Adamanzane based alkaline earthides with excellent nonlinear optical response and ultraviolet transparency

Strategies to design alkalides with excellent NLO responses are well established in the literature. On the contrary, alkaline earthides are quite scarce mainly because of low electron affinities of alkaline earth elements. Herein, we report two series of alkaline earthides M+(2(6)Adz)M- (M+ = Li and Na; M- = Be, Mg and Ca), based on 2(6) Adz as a complexant and alkali metals as sources of electrons for alkaline earth metals. These complexes are characterized as alkaline earthides by negatives charges on alkaline earth metals (NBO analysis) and presence of polarized-p HOMO orbital on alkaline earth meals. Presence of HOMO on alkaline earth metals is also confirmed through PDOS spectra. HOMO-LUMO gaps of the compounds are reduced significantly and range from 2.15 to 2.85 eV as compared to 5.17 eV HOMO-LUMO gap of pure cage. Besides H-L gaps, these compounds possess remarkably large nonlinear optical responses with the first hyperpolarizabilities (beta(o)) up to 1.0 x 10(6) au for Na (+)(2(6)Adz)Mg-. The nonlinear optical response of alkaline earthides is superior than those of alkalides and it is attributed to the presence of excess electron on alkaline earth metals in polarized p-orbitals. The hyperpolarizabilities are rationalized through two level method. The projection of hyperpolarizability on the dipole moment, beta(vec) for both series are comparable to beta(o) (static first hyperpolarizability) which indicate that the charge transfer is parallel to the molecular dipole moments. The alkaline earthides possess very small transition energies (Delta E) ranging from 0.6 to 2.4 eV. The UV-Vis analysis reveals that these alkaline earthides are transparent in ultraviolet region of the electromagnetic spectrum. The designed alkaline earthides are potential candidates for UV-transparent high performance NLO materials for applications in optical communication, optical computing and other laser devices. Present results will certainly inspire experimentalists and theoreticians to design and synthesize other alkaline earthides-based NLO materials with even better performance.