How To Maximize Birefringence and Nonlinearity of π-Conjugated Cyanurates.

Materials with large birefringence (Δ) are highly needed by fiber-optic isolators, whereas crystals showing strong second-order harmonic generation (SHG) are the key component for all-solid-state laser devices. Cyanurate constructed by the planar π-conjugated ring (CNO, CY) is a class of fascinating materials exhibiting not only very large Δ (larger than that of calcite) but also strong SHG (2 times that of β-BaBO, BBO). Here, we report five new cyanurates (-) and their single-crystal structures; among them, realizes a Δ = 0.4, the maximum in the cyanurate family, and realizes a = 6.69 pm/V, one of the highest values in the cyanurate family. We discover a dependence between Δ and the coplanarity of the CY rings that is explicitly described by a Boltzmann function, in which the coplanarity is defined by the dihedral angle (γ) between the CY plane and the principal optical axes plane. realizes the maximum Δ due to its zero γ that indicates perfect coplanarity. Such a Δ-γ dependence also allows the Δ prediction of cyanurates. Furthermore, we uncover that the SHG intensity of cyanurates increases monotonically as the angle (θ) between the maximum hyperpolarizability (β) vector and the crystal 2 polar axis decreases. We predict the to extend well beyond such a value and to maximize at θ = 0°. Our results have significant implications for the future rational design and discovery of high-performance materials of π-conjugated and other related systems.