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.