18-Crown-6 Coordinated Metal Halides with Bright Luminescence and Nonlinear Optical Effects

Abstract

The crown-ether coordination compounds ZnX₂(18-crown-6), EuX₂(18-crown-6) (X: Cl, Br, I), MnI₂(18-crown-6), Mn₃Cl₆(18-crown-6)₂, Mn₃I₆(18-crown-6)₂, and Mn₂I₄(18-crown-6) are obtained by ionic-liquid-based synthesis. Whereas MX₂(18-crown-6) (M: Zn, Eu) show conventional structural motives, Mn₃Cl₆(18-crown-6)₂, Mn₃I₆(18-crown-6)₂, and Mn₂I₄(18-crown-6) exhibit unusual single MnX₄ tetrahedra coordinated to the crown-ether complex. Surprisingly, some compounds show outstanding photoluminescence. Thus, rare Zn²⁺-based luminescence is observed and unexpectedly efficient for ZnI₂(18-crown-6) with a quantum yield of 54%. Unprecedented quantum yields are also observed for Mn₃I₆(18-crown-6)₂, EuBr₂(18-crown-6), and EuI₂(18-crown-6) with values of 98, 72, and 82%, respectively, which can be rationalized based on the specific structural features. Most remarkable, however, is Mn₂I₄(18-crown-6). Its specific structural features with finite sensitizer–activator couples result in an extremely strong emission with an outstanding quantum yield of 100%. Consistent with its structural features, moreover, anisotropic angle-dependent emission under polarized light and nonlinear optical (NLO) effects occur, including second-harmonic generation (SHG). The title compounds and their optical properties are characterized by single-crystal structure analysis, X-ray powder diffraction, chemical analysis, density functional theory (DFT) calculations, and advanced spectroscopic methods.