How To Maximize Birefringence and Nonlinearity of π-Conjugated Cyanurates

Abstract

Materials with large birefringence (Δn) are highly needed by fiber optics isolators, whereas crystals showing strong second order harmonic generation (SHG) are the key component for all-solid-state laser devices. Cyanurate constructed by the pla-nar π-conjugated ring (C3N3O3, CY) is a class of fascinating material exhibiting not only very large Δn (larger than that of cal-cite), but also strong SHG (2 times that of BBO). Here we report five new cyanurates (I–V) and their single crystal structures, among them II realizes a Δn = 0.4, the maximum in the cyanurate family, and IV realizes a d33 = 6.69 pm/V, one of the highest values in the cyanurate family. And we discover a dependence between Δn and 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 XY plane. And II realizes the maximum Δn due to its zero γ that indicates perfect coplanarity. Such a Δn–γ dependence also allows the Δn-prediction of cyanurates. Furthermore, we uncover the SHG intensity of cyanurates increases monotonically as the angle (θ) between the maximum hyperpolarizability (max) vector and the crystal 21 polar axis decreases. We predict the dij to extend well beyond such a value and maximizes at θ = 0°. Our results have significant implica-tions for the future rational design and discovery of high-performance materials of π-conjugated and other related systems.