Enhanced non-linear optical response of calix[4]pyrrole complexant based earthides in the presence of oriented external electric field

Abstract

We report here rationally designed alkaline earthides using calix[4]pyrrole complexant. Alkali metal atoms are intercalated into calix[4]pyrrole while alkaline earth metals are kept outside the cavity to form M+(calix[4]pyrrole)M- (M+ = Li, Na & K; M- = Be, Mg & Ca). Under the action of four N atoms, the s valence electron of the alkali metal atoms overflows from the bottom of the cup to becomes excess electrons and enwraps the alkaline earth metal forming negative charge on alkaline earth metal. Nine complexes with general formula, M+(calix[4]pyrrole)M- (M+ = Li, Na and K; M- = Be, Mg and Ca) are designed and their geometric, electronic and nonlinear optical (NLO) properties are studied in detail. The alkaline earthide identities of the designed compounds are determined by distribution of electron densities in highest occupied molecular orbitals (HOMOs) and natural bond orbital (NBO) analyses. Presence of negative charges and localization of HOMOs on alkaline earth metals in all these complexes shows the alkaline earthide nature of these complexes. The spectra of partial density of states (PDOS) further confirm the presence of excess electrons on alkaline earth metals. These alkaline earthides show considerably high nonlinear optical responses with the first hyperpolarizabilities (βo) up to 1.98x104 au. Such high βo values are attributed to the presence of excess electron on alkaline earth metals. Moreover, their low transition energies from 2.06 to 3.87 eV reveal the susceptible and diffuse nature of excess electron. We also calculated the NLO response of designed complexes under the influence of external electric field of strength 0.001 a.u and found that NLO response of Li+[Calix4pyrrole]Ca- has increased from 1.90 × 104 a.u to 8.19 x106 a.u. We expect that our work may inspire experimental chemists to explore synthetic, chemical and physical characterization of these new alkaline earthides.