N-(O-methoxyphenyl)aza-15-crown-5-ether derivatives: Highly efficient and wide range nonlinear optical response based cation recognition

Abstract

In this work, we demonstrate the recognition of cations by complexation of N-(O-methoxyphenyl)aza-15-crown-5-ether derivative (TZC) with alkali (Li+, Na+, K+), alkaline earth (Mg2+, Ca2+) and transition metal cations (Ni2+, Zn2+). The doping of mono- and divalent metal cations are used to significantly enhance the first hyperpolarizability values as compared to TZC. By utilizing density functional (DFT) methods, the parent molecule and metal-doped complexes have been optimized using B3LYP-D3/6-31G* level of theory. The amplitudes of first hyperpolarizability βtot have been obtained CAM-B3LYP and M06-2X functional for all studied systems. Interestingly, the atomic number, size, and charge of the metal cations and the O- and N-atoms of the crown-ether are found to be the major factor that gives rise to higher NLO response properties. From the natural bond orbital (NBO), localized orbital locator (LOL) and interactional energy analyses, alkali metal cations possessing a higher degree of ionic characteristics, whereas, the alkaline earth and transition metal cations possessing significant covalent characters. Furthermore, the frontier molecular orbital analysis and the TD-DFT studies reveal that the transition energies as a function of metal size and charge are responsible for remarkably enhanced and distinctive βtot amplitudes as large as TZC-Li+ 10,158 a.u., TZC-Mg2+ 21,948 a.u. and TZC-Ni2+ 25,483 a.u. Additionally, the electrostatic potential maps (ESP) and the difference of dipole moments (Δμ) from the ground to the first excited state have also been carried out to understand the abilities of these systems as highly efficient and selective metal cation sensors.