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

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 pi-groups by structural design, and further proposed a general strategy wherein the introduction of hydrogen into the pi-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 A(2)CO(3) (A = Na, K, Rb, Cs) and K3CO3F, with the [CO3](2-) group, and AHCO(3) (A = Li, Na, K, Rb) and K2HCO3F center dot 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/Ca-3(C3N3O3)(2) to Na2HBO3/Rb-2(HC3N3O3). To further clarify the origin of the enhancement in band gap and birefringence, the bonding electron density difference Delta rho 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.

Automated summary

In this study, a general strategy was proposed to improve birefringence by introducing hydrogen into traditional pi-conjugated groups. The compounds containing [HCO3]- group showed enhanced optical anisotropy and comparable band gaps. Experimental results demonstrated a significant improvement in the optical anisotropy and birefringence of these compounds compared to previous ones.