NaB(OH)3CH3: a deep-ultraviolet optical crystal with unprecedented methyl-modified [B(OH)3CH3] units

Abstract

Borate is a rich source for the exploration of high-performance optical materials, but non-π-conjugated [BO₄] units behave with limited anisotropy. Non-π-conjugated [B(OH)₃CH₃] units as novel deep-ultraviolet (DUV) building blocks are found by the intrinsic modification of the tetrahedra. The first methyl borate DUV optical crystal, NaB(OH)₃CH₃, is successfully synthesized, which improves the problem of the non-π-conjugated tetrahedra. Compared to the hydroxyborates NaB(OH)₄ with the same space group, NaB(OH)₃CH₃ not only has a short absorption edge of 191 nm but also exhibits enhanced methyl-driven birefringence (≥0.051 at 546 nm). A NaB(OH)₃CH₃ crystal can be grown from a simple solution evaporation process. Additionally, the [B(OH)₃CH₃] unit is available as a new DUV birefringence-active functional group, which can balance the bandgap and birefringence. The [B(OH)₃CH₃] unit is discovered to be useful for deep-ultraviolet optical crystal exploration and can combine with other deep-ultraviolet borate primitives to form rich DUV optical materials, which greatly promote the development of new DUV optical crystals with excellent properties and versatile structures.