Enhancement of band gap and birefringence induced via π-conjugated chromophore with “tail effect”

Abstract

Birefringent materials, crucial components in modulating the polarization of light, are of great significance in optical communication and the laser industry. However, it is difficult to dig out excellent material motifs for designing and fabricating novel birefringent materials. In this work, we reassembled the traditional π-groups by structural design, and further proposed a general strategy wherein the introduction of hydrogen into the π-conjugated groups is beneficial to improving birefringence. The planar [HCO₃]⁻/[HBO₃]²⁻/[HC₃N₃O₃]²⁻ groups, obtained by introducing H into the [CO₃]²⁻/[BO₃]³⁻/[C₃N₃O₃]³⁻ groups, exhibit enhanced polarizability anisotropy and comparable HOMO–LUMO energy gap. The carbonates A₂CO₃ (A = Na, K, Rb, Cs) and K₃CO₃F, with the [CO₃]²⁻ group, and AHCO₃ (A = Li, Na, K, Rb) and K₂HCO₃F·H₂O, with the [HCO₃]⁻ group, were screened out as targeted materials and investigated by first-principles calculations and experimental verification. With the evolution of the structure of the compounds containing [CO₃]⁻ to the compounds containing [HCO₃]⁻, the band gaps and birefringence increase significantly. Particularly, the enhancement in optical anisotropy of CsHCO₃ can reach 59.76% as compared to Cs₂CO₃. Consistent results were also found in Na₃BO₃/Ca₃(C₃N₃O₃)₂ to Na₂HBO₃/Rb₂(HC₃N₃O₃). To further clarify the origin of the enhancement in band gap and birefringence, the bonding electron density difference Δρ and electron density difference were analyzed. It is indicated that the enhanced optical anisotropy can be attributed to the “tail effect” of the extended electronic distribution from the [CO₃]²⁻/[BO₃]³⁻/[C₃N₃O₃]³⁻ groups to the [HCO₃]⁻/[HBO₃]²⁻/[HC₃N₃O₃]²⁻ groups. This study offers a new guide to the exploration of outstanding genetic candidates for birefringent materials.