In this modern high-tech era, nonlinear optical (NLO) materials are playing a very decisive part attributed to their potential applications and excellent properties in different fields including optical computing, optical communicating equipment, digital image automation, and laser appliances. Both experimental and theoretical methods are used for exploring NLO materials [[1], [2], [3], [4], [5]]. For designing NLO materials those materials are preferably used which have a structural design consisting of a highly delocalized π-conjugated framework for example, graphdiyne, graphyne, graphene and doped with alkali metals (AM) i.e. K, Li, Na show hyper polarizabilities up to 3.93 × 105 au [6] whereas graphdiyne doped with Li3NM (M = K, Li, Na) shows hyperpolarizability up to 2.88 × 105 au [7]. Metal ligation in organic π-systems, di-radical character, transition metal doping are other strategies used to enhance the non-centrosymmetric character of compounds [[8], [9], [10], [11], [12], [13]]. The introduction of excess electrons in an organic or inorganic system is a recently developed advanced technique that is now used for enhancing NLO characteristics of materials [[14], [15], [16], [17], [18], [19]]. Notable role of electride (excess electrons) in increasing first hyperpolarizability is revealed by solvated electron systems [[20], [21], [22]]. The hyperpolarizability βo is the NLO property of a molecule. Through solvated electron systems, a comparison between the values of hyperpolarizability of the molecular complex having excess electron (H2O)3e with that of the equivalent molecular complex without having excess electron (H2O)3 revealed that hyperpolarizability (βo) for free, excess electron containing system is six times higher in magnitude than the system without having excess electrons [[23], [24], [25]]. With the discovery of the more sensitive and responsive nature of excess electrons, a new door has been opened to propose new strategies for the production of NLO molecules characterized by free electrons [26,27].

Mainly electrides and alkalides are those compounds that have excess electrons and they represent the large NLO characteristics mainly the first hyperpolarizability [[28], [29], [30], [31], [32]]. Electrides are ionic compounds having complex electron donating basis and trapped electrons as obverse ions. The elementary design principle of alkalides comprises the incorporation of a complexant that includes two AM in a system where one AM act as an electron source while another AM is surrounded by diffuse excess electrons forming an anion placed at the anionic site. The low electron affinity of AM binds the electrons of alkali ions loosely, dispersed in space, and less confined to the nucleus. When comparing electrides and alkalides, a significant difference is found between the properties of single electron ion as in electride and the AM anion as in alkalides. Even AM anions with lower values of transition energy show higher oscillator strength and therefore alkalides exhibit larger NLO response than electrides [21,33].

Finding better electron sources is an effective way to design NLO materials that work better having superior functioning properties. This approach was also designed for coinage metals-based alkalides with significant NLO characteristics [34]. Another significant factor that is responsible for the enhancement of hyperpolarizability is the shape of the complexant. The βo of Li+(NH3)4M− with loop-shaped complexant (NH3)4 is around six folds greater than Li+(Calix [4] pyrrole)M− with cup-shaped complexant [35,36]. This shows that key factors affecting the NLO parameters of materials to a large extent are the size and shape of complexant and electron sources.

All the given hypothetically reported alkalides comprise anions as AM (deficient in electrons) while the complexant and electron sources are different in different alkalides. Recently it has been reported that similar compounds that accept electrons as AEM instead of AM exhibit more NLO characteristics in which partially filled p-electron takes part in the formation of excess electrons contradictory to alkalides where partial p-electron does so [37]. In presence of AM, it is not easier for AEM to be negatively charged due to ns2 (closed shell) configuration and it is also because AEM has lower electron affinity as compared to AM. To maintain a stable both the cation of AM and anion of AEM at the same time it is important to choose an appropriate complexant.

Based on these characteristics, hexaammine is employed as a complexant in this study for scheming innovative AEM as an undoped reference, we have applied DFT calculations on the alkali (Li, Na, K) and AEM (Be, Ca, Mg) doped hexaammine structures to study their NLO characteristics. The intended outcomes showed that the assimilation of AM and AEM can inflate the NLO attributes of hexaammine. Nine combinations of hexaammine after doping with AM and AEM formed in the present work are given as; Be−(NH3)6K+(HEAM-1), Be−(NH3)6Li+(HEAM-2), Be−(NH3)6Na+(HEAM-3), Ca−(NH3)6K+(HEAM-4), Ca−(NH3)6Li+(HEAM-5), Ca−(NH3)6Na+ (HEAM-6), Mg−(NH3)6K+ (HEAM-7), Mg−(NH3)6Li+(HEAM-8) and Mg−(NH3)6Na+(HEAM-9). The consequences of the present research work can suggest intuitions for the planning of novel and unexpected NLO materials positioned on doped hexaammine (HEAM).