Robust fluorogenic non-porphyrin interaction of Zn(II) and Hg(II) naphthadiaza-crown macrocyclic complexes with C60: Spectroscopic and dispersion-corrected DFT study

Abstract

A family of Zn(II) complexes with naphthadiaza-crown macrocyclic host molecules (Ln = L1-L4) were prepared and characterized employing mass spectrometry, elemental microanalysis, 1H, 13C NMR, IR, UV–vis, fluorescence spectroscopy. The fluorescence study on [M(II)Ln]2+(M = Zn with n = 1–4, Hg with n = 5) complexes denoted on the presence of a notable selective supramolecular interaction with C60 in two different solvents, namely N-Methyl-2-pyrrolidone (NMP) and chloroform for both [Zn(II)L4]2+ and [Hg(II)L5]2+. Employing both UV–vis and fluorescence spectroscopy, 1:1 mol ratio were determined for [M(II)Ln]2+/C60 by applying the Job's plots. Comparing with their free ligands Ln interactions, the binding constant measurements (Kb) at λem = 380 for [M(II)Ln]2+/C60 indicated remarkable increases as 22 and 30% for [Zn(II)L4]2+ and [Hg(II)L5]2+, respectively. The observed quenching of the fluorescence bands in [M(II)Ln]2+ can be described in terms of a static process mechanism, attributed to a non-fluorescent supramolecular interaction of [M(II)Ln]2+ with C60. DFT calculations alongside with NMR experiments imply remarkable π-π interactions of the naphthalene functional groups on the aza-crown macrocycles of [M(II)Ln]2+ complexes with C60. The aforesaid promotions in Kb values are rare among the reported non-porphyrin and –phthalocyanine metal-ion-complexes as the host molecules for C60. Based on DFT/M06-2X and B97-D3 result, L4 represented the highest binding energy in the series, assignable to appropriate goodness of fit for C60 due to the larger macrocycle ring. Along with more negative binding energies, more than seven times higher deformation energy was calculated by B97-D3 for the [Zn(L4)]2+/C60 dyad, comparing with the parent Ln/C60. Comparisons of NCI and RDG plots of [Zn(L4)]2+/C60 with Ln/C60, predicted strong supramolecular interaction between the metal centre of [Zn(L4)]2+ with C60, confirmed by NBO analysis.