Nonlinear optical properties of alkalides Li⁺(calix[4]pyrrole) M^- (M = Li, Na, and K): Alkali anion atomic number dependence

A new type of alkalide compound, Li⁺(calix[4]pyrrole)M^- (M = Li, Na, and K), is presented in theory, which may be stable at room temperature. It has been shown by our calculations that the first hyperpolarizability (β) is considerably large by means of the density functional theory method. The β values are determined at the B3LYP/6-311++G level (for the alkali atoms the 6-311++G(3df) basis set is employed) as 8.9 × 10³, 1.0 × 10⁴, and 2.4 × 10⁴ au for M = Li, Na, and K, respectively. These β values are much larger than that of electride Li⁺(calix[4]pyrrole)e^- (β = 7.3 × 10³ au) by a factor of 1.2 to 3.4. Comparing to the cryptand calix[4]pyrrole, the β values of Li⁺(calix[4]pyrrole)M^- are enhanced by 20-60 times. It is revealed, for the first time, that the β value of alkalide compounds depends on the atomic number of the alkali anion, and it can be enhanced by choosing the akali anions with larger atomic numbers. The alkali anion in the alkalide compound decreases the transition energy and also increases the oscillator strength of the main transition, consequently the β value is enhanced. This study proposes such a novel way to synthesize and design new NLO materials by using the alkali atom with a larger atomic number to create an anion in alkalide compounds.