A nonlinear optical crystal with deep-ultraviolet transparency and appropriate birefringence achieved using π-conjugated confined [B3O3F4(OH)]2−

Finding a deep-ultraviolet (deep-UV, λ < 200 nm) nonlinear optical (NLO) crystal is vital to a number of advanced optical technologies, yet it remains a great challenge. Herein, we report a deep-UV NLO crystal, K2B3O3F4(OH) (KBOFH), composed of π-conjugated confined groups. It exhibits a relatively large birefringence of 0.054@550 nm and a wide transparency window with an absorption edge below 200 nm. Theoretical calculations reveal that the observed optical properties of KBOFH originate from the confined π-conjugated [B3O3F4(OH)] unit, in which the π-conjugated interactions are decoupled by the tetrahedral non-π-conjugated [BO2F2] unit. This work provides an opportunity to design deep-UV NLO crystals containing π-conjugated confined groups.