Role of Metal Ions (M = Li+, Na+, and K+) and Pore Sizes (Crown-4, Crown-5, and Crown-6) on Linear and Nonlinear Optical Properties: New Materials for Optical Birefringence

Density functional theory (DFT) calculations are performed on Li +, Na +, and K + complexes of crown-4, crown-5, and crown-6, respectively. Calculations show that, for all three structures, the rings are puckered to maintain the staggered conformations along the C−C dihedral angles. The Li +, Na +, and K + are deflected from the center-of-mass of the crown ethers by 0.82, 0.98, and 0.33 Å, respectively, toward the surface more exposed to O atoms. These M + complexes have different linear (α) and nonlinear (β) polarizations on the top and bottom surfaces for equal distortions on either surface. The variation in α and β is also studied with respect to the movement of the cations along the pore of the crown ether. The inherent asymmetry in the in-plane (α ip ) and out-of-plane (α op ) components of the polarizations leads to a single-humped profile of optical birefringence with respect to the motion of the M + ion along the pore. Our calculations show that the optical birefringence for these molecules, in their equilibrium geometry, is very close to their maximum possible theoretical limit.