PbBe 2 B 2 O 6 : an ultraviolet nonlinear-optical crystal with unprecedented π-π interacting BeBO 5 group.

π-Conjugated-group systems are fascinating and have been pursued for nonlinear materials owing to their superior optical and electronic properties, but are so far quite limited. Here, we report a new ultraviolet (UV) nonlinear-optical (NLO) beryllium borate crystal PbBe 2 B 2 O 6 that manifests an unprecedented π-π interacting BeBO 5 group, constructed by interconnected π-conjugated BeO 3 and BO 3 units. The PbBe 2 B 2 O 6 crystal has a short UV cutoff wavelength of 249 nm, phase-matching behavior in the whole transmission region from UV to IR light region (249-3166 nm), and remarkably the largest NLO effect (18.5 × KDP) among the π-conjugated UV NLO crystal materials. Theoretical calculation unravels that the highly NLO-active BeBO 5 group has strong π-π interactions. This work offers an innovative source of BeO 3 -participating NLO materials and expands the frontier of NLO research.