Large second harmonic generation and birefringence from extended octupolar π-conjugated structures

Abstract

The exploration of crystal materials for optical manipulation by nonlinear optical (NLO) and anisotropic light-matter interaction is of paramount importance in modern science and technology. However, in such crystal materials, finding the right balance between second harmonic generation (SHG), birefringence, and the bandgap presents a significant challenge. In this contribution, we employ extended octupolar π-conjugated groups devoid of intrinsic dipole moments to construct melonate-based inorganic-organic hybrid crystals, thereby achieving simultaneous large optical nonlinearity and anisotropy. In accordance with this strategy, Rb₃[C₆N₇(NCN)₃]·3H₂O (I) and Cs₃[C₆N₇(NCN)₃]·3H₂O (II) were obtained and subjected to detailed investigation. Strong SHG responses of ∼9× KH₂PO₄ and a large birefringence of at least 0.6@546 nm were observed for I and II crystals, respectively, together with a suitable bandgap for visible-UV application. Theoretical calculations indicated that octupolar [C₆N₇(NCN)₃]^3- groups of I and II arranged in a near parallel configuration exhibit a discrete π electron distribution, resulting in enhanced NLO susceptibilities and maximal polarizability difference. This work underscores the potential of octupolar structures with extended π-conjugation as a promising avenue for the discovery of NLO and birefringence crystals.