=O vibration at 1791 cm−1 in the infrared spectra of 4HC downshifts to 1724 cm−1 upon complexation with the TSCX4. The noncovalent interactions reduced density gradient method was employed to characterize non-bonding interactions in the complex in conjunction with quantum theory of atoms in molecules. Steady state fluorescence measurements reveal that addition of TSCX4 in DMSO to 4HC results in quenching of the 397 nm band. Furthermore, the Stern-Volmer quenching constant of the complex determined to be Ksv = 2.19 × 104 M−1 has static and the upward curvature concave towards the I0/I axis in Stern-Volmer plots thus was noticed. The recorded excited state lifetime of complex led to the bimolecular quenching constant kq to be 1.98 × 104 M−1 s-1. Upon complexation the hydroxyl protons (Hb) in 4HC engender large deshielding as opposed to aromatic protons of TSCX4 macrocycle which emerge with up-field signals in the measured 1H NMR spectra. The noncovalent binding manifests in NOESY spectrum of the 4HC⊂TSCX4 complex. The association constant of the complex obtained from the UV–Vis and steady state fluorescence correlate well with those from 1H NMR titration experiments which establish that the inclusion complex is stable.